The α_2A-AR and α_2C-AR are the main autoreceptors involved in presynaptic feedback inhibition of NA, with the α_2B-AR making no significant contribution to NA feedback inhibition (14). However, the potency and affinity of NA at the α_2C-AR is higher than that for the α_2A-AR (14, 29, 69), and evidence from peripheral and CNS tissue demonstrates that the α_2C-AR would inhibit NA release at low [10–100 nM, adapted from Ref. (14)] endogenous concentrations of NA as opposed to high [0.1 –10 μM, adapted from Ref. (14)] concentrations for the α_2A-AR (14, 26). Deactivation kinetics also differs for the α_2A-AR and α_2C-AR, with the α_2C-AR displaying much slower deactivation upon removal of NA than the α_2A-AR (29). Despite their more modest presentation in the CNS, α_2C-ARs will, therefore, have distinct effects on a number of important neurotransmitters (see below), while its effects on NA’ergic transmission cannot be underestimated. Along with the α_2A-AR, α_2C-ARs are involved in the presynaptic negative feedback loop on NA release in the cortex, although α_2C-AR-mediated presynaptic inhibition occurs more slowly than that mediated by α_2A-ARs (26). Figure 1 depicts this proposed differential regulation on NA feedback and receptor pharmacodynamics mediated by α_2A-ARs and α_2C-ARs. Furthermore, the α_2C-AR produces a limited inhibition of NA release (maximum 20–30% in hippocampal tissue) in contrast to the α_2A-AR (26), which would suggest that from a therapeutic perspective, α_2C-AR modulation would provide a more subtle and targeted effect on NA release, while limited effects on NA release could potentially dampen the potential for cardiovascular side effects, which are a significant concern with α_2A-AR antagonism (26). Ordway and co-workers demonstrated that the density of α_2C-AR binding sites increases 3 weeks after the destruction of NA terminals in the rodent cerebral cortex, which suggests that α_2C-AR density is regulated by the synaptic availability of NA. In contrast, altered α_2A-AR density was not observed under the same conditions (30). This effect of synaptic availability on α_2C-AR expression might imply a unique role for the α_2C-AR in disorders characterized by noradrenergic dysregulation, such as MDD and schizophrenia.

The α_2C-AR has also been implicated in α₂-autoreceptor-mediated modulation of hippocampal and cortical DA and NA synthesis via feedback inhibition on tyrosine hydroxylase, which converts tyrosine to the DA precursor 3,4-dihydroxyphenylalanine (DOPA) (31). These authors used early subtype-specific antagonists and agonists to measure levels of DOPA and NA in rodent hippocampus and cerebral cortex, with α_2B/C-AR antagonists increasing synthesis of DOPA and α_2B/C-AR agonists decreasing its synthesis. Although the ligands used in this study were α_2B/C-AR specific ligands, the expression of α_2B-ARs is limited to the hypothalamus and does not seem to contribute to auto- and heteroreceptor function in the CNS (53). This study also reported that α_2A-AR specific antagonism and agonism were devoid of effects on DOPA. However, a limitation of this study is that the subtype-specific ligands used also present with some antagonist activity at 5-HT_1A receptors (32). α_2C-AR selective antagonism could, however, play a role in increasing DA and NA levels and thus be of benefit in the treatment of neuropsychiatric illness. Nevertheless, these findings need to be confirmed using novel, highly subtype-selective α₂-AR ligands.

The high expression of α_2C-ARs in the striatum allows it to modulate presynaptic DA release and DA-mediated behaviors (26). Of particular interest is that Zhang and co-workers (64) provided early evidence for the ability of DA to function as an activating ligand on striatal α_2C-ARs, while Sallinen and co-workers (18) used a novel α_2C-AR selective antagonist (ORM-10921) to show increased in vitro α_2C-AR potency and selectivity ratios in the presence of DA as agonist (Figure 2). These authors also reported that ORM-10921 increases extracellular DA levels in the rodent PFC. In support of the correlation between DA activity and α_2C-AR activity, early studies indicated changes in brain DA metabolism in α_2C-KO and α_2C-OE mice (67) (Figure 3). α_2C-OE mice show higher levels of the DA metabolite homovanillic acid (HVA) in the frontal cortex but not in the striatum compared to wild-type controls, whereas α_2C-KO animals showed lower HVA concentrations in the striatum (67), although not in the frontal cortex (Figure 3). These findings suggest decreased striatal but not frontal cortical DA turnover in response to α_2C-AR deactivation and increased cortical DA turnover in response to α_2C-AR stimulation. Therefore, an important relationship exists between DA and the α_2C-AR. The therapeutic potential of this can be realized in the targeting of α_2C-ARs in disorders characterized by mesolimbic-cortical DA imbalance, such as schizophrenia or as demonstrated in SIR rats (20).

α_2C-ARs also modify d-amphetamine-induced hyperlocomotion. Here d-amphetamine administration is associated with increased DA and NA release in the caudate nucleus and nucleus accumbens of the dorsal and ventral striatum, respectively, as well as in the PFC, together with co-presenting hyperactive behavior (70–72). Hyperlocomotion was further increased in α_2C-KO mice following d-amphetamine administration, while d-amphetamine-induced hyperlocomotion was attenuated in α_2C-OE mice (66). Subsequent studies with methylphenidate, a drug which also increases DA release and blocks DA and NA reuptake, showed increased response rates in a cognitive task sensitive to alterations in striatal DA levels in α_2C-KO mice (73). The effects of drugs that increase synaptic DA could, therefore, be enhanced by antagonism of the α_2C-AR, further emphasizing the role of α_2C-ARs in regulating DA release and metabolism.

