Plasma catecholamine and cardiovascular responses to morphine and D-ala2-d-leu5-enkephalin in conscious rats.

The effects of morphine and DADL on cardiovascular parameters and plasma catecholamine levels were studied in conscious unrestrained rats. Morphine 30 micrograms icv increased blood pressure over the 3 hr recording period, produced an initial bradycardia followed by a tachycardia and increased plasma epinephrine and norepinephrine levels 20- and 2-fold respectively. These responses were not altered by systemic naloxone 0.8 mg/kg ia or naloxone injected icv 110 micrograms. In adrenalectomized rats blood pressure, heart rate and plasma norepinephrine responses were not significantly altered. Lower doses of morphine 1, 3 and 10 micrograms icv produced dose-related increases in plasma catecholamines, in the absence of any signs of respiratory depression, which were antagonized by naloxone 110 micrograms icv. 1m Morphine 10 mg/kg produced effects similar to those of icv morphine and these were readily antagonized by naloxone 0.8 mg/kg ia. DADL 10 micrograms icv also produced effects similar to those of morphine but of a shorter duration, and these were also inhibited by naloxone 0.8 mg/kg ia. The results are consistent with an action of opiates on a specific opiate receptor in the brain mediating an increase in catecholamine release. The rise in blood pressure may in part be a consequence of the increase in circulating catecholamines.