Enkephalins induce a centrally mediated rise in blood pressure in rats

The cardiovascular effects of enkephalins after administration directly into the nucleus tractus solitarii (NTS) of urethane anesthetized rats were investigated. Unilateral microinjection of Met-enkephalin and its stable analogue D-Ala2-Met5-enkephalin resulted in a dose-related rise in mean arterial pressure which in the case of the analogue was accompanied by tachycardia. The elevation in blood pressure was anatomically specific and restricted to the intermediate third of the NTS, as verified histologically. These cardiovascular changes were prevented by pretreatment with locally applied naloxone (10 ng). A similar pressor effect was obtained with Leu-enkephalin. Antiserum to Met-enkephalin (1:50 dilution) caused a fall in blood pressure on injection into the NTS, and completely blocked the pressor response and tachycardia induced by D-Ala2-Met5-enkephalin. These results suggest that enkephalins have a pressor role in the central nervous system.