Baclofen reduces tone-evoked activity of cochlear nucleus neurons

Influence of GABAA and GABAB receptor activation on auditory sensory gating and its association with anxiety in healthy volunteers

BACKGROUND

Dysfunctional sensory gating in anxiety disorders, indexed by the failure to inhibit the P50 event-related potential (ERP) to repeated stimuli, has been linked to deficits in the major inhibitory neurotransmitter γ-aminobutyric acid (GABA).

AIMS/METHODS

This study, conducted in 30 healthy volunteers, examined the acute effects of GABAA (lorazepam: 1 mg) and GABAB receptor (baclofen: 10 mg) agonists on P50 measures of auditory sensory gating within a paired-stimulus (S1-S2) paradigm and assessed changes in gating in relation to self-ratings of anxiety.

RESULTS

Compared to placebo, lorazepam reduced ERP indices of sensory gating by attenuating response to S1. Although not directly impacting P50 inhibition, baclofen-induced changes in gating (relative to placebo) were negatively correlated with trait but not state anxiety.

CONCLUSIONS

These preliminary findings support the involvement of GABA in sensory gating and tentatively suggest a role for GABAB receptor signaling in anxiety-associated gating dysregulation.

The role of GABAB receptors in the subcortical pathways of the mammalian auditory system

GABAB receptors are G-protein coupled receptors for the inhibitory neurotransmitter GABA. Functional GABAB receptors are formed as heteromers of GABAB1 and GABAB2 subunits, which further associate with various regulatory and signaling proteins to provide receptor complexes with distinct pharmacological and physiological properties. GABAB receptors are widely distributed in nervous tissue, where they are involved in a number of processes and in turn are subject to a number of regulatory mechanisms. In this review, we summarize current knowledge of the cellular distribution and function of the receptors in the inner ear and auditory pathway of the mammalian brainstem and midbrain. The findings suggest that in these regions, GABAB receptors are involved in processes essential for proper auditory function, such as cochlear amplifier modulation, regulation of spontaneous activity, binaural and temporal information processing, and predictive coding. Since impaired GABAergic inhibition has been found to be associated with various forms of hearing loss, GABAB dysfunction could also play a role in some pathologies of the auditory system.

GABAB Receptor Agonist R-Baclofen Reverses Altered Auditory Reactivity and Filtering in the Cntnap2 Knock-Out Rat

Altered sensory information processing, and auditory processing, in particular, is a common impairment in individuals with autism spectrum disorder (ASD). One prominent hypothesis for the etiology of ASD is an imbalance between neuronal excitation and inhibition. The selective GABA_B receptor agonist R-Baclofen has been shown previously to improve social deficits and repetitive behaviors in several mouse models for neurodevelopmental disorders including ASD, and its formulation Arbaclofen has been shown to ameliorate social avoidance symptoms in some individuals with ASD. The present study investigated whether R-Baclofen can remediate ASD-related altered sensory processing reliant on excitation/inhibition imbalance in the auditory brainstem. To assess a possible excitation/inhibition imbalance in the startle-mediating brainstem underlying ASD-like auditory-evoked behaviors, we detected and quantified brain amino acid levels in the nucleus reticularis pontis caudalis (PnC) of rats with a homozygous loss-of-function mutation in the ASD-linked gene Contactin-associated protein-like 2 (Cntnap2) and their wildtype (WT) littermates using Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (MALDI MS). Abnormal behavioral read-outs of brainstem auditory signaling in Cntnap2 KO rats were accompanied by increased levels of GABA, glutamate, and glutamine in the PnC. We then compared the effect of R-Baclofen on behavioral read-outs of brainstem auditory signaling in Cntnap2 KO and WT rats. Auditory reactivity, sensory filtering, and sensorimotor gating were tested in form of acoustic startle response input-output functions, short-term habituation, and prepulse inhibition before and after acute administration of R-Baclofen (0.75, 1.5, and 3 mg/kg). Systemic R-Baclofen treatment improved disruptions in sensory filtering in Cntnap2 KO rats and suppressed exaggerated auditory startle responses, in particular to moderately loud sounds. Lower ASR thresholds in Cntnap2 KO rats were increased in a dose-dependent fashion, with the two higher doses bringing thresholds close to controls, whereas shorter ASR peak latencies at the threshold were further exacerbated. Impaired prepulse inhibition increased across various acoustic prepulse conditions after administration of R-Baclofen in Cntnap2 KO rats, whereas R-Baclofen did not affect prepulse inhibition in WT rats. Our findings suggest that GABA_B receptor agonists may be useful for pharmacologically targeting multiple aspects of sensory processing disruptions involving neuronal excitation/inhibition imbalances in ASD.

