Electrophysiological evidence for a projection from the arcuate nucleus to the supraoptic nucleus.

The electrical activity of single neurones in the hypothalamic arcuate and supraoptic nuclei was recorded in urethane-anaesthetized rats. Stimulus pulses applied to the supraoptic nucleus antidromically activated 3 out of 41 cells recorded in the ipsilateral arcuate nucleus, confirming that there is a projection from the arcuate nucleus to the region of the supraoptic nucleus. Stimulation of the arcuate nucleus inhibited 17 out of 19 continuously firing (putative oxytocin) supraoptic neurones. Inhibition was followed by a marked post-stimulus excitation in 12 cells. The responses were not abolished by i.v. injection of the opioid antagonist naloxone. Thus at least part of the input to the magnocellular oxytocin system that arises from or passes through the arcuate nucleus, is not mediated by opioid peptides.