Seasonal and state-dependent changes in brain TRH receptors in hibernating ground squirrels.

Quantitative autoradiography was used to localize and quantify thyrotropin-releasing hormone (TRH) receptors in the brain of hibernating (H), winter euthermic (WE), and summer euthermic (SE) animals to further explore the state-dependent physiological and behavioral effects of TRH in ground squirrels. The pattern of [3H]MeTRH binding (Kd 6.7 +/- 0.1 nM) was heterogeneous and highly concentrated in structures primarily associated with the limbic forebrain. Statistically significant seasonal changes (SE vs. WE) were reflected by decreases in TRH receptor binding in the arcuate nucleus, dorsomedial nucleus, and ventral pallidum of WE animals. Increased binding in WE animals was evident in the suprachiasmatic nucleus and choroid plexus of the lateral ventricles. Significant state-dependent changes (WE vs. H) were characterized by decreases in TRH receptor binding in the hypothalamic paraventricular nucleus, medial preoptic area, ventral tegmental area, and choroid plexus of the lateral ventricles of H animals. Increased binding occurred in the anterior cortical nucleus of the amygdala in H animals. The results suggest that naturally occurring changes in central TRH systems may be important in the mediation of physiological and behavioral processes that undergo seasonal and state-dependent adjustments in hibernating mammals.