Calcium channel types contributing to excitatory and inhibitory synaptic transmission between individual hypothalamic neurons.

The contribution of L-, N-, P- and Q-type Ca2+ channels to excitatory and inhibitory synaptic transmission and to whole-cell Ba2+ currents through Ca2+ channels (Ba2+ currents) was investigated in rat hypothalamic neurons grown in dissociated cell culture. Excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) were evoked by stimulating individual neurons under whole-cell patch-clamp conditions. The different types of high-voltage-activated (HVA) Ca2+ channels were identified using nifedipine, omega-Conus geographus toxin VIA (omega-CTx GVIA), omega-Agelenopsis aperta toxin IVA (omega-Aga IVA), and omega-Conus magus toxin VIIC (omega-CTx MVIIC). N-, but not P- or Q-type Ca2+ channels contributed to excitatory as well as inhibitory synaptic transmission together with Ca2+ channels resistant to the aforementioned Ca2+ channel blockers (resistant Ca2+ channels). Reduction of postsynaptic current (PSC) amplitudes by N-type Ca2+ channel blockers was significantly stronger for IPSCs than for EPSCs. In most neurons whole-cell Ba2+ currents were carried by L-type Ca2+ channels and by at least two other Ca2+ channel types, one of which is probably of the Q-type and the others are resistant Ca2+ channels. These results indicate a different contribution of the various Ca2+ channel types to excitatory and inhibitory synaptic transmission and to whole-cell currents in these neurons and suggest different functional roles for the distinct Ca2+ channel types.