Less evidence is available to delineate the role of the α_2C-AR on serotonergic function. The hippocampal and cortical synthesis of the serotonin (5-HT) precursor, 5-hydroxytryptophan (5-HTP), via the rate-limiting enzyme tryptophan hydroxylase, seems to be dependent on both α_2A-ARs and α_2C-ARs in the rodent, with α_2A-ARs emerging as the main α₂-AR modulating 5-HT synthesis (31). Non-selective α₂-AR agonism decreases 5-HTP levels in rodent hippocampus and cerebral cortex, while an increase in cortical 5-HTP levels seems to be largely induced by α_2A-specific antagonism, with a α_2B/C-AR antagonist producing an increase in 5-HTP levels (markedly less than that by a α_2A-AR antagonist). These effects were not mirrored in the hippocampus, although α_2B/C-AR specific antagonism decreased hippocampal 5-HTP levels in this brain region (31). Similarly, α₂-AR-agonist-induced inhibition of 5-HT release is dependent on both α_2A-ARs and α_2C-ARs, although the α_2C-AR exerts a more subtle effect on 5-HT release (33). These authors demonstrated that α_2C-KO mice present with lower disinhibition of agonist-induced 5-HT release in hippocampal and occipito-parietal cortex slices compared to α_2A-KO mice. The α_2A-AR is, therefore, the main α₂-AR regulating 5-HT release and possibly 5-HT synthesis. Nevertheless, selective antagonism of the α_2C-AR could result in meaningful increases in 5-HT release and region-specific 5-HT synthesis (e.g., provoking serotonergic behaviours in Flinders Sensitive Line (FSL) rats (21)), which may be of importance in various neuropsychiatric illnesses characterized by altered serotonergic neurotransmission, such as obsessive compulsive disorder, MDD, and schizophrenia. Confirmation of these findings using highly selective α_2C-AR subtype ligands and in appropriate animal models is, therefore, warranted (e.g., FSL rats; 21).

Apart from effects on the synthesis and release of monoamines, the α_2C-AR is an important mediator of striatal, but not hippocampal GABA release (65) via heteroreceptor actions. While α_2C-ARs and α_2A-ARs are located on different striatal neurons, almost all GABAergic projection neurons in the striatum contains α_2C-ARs (60), which project to the globus pallidus and substantia nigra (74). Inhibition of striatal GABA release by an α₂-AR antagonist (RX821002) is completely blocked in α_2C-KO mice, while enhancement of striatal GABA release by an α₂-AR agonist is maintained in these mice, suggesting that inhibition of striatal GABA release is strongly mediated by the α_2C-AR (65). Striatal GABA’ergic transmission and response to α₂-AC modulation is depicted in Figure 3. This response was not found with respect to hippocampal GABA release (65). These findings could suggest that selective blockade of the α_2C-AR may mediate disinhibited GABA release in brain regions with dense dopaminergic innervation and low noradrenergic innervation (3). Considering the presence of α_2C-ARs in the striatum (particularly the reward centers), and the role of GABAergic transmission in mania and the action of mood stabilizers (75), selective α_2C-AR antagonism could be of value in disorders like schizophrenia in which deficient GABAergic transmission may play a pathophysiological role (76).

Although it is known that α_2A-AR modulate glutamate release via heteroreceptor-mediated cross-talk at glutamatergic neurons (77, 78), very little information is available on the specific role of the α_2C-AR on central glutamatergic neurotransmission. Additional studies delineating the role of the α_2C-AR on glutamatergic neurotransmission is warranted. Non-selective α₂-AR antagonism per se does not seem to be beneficial in reversing NMDA-antagonist-induced cognitive impairment in rodent models (79), while non-selective α₂-AR agonism may ameliorate these impairments (80–82). Contrasting the aforementioned findings, α_2C-AR selective antagonists JP-1302, ORM-10921 and ORM-12741 reverse cognitive and social dysfunction in NMDA-antagonist-induced animal models of neuropsychiatric illness (17–19), indicating a beneficial role of selective α_2C-AR antagonism (and not agonism) in attenuating symptoms induced by hypoglutamatergic states, although the mechanism is uncertain.

Disturbances in glutamate are well described in MDD and schizophrenia, while glutamatergic transmission represents an important target in pharmacological management of these disorders (68). Non-selective activation of α₂ heteroreceptors on glutamatergic neurons by NA reduces glutamate release in various brain areas implicated in MDD and schizophrenia, including the frontal cortex, ventral tegmental area, hippocampus, and nucleus accumbens (77, 83). Moreover, the treatment arsenal for both MDD and schizophrenia include drugs that are α₂-AR antagonists that would thus facilitate disinhibition of glutamate release. In support of this notion, the addition of a non-selective α₂-AR-antagonist to a D₂-blocker increases frontal cortical glutamatergic neurotransmission in rodents to a similar extent as the atypical antipsychotic clozapine, while at the same time improving cognitive and negative symptoms (3, 7, 84). Notably, clozapine has a threefold to fourfold higher α_2C over α_2A selectivity ratio and one of the highest α_2C over D₂ selectivity ratios compared to other antipsychotics. The novel antipsychotic asenapine also presents with increased affinity for the α_2C-AR and has good efficacy in treating both positive and negative symptoms of schizophrenia (4). Like that observed with clozapine and following the combination of a α₂-AR lytic with a DA antagonist (7, 84), asenapine enhances frontal cortical glutamate transmission via DA activation of D₁ receptors (85). Considering the above described effects of α₂-lytic activity on prefrontal cortical glutamatergic transmission (84), measuring frontal cortical NMDA currents in NMDA-antagonist model of schizophrenia might elucidate the effects whereby α_2C-AR selective antagonists improve NMDA-induced behavioral deficits.

