Homeostatic changes of the endocannabinoid system in Parkinson's disease

Endocannabinoids (eCBs) are endogenous lipids that bind principally type-1 and type-2 cannabinoid (CB(1) and CB(2)) receptors. N-Arachidonoylethanolamine (AEA, anandamide) and 2-arachidonoylglycerol (2-AG) are the best characterized eCBs that are released from membrane phospholipid precursors through multiple biosynthetic pathways. Together with their receptors and metabolic enzymes, eCBs form the so-called "eCB system". The later has been involved in a wide variety of actions, including modulation of basal ganglia function. Consistently, both eCB levels and CB(1) receptor expression are high in several basal ganglia regions, and more specifically in the striatum and in its target projection areas. In these regions, the eCB system establishes a close functional interaction with dopaminergic neurotransmission, supporting a relevant role for eCBs in the control of voluntary movements. Accordingly, compelling experimental and clinical evidence suggests that a profound rearrangement of the eCB system in the basal ganglia follows dopamine depletion, as it occurs in Parkinson's disease (PD). In this article, we provide a brief survey of the evidence that the eCB system changes in both animal models of, and patients suffering from, PD. A striking convergence of findings is observed between both rodent and primate models and PD patients, indicating that the eCB system undergoes dynamic, adaptive changes, aimed at restoring an apparent homeostasis within the basal ganglia network.