Cervical stimulation in the rat institutes a pattern of prolactin secretion consisting of two daily surges, one diurnal and one nocturnal, which continue for the greater part of pseudopregnancy. This contrasts with the diestrous day 1-2 interval of the estrous cycle wherein prolactin levels are low and relatively unchanging. A variety of noxious procedures such as aortic cannulation and serial blood sampling, sham ovariectomy and sham surgical cuts in the hypothalamus selectively and temporarily suppresses the diurnal prolactin surges of pseudopregnancy; the nocturnal surges remained unaffected. In the absence of the ovaries (removed on day 0 of pseudopregnancy) the diurnal prolactin surges have disappeared no later than day 5, whereas the nocturnal surges are present at day 6 but have disappeared by day 10. Hypothalamic retrochiasmatic cuts abolish immediately both surges. These results demonstrate the existence of an apparently unique kind of neuroendocrine reflex, copulation-induced, which results in repeated nocturnal surges of prolactin secretion at daily intervals for a prolonged period of time in response to a brief 1-min stimulus. To account for these observations, a hypothalamic mnemonic system has been postulated which is activated by cervical stimulation and which retains and expresses this information repeatedly even in the absence of the ovaries.
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