Thus, the above findings suggest that α_2C-AR antagonism allows the regulation of cortical glutamatergic transmission, which may underscore a therapeutic option in schizophrenia and cognitive dysfunction in particular. The involvement of α_2C-ARs in the inhibition of striatal GABA release as mentioned above (65), could also indicate an indirect role of the α_2C-AR in glutamate release, since glutamate release is also tonically regulated by GABAergic interneurons (86).

Dysfunctional central cholinergic transmission has been implicated in the underlying pathophysiology of mood disorders, cognitive dysfunction, and schizophrenia [reviewed in Ref. (87)], while various drugs target the cholinergic system in an attempt at improving the above symptoms (88–90). Deficits in cholinergic transmission are also central to cognitive and memory dysfunction evident in Alzheimer’s disease (91). α₂-adrenergic heteroreceptors, as well as D₂ receptors, inhibit the release of acetylcholine (1). Similarly, the α_2C-AR might be involved in the presynaptic regulation of cholinergic transmission. Since acetylcholine inhibits GABA release (92), Zhang and Ordway (65) have posited that α_2C-AR effects on striatal GABA release (described above) might be attributed to the location of α_2C-ARs on striatal cholinergic neurons. These authors have also reported that the α_2C-AR mediates inhibition of striatal adenylyl cyclase and acetylcholine release, while these effects might be related to tonic activation of the α_2C-AR by DA (64, 65). A selective α_2C-AR antagonist might thus disinhibit striatal acetylcholine release that in turn may decrease extracellular striatal DA (87) (Figure 3). The findings of Zhang and Ordway (65) might thus be applicable to a neuropsychiatric disorder characterized by striatal dopaminergic over-activity, such as schizophrenia. A complex interplay of cortico-striatal cholinergic, GABAergic, and glutamatergic transmission has been described in the pathophysiology of schizophrenia (87), along with cholinergic regulation of dopaminergic and serotoninergic transmission and vice versa. However, more evidence in this regard using α_2C-AR selective ligands is required to enable more definitive conclusions regarding the interplay of the α_2C-AR, the cholinergic system and the effect of this interplay in neuropsychiatric disorders. Importantly though, the selective α_2C-AR antagonist, ORM-12741, has demonstrated favorable effects on episodic memory in patients with Alzheimer’s disease (25), thus providing proof of concept regarding targeting of the α_2C-AR in disorders of cognition, possibly via beneficial effects on cholinergic neurotransmission.

The α_2C-AR thus seems to play a distinct role in monoaminergic, GABAergic, glutamatergic, and possibly cholinergic neurotransmission, making it a promising target in several neuropsychiatric illnesses characterized by dysregulation in the aforementioned pathways, in particular MDD, schizophrenia, and conditions associated with cognitive decline. The potential therapeutic role of the α_2C-AR in these conditions, including an overview of evidence implicating its involvement in associated cognitive processes, will now be presented.

A genetic polymorphism of the α_2C-AR has been associated with emotional dysfunction in MDD (22). The α_2C-AR is densely expressed in the hippocampus, an area that is prominent in the pathophysiology of MDD (93). MDD is thought to be characterized, at least in some patients, by deficits in monoamine activity and diminished inhibitory neural control of the hippocampus and PFC over the hypothalamic–adrenal–pituitary axis (HPA-axis), resulting in HPA-axis over-activity with reduced negative feedback and hypercortisolaemia (94). Additionally sleep alterations, deficient neurotrophic signaling and the effects of chronic stress on neurotrophic factors and hippocampal atrophy has been hypothesized to underlie the complex pathophysiology of the disorder (95, 96). Aside from limbic function, the hippocampus plays an important role in learning and memory, and hippocampal atrophy could account for the cognitive deficits that accompany MDD (93).

Antidepressants generally increase the levels of NA, 5-HT and DA to varying degrees depending on the class of antidepressant (97). However, about 40% of patients do not respond to the most commonly used conventional antidepressants (98, 99). Considering that α_2C-ARs are densely expressed in the hippocampus, this AR subtype might be a potential target to address hippocampal-related disturbances in MDD. α₂-AR dysregulation in depressive disorders is widely described in the literature [Ref. (46) for review], with increased α₂-AR density found in platelets and in post-mortem brain tissue of depressed suicide completers in the locus coeruleus, temporal and frontal cortex, hippocampus and hypothalamus (100–103). Moreover, receptor upregulation has been specifically associated with the α_2A-AR subtype in depressed states (104–106). The role of the α₂-AR in the action of antidepressants is also fairly well described, of particular relevance being the α₂-AR antagonist antidepressants, mirtazapine and mianserin (107, 108). Indeed, α₂-AR downregulation is induced by tricyclic antidepressants (TCAs) and mirtazapine in rodents and depressed humans (brain and platelets), although regional differences in α₂-AR downregulation have been noted in the CNS [reviewed in Ref. (46)].

The rodent forced swim test (FST) is a well-described predictive model for antidepressant drug screening (109, 110). In this test, rodents are exposed to inescapable swim stress where the adoption of an immobile posture during re-exposure is thought to reflect failure in persistent escape-directed behavior, purported to model certain behavioral aspects of MDD such as the psychological feeling of “entrapment” and the replacement of active coping strategies with passivity (109, 111, 112), also resembling avolition and despair noted in MDD. Specifically, an increase in immobility time is considered to reflect the aforementioned depressive-like manifestations. The majority of antidepressants reduce immobility time in the FST (109).

