Delta-opioid receptor gene expression in the mouse forebrain: localization in cholinergic neurons of the striatum

Opioid peptides and opioid receptors, particularly the delta receptor, are abundant in the striatum where they contribute to the neuronal interactions, and are involved in various behavioral effects. The recent cloning of the delta-opioid receptor now allows the identification of the striatal neurons that express it, and that are direct targets of endogenous opioid peptides such as enkephalins. In this context, we have used in situ hybridization histochemistry to determine the distribution of the delta-opioid receptor messenger RNA in the forebrain, and especially the phenotype of the neurons expressing the delta-opioid receptor gene in the striatum. We show that the topgraphy of the neurons containing the delta-opioid receptor messenger RNA is similar to the topography of the neurons containing the choline acetyltransferase messenger RNA in the mouse forebrain. Comparison of adjacent serial sections demonstrates that the delta-opioid receptor gene is indeed expressed exclusively in cholinergic interneurons in the striatum. As these neurons also selectively express the substance P receptor gene, our data suggest that the striatal cholinergic interneurons are a common link in the interactions between the two striatal efferent populations, namely enkephalin and substance P neurons.