Gamma-aminobutyric acid type B receptors (GABA_BRs) are G-protein coupled receptors (GPCR) for GABA, which together with glycine represent the major inhibitory transmitters in the mammalian nervous system. They are widely distributed in nervous tissue, where they regulate neuronal excitability, oscillatory activity and neurogenesis, and are involved in processes such as synaptic plasticity, memory formation and nociception (reviewed in (1–7). Alterations in GABA_B functions have been linked to a variety of neurological states and psychiatric disorders including drug addiction, anxiety, cerebral ischemia, depression, epilepsy, neuropathic pain, and spasticity, and Alzheimer’s disease (7–10). In the auditory system, GABA_BR subunit expression generally shows high levels, particularly in the cochlea, cochlear nucleus, inferior colliculus, medial geniculate nucleus and auditory cortex (11–18). Increasing evidence suggests their functional involvement in the neural circuits that make up these areas, and it is emerging that dysfunction of GABA_BRs could play a role in some pathologies of the auditory system. In this review, we summarize the current knowledge on the distribution and functions of GABA_BRs in the peripheral and subcortical parts of the central auditory system obtained in animal studies, mostly using various rodent models.

GABA_BRs are obligatory heteromers of GABA_B1 and GABA_B2 subunits (19) ( Figure 1A ). The subunits heterodimerize through a C-terminal coiled-coil domain which displaces an ER retention signal protein from GABA_B1, thus allowing expression of the assembled complexes on the plasma membrane (20, 21). Despite their structural similarity, the subunits play different functional roles in the activation of the receptor heteromer. During this process, GABA_B1 mediates agonist binding to GABA_BR through its N-terminal ‘flytrap’ domain. This induces a series of conformational changes of the receptor and activation of the G_i/o protein via GABA_B2 intracellular loops (for a review, see (22–24). Upon activation, the G protein G_α subunits inhibit adenylyl cyclase to decrease cytosolic cAMP levels while the G_βγ subunits inhibit voltage-gated Ca²⁺ channels (VGCC) or open inwardly rectifying Kir₃ K⁺ channels (GIRK) (25–27). Through coupling to these effector enzymes and ion channels, GABA_BRs act as important regulators of neurotransmitter release, neuronal excitability and propagation of dendritic spikes (28–32) but see (33).

The physiological functions of GABA_BRs critically depend on their density and location in specific neural compartments, as well as on the kinetics of their signaling. These properties of GABA_BR are, on the other hand, significantly modulated by its post-translational modifications and interactions with associated proteins. First, ample evidence exists that phosphorylation of GABA_BR subunits by serine/threonine protein kinases bidirectionally affects both cell surface receptor expression and the magnitude of its responses (for details, see (34, 35). Phosphorylation of GABA_B1 at S867 by calcium/calmodulin-dependent kinase II or at an unidentified site by protein kinase C triggers GABA_BR internalization or desensitization (36, 37). Conversely, phosphorylation of GABA_B2 by adenosine monophosphate-activated protein kinase (S783) or cAMP-dependent protein kinase (S892) stabilizes GABA_BRs at the cell surface (38, 39). Second, the subcellular localization of GABA_BR and the dynamics of its intracellular trafficking depend, at least in some brain areas, on the type of GABA_B1 isoform ( Figure 1B ). The GABA_B1a variant containing two extracellular sushi domains (SDs) at its N-terminus is predominantly expressed in glutamatergic axonal terminals whilst the GABA_B1b lacking SDs expresses in somatodendritic parts (31, 40–42). GABA_B1a is also present in dendritic shafts but seems to be excluded from the spine heads (40). Recent studies have identified several proteins that interact with SDs ( Figure 1A ) and promote axonal trafficking of GABA_B1a and/or receptor stabilization at the presynaptic plasma membrane (43–45). Among these proteins, the amyloid-precursor protein (APP) appears to play a key role, linking the APP/GABA_BR complex to the axonal trafficking motor (for details, see (19, 23). Moreover, a secreted cleaved APP fragment has been shown to regulate synaptic transmission in the hippocampus of mice in a GABA_BR-dependent manner (45), but it appears to act through a more complex mechanism than as a functional receptor ligand (46). GABA_B1 subunits have also been reported to associate with 14-3-3 proteins and the capsaicin receptor TRPV1 (47, 48), with both interactions being implicated in chronic pain conditions. 14-3-3 binds to the C-terminus of GABA_B1 to dissociate the GABA_B heterodimer, resulting in impaired GABA_B signaling and reduced control of TRPV1 sensitization in spinal neurons (47–49).