NMDA Receptors Mediate Stimulus-Timing-Dependent Plasticity and Neural Synchrony in the Dorsal Cochlear Nucleus

Auditory information relayed by auditory nerve fibers and somatosensory information relayed by granule cell parallel fibers converge on the fusiform cells (FCs) of the dorsal cochlear nucleus, the first brain station of the auditory pathway. In vitro, parallel fiber synapses on FCs exhibit spike-timing-dependent plasticity with Hebbian learning rules, partially mediated by the NMDA receptor (NMDAr). Well-timed bimodal auditory-somatosensory stimulation, in vivo equivalent of spike-timing-dependent plasticity, can induce stimulus-timing-dependent plasticity (StTDP) of the FCs spontaneous and tone-evoked firing rates. In healthy guinea pigs, the resulting distribution of StTDP learning rules across a FC neural population is dominated by a Hebbian profile while anti-Hebbian, suppressive and enhancing LRs are less frequent. In this study, we investigate in vivo, the NMDAr contribution to FC baseline activity and long term plasticity. We find that blocking the NMDAr decreases the synchronization of FC- spontaneous activity and mediates differential modulation of FC rate-level functions such that low, and high threshold units are more likely to increase, and decrease, respectively, their maximum amplitudes. Three significant alterations in mean learning-rule profiles were identified: transitions from an initial Hebbian profile towards (1) an anti-Hebbian; (2) a suppressive profile; and (3) transitions from an anti-Hebbian to a Hebbian profile. FC units preserving their learning rules showed instead, NMDAr-dependent plasticity to unimodal acoustic stimulation, with persistent depression of tone-evoked responses changing to persistent enhancement following the NMDAr antagonist. These results reveal a crucial role of the NMDAr in mediating FC baseline activity and long-term plasticity which have important implications for signal processing and auditory pathologies related to maladaptive plasticity of dorsal cochlear nucleus circuitry.

Anticonvulsant effects of structurally diverse GABA(B) positive allosteric modulators in the DBA/2J audiogenic seizure test: Comparison to baclofen and utility as a pharmacodynamic screening model

The GABA(B) receptor has been indicated as a promising target for multiple CNS-related disorders. Baclofen, a prototypical orthosteric agonist, is used clinically for the treatment of spastic movement disorders, but is associated with unwanted side-effects, such as sedation and motor impairment. Positive allosteric modulators (PAM), which bind to a topographically-distinct site apart from the orthosteric binding pocket, may provide an improved side-effect profile while maintaining baclofen-like efficacy. GABA, the major inhibitory neurotransmitter in the CNS, plays an important role in the etiology and treatment of seizure disorders. Baclofen is known to produce anticonvulsant effects in the DBA/2J mouse audiogenic seizure test (AGS), suggesting it may be a suitable assay for assessing pharmacodynamic effects. Little is known about the effects of GABA(B) PAMs, however. The studies presented here sought to investigate the AGS test as a pharmacodynamic (PD) screening model for GABA(B) PAMs by comparing the profile of structurally diverse PAMs to baclofen. GS39783, rac-BHFF, CMPPE, A-1295120 (N-(3-(4-(4-chloro-3-fluorobenzyl)-6-methoxy-3,5-dioxo-4,5-dihydro-1,2,4-triazin-2(3H)-yl)phenyl)acetamide), and A-1474713 (N-(3-(4-(4-chlorobenzyl)-3,5-dioxo-4,5-dihydro-1,2,4-triazin-2(3H)-yl)phenyl)acetamide) all produced robust, dose-dependent anticonvulsant effects; a similar profile was observed with baclofen. Pre-treatment with the GABA(B) antagonist SCH50911 completely blocked the anticonvulsant effects of baclofen and CMPPE in the AGS test, indicating such effects are likely mediated by the GABA(B) receptor. In addition to the standard anticonvulsant endpoint of the AGS test, video tracking software was employed to assess potential drug-induced motor side-effects during the acclimation period of the test. This analysis was sensitive to detecting drug-induced changes in total distance traveled, which was used to establish a therapeutic index (TI = hypoactivity/anticonvulsant effects). Calculated TIs for A-1295120, CMPPE, rac-BHFF, GS39783, and A-1474713 were 5.31x, 5.00x, 4.74x, 3.41x, and 1.83x, respectively, whereas baclofen was <1. The results presented here suggest the DBA/2J mouse AGS test is a potentially useful screening model for detecting PD effects of GABA(B) PAMs and can provide an initial read-out on target-related motor side-effects. Furthermore, an improved TI was observed for PAMs compared to baclofen, indicating the PAM approach may be a viable therapeutic alternative to baclofen.

