Estradiol, TRH and striatal dopaminergic mechanisms

1. Estrogens have been shown to decrease the effect of apomorphine in a variety of animal behavioral models reflecting the sensitivity of striatal and mesolimbic dopamine receptors. 2. These include circling, and locomotor activity, in rats and suppression of midbrain tremor as well as lingual dyskinesia in monkeys. 3. Estradiol also increases the haloperidol-induced catalepsy in rats. Moreover estradiol increases 3H spiroperidol specific binding in the rat striatum and potentiates the increase caused by haloperidol or denervation with 6-hydroxydopamine. 4. These findings point to an action of estradiol similar to a week neuroleptic. 5. Thyrotropin-releasing hormone when injected into the head of the caudate nucleus in cats induces a head turning response which may be ipsilateral or contralateral depending upon the injection site. The response is similar to the effect of dopamine injected into the same site. 6. The effect of dopamine but not that of TRH is blocked by prior administration of haloperidol indicating that although TRH has a dopamine-like action in the caudate nucleus, it is not mediated via the dopamine terminals or the dopamine receptors.