Animal studies
ECS involvement
In adolescent (postnatal day 25-39) rats daily injections of synthetic CB1/CB2 agonist CP55,940 (0.4 mg/kg) or CB1 antagonist AM251 (0.5 mg/kg) both reduced sociability, but this did not persist into adulthood suggesting a transient effect (Cossio et al., 2019).
However, in adolescent (postnatal day 35-45) rats treated with 5 mg/kg AM251 did result in changed adult corticolimbic endocannabinoid signaling with a significant decrease in amygdalar AEA, an increase in hypothalamic AEA and an increase in prefrontal cortical CB1R expression (Lee et al., 2015).
Intermittent alcohol exposure in adolescent rats lowered CB1 and PPARα and increased CB2 in the spleen in adult rats. In addition PEA-degrading phospholipase D expression was higher in alcohol-exposed female rats and lower in males (Pavón et al., 2016).
In rats, the CB1 F238L gain of function mutation causes rats to display adolescent like behavior (typical high risk/novelty seeking, increased peer interaction, enhanced impulsivity, and augmented reward sensitivity for drug and nondrug reward) into adulthood, while partial inhibition of CB1 activity normalized behavior to wildtype phenotype (Schneider et al., 2015). This raises the interesting hypothesis that CB1 activity may mediate/drive adolescent behavior.
Exposing adolescent rats to the cannabinoid receptor agonist WIN 55, 212-2 (0.1, 0.3 or 1.0 mg/kg, i.p) for 14 days during adolescence (i.e., from post-natal day 30-44) resulted in significant sleep disturbances when the animals became adult (post-natal day 80). These included decreased wakefulness and enhanced rapid eye movement sleep. Furthermore, labeling for NeuN, a marker of postmitotic neurons, was significantly increased the dorsomedial hypothalamic nucleus of rats treated with WIN 55, 212-2 (Macías-Triana et al., 2020), suggesting that excessive cannabinoid receptor activation during adolescence can persistently influence sleep patterns and neuronal activity later in life.
In acute preparations of the nucleus accumbens of adolescent, but not adult, mice WIN 55, 212-2 induced long-term depression. In the dorsolateral striatum, WIN 55, 212-2 failed to induce long-term depression in both adolescents and adults. These effects were accompanied by reduced expression of CB1 and increased endocannabinoid tone in both brain regions (Zhang et al., 2015). Together these results may provide some mechanistic insights into the relative vulnerability of adolescents to cannabis/cannabinoid abuse.