Effects of early and late treatment with L-baclofen on the development and maintenance of tinnitus caused by acoustic trauma in rats

Subjective tinnitus is a chronic neurological disorder in which phantom sounds are perceived. Recent evidence supports the hypothesis that tinnitus is related to neuronal hyperactivity in auditory brain regions, and consequently drugs that increase GABAergic neurotransmission in the CNS, such as the GABA(B) receptor agonist L-baclofen, may be effective as a treatment. The aim of this study was to investigate the effects of early (5 mg/kg s.c., 30 min and then every 24 h for 5 days following noise exposure) and late treatment (3 mg/kg/day s.c. for 4.5 weeks starting at 17.5 weeks following noise exposure) with l-baclofen on the psychophysical attributes of tinnitus in a conditioned lick suppression model following acoustic trauma in rats. Acoustic trauma (a 16-kHz, 115-dB pure tone presented unilaterally for 1h) resulted in a significant decrease in the suppression ratio (SR) compared to sham controls in response to 20-kHz tones at 2, 10 and 17.5 weeks post-exposure (P ≤ 0.009, P ≤ 0.02 and P ≤ 0.03, respectively). However, l-baclofen failed to prevent the development of tinnitus when administered during the first 5 days following the acoustic trauma and also failed to reverse it when treatment was carried out every day for 4.5 weeks. We also found that treatment with L-baclofen did not alter the expression of the GABA(B)-R2 subunit in the cochlear nucleus of noise-exposed animals.

Localization of the GABAB receptor 1a/b subunit relative to glutamatergic synapses in the dorsal cochlear nucleus of the rat

Metabotropic gamma-aminobutyric acid receptors (GABA(B)) are involved in pre- and postsynaptic inhibitory effects upon auditory neurons and have been implicated in different aspects of acoustic information processing. To understand better the mechanisms by which GABA(B) receptors mediate their inhibitory effects, we used pre-embedding immunocytochemical techniques combined with quantification of immunogold particles to reveal the precise subcellular distribution of the GABA(B1) subunit in the rat dorsal cochlear nucleus. At the light microscopic level, GABA(B1) was detected in all divisions of the cochlear complex. The most intense immunoreactivity for GABA(B1) was found in the dorsal cochlear nucleus, whereas immunoreactivity in the anteroventral and posteroventral cochlear nuclei was very low. In the dorsal cochlear nucleus, a punctate labeling was observed in the superficial (molecular and fusiform cell) layers. At the electron microscopic level, GABA(B1) was found at both post- and presynaptic locations. Postsynaptically, GABA(B1) was localized mainly in the dendritic spines of presumed fusiform cells. Quantitative immunogold immunocytochemistry revealed that the highest concentration of GABA(B1) in the plasma membrane was in dendritic spines, followed by dendritic shafts and somata. Thus, the most intense immunoreactivity for GABA(B1) was observed in dendritic spines with a high density of immunogold particles at extrasynaptic sites, peaking around 300 nm from glutamatergic synapses. This is in contrast to GABAergic synapses, in which GABA(B1) was only occasionally found. Presynaptically, receptor immunoreactivity was detected primarily in axospinous endings, probably from granule cells, in both the active zone and extrasynaptic sites. The localization of GABA(B1) relative to synaptic sites in the DCN suggests a role for the receptor in the regulation of dendritic excitability and excitatory inputs.

GABAergic inputs shape responses to amplitude modulated stimuli in the inferior colliculus

The inferior colliculus (IC) is an important auditory processing center receiving inputs from lower brainstem nuclei, higher auditory and nonauditory structures, and contralateral IC. The IC, along with other auditory structures, is involved in coding information about the envelope of complex signals. Biologically relevant acoustic signals, including animal vocalizations and speech, are spectrally and temporally complex and display amplitude and frequency variations over time. Certain IC neurons respond selectively over a narrow range of modulation frequencies to sinusoidally amplitude modulated (SAM) stimuli. Responses to SAM stimuli can be measured in terms of discharge rate, with rate plotted against the modulation frequency to generate rate modulation transfer functions (rMTF). A role for the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), in shaping selective responses to SAM stimuli has been suggested. The present study examined the role of GABA in shaping responses to SAM stimuli in the IC of anesthetized chinchilla. Responses from 94 IC neurons were obtained before, during and after iontophoretic application of the GABA(A) receptor antagonist bicuculline methiodide. Complete responses to SAM stimuli were obtained from 55 extensively tested neurons, displaying band-pass (38) and low-pass rMTFs (17). For neurons showing band-pass rMTFs, GABA(A) receptor blockade selectively increased discharge rate at low modulation frequencies for 14 units, increased discharge near the best modulation frequency for 12 units. For neurons showing low-pass rMTFs, GABA(A) receptor blockade selectively increased discharge rate at low modulation frequencies for nine units. GABA(A) receptor blockade consistently reduced peak modulation gain, producing low-pass gain functions in a subset of IC neurons. In support of previous findings suggesting that selective temporal responses to SAM stimuli are coded in lower brainstem nuclei, temporal responses to SAM stimuli were relatively unaffected by GABA(A) receptor blockade. These findings support a role for GABA in shaping selective rate responses to SAM stimuli for a subset of chinchilla IC neurons.

