Chronic prolactin, gonadal and thyroid hormone treatments in vivo alter levels of TRH and muscarinic receptors in male and female rat tissues.

Chronic subcutaneous administration of prolactin into female rats during proestrus led to a 20% (P less than 0.05) decrease in retinal and a 32% (n = 20; P less than 0.01) decrease in pituitary TRH receptors as compared to controls. In parallel experiments prolactin treatment during diestrus failed to influence TRH receptor levels in both tissues compared to vehicle-treated rats. Intraperitoneal injections of triiodothyronine for 8 weeks resulted in a selective 41% increase (P less than 0.02) in retinal TRH receptor levels without any changes in the pituitary and 4 other brain regions. Administration of 17-beta-estradiol for 2 weeks into male rats 1 month after castration resulted in a 68% increase (P less than 0.02) in pituitary TRH receptor content without any changes in the retina, amygdala or hypothalamus. Testosterone administration for 2 weeks into castrated male rats 30 days post-castration did not alter TRH receptor content in the latter 4 tissues but caused a 27% (n = 10; P less than 0.05) and a 40% increase (n = 5; P less than 0.05) in muscarinic cholinergic receptor levels in the superior cervical ganglia and anterior pituitary gland, respectively. In conclusion, these data have demonstrated that chronic administration of exogenous hormones selectively up- or down-regulates TRH and muscarinic receptors in a region-specific manner depending on the physiological state of the animal and the tissue under study, and provide further new evidence for feedback hormonal control of these neurotransmitter receptors.