Behavioural and biochemical effects in C57BL/6J mice after a prolonged treatment with the δ‐opiate antagonist ICI 154129

A long term treatment with the δ‐selective opiate antagonist NN‐bisallyl‐Tyr‐Gly‐Gly‐ψ‐(CH2S)‐Phe‐Leu‐OH (ICI 154129) produces an increase in the number of δ‐opiate binding sites, whereas the same treatment with the non selective opiate antagonist naloxone results in an enhancement of both μ‐ and δ‐binding sites. This biochemical effect in naloxone‐pretreated mice is paralleled by a more pronounced increase in locomotor activity induced by a challenge dose of morphine. In contrast, no difference in the effect of morphine was seen in ICI 154129‐pretreated mice with respect to control. These data suggest that the locomotor response to morphine in C57 mice is not mediated through δ‐opiate receptors. 1984 Royal Pharmaceutical Society of Great Britain