Role of central cannabinoid receptors in cerebellar dependent learning

Cannabinoid receptors (CBR) are the most abundant G-protein coupled receptors in the mammalian brain with the highest densities within the cerebellum (Herkenham et al., 1990). Cannabinoid manipulations have been reported to cause deficits in cerebellar-dependent learning (Kishimoto and Kano, 2006; Skosnik et al., 2007; Steinmetz and Freeman, 2010; 2013). Cannabinoid receptors-1 (CB1R) have been hypothesized to be important in the establishment of long-term depression within the cerebellar cortex (Levenes et al., 1998; Safo and Regehr, 2005). However, all investigations in vivo have used global manipulations and have not attempted to localize or characterize these receptors during cerebellar-dependent learning. Chapter 2 systematically examined localization within the cerebellar cortex of cannabinoid effects on eyeblink conditioning, a type of cerebellum-dependent learning. Local infusions into a specific portion of the cerebellar cortex, the eyeblink conditioning microzone, resulted in deficits in learning similar to systemic injections. Additionally, infusions of cannabinoids into the eyeblink conditioning microzone, and no other parts of the cerebellar cortex or deep nuclei, were responsible for the deficits. Finally, tetrode recordings were made in Purkinje cells while receiving either CBR agonist or vehicle injections prior to training. Fewer Purkinje cells exhibited learning-related decreases in activity when the rat was administered a CBR agonist as compared to when it was injected with the vehicle. The CBR administered Purkinje cells also showed earlier onsets and smaller amplitudes in their learning-related activity. Purkinje cells that show a learning-related increase in activity were not affected by cannabinoid administration. The impairment in Purkinje cell plasticity was not observed after the rats reached asymptotic levels of learning. These results indicate that CBR agonist administration disrupts the induction of plasticity within the cerebellar cortex and this may account for the behavioral deficit in eyeblink conditioning. Chapter 3 examined whether infusions of the CBR agonist into the cerebellar cortex impaired forebrain-dependent learning as well as forebrain-independent associative learning. Similar to subcutaneous injections, forebrain-dependent trace eyeblink conditioning was unimpaired, whereas forebrain independent delay eyeblink conditioning was impaired. These findings provide evidence that plasticity mechanisms that are modulated by cannabinoids do not play a significant role in trace eyeblink conditioning. Finally, in Chapter 4 the role of CBRs and endocannabinoids during memory consolidation were examined. CBR and endocannabinoid manipulations prior to training resulted in impaired eyeblink conditioning. However, a CBR agonist or a drug increasing endocannabinoid levels resulted in enhanced consolidation when administered 1 hour post-training. In contrast, a CBR antagonist or an endocannabinoid decreasing drug resulted in impairments 1 hour post-training. Thus, CBRs and endocannabinoids appear to be important in learning and consolidation of cerebellar-dependent learning.