MEMBRANE-PROPERTIES AND MORPHOLOGY OF VASOPRESSIN NEURONS IN SLICES OF RAT SUPRACHIASMATIC NUCLEUS

Vasopressin (VP) neurons in the suprachiasmatic nucleus (SCN) are thou ght to be closely linked to neural mechanisms for circadian timekeepin g. To gain insight into the cellular-physiological principles that gov ern spike-driven VP release and to examine whether VP cells can be ele ctrophysiologically and morphologically identified by a unique combina tion of features, we recorded membrane properties by whole cell patch- clamp methods and stained the cells with biocytin. In current-clamp mo de, VP neurons recorded during subjective daytime expressed a clear ti me-dependent inward rectification but no pronounced low-threshold Ca2 potential after hyperpolarizing current pulses. Their spontaneous fir ing rate varied between 0.6 and 13.4 Hz and was generally tonic and ir regular. Spike afterhyperpolarizations (AHPs) were steeply rising and monophasic. Spikes were preceded by depolarizing ramps mediated by a s low component of Na+ current. Spike trains evoked by depolarizing curr ent pulses displayed frequency adaptation and were usually followed by an AHP lasting 0.5-2.0 s. Spontaneous postsynaptic potentials were pr esent in a majority of cells. Voltage-clamp recordings revealed a Ba2-sensitive K+ current that exerts a tonic, hyperpolarizing influence o n the membrane potential. This set of membrane properties was not sign ificantly different from other cells in the dorsomedial region and is characteristic for cluster I cells, which were described previously an d are widely encountered throughout the SCN. None of the cells could b e classified as belonging to cluster II or III, which were indeed foun d mainly outside the dorsomedial region. Morphologically, single VP ne urons were characterized by compact, mono- or bipolar dendritic branch ing patterns and numerous varicosities throughout the dendrites. They generally possessed few axon collaterals, most of which remained insid e the boundaries of the SCN but were occasionally seen to project to S CN target areas. In conclusion, VP neurons in the SCN express several active membrane poperties, including time-dependent inward rectificati on, frequency adaptation in spike trains, monophasic spike AHPs, and B a2+-sensitive K+ current. VP release is proposed to be governed by ton ic and irregular patterns of spontaneous firing. The electrophysiologi cal and cytological properties of VP neurons are representative for a majority of SCN cells and define them as a subset of previously define d cluster I cells.