The α5GABAA receptor modulates the induction of long-term potentiation at ventral but not dorsal CA1 hippocampal synapses.

The hippocampal synapses display conspicuous ability for long-term plasticity which is thought to underlie learning and memory. Growing evidence shows that this ability differs along the long axis of the hippocampus, with the ventral CA1 hippocampal synapses displaying remarkably lower ability for long-term potentiation (LTP) compared with their dorsal counterpart when activated with high-frequency stimulation. Here, we show that low frequency, 10 Hz stimulation induced LTP more reliably in dorsal than in ventral CA1 field. Blockade of alpha5 subunit-containing GABAA receptors eliminated the difference between dorsal and ventral hippocampus. We propose that α5GABAA receptor-mediated activity plays a crucial role in regulating the threshold for induction of LTP especially at the ventral CA1 hippocampal synapses. This might have important implications for the functional specialization along the hippocampus.

Changes in social, sexual, and hedonic behaviors in rats in response to stress and restoration by a negative allosteric modulator of α5-subunit containing GABA receptor.

Major depressive disorder (MDD) is a debilitating and costly human condition. Treatment for MDD relies heavily on the use of antidepressants that are slow to produce mood-related changes and are not effective in all patients, such as selective serotonin reuptake inhibitors (SSRIs). Several novel compounds, including negative allosteric modulators of GABA-A receptors containing the α5-subunit (GABA-NAMs), are under investigation for potential fast acting therapeutic use in MDD. Preclinical evidence that these compounds produce a rapid antidepressant-like response comes primarily from simple tests of escape behavior and preference for rewarding stimuli after chronic stress. To increase the ethological relevance of these compounds, we tested the hypothesis that the GABA-NAM, L-655,708, would produce an antidepressant-like response in more complex stress-sensitive social and sex behaviors, which are of relevance to the symptoms of human depression. In male rats subjected to chronic restraint stress, injection of L-655,708 increased reward in a sexual conditioned place preference task, increased male sexual activity with a receptive female, and re-established male social dominance hierarchies within 24 h. We also report increased sucrose preference in the social defeat stress (SDS) model of depression following GABA-NAM administration, demonstrating that its antidepressant-like actions are independent of the type of chronic stress administered. This work extends the impact of GABA-NAMs beyond traditional tests of anhedonia and further supports the development of alpha5 subunit-selective GABA-NAMs as a potential fast-acting therapeutic approach for treating human MDD.

Increased expression of GABAA receptor subunits associated with tonic inhibition in patients with temporal lobe epilepsy.

Epilepsy animal models indicate pronounced changes in the expression and rearrangement of GABAA receptor subunits in the hippocampus and in para-hippocampal areas, including widespread downregulation of the subunits α5 and δ, and upregulation of α4, subunits that mediate tonic inhibition of GABA. In this case-control study, we investigated changes in the expression of subunits α4, α5 and δ in hippocampal specimens of drug resistant temporal lobe epilepsy patients who underwent epilepsy surgery. Using in situ hybridization, immunohistochemistry and α5-specific receptor autoradiography, we characterized expression of the receptor subunits in specimens from patients with and without Ammon's horn sclerosis compared to post-mortem controls. Expression of the α5-subunit was abundant throughout all subfields of the hippocampus, including the dentate gyrus, sectors CA1 and CA3, the subiculum and pre- and parasubiculum. Significant but weaker expression was detected for subunits α4 and δ notably in the granule cell/molecular layer of control specimens, but was faint in the other parts of the hippocampus. Expression of all three subunits was similarly altered in sclerotic and non-sclerotic specimens. Respective mRNA levels were increased by about 50-80% in the granule cell layer compared with post-mortem controls. Subunit α5 mRNA levels and immunoreactivities were also increased in the sector CA3 and in the subiculum. Autoradiography for α5-containing receptors using [3H]L-655,708 as ligand showed significantly increased binding in the molecular layer of the dentate gyrus in non-sclerotic specimens. Increased expression of the α5 and δ subunits is in contrast to the previously observed downregulation of these subunits in different epilepsy models, whereas increased expression of α4 in temporal lobe epilepsy patients is consistent with that in the rodent models. Our findings indicate increased tonic inhibition likely representing an endogenous anticonvulsive mechanism in temporal lobe epilepsy.

Contribution of GABAA receptor subunits to attention and social behavior.

