DESCENDING ANTINOCICEPTIVE PATHWAY FROM THE ROSTRAL VENTROLATERAL MEDULLA - A CORRELATIVE ANATOMICAL AND PHYSIOLOGICAL STUDY

Microinjections of the excitatory amino acid L-glutamate were made int o the rostral ventrolateral medulla (RVLM) of anesthetised cats, to ma p the sites at which selective stimulation of cell bodies elicited a s ignificant antinociceptive response (greater than or equal to 15% inhi bition of the increase in L(7) ventral root activity reflexly evoked b y stimulation of C-fiber afferents). Antinociceptive sites were largel y confined to the RVLM subregion ventromedial to the retrofacial nucle us, extending from the caudal pole of the facial nucleus to the level approximately 2.5 mm more caudal. Increases in arterial pressure were also elicited from some sites in the RVLM, but these were mainly later al to the antinociceptive sites. In a second series of experiments, rh odamine labeled microspheres or cholera toxin B-gold (CTB-gold) were i njected into the dorsal horn of the L(7) segment. In three of these ex periments in which the injection sites were restricted to the dorsal h orn, retrogradely labeled cells in the caudal pens and medulla were vi rtually all within either the nucleus raphe magnus or the RVLM. Furthe rmore, the labeled cells in the RVLM were virtually confined to a disc rete group located just ventromedial to the retrofacial nucleus, i.e. within the antinociceptive region as mapped by glutamate microinjectio n. The results of the present study indicate that antinociceptive effe cts are elicited by stimulation of a subregion in the RVLM, which is l ocated medial to the presser region. Further, the antinociceptive effe cts may be mediated, at least in part, by cells projecting directly to the dorsal horn in the spinal cord.