Abstract
The aim of the present experiment was to evaluate the hypothesis that raising and lowering the level of brain serotonin (5-HT) modified somesthetic thresholds to painful stimuli. Moreover, since 5-HT has been postulated to vary in relation to the states of vigilance, a second purpose was to determine whether endogenous 5-HT variations and experimental manipulations would have cumulative effects in determining thresholds. 5-HT was raised and lowered in cats by IP or SC injections of DL-Tryptophan (TRY) or parachlorophenylalanine (PCPA) respectively. Painful electric shocks were delivered SC and thresholds were evaluated which produced successively more complex motor responses (orienting, righting and escape responses) in sleeping (slow wave and paradoxical sleep) and waking cats. The results confirm the first but not the second hypothesis in that thresholds were indeed modified by the pharmacological manipulations, response thresholds being lower for PCPA treated animals and higher for TRY injected animals when compared to control animals. No significant interaction effects between states of vigilance and treatment condition were, however, evident.
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