Hypothyroidism inhibits the formation of inositol phosphate in response to carbachol in the striatum of adult rat.

The effects of hypothyroidism on the muscarinic cholinergic receptor-coupled inositol phospholipid hydrolysis in the adult rat brain were examined. Tissue slices of striatum, hippocampus, and cortex from either euthyroid or hypothyroid rats were labeled with [3H]myoinositol and incubated with carbachol, a muscarinic cholinergic agonist. In other experiments, crude plasma membranes of each brain region obtained from either euthyroid or hypothyroid rats were incubated with [3H]N-methylquinuclidinyl benzilate ([3H]NMeQNB), a muscarinic cholinergic antagonist, in the presence or absence of atropine. Carbachol produced a significant increase in [3H]inositol phosphate ([3H]IP) formation in each brain region in a dose dependent manner. Hypothyroidism caused a marked decrease in carbachol-stimulated [3H]IP formation in the striatum, whereas it did not affect the formation of [3H]IP in the cortex or hippocampus. In contrast, the affinity constant and the maximal binding of [3H]NMeQNB to plasma membranes in these regions were not changed by hypothyroidism. The present results suggest that thyroid hormones might participate in regulating the muscarinic cholinergic neurotransmission in the striatum of adult rat.