Activation of protein kinase C suppresses the gamma-aminobutyric acidB receptor-mediated inhibition of the vesicular release of noradrenaline and acetylcholine

GABAB receptor attenuation of GABAA currents in neurons of the mammalian central nervous system

Ionotropic receptors are tightly regulated by second messenger systems and are often present along with their metabotropic counterparts on a neuron's plasma membrane. This leads to the hypothesis that the two receptor subtypes can interact, and indeed this has been observed in excitatory glutamate and inhibitory GABA receptors. In both systems the metabotropic pathway augments the ionotropic receptor response. However, we have found that the metabotropic GABA_B receptor can suppress the ionotropic GABA_A receptor current, in both the in vitro mouse retina and in human amygdala membrane fractions. Expression of amygdala membrane microdomains in Xenopus oocytes by microtransplantation produced functional ionotropic and metabotropic GABA receptors. Most GABA_A receptors had properties of α‐subunit containing receptors, with ~5% having ρ‐subunit properties. Only GABA_A receptors with α‐subunit‐like properties were regulated by GABA_B receptors. In mouse retinal ganglion cells, where only α‐subunit‐containing GABA_A receptors are expressed, GABA_B receptors suppressed GABA_A receptor currents. This suppression was blocked by GABA_B receptor antagonists, G‐protein inhibitors, and GABA_B receptor antibodies. Based on the kinetic differences between metabotropic and ionotropic receptors, their interaction would suppress repeated, rapid GABAergic inhibition.

GABAergic alterations in neocortex of patients with pharmacoresistant temporal lobe epilepsy can explain the comorbidity of anxiety and depression: the potential impact of clinical factors

Temporal lobe epilepsy (TLE) is a chronic neurodegenerative disease with a high prevalence of psychiatric disorders. Temporal neocortex contributes to either seizure propagation or generation in TLE, a situation that has been associated with alterations of the γ-amino-butyric acid (GABA) system. On the other hand, an impaired neurotransmission mediated by GABA in temporal neocortex has also been involved with the pathophysiology of psychiatric disorders. In spite of these situations, the role of the necortical GABA system in the comorbidity of TLE and mood disorders has not been investigated. The present study was designed to identify alterations in the GABA system such as binding to GABA_A and GABA_B receptors and benzodiazepine site, the tissue content of GABA and the expression of the mRNA encoding the α1–6, β1–3, and γ GABA_A subunits, in the temporal neocortex of surgically treated patients with TLE with and without anxiety, and/or depression. Neocortex of patients with TLE and comorbid anxiety and/or depression showed increased expression of the mRNA encoding the γ2-subunit, reduced GABA_B-induced G-protein activation in spite of elevated GABA_B binding, and lower tissue content of GABA when compared to autopsy controls. Some of these changes significantly correlated with seizure frequency and duration of epilepsy. The results obtained suggest a dysfunction of the GABAergic neurotransmission in temporal neocortex of patients with TLE and comorbid anxiety and/or depression that could be also influenced by clinical factors such as seizure frequency and duration of illness.

GABAergic alterations in neocortex of patients with pharmacoresistant temporal lobe epilepsy can explain the comorbidity of anxiety and depression: the potential impact of clinical factors

Temporal lobe epilepsy (TLE) is a chronic neurodegenerative disease with a high prevalence of psychiatric disorders. Temporal neocortex contributes to either seizure propagation or generation in TLE, a situation that has been associated with alterations of the γ-amino-butyric acid (GABA) system. On the other hand, an impaired neurotransmission mediated by GABA in temporal neocortex has also been involved with the pathophysiology of psychiatric disorders. In spite of these situations, the role of the necortical GABA system in the comorbidity of TLE and mood disorders has not been investigated. The present study was designed to identify alterations in the GABA system such as binding to GABAA and GABAB receptors and benzodiazepine site, the tissue content of GABA and the expression of the mRNA encoding the α1-6, β1-3, and γ GABAA subunits, in the temporal neocortex of surgically treated patients with TLE with and without anxiety, and/or depression. Neocortex of patients with TLE and comorbid anxiety and/or depression showed increased expression of the mRNA encoding the γ2-subunit, reduced GABAB-induced G-protein activation in spite of elevated GABAB binding, and lower tissue content of GABA when compared to autopsy controls. Some of these changes significantly correlated with seizure frequency and duration of epilepsy. The results obtained suggest a dysfunction of the GABAergic neurotransmission in temporal neocortex of patients with TLE and comorbid anxiety and/or depression that could be also influenced by clinical factors such as seizure frequency and duration of illness.

