Localization of mu-opioid receptors on amygdaloid projection neurons in the parabrachial nucleus of the rat.

The parabrachial nucleus (PB) is a major relay of noxious and non-noxious visceral sensory information from the nucleus of the solitary tract, spinal cord, and spinal trigeminal nucleus to the forebrain. The nucleus of the solitary tract, spinal cord, and trigeminal dorsal horns contain many enkephalin- and dynorphin-immunoreactive neurons that project to the PB. To study the role of mu-opioid receptors in relaying these inputs, we examined the distribution of mu-opioid receptor immunoreactivity in the PB. The most intense staining was in the external lateral parabrachial subnucleus (PBel), including dendrites extending from the PBel into the lateral crescent subnucleus. Because the Pbel is a major source of projections to the amygdala, we combined retrograde tracing from the central nucleus of the amygdala with immunohistochemistry for mu-opioid receptors. These experiments showed that mu-opioid receptors are expressed by Pbel neurons that project to the amygdala, including those Pbel neurons whose dendrites extend into the lateral crescent subnucleus. These results indicate that mu-opioid receptors in the PB may mediate or modulate nociceptive information relayed to the amygdala from medullary or spinal cord neurons that terminate not only in the Pbel, but also in the adjacent lateral crescent parabrachial subnucleus.