The α₂-AR has been implicated in mediating the antidepressant (or anti-immobility) effects of TCAs in the FST, while activation of the α_2A-AR subtype seems to be essential in this regard (41, 46). Interestingly, the α_2C-AR plays an opposite role in regulating antidepressant effects in the FST. Early studies in α_2C-OE models in mice have suggested that α_2C-AR activation increases depressive behaviour in the FST, with α_2C-OE mice displaying increased immobility compared to wild-type-controls (40) (Figure 3B), an effect not attributed to altered locomotor activity (67). On the other hand, α_2C-KO mice demonstrate an antidepressive phenotype (40) (Figure 3A). These findings might explain why relatively non-selective α₂-AR agonists (113–115) and certain non-selective α₂-AR antagonists have both shown antidepressant-like effects in the FST. Recently these findings have been confirmed in rodents using subtype-selective α_2C-AR antagonists. Acute administration of highly subtype-selective α_2C-AR antagonists, JP-1302 (17), ORM-10921 (18, 21) and ORM-12741 (19) to Sprague Dawley and Han-Wistar rats was found to decrease immobility in the FST (see Table 2), providing evidence that selective α_2C-AR antagonism harbors therapeutic antidepressant effects. Although the aforementioned findings were predominantly from acute studies, we recently reported that chronic ORM-10921 reduced FST immobility time in the FSL rat, a genetic rodent model of MDD (21). Moreover, these effects were not seen with the non-selective α₂-AR antagonist idazoxan (21). These findings constitute the first findings for an antidepressant-like effect of an α_2C-AR antagonist within a translational and pathological construct-driven approach (16). The beneficial effect of α_2A-AR agonism on immobility in the FST as mentioned earlier and the increased immobility of α_2C-OE mice observed in this test emphasizes that both the absence/minimization of α_2A-AR antagonism and the presence of α_2C-AR antagonism might be required for antidepressant-like effects. Earlier, we discussed how α_2A-AR antagonism bolsters 5-HT transmission (33). Various studies have supported a therapeutic advantage for augmenting conventional antidepressants with α₂-AR antagonists. Dhir and Kulkarni demonstrated potentiated anti-immobility effects in the FST when fluoxetine and venlafaxine are augmented with yohimbine (9). This effect is mirrored in the clinic, where the addition of yohimbine to selective serotonin reuptake inhibitor (SSRI) treatment hastens antidepressant response and increases the number of responders compared to SSRI treatment alone (116). Enhanced clinical response to SSRI’s, venlafaxine, and bupropion is also evident following augmentation with the α₂-AR antagonist antidepressant mirtazapine, showing an early-onset of action (107, 117) as well as an almost doubling of the remission rate (118, 119). Clearly there is strong argument for adding an α₂-AR antagonist to conventional antidepressant therapy.

The Novel Object Recognition Test (NORT) (see Cognitive Deficits Associated With MDD and Schizophrenia) measures recognition memory and is reliant on hippocampal function, while both this cognitive parameter (120, 121) and hippocampal function has been shown to be deficient in patients with MDD (93). Recently, an important role for the α_2C-AR in this test has been demonstrated in the FSL rat, using the selective α_2C-AR antagonist ORM-10921 in a chronic treatment paradigm (21). This study found that selective α_2C-AR antagonism reversed deficits in novel object recognition memory in FSL rats, constituting the first findings for a pro-cognitive effect of a selective α_2C-AR antagonist using an illness-specific construct-driven translational model of MDD.

Altered circadian rhythm is a well-recognized biomarker of MDD (68), with HPA-axis dysregulation and hypercortisolaemia underlying the pathophysiology of the disorder (94). Since stress and MDD are causally linked, stress-induced increases in glucocorticoids have been suggested to mediate hippocampal atrophy and neurodegeneration evident in depressed individuals (93, 122). This incapacitation of the hippocampus leads to impaired cognitive function as well as a perpetuation of the stress response, the latter due to an inability of the hippocampus to exert top-down control over the HPA-axis (122). Long-term exposure to elevated cortisol levels induces regional upregulation of α₂-ARs (123), which in turn could result in further decreased NA levels. In this regard, the α₂ antagonist and antidepressant mirtazapine has been associated with amelioration of HPA-axis hyperactivity in depressed patients (124, 125), albeit not necessarily related to clinical improvement. Interestingly, this amelioration of HPA-axis hyperactivity is not mirrored in rodents (126). In healthy volunteers, the acute administration of the α₂-AR antagonist idazoxan has been associated with an attenuated normal diurnal fall in plasma cortisol, although dissipated following chronic treatment (127). Earlier studies on the other hand have shown that depressed patients exhibited much greater cortisol responses to yohimbine than controls (128). The α_2C-KO mouse demonstrates attenuated plasma corticosterone elevations vs. wild-type controls following different stressors, while α_2C-OE mice show more intense corticosterone responses compared to α_2C-KO (40) (Figures 3A,B). Interestingly, non-selective α₂-AR antagonism seems to elevate plasma corticosterone levels and to potentiate corticosterone responses to restraint stress in rodents (129). More selective α_2C-AR antagonism might, therefore, elicit beneficial effects on HPA-axis functioning in depressive states. Previous studies have shown that both inhibition of corticosterone synthesis and injection of glucocorticoid receptor antisense oligonucleotides into the dentate gyrus of the hippocampus decreases immobility in the FST (130, 131). That the α_2C-AR is the only α₂-AR subtype expressed in this region in mice (67), together with the effects of α_2C-AR modulation on corticosterone levels and FST immobility, consolidates a valuable role for α_2C-AR antagonism in the treatment of MDD. Therefore, hypercortisolism in MDD may underscore a central dysfunctional adrenocortical feedback mechanism, with α₂-ARs, and indeed the α_2C-AR subtype specifically, being important in regulating glucocorticoid responses.

