[Met5]Enkephalin content in brain regions of rats treated with lithium.

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Abstract
In rats, chronic treatment with Li elicits a dose-dependent increase in the [Met5]enkephalin content of nucleus caudatus and globus pallidus. A single injection of Li fails to change the striatal [Met5]enkephalin content. The increase in [Met5]enkephalin caused by chronic Li is proportional to the serum Li level. The extent of the increase in striatal [Met5]enkephalin content levels off at a value of about 250% that of untreated rats. This increase has a time latency of 2-3 days and reaches a plateau at 5 days. The increase that was present at 5 days was no longer evident if the treatment was continued for 2 wk. Li also increases striatal [Leu5]enkephalin content by an extent equal to the increase of [Met5]enkephalin. Based on the characteristics of the Li-induced increase in [Met5]enkephalin content, Li may reduce the rate of release of [Met5]enkephalin from the small enkephalinergic neurons that are intrinsic to the striatum; this action may be related to a change in the regulation of striatal neurons.
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