Auditory plasticity and hyperactivity following cochlear damage

This paper will review some of the functional changes that occur in the central auditory pathway after the cochlea is damaged by acoustic overstimulation or by carboplatin, an ototoxic drug that selectively destroys inner hair cells (IHCs) in the chinchilla. Acoustic trauma typically impairs the sensitivity and tuning of auditory nerve fibers and reduces the neural output of the cochlea. Surprisingly, our results show that restricted cochlear damage enhances neural activity in the central auditory pathway. Despite a reduction in the auditory-nerve compound action potential (CAP), the local field potential from the inferior colliculus (IC) increases at a faster than normal rate and its maximum amplitude is enhanced at frequencies below the region of hearing loss. To determine if this enhancement was due to loss of sideband inhibition, we recorded from single neurons in the IC and dorsal cochlear nucleus before and after presenting a traumatizing above the unit's characteristic frequency (CF). Following the exposure, some neurons showed substantial broadening of tuning below CF, less inhibition, and a significant increase in discharge rate, consistent with a model involving loss of sideband inhibition. The central auditory system of the chinchilla can be deprived of some of its cochlear inputs by selectively destroying IHCs with carboplatin. Selective IHC loss reduces the amplitude of the CAP without affecting the threshold and tuning of the remaining auditory nerve fibers. Although the output of the cochlea is reduced in proportion to the amount of IHC loss, the IC response shows only a modest amplitude reduction, and remarkably, the response of the auditory cortex is enhanced. These results suggest that the gain of the central auditory pathway can be up- or down regulated to compensate for the amount of neural activity from the cochlea.

Gamma-aminobutyric acidergic and glycinergic inputs shape coding of amplitude modulation in the chinchilla cochlear nucleus

Amplitude modulation is a prominent acoustic feature of biologically relevant sounds, such as speech and animal vocalizations. Enhanced temporal coding of amplitude modulation signals is found in certain dorsal and posteroventral cochlear nucleus neurons when they are compared to auditory nerve. Although mechanisms underlying this improved temporal selectivity are not known, involvement of inhibition has been suggested. gamma-Aminobutyric acid- and glycine-mediated inhibition have been shown to shape the dorsal cochlear nucleus and posteroventral cochlear nucleus response properties to other acoustic stimuli. In the present study, responses to amplitude modulation tones were obtained from chinchilla dorsal cochlear nucleus and posteroventral cochlear nucleus neurons. The amplitude modulation carrier was set to the neuron's characteristic frequency and the modulating frequency varied from 10 Hz. Rate and temporal modulation transfer functions were compared across neurons. Bandpass temporal modulation transfer functions were observed in 74% of the neurons studied. Most cochlear nucleus neurons (90%) displayed flat or lowpass rate modulation transfer functions to amplitude modulation signals presented at 2540 dB (re: characteristic frequency threshold). The role of inhibition in shaping responses to amplitude modulation stimuli was examined using iontophoretic application of glycine or gamma-aminobutyric acidA receptor agonists and antagonists. Blockade of gamma-aminobutyric acidA or glycine receptors increased stimulus-evoked discharge rates for a majority of neurons tested. Synchronization to the envelope was reduced, particularly at low and middle modulating frequencies, with temporal modulation transfer functions becoming flattened and less bandpass in appearance. Application of glycine, gamma-aminobutyric acid or muscimol increased the modulation gain over the low- and mid-modulation frequencies and reduced the discharge rate across envelope frequencies for most neurons tested. These findings support the hypothesis that glycinergic and gamma-aminobutyric acidergic inputs onto certain dorsal cochlear nucleus and posteroventral cochlear nucleus neurons play a role in shaping responses to amplitude modulation stimuli and may be responsible for the reported preservation of amplitude modulation temporal coding in dorsal cochlear nucleus and posteroventral cochlear nucleus neurons at high stimulus intensities or in background noise.

Glycinergic and GABAergic inputs affect short-term suppression in the cochlear nucleus

Most cochlear nucleus (CN) neurons exhibit short-term response suppression to a second stimulus in a paired-pulse (click), forward-masking, paradigm. The magnitude of suppression, which appears to be greater than that observed in acoustic nerve, is dependent on the temporal separation and/or relative intensities of the two stimuli. Recent evidence suggests that inhibitory circuitry ending on CN neurons may mediate this response suppression. Using extracellular recordings from single CN neurons, suppression was evaluated using a forward-masking paradigm. Responses to paired acoustic clicks (i.e., a 'masker' followed by an identical 'probe' click) were measured while the time interval between the masker and probe was varied systematically. The role of inhibitory circuitry in forward-masking in the CN was assessed by pharmacologic manipulation of the GABA(A) and glycine(I) (strychnine-sensitive) receptors. Blockade of glycinergic or GABAergic receptors by iontophoretic application of the antagonists, strychnine and bicuculline methiodide, decreased the effects of forward-masking by shortening recovery times of the probe response in 2/3 of the neurons tested. Conversely, agonist application (glycine, and GABA or muscimol) increased the magnitude of suppression and delayed recovery of the probe response relative to control values. These findings suggest that known circuits releasing glycine and/or GABA mediate short-term response suppression in some CN neurons.

