Nyctohemeral rhythms of gonadal, thyroid and pineal function in the hyperprolactinemic Male rat

In young adult male rats bearing a donor anterior pituitary gland grafted for 3 weeks under a kidney capsule, serum prolactin (PRL) concentrations were elevated and exhibited a rhythm with the highest values in the light phase. Serum PRL in control animals did not exhibit a significant rhythm. Eutopic pituitary PRL content, manifesting a biphasic (12-hr) rhythm with crests during the day and night in controls, exhibited a similar pattern in grafted rats though an overall reduction in pituitary PRL content was seen in the grafted animals. Neither the normal biphasic serum testosterone rhythm nor the normal 24-hr rhythm (nocturnal surge) of pineal N-acetyltransferase activity and melatonin content were altered in the hyperprolactinemic rats. Serum thyroxine (T₄) and triiodothyronine (T₃) and their free indices (FT₄ I, FT₃ I) and serum thyrotropin (TSH) were highest during the day in controls and grafted rats and a 12-hr rhythmic component was detected in data for these variables. In the grafted animals, the 12-hr component was reflected in an additional peak at night detectable by testing of means. The overall serum T₄ FT₄ I, and TSH levels were lower in grafted rats though overall T₃ and FT₃I levels did not differ between grafted and controls. T₃ uptake (T₃ U) values were similar between controls and grafted rats, in both cases exhibiting a fall during the night. Changes in serum thyronines could not be explained by changes in serum binding as assessed by the T₃U₃ and thus may represent changes in thyroidal secretion of T₄. The rhythm in serum PRL of grafted rats suggests the presence of rhythmic circulating factor(s) capable of influencing ectopic lactotrophs. The reduced eutopic pituitary PRL content suggests a role for PRL in influencing eutopic lactotrophs in the pituitary-grafted hyperprolactinemic male rat model. Though circulating testosterone and pineal melatonin synthesis were not altered in this model, thyroid function appeared to be so.