Modulation of oxytocin secretion in the late pregnant and parturient rats by the mu and cappa opioid receptor

2. ABSTRACT Opioids have inhibitory effects on the neurosecretory functions of the oxytocin cells in the hypothalamus. Different insights have been suggested about the mechanism of this effect. The preseni study was carried out to investigate the effects of p. and k opioid receptors on the noradrenergic neurotransmission in the hypothalamus and the blood levels of oestrogen, progesterone, oxytocin and (3-endorphin in late pregnant and parturient rats. The blood hormone levels were determined (Oxytocin and (3 endorphin by radioimmunoassay, oestrogen and progesterone by enzyme immunoassay) in 20 and 21 daily pregnant and parturient rals following decapitation. The others in the 20lh day of pregnancy separated to five groups. The animals were anaesthetised with chloral hydrate, u. and k receptor agonists and antagonists were inlraccrebrovcnlricularly administered at 50 ug/kg/10 pi saline doses in a period of two minutes. The control group received saline alone. After 20 minutes, all rats were decapitated and trunk blood was collected for determination of serum oxytocin levels. The brains were rapidly removed and frozen on dry ice. Brain slices were cut, and supraoptic and paraventricular nuclei were isolated by micro-punch. The neurohypophysis was removed by dissection. Noradrenaline and dihydroxyphenil glycol concentrations in both hypothalamic nuclei and neurohypophysis were detected by HPLC-ECD. Progesterone and $ endorphin levels were significantly reduced in parturient group, while oxytocin and oestrogen levels were similar in late pregnant and parturient rats. An apparent increase occurred in oxytocin levels in the p. antagonist -treated animals and there was a decrease in k agonist group. Oxytocin levels did not significantly differ in all other groups. Noradrenaline concentrations were elevated in only the p. antagonist group. There was no significant difference in the dihydroxyphenilglycol levels between the groups-. In conclusion, it is suggested that there is a u. opioid restriction mediating noradrenergic neurotransmission on the oxytocin neurones in the late period of pregnancy. A decrease in P endorphin (which acts through p. opioid receptors) levels towards parturition appears to support to this suggestion. It is possible that the dramatically decreased serum progesterone levels may be the cause for reduced |3 endorphin tone. Key words: Opioid, oxytocin, pregnancy, rat.