The main finding of the present study is that chronic adolescent exposure to low-dose THC leads to persistent behavioral abnormalities related to some but not all aspects of depressive reactivity. In particular, THC induces a decrease in sucrose consumption and a decreased latency to the first immobility in the FST. THC also induces some anxious behavior in the EPMT and a decrease in nose dips (exploratory behavior). In addition, we found altered 5-HT firing rate activity in rats treated with THC after adolescent exposure, but there was no change in NE firing activity, thus suggesting that the THC effects are quite specific for areas implicated in mood regulation. Importantly, these depressive phenotypes were not observed in adult exposure, suggesting that the emotional liabilities during adolescence were linked to an age-dependent vulnerability of the developing serotonergic system.

However, we found that serotonergic activity was also decreased in rats exposed during adulthood, even if it was not paralleled by significant changes in depressive/anxiety behaviors, except for an anxious phenotype in some parameters of the EPMT. Depression is a complex disease and several core symptoms such as feelings of hopelessness and low self-esteem are hardly reproduced in animals (Matthews et al., 2005). The FST is mostly used to assess one of the main “symptoms” of depression, which is “behavioral despair,” characterized by an increased immobility time or change in the latency to the first immobility episode; it mostly has been used to screen the effect of antidepressant drugs and not a depression model per se (Castagné et al., 2010). However, several studies have shown that depressogenic manipulations (e.g., chronic unpredictable stress, genetic deletion) may increase the immobility time in the FST, thus suggesting that the test can measure a depressive phenotype (Garza et al., 2012; Hao et al., 2019). Nevertheless, one cannot rule out the possibility that the immobility duration or the decreased latency of immobility may represent a resilient behavior or coping strategy, as previously reported (Molendijk and de Kloet, 2015, 2019; Commons et al., 2017). Following these recent interpretations, the decreased latency to swim observed after adolescent THC exposure can be interpreted as a coping strategy or resilience, even if further studies are required to better characterize the behavioral effects of THC on despair-like behaviors.

Anhedonia is another core symptom of depression, which is characterized by the inability to experience pleasure from rewarding or enjoyable activities (Der-Avakian and Markou, 2012). We used the SPT, which does not directly measure anhedonia but rather the reduction in the preference for sucrose (a pleasant and rewarding stimuli) over water (neutral stimuli) (Liu et al., 2018). Compared with exposure during adulthood, THC exposure during adolescence decreased the consumption of sucrose, which is interpreted as increased anhedonic behavior. Since SPT experiments were performed in animals after food deprivation for 24 hours and considering the involvement of the cannabinoid system in appetite and food intake (Berry and Mechoulam, 2002), this behavioral condition might have masked the anhedonia outcome in the SPT. Nevertheless, NSFT revealed no changes in the latency to feed in the home cage between animals treated with vehicle or THC during adolescence or adulthood, suggesting that food deprivation had minimal impact on the anhedonia behavior. Adolescence is marked by typical neurobehavioral characteristics that include distinct cognitive and social behavior (e.g., increased play and interaction), emotional volatility (e.g., anxiety and self-consciousness), disproportionately extensive recklessness, novelty- or sensation-seeking, and risk-taking behaviors (Buchanan and Holmbeck, 1998). Mounting evidence also suggests that the adolescent nervous system undergoes continuous structural modification in gray and white matter as well as progressive and regressive refinements in network connectivity, migration and pattern formation, differentiation, and cytogenesis (Paus, 2005). Age-dependent differences in the transmission of key neuromodulators and transmitters involved in emotional processing, most notably monoamines and endocannabinoids, have also been reviewed (Crews et al., 2007). These differences include low levels of 5-HT activity (Depue and Spoont, 1986) and 5-HT_1A receptor binding (Dillon et al., 1991) in adolescence and high levels of 5-HT_2A expression (Morilak and Ciaranello, 1993). The constitution of cannabinoid receptors, particularly the CB1Rs, mature slowly during the post-natal period, and receptor binding in the hippocampus reaches higher levels in adolescence than in adults (de Fonseca Rodríguez et al., 1993). Importantly, CB1Rs play a critical role in neuronal development due to their ability to modulate the release of neurotransmitters and expression, establishment of synaptic connection (Kano et al., 2009), and epigenetic regulation (Prini et al., 2018). The pharmacological exogenous manipulation of the endocannabinoid system by THC led to changes in neuronal development and homeostasis, pointing out the role of the endogenous cannabinoid system in neurodevelopment (Viveros et al., 2012).

