The effects of nociceptive stimuli on the metabolism of brain histamine (HA) were examined, because of a previous observation that the exposure of mice to electric foot shock increased the brain HA turnover. In mice, the exposure to tail pinch markedly increased the brain level of te/e-methylhistamine (t-MH), a predominant metabolite of brain HA, while the level of HA was not changed. The HA turnover, measured either by the accumulation of t-MH after pargyline injection or by the HA depletion after the treatment with a-fluoromethylhistidine, a specific inhibitor of histidine decarboxylase, was enhanced by the exposure to tail pinch, like the enhancement produced by the exposure to foot shock. The exposure of rats to tail pinch increased the t-MH level in the telencephalon and the midbrain. Other types of noxious stimuli, such as placing mice on a hot plate or subjecting mice to acetic acid-induced writhing, also significantly elevated the level of t-MH but not that of HA in the mouse brain. These results suggest that nociceptive stimuli produce an increase in the brain HA turnover.
