Heterogeneous presynaptic Ca2+ channel types triggering GABA release onto medial preoptic neurons from rat

1 Voltage-dependent Ca2+ channels triggering GABA release onto neurons from the medial preoptic nucleus of rat were investigated. Acutely dissociated neurons with adherent functional synaptic terminals were investigated by tight-seal whole-cell recordings from the postsynaptic cells. 2 Spontaneous current events similar to miniature postsynaptic currents were recorded. They were blocked by bicuculline (100 μM), showed a roughly unimodal amplitude distribution and a reversal potential consistent with a Cl− current, and were therefore attributed to GABAA receptors activated by synaptically released GABA. 3 Application of 140 mM KCl, expected to depolarize presynaptic terminals, evoked currents that were ascribed to a more massive release of GABA. The KCl-induced synaptic currents were abolished in Ca2+-free solutions and showed a roughly hyperbolic relation to external Ca2+ concentration with half-saturation at 0.15 mm. They further depended on the concentration of applied KCl in a way expected for high-threshold Ca2+ channels. 4 The KCl-evoked synaptic currents were completely blocked by 200 μm Cd2+, but only partially blocked by 200 μm Ni2+. 5 The KCl-evoked synaptic currents were insensitive to the L-type Ca2+ channel blocker nifedipine (10 μm). However, the synaptic currents were sensitive to either 1 μmω-conotoxin GVIA, 25 nmω-agatoxin IVA or 1 μmω-conotoxin MVIIC. 6 It was concluded that, in many presynaptic terminals, the Ca2+ influx triggering GABA release onto medial preoptic neurons is mainly mediated by one predominant type of high- threshold Ca2+ channel that may be either of N-, P- or Q-type. 7 It was further concluded that terminals with similar predominant channel types often were clustered on the same postsynaptic cell.