Perinatal Δ9-tetrahydrocannabinol exposure reduces proenkephalin gene expression in the caudate-putamen of adult female rats

Perinatal Delta(9)-tetrahydrocannabinol (Delta(9)-THC) exposure in rats affects several behavioral responses, such as opiate self-administration behavior or pain sensitivity, that can be directly related to changes in opioidergic neurotransmission. In addition, we have recently reported that the administration of naloxone to animals perinatally exposed to Delta(9)-THC produced withdrawal responses, that resemble those observed in opiate-dependent rats. The purpose of the present study was to examine the basal opioid activity in the brain of adult male and female rats that had been perinatally exposed to Delta(9)-THC. To this aim, proenkephalin mRNA levels were measured, by using in situ hybridization histochemistry, in the caudate-putamen, nucleus accumbens, central amygdala and prefrontal cingulate cortex. The results showed a marked reduction in proenkephalin mRNA levels in the caudate-putamen of Delta(9)-THC-exposed females as compared to oil-exposed females, whereas no changes were observed between Delta(9)-THC- and oil-exposed males. There were no differences in proenkephalin mRNA levels in the nucleus accumbens, central amygdala and prefrontal cingulate cortex between males and females perinatally exposed to Delta(9)-THC and their respective controls, although a certain trend to decrease was observed in Delta(9)-THC-exposed females. In summary, perinatal exposure to Delta(9)-THC exposure decreased proenkephalin gene expression in the caudate-putamen of adult rats, although this effect exhibited a marked sexual dimorphism since it was only seen in females. This result is in agreement with a previous observation fi om our laboratory that females, but not males, that had been perinatally exposed to Delta(9)-THC, self-administered more morphine in adulthood. This suggests that low levels of proenkephalin mRNA may be used as a predictor of greater vulnerability to opiates.