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 magnocellular neurosecretory cells of the hypothalamus, and in particular, in the regulation of neurons secreting oxytocin. The present study investigated the hypothesis that this medial septal pathway originates in the subfornical organ. Brief electrical stimulation of the subfornical organ or of the medial septum both evoked a transient rise in intramammary 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 stimulation also elicited at its onset a single transient response, similar 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 reflex milk ejection induced by suckling. Single-pulse stimulation of the subfornical organ rarely produced excitation, but short trains of stimuli evoked a large excitation in most oxytocinergic and vasopressinergic neurons. To delineate further the pathway from the subfornical organ to the magnocellular neurons, stimulations were combined with various 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. Stimulation of the medial septum no longer had an effect after the subfornical organ had been lesioned a week prior to experiments, a period sufficient to allow degeneration of subfornical efferents. This study shows that the excitatory afferent input to the oxytocin and vasopressin-secreting neurons of the hypothalamus from the medial septal area originates in the subfornical organ. This input is not involved in the main afferent control of the milk ejection reflex since lesions of the subfornical organ and of the medial septum had no effect on the reflex. It is suggested, therefore, that the subfornical input to both oxytocin and vasopressin cells intervenes to facilitate synergistic action of both hormones in non-reproductive functions.