None of these behaviors were induced by the naloxone challenge. Although this test suggested that the rewarding effect of morphine in these animals was not accompanied by physical dependence, it was possible that undetected dependence was sufficient to motivate drug-taking behavior. Morphine was also infused into the periventricular gray substance and the ventricular space dorsal to this region. Some dependence signs were also seen after infusions into the ventral tegmentum, which lies 2 mm ventral to the injection site in the periventricular gray. To assess this possibility, additional animals were tested with cannulas angled to avoid penetration of the periventricular gray.
Error bars depict standard errors of the means.
Acknowledgements Naloxone hydrochloride was a gift of the Endo Laboratories. We thank M. Asselin for her technical assistance. Rats were tested in three 4-hour sessions. Animals in the yoked control condition rarely pressed the lever. Other subjects were tested after long-term infusions into the dorsal aspect of the third ventricle to determine if the dependence resulting from periventricular gray infusions were produced by ventricular diffusion.