CALBINDIN-D-28K - ROLE IN DETERMINING INTRINSICALLY GENERATED FIRING PATTERNS IN RAT SUPRAOPTIC NEURONS

1. Physiological activation of rat supraoptic nucleus (SON) neurones l eads to phasic firing in vasopressin neurones and fast, continuous fir ing in oxytocin neurones. Using whole-cell patch clamp methods in brai n slices, we investigated the role of endogenous calbindin-D-28k (calb indin) in determining these intrinsically generated patterns of firing . 2. Direct introduction of calbindin (0.1-0.2 mar) into twelve of twe lve phasically firing neurones suppressed Ca2+-dependent depolarizing after-potentials (DAPs) and changed activity from phasic to continuous firing. Bovine calcium binding protein (0.3 mar), an analogue of calb indin, had similar effects on both DAPs and firing patterns in five of five cells tested. 3. Introduction of anti-calbindin antiserum (1:200 0-5000) into thirteen of thirteen continuously firing neurones unmaske d DAPs and converted continuous into phasic firing. Such effects could not be mimicked either by diffusion of normal rabbit serum or antibod ies directed against glial fibrillary acidic protein or against neurop hysin. 4. Immunocytochemical staining with antisera directed against c albindin revealed more intense staining in the dorsal, oxytocin-rich a nd less intense staining in the ventral, vasopressin-rich areas of the SON. 5. Elevated intracellular Ca2+ concentration ([Ca2+](i); 0.1 mM) induced DAPs and phasic firing in all twenty-nine SON cells recorded. During chelation of intracellular Ca2+ with (1.1-11 mM) BAPTA, fifty- eight of fifty-eight neurones recorded displayed regular continuous ac tivity and had no DAPs. 6. These data suggest that firing activities i n SON cells are dependent on [Ca2+](i) and that calbindin, acting as a n endogenous Ca2+ buffer, is involved in regulation of intrinsic firin g patterns. It is likely that calcium binding proteins have a similar influence on the firing patterns of many neuronal types throughout the nervous system.