Enantioselective modulation of GABAergic synaptic transmission by steroids and benz[e]indenes in hippocampal microcultures

Abstract

The effects of enantiomers of the neurosteroid analogues, 3alpha-hydroxy-5alpha-pregnan-20-one (DHP) and 3alpha-hydroxy-5alpha-androstane-17beta-carbonitrile (ACN), and the benz[e]indene, BI-1, on synaptic currents were examined in microcultures of rat hippocampal neurons. Over the range of 0.1-10 microM, the (+)-enantiomers were more potent and effective than their (-)-enantiomeric counterparts in enhancing gamma-aminobutyric acid (GABA)A receptor-mediated evoked synaptic currents. The (+)-enantiomers had small effects on peak currents, but slowed the decay of inhibitory synaptic currents, resulting in 2-3-fold increases in charge transfer during inhibitory synaptic events at 10 microM. Similar prolongations of spontaneous miniature inhibitory postsynaptic currents (IPSCs) and responses to brief GABA pulses to outside-out patches suggest that the prolongations of evoked synaptic currents result primarily from postsynaptic effects. In contrast, the (-)-enantiomers had little effect on evoked IPSCs at concentrations < or = 1 microM, but enhanced inhibitory transmission at 10 microM. At concentrations < or = 1 microM, neither the (+)- nor (-)-enantiomers altered glutamate-mediated excitatory synaptic currents. At 10 microM, (+)-DHP and (+)-ACN depressed excitatory responses in a bicuculline-sensitive fashion, suggesting that direct chloride channel gating by the steroids contributed to the depression. These data indicate that certain steroids and benz[e]indenes augment inhibitory synaptic transmission enantioselectively and provide strong support for the hypothesis that steroids act at specific sites on synaptic GABA(A) receptors rather than via alteration of membrane lipids.

The γ-aminobutyric acid type A (GABA A ) receptor-associated protein (GABARAP) promotes GABA A receptor clustering and modulates the channel kinetics

“…Modulation of kinetics is an important mechanism to modulate synaptic transmission (25)(26)(27)(28). Many drug actions are based on this concept (29)(30)(31)(32)(33)(34). There are several ways to modulate channel kinetics, such as changing subunit composition (17,35), phosphorylating channel proteins (28,(36)(37)(38)(39), binding of modulator to specific modulatory sites (e.g., the glycine site for N-methyl-D-aspartate receptor and neurosteroid binding site for GABA A receptor), etc.…”

Mechanisms of neurosteroid interactions with GABAA receptors

“…To determine whether steroids may act at protein sites, Covey and colleagues synthesized and tested a series of steroid enantiomers (Hu et al, 1997;Nilsson et al, 1998;Wittmer et al, 1996;Zorumski et al, 1998). These are important agents because prior studies used diastereomers (or epimers) to probe steroid actions (Im et al, 1990;Woodward et al, 1992).…”

“…7), potentiation of GABA A receptors, direct gating of chloride channels, effects in the TBPS binding assay, and anesthetic effects in tadpoles and mice show significant enantioselectivity. In some cases enantiomer pairs differ by an order of magnitude or more in potency Wittmer et al, 1996;Zorumski et al, 1998). In general, (+)-enantiomers that have the absolute configuration of natural steroids are more potent (and in physiological assays are probably more effective) than (-)-enantiomers.…”

“…ketamine, isoflurane, barbiturates) have similarly been used to argue that anesthesia results from specific interactions of drugs with protein receptors (Andrews & Mark, 1982;Franks & Lieb, 1991;Hall et al, 1994). However, the differences in enantiomeric anesthetic potencies observed with these other agents have rarely achieved the order of magnitude separation we found with ACN enantiomers Zorumski et al, 1998).…”

Comparison of Steroid Modulation of Spontaneous Inhibitory Postsynaptic Currents in Cultured Hippocampal Neurons and Steady-State Single-Channel Currents from Heterologously Expressed α1β2γ2L GABAA Receptors

“…2B). This is similar to several previous estimates for decay times of miniature and spontaneous IPSCs from hippocampal neurons (e.g., Poisbeau et al, 1997;Zorumski et al, 1998;Banks and Pearce, 1999;Park et al, 2011). The amplitudes of sIPSCs varied considerably from cell to cell.…”