Associations between genetic polymorphism of the α_2C-AR and certain aspects of psychotic disorders have been reported (24). Furthermore, α_2C-ARs are most densely expressed in the striatum (132), where they are thought to play an inhibitory role (133). This dense expression has distinct importance when striatal dysfunction in schizophrenia is considered, especially its intricate connection to frontal cortical cognitive deficits (134). The α_2C-AR, therefore, represents a potentially beneficial pharmacological approach to modulate striatal deficits in schizophrenia and possibly other psychotic disorders. The PFC, striatum and hippocampus are implicated in schizophrenia, where noradrenergic and dopaminergic terminals presenting with α_2C auto and heteroreceptors are well-represented (27, 59). Despite the prominence of the DA hypothesis of schizophrenia, a hypothesis implicating noradrenergic dysfunction also has significant support in the literature (135).

The DA paradox is well described in schizophrenia (136), with mesolimbic hyperdopaminergic and mesocortical hypodopaminergic states being postulated. Excessive striatal DA is linked to positive symptoms, while cognitive dysfunction is linked to deficits in cortical dopaminergic function (137). In Section “Role of the α_2C-AR in Regulating Key Neurotransmitters,” we discussed findings that suggest decreased striatal but not frontal cortical DA turnover in response to α_2C-AR deletion (Figures 3A,B), while increased cortical DA turnover has been noted in response to α_2C-AR overexpression (67). These early findings suggest a positive role for α_2C-AR antagonism in regulating mesolimbic-cortical dopaminergic imbalances, which may have therapeutic value in schizophrenia. GABAergic and glutamatergic deficits are also implicated in schizophrenia disease pathology, where loss of GABAergic output onto secondary glutamatergic cortical neurons required for tonic control over subcortical dopaminergic neurons, results in increased mesolimbic dopaminergic firing (increased striatal DA release) and consequently the presentation of psychotic symptoms (86). As discussed earlier, the α_2C-AR strongly mediates striatal GABA release, while α_2C-AR deletion seems to disinhibit α₂-AR antagonist-induced inhibition of GABA release (65). Here, α_2C-AR subtype-selective antagonism might present with more beneficial effects on striatal GABA release than non-selective α₂-AR antagonism when applied as pharmacological treatment of schizophrenia.

The atypicality of antipsychotic drugs primarily reflects their reduced risk of extra-pyramidal side effects and to some extent improved efficacy against negative and cognitive symptoms of schizophrenia (138), over and above their efficacy against positive symptoms. Atypicality has, apart from actions at serotonergic receptors, been proposed to revolve around α-AR modulation, with α₁ and α₂-AR antagonism suggested to contribute to stabilization of dysregulated dopaminergic activity (139). Indeed, in a thorough comparative study employing human receptor binding data, Shahid and colleagues (4) have shown that a number of atypical antipsychotics (clozapine, quetiapine, asenapine, risperidone, ziprasidone) possess significant α₂-AR antagonist properties. Furthermore, quetiapine and in particular clozapine showed prominent α_2C over D₂ as well as α_2C over α_2A receptor selectivity. A pharmacological profile constituting a higher α₂ vs. D₂ receptor binding ratio (139), and specifically a higher α_2C vs. D₂ receptor selectivity ratio (3, 4), has been suggested to mediate the improved efficacy of drugs like clozapine that exhibit lower D₂-receptor occupancy. The α₂ over D₂ receptor subtype selectivity ratios for various antipsychotics as well as the α_2C-AR selective antagonist, ORM-10921 (which as described below has shown antipsychotic-like activity in animal models), are depicted in Figures 4A,B. Thus, reduced D₂-receptor occupancy might be possible in therapy because of the beneficial effects of additional α₂-AR antagonism on dysregulated dopaminergic activity, allowing for improved efficacy with less motor side effects. Support for this hypothesis has been demonstrated in studies employing non-selective α₂-AR antagonists (e.g., idazoxan) as augmentation to D₂-receptor antagonist antipsychotic treatment (7, 84, 140). While this combination of α₂-AR and D₂ receptor antagonism presents with improved antipsychotic-like effects in mouse models of schizophrenia, it also resulted in enhanced cortical glutamatergic transmission and increased dopaminergic output in the PFC, with subsequent improvement in cognitive parameters in rats (84). The effects of this augmentation strategy were comparable to that of clozapine. While clozapine requires approximately 45% D₂ receptor occupancy compared to >70% required by other D₂ receptor antagonists for antipsychotic efficacy (141, 142), the combination of idazoxan with a D₂ receptor antagonist exhibited potent antipsychotic effects similar to that of clozapine at similar low D₂ receptor occupancy rates (84).

Sensorimotor gating refers to the ability to integrate and process sensorimotor information, deficits of which are suggested to underlie the fragmentation of reality evident in schizophrenia (143). The prepulse inhibition (PPI) of startle test refers to the attenuation of a startle response produced by the presentation of a smaller prepulse, and is used to study the gating of sensorimotor information by the brain (143, 144). A typical example of the PPI test in humans employs the somatosensory eye blink reflex in response to acoustic, tactile (e.g., air puffs) or light stimuli (143, 145, 146). A PPI deficit can be induced in humans and animals by various psychotomimetic drugs, including dopaminergic and antiglutamatergic drugs. Animal models of schizophrenia, such as SIR (147–149) and various transgenic models including mice with altered DA, 5-HT, and glutamate receptor expression (150), present with deficits in PPI. Importantly, antipsychotic drugs normalize disrupted PPI in animals and humans (151–155). While the contribution of non-selective α₂-blockade to modulation of PPI has been proposed, the literature is somewhat inconclusive in this regard. In fact, some papers have suggested that antagonism of the α_2A-AR does not contribute to enhancement of the PPI (44, 156–158).

