Morphine and endorphins modulate dopamine turnover in rat median eminence.

Abstract

Some actions of endogenous and exogenous opioids (e.g., stimulation of prolactin release) are mediated by interaction with catecholaminergic systems. Morphine (1.67, 5 and 15 mg/kg of body wt, i.p.) altered dopamine turnover as measured by the .alpha.-methyl-p-tyrosine method in the median eminence, neostriatum and frontal cortex of male Sprague-Dawley rats. The turnover rate of dopamine was reduced in the neostriatum. In the frontal cortex all doses were effective in decreasing dopamine turnover; in the median eminence the lowest dose of morphine did not alter dopamine turnover. All 3 doses accelerated dopamine turnover in the neostriatum. Naloxone effectively reversed the effects of morphine at all doses in all brain areas, but did not effect turnover when given alone. In the median eminecne, neostriatum and frontal cortex intraventricular injection of [D-Ala2,D-Leu5]-enkephalin (25 .mu.g) or .beta.-endorphin (15 .mu.g) produced the same effects on dopamine turnover as morphine. The actions of peptides were blocked by naloxone. Apparently opiates and opioid peptides increase prolactin release by reducing the activity of the tuberoinfundibular dopaminergic system.