PREPROENKEPHALIN MESSENGER-RNA-EXPRESSING NEURONS IN THE RAT THALAMUS

WE investigated the expression of preproenkephalin mRNA in the rat tha lamus by in situ hybridization histochemistry using a radiolabelled fu ll-length cRNA probe. Enkephalinergic neurones were present in the ven tral 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 gen iculate, thalamic reticular, central medial and central lateral nuclei . Sparse labelling was present in the parafascicular, paracentral, par aventricular, and lateral habenular nuclei. The principal nuclei of th e dorsal thalamus were unlabelled. The distribution of the enkephaline rgic neurones suggests an involvement in arousal and attentional mecha nisms, being in line with the sedative effects of exogenous opioids.