gamma-Aminobutyric acid and glycine in the baboon cochlear nuclei: an immunocytochemical colocalization study with reference to interspecies differences in inhibitory systems

Previous studies of the cochlear nuclei in cat, rat, and guinea pig have demonstrated neural structures that are enriched in the inhibitory neurotransmitter amino acids gamma-aminobutyric acid (GABA) and glycine. In these mammals, inhibitory terminals are widely distributed throughout the nuclear complex, but somata of inhibitory neurons are concentrated in the dorsal cochlear nucleus, in granule cell regions, and in the cap area. Because these are the subdivisions that undergo the most pronounced phylogenetic changes in primates, we wanted to see whether the inhibitory systems are influenced by changes in cytoarchitecture. Therefore, we applied light microscopic postembedding immunostaining and optical densitometry to the cochlear nuclei of an anthropoid primate, the Senegalese baboon (Papio anubis). Our results demonstrate that, in baboon 1) glycinergic neurons and axons in the ventral cochlear nucleus seem to form a commissural system similar to that of other mammals; 2) the tuberculoventral system appears to be unchanged in morphology but exhibits a higher level of colocalization of GABA with glycine; 3) there is a reduction of the granule/cartwheel cell system, which is reflected in lesser numbers of inhibitory cartwheel, Golgi, and molecular layer stellate cells; 4) the cap area is larger than in rodents and carnivores and contains many neurons that colocalize GABA and glycine; and 5) throughout the nuclear complex, a higher proportion of the inhibitory terminals colocalize GABA and glycine. We conclude that modulation of the ascending auditory pathway in baboon is likely to differ from that in rodents and cat.

Excitatory acoustic responses in the inferior colliculus of the rat are increased by GABAB receptor blockade

This study sought to investigate the influence of GABAB receptor activation on acoustically induced excitation within the rat inferior colliculus. To this end, the GABAB receptor antagonist, CGP 35348, was applied systemically and iontophoretically. Single and multibarrel electrodes were used for extracellular recordings within the central nucleus of the inferior colliculus. The experimental model, a paired-pulse stimulus paradigm, applied two identical acoustic stimuli, 200 msec apart, evoking corresponding responses characterized by the second being consistently weaker than the first. Abolishment of the acoustically evoked response, following iontophoretic application of the GABAB receptor agonist, L-baclofen, verified the existence of GABAB receptors in all inferior colliculus cells tested. Intravenous application of CGP 35348 (200 mg/kg) evoked a 24% overall increase in stimulus responses. Likewise, a 13% increase in total evoked excitation was observed, following iontophoretic application. There was no significant reduction of inhibition on the second evoked response in the paired-pulse model, following either systemic or iontophoretic application of CGP 35348. This result implies that the decreased magnitude of the second response, with an interpulse interval of 200 msec, is not influenced by GABAB receptor mediated inhibition. These findings do indicate, however, that GABAB receptors play a small, but significant role during the processing of acoustic information, within the inferior colliculus.

GABA alters the discharge pattern of chopper neurons in the rat ventral cochlear nucleus

The effect of microiontophoretically applied gamma-aminobutyric acid (GABA) on chopper neurons in the ventral cochlear nucleus of the rat is described. The predominantly inhibitory effect of GABA resulted in a change of the regular discharge pattern. The interspike interval increased and the pattern became less regular as indicated by an increase of its coefficient of variation. These results suggest that the release of GABA may be responsible for the transient chopper behavior of some neurons which loose their regular discharge pattern within 20 ms after onset of the response to pure-tone stimulation.

Effects of L-baclofen and D-baclofen on the auditory system: a study of click-evoked potentials from the inferior colliculus in the rat

The drug baclofen is a potential treatment for severe tinnitus, but its action in relieving tinnitus is not known. Baclofen is available as an approved drug only in racemic form with about equal content of the two enantiomers. In the present paper we show that L-baclofen causes a considerable (40.7%) suppression of the amplitude of the second peak in the click-evoked response from the cochlear nucleus. Bipolar recordings from the external nucleus of the inferior colliculus showed that L-baclofen caused a reduction in the amplitude of three or four distinct peaks in this response. D-Baclofen had no detectable effect on the response from the cochlear nucleus, and had only a slight effect on one component of the response from the external nucleus of the inferior colliculus. The demonstrated effect of L-baclofen on excitation in the ascending auditory pathway indicates that this drug may be a potential treatment for hyperactive auditory disorders such as tinnitus and hyperacusis.

