Calcium spikes and calcium currents in neurons from the medial preoptic nucleus of rat.

Ca2+ spikes, their contribution to firing patterns, and the underlying Ca2+ currents in neurons of the medial preoptic nucleus of rat were investigated by tight-seal whole-cell recordings in a slice preparation. Two different types of spikes were recorded: Low-threshold spikes were generated from membrane potentials <-75 mV. High-threshold spikes were recorded when K+ currents were reduced, and were readily evoked from membrane potentials near -40 mV. Both types of spikes were blocked by substitution of Co2+ for Ca2+ in the external medium, but were insensitive to 2.0 microM TTX. Under voltage-clamp conditions, two main types of Ca2+ currents were characterized: low-threshold currents that activated at membrane potentials >-60 mV, and high-threshold currents that activated at potentials >-30 mV. The low-threshold current and the low-threshold spike were more sensitive to block by external Ni2+ than to block by Cd2+, whereas the high-threshold current and the high-threshold spike were more sensitive to block by external Cd2+ than to block by Ni2+. Significant fractions of the high-threshold currents were blocked by 10 microM nifedipine, 1.0 microM omega-conotoxin GVIA, 50 nM omega-agatoxin IVA and 1.0 microM omega-conotoxin MVIIC, suggesting the presence of L-, N-, P- and Q-type Ca2+ channels. There were also a high-threshold current component insensitive to the above mentioned toxins. It is proposed that the low-threshold current serves as a trigger for short bursts of fast spikes from hyperpolarized levels, whereas the high-threshold current is involved in the Cd2+-sensitive burst firing seen in relatively depolarized neurons.