Kappa opioid agonists alter dopamine markers and cocaine-stimulated locomotor activity

To better understand the influence of kappa -opioid agonists on the effects of cocaine, rats were treated with daily injections of the selective ic-op ioid agonist U-69593 or bremazoeine. In combination with 10 mg/kg cocaine, both compounds, at a dose of 0.32 mg/kg, greatly diminished locomotor activ ity, and these effects were maintained over a period of 5 days. In addition , the response to a challenge injection of 10 mg/kg cocaine several days af ter the end Of kappa -opioid agonist treatment with or without cocaine was markedly reduced. When naltrexone was given in combination with U-69593, it blocked the reduction in cocaine-induced locomotor activity after U-69593 treatment alone. However, a single injection of either kappa -opioid agonis t alone had no effect on cocaine-induced locomotion several days later (i.e . no long-term effects), suggesting that multiple injections of the kappa - opioid agonist are needed to reduce the locomotor activating effects of coc aine other than acutely. In addition, treatment with the kappa -opioid agon ist U-69593 (0.32 mg/kg) over a 5-day period decreased dopamine transporter densities in the caudate putamen, and this was also blocked by co-administ ration of naltrexone. (C) 2001 Lippincott Williams & Wilkins.