Melanotropic activity in extrahypophyseal regions of rodent brain: effect of age, hormones, and drugs

Abstract

Two or more melanotropic peptides are present in extrahypophyseal regions of mammalian brain. Previous studies showed that extrahypophyseal melanotropic activity is not influenced by hypophysectomy, adrenalectomy, or exogenous glucocorticoid. The present study investigated the possible influence of the following factors on the level of melanotropic activity in whole brain, cerebral cortex, cerebellum, midbrain, and brainstem of mouse and rat: age, sex, starvation; and of the following hormones or drugs administered by the intraperitoneal or intracerebral route: norepinephrine, dihydroxyphenylalanine, pargyline, 6-hydroxy-dopamine, alpha-methyltyrosine methyl ester, reserpine, acetylcholine, pilocarpine, atropine, serotonin, p-chlorophenylalanine, pentobarbital, pentylenetetrazol, insulin, melatonin, and cycloheximide. Only age influenced extrahypophyseal melanotropic activity. The activity per unit of tissue wet weight or of tissue protein increased in all regions progressively from birth to 1 yr of age. Extrahypophyseal melanotropic activities perunit wet weight of tissue at 50 wk averaged 4.3 times those at birth. When brain of adult rodents was fractionated by differential centrifugation, the major proportion of melanotropic activity was recovered in myelin (27-35 percent), nerve endings (20-22 percent), and mitochondria (25-30 percent). The lower activity in newborn brain resulted not onlyfrom absence of a myelin fraction, but also from lower activity at birth in nerve endings and mitochondria.