GABAA and glycine receptor-mediated transmission in rat lamina II neurones: relevance to the analgesic actions of neuroactive steroids

Analgesic neurosteroids such as 5a-pregnan-3a-ol-20-one (5a3a) are potent selective endogenous modulators of the GABAA receptor (GABAAR) while certain synthetic derivatives (i.e. minaxolone) additionally enhance the function of recombinant glycine receptors (GlyR). Inhibitory transmission within the superficial dorsal horn has been implicated in mediating the analgesic actions of neurosteroids. However, the relative contribution played by synaptic and extrasynaptic receptors is unknown. In this study, we have compared the actions of 5a3a and minaxolone upon inhibitory transmission mediated by both GABAA and strychnine-sensitive GlyRs in lamina II neurones of juvenile (P15–21) rats. At the near physiological temperature of 35°C and at a holding potential of -60 mV we recorded three kinetically distinct populations of miniature IPSCs (mIPSCs): GlyR-mediated, GABAAR-mediated and mixed GABAAR-GlyR mIPSCs, arising from the corelease of both inhibitory neurotransmitters. In addition, sequential application of strychnine and bicuculline revealed a small (5.2 ± 1.0 pA) GlyR- but not a GABAAR-mediated tonic conductance. 5a3a (1–10 µm) prolonged GABAAR and mixed mIPSCs in a concentration-dependent manner but was without effect upon GlyR mIPSCs. In contrast, minaxolone (1–10 µm) prolonged the decay of GlyR mIPSCs and, additionally, was ~10-fold more potent than 5a3a upon GABAAR mIPSCs. However, 5a3a and minaxolone (1 µm) evoked a similar bicuculline-sensitive inhibitory conductance, indicating that the extrasynaptic GABAARs do not discriminate between these two steroids. Furthermore, ~92% of the effect of 1 µm 5a3a upon GABAergic inhibition could be accounted for by its action upon the extrasynaptic conductance. These findings are relevant to modulation of inhibitory circuits within spinally mediated pain pathways and suggest that extrasynaptic GABAARs may represent a relevant molecular target for the analgesic actions of neurosteroids.