Oxytocin (OT) and vasopressin (VP) were measured by radioimmunoassay in hypophysial portal and peripheral blood from male Wistar rats and heterozygous and homozygous Brattleboro rats anaesthetized with urethane. In Wistar rats the concentrations of OT and VP were about 50 times greater than the concentrations in peripheral blood, whether or not the pituitary gland was left in situ during collection, and also considerably greater than the reported concentrations of the peptides in the cerebrospinal fluid. The release of both peptides was increased significantly by a lesion of the supraoptico-hypophysial tract that led to diabetes insipidus, but which left intact the external layer of the median eminence (ME). Concentrations of VP were undetectable in plasma from homozygous Brattleboro rats, but the portal plasma concentrations of VP in heterozygous Brattleboro rats were not significantly lower than in Wistar rats. The concentrations of OT in portal plasma from both types of Brattleboro rat were significantly higher than in Wistar rats. The output of VP and OT into hypophysial portal blood of Wistar rats was not significantly affected by electrical stimulation of the suprachiasmatic, supraoptic or paraventricular nuclei or the ME using two types of stimuli, one of which produced an increase in peripheral plasma concentrations of VP and OT in intact rats and a significant increase in the release of LH-releasing hormone into hypophysial portal blood. The output of VP and OT into portal blood was also not significantly affected by either adrenalectomy with or without injection of dexamethasone or the injection of either the 5-hydroxytryptamine (5-HT) synthesis blocker, parachlorophenylalanine, or the 5-HT uptake blockers, alaproclate or zimelidine. These results show that large amounts of OT as well as VP are released into hypophysial portal blood from fibres of the hypothalamo-neurohypophysial system that terminate in the external layer of the ME. Although distinct from the fibres that terminate in the pars nervosa (PN), the findings in Brattleboro rats show that the VP fibres of the ME system originate in neurones with a genomic mechanism for VP synthesis similar to that of the VP neurones that project to the PN. The lack of effect of adrenalectomy and the administration of 5-HT synthesis and uptake blockers must be interpreted with caution since the results obtained with electrical stimulation suggest that when the pituitary stalk is cut the release of OT and VP into portal blood approaches a maximum and may therefore be difficult to alter by experimental manipulation.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of the 5-hydroxytryptamine (5-HT) uptake blockers on the surges of LH and prolactin has been investigated in pro-oestrous rats and various experimental models used frequently to study the effects of steroids on LH and prolactin secretion in female rats. The steroid models were: rats ovariectomized on dioestrus, injected immediately with oestradiol benzoate (OB) and at 12.00 h on the next day (presumptive pro-oestrus) with progesterone (model 1); long-term ovariectomized rats injected with a single injection of OB and 72 h later with either progesterone (model 2) or OB (model 3); long-term ovariectomized rats injected daily with OB (model 4). The uptake blockers alaproclate (3-30 mg/kg) and zimelidine (20 mg/kg) were injected and blood samples withdrawn from previously implanted intra-atrial cannulae. Plasma LH and prolactin concentrations were determined by radioimmunoassay. The present study confirmed that a surge of LH occurs at about 17.00-18.00 h of the presumptive day of pro-oestrus in model 1, at about 5 h after (approximately 17.00 h) the injection of either progesterone or the second injection of OB in models 2 and 3, and diurnally in model 4, and the simultaneous occurrence of a prolactin surge in models 2 and 4. A surge of prolactin at the same time as the LH surge was shown to occur also in models 1 and 3. Alaproclate (30 mg/kg) administered at 15.00 h delayed significantly the peak of the prolactin surge in the pro-oestrous rat and models 1, 3 and 4, and in the latter the magnitude of the prolactin surge was also significantly reduced. By contrast, the peak of the prolactin surge in model 2 was significantly prolonged by alaproclate. Alaproclate had no significant effect on either the timing or the magnitude of the LH surge in the pro-oestrous rat, and models 3 and 4. The peak of the LH surge was delayed by alaproclate in model 1 and abolished in model 2, providing further evidence for the possible importance of interactions between 5-HT and progesterone in neuroendocrine control. Zimelidine had no significant effect on either the LH or prolactin surge in the pro-oestrous rat and in models 1 and 2. These results show that normal 5-HT uptake is necessary for the normal timing and/or magnitude of the spontaneous and steroid-induced prolactin surge but is not essential for the normal timing and magnitude of the spontaneous surge of LH and the LH surge in some but not all steroid models.(ABSTRACT TRUNCATED AT 400 WORDS)

The possible role of thyrotrophin-releasing hormone (TRH) in causing the pro-oestrous surge of prolactin was investigated in conscious female rats by passive immunization with a specific anti-TRH serum raised in sheep. Blood samples were withdrawn through a previously implanted intra-atrial cannula. The i.p. injection of 1 ml anti-TRH serum, but not non-immune sheep serum, at 13.00 h of pro-oestrus delayed by about 1 h the onset of the prolactin surge, but the peak of the surge was similar to that in animals injected with the non-immune serum. The plasma concentrations of TSH were significantly reduced by the anti-TRH serum, but plasma concentrations of LH were not significantly affected. These results show that TRH may play an important role in the timing and initiation, but not the maintenance of the prolactin surge in the pro-oestrous rat.

Vasopressin and oxytocin immunoreactivity (AVP-IR, OT-IR) have been detected in the trigeminal and dorsal root ganglia (TG, DRG) of the rat. We have investigated whether AVP or OT have any neurotransmitter role in these tissues by measuring the effects of the peptides on levels of intracellular second messengers. AVP and OT at concentrations up to 3 x 10(-6) M have no effect on the accumulation of cAMP. However, in tissue prelabelled with 3H-inositol, and in the presence of 10 mM Li+, AVP and OT cause an increase in the accumulation of inositol phosphates (IP), in a dose-dependent manner. AVP causes a maximum stimulation of 1.7 fold of control in TG, (p less than 0.01) and of 2.5 fold in DRG (p less than 0.01) at a concentration of 3 x 10(-7) M. OT causes a maximum stimulation of 1.3 fold of control in TG, (p less than 0.01), and of 1.75 fold of control in DRG, at a concentration of 3 x 10(-6) M. The stimulation of IP turnover by AVP in both tissues is inhibited by the specific V1-antagonist, (CH2)5Tyr(Me)AVP, at a concentration of 2 x 10(-5) M. The V2-agonist, DDAVP, has no effect on IP accumulation in either tissue at concentrations up to 3 x 10(-6) M. The response to exogenous AVP is still present in ganglia incubated in media without added CaCl2. We conclude that the rat TG and DRG contain receptors for AVP, and that these receptors have characteristics associated with the V1 subtype.

The effect of the 5-hydroxytryptamine synthesis inhibitor, parachlorophenylalanine (PCPA), on the spontaneous, pro-oestrous surges of prolactin and LH was investigated in conscious female rats implanted with an intra-atrial cannula. The LH surge was blocked in all animals treated with PCPA, but the prolactin surge was blocked in six out of ten animals and unaffected in four out of ten animals. There was a significant correlation between the plasma concentrations of prolactin and oestradiol-17 beta, surges of prolactin occurred in animals with plasma oestradiol-17 beta concentrations of 120 pmol/l and above, and the LH surge was blocked in animals in which the oestradiol-17 beta concentration was less than 188 pmol/l plasma. These results show that (i) PCPA can block the pro-oestrous prolactin as well as the LH surge, (ii) in addition to a central action PCPA may depend also on its peripheral effect on the secretion of oestradiol-17 beta and (iii) on average, lower plasma concentrations of oestradiol-17 beta are required for triggering the prolactin compared with the LH surge.