Long-lasting visceral pain was produced in rats by the intraperitoneal injection of dilute acetic acid, and the response to this stimulus measured by observing the number of writhes over a subsequent 50 min. period. Prior treatment of the animals with naloxone or saline showed that naloxone increased the frequency of writhing. The enhancement by naloxone of the response to this nociceptive stimulus suggests that the binding of an endogenous opioid to the opiate receptors increases during prolonged nociceptive stimulation. © 1977.
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