Particularly notable is FAAH’s ability to hydrolyse bioactive fatty amides, which do not bind to any of the known cannabinoid receptors: these include the satiety factor oleoylethanolamide and the anti inflammatory/ analgesic mediator palmitoylethanolamide. FAAHtightly controls brain concentrations of these compounds, but the functional significance of this regulation is unknown. FAAH is widely distributed in the rat brain, where it is expressed at high concentrations in cell bodies and dendrites of principal neurons. In the hippocampus, neocortex and cerebellum, FAAH-positive cell bodies are juxtaposed to axon terminals that contain CB1 receptors, indicating not only that FAAH participates in the inactivation of neurally generated anandamide, but also that this process occurs post synaptically. This idea can now be tested in FAAH-deficient mice or using selective FAAH inhibitors with long-lasting systemic actions.The pig brain contains two chromatographically distinct 2-AG-hydrolysing activities, one of which is probably due to the enzyme monoacylglycerol lipase . The rat brain isoform of this cytosolic serine hydrolase has been characterized both molecularly and morphologically. It has a broad distribution in the central nervous system , which partially overlaps with that of FAAH; however, whereas FAAH is predominantly found in postsynaptic structures, MGL might be mostly associated with nerve endings. In the hippocampal CA1 field, MGL-positive axon terminals surround cell bodies of pyramidal neurons containing FAAH. This localization could reflect a functional role of presynaptic MGL in terminating RETROGRADE SIGNALLING events mediated by 2-AG .CB1 is considered to be the most abundant G-protein coupled receptor in the mammalian brain, and its presence in the neocortex, hippocampus, basal ganglia, cerebellum and brainstem accounts for most of the behavioural actions of cannabinoid drugs. The four symptoms that are often used to define cannabinoid intoxication in the rodent __ hypothermia, rigid immobility,weed drying room analgesia and decreased motor activity __ are strikingly absent in mice in which the cb1 gene has been deleted by targeted recombination.

Aside from its unusually high concentrations in the brain, CB1 is a standard Gi/o-coupled receptor and can initiate signalling events typical of this class of transducing proteins. These include closure of Ca2+ channels, opening of K+ channels, inhibition of adenylyl cyclase activity and stimulation of kinases that phosphorylate tyrosine, serine and threonine residues in proteins. Each of these mechanisms seems to have distinct functions in translating CB1 -receptor occupation into biological responses. Cannabinoid agonists inhibit N- and P/Q-type voltage-activated Ca2+ channels. This effect, which has been suggested to result from a direct interaction of Gi/o-protein β−γ subunits with the channels, might underlie CB1 -mediated depression of transmitter release at GABA synapses in the CA1 field of the hippocampus and at glutamatergic synapses in the dorsal striatum. Importantly,however, endocannabinoid-mediated suppression of GABA release in hippocampal slices seems primarily to involve N-type Ca2+ channels. Cannabinoid regulation of voltage-gated K+ currents is also implicated in presynaptic inhibition at GABA and glutamate synapses. The latter include PARALLEL FIBRE–Purkinje cell synapses in the cerebellum, as well as synapses in the nucleus accumbens and lateral amygdala. The sensitivity of these responses to PERTUSSIS TOXIN implies that they are mediated by Gi/o proteins, but it is still unclear whether transduction is direct or indirect . Inhibition of cAMP formation does not seem to be involved. On the other hand, cAMP can contribute to the regulation of neuronal gene expression by CB1 . This process, which is necessary to produce lasting changes in synaptic strength, depends on the recruitment of complex networks of intracellular protein kinases87. Two components of these networks, extracellular signal-regulated kinase and focal adhesion kinase , become activated when hippocampal slices are treated with cannabinoid agonists. This activation is mimicked by inhibitors of cAMP-dependent kinase and is lost when the slices are exposed to cell-permeant cAMP analogues, implying that it might result from a decrease in intracellular cAMP concentrations.

The involvement of ERK and FAK in synaptic plasticity indicates that these protein kinases could participate in the changes in gene expression and the persistent neural adaptations that accompany cannabinoid administration90.In the rodent and human cortices, CB1 receptors are primarily found on axon terminals of cholecystokinin-8 -positive GABA interneurons. This expression pattern dominates the neocortex, hippocampal formation and amygdala, where nerve terminals that form excitatory synapses are ostensibly devoid of CB1 immunoreactivity. However, there is evidence that excitatory terminals in these regions do contain the receptor; for example, cannabinoid agonists reduce glutamatergic transmission in the amygdala of normalmice, but fail to do so in CB1 -deficient mutants. In addition, low concentrations of CB1 messenger RNA have been found in many neurons of the cortex that do not contain GABA. CB1 receptors are also expressed at very high levels throughout the basal ganglia. In the striatum they are localized to three distinct neuronal elements: glutamatergic terminals originating in the cortex, local-circuit GABA interneurons and axon terminals of GABA projection neurons. Medium spiny neurons project to striatal outflow nuclei, where CB1 receptors are especially abundant; for example, in the globus pallidus they outnumber dopamine D1 receptors by a factor of 45 . In the cerebellum, CB1 is present on excitatory terminals of climbing and parallel fibres as well as on GABA interneurons. Smaller numbers of CB1 receptors are also found in the thalamus , hypothalamus , midbrain , medulla and spinal cord. Last, CB1 is expressed in peripheral sensory neurons, where it is localized in cells that express N52, a protein marker of mechanosensitive Aβ fibres.A few cannabinoid effects persist in CB1 -null mice, implying that this receptormight not act alone in mediating brain cannabinoid signalling. Although cannabinoid agonists lose their ability to inhibit GABA and glutamate transmission in some brain regions of adult CB1 -knockout mice, they can still reduce excitatory transmission in the hippocampal CA1 field of these animals. This discrepancy is reinforced by the finding that GABA and glutamate synapses in CA1 respond in different ways to cannabinoid drugs. For example, cannabinoid depression of excitatory currents is blocked by CAPSAZEPINE, whereas depression of inhibitory currents is not.

