Role of the supraoptic nucleus in regulation of parturition and milk ejection revisited

Abstract

This review will focus on the activity of oxytocin neurons in the supraoptic nucleus (SON) and some factors that regulate their function during parturition and milk ejection in the rat. The level of oxytocin increases in the blood during parturition following a regression of the corpus luteum. The increase in oxytocin secretion is presumably a consequence of releasing the oxytocin neurons from restraining inhibitory influences of endogenous opioids-, nitric oxide-, and GABA-containing neurons following declining blood levels of progesterone on the one hand and increasing levels of estrogen on the other during late pregnancy. However, the principal stimulus for the increased oxytocin release is believed to originate, at least in part, from mechanical stimulation to the uterine cervix by fetuses near term, the resultant uterine contractile activity, and the fetal expulsion reflex. Hence, the contractile activity of the uterus acts through positive feedback mechanisms during parturition to stimulate oxytocin neurons as well, and this further increases the secretion of oxytocin. During suckling in lactating rats, somatosensory stimuli from the pups induce intermittent synchronized burst firing of oxytocin neurons, resulting in pulsatile increases in blood oxytocin concentrations to cause milk ejection. The oxytocin neurons appear to have an intrinsic capability to fire in a bursting fashion as determined by observation of this phenomenon in brain slice or tissue culture preparations. The release of oxytocin within the microenvironment of the SON and paraventricular nucleus coupled with morphological reorganization in these nuclei play important roles in the bursting activity of each oxytocin neuron and synchronization in vivo. However, the mechanism responsible for the synchronization of electrical activity in oxytocin neurons in the four discrete hypothalamic nuclei remains an interesting unanswered question.

Central Nervous System Control of Oxytocin Secretion during Lactation

“…209,210 However, tonic excitatory synaptic activity in hypothalamic slices is low, is largely from miniature EPSPs and is insufficient to alter OT firing patterns. Since the sex of the donors is unknown, it is unlikely to involve steroid preconditioning or other sex-dependent electrophysiological and morphological changes known to occur with OT neurons during pregnancy and lactation (see "Synaptic Plasticity During Lactation" below).…”

Physiological consequences of membrane-initiated estrogen signaling in the brain

“…The internalization of MOR occurs rapidly (< 30 min of E2 treatment) and is correlated with an initial inhibition of the lordosis behavior in females (74,75). Membrane-impermeant E2 administered into the arcuate nucleus also initiates the MOR internalization (76).…”

The hippocampal continuation (indusium griseum): its connectivity in the hedgehog tenrec and its status within the hippocampal formation of higher vertebrates

“…The majority of these fibers may just pass across this area. The possibility of HCt efferents terminating on supraoptic neurons, however, should also be considered, particularly in view of the HCt stimulation experiments resulting in oxytocic milk ejection (Woods et al 1969;Higuchi and Okere 2002).…”