Therapeutic potential of GABA(B) receptor ligands in drug addiction, anxiety, depression and other CNS disorders

Glutamate and γ-aminobutyric acid (GABA) are the major excitatory and inhibitory neurotransmitter systems, respectively in the central nervous system (CNS). Dysregulation, in any of these or both, has been implicated in various CNS disorders. GABA acts via ionotropic (GABA(A) and GABA(C) receptor) and metabotropic (GABA(B)) receptor. Dysregulation of GABAergic signaling and alteration in GABA(B) receptor expression has been implicated in various CNS disorders. Clinically, baclofen-a GABA(B) receptor agonist is available for the treatment of spasticity, dystonia etc., associated with various neurological disorders. Moreover, GABAB receptor ligands has also been suggested to be beneficial in various neuropsychiatric and neurodegenerative disorders. The present review is aimed to discuss the role of GABA(B) receptors and the possible outcomes of GABA(B) receptor modulation in CNS disorders.

After damage of large bile ducts by gamma-aminobutyric acid, small ducts replenish the biliary tree by amplification of calcium-dependent signaling and de novo acquisition of large cholangiocyte phenotypes

Large cholangiocytes secrete bicarbonate in response to secretin and proliferate after bile duct ligation by activation of cyclic adenosine 3', 5'-monophosphate signaling. The Ca(2+)-dependent adenylyl cyclase 8 (AC8, expressed by large cholangiocytes) regulates secretin-induced choleresis. Ca(2+)-dependent protein kinase C (PKC) regulates small cholangiocyte function. Because gamma-aminobutyric acid (GABA) affects cell functions by activation of both Ca(2+) signaling and inhibition of AC, we sought to develop an in vivo model characterized by large cholangiocyte damage and proliferation of small ducts. Bile duct ligation rats were treated with GABA for one week, and we evaluated: GABA(A), GABA(B), and GABA(C) receptor expression; intrahepatic bile duct mass (IBDM) and the percentage of apoptotic cholangiocytes; secretin-stimulated choleresis; and extracellular signal-regulated kinase1/2 (ERK1/2) phosphorylation and activation of Ca(2+-)dependent PKC isoforms and AC8 expression. We found that both small and large cholangiocytes expressed GABA receptors. GABA: (i) induced apoptosis of large cholangiocytes and reduced large IBDM; (ii) decreased secretin-stimulated choleresis; and (iii) reduced ERK1/2 phosphorylation and AC8 expression in large cholangiocytes. Small cholangiocytes: (i) proliferated leading to increased IBDM; (ii) displayed activation of PKCbetaII; and (iii) de novo expressed secretin receptor, cystic fibrosis transmembrane regulator, Cl(-)/HCO(3)(-) anion exchanger 2 and AC8, and responded to secretin. Therefore, in pathologies of large ducts, small ducts replenish the biliary epithelium by amplification of Ca(2+)-dependent signaling and acquisition of large cholangiocyte phenotypes.

Functional modulation of GABAB receptors by protein kinases and receptor trafficking

