α5GABAA Receptors Mediate Tonic Inhibition in the Spinal Cord Dorsal Horn and Contribute to the Resolution Of Hyperalgesia.

Neuronal inhibition mediated by GABAA receptors constrains nociceptive processing in the spinal cord, and loss of GABAergic inhibition can produce allodynia and hyperalgesia. Extrasynaptic α5 subunit-containing GABAA receptors (α5GABAA Rs) generate a tonic conductance that inhibits neuronal activity and constrains learning and memory; however, it is unclear whether α5GABAA Rs similarly generate a tonic conductance in the spinal cord dorsal horn to constrain nociception. We assessed the distribution of α5GABAA Rs in the spinal cord dorsal horn by immunohistochemical analysis, and the activity and function of α5GABAA Rs in neurons of the superficial dorsal horn using electrophysiological and behavioral approaches in male, null-mutant mice lacking the GABAA R α5 subunit (Gabra5-/-) and wild-type mice (WT). The expression of α5GABAA Rs in the superficial dorsal horn followed a laminar pattern of distribution, with a higher expression in lamina II than lamina I. Similarly, the tonic GABAA current in lamina II neurons had a larger contribution from α5GABAA Rs than in lamina I, with no significant contribution of these receptors to synaptic GABAA current. In behavioural tests, WT and Gabra5-/- mice exhibited similar acute thermal and mechanical nociception, and similar mechanical sensitization immediately following intraplantar capsaicin or Complete Freund's Adjuvant (CFA). However, Gabra5-/- mice showed prolonged recovery from sensitization in these models, and increased responses in the late phase of the formalin test. Overall, our data suggest that tonically-active α5GABAA Rs in the spinal cord dorsal horn accelerate the resolution of hyperalgesia and may therefore serve as a novel therapeutic target to promote recovery from pathological pain. © 2016 Wiley Periodicals, Inc.

Sustained increase in α5GABAA receptor function impairs memory after anesthesia.

Many patients who undergo general anesthesia and surgery experience cognitive dysfunction, particularly memory deficits that can persist for days to months. The mechanisms underlying this postoperative cognitive dysfunction in the adult brain remain poorly understood. Depression of brain function during anesthesia is attributed primarily to increased activity of γ-aminobutyric acid type A receptors (GABA(A)Rs), and it is assumed that once the anesthetic drug is eliminated, the activity of GABA(A)Rs rapidly returns to baseline and these receptors no longer impair memory. Here, using a murine model, we found that a single in vivo treatment with the injectable anesthetic etomidate increased a tonic inhibitory current generated by α5 subunit-containing GABA(A)Rs (α5GABA(A)Rs) and cell-surface expression of α5GABA(A)Rs for at least 1 week. The sustained increase in α5GABA(A)R activity impaired memory performance and synaptic plasticity in the hippocampus. Inhibition of α5GABA(A)Rs completely reversed the memory deficits after anesthesia. Similarly, the inhaled anesthetic isoflurane triggered a persistent increase in tonic current and cell-surface expression of α5GABA(A)Rs. Thus, α5GABA(A)R function does not return to baseline after the anesthetic is eliminated, suggesting a mechanism to account for persistent memory deficits after general anesthesia.

Inhibition of α5 γ-Aminobutyric acid type A receptors restores recognition memory after general anesthesia.

BACKGROUND: General anesthetics cause cognitive deficits that persist much longer than would be expected on the basis of their pharmacokinetics. The cellular mechanisms underlying these postanesthetic cognitive deficits remain unknown. γ-Aminobutyric acid type A (GABA(A)) receptors are principal targets for most anesthetics. In particular, the α5GABA(A) receptor subtype has been implicated in acute memory blockade during anesthesia and memory deficits in the early postoperative period. We first sought to determine whether working memory and short-term recognition memory are impaired after isoflurane anesthesia. The second aim of the study was to determine whether memory deficits after isoflurane can be reversed by inhibiting α5GABA(A) receptors. We also sought to determine whether the expression of α5GABA(A) receptors is necessary for the development of memory dysfunction after isoflurane. Lastly, the effect of sevoflurane on memory was studied. METHODS: Wild-type and α5GABA(A) receptor null-mutant (Gabra5-/-) mice were treated with isoflurane (1.3%; 1 minimum alveolar concentration [MAC]) or sevoflurane (2.3%; 1 MAC) or vehicle gas for 1 hour. Memory performance was assessed with a novel object recognition task. Mice were trained on the recognition task either 24 hours or 72 hours after isoflurane anesthesia. Working memory and short-term memory were tested 1 minute and 1 hour after training, respectively. To determine whether inhibition of α5GABA(A) receptors reverses memory deficits, we treated a subset of mice with L-655,708 (0.35 mg/kg or 0.7 mg/kg) 23.5 hours after isoflurane and 30 minutes before behavioral training. RESULTS: Short-term memory was impaired in wild-type mice 24 hours after isoflurane as evidenced by a decrease in the discrimination ratio (control 0.66 ± 0.03 vs isoflurane 0.51 ± 0.03, P = 0.0005). In contrast, working memory was not impaired by isoflurane (control 0.68 ± 0.05 vs isoflurane 0.67 ± 0.04, P = 0.979). The deficit in short-term memory was fully reversed by L-655,708 (effect of isoflurane × L-655,708, F(2,102) = 3.59, P = 0.032; isoflurane 0.51 ± 0.03 vs isoflurane + L-655,708 at 0.35 mg/kg 0.67 ± 0.03, P < 0.05). By 72 hours, the deficits in short-term memory resolved spontaneously (control 0.65 ± 0.05 vs isoflurane 0.60 ± 0.04, P = 0.441). Gabra5-/- mice showed no short-term memory deficits 24 hours after isoflurane (effect of isoflurane F(1,47) = 0.375, P = 0.544). Sevoflurane also caused memory deficits 24 hours after anesthesia, as evidence by a reduction in the discrimination ratio (control 0.63 ± 0.02 vs sevoflurane 0.53 ± 0.03, P = 0.039). CONCLUSIONS: Inhalational anesthetics cause deficits in anterograde recognition memory. This proof-of-concept study shows that α5GABA(A) receptors are necessary for the development of postanesthetic deficits in recognition memory and that these receptors can be targeted to restore memory even after the anesthetic has been eliminated.