Lastly, four cytosolic K⁺-channel tetramerization-domain (KCTD) proteins KCTD8, KCTD12, KCTD12b and KCTD16, which constitutively bind to the GABA_B2 C-terminal domain as pentamers, increase receptor surface expression and show multiple effects on its response kinetics (50–52). The KCTD proteins comprise the N-terminal T1 and C-terminal H1 domains (53, 54), capable of simultaneous interactions with GABA_B2 and G_βγ subunits of the G protein, respectively (55, 56). The preassembled complex is characterized by elevated potency and accelerated kinetics of G-protein signaling (50, 55). In addition, KCTD12 and 12b induce pronounced desensitization of GABA_BR responses by activity-dependent stripping of G_βγ from GIRK or VGCC channels (55, 56). The desensitization is in turn regulated by phosphorylation of serine-892 on GABA_B2 or by heteromerization of KCTD12 with KCTD16 (57). KCTD16 itself slows down the deactivation phase of GABA_BR activated GIRK currents by unknown mechanism (58). Moreover, KCTD8 and KCTD16 contain a C-terminal H2 domain that binds secondary GABA_BR interacting proteins, such as VGCC or hyperpolarization-activated cyclic nucleotide-gated channels (43). Because many neurons in the brain simultaneously express several KCTDs and some of them possibly all KCTDs, assembly of distinct KCTDs at the receptor increases the molecular and functional repertoire of native GABA_BRs (58, 59).

In the mammalian cochlea, the sensory organ of Corti comprises one row of inner and three rows of outer hair cells (IHCs and OHCs) ( Figure 2 ). Their main tasks are to amplify the incoming auditory signals (OHC) and to relay auditory information to the brain (IHC) (for a review, see (60, 61). Hair cells receive both afferent and efferent innervation. The afferent innervation is carried by spiral ganglion neurons type I and II, which respond to glutamate released by IHCs and OHCs, respectively, and initiate and conduct action potentials to the cochlear nucleus (62–65). GABA_BRs have been found in both type I and type II spiral ganglion neurons and their afferent terminals at IHCs and OHCs that do not express GABA_BRs themselves (66–70). Activation of the receptors with baclofen diminished glutamate induced increases of intracellular Ca²⁺ concentration in cultured spiral ganglion neurons (66). The mechanism underlying the inhibition has not been revealed but these observations would be consistent with GABA_BR-mediated regulation of the excitability of IHC afferents by reducing the Ca²⁺ permeability of postsynaptic NMDA receptors (32, 71). In addition, GABA_BRs could also inhibit Ca²⁺-dependent K⁺ conductance in a way found in vestibular hair cell afferents (72–75), thereby modulating the spike frequency in spiral ganglion neurons (73, 76).

The efferent fibers originate from the superior olivary complex and allow feedback control of cochlear activity by the auditory brainstem (for reviews, see (77–79). There are two groups of olivocochlear efferents (OC): medial OC (MOC), large-diameter myelinated fibers arising from regions near the medial superior olive (MSO) and releasing acetylcholine to reduce the gain of the OHC amplifier, and lateral OC (LOC), thinner unmyelinated fibers that arise near the lateral superior olive (LSO) and modulate the excitability of type I afferents by releasing multiple transmitters, including acetylcholine and GABA. Functional GABA_BRs have been found in OC bundles, where their expression appeared to be developmentally regulated (80) ( Figure 2 ). Mice during the second postnatal week express GABA_B1a/2 receptors at axonal terminals of OC fibers innervating somata of OHCs and, at this developmental stage, also IHCs (68) ( Figure 2A ). The receptors control acetylcholine release at efferent synapses via inhibition of presynaptic P-/Q-type VGCC (68) ( Figure 2C ). In adult mice, however, no GABA_BRs or their disinhibitory effects were observed in MOC terminals or OHCs, respectively (67) ( Figure 2B ).