Paired tone facilitation in dorsal cochlear nucleus neurons: a short-term potentiation model testable in vivo

It has been suggested that the dorsal cochlear nucleus (DCN) is involved in coding stimulus history or prior auditory activity [Manis (1989) J. Neurophys., 61, 149-161; Manis (1990) J. Neurosci., 10, 2338-2351]. The major output neurons of the DCN are the fusiform (pyramidal) cells which are thought to receive excitatory inputs from the descending branch of the acoustic nerve onto their basal dendrites and significant inhibitory glycinergic and GABAergic inputs to the soma and dendrites. The apical dendrites of these neurons lie within the molecular layer of the DCN and encounter parallel fibers which are thought to utilize the excitatory amino acid neurotransmitter glutamate. In this study of anesthetized chinchillas, we found that, in contrast to the responses of acoustic nerve fibers and most cochlear nucleus neurons which are masked by an appropriate preceding signal, many DCN principal cells are facilitated during the second of two identical stimuli. Facilitated DCN responses often have a reduced interspike interval and a more chopper-like temporal response pattern to the second characteristic frequency tone. This paired tone facilitation in the chinchilla DCN provides as in vivo model of short-term potentiation elicited by sensory stimulation similar to the paired-pulse facilitation observed with electrical stimulation in other models.

Distribution of GABAA and GABAB binding sites in the cochlear nucleus of the guinea pig

We compared the distribution of GABAA and GABAB binding sites in the cochlear nucleus using quantitative receptor autoradiography with [3H]GABA. To visualize GABAA binding sites, GABAB binding sites were blocked with +/- baclofen. To visualize GABAB binding sites, isoguvacine was used to block GABAA binding sites. GABAA binding sites predominated over GABAB, although there were marked regional differences in the distribution of binding. In the ventral cochlear nucleus, GABAA and GABAB binding sites were concentrated in the peripheral granule cell cap, with low binding levels in the central region. In the dorsal cochlear nucleus, binding was concentrated in the superficial (fusiform and molecular) layers, with a distinct laminar pattern. GABAA binding sites predominated in the fusiform cell layer. The molecular layer contained the highest level of GABAB binding sites in the entire cochlear nucleus. These results suggest that GABAergic inhibition in the cochlear nucleus is mediated both by GABAA and GABAB receptors, particularly in the dorsal cochlear nucleus. However, low levels of binding in areas such as the magnocellular regions of the ventral cochlear nucleus, known to contain abundant GABAergic synapses, suggest heterogeneity of GABA receptors in this auditory nucleus.

Varieties of inhibition in the processing and control of processing in the mammalian cochlear nucleus

Seven-barrel micropipettes were used to apply drugs microiontophoretically to single units in the dorsal cochlear nucleus (DCN) in chloralose-anaesthetised guinea-pigs. While both agonists and antagonists of putative neurotransmitters in the cochlear nucleus have been investigated in these experiments, the main thrust has been to explore the influence of specific antagonists on cells' spectral and temporal properties, thus elucidating the effects of naturally occurring inhibitory transmitters. At least five types of inhibition appear to be pharmacologically/physiologically separable: (1) Stimulus-evoked tonic "lateral/sideband" inhibition: glycinergic; (blocked by strychnine); responsible for the lateral inhibition of dorsal cochlear nucleus (DCN) type III and IV cells. Strychnine has its predominant effect on sustained (lateral) inhibition compared with the more transient forms of inhibition. Subtraction of receptive field maps enables us to visualise the extent of the inhibitory receptive field. It extends virtually throughout the unit's response field for both these classes but is generally, especially in type IV cells, maximal at the characteristic frequency (CF). This type of inhibition will primarily be responsible for enhancing spectral contrasts in the way that, in the visual system, surround inhibition enhances visual contrast. Furthermore, lateral inhibitory sidebands can "bias" the "working point" of a cell's response so that the dynamic range of effective stimuli and response can be extended. (2) "Background" tonic inhibition: GABAAergic; (blocked by bicuculline). Blocking this inhibition generally results in an increase in the background (i.e., spontaneous) activity. This inhibition is probably responsible for adjusting excitatory-inhibitory contrasts in both spectral and temporal domains. (3) Stimulus-related off-inhibition appears to be neither glycinergic nor GABAAergic. Blocking these receptors actually enhances off-inhibition. Nicotinic cholinergic blockers may have a small effect on off-inhibition, but so far we have not been able to block it entirely. This off-inhibition is important for enhancing temporal contrast. This inhibition must, therefore, be mediated by other transmitters, yet undetermined, or by a local feedback circuit or, less likely, be a membrane-based after-effect of stimulation. (4) Pre-synaptic inhibition, mediated by GABAB receptors presumed to act on primary afferent terminals, thus controlling afferent input to DCN principal cells. (5) Short-latency contralateral inhibition, mediated by glycine.

