The catecholaminergic system, and particularly dopamine and noradrenaline, are substantially involved in state-dependent information processing in the central nervous system. The ventral tegmental area and the locus coeruleus are the major sources of hippocampal dopamine and noradrenaline, respectively. Both structures increase their firing rates when an animal is presented with novel stimuli (Aston-Jones and Bloom, 1981; Horvitz et al., 1997; Kitchigina et al., 1997) and dopamine and noradrenaline levels in the hippocampus increase during exploration of a novel environment (Ihalainen et al., 1999). In addition, the dopaminergic system is involved in reward-learning behavior (Schultz et al., 1993; Ungless, 2004).

In the hippocampus, activation of either dopamine D1/D5 receptors or beta-adrenergic receptors reduces the threshold for induction of synaptic plasticity that lasts for very long (>24 h) periods of time in rodents (Hansen and Manahan-Vaughan, 2014; Hagena et al., 2016). Correspondingly, weak synaptic potentiation (short-term potentiation, STP) is prolonged into persistent long-term potentiation (LTP) if patterned afferent stimulation occurs in the presence of dopamine D1/D5 receptor or beta-adrenergic receptor agonists (Kusuki et al., 1997; Li et al., 2003; Almaguer-Melian et al., 2005; Lemon and Manahan-Vaughan, 2006; Tenorio et al., 2010; Hagena and Manahan-Vaughan, 2012; Hansen and Manahan-Vaughan, 2015b). Moreover, hippocampal LTP is impaired by dopamine D1/D5 receptor or beta-adrenergic receptor antagonists (Stanton and Sarvey, 1985; Straube and Frey, 2003; Hansen and Manahan-Vaughan, 2015b). LTP is tightly associated with spatial learning and exploration of a novel environment results in the strengthening of STP into persistent (>24 h) LTP in rodents in vivo (Kemp and Manahan-Vaughan, 2004, 2008a; Hagena and Manahan-Vaughan, 2012). This facilitation of LTP through novel learning requires activation of both dopamine D1/D5 and beta-adrenergic receptors (Straube et al., 2003; Lemon and Manahan-Vaughan, 2006; Kemp and Manahan-Vaughan, 2008b; Hansen and Manahan-Vaughan, 2015a, b).

The catecholaminergic system may change with age (Popova and Petkov, 1989; Araki et al., 1997; Suzuki et al., 2001). But aging, despite the manifestation of attrition in cognition, including memory deficits (Perlmutter et al., 1981; Larrabee and McEntee, 1995), is not always a process of degeneration. Decline in cognitive ability with aging is related to changes in the hippocampus (Driscoll et al., 2003). These cognitive changes begin surprisingly early in mature adulthood in healthy humans (Ballesteros et al., 2013). We have observed deficits in object-place learning and in extinction learning that emerge in healthy middle-age in rats (Wiescholleck and Manahan-Vaughan, 2014). We have proposed that these changes in learning ability in middle-age relate less to a cognitive decline per se, but rather may reflect less flexibility in revising hard-wired knowledge or reduced adaptability to new learning challenges (Wiescholleck and Manahan-Vaughan, 2014). These age-related changes may be state-dependent: rats (as well as humans) can be expected to be less enthused in middle-age by stimuli that would have been perceived as very exciting in youth (“been there, done that”) and thus, higher excitation thresholds may need to be surpassed in driving the hippocampus towards the synaptic encoding of experience. The dopaminergic and noradrenergic systems are pivotal in the signaling of novelty, motivation and arousal and both noradrenaline and dopamine release in the hippocampus drives elevations of neuronal excitability (Harley, 2007; Hamilton et al., 2010). We speculated that the loss of learning ability in middle-age may relate to changes in neuromodulatory control of the hippocampus by these components of the catecholaminergic system. We report here that although LTP is equivalent in young and middle-aged animals, and although middle-aged animals respond to a change in spatial environment with LTP, we detected a failure of dopamine D1/D5 receptor and beta-adrenergic receptor agonists to facilitate weak potentiation into persistent (>24 h) LTP, as was the case in young rats.

The present study was carried out in accordance with the European Communities Council Directive of 22 September 2010 (2010/63/EU) for care of laboratory animals and after approval of the local government ethics committee (Landesamt für Naturschutz, Umweltschutz und Verbraucherschutz, Nordrhein Westfalen). All efforts were made to minimize the number of animals used.