GABA(B) receptors (GABA(B)R) are heterodimeric G protein-coupled receptors (GPCRs) that mediate slow and prolonged inhibitory signals in the central nervous system. The signaling of GPCRs is under stringent control and is subject to regulation by multiple posttranslational mechanisms. The beta-adrenergic receptor is a prototypic GPCR. Like most GPCRs, prolonged exposure of this receptor to agonist induces phosphorylation of multiple intracellular residues that is largely dependent upon the activity of G protein-coupled receptor kinases (GRKs). Phosphorylation terminates receptor-effector coupling and promotes both interaction with beta-arrestins and removal from the plasma membrane via clathrin-dependent endocytosis. Emerging evidence for GABA(B)Rs suggests that these GPCRs do not conform to this mode of regulation. Studies using both native and recombinant receptor preparations have demonstrated that GABA(B)Rs do not undergo agonist-induced internalization and are not GRK substrates. Moreover, whilst GABA(B)Rs undergo clathrin-dependent constitutive endocytosis, it is generally accepted that their rates of internalization are not modified by prolonged agonist exposure. Biochemical studies have revealed that GABA(B)Rs are phosphorylated on multiple residues within the cytoplasmic domains of both the R1 and R2 subunits by cAMP-dependent protein kinase and 5'AMP-dependent protein kinase (AMPK). Here we discuss the role that this phosphorylation plays in determining GABA(B)R effector coupling and their trafficking within the endocytic pathway and go on to evaluate the significance of GABA(B)R phosphorylation in controlling neuronal excitability under normal and pathological conditions.

Modulation of hypothalamic neuronal activity through a novel G-protein-coupled estrogen membrane receptor

Estrogens are involved in the hypothalamic control of multiple homeostatic functions including reproduction, stress responses, energy metabolism, sleep cycles, temperature regulation and motivated behaviors. The actions of 17beta-estradiol (E(2)) in the brain have been attributed to the activation of estrogen receptors alpha and beta, as well as G-protein-coupled or other membrane-associated estrogen receptors. Recently, we have identified a putative membrane-associated estrogen receptor that is coupled to desensitization of GABA(B) receptors in guinea pig and mouse hypothalamic neurons including proopiomelanocortin (POMC) neurons. We have synthesized a new nonsteroidal compound, STX, which selectively targets the Galphaq-coupled phospholipase C-protein kinase C-protein kinase A pathway, and have established that STX is more potent than E(2) in mediating this desensitization in an ICI 182,780-sensitive manner in both guinea pig and mouse neurons. Both E(2) and STX are fully efficacious in estrogen receptor alpha, beta knock-out mice. Finally, we observed that the putative membrane-associated estrogen receptor is different from GPR30 in arcuate neurons using whole-cell patch recording in hypothalamic slices from GPR30 knock-out mice. Collectively, these findings suggest that the mER is distinct from ERalpha, ERbeta or GPR30.

Molecular structure and physiological functions of GABA(B) receptors

GABA(B) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. The cloning of the first GABA(B) receptor cDNAs in 1997 revived interest in these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was made in our understanding of the GABA(B) system. This led to the surprising discovery that GABA(B) receptors need to assemble from distinct subunits to function and provided exciting new insights into the structure of G protein-coupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can exist as heterodimers. The cloning of GABA(B) receptors allowed some important questions in the field to be answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABA(B) receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate, an emerging drug of abuse. There are now the first indications linking GABA(B) receptor polymorphisms to epilepsy. Significantly, the cloning of GABA(B) receptors enabled identification of the first allosteric GABA(B) receptor compounds, which is expected to broaden the spectrum of therapeutic applications. Here we review current concepts on the molecular composition and function of GABA(B) receptors and discuss ongoing drug-discovery efforts.

Rapid signaling of estrogen in hypothalamic neurons involves a novel G-protein-coupled estrogen receptor that activates protein kinase C

Classically, 17beta-estradiol (E2) is thought to control homeostatic functions such as reproduction, stress responses, feeding, sleep cycles, temperature regulation, and motivated behaviors through transcriptional events. Although it is increasingly evident that E2 can also rapidly activate kinase pathways to have multiple downstream actions in CNS neurons, the receptor(s) and the signal transduction pathways involved have not been identified. We discovered that E2 can alter mu-opioid and GABA neurotransmission rapidly through nontranscriptional events in hypothalamic GABA, proopiomelanocortin (POMC), and dopamine neurons. Therefore, we examined the effects of E2 in these neurons using whole-cell recording techniques in ovariectomized female guinea pigs. E2 reduced rapidly the potency of the GABAB receptor agonist baclofen to activate G-protein-coupled, inwardly rectifying K+ channels in hypothalamic neurons. These effects were mimicked by the membrane impermeant E2-BSA and selective estrogen receptor modulators, including a new diphenylacrylamide compound, STX, that does not bind to intracellular estrogen receptors alpha or beta, suggesting that E2 acts through a unique membrane receptor. We characterized the coupling of this estrogen receptor to a Galpha(q)-mediated activation of phospholipase C, leading to the upregulation of protein kinase Cdelta and protein kinase A activity in these neurons. Moreover, using single-cell reverse transcription-PCR, we identified the critical transcripts, PKCdelta and its downstream target adenylyl cyclase VII, for rapid, novel signaling of E2 in GABA, POMC, and dopamine neurons. Therefore, this unique Gq-coupled estrogen receptor may be involved in rapid signaling in hypothalamic neurons that are critical for normal homeostatic functions.

