Effects of thyroid status on presynaptic?2-adrenoceptor function and?-adrenoceptor binding in the rat brain

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Abstract
The effect of thyroid status on noradrenergic synaptic function in the mature rat brain was examined by measuring presynapticα2- and post-synapticβ-adrenoceptors. Repeated triiodothyronine (T3) administration to rats (100μg/kg×14 days: hyperthyroid) caused an 18% increase in striatalβ-adrenoceptors as shown by [3H]-dihydroalprenolol binding with no change in membranes from cerebral cortex or hypothalamus. In contrast, hypothyroidism (propyl-thiouracil, PTU×14 days) produced significant 12% and 30% reductions in striatal and hypothalamicβ-adrenoceptors respectively with no change in the cerebral cortex. Presynapticα2-adrenoceptor function was measured in the two dysthyroid states using the clonidine-induced hypoactivity model. Experimental hyperthyroidism increased the degree of clonidine-induced hypoactivity, and suggests increased presynapticα2-adrenoceptor function compared with control rats, whereas hypothyroidism suppressed presynapticα2-adrenoceptor function. These results show firstly that changes in thyroid status in the mature rat may produce homeostatic alterations at central noradrenergic synapses as reflected by changes in pre- and postsynaptic adrenoceptor function. Secondly, there appear to be T3-induced changes inβ-adrenoceptors in the striatum where changes in dopaminergic neuronal activity have previously been demonstrated.