Acute effects of morphine on neurochemical estimates of activity of incertohypothalamic dopaminergic neurons in the male rat.

The effects of an acute injection of morphine on the activities of mesotelencephalic, tuberoinfundibular and incertohypothalamic dopamine (DA) neurons was estimated by measuring: the rate of turnover of DA (decline after alpha-methyltyrosine); and the concentration of the DA metabolite, dihydroxyphenylacetic acid (DOPAC), in brain regions containing cell bodies or terminals of these neurons (i.e. nucleus accumbens, striatum, median eminence and various hypothalamic nuclei). The rate of turnover of DA and the concentration of DOPAC were increased in nucleus accumbens and striatum and decreased in the median eminence 60 min after the administration of morphine (10 mg/kg, s.c.). Morphine increased the rate of turnover of DA and the concentration of DOPAC in brain regions containing both cell bodies (periventricular nucleus and medial zona incerta) and terminals (medial preoptic, preopticosuprachiasmatic and dorsomedial nuclei) of incertohypothalamic DA neurons. The effects of morphine in all brain regions were blocked by pretreatment with naltrexone. These results indicate that incertohypothalamic DA neurons are stimulated by the acute administration of morphine, and in this respect they resemble the extrahypothalamic mesotelencephalic DA neurons rather than hypothalamic tuberoinfundibular DA neurons.