Changes in concentration of hypothalamic cytosolic progestin receptors at the initiation of pseudopregnancy in rats.

Abstract

Pseudopregnancy (psp) can be induced by a single injection of progesterone in cyclic rats and the administration of anti-progesterone serum can block the establishment of psp in cervically stimulated rats. To further investigate neuroendocrine mechanisms for the initiation of either psp or prolactin (PRL) surges in connection with the neurotropic action of progesterone, cytosolic progestin receptors (PRc) were identified and measured in preoptic (POA) and basal hypothalamic (BH) areas. Chronological determinations of PRc concentrations in both areas revealed large fluctuations in the morning of the estrous day but not in the morning of the diestrous day. The differences between the maximal and the minimal PRc concentrations observed were more than 100-fold in POA and 3-fold in BH. Cervical stimulation applied in the afternoon of the proestrous day significantly altered the changing pattern of PRc concentrations in POA but not in BH. One of the two peaks of PRc concentrations in POA was magnified and advanced earlier to 0300 h, and then the 1st PRL surge peaking at 0700 h occurred. This PRL release seemed to stimulate progesterone secretion and an elevation of peripheral progesterone levels coincided with the 2nd peak of PRc concentrations in POA at 0900 h. This coincidence may be a prerequisite for the further continuation of PRL surges. These results strongly suggest that the spontaneous oscillation of the PRc concentration in hypothalamic neurons is involved in the regulation of PRL secretion and that cervical stimulation shifts the phase and changes the amplitude.