Enhancement of stress-induced increase in hypothalamic noradrenaline turnover by pretreatment with naloxone in rats.

The relationship between the increase in noradrenaline (NA) turnover and endogenous opioid peptides in the hypothalamus under stressful situations was investigated in rats using a specific antagonist of opioids, naloxone. Male Wistar rats were injected subcutaneously with saline, naloxone at 1 mg/kg or 5 mg/kg 10 min before exposure to immobilization stress for 1 hour. Levels of NA and its major metabolite, 3-methoxy-4-hydroxyphenylethyleneglycol sulfate (MHPG-SO4) in the hypothalamus were determined fluorometrically. Immobilization stress caused both significant decrease in NA level and increase in MHPG-S04 level in rats treated with saline, naloxone at 1 mg/kg and 5 mg/kg in comparison to controls. In spite of the finding that 5 mg/kg of naloxone by itself significantly reduced hypothalamic NA level, pretreatment with 5 mg/kg of the drug significantly enhanced both the decrease in NA level and increase in MHPG-S04 level induced by stress. Enhancement of the stress-induced increase in the hypothalamic NA turnover is believed to be due to blockade of opiate receptors by the drug. These results suggest that endogenous opioid peptides in the hypothalamus might be partially involved in stress process by attenuating the increase in NA turnover induced by stress.