Cyclic steroid replacement alters auditory brainstem responses in young women with premature ovarian failure

To determine the independent contributions of estradiol and progesterone to the auditory brainstem response (ABR) latency changes associated with the menstrual cycle, we obtained ABRs on young women with premature ovarian failure who were undergoing cyclic hormone replacement therapy (HRT). We evaluated the influence of cyclic HRT on the ABRs of young women in three controlled phases of the same replacement cycles: 1) no steroid replacement, 2) estrogen-only replacement (E2-only), and 3) estrogen-plus-progesterone replacement (E2-plus-P). A significantly lengthening of wave V peak latency and I-V interpeak interval was found during E2-only replacement. Despite equivalent circulating estradiol levels, both wave V peak latencies and wave I-V interpeak intervals significantly decreased during the E2-plus-P replacement phase as compared to the E2-only replacement phase. These findings are compatible with the hypothesis that estradiol potentiates secretion of the inhibitory neurotransmitter gamma-amino-butyric acid (GABA) at auditory nerve synapses, leading to delayed synaptic conduction time. Progesterone is known to blunt E2-potentiated GABA release and may antagonize its prolongation of wave V latency.

GABAA receptors in auditory brainstem nuclei of the chick during development and after cochlea removal

The presence of GABAA receptors (GABARs) in auditory brainstem nuclei of the chick was determined by immunocytochemical (ICC) and receptor autoradiographic techniques. A monoclonal antibody to the GABAR/benzodiazepine/chloride channel complex and radiolabeled ligand binding using [3H]-muscimol, a GABA agonist, revealed labeling in nucleus magnocellularis (NM), nucleus laminaris (NL), nucleus angularis (NA), and the superior olive (SO) in both posthatch and embryonic chicks. GABAR-immunoreactivity (GABAR-I), as well as [3H]-muscimol binding, appear homogeneous throughout these nuclei at all ages studied. During development, GABAR-I is first observed in these nuclei around embryonic day 13 (E13). GABAR-I, which appears heavier in embryos than in posthatch chicks, becomes less intense with age in all 4 nuclei. Levels of receptor binding are also greater in embryos compared to posthatch chicks. [3H]-Muscimol binding is consistently greatest in SO followed by that in NL. NM and NA exhibit the least amount of binding at all ages studied. [3H]-Muscimol binding decreases in auditory brainstem nuclei as a function of age. Two days after unilateral cochlea removal, there is an apparent increase in GABAR-I in the ipsilateral NM compared to controls. This, however, may be the result of a decrease in the cross-sectional area of NM neurons as a result of de-afferentation (Born and Rubel, 1985). In contrast, there is a 28% decrease in [3H]-muscimol binding in the ipsilateral NM compared to controls probably reflecting the 30% reduction in the number of NM neurons due to cochlea removal (Born and Rubel, 1985). Fourteen days after cochlea removal, there is still a small, but not significant, decrease in [3H]-muscimol binding in the ipsilateral NM. In the contralateral NM, GABAR-I is less intense compared to that in the ipsilateral NM and controls. Additionally, there is a slight but insignificant decrease in [3H]-muscimol binding compared to that in controls 2 days after cochlea removal. After 14 days survival, however, the average binding is similar to that in controls. Thus, cochlea removal appears to transiently decrease the number of GABARs in the ipsilateral NM and may have a similar, but not as dramatic, effect in the contralateral NM. These GABARs are most likely to be postsynaptic, that is, located on NM neurons.

GABAergic inhibition upon auditory response properties of neurons in the dorsal cochlear nucleus of the rat

It is well known that the superficial layers of the dorsal cochlear nucleus (DCN) are rich in GABAergic neurons. We investigated the effects of topical application of GABA receptor agonists and/or antagonists upon the auditory response properties of DCN neurons in rats anesthetized with alpha chloralose-urethane. Auditory stimuli consisted of 20 ms tone bursts presented in a free field. Response properties of DCN neurons were studied before and during iontophoretic application of GABA, bicuculline methiodide (BIC) and muscimol (MUS) alone and GABA with MUS or BIC through triple barrel electrodes glued to the recording microelectrode. Of 68 DCN neurons studied, 27 were sensitive to topical application of the GABA agonists or antagonist. In these neurons, BIC enhanced spontaneous activity as well as auditory responses and decreased the Q-30 quality factor values. MUS reduced auditory responses. BIC often increased the width of the turning curve but GABA and/or MUS reduced it. Without drug application, GABA sensitive neurons tended to have longer response latencies and larger tuning widths at 30 dB above threshold as well as larger Q-30 values as compared with neurons that were insensitive to GABA. These findings suggest that: 1) GABAergic neurons determine the width of the tuning curve in neurons with GABA receptors by curtailing the excitatory response area, and 2) such neurons receive tonic inhibition from intrinsic GABAergic neurons.