Considering the important role for α₂-AR antagonism in managing schizophrenia (139), earlier studies in transgenic mouse models have demonstrated that antipsychotic-like effects are subtype dependent. In this regard, α_2C-KO mice demonstrated clear PPI deficits compared to wild-type controls, while α_2C-OE mice had markedly higher PPI scores than their wild-type controls (43), suggesting that α_2C-receptor agonism may induce antipsychotic-like effects. However, this extrapolation from transgenic mouse studies has since been disproven following experiments with selective α_2C-AR antagonists. JP-1302, ORM-10921, and ORM-12741 that consistently show improved PPI in Sprague Dawley and Wistar rats in NMDA-antagonist-induced models of schizophrenia (17–19). More recent findings in SIR rats, a putative neurodevelopmental model of schizophrenia (159, 160), corroborate these earlier findings, with ORM-10921 found to significantly improve SIR-associated PPI deficits in a manner comparable to clozapine (20) (Figure 3C). Moreover, ORM-10921 also enhanced the effects of haloperidol on the above-mentioned deficits in PPI (20). Earlier, in Section “Distinct Roles for α₂-AR Subtypes,”, we highlighted this discrepancy, emphasizing the need to corroborate findings from transgenic mouse models with studies employing subtype-selective ligands in suitable animal models.

Cognitive deficits in schizophrenia make up some of the core elements of the disorder (161) and are often refractory to treatment (162). These impairments include deficits in working, recognition and spatial memory, cognitive flexibility, learning, and attention (163–165). However, antipsychotic treatments are not always reproducibly effective in reversing these cognitive deficits in animal models (166–170), which in fact reflects the relative lack of efficacy displayed by antipsychotics in treating cognitive impairment in the clinic (164, 165). Recently, the highly selective α_2C-AR antagonist ORM-12741 showed improved effects on NMDA-antagonist-induced disruptions in working memory and spatial learning, navigation and memory in rodents (19). NMDA-antagonist models include the administration of the glutamate NMDA-receptor antagonists dizolcilpine (MK-801) or PCP which are known to induce behavioral, cognitive, and neurochemical disruptions in behavior akin to those seen in schizophrenia (171). ORM-12741 attenuates the disruption of learning in the Morris Water Maze (MWM) induced by MK-801, while also improving PCP-induced memory deficits in the 8-arm radial maze (19). Similar findings were reported for the selective α_2C-AR antagonist ORM-10921 which attenuates MK-801-induced spatial navigation in the MWM (18), a finding consistent with effects described for atypical (167, 172) but not typical antipsychotics such as haloperidol (173). Additionally, ORM-10921 significantly improved object recognition memory in SIR rats, comparable to the atypical antipsychotic clozapine, while also significantly improving the efficacy of haloperidol in this regard (20) (Figure 3C). Evidence of improved cognition in NMDA-antagonist and neurodevelopmental models of schizophrenia with novel highly selective α_2C-AR antagonists, therefore, demonstrates the therapeutic potential of targeting the α_2C-AR in treating cognitive deficits associated with schizophrenia.

Another interesting observation concerns the neurotrophic hypothesis of schizophrenia, where reduced brain-derived neurotrophic factor (BDNF) is widely evident in the illness (174, 175), as well as being associated with the above-mentioned cognitive deficits (68). Although chronic treatment with the α_2C-AR antagonist, ORM-10921, alone did not significantly reverse lowered BDNF levels in SIR rats on its own, combining haloperidol with ORM-10921 showed a significant increase in BDNF levels that exceeded that of either drug alone (20) (Figure 3C). These preliminary results further support a therapeutic role for α_2C-AR antagonism in improving cognitive symptoms in schizophrenia.

Social isolation, decreased social cognition, and impaired social skills form part of the negative symptoms of schizophrenia and are refractory to most antipsychotic treatments (176). The social interaction test measures deficits in social motivation and self-directed behavior in rats and is used to measure predictive validity of antipsychotics in rodent models of schizophrenia (177). Although there are mixed results, generally atypical antipsychotics are more effective than typical antipsychotics at attenuating social deficits in these models (177, 178). In this regard, the α_2C-AR antagonists ORM-10921 and ORM-12741 significantly attenuate PCP-induced deficits in social interaction in short-term single-housed and pair-housed rats (18, 19).

Since especially atypical antipsychotics have activity at the α_2C-AR, it is important to consider data from functional assays on these compounds using cloned receptors in Chinese hamster ovary cell lines. Kalkman and Loetscher (3) found α_2C over α_2A receptor selectivity ratios for clozapine, chlorpromazine, risperidone, quetiapine, and iloperidone to be between 3 and 12, indicating that most atypical antipsychotics present with higher α_2C-AR antagonist activity than α_2A-AR antagonist activity. Additionally, the novel antipsychotics asenapine and lurasidone both present with potent α_2C-AR binding affinity (4, 179). α_2C over D₂ selectivity ratio has, therefore, been suggested to be an important factor in antipsychotic efficacy (3). Clozapine, arguably the most efficacious antipsychotic in treatment refractory schizophrenia (180), presents with an α_2C over D₂ selectivity ratio of 85 compared to ratios of 0.01–11 for other tested antipsychotics (4) (see Figure 4A). Haloperidol, on the other hand, has the lowest potency at the α_2C-AR as well as the lowest α_2C over D₂ ratio of tested compounds (3, 4), and is not regarded as an atypical antipsychotic. However, bolstering its antipsychotic-like and pro-cognitive effects with a selective α_2C-AR-antagonist (20) supports the notion that an increased α_2C over D₂ ratio will translate to superior antipsychotic effects. Taken together, α_2C-AR antagonism is involved in the mechanism of improved sensorimotor gating, cognitive, and social functioning in pharmacological and neurodevelopmental models of schizophrenia. These data are indicative of a therapeutic role for α_2C-AR antagonism in the treatment of schizophrenia, and further study with more subtype-selective ligands is encouraged.