In this study, we show that agonist activation of dopamine D1/D5 receptors, or of beta-adrenergic receptors, fail to prolong STP into LTP in the dentate gyrus of middle-aged rats, even though these doses are effective in young animals. This absence of effect does not appear to relate to putative deficits in hippocampal plasticity: a stronger tetanization protocol is successful in inducing persistent (>24 h) LTP that is equivalent in profile and magnitude to that elicited in young animals. Furthermore, the coupling of weak afferent stimulation with novel spatial exploration prolongs STP into LTP in middle-aged animals, as has been previously reported for young animals (Kemp and Manahan-Vaughan, 2008a). Taken together, these results indicate that, with increasing age, the dopaminergic and noradrenergic systems become less effective in supporting memory encoding through LTP. This may relate to state-dependent changes in the activation of the catecholaminergic system.

Receptors, such as the NMDA, dopamine D1/D5 and beta-adrenergic receptors, are important elements in mediating synaptic plasticity (Herron et al., 1986; Frey et al., 1990; Pöschel and Manahan-Vaughan, 2007; Hansen and Manahan-Vaughan, 2015b; Chay et al., 2016). Male rats show lower hippocampal NMDA receptor subunit expression and decreased NMDA receptor function as early as 8–14 months postnatally (Monfort and Felipo, 2007; Guidi et al., 2015). Although, NMDA receptors are important in initiating synaptic potentiation (Herron et al., 1986; Pöschel and Manahan-Vaughan, 2007) we did not find any difference in the magnitude or duration in the early or late phase of LTP induced by high frequency afferent stimulation (HFS) in middle-aged animals, compared to young adult animals. This finding is in line with other studies of LTP in the dentate gyrus of middle-aged rats (Barnes et al., 2000). This suggests that at this stage of maturity, changes in NMDA receptor levels are not yet adequate to mediate a change in LTP. Alternatively compensatory mechanisms are already underway: in the dentate gyrus, LTP induction is also supported by L-type voltage gated Ca²⁺ channels (Manahan-Vaughan et al., 1998).

Both dopamine and noradrenaline release in the hippocampus results in an increase of neuronal excitability in the dentate gyrus that predisposes this hippocampal subfield towards expression of LTP (Harley, 2007; Hamilton et al., 2010). Dopamine D1/D5 and beta-adrenergic receptors are also highly important for the maintenance of the later phases of LTP in young adult rodents (Stanton and Sarvey, 1985; Straube and Frey, 2003; Hansen and Manahan-Vaughan, 2015b). In the present study we found that pharmacological activation of either dopamine D1/D5 or beta-adrenergic receptors fail to facilitate LTP in middle-aged animals. Both dopamine D1/D5 and beta-adrenergic receptors exhibit an age-dependent decrease in ligand binding and reduced amount of total receptor expression in the hippocampus that is evident at 12 months (Popova and Petkov, 1989; Araki et al., 1997; Suzuki et al., 2001). An inhomogeneous shift in D1 receptor expression has been reported in 12–24 month old rats compared to young (3 month old) adults, however (Amenta et al., 2001): here, increases in receptor expression emerged in the CA1 region and decreases occurred in the dentate gyrus. This could explain why agonist activation of D1/D5 receptors was ineffective in modulating dentate gyrus LTP in middle-aged rats, as seen in our study. Interestingly, another study (Reis et al., 2005) showed that not only beta-adrenergic receptor mediated signaling, but also adenylate-cyclase mediated signaling is directly impaired in 24 month old animals. Therefore, the lack of LTP facilitation by dopamine D1/D5 receptor, or beta-adrenergic receptor agonists in middle-aged animals, that we observed, might reflect the gradual development of changes in receptor expression/agonist binding as well as in signaling efficacy through adenylate cyclase. Since receptors of the dopaminergic and noradrenergic systems have been proposed to act synergistically with NMDA receptors to enable long-lasting synaptic plasticity (Kauer et al., 1988; Frey et al., 1990; Chay et al., 2016), changes in NMDA receptor signaling may also contribute to these effects.