INTRODUCTION: GABA dysfunction is associated with a number of psychiatric conditions including schizophrenia, autism and depression. Blocking cortical GABAA receptors in rodents causes behavioral deficits, including impaired attention and sociability, that are consistent with the symptoms of these conditions. The subunit composition of GABAA receptors is diverse and can affect receptor function. The current experiment examined the role of GABAA receptors containing different α-subunits in social behavior and attention. METHODS: Male Sprague-Dawley rats were administered FG7142 (0.0-5.0 mg/kg; a non-selective GABAA receptor inverse agonist), L-655,708 (0-1.0 mg/kg; a low efficacy inverse agonist at α5-containing GABAA receptors), MRK-016 (0.0-2.0 mg/kg; a high efficacy inverse agonist at α5-containing GABAA receptors), or L-838,417 (0.0-3.0 mg/kg; an antagonist at α1-containing receptors and a partial agonist at α2, α3, α5-containing GABAA receptors) and either tested on the social interaction and social preference tests or the 5-choice serial reaction time task. RESULTS: FG7142 decreased social interactions and impaired attention. MRK-016 impaired attention but did not affect social behavior. Neither L-655,708 nor L-838,417 significantly affected either social behavior or attention. DISCUSSION: Systemic reduction in GABAA receptor signaling decreased sociability and attention, a result consistent with past research demonstrating cortical GABAA receptor blockade impairs social behavior and attention. Overall, the effects of the receptor subtype selective ligands were minimal; α5-containing GABAA receptors may contribute to the attentional deficit but do not contribute to the decrease in sociability. Further research is needed to determine the GABAA receptor subunits that contribute to social behavior and attention.

L-655,708 Does not Prevent Isoflurane-induced Memory Deficits in Old Mice.

BACKGROUND: General anesthesia and increasing age are two main risk factors for postoperative cognitive dysfunction (POCD). Effective agents for the prevention or treatment of POCD are urgently needed. L-655,708, an inverse agonist of α5 subunit-containing γ-aminobutyric acid subtype A (α5GABAA) receptors, can prevent anesthesia-induced memory deficits in young animals. However, there is a lack of evidence of its efficacy in old animals. METHODOLOGY: Young (3- to 5-month-old) and old (18- to 20-month-old) mice were given an inhalation of 1.33% isoflurane for 1 hour and their associative memory was evaluated 24 hours after anesthesia using fear-conditioning tests (FCTs). To evaluate the effect of L-655,708, mice received intraperitoneal injections of L-655,708 (0.7 mg/kg) or vehicle 30 minutes before anesthesia. RESULTS: Old mice exhibited impaired memory and lower hippocampal α5GABAA levels than young mice under physiological conditions. Pre-injections of L-655,708 significantly alleviated isoflurane-induced memory decline in young mice, but not in old mice. CONCLUSIONS: L-655,708 is not as effective for the prevention of POCD in old mice as it is in young mice. The use of inverse agonists of α5GABAA in preventing POCD in old patients should be carefully considered.

Evidence for the participation of peripheral α5 subunit-containing GABAA receptors in GABAA agonists-induced nociception in rats.

The activation of GABAA receptor by γ-amino butyric acid (GABA) in primary afferent fibers produces depolarization. In normal conditions this depolarization causes a reduction in the release of neurotransmitters. Therefore, this depolarization remains inhibitory. However, previous studies have suggested that in inflammatory pain, GABA shifts its signaling from inhibition to excitation by an increased GABA-induced depolarization. The contribution of peripheral α5 subunit-containing GABAA receptors to the inflammatory pain is unknown. The purpose of this study was to investigate the possible pronociceptive role of peripheral α5 subunit-containing GABAA receptors in the formalin test. Formalin (0.5%) injection into the dorsum of the right hind paw produced flinching behavior in rats. Ipsilateral local peripheral pre-treatment (-10min) with exogenous GABA (0.003-0.03µg/paw) or common GABAA receptor agonists muscimol (0.003-0.03µg/paw), diazepam (0.017-0.056µg/paw) or phenobarbital (1-100µg/paw) significantly increased 0.5% formalin-induced nociceptive behavior. The pronociceptive effects of GABA (0.03µg/paw), muscimol (0.03µg/paw), diazepam (0.056µg/paw) and phenobarbital (100µg/paw) were prevented by either the GABAA receptor antagonist bicuculline (0.01-0.1µg/paw) or selective α5 subunit-containing GABAA receptor inverse agonist L-655,708 (0.017-0.17µg/paw). The α5 subunit-containing GABAA receptor protein was expressed in dorsal root ganglion (DRG) and dorsal spinal cord of naïve rats. The formalin injection did not modify α5 subunit-containing GABAA receptor expression. Overall, these results suggest that peripheral α5 subunit-containing GABAA receptors play a pronociceptive role in the rat formalin test.