Endocannabinoid System Biomarkers in Alzheimer's Disease

Background: Alterations in the endocannabinoid system (ES) have been described in Alzheimer's disease (AD) pathophysiology. In the past years, multiple ES biomarkers have been developed, promising to advance our understanding of ES changes in AD. Discussion: ES biomarkers, including positron emission tomography with cannabinoid receptors tracers and biofluid-based endocannabinoids, are associated with AD disease progression and pathological features. Conclusion: Although not specific enough for AD diagnosis, ES biomarkers hold promise for prognosis, drug-target engagement, and a better understanding of the disease. Here, we summarize currently available ES biomarker findings and discuss their potential applications in the AD research field.

Quantification of the Cannabinoid Type 1 Receptor Availability in the Mouse Brain

Introduction: The endocannabinoid system is involved in several diseases such as addictive disorders, schizophrenia, post-traumatic stress disorder, and eating disorders. As often mice are used as the preferred animal model in translational research, in particular when using genetically modified mice, this study aimed to provide a systematic analysis of in vivo cannabinoid type 1 (CB1) receptor ligand-binding capacity using positron emission tomography (PET) using the ligand [¹⁸F]MK-9470. We then compared the PET results with literature data from immunohistochemistry (IHC) to review the consistency between ex vivo protein expression and in vivo ligand binding.

Methods: Six male C57BL/6J (6–9 weeks) mice were examined with the CB1 receptor ligand [¹⁸F]MK-9470 and small animal PET. Different brain regions were evaluated using the parameter %ID/ml. The PET results of the [¹⁸F]MK-9470 accumulation in the mouse brain were compared with immunohistochemical literature data.

Results: The ligand [¹⁸F]MK-9470 was taken up into the mouse brain within 5 min after injection and exhibited slow kinetics. It accumulated highly in most parts of the brain. PET and IHC classifications were consistent for most parts of the telencephalon, while brain regions of the diencephalon, mesencephalon, and rhombencephalon were rated higher with PET than IHC.

Conclusions: This preclinical [¹⁸F]MK-9470 study demonstrated the radioligand’s applicability for imaging the region-specific CB1 receptor availability in the healthy adult mouse brain and thus offers the potential to study CB1 receptor availability in pathological conditions.

Examining the effects of former cannabis use on cerebellum-dependent eyeblink conditioning in humans

RATIONALE

Previous work in humans has shown that chronic cannabis users exhibit disruptions in classical eyeblink conditioning (EBC), a form of associative learning that is known to be dependent on the cerebellum. Based upon previous work in animals, it was hypothesized that these learning deficits were related to cannabinoid receptor (CB1R) downregulation. However, it remains unclear whether there is a recovery of cerebellum-dependent learning after the cessation of cannabis use.

METHODS

Therefore, former cannabis users (n=10), current cannabis users (n=10), and cannabis-naïve controls (n=10), all free of DSM-IV Axis-I or -II disorders, were evaluated. A standard delay EBC procedure was utilized in which paired presentations of a conditioned stimulus (CS; e.g., tone) and a co-terminating unconditioned stimulus (US; e.g., ocular airpuff) were administered, thus eliciting a conditioned eyeblink response (CR). The primary dependent measures were percentage of CRs and CR latency across conditioning blocks.

RESULTS

Similar to prior studies, current cannabis users exhibited marked impairments in both the acquisition and timing of CRs compared to controls. Although former cannabis users showed intact CR acquisition compared to controls, they exhibited significantly impaired (shorter) CR latencies. In both cannabis groups, UR amplitude did not differ from controls, indicating normal US processing.

CONCLUSIONS

These data suggest that a recovery of function has occurred for the learning of the CS-US association, while the accurate timing of the CR shows lasting impairments. Taken together, these results suggest that heavy cannabis use can disrupt timing-related synaptic plasticity within the cerebellum, even after the cessation of cannabis use.