Plant cannabinoids
In adolescent (postnatal day 35), but not adult (postnatal day 65) rats, THC (2.5 mg/kg escalating to 10 mg/kg, twice daily, i.p.) induced long-term behavioral changes reminiscent of human schizophrenic behavior (Renard et al., 2014, 2016). While this suggests THC to be a risk factor for developing schizophrenia in adolescents, it must be noted that the used doses are incredibly high for human standards.
Adolescent (postnatal day 29-43) mice, injected with THC (2.5 mg/kg escalating to 10 mg/kg, i.p.) showed a hyperpolarized resting membrane potential, decreased spontaneous firing rate, increased current-induced firing threshold, and decreased depolarizing response to NMDA in deep-layer prelimbic prefrontal cortex neurons analyzed by current-clamp recordings (Pickel et al., 2019). While these results may provide a mechanistic explanation for social dysfunction observed in teenagers chronically using cannabis it must be noted that the used doses are incredibly high for human standards.
In adolescent rats the effect of voluntary oral THC consumption (ad lib) on behavioral development was tested. Adolescent rats of both sexes consumed enough THC to trigger acute hypothermia, analgesic, and locomotor responses, and that 15 days of access to THC-gelatin in adolescence resulted in the down-regulation CB1 in adulthood in a sex and brain area specific manner. Remarkably, THC consumption by adolescent male rats and not female rats led to impaired Pavlovian reward-predictive cue behaviors in adulthood consistent with a male-specific loss of CB1-expressing vGlut-1 synaptic terminals in the ventral tegmental area (VTA). Thus, voluntary oral consumption of THC during adolescence is associated with sex-dependent behavioral impairment in adulthood (Kruse et al., 2019).
Adolescent (postnatal day 35-49) mice injected with THC (3 mg/kg escalating to 12 mg/kg, s.c.) and simultaneously exposed to stress (forced swimming, tail suspension or restraint) displayed impaired fear extinction at adulthood. This was associated with decreased neuronal activity in the basolateral amygdala (BLA) and the infralimbic prefrontal cortex, suggesting a long-term dysregulation of the fear circuit. This was supported by an increase of immature dendritic spines in pyramidal neurons of the BLA (Saravia et al., 2018). While the results suggest that combined adolescent cannabis use and stress may lead to long-term anxiety disorders it must be noted that the used doses are incredibly high for human standards.
In rats, adolescent THC exposure can lead to abnormal behavior such as depression in adults. This depression can be rescued/suppressed by FAAH inhibition (preventing breakdown of AEA). FAAH-mediated rescue of depression requires CB1 activation and involves restoration of cortical synaptic plasticity and hippocampal neurogenesis (Cuccurazzu et al., 2018).
Adult (postnatal day 70) and adolescent (day 37) rats show different pharmacokinetic responses to injected THC (ascending doses of 0.5, 1.6 and 5 mg/kg, i.p.). At 5 mg/kg THC reached 50% higher plasma concentrations in adolescents and in adults. A similar effect was seen in white adipose tissue. Conversely, brain THC levels were 40-60% lower in adolescents. Liver microsomes from adolescent mice converted Δ9-THC into 11-COOH-THC twice as fast as adult microsomes. Moreover, the brain of adolescent mice contained higher mRNA levels of the multi-drug transporter Abcg2, which may extrude Δ9-THC from the brain, and of claudin-5, a protein that contributes to blood-brain barrier integrity. The results reveal the existence of multiple dissimilarities in the distribution and metabolism of Δ9-THC between adolescent and adult male rodents (Torrens et al., 2020).
Nicotine/tobacco and THC/cannabis are often consumed together. In rats, THC-induced suppression of locomotor activity was attenuated by nicotine pre-exposure in adult but not adolescent males. THC-induced suppression of locomotor activity was potentiated by nicotine pre-exposure in female adolescents, with no effects of THC or nicotine observed in female adults. THC increased c-Fos IR in the caudate, nucleus accumbens, stria terminalis, septum, amygdala, hypothalamus, and thalamus. Nicotine pre-exposure potentiated this effect in all regions. Several brain regions showed age and sex differences in c-Fos IR such that expression was greater in adults than adolescents and in females than males (Miladinovic et al., 2020). Thus, the amount of crosstalk between nicotine and THC appears to be dependent on age and sex.

Human studies
Plant cannabinoids
A systematic review of adolescent cannabis use conclude that: a) cannabis use can have detrimental effects on cognition beyond acute intoxications, and b) sustained abstinence can improve impaired cognitive function (Lorenzetti et al., 2020).
A psychological study compared the acute effects of cannabis in human adolescent (n=20; 16-17 years old) and adult (n=20; 24-28 years old) male cannabis users, in a placebo-controlled, double-blind cross-over design. After inhaling vaporized active or placebo cannabis, participants completed tasks assessing spatial working memory, episodic memory and response inhibition, alongside measures of blood pressure and heart rate, psychotomimetic symptoms and subjective drug effects (for example, 'stoned', 'want to have cannabis'). Results showed that on active cannabis, adolescents felt less stoned and reported fewer psychotomimetic symptoms than adults. Further, adults but not adolescents were more anxious and less alert during the active cannabis session (both pre- and post-drug administration). Following cannabis, cognitive impairment (reaction time on spatial working memory and prose recall following a delay) was greater in adults than adolescents. By contrast, cannabis impaired response inhibition accuracy in adolescents but not in adults. Moreover, following drug administration, the adolescents did not show satiety; instead they wanted more cannabis regardless of whether they had taken active or placebo cannabis, while the opposite was seen for adults. These contrasting profiles of adolescent resilience (blunted subjective, memory, physiological and psychotomimetic effects) and vulnerability (lack of satiety, impaired inhibitory processes) show some degree of translation from preclinical findings, and may contribute to escalated cannabis use by human adolescents (Mokrysz et al., 2016).

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Cuccurazzu, B., Zamberletti, E., Nazzaro, C., Prini, P., Trusel, M., Grilli, M., Parolaro, D., Tonini, R., and Rubino, T. (2018). Adult cellular neuroadaptations induced by adolescent THC exposure in female rats are rescued by enhancing Anandamide signaling. Int. J. Neuropsychopharmacol.
Kruse, L.C., Cao, J.K., Viray, K., Stella, N., and Clark, J.C. (2019). Voluntary oral consumption of Δ9-tetrahydrocannabinol by adolescent rats impairs reward-predictive cue behaviors in adulthood. Neuropsychopharmacol. Off. Publ. Am. Coll. Neuropsychopharmacol.
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