Preproenkephalin mRNA-expressing neurones in the rat thalamus.

We investigated the expression of preproenkephalin mRNA in the rat thalamus by in situ hybridization histochemistry using a radiolabelled full-length cRNA probe. Enkephalinergic neurones were present in the ventral thalamus, epithalamus, and intralaminar and midline nuclei of the dorsal thalamus. The largest number of labelled cell bodies was found in the zona incerta, the fields of Forel, and the ventral lateral geniculate, thalamic reticular, central medial and central lateral nuclei. Sparse labelling was present in the parafascicular, paracentral, paraventricular, and lateral habenular nuclei. The principal nuclei of the dorsal thalamus were unlabelled. The distribution of the enkephalinergic neurones suggests an involvement in arousal and attentional mechanisms, being in line with the sedative effects of exogenous opioids.