Inhibition of rat spinothalamic tract neuronal responses to noxious skin heating by stimulation in midbrain periaqueductal gray or lateral reticular formation.

The descending inhibitory effects of electrical midbrain stimulation on identified lumbar spinothalamic tract (STT) neurons were investigated in rats anesthetized with sodium pentobarbital. STT units were identified by their antidromic response to stimulation in the contralateral ventrobasal thalamus or medial lemniscus (mean conduction velocity: 18-20 m/sec). Thirty-two of 43 dorsal horn multireceptive STT units gave reproducible responses to noxious heat stimuli (42-56 degrees C, 10 sec) applied to the ventral hind paw. All STT units' responses were suppressed during stimulation (100 msec trains at 100 Hz, 3/sec, 15-300 microA) in PAG or LRF bilaterally. On- and offset of inhibition was rapid (less than 1 sec). STT unit responses to noxious heat were more effectively suppressed by LRF than PAG stimulation, based on statistically significant differences in mean thresholds for inhibition, slopes of current-inhibition plots, and mean current intensities evoking 50% inhibition. Mapping experiments revealed lowest-threshold (less than 25 microA) sites for inhibition to be in ventral PAG, subjacent tegmentum, and LRF bilaterally. STT unit responses increased with graded increases in stimulus temperature from threshold (mean: 46 degrees C) to 56 degrees C. Slopes of temperature-response functions were significantly reduced with little change in threshold during PAG stimulation, whereas these functions were shifted in a parallel manner toward higher temperatures during LRF stimulation. The serotonin antagonist methysergide reduced or blocked PAG- or LRF-evoked inhibition in 3/5 and 5/12 STT units, respectively. These results indicate that STT units are subject to different inhibitory controls from PAG and LRF.