Chronic changes in thyroid hormones do not affect brain adenosine receptors.

1. In this study, the authors examined the effects of chronic (14 days) changes in thyroid function on a major neuromodulatory receptor system in the brain- the adenosinergic system. While previous investigators have examined the effects of alteration in thyroid function on adenosine receptors in peripheral tissues (adipocytes), this is the first study to examine such effects in brain. 2. Three groups of male Sprague-Dawley rats were treated for 14 days with either a) oral PTU (0.00625%), iodine-free diet, and i.p. saline injections, b) i.p. saline injections, or c) i.p. triiodothyronine (25 micrograms/100 g) injections. 3. These manipulations reliably resulted in the production of hypothyroidism (TSH 30.2 +/- 8.6 ng/ml), euthyroidism (TSH 2.1 +/- 0.9), and hyperthyroidism (TSH < 0.4). 4. Treatment had no significant effect on the Bmax or Kd of [3H]DPCPX (A1-antagonist) binding to homogenates from cerebral cortex, cerebellum or hippocampus; similarly, no effect on [3H]CGS-21680 (A2-agonist) binding to striatal homogenates was noted. 5. Similarly, quantitative autoradiographic studies failed to reveal consistent regional alterations unique to either hypo- or hyperthyroidism. 6. Incubation of sections with GppNHp resulted in the expected reduction (approximately 40%) in agonist binding, but there was no differential effect seen for either the hypo- or hyperthyroid tissues. 7. These preliminary findings suggest that alterations in brain adenosine receptors or G-protein-receptor coupling are unlikely to be requisite correlates of abnormal thyroid hormone levels.