Opioid modulation of the gamma-aminobutyric acid-controlled inhibition of exercise-stimulated growth hormone and prolactin secretion in normal men

The possible involvement of endogenous opioids in the gamma-aminobutyric acid-controlled (GABAergic) inhibition of growth hormone (GH) and prolactin (PRL) during physical exercise was evaluated in normal men. After fasting overnight, seven subjects were tested on four mornings at least 1 week apart. Exercise was performed on a bicycle ergometer. The workload was gradually increased at 3-min intervals until exhaustion and lasted about 15 min in all subjects. Tests were carried out under administration of placebo, the opioid antagonist naloxone (10 mg as an iv bolus injection), the GABAergic agonist sodium valproate (600 mg in three divided doses orally) or naloxone plus sodium valproate