GABAB receptor transduction mechanisms, and cross-talk between protein kinases A and C, in GABAergic terminals synapsing onto neurons of the rat nucleus basalis of Meynert

The transduction mechanisms underlying presynaptic GABAB receptor-mediated inhibition of transmitter release have been characterized for a variety of synapses in the central nervous system (CNS). These studies have suggested a range of transduction mechanisms, including a role for second messengers such as protein kinases A (PKA) and C (PKC). In the present study, we have examined the intracellular signalling pathways underlying baclofen-induced inhibition of GABA release from terminals synapsing onto rat basalis of Meynert neurons using patch-clamp recordings. Baclofen, a selective GABAB receptor agonist, reversibly decreased both evoked and spontaneous, miniature, GABAergic inhibitory postsynaptic currents (eIPSCs and mIPSCs, respectively). Such baclofen actions were completely abolished by CGP55845A, a selective GABAB receptor antagonist, and by staurosporine, a non-selective PKA and PKC inhibitor. The mIPSC frequency was still decreased by baclofen even in the presence of 4 AP, a K+ channel blocker, and Cd2+, a voltage-dependent calcium channel blocker. Pharmacological activation or inhibition of PKC activity affected basal GABA release and mildly affected the response to baclofen. Inhibition of the cAMP/PKA cascade also affected basal GABA release and, in a subset of neurons, occluded the effects of baclofen, suggesting that the GABAB receptor-mediated inhibitory action on GABA release was mediated via decreases in PKA activity. In addition, PKA inhibition occluded the effects of PKC modulation on both basal GABA release and on the response to baclofen. Our results characterize the transduction pathway of baclofen at these nucleus basalis of Maynert (nBM) synapses and show, for the first time, some cross-talk between the cAMP/PKA and PKC pathways in mammalian presynaptic nerve terminals.

GABA transmission in the nucleus accumbens is altered after withdrawal from repeated cocaine

Repeated cocaine causes enduring changes in dopamine and glutamate transmission in the nucleus accumbens, and dopamine and glutamate terminals synapse on GABAergic accumbens neurons. The present study demonstrates that there are changes in GABA transmission in the accumbens at 3 weeks after discontinuing daily cocaine injections. No-net flux microdialysis revealed a significant increase in the basal levels of extracellular GABA in the accumbens of cocaine-treated rats. The elevated extracellular GABA was normalized by blocking voltage-dependent Na+ channels and provided increased tone on GABA(B) presynaptic autoreceptors and heteroreceptors because blocking GABA(B) receptors produced a greater elevation in extracellular GABA, dopamine, and glutamate in cocaine-treated compared with control subjects. For many G-protein-coupled receptors, increased agonist can cause receptor desensitization. Consistent with GABA(B) receptor desensitization, baclofen-stimulated GTPgammaS binding was reduced, and the reduction in G-protein coupling was accompanied by reduced Ser phosphorylation of the GABA(B2) receptor subunit. No effect by repeated cocaine was found in the levels of total GABA(B1) or GABA(B2) protein. Together, these data demonstrate that withdrawal from repeated cocaine treatment produces an increase in the basal levels of extracellular GABA in the accumbens that depends on neuronal activity. The increase may be mediated in part by functional desensitization of GABA(B) receptors, likely the result of diminished Ser phosphorylation of the GABA(B2) receptor.

