Long-term effects of adolescent Δ9-THC exposure in Substance Use Disorder liability assessed through behavioural, neuroimaging and transcriptome studies

Resumen

Different lines of evidence have associated cannabis exposure during adolescence with an enhanced risk of developing psychiatric disorders. Furthermore, the idea that cannabis use may be a gateway to other drugs has been investigated by several approaches. However, despite decades of research, it is not clear if cannabis consumption could enhance the liability towards substance use disorders (SUDs). In this work we aim to increase our understanding of the protracted effects of adolescent cannabinoid exposure, exploring the diverse causal mechanisms involved in the modulation of SUD liability. As such, male and female adolescent Wistar rats were administered 9 injections of THC (3 mg/kg) or the vehicle alone every other day from postnatal day (PND) 28 to 42 and they were then left undisturbed until adulthood PND90. Different sets of rats were then subjected to five different experimental regimes: (1) studies of the structural (MRI) and metabolic ([1H]-spectroscopy) changes produced in the brain in vivo. Behavioural experiments aimed at (2) measuring the ability of conditioned cues to influence instrumental responses (Pavlovian to instrumental transfer, PIT) and motor impulsivity (two-choice serial reaction time task, 2-CSRTT), or (3) to assess the propensity of individuals to engage with a conditioned stimulus (Pavlovian conditioned approach) and their habit formation propensity. (4) A multi-component cocaine self-administration (CSA) protocol was used to evaluate alterations to cocaine addiction-like behaviours. Moreover, (5) a Ribonucleic acid sequencing (RNA-seq) study was undertaken to explore the protracted effects on gene expression in the NAc Shell after adolescent THC exposure. Adult THC-treated animals displayed (1) volumetric and microstructural alterations to subcortical regions, and complimentary brain ventricle volumetry showed reductions in the size of their lateral ventricles. A white matter analysis found a reduced fractional anisotropy in several tracts due to THC administration, prominently in rostral sections, while in vivo 1H MR spectroscopy identified lower levels of cortical choline compounds in these animals. (2) In males that received THC there was enhanced PIT and weaker motor impulsivity, whereas females that received THC displayed enhanced motor impulsivity. (3) THC-treated animals were more goal-directed but showed no differences in habit formation compared to the control rats. (4) Cocaine addiction-like behaviours were mostly unaltered, although significantly, males administered THC showed a higher intake under progressive ratio and females a higher rebound of cocaine intake after re-establishing low-effort conditions. (5) RNA-seq revealed THC-induced alterations in gene expression with a marked sex-specific character. The differentially expressed genes highlighted changes to glutamatergic synapses, and in ion binding, axonal growth and hormonal activity, among other categories. These results show that mild THC exposure during adolescence leaves a lingering mark on brain structure and function, reflected in adult behaviour, and that is relevant to the motivational aspects of behaviour and SUDs even after prolonged drug-free periods. Some of the changes found mimic those evident in human epidemiology and they highlight the importance of sex-specific effects in cannabis research. Adolescent THC exposure changes the reactivity to reward-related cues and affects the expression of impulsive behaviours, protracted effects that also influence drug administration patterns in a sex dependent manner. However, despite the evident alterations to brain development and the impact on adult psychological traits, a deterministic direction towards increased vulnerability to substance use disorders cannot be inferred from these changes.