Detailed analysis of blood oxytocin levels during suckling and parturition in the rat

Abstract

A detailed secretory profile of oxytocin during suckling and parturition was determined in unanaesthetized freely moving rats. Ten pups were reunited with their mothers after 12-15 h of separation. Unless the milk-ejection reflex occurred, there was no difference in serum oxytocin levels before separation and during the suckling of either four or five, or nine or ten pups. Serum oxytocin levels increased abruptly by 50.1 +/- 4.2 (S.E.M.) pmol/l (n = 9) at milk ejection, and declined rapidly with a half-life of about 1.5 min. The peak concentration of blood oxytocin at each milk ejection was independent of the previous suckling period; values from the first three milk-ejection reflexes following the introduction of the pups and those observed 3-5 h after introduction were similar. The process of parturition was monitored by recording intra-uterine pressure with a balloon implanted in the uterus. On day 22 or 23 of pregnancy, continuous and rhythmical contractions of the uterus occurred (onset of parturition), but serum levels of oxytocin (21.1 +/- 1.9 pmol/l; n = 13) did not alter until the expulsive phase. During the expulsive phase, fetuses were delivered after fetus-expulsion reflexes which were recorded as sudden large increases in intra-uterine pressure. Basal levels of oxytocin in the blood increased during this phase (32.5 +/- 4.4 pmol/l; n = 13) and, in addition, rose by about 15 pmol/l and declined slowly after fetus-expulsion reflexes. The increase, however, was quite different from that seen at milk-ejection reflexes.

Oxytocin does not directly affect vascular tone in vessels from nonpregnant and pregnant rats

“…However, in these previous studies, only high concentrations of OT (Ͼ10 Ϫ10 M) were used. Circulating concentrations of OT in either the pregnant or nonpregnant state are less than 10 Ϫ10 M in both rat (21) and human (16,44). In our studies, there was no vasorelaxant response at any concentration as low as 10 Ϫ14 M. In other studies (data not shown), we measured no effect of OT with concentrations as low as 10 Ϫ25 M. Several in vitro studies have provided support for a possible direct vasorelaxation response to OT.…”

Electrical Activities of Neurones in the Dorsomedial Hypothalamic Nucleus Projecting to the Supraoptic Nucleus During Milk-ejection Reflex in the Rat

“…56: [336][337][338][339][340] 2010) xytocin (OT) is synthesized in the cell body of the OT cell in the hypothalamic paraventricular (PVN) and supraoptic nuclei (SON) and released into blood circulation from axon terminals in the neurohypophysis. Various kinds of physiological stimuli such as parturition, lactation, osmotic stimulus and stress are known to stimulate OT secretion [1][2][3][4]. In lactating rats, OT is released intermittently as a result of periodic and synchronized activation of OT cells distributed in both sides of the hypothalamus, whereas pups suck nipples continuously [5].…”

Oxytocin: recent developments

“…Oxytocin was first recognised for its roles in female physiology including the induction of labour and milk ejection, roles that are evidenced by increasing oxytocin levels during lactation and labour (18). Additionally, oxytocin is the strongest uterogenic substance known to man and its agonists and antagonists have been used clinically to induce labour or halt premature labour, respectively (1).…”