In young rats, both patterned stimulation of hippocampal afferents and novel spatial exploration induce an increase in extracellular levels of noradrenaline and dopamine in the hippocampus (Ihalainen et al., 1999; Neugebauer et al., 2009), most likely through increased activity of the locus coeruleus and ventral tegmental area (Sara et al., 1994; Horvitz et al., 1997; Kitchigina et al., 1997; Lemon et al., 2009). In healthy rodents, locus coeruleus projections to diverse brain structures begin to decline by 7–15 months of age (Ishida et al., 2000; Shirokawa et al., 2000). Compensatory mechanisms in the form of increased axonal branching and physiological changes of those projections have been described (Ishida et al., 2000; Shirokawa et al., 2000). Similarly, age-related changes in the ventral tegmental area have been reported (Schuligoi et al., 1993; Siddiqi et al., 1999). The locus coeruleus and ventral tegmental area are anatomically and functionally connected (Simon et al., 1979; Ornstein et al., 1987). Lesions of one of those nuclei increases the vulnerability of the other nucleus (Szot et al., 2012), which suggests that aging-related deficits in the locus coeruleus may have an impact on ventral tegmental area function. In 22–24 month old rats decreased synthesis of noradrenaline and, to a lesser extent, dopamine have been described (Luine et al., 1990; Dickerson et al., 2009). Taking into account the necessity for dopamine D1/D5 and beta-adrenergic receptor activation for hippocampus-dependent learning (Straube et al., 2003; Hansen and Manahan-Vaughan, 2015a, b), age-dependent reductions in noradrenaline and dopamine levels are likely to lead to deficits in spatial learning. In line with this, middle-aged rats exhibit deficits in hippocampus-related learning such as context-dependent renewal of extinguished memory, or object-place memory (Cavoy and Delacour, 1993; Wiescholleck et al., 2014). A decrease in attention and vigilance is also evident in middle-aged rats (Jones et al., 1995; Guidi et al., 2015). Our observation that a loss of sensitivity of dopamine D1/D5 and beta-adrenergic receptors occurs in LTP processes in middle-age may comprise a cellular mechanism that underlies these behavioral changes.

Interestingly, LTP appears unchanged in its magnitude and profile in middle-aged animals compared to young adults. We found this to be the case both for LTP that is induced by strong tetanic stimulation of hippocampal afferents, and for LTP that is facilitated by the conjunction of weak afferent stimulation and novel spatial learning. In line with this, other in vivo studies of dentate gyrus LTP reported that 6–12 month old F344 rats express robust LTP that lasts for several days, although 22–30 month old rats show impaired, or no, LTP expression (Orr et al., 2001; Sierra-Mercado et al., 2008). Both forms of persistent LTP that we examined are likely to be related to experience-dependent memory encoding (Martin et al., 2000; Straube and Frey, 2003; Kemp and Manahan-Vaughan, 2008a). Age-dependent changes in LTP may not occur in synchrony across the different hippocampal subfields, however: a decrease in LTP magnitude at Schaffer-Collateral CA1 synapses of 6 month vs. 22 month old rats (Monfort and Felipo, 2007) and a loss of late (>1 day) LTP at perforant path-CA3 synapses in 2 month vs. 8 month old rats (Dieguez and Barea-Rodriguez, 2004) have been reported. These subfield-specific alterations may perhaps explain why middle-aged rats show deficits in specific forms of hippocampus-dependent memory (Wiescholleck et al., 2014).

It is also striking that spatial novelty remained effective in promoting weak potentiation into persistent LTP in middle-aged rats, despite the fact that the D1/D5 and beta-adrenergic receptor agonists failed to pharmacologically enhance STP into LTP in this age-group. Taken together with our observation that strong afferent stimulation resulted in persistent LTP in the middle-aged animals, this finding suggests that if a behavioral (or synaptic) impetus is salient enough, LTP will result in these older rats. Transduction of behavioral saliency into cellular signals that promote hippocampal information encoding is very likely to be mediated by the noradrenergic and dopaminergic systems (Hansen and Manahan-Vaughan, 2014; Hagena et al., 2016). So why then, does agonist activation of these receptors fail to strengthen weak potentiation in the middle-aged animals? Learning-facilitated LTP is strongly dependent on the activation of both of these receptors in younger rats (Kemp and Manahan-Vaughan, 2008a; Hagena and Manahan-Vaughan, 2012; Lemon and Manahan-Vaughan, 2012). As mentioned above, one possible explanation for the lack of effect of D1/D5 and beta-adrenergic receptor activation on STP in the older rats may relate to changed expression of the catecholamine receptors that develops with aging (Popova and Petkov, 1989; Araki et al., 1997; Suzuki et al., 2001). Prolonged agonist activation of the receptors may serve to desensitize, or promote tachyphylaxis of, a significant pool of those receptors that still remain functional in aging animals (Benovic et al., 1988; Bouvier et al., 1988; Hausdorff et al., 1989; Ng et al., 1994), thereby negating the kind of positive effects on LTP that are detectable (following agonist treatment) in younger animals. However, it may also be the case that the temporal pattern of catecholamine receptor activation that is enabled by means of burst firing of the locus coeruleus (Sara, 2015) and/or ventral tegmental area (Schultz, 1998; Bromberg-Martin et al., 2010; Paladini and Roeper, 2014) is decisive in enabling synaptic information encoding. Whereas the lack of effect of the agonists in middle-aged animals may herald the onset of deficits in catecholamine receptor-related hippocampal information processing, the retention of the ability to respond to spatial novelty with persistent LTP suggests that, at this time-point in aging, the catecholaminergic system has not yet become entirely dysfunctional.

