THE MAGNITUDE OF SHOCK-INDUCED ANTINOCICEPTION OBSERVED ON THE TAIL-FLICK TEST IN RATS DEPENDS ON WHERE RADIANT HEAT IS APPLIED. T.W. Prentice, J.W. Grau, and M.W. Meagher, Dept. of Psychology, Texas A&M University, College Station, TX 77843.
Exposure to 3 2-s 3.0-mA tailshocks produces a strong naltrexone-reversible, "opioid", antinociception on the tail-flick test in both spinalized and intact rats. Recently, we had difficulty replicating this basic effect. Systematic studies revealed that the primary variable determining the magnitude of antinociception observed across researchers was where the radiant heat was applied. To characterize this effect, spinalized rats received tail-flick tests at 4 different sites, designated A (proximal) to D (distal). Rats experienced tailshock, or nothing, through electrodes attached at site C. Tail-flick latencies were assessed 2-8 min after shock in a counterbalanced order. Shock produced a significant antinociception at C and D, but not at the more proximal site. The remaining experiments evaluated whether other examples of shock-induced antinociception were similarly sensitive to test location. Exposure to much longer (75-s) 3 mA tailshocks produces a naltrexone-insensitive, "nonopioid", antinociception in spinalized rats. This effect too was only observed at the location where shock was applied and at more distal sites. Exposure to 3 much milder (0.75-s, 1.0 mA) tailshocks produces a forebrain-mediated nonopioid and opioid analgesia in intact rats. This analgesic effect varied in a similar fashion as a function of test location. Finally, 3 longer (25-s) 1.0-mA tailshocks have been shown to produce a brainstem-mediated nonopioid analgesia. This shock schedule produced a significant analgesia at all test locations. Supported by NIMH grant MH 48994 to J.W.G.
Published in Society for Neuroscience Abstracts, 20, 1994, 764.