Frontal, dorsal, or sham deafferentations were placed at various locations within the hypothalamus in order to study the neural pathways involved in pseudopregnancy (PSP), estrous cyclicity, and prolactin (PRL) secretion in the rat. Dorsal or sham transections did not interfere with PSP or estrous cyclicity. Frontal cuts placed on day 3–4 of PSP between the posterior border of the optic chiasm and the anterior tip of the mediobasal hypothalamus (MBH) led to interruption of diestrus within 3–5 days. With frontal cuts placed more caudally in the MBH, and with frontal cuts placed rostrally at the anterochiasmatic area, the duration of PSP was within a normal range. Irrespective of their effects on PSP, anterochiasmatic and retrochiasmatic cuts were associated with onset of persistent estrus, and MBH transections resulted in either persistent estrus in some rats or regular estrous cycles in the others. In deafferentated rats that showed persistent estrus, the basal plasma concentrations of PRL measured 3–4 weeks after ovariectomy were 2- to 3-fold higher than in deafferentated and sham-deafferentated animals that were cyclic before ovariectomy. Electrical vaginocervical stimulation induced secretion of nocturnal PRL surges in long-term ovariectomized rats with dorsal or sham transections, but not in those bearing frontal cuts, regardless of the neuroanatomical location of the frontal cut. These results suggest that (1) impulses generated at the uterine cervix must reach the medial preoptic area, a putative ‘anti-surge center’, and proceed from there to the MBH, in order to allow initiation of nocturnal PRL release; (2) the ‘surge center’ that transmits the nightly signal for PRL release is located above the optic chiasm and projects caudally to the MBH, and (3) preoptic-hypothalamic pathways controlling PSP and nocturnal PRL surges are dissociated from gonadotropin-releasing hormone pathways.

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