The role of cochlear GABA_BRs in the auditory function remains to be understood. The receptors are thought to be activated in vivo by GABA co-released with acetylcholine from LOC and MOC fibers (79–81) ( Figure 2C ). The firing frequency of the fibers and thus the secretory activity of their efferent synapses depend on the intensity of sound stimulation (77, 82). During low-intensity acoustic stimulation, GABA_BRs can be activated predominantly by the background GABA (83) and tonically suppress the release of acetylcholine at OC terminals. Consistent with this expectation, selective GABA_BR antagonists increase the amplitudes of postsynaptic currents elicited in hair cells by low-frequency stimulation of MOC fibers (68). The low basal probability of acetylcholine release then allows MOC-OHC synapses to respond to high-frequency stimulation by strongly facilitating cholinergic postsynaptic currents (78, 84). It has been proposed that the presynaptic facilitation together with a summation of repetitive postsynaptic currents significantly increase the reliability and strength of cholinergic synaptic transmission during high-intensity acoustic stimulation (78). This, in turn, would lead to a greater reduction in the gain of the OHC amplifier when exposed to intense sounds and to the protection of the immature cochlea from acoustic trauma (80). Adult mice with deleted GABA_BRs exhibit increased hearing thresholds as measured by auditory brainstem responses and distortion product otoacoustic emissions (67). This indicated the importance of GABA_BRs for the normal function of the mature cochlea. It has been suggested that deletion of GABA_BRs in spiral ganglion and brainstem neurons led to increased spontaneous activity and elevation of the thresholds (67).

Finally, GABA_BR-associated KCTD12 proteins have been found to be highly expressed in the mammalian cochlea during the early developmental stages (11, 59). Immunostaining experiments located KCTD12 in spiral ganglion neurons and also in cochlear supporting cells and fibrocytes (11). As these non-neuronal cells have been implicated in the K⁺ recycling pathways, KCTD12 could play a role in ion transport or ionic content regulation in the cochlea (11, 85, 86).

Dysfunction of the GABA_BR, an important modulator of cellular excitability, is expected to be part of the mechanisms underlying the neuronal hyperactivity that accompanies some of the known auditory pathologies. Examples of such conditions include noise-induced hearing loss caused by sudden acoustic trauma or prolonged exposure to noise levels above 85 dB (183). It is well documented that this overexposure can lead to death of the IHCs and OHCs, reduction of synaptic ribbons, death of spiral ganglion neurons, or degeneration of the auditory nerve, causing cochlear dysfunction and reduced sensory output (184–186). The peripheral deficit is then thought to trigger mechanisms of homeostatic plasticity in the central auditory pathway to regulate its gain and thereby compensate for the amount of neural activity from the cochlea (187–190). Although the mechanisms underlying neuronal gain modification are not fully understood, one possibility is a reduction in synaptic inhibition mediated by neurotransmitters such as GABA and glycine (191). Consistent with this assumption, studies in animal models of acoustic trauma indicated an imbalance between excitation and inhibition in the auditory system due to impaired GABAergic neurotransmission (192–196), including reduced GABA_BR expression in the auditory brainstem (121, 122). The latter showed that neurons in the molecular and fusiform layers of the DCN exhibit reduced GABA_BR density in mice after acoustic trauma, presumably as a result of receptor internalization due to its phosphorylation by protein kinase C gamma (121, 122). This is consistent with the increased excitability of these neurons in animals with noise-induced hearing loss (90, 197, 198). Thus, changes in GABA_B function may be part of the mechanisms underlying maladaptive plasticity in the DCN, which is known to lead to hyperactivity of DCN neurons and the development of tinnitus (199, 200). Changes in receptor expression or distribution could also occur in other regions of the subcortical auditory pathway, as suggested by trauma-induced changes in temporal and binaural processing or adaptive coding of sound stimuli, processes that require proper GABA_BR function (165, 193, 201–204).