The form and distribution of GABAergic synapses on the principal cell types of the ventral cochlear nucleus of the cat

The distribution of GABAergic endings was examined histochemically in the ventral cochlear nucleus (VCN) of the cat using an antibody to glutamate decarboxylase (GAD), the synthetic enzyme for GABA. Immunoreactive (GAD+) endings appeared in all subdivisions of the cat VCN. Each of the principal cell types had a characteristic labeling pattern, based on the size, concentration, and distribution of GAD+ endings on its soma. Spherical bushy cell somata were typically contacted by many small (less than 1.5 microns in diameter) and medium-sized (1.5-2 microns in diameter) endings, many of which aggregated into tight clusters. Globular bushy cells had a similar pattern, but the clusters of GAD+ endings were less tightly packed. Reactive endings on stellate cells were more evenly distributed. GAD+ endings on octopus cells were larger (up to 2.5 microns in diameter) than those on the bushy cells and tended to aggregate into small clusters or rows on the somata and dendrites. Reactive endings contained small pleomorphic vesicles and formed symmetrical synaptic contacts on each of the cell types examined. The patterns formed by GAD+ endings on each type of neuron resemble those of certain types of non-cochlear axons previously described with the Golgi methods as projecting from the dorsal cochlear nucleus and the trapezoid body.

Neuroanatomical distribution of receptors for three potential inhibitory neurotransmitters in the brainstem auditory nuclei of the cat

In order to visualize the relative abundance of each of three potentially inhibitory neurotransmitters in the nuclei of the brainstem auditory pathway, receptor sites for glycine, GABA-A, and muscarinic acetylcholine (ACh) have been localized in the cat's brainstem auditory system. Conventional autoradiographic receptor-binding procedures were used and the distributions of the receptors were inferred from the respective distributions of tritiated strychnine, muscimol, and quinuclidinyl benzilate (QNB) binding sites. The results show that glycine may be the major inhibitory neurotransmitter in the auditory system as it ascends to the midbrain in that relatively high levels of strychnine binding are present in every major nucleus of the system. In contrast, high levels of muscimol binding of high-affinity GABA-A receptors are confined mostly to the dorsal cochlear nucleus, the dorsal nucleus of the lateral lemniscus, and the central and cortical regions of the inferior colliculus, while high levels of QNB binding of muscarinic ACh receptors are seen only in the central and cortical regions of the inferior colliculus.

Strychnine alters the fusiform cell output from the dorsal cochlear nucleus

Anatomical and physiological evidence suggests that fusiform cells, the major output neurons of the dorsal cochlear nucleus (DCN), receive significant inhibitory input. Fusiform cells often display strongly non-monotonic rate-intensity functions and pauser-buildup or buildup tone-evoked temporal responses, patterns which may be mediated by inhibitory neurotransmitters. Other neurons located within the fusiform cell layer or in the more superficial molecular layer display varied rate-intensity functions and temporal responses. Neurons displaying response properties characteristic of fusiform cells are sensitive to iontophoretic application of the inhibitory amino acid neurotransmitter, glycine. Application of the glycine receptor antagonist, strychnine, alters the non-monotonic portion of the rate-intensity function at doses which do not alter spontaneous activity or near-threshold tone-evoked responses. These neurons are also sensitive to GABA and the GABAB agonist, (-)-baclofen, but are insensitive to the GABAA antagonist, bicuculline. DCN neurons which display monotonic rate-intensity functions and temporal response properties different than those associated with fusiform cells are sensitive to bicuculline, (-)-baclofen, and GABA. These data suggest that a glycinergic input onto fusiform cells may control the non-monotonic nature of the response of these neurons near characteristic frequency and therefore may contribute significantly to the nature of the output of the DCN.

Excitatory amino acid pharmacology of the auditory nerve and nucleus magnocellularis of the chicken

Experiments were performed on the nucleus magnocellularis and auditory nerve in tissue slices of 19-20-day-old chick embryos. Bath-applied kainate, quisqualate and N-methyl-D-aspartate induced dose-dependent alterations in the antidromic responses of nucleus magnocellularis neurons. The sensitivity of these agonist-induced responses to 2,3-cis-piperidine dicarboxylate, glutamate diethylester and D-alpha-aminoadipate were tested, as was the sensitivity of auditory nerve transmission. The data suggest that receptors for all three agonists are present on nucleus magnocellularis neurons and that the postsynaptic receptor of the nucleus magnocellularis-auditory nerve synapse is of the kainate type. The effects of bath-applied baclofen were also studied. Baclofen blocked orthodromic responses suggesting that an excitatory amino acid is released from the presynaptic terminal.