Several animal studies have highlighted that adolescent cannabinoid exposure persistently impairs cognition, social behavior, motivation and anxiety, and produces long-lasting cross-tolerance to psychostimulants and other cannabinoid agonists (Pistis et al., 2004). Our previous results using the potent CB₁R full agonist WIN55,212-2, found instead a depressive behavior and anxious behaviors after adolescent exposure, but not after adult exposure. This difference could account for the fact that THC is a partial agonist at CB₁R with a longer half-life compared with WIN55,212-2, gating stronger adverse anxiety-related effects in adults (Bambico et al., 2010). Our findings are in agreement with Silva and collaborators (Silva et al., 2016) showing that chronic THC treatment (3 mg/kg per day) during adolescence produced anxiolytic and antidepressant behavior as measured by the EPMT and FST. In contrast, Rubino and colleagues observed that chronic administration of THC (2.5–10 mg/kg per day during adolescence) increased anhedonia but not despair in males (Rubino et al., 2008) without anxiety in both sexes. More recently, it was reported that THC in adolescent male rodents produced impaired Pavlovian reward-predictive cue behaviors, paralleled by a loss of CB₁R-expressing vGlut-1 synaptic terminals in the ventral tegmental area in males during adulthood (Kruse et al., 2019).

Our behavioral data were coupled with a dysfunction in serotonergic neuronal activity in the DR and noradrenergic activity in the LC, both implicated in depression and anxiety. In particular, we found a parallel between a significantly decreased 5-HT activity and a depressive phenotype after THC adolescent exposure, as also reported in our meta-analysis on humans (Gobbi et al., 2019), pointing out a preferential role of cannabis in the dysregulation of mood. The significant alteration induced by THC on 5-HT firing activity may be due to the activation of CB1 receptors expressed on GABAergic neurons of the DR (Häring et al., 2007) or via an increased activity of GABAergic neurons in the hippocampus (Higuera-Matas et al., 2012) or glutamateric neurons in the medial prefrontal cortex (Bambico et al., 2007; Geddes et al., 2016) that regulate the synaptic excitatory/inhibitory balance governing the excitability of 5-HT neurons. The attenuation of 5-HT neurotransmission could also occur via a dopaminergic hyperactivity in the prefrontal cortex (Renard et al., 2017) or indirectly through a decreased activation of dopamine neurons in the ventral tegmental area (Diana et al., 1999; Pistis et al., 2004) or via an indirect opioid receptor activation (Ellgren et al., 2007).

Recently, it was observed that increasing doses of THC in adolescence (0.3–3 mg/kg twice per day from PND 35 to 45) 5-HIIA/5-HT rate in the hippocampus (Poulia et al., 2019), which could be interpreted as a compensation for the low 5-HT rate. More studies are needed to establish this direct correlation between cannabinoid receptors and 5-HT firing activity.

Interestingly, when THC was administered during adulthood, we found a decreased 5-HT firing activity that was not associated with any reduction of sucrose consumption or with a decreased latency to the first FST immobility, but only with an increased time and exploration of the open arms in the EPMT. This low 5-HT could be a neurobiological hallmark of a particular susceptibility to stress or trauma, as observed by (Lee et al., 2018), who found that people consuming cannabis have an elevated risk of developing post-traumatic stress disorder after exposure to trauma. For instance, several studies have demonstrated that animal models of chronic stress are correlated with a decrease in the firing activity of monoamine neurons, including 5-HT (Pavcovich et al., 1990; Bambico et al., 2009). Nevertheless, further studies must be conducted to determine the link between cannabis exposure, vulnerability to mental health problems in adulthood, and the 5-HT system.

While in previous investigations with WIN55,212-2 we found that an increase in spontaneous LC NE activity was associated with enhanced anxiety response (Bambico et al., 2010), here this dose of THC also did not have the capacity to modify NE firing, even if the coefficient of variation in NE neurons was changed in both adolescent and adult exposure; this indicates a drastic change in firing pattern regularity also in NE neurons.

We cannot rule out that this NE activity pattern disturbance might have conveyed some degree of vulnerability indicated by the observed changes in EPMT behaviors after both adolescent and adulthood exposure.

Pertaining to the generally inert effect of chronic THC exposure during adulthood, it is worth mentioning that we have published that an identical dose given sub-chronically (4 days) during adulthood (PND 70) instead yielded potent antidepressant activity when assayed immediately after the treatment (Bambico et al., 2012). In the current study, the FST was done after a long period of washout (20 days). This suggests that adult exposure to cannabis does not produce any significant long-term effects on mood, thus indicating that the period from adolescence to young adulthood is critical for developing depression after THC consumption. Remarkably, this age difference on mood by THC was also reported in humans (Schoeler et al., 2018), indicating that only after 27 years of age could chronic cannabis use produce positive effects on mood, while before 27 years of age, cannabis is associated with depression.

In summary, both chronic adolescent and adult exposure to a low dose of THC impairs monoaminergic transmission, with mild anxiogenic effects, but a more profound anhedonia is produced in rats treated during adolescence. A limitation of our study is the lack of data on female rodents, and further studies should be aimed at establishing whether these neurobiological THC-induced differences are also detectable in females or if other dysfunctions are present, as already observed by (Rubino et al., 2008; Poulia et al., 2019). This study in rodents shows that the daily administration of THC alters emotional behavior and impacts monoaminergic neurotransmission. More preclinical and clinical investigations are needed to better understand the impact of the daily and weekly doses of cannabinoids on the brain and their potential risk of increasing suicidality and depression (Silins et al., 2014).