Our study showed that THC and CGS-21680 caused anxiety-like behaviors in mice, whereas Istradefylline diminished such behaviors. THC notably elevated the levels of adenosine A2AR gene expression in the hippocampal regions, while it also led to a slight reduction in the expression of CB1R. These findings suggest an intricate interplay between the adenosinergic and cannabinoidergic systems in the modulation of anxiety-like behaviors in mice. This notion relies on the measurements of behavior tests such as OF test and EPM test and molecular response via qPCR, allowing us to make a connection between anxiety-like behaviors and expression levels of adenosine A2AR and CB1R genes.
Cannabis use is a widespread phenomenon that leads to behavioral effects, including addiction, in millions of individuals across all races, ethnicities, and socioeconomic groups worldwide. Numerous scientific studies on cannabis indicate that THC creates addiction by influencing various neurophysiological mechanisms in the brain. The information obtained about the cellular and molecular mechanisms underlying the effects of cannabis is still limited today. Additionally, the perspective that the adenosinergic system could be involved in the physiopathology of addiction has gained significant importance in recent years. As indicated by epidemiological studies, approximately half of the participants in survey studies have reported using cannabis instead of prescription medications for anxiety (Corroon et al. 2017; Turna et al. 2019). D'Souza et al. (2004), in a study conducted with human subjects, demonstrated that THC induces a variety of symptoms such as anxiety, paranoia, depersonalization, and time slowing, which are observed in schizophrenia and other psychotic conditions, in healthy individuals.. It has been observed that the level of anxiety in individuals using THC increases dose-dependently compared to control groups (D'souza et al. 2008). In contrast to the anxiogenic effects of THC, studies also indicate that CBD in the cannabis content produces anxiolytic effects (Bergamaschi et al. 2011; Crippa et al. 2011). It has been observed that a cannabis plant containing equal amounts of THC and CBD leads to less anxiety compared to THC-dominant cannabis (Hutten et al. 2022). Additionally, it has been noted in recent years that the THC content in the cultivated cannabis plant has been increasing (Chandra et al. 2019). Therefore, studies on the effects of THC constitute an important area of interest.
In experimental modeling, OF and EPM tests are used to evaluate anxiety-like behaviors in rodents (Akillioglu at al. 2012). It is known that high-dose THC administration affects emotional and cognitive behaviors in experimental animals. It has been shown that THC administration in mice can produce different effects in a dose-dependant manner (Patel and Hillard 2006). It has been observed that THC administration in mice produces anxiolytic effects at low doses and anxiogenic effects at high doses (Bruijnzeel et al. 2016; Klein et al. 2011). Similarly, Valjent et al. (2002) observed that THC administration in mice produced an anxiolytic-like response at a low dose (0.3 mg kg− 1) for 5 days, while it caused an anxiogenic effect at a high dose (5 mg kg− 1).. Additionally, Murphy et al. (2017) demonstrated that high-dose chronic THC administration (10 mg kg− 1) in adolescent or adult rats resulted in an anxiogenic effect measured through the EPM. However, it has been observed that THC administered i.p to rats is anxiogenic at low doses and anxiolytic at high doses (Fokos and Panagis 2010). These findings indicate that the effect of THC on anxiety-like behaviors in animal models varies depending on multiple factors such as dose, route of administration, and duration of administration. In the present study, mice treated with THC (10 mg kg− 1) for 5 days showed a decrease in the frequency of center crossings in the OF test. Additionally, in the EPM test, the time spent in the open arm decreased while the time spent in the closed arm increased. These findings support the results obtained by Kasten et al. (2019) in the OF and EPM tests. Similar effects were obtained with the administration of CGS-21680, an adenosine A2AR agonist. Conversely, in our study, Istradefylline, an adenosine A2AR antagonist, reduced the anxiogenic effect induced by THC. Recently a notable research topic has been the close relationship between the endocannabinoid system and the adenosinergic system, and the idea that receptors can form heterodimers (Carriba et al. 2007). Our findings are supportive of previous studies suggesting adenosine A2AR may contribute to the anxiogenic effects of THC.
Adenosine A2AR and cannabinoid CB1R are found in common regions of the CNS, including the striatum, hippocampus, and cerebellum (Herkenham et al. 1990; Svenningsson et al. 1999). However, the role of these receptors in both the behavioral effects of THC and the role of the adenosinergic system in the effects of THC in the hippocampus are not yet well understood. Carriba et al. (2007) suggested that the signaling of CB1R is dependent on A2AR activation and that A2ARs form heteromers with CB1R using in vitro neuroblastoma models. In the present study, to evaluate the changes on CB1R and adenosine A2AR expression levels in response to THC treatment, hippocampal tissue of mice were isolated and qPCR experiment was conducted. A significant increase in A2AR expression was observed in the hippocampal tissues of animals treated with THC. However, a partial decrease in CB1R gene expression was observed. These findings suggest an interaction between the adenosinergic system and the endocannabinoid system.
In conclusion, findings from our study suggest that THC administration at high doses given for 5 days, induced anxiety-like behaviors in mice, and that the adenosinergic system may contribute to this effect.