Lack of effects of thyroid hormones on muscarine cholinergic receptors in rat brain and heart.

The muscarinic cholinergic antagonist 3H-quinuclidinyl benzilate was used to study muscarinic cholinergic receptors in the brain and the heart from triiodothyronine, thyroxine and 6-propyl-2-thiouracil treated rats. Scatchard analysis of the saturation binding studies revealed for each rat in both brain and heart tissue a single group of muscarinic cholinergic receptor binding sites of high affinity. The density and the affinity of muscarinic cholinergic receptors in brain and heart homogenates from treated rats were not different from values obtained in control rats with the exception of triiodothyronine treated rats which showed a slightly but significantly increased equilibrium dissociation constant in the brain homogenates. Furthermore, we observed that the density of the muscarinic cholinergic receptors in the brain was significantly lower in the month of August as compared to March-April, which suggests a seasonal variation. No similar change was observed in heart homogenates. Our results suggest that cardiovascular and central nervous symptoms in patients with thyroid diseases cannot be ascribed to changes in muscarinic cholinergic receptors.