Dissociation of antinociceptive from cardiovascular effects of stimulation in the lateral reticular nucleus in the rat.

The lateral reticular nucleus (LRN) in the caudal ventrolateral medulla has been implicated in the regulation of spinal nociceptive transmission and hemodynamics. Experiments were undertaken to examine the relationship between inhibition of the tail flick reflex and cardiovascular effects produced by electrical stimulation in the LRN in rats lightly anesthetized with pentobarbital. Intensity- and frequency-dependent increases in mean arterial pressure and vascular resistance in the hindquarter, mesenteric, renal and caudal arterial beds were observed. Inhibition of the tail flick reflex, however, occurred at intensities of electrical stimulation which produced no significant changes in mean arterial pressure or vascular resistance in any of the arterial beds studied. Selective stimulation of cell bodies in the LRN by microinjection of glutamate similarly inhibited the tail flick reflex but produced significant reductions in mean arterial pressure, without substantially affecting regional vascular resistances. These results suggest that the antinociceptive and depressor effects of stimulation in the LRN are mediated by activation of cell bodies, while pressor effects produced by focal electrical stimulation are mediated by activation of fibers of passage. The descending inhibition produced by stimulation in the LRN is independent of stimulation-produced cardiovascular responses.