ROLE OF A T-TYPE CALCIUM CURRENT IN SUPPORTING A DEPOLARIZING POTENTIAL, DAMPED OSCILLATIONS, AND PHASIC FIRING IN VASOPRESSINERGIC GUINEA-PIG SUPRAOPTIC NEURONS

Guinea pig magnocellular neurosecretory cells (MNCs) of the supraoptic nucleus (SON) were studied using the in vitro slice preparation. Intr acellular recordings were made with biocytin-filled electrodes, permit ting immunocytochemical identification of the recorded cells as argini ne vasopressin- (AVP) versus oxytocin- (OT) containing. Only AVP cells displaying a depolarizing potential (DP) fired phasically. The DP was associated with a transient inward current measured in voltage clamp, which exhibited a number of properties of the T-type calcium current: activation threshold of -64 mV, time course of up to 250 ms, blockade by nickel and augmentation by barium chloride. This current has not b een reported previously in SON neurons. The T-type current (I(T)) was always associated with a damped oscillation of the membrane following the offset from hyperpolarizing steps. In all cells tested, an apamin- sensitive afterhyperpolarization (AHP) was observed, similar to the ca lcium-dependent potassium current (I(K,Ca)) described in rat SON and o ther CNS regions. Therefore, as with other CNS regions displaying damp ed oscillations, guinea pig SON cells possess both an I(T) and an I(K, Ca). We have previously described an I(h) activating at hyperpolarized potentials in these cells, which depolarizes the membrane to a range in which the I(T) and I(K,Ca) can interactively support oscillations. In summary, the I(T) and associated depolarizing potential appears to be a requisite feature for phasic firing in AVP cells of guinea pig SO N.