Role of cholinergic systems in pain modulation: I. Impact of scopolamine on environmentally induced hypoalgesia and pain reactivity.

Scopolamine was found to block both brief shock-induced (3 0.75-s, 1.0-mA shocks) and conditioned hypoalgesia on the tail-flick test in rats. The drug also produced a general increase in pain reactivity as measured by both the tail-flick test and shock-induced vocalization. It was shown that this hyperalgesia cannot account for the effect of the drug on brief-shock or conditioned hypoalgesia. Scopolamine did not block the nonopioid analgesia observed after long shock (3 25-s, 1.0-mA shocks). When the effect of the drug on baseline levels of pain reactivity was controlled, it potentiated long shock-induced hypoalgesia. Scopolamine also increased reactivity to tactile stimulation, which suggests the hyperalgesia reflects a general increase in arousal. None of these effects were observed with methylscopolamine, which suggests they are not peripherally mediated.