ELECTROPHYSIOLOGICAL CHARACTERISTICS OF IMMUNOCHEMICALLY IDENTIFIED RAT OXYTOCIN AND VASOPRESSIN NEURONS IN-VITRO

1. Intracellular recordings were made from supraoptic neurones in vitr o from hypothalamic explants prepared from adult male rats. Neurones w ere injected with biotinylated markers, and of thirty-nine labelled ne urones, nineteen were identified immunocytochemically as containing ox ytocin-neurophysin and twenty as containing vasopressin-neurophysin. 2 . Vasopressin and oxytocin neurones did not differ in their resting me mbrane potential, input resistance, membrane time constant, action pot ential height from threshold, action potential width at half-amplitude , and spike hyperpolarizing after-potential amplitude. Both cell types exhibited spike broadening during brief, evoked spike trains (6-8 spi kes), but the degree of broadening was slightly greater for vasopressi n neurones. When hyperpolarized below -75 mV, all but one neurone exhi bited a transient out-ward rectification to depolarizing pulses, which delayed the occurrence of the first spike. 3. Both cell types exhibit ed a long after-hyperpolarizing potential (AHP) following brief spike trains evoked either with a square wave pulse or using 5 ms pulses in a train. There were no significant differences between cell types in t he size of the AHP evoked with nine spikes, or in the time constant of its decay. The maximal AHP evoked by a 180 ms pulse was elicited by a n average of twelve to thirteen spikes, and neither the size of this m aximal AHP nor its time constant of decay were different for the two c ell types. 4. In most oxytocin and vasopressin neurones the AHP, and c oncomitantly spike frequency adaptation, were markedly reduced by the bee venom apamin and by d-tubocurarine, known blockers of a Ca2+-media ted K+ conductance. However, a minority of neurones, of both cell type s, were relatively resistant to both agents. 5. In untreated neurones, 55% of vasopressin neurones and 32% of oxytocin neurones exhibited a depolarizing after-potential (DAP) after individual spikes or, more co mmonly, after brief trains of spikes evoked with current pulses. For e ach neurone with a DAP, bursts of spikes could be evoked if the membra ne potential was sufficiently depolarized such that the DAP reached sp ike threshold. In four out of five vasopressin neurones a DAP became e vident only after pharmacological blockade of the AHP, whereas in six oxytocin neurones tested no such masking was found. 6. The firing patt erns of neurones were examined at rest and after varying the membrane potential with continuous current injection. No identifying pattern wa s strictly associated with either cell type, and a substantial number of neurones were silent at rest. Although more vasopressin than oxytoc in neurones exhibited phasic bursting, two of six oxytocin neurones wi th a DAP adopted phasic bursting after manipulating the membrane poten tial. Many vasopressin and oxytocin neurones could fire continuously f or long periods, and in a few cases continuous activity was elicited b y evoking a DAP in an otherwise quiescent neurone. During slower firin g, spike clustering was evident in both cell types, even in neurones w ithout a DAP. 7. Oxytocin and vasopressin neurones in male rats share many electrophysiological features which are quantitatively and qualit atively similar in vitro. Although the largest difference noted was in the percentage of neurones with a DAP, this potential is labile and i s not a signature for a cell type. The similarity of these two cell ty pes in vitro suggests a greater role for synaptic inputs in vivo in de termining the distinctive firing patterns displayed during increased h ormone release.