Localization of central cannabinoid CB1 receptor messenger RNA in neuronalsubpopulations of rat dorsal root ganglia: A double-label in situ hybridization study

In situ hybridization histochemistry was used to show the distribution of m essenger RNA for central cannabinoid CB1 receptors in dorsal root ganglia o f the rat. CB1 messenger RNA was highly expressed in neuronal subpopulation s of rat dorsal root ganglia. The phenotypes of neurons that express messen ger RNA for CB1 were subsequently examined by combining a S-35-labeled ribo nucleotide probe for CB1 messenger RNA with digoxigenin-labeled riboprobes for preprotachykinin A (substance P precursor), alpha-calcitonin gene-relat ed peptide and preprosomatostatin (somatostatin precursor) messenger RNAs. Qualitative examination revealed expression of CB1 messenger RNA predominan tly in medium- and large-sized cells distributed throughout the dorsal root ganglia. The majority of neurons expressing substance P messenger RNA were CBI messenger RNA negative and smaller in size than the CB1 messenger RNA- positive cells. Only 13% of substance P messenger RNA-positive cells expres sed CB1 messenger RNA. A similar degree of co-localization was observed wit h alpha-calcitonin gene-related peptide: 10% of cells expressing messenger RNA for this neuropeptide were CB1 messenger RNA positive. Colocalization o f CB1 and somatostatin messenger RNAs was observed in less than 0.5% of som atostatin messenger RNA-positive cells. The data suggest that subpopulation s of neurons in rat dorsal root ganglia are capable of synthesizing cannabi noid receptors and inserting them on terminals in the superficial dorsal ho rn. These findings provide anatomical evidence for cannabinoid modulation of pr imary afferent transmission. Although an anatomical basis for cannabinoid-m ediated suppression of release of neurogenic peptides from nociceptive prim ary afferents is provided, our results demonstrate that the majority of CB1 messenger RNA-positive neurons in the dorsal root ganglia contain transmit ters and/or neuromodulators other than the neuropeptides examined herein.