The acute and chronic effects of naloxone on motor activity was studied in rats. No difference in motor activity was noted between naloxone-treated (2.5, 5.0, 10.0 mg/kg) and saline-treated rats on the first day of testing. By contrast, naloxone, in all three doses tested, blocked the increase in motor activation noted in saline-treated animals upon repeated testing. These data may suggest that endogenous opioid systems may play a role in behavioral activation and adaptation and provide further support to the proposed involvement of endogenous ligands for the opiate receptor in emotional reactivity. © 1979.
Amir, S., Solomon, M., & Amit, Z. (1979). The effect of acute and chronic naloxone administration on motor activation in the rat. Neuropharmacology, 18(2), 171–173. https://doi.org/10.1016/0028-3908(79)90058-3