Oxytocin-immunoreactive terminals synapse on oxytocin neurones in the supraoptic nucleus.

The neuropeptide oxytocin, synthesized by magnocellular neurones in the hypothalamus, is well known for its peripheral action during lactation and parturition after its release into the bloodstream from axons in the neurohypophysis. However, it may also be released centrally to control the activity of oxytocinergic neurones themselves. Oxytocin release has been measured from isolated magnocellular nuclei in vitro and in the cerebrospinal fluid. When injected into the third ventricle, the peptide increases the basal firing rate of oxytocinergic neurones as well as the frequency and amplitude of the bursts of action potentials they normally show before each reflex milk ejection. Oxytocin also excites magnocellular neurones when applied microiontophoretically. I now report that immunocytochemical staining reveals synapses in the supraoptic nucleus of the hypothalamus where both the pre- and postsynaptic elements contain oxytocin. These oxytocinergic synapses, impinging on their own neurones, may represent the anatomical basis for the hypothalamic release of this peptide and for the facilitatory action on its own secretion.