SUMMARY1. Extracellular recordings in pentobarbitone anaesthetized male Long-Evans rats examined the influence of electrical stimulation in the diagonal band of Broca on the excitability of 113 putative vasopressin-secreting and 22 putative oxytocinsecreting neurosecretory neurones in the hypothalamic supraoptic nucleus.2. Single pulse or repetitive (5-20 Hz) stimulation in the ventral part of the diagonal band evoked a prominent reduction in the excitability of83 % ofvasopressinsecreting neurones with no effect on the remainder. Amongst oxytocin-secreting neurones, 59 % were unresponsive, 27 % responded with an increase in activity while only 14 % revealed an inhibitory pattern similar to vasopressin-secreting neurones.3. Diagonal band stimulation-evoked inhibitions were reversibly abolished by local pressure applications of bicuculline methiodide (100 fM) to twenty out of twenty vasopressin-secreting cells tested, whereas strychnine sulphate (100 FtM) was without effect on four out of four cells tested. 4. In five out of five vasopressin-secreting cells tested, bicuculline applications reversibly abolished the reduction in their activity that follows peripheral baroreceptor activation. Failure to alter baroreflex-evoked depressions in firing during similar trials with prazosin hydrochloride (10 #M, six cells tested), timolol maleate (20 /tM, six cells tested) or strychnine sulphate (100 /M, three cells tested) indicated the specificity of bicuculline's action.5. These findings suggest that a GABAergic pathway from the diagonal band of Broca preferentially innervates vasopressin-secreting neurosecretory supraoptic nucleus (s.o.n.) neurones, and support the view that the baroreflex-induced depression in firing of s.o.n. vasopressin-secreting neurones is mediated in large part through this input.

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Antibodies to oxytocin and noradrenalin were utilized in an immunocytochemical study of the caudal ventrolateral medulla of the rat brainstem. Noradrenalin was visualized by using antibodies to noradrenalin and by means of a silver-gold intensification of diaminobenzidine, whereas oxytocin could be demonstrated in the same section by using the diaminobenzidine precipitate as a marker. At the light microscopic level, oxytocin fibers were densely distributed around the A1 cell bodies. At the ultrastructural level, oxytocin-containing fibers were seen to terminate synaptically onto noradrenalin-containing neurons. Previous studies have shown that electrical stimulation of A1 neurons selectively activates vasopressin-secreting neurons in the supraoptic nucleus. Therefore, separate electrophysiological studies were set up, in which we observed that oxytocin infusions (100-200 pg) into the A1 area enhanced the activity of 16 out of 19 putative vasopressin-secreting neurons and elicited no response from any of 10 oxytocin-secreting neurons. This finding suggests that some of the parvicellular neurons in the paraventricular nucleus of the hypothalamus, from which the A1 neurons derive their oxytocin innervation, can activate the A1 cell group via this peptidergic neurotransmitter. One of the consequences of A1 neuronal activation is enhanced firing of hypothalamic supraoptic (and paraventricular) vasopressin-secreting neurons, and a consequent rise in plasma vasopressin.