The endocannabinoid system consists of the endogenous cannabinoids (endocannabinoids), cannabinoid receptors and the enzymes that synthesise and degrade endocannabinoids. Many of the effects of cannabinoids and endocannabinoids are mediated by two G protein-coupled receptors (GPCRs), CB(1) and CB(2), although additional receptors may be involved. CB(1) receptors are present in very high levels in several brain regions and in lower amounts in a more widespread fashion. These receptors mediate many of the effects (including the psycho-active effects) of cannabinoids. CB(2) receptors have a more restricted distribution, being found in a number of immune cells and in a few neurones. Both CB(1) and CB(2) couple primarily to inhibitory G proteins and are subject to the same influences as other GPCRs. Thus, partial agonism, functional selectivity and inverse agonism all play important roles in determining the cellular response to specific cannabinoid receptor ligands.

The endocannabinoid system (ECS) is a group of endogenous cannabinoid receptors located in the mammalian brain and throughout the central and peripheral nervous systems, consisting of neuromodulatory lipids and their receptors. Known as "the body’s own cannabinoid system", the ECS is involved in a variety of physiological processes including appetite, pain-sensation, mood, and memory, and in mediating the psychoactive effects of cannabis. The ECS is also involved in voluntary exercise and may be related to the evolution of the runner's high in human beings and related aspects of motivation or reward for locomotor activity in other animals.

The endocannabinoid system, broadly speaking, includes:

  • The endogenous arachidonate-based lipids, anandamide (N-arachidonoylethanolamide, AEA) and 2-arachidonoylglycerol (2-AG); these are known as "endocannabinoids" and are physiological ligands for the cannabinoid receptors. Endocannabinoids are all eicosanoids.
  • The enzymes that synthesize and degrade the endocannabinoids, such as fatty acid amide hydrolase or monoacylglycerol lipase.
  • The cannabinoid receptors CB1 and CB2, two G protein-coupled receptors that are located in the central and peripheral nervous systems.

The neurons, neural pathways, and other cells where these molecules, enzymes, and one or both cannabinoid receptor types are all colocalized from the endocannabinoid system.

The endocannabinoid system has been studied using genetic and pharmacological methods. These studies have revealed that cannabinoids act as neuromodulators for a variety of processesThe localization of the CB1 receptor in the endocannabinoid system also has a very large degree of overlap with the orexinergic projection systemMoreover, CB1 is colocalized on orexin projection neurons in the lateral hypothalamus and many output structures of the orexin system, where the CB1 and orexin receptor 1 (OX1) receptors physically and functionally join together to form the CB1–OX1 receptor heterodimer.