CONNECTIONS FROM THE SUBFORNICAL ORGAN TO THE OXYTOCIN AND VASOPRESSIN SYSTEMS IN THE LACTATING RAT - A STUDY USING ELECTRICAL STIMULATIONS, LESIONS AND ELECTROPHYSIOLOGICAL RECORDINGS

The medial septal area has been implicated in the control of the magno cellular neurosecretory cells of the hypothalamus, and in particular, in the regulation of neurons secreting oxytocin, The present study inv estigated the hypothesis that this medial septal pathway originates in the subfornical organ. Brief electrical stimulation of the subfornica l organ or of the medial septum both evoked a transient rise in intram ammary pressure equivalent to that caused by an i.v. injection of 1 mU oxytocin. The optimal frequency was 5-20 Hz for 5-10 s. Prolonged sti mulation also elicited at its onset a single transient response, simil ar to that evoked by brief stimulation, Extracellular recordings were made from neurosecretory cells of the supraoptic nucleus identified by antidromic stimulation of the neural stalk and further classified as vasopressinergic and oxytocinergic by their reaction at the time of re flex milk ejection induced by suckling. Single-pulse stimulation of th e subfornical organ rarely produced excitation, but short trains of st imuli evoked a large excitation in most oxytocinergic and vasopressine rgic neurons. To delineate further the pathway from the subfornical or gan to the magnocellular neurons, stimulations were combined with vari ous lesions of the medial forebrain. The effects of stimulation of the subfornical organ were abolished after a section immediately rostral to the organ, and in most cases after lesion of the medial septum. Sti mulation of the medial septum no longer had an effect after the subfor nical organ had been lesioned a week prior to experiments, a period su fficient to allow degeneration of subfornical efferents. This study sh ows that the excitatory afferent input to the oxytocin and vasopressin -secreting neurons of the hypothalamus from the medial septal area ori ginates in the subfornical organ. This input is not involved in the ma in afferent control of the milk ejection reflex since lesions of the s ubfornical organ and of the medial septum had no effect on the reflex. It is suggested, therefore, that the subfornical input to both oxytoc in and vasopressin cells intervenes to facilitate synergistic action o f both hormones in non-reproductive functions.