Influence of opioid system on behavioral sensitization induced by cocaine in the rat

In rats, repeated daily ip administrations of a low dose (10 mg/kg) of cocaine HCl (COC) produced enhanced locomotor activity; a higher dose (50 mg/kg) led to seizures (pharmacological kindling). Daily pretreatment with naltrexone (NTX) (2 mg/kg/sc), injected 15 min prior to COC, completely antagonized COC-induced motor and convulsant activity, except on the first day when a potentiation of seizures was observed. Using the same treatment schedule, COC 50 mg/kg failed to induce sensitization to the well known local anesthetic properties evaluated through the loss of abdominal tone; this effect was not reversed by NTX pretreatment. These findings provide further evidence of the modulatory role of the opiate system on cocaine sensitization.