CNS GABA neurons express the μ-opioid receptor: Immunocytochemical studies

IT has been proposed that μ-opioid receptors excite neurons in hippocampus and nucleus raphe dorsalis (NRD) by decreasing GABAergic tone. In the present study, we examined whether immunocytochemical evidence of interaction between GABAergic neurons and the μ-opioid receptor could be found in the CNS. Portions of rat brain were sectioned and stained for GABA and for the cloned μ-opioid receptor (MOR1) using two-color immunofluorescence. Neurons double-labeled for GABA and MOR1 were present in hippocampus and NRD, as well as in olfactory bulb, dorsal lateral periaqueductal gray matter, nucleus raphe medianis, nucleus raphe obscurus, and the spinal trigeminal nucleus and tract. We conclude that expression of the μ-opioid receptor by GABAergic neurons is common in the rat CNS.