Electrophysiological evidence for neural connections between the supraoptic nuclei

Effects of electrical stimulation of the contralateral supraoptic nucleus (SON) on the activity of neurosecretory neurons in the SON were studied in urethane-anesthetized lactating rats. Thirty-one out of 41 oxytocin neurons were excited and only two neurons were inhibited by contralateral SON stimulation. Eight out of 26 vasopressin neurons were excited and 8 were inhibited. These responses were not affected by suckling stimuli. Neither oxytocin nor vasopressin neurons tested were antidromically activated by contralateral SON stimulation. Thus monosynaptic connections between the bilateral SON neurosecretory neurons seem to be very few, if any. These results suggest that neural connections exist between the bilateral SON and that they are mainly polysynaptic.