Modulation of antinociceptive responses following tail pinch stress

Mild, non-noxious, oscillating pinches to a rat's tail elicits hyperphagia. The present study examined whether tail-pinch (TP) would exert hyperalgesic and hyperactive effects in rats that also exhibit the overeating response. The first experiment assessed TP effects upon reactivity to electric shock as measured by flinch-jump thresholds. Significant decreases in jump thresholds were observed 0 and 15, but not 30, min following TP. This effect persisted regardless of whether food was present or absent during TP. The second experiment assessed TP effects upon reactivity to heat as measured by hot-plate latencies. In contrast to jump thresholds, the shortened hot-plate latencies observed following TP persisted into the recovery period. In examining TP effects upon activity levels (Experiment 3), it was found that animals display similar patterns of temporally-declining activity regardless of whether TP was administered or not. Finally, TP selectively decreased the analgesic responses to two different doses of morphine and two different cold-water swim temperatures (Experiment 4). The TP-induced reductions occurred when TP was administered either before or after the analgesic manipulation. These data are discussed in terms of the nociceptive selectivity of the TP effect, and its influences upon analgesic processes.