Calcium-dependent prepotentials contribute to spontaneous activity in rat tuberomammillary neurons.

1. Intracellular recordings from histaminergic neurons of the tuberomammillary (TM) nucleus reveal subthreshold depolarizing potentials (DPs) which persist in the presence of tetrodotoxin. 2. Block of hyperpolarization-activated current by 1-4 mM Cs+ failed to reduce spontaneous activity or DPs. 3. In the presence of tetrodotoxin DPs are voltage dependent and are depressed by Cd2+ and Co2+. 4. Ba2+ (100 microM) treatment enhances DP amplitude and converts low-amplitude potentials to tetrodotoxin-insensitive action potentials. 5. In the presence of TTX, DPs are reduced by Ni2+. Spontaneous action potentials are also reduced by Ni2+ (100-300 microM). A low-threshold Ca2+ current is present which is sensitive to Ni2+. These results indicate the presence of calcium currents, perhaps of the low-threshold type, which contribute to activation of action potentials in TM neurons.