Cyclic AMP-dependent protein kinase phosphorylation facilitates GABA(B) receptor-effector coupling

GABA (gamma-aminobutyric acid)(B) receptors are heterodimeric G protein-coupled receptors that mediate slow synaptic inhibition in the central nervous system. Here we show that the functional coupling of GABA(B)R1/GABA(B)R2 receptors to inwardly rectifying K(+) channels rapidly desensitizes. This effect is alleviated after direct phosphorylation of a single serine residue (Ser892) in the cytoplasmic tail of GABA(B)R2 by cyclic AMP (cAMP)-dependent protein kinase (PKA). Basal phosphorylation of this residue is evident in rat brain membranes and in cultured neurons. Phosphorylation of Ser892 is modulated positively by pathways that elevate cAMP concentration, such as those involving forskolin and beta-adrenergic receptors. GABA(B) receptor agonists reduce receptor phosphorylation, which is consistent with PKA functioning in the control of GABA(B)-activated currents. Mechanistically, phosphorylation of Ser892 specifically enhances the membrane stability of GABA(B) receptors. We conclude that signaling pathways that activate PKA may have profound effects on GABA(B) receptor-mediated synaptic inhibition. These results also challenge the accepted view that phosphorylation is a universal negative modulator of G protein-coupled receptors.

Regulation of muscarinic acetylcholine receptor-mediated synaptic responses by GABA(B) receptors in the rat hippocampus

1. Both GABA(B) and muscarinic acetylcholine receptors (mAChRs) influence hippocampal-dependent mnemonic processing. Here the possibility of a direct interaction between GABA(B) receptors and mAChR-mediated synaptic responses has been studied using intracellular recording in rat hippocampal slices. 2. The GABA(B) receptor agonist (-)-baclofen (5-10 microM) depressed an atropine-sensitive slow EPSP (EPSP(M)) and occluded the GABA(B)-receptor-mediated IPSP (IPSP(B)) which preceded it. These inhibitory effects were accompanied by postsynaptic hyperpolarization (9 +/- 2 mV) and a reduction in cell input resistance (12 +/- 3 %). 3. The selective GABA(B) receptor antagonist CGP 55845A (1 microM) fully reversed the depressant effects of (-)-baclofen (5-10 microM) such that in the combined presence of (-)-baclofen and CGP 55845A the EPSP(M) was 134 +/- 21 % of control. 4. (-)-Baclofen (5-10 microM) caused a small (28 +/- 11 %) inhibition of carbachol-induced (3.0 microM) postsynaptic depolarizations and increases in input resistance. 5. CGP 55845A (1 microM) alone caused an increase in the amplitude of the EPSP(M) (253 +/- 74 % of control) and blocked the IPSP(B) that preceded it. 6. In contrast, the selective GABA uptake inhibitor NNC 05-0711 (10 microM) increased the amplitude of the IPSP(B) by 141 +/- 38 % and depressed the amplitude of the EPSP(M) by 58 +/- 10 %. This inhibition was abolished by CGP 55845A (1 microM). 7. Taken together these data provide good evidence that synaptically released GABA activates GABA(B) receptors that inhibit mAChR-mediated EPSPs in hippocampal CA1 pyramidal neurones. The mechanism of inhibition may involve both pre- and postsynaptic elements.

Modulation of GABA-augmented norepinephrine release in female rat brain slices by opioids and adenosine

GABAA receptor activation augments electrically-stimulated release of norepinephrine (NE) from rat brain slices. Because this effect is not observed in synaptoneurosomes, GABA probably acts on inhibitory interneurons to disinhibit NE release. To determine whether opioids or adenosine influence GABA-augmented NE release, hypothalamic and cortical slices from female rats were superfused with GABA or vehicle in the presence and absence of 10 microM morphine or 100 microM adenosine. GABA augments [3H]NE release in the cortex and hypothalamus. Morphine alone has no effect on [3H]NE release, but attenuates GABA augmentation of [3H]NE release in both brain regions. Adenosine alone modestly inhibits [3H]NE release in the cortex, but not in the hypothalamus. Adenosine inhibits GABA-augmented [3H]NE release in both brain regions. The general protein kinase inhibitor H-7, augments [3H]NE release in both brain regions and may have additive effects with GABA in cortical slices. These results implicate opioid and adenosine interneurons and possibly protein kinases in regulating GABAergic influences on NE transmission.

