Lateral reticular formation as a site for morphine- and clonidine-induced hypotension.

The possible role of the lateral reticular formation (LRF) in morphine-induced hypotension was investigated. Morphine (5-10 micrograms) microinjected into 15 of 25 sites in the LRF of anesthetized cats reduced mean (+/- S.E.) arterial pressure by 18.4 +/- 2.1 mmHg. Morphine applied microiontophoretically on LRF neurons had a predominately inhibitory effect on LRF cells, decreasing spontaneous rate in 51%. Since the LRF has been suggested as a site of action for clonidine-induced hypotension, clonidine was also applied microiontophoretically to these neurons. Of the cells evaluated, 43% were inhibited by clonidine. Individual cells typically responded in the same manner to morphine and clonidine. An inhibition of LRF cells by morphine and clonidine is consistent with their hypotensive action and the tonic vasopressive role assigned to the LRF. Collectively, these results suggest that the LRF is a site at which morphine and clonidine may produce their hypotensive effect.