Neonatal and preweanling rats are able to express short-term behavioral sensitization to cocaine

Abstract

The present study assessed the ability of suckling rats to express short-term behavioral sensitization to cocaine prior to weaning. Rat pups, aged either 3, 5, 10, 12, 17 or 19 days at the beginning of the experiment, were placed in a chamber after daily injection with cocaine (7.5 or 15 mg/kg, i.p.) for either 2 or 4 consecutive days, and were tested for behavioral responsiveness to cocaine in the same chamber 24 h later (at either 7, 14 or 21 days of age). Such a short post-treatment interval was adopted, along with a consistent pairing of the testing context with the drug effect and a sensitive technique of behavioral measurement (video recording), in order to maximize the possibility of detecting any cocaine sensitization. Locomotion was sensitized at all ages, after both regimens in 14-day-old pups, but solely after 2 injections in 21- and 4 injections in 7-day-old pups. Sensitization was also expressed via behaviors specific to each age. Four cocaine injections augmented cocaine-induced uncoordinated movements of head, paws and body (horizontal activity) in 7-day-old pups, and mouth movements in 14-day-old pups. In 21-day-old pups, sensitization was dose- and regimen-dependently expressed via adult-like stereotyped head movements. In neonatal 7-day-old pups, cocaine sensitization was also visible as reductions in immobility (both injection regimens). Contrary to previous studies, these results indicate that, given the use of an appropriate methodology, short-term sensitization to the motoric effects of cocaine can be expressed by suckling rats prior to weaning, even after relatively short regimens of daily injections.