In vivo interaction of baclofen, TRH and serotonin on PRL and TSH secretion in the developing and adult male and female rats

Gamma-aminobutyric acid (GABA) is involved in the neural control of hypophyseal hormones, including PRL and TSH. In the present work we investigated the ontogeny of the effect of baclofen, a GABA B agonist, on basal PRL and TSH release and in the presence of releasing stimulus which act at two different levels: TRH, at the hypophyseal level, and serotonin, at the central nervous system. Ages studied were 4, 12, 20, 28-29, 37-38 day-old and adult male and female animals. Rats of each age and sex were separated in groups and each group received two intraperitoneally injections, one 45 minutes after the other: saline-saline, saline-TRH, baclofen-saline, baclofen-TRH, saline-serotonin or baclofen-serotonin. Rats were decapitated 15 minutes after the last injection and serum hormones were measured by RIA. Baclofen (7 mg/kg) significantly elevated basal prolactin levels at 4, 12 and 20 days of age and the stimulating effect increased with age. At 28 days of age baclofen significantly inhibited PRL whereas from 38 days of age onwards it had no effect on basal PRL levels. No sex differences were evident. Interaction of TRH (4 microg/kg) and baclofen on PRL secretion resulted in an additive effect on days 4 and 12, this effect was not observed when baclofen was administered with serotonin (10 mg/kg). In 28 day-old and older animals baclofen completely blunted the PRL releasing effect of TRH or serotonin. Again, no sex differences were observed. With regard to TSH, baclofen did not alter either basal or TRH stimulated TSH secretion regardless of sex and age. The present experiments indicate that GABA B receptors are involved in the regulation of basal and stimulated PRL secretion from the first days of life to adulthood. Receptor activation results in stimulation or inhibition of PRL release depending on the age of the animals and the site of action. This GABA B regulation of PRL secretion is sex independent. In contrast, pituitary GABA B receptors do not seem to be involved in the regulation of TSH secretion.

Autoprotective mechanisms in the CNS: some new lessons from white matter

Anoxia/ischemia in the CNS is a common and devastating phenomenon. It is possible that the best hopes for protection against anoxic/ischemic injury may involve recruiting and/or augmenting any autoprotective systems that evolution has provided for the CNS. We describe here the existence of such an autoprotective system present in CNS white matter. White matter is both well suited to studying extrasynaptic systems, such as the system we describe here, and is a highly appropriate target for research into anoxic-ischemic injury in its own right. We show that white matter contains functional GABAB and adenosine receptors that respond to an anoxic efflux of GABA and adenosine by recruiting a convergent intracellular mechanism involving protein kinase C (PKC). The net result of this receptor-mediated cascade is an increase in resistance to anoxia, which presumably allows CNS white matter to tolerate better a common class of ischemic events that are located solely in white matter and that comprises approximately 25% of all strokes seen clinically.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

Age-related decrease in GABAB receptor binding in the Fischer 344 rat inferior colliculus

Quantitative receptor autoradiography was used to assess GABAB receptor binding in three primary subdivisions of the inferior colliculus (IC): dorsal cortex (DCIC), external cortex (ECIC), and the central nucleus (CIC) of 3-, 18-20-, and 26-month-old Fischer 344 rats. GABAB binding sites were localized using [3H]GABA in the presence of a saturating concentration of isoguvacine, a selective GABAA receptor agonist, to displace [3H]GABA bound to GABAA receptor sites. In the three IC subdivisions examined, GABAB receptor binding was significantly reduced in 26-month-old rats when compared to 3-month-old rats (DCIC, -44%; ECIC, -36%; CIC, -32%; p < 0.05). For comparison, GABAB binding was determined in the portion of cerebellum located in the recess of the IC. In the molecular layer of this region, there was no statistically significant differences in receptor binding between 3, 18-20-, and 26-month-old rats. In addition, there was not a significant age-related change in the cross-sectional area of the IC. These findings provide additional evidence to support the existence of selective age-related changes in GABA neurotransmitter function in the rat IC.