Localized effects of naloxone on local cerebral glucose utilization in rat cerebral nuclei with Met-enkephalinergic neurons.

The alterations of local glucose utilization in 101 cerebral nuclei following the subcutaneous administration of the specific opioid antagonist naloxone were measured using the quantitative autoradiographic 14C-deoxy-D-glucose technique in conscious rats. Met5-enkephalin-like immunoreactivity (MELI) and ME receptor binding (MERB) levels in 82 cerebral nuclei were assayed quantitatively by microdensitometry of fluorescence micrographs and autoradiographs of the brain slices. Naloxone administration significantly reduced glucose utilization rate in 18 lower brainstem nuclei including the n. tractus solitarii, n. dorsalis nervi vagi, substantia grisea centralis, n. parabrachialis dorsalis and ventralis, and n. interpeduncularis. These nuclei contained ME perikarya with high levels of MELI and MERB. However, naloxone did not alter glucose utilization of other lower brainstem nuclei containing ME neurons and all thalamic, epithalamic, hypothalamic and limbic nuclei with ME perikarya. The distribution and magnitude of the neuronal response of cerebral nuclei to naloxone were apparently not related to the distributions of ME neurons. The present study offers direct evidence for the selective action of naloxone per se in some lower brainstem nuclei with ME perikarya.