Tinnitus, phantom perception in the absence of sound stimuli, is another example of audiological conditions involving hyperactivity of neurons in the auditory pathway (see (90, 198, 205, 206) for reviews). It is generally ignited by hearing loss and very often by noise exposure (207) and, accordingly, animal models of tinnitus show reduced GABA/glycinergic inhibition in various regions of the auditory system (reviewed in (206, 208). Initial work focusing on the relationship between GABA_BRs and tinnitus explored the possibility of compensating for the loss of GABAergic inhibition by pharmacological activation of receptors with baclofen (165, 209). The results show that baclofen is a potent modulator of neuronal excitability in the ascending auditory pathway that dose-dependently reduces behavioral symptoms of chronic tinnitus in an animal model of acoustic trauma. Later work then indicated a closer link between the receptor and tinnitus by finding that intraperitoneal injection of sodium salicylate, which is known to elicit behavioral measures of tinnitus in animal studies (210), reduced levels of GABA_B1 and GABA_B2 subunits in the rat IC (211). These observations suggested that baclofen could represent a potentially effective agent in the treatment of tinnitus. However, this assumption failed to be demonstrated in a clinical study of this substance in patients with subjective tinnitus, in which no significant difference was found between the drug and placebo groups (212). Smith et al. (213) addressed the reasons for this failure and suggested that the efficacy of novel GABA_BR agonists, which do not have the undesirable side effects of baclofen, should be investigated against tinnitus. It is also likely that tinnitus-associated changes in GABA_BR expression occur only in selected cell subpopulations, as has been proposed, for example, for the inhibitory interneurons of the DCN, cartwheel cells (121). Therefore, more specific substances that selectively modulate the activity of GABA_BRs in specific cell types or compartments should be found for effective treatment. Interestingly, a variation in KCTD12 gene has been identified as a risk modifier in chronic tinnitus (214). As mentioned above, KCTD12 proteins exhibit multiple modulatory effects on GABA_BR activity and are expressed in both the peripheral and central parts of the auditory system (11, 59). Their high expression in the stria vascularis suggests their possible involvement in the processes responsible for cochlear K+ transport necessary for the maintenance of the endocochlear potential (11, 85). Therefore, changes in KCTD12 action could contribute to sensorineural hearing loss due to impaired cochlear K+ homeostasis (215, 216) and impaired GABA_B function in the auditory pathway, leading to tinnitus. KCTD12 may thus represent a potential target in the therapy of hearing disorders, and it is therefore essential to understand both the mechanisms of its effects in the auditory system and its role in animal models of tinnitus.

The above data illustrate the involvement of GABA_BRs in mechanisms that are important for proper hearing function in mammals. These include both peripheral and central processes such as cochlear amplification, sound source localization, temporal processing, adaptive coding, or suppression of spontaneous activity. Because of their relatively slow kinetics, GABA_BRs are thought to primarily regulate the overall excitability of neurons and thus determine the gain in signal processing. The relatively broad involvement of receptors in auditory processing could then be due to their specific localization in neuronal circuits and the multitude of interacting proteins that serve as their effectors or through which other signaling pathways modulate GABA_BR activity and thus neuronal responses. The presence of numerous modulation sites on the subunits of GABA_BRs and their effectors also provides an opportunity to influence receptor responses with pharmacological agents, for example those that restore GABA_B function in certain pathological conditions of the auditory system.

As discussed in Section 5, noise or salicylate-induced hearing loss may be accompanied by changes in central gain, and it is likely that attenuation of GABA_BR signaling contributes to this. Similarly, GABA_BRs seem to be involved in mechanisms of age-related hearing loss, as suggested by the decline in their levels in the auditory pathway of aging rats (163, 217). By increasing the severity of the auditory deficit, reduced GABA_BR expression could also indirectly promote pathophysiological processes in other, non-auditory parts of the brain. This assumption is based on the fact that the auditory pathway contains numerous inputs from other sensory systems (see also Sections 4.1 and 4. 3), through which it receives neuromodulation that affects sound processing, and through which it in turn elicits auditory-driven neural responses in non-auditory areas (see (161, 218, 219) for reviews and (220) for recent evidence). Growing evidence suggests a strong association between hearing impairment and cognitive decline, and age-related hearing loss is considered one of the greatest risk factors for the development of dementias, including Alzheimer’s disease (221, 222). The mechanisms linking these states are not entirely clear, but they appear to involve a cascade of events altering the activity of neural circuits in areas important for cognitive function, such as the hippocampus (223–226). Therefore, changes in the activity of auditory neurons due to GABA_BR dysfunction may potentially play a role in these processes. Hearing loss could be widely treated with hearing aids or cochlear implants and is therefore considered a modifiable risk factor (225), but treatment targeting GABA_BRs in the auditory pathway could represent a pharmacological alternative. However, the mere systemic use of stimulators of GABA_BR or its effectors could lead to contradictory effects, as pharmacological inhibition of GABA_BRs or their G-protein-dependent signaling has been shown to improve hippocampus-dependent memory and decision making, respectively (227–229). This suggests the need for new specific agents targeting GABA_BRs expressed in the auditory system. The current detailed knowledge of GABA_BR structure supports advances in pharmacological methods, enabling the investigation of receptor function, as well as the development of new drugs, such as specific allosteric modulators and peptide-based inhibitors that target protein-protein interactions in the receptor complex (230, 231).

RT wrote the paper with input from the other authors. All authors contributed to the article and approved the submitted version.