Changes in object-recognition memory and in context-dependent extinction learning become apparent in middle-aged rodents (Wiescholleck et al., 2014). With increasing age, exploratory behavior of rats also declines, and although object-place memory is impaired, older rats detect novelty related to newly included environmental features (Shukitt-Hale et al., 2001). Older rats are more vulnerable to distraction than younger animals (Mishra et al., 2014). This kind of attention deficit can be ameliorated by training, however (Mishra et al., 2014). These findings are in line with our interpretation that early age-related deficits in hippocampal information processing may less relate to a failure of learning per se and more to a decline in motivation and attention. This suggests that neuromodulatory systems that support attention, such as the dopaminergic and noradrenergic systems are not completely disabled, but may be become increasingly ineffective with aging. Their involvement in raising hippocampal excitability during e.g., novel experience and in lowering the thresholds for induction of hippocampal synaptic plasticity (Sara et al., 1994; Li et al., 2003; Lemon et al., 2009; Hansen and Manahan-Vaughan, 2015a, b) becomes progressively weaker, however, necessitating that a higher degree of arousal (than that required in young animals) must be elicited in order for the animal to learn effectively.

In young rats, the dopaminergic and noradrenergic systems exert a very potent control on the longevity and magnitude of LTP (Hansen and Manahan-Vaughan, 2014; Hagena et al., 2016). This regulation is very tightly linked to hippocampus-based learning: antagonism of dopamine D1/D5 receptors prevents the facilitation of long-term depression (LTD) by spatial content learning in the CA1 region (Lemon and Manahan-Vaughan, 2006) and the dentate gyrus (Wiescholleck and Manahan-Vaughan, 2014). The dopamine D1/D5 receptors are required for very long-lasting forms of LTP and LTD in behaving rats (Kulla and Manahan-Vaughan, 2000; Lemon and Manahan-Vaughan, 2006), as are beta-adrenergic receptors (Kemp and Manahan-Vaughan, 2008b; Hagena and Manahan-Vaughan, 2012; Hansen and Manahan-Vaughan, 2015a, b). Thus, it was all the more surprising to find that activation of neither receptor could prolong LTP in middle-aged rats. This deficit does not appear to relate to failure of LTP machinery: strong afferent stimulation resulted in LTP that was equivalent in young and middle-aged rats. Furthermore, weak afferent stimulation during novel spatial exploration resulted in LTP. This latter phenomenon has been extensively described in young rats (Kemp and Manahan-Vaughan, 2008a). The facilitation of LTP by novel spatial exploration is very tightly linked to the spatial learning event (Kemp and Manahan-Vaughan, 2008a) and is prevented in young animals by antagonism of either D1/D5 receptors (Wiescholleck and Manahan-Vaughan, 2014) or beta-adrenergic receptors (Kemp and Manahan-Vaughan, 2008b; Hagena and Manahan-Vaughan, 2012; Hansen and Manahan-Vaughan, 2015b) that prevent the learning event itself. The fact that robust LTP can be either induced by afferent stimulation or by spatial learning in middle-aged rats suggests that the ability of the hippocampus to express persistent LTP is intact. Changes in the sensitivity of the catecholaminergic receptors to their respective agonists (Suzuki et al., 2001) may reflect a more subtle loss of the state-dependent regulation of learning and synaptic encoding by these neuromodulatory systems. This desensitization of hippocampal LTP to catecholaminergic control may reflect the functional basis for changes in attention and motivation-based learning that emerge with increasing age.

DM-V created the concept and strategy of the study. HT conducted the electrophysiological experiments and prepared the corresponding figures. Data analysis and interpretation were conducted jointly. DM-V wrote the article, with contributions from HT.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.