Many neuropsychiatric illnesses, including MDD and schizophrenia, present with cognitive deficits and memory impairments (122, 162, 163, 165). The α_2C-AR has been shown to be involved in cognitive deficits evident in both non-pathological (34–37) and pathological (20, 21) animal models, with findings implicating a significant role in the treatment of cognitive deficits associated with these disorders. Although α₂-AR agonists are associated with improved cognitive processing in humans and animals (38, 181–184) and in the treatment of cognitive decline associated with aging (185), these effects have been shown to be mediated via activation of the α_2A-AR (37–39), which is also responsible for sedative and hypotensive effects (51, 186). In contrast, genetic deletion of the α_2C-AR subtype, or by extrapolation selective α_2C-AR antagonism, has been demonstrated to improve memory and cognition in the MWM, the 8-arm radial maze and the NORT, as discussed below. Furthermore, α_2C-AR antagonism has been found to benefit neurotrophins and other biomarkers of neuronal resilience associated with cognition (20).

The MWM is a spatial water navigation task requiring the rodent to learn and remember the location of an escape platform in a water arena in order to locate a hidden (submerged) platform in subsequent trials by using various spatial cues. The escape latency is a measure of spatial working memory. The test is a reliable tool correlating with hippocampal synaptic plasticity as well as intact glutamate NMDA-receptor function (187). In early transgenic mouse studies, α_2C-OE mice showed impaired spatial and non-spatial escape strategies and search patterns in the MWM. Administration of an α₂-AR antagonist could reverse these impairments to a greater extent in α_2C-OE than in wild-type mice, suggesting that α_2C-AR antagonism might play a more prominent role than α_2A-AR antagonism in brain areas involved in spatial navigation (34–36). Considering the dense expression of the α_2C-AR in the hippocampus and striatum and that hippocampal (188) and striatal lesions (189) impair aspects of MWM navigation, α_2C-AR selective antagonism may mediate information processing and behavioral adaptation to environmental change. α_2C-OE mice display normal passive avoidance learning, suggesting that impaired water maze navigation in α_2C-OE mice does not reflect defective stimulus-response learning and that α_2C-AR deactivation is, therefore, associated with the processes underlying complex organization of escape behavior (34). This effect of α_2C-AR antagonism might partially explain previous findings for pro-cognitive effects of the non-selective α₂-AR antagonist, idazoxan, on planning, attention, episodic memory and verbal fluency in patients with frontal lobe dysfunction (11).

The radial-arm maze is a test used to measure reference and working memory in rodents and relies on intact functioning of the prefrontal cortical, hippocampal and striatal interconnections to locate food rewards hidden in various radial-arm target sites (190). Björklund and co-workers (37) demonstrated that the non-selective α₂-AR agonist dexmedetomidine improves working memory in the radial-arm maze, and that this improvement is greater in α_2C-KO mice, suggesting that the absence of α_2C-AR agonism (or simultaneous α_2C-AR antagonism) might result in enhanced performance with respect to working memory.

The NORT is a two-trial behavioral measure that relies on the rodent’s innate preference to explore novel objects over familiar objects, thereby enabling measurement of recognition memory (191, 192). The declarative memory processes underlying the NORT relies on the perirhinal cortex and the hippocampal complex (193–195). Uys and colleagues have demonstrated that selective α_2C-AR antagonism with ORM-10921 markedly improves recognition memory in pathological animal models of schizophrenia (20) and MDD (21).

The above-mentioned benefits of selective α_2C-AR antagonism on cognitive parameters have been corroborated with studies employing highly selective α_2C-AR antagonists in animal models of schizophrenia, MDD, and age-related cognitive impairment (18–21), as described in Sections “Behavioural Deficits Associated With MDD” and “Behavioural Deficits Associated With Schizophrenia,” as well as in clinical trials investigating novel therapy for Alzheimer’s disease (25) (see Evidence for Targeting the α_2C-AR in Other Neuropsychiatric Disorders).

Brain-derived neurotrophic factor is the most prevalent neurotrophic growth factor in the CNS where it is especially important in regulating synaptic plasticity and various aspects underlying cognitive performance, memory, and mood (196, 197). Acute and chronic stress purportedly have detrimental effects on rodent BDNF expression in the hippocampus, while altered BDNF levels are evident in depressive disorders (68, 198) and in schizophrenia (174, 175). While both antipsychotics and antidepressants alter BDNF levels to varying degrees (199–203), non-selective α₂-AR antagonism has also been associated with neurogenesis and increased BDNF levels in the hippocampus (204, 205). Noradrenergic (202, 206), dopaminergic (207), serotonergic (208), and GABA-glutamate (209) interactions are involved in the expression of BDNF. With the α_2C-AR acting as a heteroreceptor to modulate the release of many of the aforementioned neurotransmitters, this receptor might play an indirect role in regulating the expression of BDNF. Evidence for the involvement of the α_2C-AR in the expression of BDNF has been demonstrated in the SIR animal model of schizophrenia, where SIR rats present with reduced striatal BDNF levels (20). While conventional antidopaminergic antipsychotics are not associated with correction of said reduced BDNF levels (201), a recent study reported that combining haloperidol with the selective α_2C-AR antagonist ORM-10921 (but not α_2C-AR antagonism per se) increases striatal BDNF levels in these animals, while at the same time improving deficits in cognition and sensorimotor gating (20). This study indicated that not only is augmentation with a α_2C-AR antagonist associated with improved BDNF expression but also that this improvement is correlated with improved cognitive performance, thus supporting a role for α_2C-AR antagonism in disorders associated with reduced cognitive flexibility and deficits in neurotrophin support.

