Reduction by beta-endorphin of efflux of dopamine and DOPAC from superfused medial basal hypothalamus

The effects of β-endorphin (β-EP) and naloxone on the efflux of dopamine (DA) and its specific deaminated metabolite, dihydroxyphenylacetic acid (DOPAC), from superfused rat medial basal hypothalamus (MBH) were determined. After an equilibration period of 2.5 hrs with Medium 199 at 37°C 0.5 ml fractions were collected. Injection of β-EP, but not medium, into the system significantly reduced (P<0.05) the effluent concentrations of both DA and DOPAC from female MBH's (N=5) by 71% and 55% respectively and from male MBH's (N=7) by 68% and 49% respectively. Conversely, administration of naloxone significantly increased DA and DOPAC efflux by 85% and 148% respectively in females and by 132% and 120% in males. When an equimolar mixture of β-EP and naloxone was injected, no detectable changes in DA and DOPAC efflux were found. The effects of β-EP and naloxone were demonstrable irrespective of the sequence of administration. We conclude that β-EP exerts a tonic inhibitory effect on tuberoinfundibular DA neurons by interaction with naloxone-sensitive opiate receptors. This action does not require the intermediation of brain centers outside the MBH.