Effects of opiate antagonist treatment into either the periaqueductal grey or nucleus accumbens on heroin-induced locomotor activation.

The role of opiate receptors in the periaqueductal grey (PAG) and nucleus accumbens (NA) in the expression of heroin-induced locomotor activation was investigated. Rats received subcutaneous injections of heroin (0.5 mg/kg) in combination with intra-PAG or intra-NA microinjections of the quaternary opiate antagonist methyl naltrexone (MN) in doses of 0.0 (saline vehicle), 0.3, 1.0 and 3.0 micrograms. When injected into the NA, all three doses of MN were found to attenuate heroin-induced locomotor activity compared to saline vehicle. In contrast, intra-PAG MN treatment did not have any significant effect on heroin-induced locomotor activity. With regard to the NA, these results confirm previous findings showing that blockade of NA opiate receptors attenuates heroin-induced locomotor activity. The lack of effect in the PAG indicates that PAG opiate receptors do not participate in the expression of heroin-induced locomotor activation. The implications of these results for the anatomical overlap found between sites mediating opiate reward and locomotor activation are discussed.