Apparent hyperalgesia in the mouse tail-flick test due to increased tail skin temperature after lesioning of serotonergic pathways.

The relationship between tail skin temperature and responsiveness to noxious radiant heat in the tail-flick test was investigated in mice. A significant negative correlation between tail skin temperature and tail-flick latency was found when the tail skin temperature was increased by elevating the ambient temperature. After intracerebroventricular injection of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT, 80 micrograms) tail skin temperatures were increased and tail-flick latencies reduced. In contrast, administration of the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA, 400 mg kg-1 for 5 consecutive days) lead to a slight lowering of tail temperatures and a tendency towards elevation of tail-flick latencies. The results show that factors which affect tail skin temperature also influence the tail-flick test in mice. The divergent effects of 5,7-DHT and PCPA on tail-flick responsiveness may be due to the different effects of these compounds on the tail skin temperature. The results suggest that the reduced tail-flick latency after partial destruction of serotonergic pathways by 5,7-DHT is due primarily to the increased tail skin temperature. The dependence of tail-flick latency on tail skin temperature limits the usefulness of the tail-flick test unless changes in tail skin temperature are controlled for.