Effects of thyroid state on brain development: muscarinic acetylcholine and GABA receptors

A study was made of the effects of age, neonatal hypothyroidism and hyperthyroidism on the development in rat brain of muscarinic cholinergic and GABA receptors. The former receptors were estimated by the binding of [3H]quinuclidinylbenzilate and the latter by binding of [3H]muscimol to crude membrane preparations from the forebrain and the cerebellum. In the normal forebrain, the density of muscarinic cholinergic receptors (in terms of unit membrane proteins) doubled during the period 6-35 days after birth. Thyroid state had relatively little effect on this development. In contrast, in the normal cerebellum the peak of the density of the receptors was attained in the early neonatal period followed by a progressive decline reaching about half of the maximum at day 35. Furthermore, in the cerebellum this development was significantly influenced by thyroid disorders: the rate of decrease in receptor density was accelerated by hyperthyroidism and retarded in thyroid deficiency. In comparison with euthyroid rats, the density of muscarinic receptors in the cerebellum was 30% lower in the hyperthyroidism (at day 21) and 40% higher in thyroid deficiency (at day 35). The increase in the density of GABA receptors with age was very small in the normal forebrain relative to the marked rise in the cerebellum. In the forebrain, thyroid state had no significant effect on this development. In contrast, in the cerebellum the ontogenesis of GABA receptors was advanced by thyroid hormone treatment and retarded in thyroid deficiency. However, by day 35 receptor density was normal in both conditions. Thyroid state had no significant influence on the affinity of either [3H]muscimol or the [3H]quinuclidinylbenzilate binding. The results suggest that thyroid hormone disorders during early life may lead to distortions rather than synchronized shifts in the relative development of several central transmitter systems.