Brain-derived neurotrophic factor is also important in regulating C-fos and JunB expression, biomarkers of neuronal activity that play an important role in synaptic function (210, 211). Upregulation of c-fos mRNA is induced by noxious stimuli, neurotransmitters, neurotrophins and other growth factors as well as learning and memory processes (212). Jun-B is also involved in the regulation of emotional memory (213). BDNF restores the expression of these transcription factors after neuronal insult (214), reinforcing BDNF’s role in neuroplasticity at gene-transcription level. Interestingly, cortical and hippocampal levels of c-fos and JunB mRNA are increased in α_2C-KO mice compared to wild-type controls (40), while this is not the case in α_2C-OE mice. Whether this increase is associated with altered BDNF levels in α_2C-KO mice has not been investigated. Nevertheless, the increase in neuronal activity in α_2C-KO mice is of interest considering the pro-cognitive behavioral characteristics of this transgenic strain and the above-mentioned beneficial effects of α_2C-AR antagonists on BDNF expression and cognitive performance.

Thus, antagonism of the α_2C-AR might benefit cognitive processes relevant to both MDD and schizophrenia. Since cognitive deficits are core symptoms of both disorders, the α_2C-AR related effects on cognition and neuronal markers of plasticity support the therapeutic potential of targeting the α_2C-AR in these disorders.

To summarize findings from transgenic mouse models and those gained from treatment with α_2C-subtype-selective ligands, Table 2 presents neurochemical and behavioral findings reported in transgenic mice and in various rodent models predicting pro-cognitive-like, antidepressant-like and antipsychotic-like effects as described in Sections “Behavioural Deficits Associated With MDD,” “Behavioural Deficits Associated With Schizophrenia,” and “Cognitive Deficits Associated With MDD and Schizophrenia”. As a GPCR that functions within the PSD, the synaptic actions of the α_2C-AR and indeed drugs that target this receptor, might involve regulatory PSD proteins to mediate the aforementioned effects.

The PSD is a specialized matrix located at excitatory postsynaptic terminals, described as a macromolecular complex of several hundreds of proteins that act as a molecular switch for multiple interacting neurotransmitter signaling pathways (215). Such proteins include those containing the PSD-95/disc large/zonula occludens-1 (PDZ) domain, and the membrane-associated guanylyl kinase, all of which comprise three PDZ peptide-binding domains (215). These proteins in turn promote binding to a variety of molecules within the PSD necessary for signal transduction (45). We have earlier noted the importance of the PSD in postsynaptic GPCR signaling. There is significant interest in the role of the PSD network in psychopharmacology and psychotropic drug action, although much of the extant evidence in support of this relates to DA and glutamate-dependent synaptic plasticity (45, 215). Nevertheless, this review has highlighted the importance of heteroreceptor-directed modulation of DA and glutamate signaling by the α_2C-AR, while at least one prominent PDZ-domain binding protein, spinophilin, has been associated with the α₂-AR (45). Spinophilin regulates α₂-AR associated G_αi coupling, membrane localization, endocytosis, receptor desensitization and calcium signaling (216–218). Despite this evidence, however, spinophilin is not yet known to be involved in major neuropsychiatric disorders or to by modulated by main psychopharmacologic treatments (215). Nevertheless, it is perhaps worth discussing how and why a PSD protein such as spinophilin may mediate important pharmacological responses following ligand binding to the α_2C-AR.

Although the specific role for PSD proteins in psychiatric illness remains speculative, clinical and preclinical studies have provided evidence for their involvement in aberrant synaptic plasticity [see Ref. (215) for review]. These processes are invariably associated with high-order cognitive alterations, which are essentially the core pathophysiology in a number of psychiatric diseases, including depression and schizophrenia (219–221).

When considering a therapeutic strategy in psychiatric diseases, psychotropic-mediated modulation of PSD molecules may occur either directly or indirectly, the latter as a consequence of drug interaction with their target non-PSD receptors. Currently, there is no known agent that directly targets a PSD protein for therapeutic effect. Since PSD molecules are modulated by antipsychotics and antidepressants (221–226), as well as play a key role in behavioral response (227, 228), they represent putative targets for pharmacological action. Moreover, concurrent administration of antipsychotics and antidepressants may induce synergistic modulation of specific PSD molecules (229–231), which provides at least conceptual support for targeting the PSD to address treatment resistance in mood and psychotic disorders. This is particularly relevant when discussing the α₂-AR, since a number of studies have described enhanced efficacy of typical and atypical antipsychotic drugs by adjunctive α₂-AR blockade (20, 84, 232). Concerning the α_2C-AR, the α_2C-AR antagonist, ORM-10921 has been found to bolster the response to haloperidol in social isolation reared rats at both the level of synaptic plasticity (i.e., BDNF) and cognition (i.e., object recognition memory) (20). That the combined response was similar to clozapine emphasizes the benefit of adjunctive α_2C-AR antagonism with regard to treatment response. Such data holds promise for application in treatment resistance, and further studies in this regard, but combined with co-assessment of PSD proteins, are warranted.

This review has focused on the therapeutic potential of targeting the α_2C-AR subtype in MDD and schizophrenia. However, cognitive dysfunction is common in patients with Alzheimer’s disease, MDD and schizophrenia, while symptoms of the latter two illnesses permeate through to patients suffering from Alzheimer’s disease. Indeed, recent preclinical and preliminary clinical evidence has revealed the promising therapeutic role for the α_2C-AR in addressing cognitive decline in Alzheimer’s disease. Co-presentation of cognitive decline in this and other disorders, and the role of the α_2C-AR, warrants brief discussion.