Thyrotropin-like immunoreactivity in the pituitary and three brain regions of the female rat: diurnal variations and the effect of thyroidectomy.

A method for extracting and assaying TSH-like immunoreactivity (TSH-LI) in regions of the central nervous systems (CNS) of individual rats was developed and validated. Serial dilutions of tissue extracts paralleled dilutions of purified rat TSH standard in the RIA, and tissue TSH-LI comigrated electrophoretically with TSH standard. TSH-LI was measured in the pituitary (PIT), the medial basal hypothalamus (MBH), the remainder of the hypothalamus (HYPO), and the cerebral cortex (CC). Female rats were maintained on a normal 12-h light, 12-h dark cycle (LD; onset of light, 0730 h) or an inverted 12-h light, 12-h dark cycle (DL; onset of light, 1930 h) for 3 weeks and then killed at 4-h intervals throughout the day. Another group of rats on the LD photoperiod were either thyroidectomized or sham thyroidectomized and killed 3 weeks later at 1000 h. Diurnal variations of TSH-LI were present in the PIT, MBH, HYPO, and CC on the LD photoperiod (P less than 0.05). The acrophases of MBH, HYPO, and CC TSH-LI diurnal variations occurred soon after the onset of darkness, whereas the PIT TSH-LI variation peaked soon after the onset of light. Inverting the photoperiod inverted the PIT TSH-LI diurnal variation, but central nervous system TSH-LI variations were not phase shifted after 3 weeks on the inverted photoperiod. Thyroidectomy elevated plasma and MBH TSH-LI levels (P less than 0.05), but PIT, HYPO, and CC TSH-LI levels were not significantly different from those in sham-operated controls. These results indicated that brain TSH concentrations could change under different physiological states independently of changes in PIT or plasma TSH. Although brain TSH exhibited diurnal variations, the data suggested that the phase of these variations was not set by the light-dark cycle.