Impaired learning in adulthood following neonatal Δ9-THC exposure

Cannabis is one of the most commonly used illicit drugs during pregnancy, but little is known about the lasting effects of early-life exposure to this drug. In this study, male Wistar rat pups were treated daily with (-)-Δ9-tetrahydrocannabinol (THC; 5 mg/kg, s.c.) or its vehicle between postnatal days (PND) 4 and 14. Drug administration during this early postnatal period in rats is analogous to the third trimester of gestation in humans, which is a major period of synaptogenesis. Rats were subsequently tested drug-free during young adulthood (PND 56) using a two-component food-motivated double Y-maze test. Each trial included distinct spatial discrimination and delayed alternation components, which permitted the simultaneous assessment of reference memory and working memory. Rats were tested for 30 trials/day, 5 days/week for 5 weeks. Results revealed no significant differences between THC- and vehicle-treated rats in the spatial discrimination task. However, compared to vehicle-treated rats, THC-treated rats committed significantly more errors, and required significantly longer to obtain 80% correct over two consecutive days in the delayed alternation task. Results suggest that neonatal THC exposure leads to a specific and lasting deficit in learning in adulthood, which is likely due to impaired working memory function.