The role played by the medial preoptic area-anterior hypothalamus (MPOA-AH) in the regulation of reproductive function of the female rat was examined in the first part of the present study. Extensive bilateral lesions placed in different locations of the MPOA-AH continuum resulted in three different patterns of vaginal smears. Lesions located most rostrally and dorsally, sparing the suprachiasmatic nuclei (SCN), resulted in series of successive pseudopregnancies. Lesions located more caudally and ventrally, still sparing the SCN, eventuated in regular oestrous cycles with only occasional pseudopregnancies. MPOA-AH lesions encroaching upon the SCN were associated with persistent vaginal cornification. MPOA-AH-lesioned females exhibiting repeated pseudopregnancies were capable of mating, conceiving, and giving birth. Parturition in these females was uniformly delayed by 1 day and was associated with a variable incidence of stillbirth, cannibalism of newborn, or incomplete placentophagy. As a rule, MPOA-lesioned mothers did not retrieve pups but they did show crouching behaviour and some of them even secreted milk. Thus, while the MPOA subserves in the regulation of oestrous cyclicity and maternal behaviour, it appears to play no essential role in mating behaviour or in ovarian function during pregnancy and lactation. The pattern of prolactin secretion in MPOA-AH-lesioned male and female rats was evaluated in the second part of this study. Both intact and orchidectomized males were incapable of generating nocturnal prolactin surges following extensive bilateral ablation of the MPOA-AH. On the other hand, 6 out of 9 ovariectomized MPOA-AH-lesioned females exhibited spontaneous nightly surges of prolactin. The absence of the surges in the remaining 3 females was associated with invasion of the lesion into the SCN and/or structures rostral to the MPOA. During the light period, prolactin levels remained within the basal range in all lesioned females, irrespective of the presence or the absence of the nocturnal surge. These lesioned ovariectomized females, in contrast to nonlesioned females, were incapable of producing a diurnal (afternoon) surge of prolactin in response to administration of oestrogen, though their basal prolactin levels were much higher than the levels seen in oestrogen-treated, nonlesioned castrated males. These results support the view that the MPOA is inhibitory to the nocturnal prolactin surge while stimulatory to the diurnal surge. The results also suggest that the male rat is functionally devoid of both a ‘diurnal-surge centre’ residing in the MPOA, and a ‘nocturnal-surge centre’ located outside the MPOA.

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