Different effects of α-chloralose on spontaneous and evoked GABA release in rat hippocampal CA1 neurons

The effects of α-chloralose on presynaptic GABA_A receptors were investigated with respect to spontaneous and evoked GABAergic transmission (sIPSCs and eIPSCs) in rat hippocampal CA1 pyramidal neurons. sIPSCs were recorded in mechanically dissociated CA1 neurons with intact GABAergic terminals, namely the "synaptic bouton preparation." eIPSCs were elicited by focal electrical stimuli of a single GABAergic bouton on an isolated CA1 neuron using the whole-cell patch recording configurations under voltage-clamp condition. We found that α-chloralose potentiated the exogenous GABA-induced Cl^- response in a concentration dependent manner, and the drug itself induced Cl^- response at high concentrations (>100μM). α-Chloralose at low concentrations (3-10μM) increased sIPSC frequency without affecting the current amplitude and kinetics, but prolonged the slow current decay time constant (τ_s) at concentrations greater than 30μM without changing either current amplitude or frequency. α-Chloralose at 10μM enhanced amplitude of eIPSCs and decreased the failure rate (Rf), but at 30μM decreased the amplitude and increased the Rf. Pretreatment with bumetanide, a blocker of NKCC-1, completely prevented the 30μM α-chloralose-induced inhibition on eIPSC amplitude and Rf. These results suggest that α-chloralose activates GABA_A receptors on GABAergic presynaptic nerve terminals and depolarizes the terminals, mediating presynaptic inhibition or autoregulation, in a concentration-dependent manner. In addition, α-chloralose at high concentrations activates not only extrasynaptic GABA_A receptors on the postsynaptic soma membrane but also synaptic GABA_A receptors resulting in prolongation of current decay phase. Thus α-chloralose induces complex and differential modulation of sIPSCs and eIPSCs in a concentration dependent manner.