Heavy adolescent cannabis consumption produces social and cognitive deficits in adulthood

Population-based studies show cannabis use doubles the risk of developing schizophrenia especially when use occurs in early adolescence (prior to age 15). However, the cause-and-effect mechanisms are largely unknown. To investigate the effect of cannabis on brain maturation and relation to the development of psychosis-like behaviours in adulthood, we treated young adolescent mice with vehicle or cannabis extract once a day for 2 weeks between postnatal days 14 and 28, and then collected hippocampal tissue for microarray analysis 12 weeks later. We identify a total of 78 differentially expressed genes (25 upregulated and 53 downregulated; p<0.05, fold change ± 1.2) and validate increases in dopamine D2 receptor (Drd2) and fatty acid amide hydrolase (Faah). Changes in Faah expression were limited to the hippocampus however Drd2 also increased in striatum but not prefrontal cortex or amygdala. When tested in adulthood with a behavioural panel relevant to schizophrenia, cannabis-treated mice displayed lower anxiety in the elevated zero-maze, decreased social preference, increased social novelty preference, mild cognitive impairments in a spatial version of the novel object recognition task and absence of latent inhibition when compared to vehicle controls. Adolescent treatment with cannabis extract thus lead to long-lasting changes in gene expression within the hippocampus which together result in behavioural deficits consistent with the negative and positive symptoms of schizophrenia. Cannabis extract was obtained from GW Pharmaceuticals (Cambridge, UK) and a dose of 20 mg/kg was administered daily for 14 days to C57BL/6J mice. A 1 mg/mL solution was prepared in vehicle consisting of 50% (v/v) sweet condensed milk containing 10% fat (Baeren Brand, Allgaeuer Alpenmilch GmbH, Muehldorf, Germany) + 0.25% (v/v) olive oil. Extract was first dissolved in 1%(v/v) EtOH at RT for 30 min, after which olive oil was added and samples allowed to dissolve overnight. The EtOH was then evaporated at 70oC and a solution of 50% sweet condensed milk was added to make the final emulsion. Vehicle solutions were made in parallel, omitting the cannabis extract. Pups were habituated to handling on postnatal days 12-13. Pups were then weighed and fed daily by hand using a blunted needle and syringe (not by gavage) from P14 until P28. Average feeding time was between 10-30 minutes per pup. Litters selected for vehicle and cannabis treatment were rotated between dams to minimize maternal care effects. Sires were never housed with pups. Animals were tested concerning behaviour, sucrose preference, elevated zero-maze (EOM), open-field, prepulse inhibition, startle reactivity, latent inhibition, social preference and novelty, Y-maze continuous alternations, novel object recognition, and drug-induced hypermotility. P28 animals were sacrificed 70-80 minutes following the beginning of feeding and in a separate room to that where per os administration took place. Adult animals were sacrificed two weeks following PCP-induced hyperlocomotion to allow sufficient time for drug washout. All mice were anaesthetized with isoflurane until loss of tail-pinch reflex then decapitated and brains removed. Brains were quickly cut into 1mm sections using a mouse brain matrix (Zivic Instruments, Pittsburgh, PA, USA) and sections placed in ice-cold ACSF ((26 mM NaHCO3, 124 mM NaCl, 5 mM KCl, 2 mM CaCl2, 1.3 mM MgCl2, 10 mM D- glucose, 100 U/ml penicillin, and 100 µg/ml streptomycin; all from Sigma-Aldrich). The following regions were dissected using a stereomicroscope: prefrontal cortex, striatum (sometimes further divided into dorsal and ventral regions), hippocampus, and amygdala; then snap frozen in liquid nitrogen and stored at -80oC until further use.