These results make a persuasive case for the existence of a hippocampal cannabinoid-sensitive site that is distinct from CB1 , but other evidence appears to contradict them; for example, in newborn CB1 -null mice, cannabinoid agonists affect neither GABA nor glutamate transmission. Although this difference could be due to the developmental stage of the preparation used __ adult versus one-week-old mice __ more studies are needed to establish whether the CB3 site is molecularly distinct from CB1 . A novel cannabinoid site has also been identified in the vascular endothelium, but seems to be different from CB3 because it is not antagonized by capsazepine or activated by the CB1 /CB2 agonist Win-55212-2 .Outside the brain, the endocannabinoids are produced on demand and act on cells located near their site of synthesis. For example, they are formed by circulating leukocytes and platelets, and induce vascular relaxation by interacting with cannabinoidreceptors on the surface of neighbouring endothelial and smooth muscle cells. Similar PARACRINE actions are thought to occur in the CNS, where the endocannabinoids might mediate a localized signalling mechanism through which principal neurons modify the strength of incoming synaptic inputs.When a pyramidal neuron in the CA1 field of the hippocampus is depolarized, the inhibitory GABA inputs received by that cell are transiently suppressed. This phenomenon, called depolarization-induced suppression of inhibition , is initiated postsynaptically by voltage-dependent influx of Ca2+ into the soma and dendrites of the neuron, but is expressed presynaptically through inhibition of transmitter release from axon terminals of GABAinterneuron. This indicates that a chemical messenger generated during depolarization of the pyramidal cell must travel backwards across the synapse to induce DSI . There is evidence that this retrograde signalling process involves an endocannabinoid substance, possibly 2-AG. First, CB1 agonists mimic DSI, whereas CB1 antagonists block it. Second, DSI is absent in CB1 – deficient mice81,109. Third, the GABA interneurons that are implicated in DSI express high levels of CB1 receptors, which are localized to their axon terminals. Fourth, neural activity and Ca2+ entry stimulate the hippocampal synthesis of 2-AG, but have no effect on anandamide concentrations. Nevertheless, we still don’t know whether the endocannabinoid actually crosses back to the presynaptic nerve ending or is produced there by the action of another, unidentified retrograde signal . The fact that DSI is induced in vitro by levels of neural activity that could also be encountered in vivo indicates that this process might have a role in normal brain function. Although this idea is still questioned, various results link DSI to the regulation of hippocampal GAMMA OSCILLATIONS. These network oscillations are coordinated by CB1 -positive GABA interneurons and are influenced by cannabinoid agonists,drying rack for weed raising the possibility that an endocannabinoid substance might modulate their expression and be involved in the organization of hippocampal cell assemblies. Another function of DSI might relate to synaptic plasticity. By weakening GABA-mediated inhibition, DSI could facilitate the induction of long-term potentiation in individual CA1 pyramidal neurons; this might contribute in turn to the formation of ‘place fields’ or to other forms of hippocampus-dependent learning. Such a cognitive-enhancing action would not contradict the well-known amnesic effects of cannabinoid drugs as the latter might result from a generalized, circuit-independent activation of CB1 receptors in the hippocampus and other brain areas. Outside the hippocampus, endocannabinoid-mediated DSI has been shown to occur at interneuron– principal cell synapses of the cerebellum and probably will soon be discovered elsewhere.

CB1 -bearing interneurons are selectively localized to a subdivision of the amygdala called the basolateral complex, a key station in the neural circuitry that processes emotions and a primary site of cannabinoid analgesia. This localization, and the fact that CB1 inactivation causes anxiety-like and aggressive responses in rodents, indicate that the endocannabinoid system might influence affective states through changes in the amygdala’s efferent activity. This idea is further supported by two findings: first, presentation of anxiogenic stimuli increases anandamide and 2-AG concentrations in the mouse amygdala; second, FAAH inhibitors exhibit marked anxiolytic-like properties in rats. Locally formed endocannabinoids could modify the amygdala’s output in two complementary ways.They could depress glutamate release from axon terminals originating in the cortex and other brain regions. In addition, by reducing GABA release from basolateral interneurons, they might disinhibit GABA cells in the adjacent intercalated nuclei and consequently decrease the activity of their postsynaptic targets, the pyramidal neurons in the central nucleus of the amygdala, which constitute the structure’s primary efferent pathway. Here, local administration of cannabinoid agonists inhibits GABA release and profoundly affects motor behaviours. Membrane depolarization and dopamine D2 -receptor activation stimulate striatal anandamide formation, indicating that this endocannabinoid might contribute to the regulation of basal ganglia function. In agreement with this hypothesis, the CB1 antagonist rimonabant enhances the stimulation of movement that is induced in rats by dopamine agonists, whereas the endocannabinoid transport inhibitor AM404 attenuates this stimulation in a CB1 -dependent manner. Anandamide might act at multiple sites in the basal ganglia, including GABA projection neurons, corticostriatal glutamatergic terminals and local-circuit interneurons. Local-circuit interneurons are particularly notable because of their functional resemblance to CB1 -positive interneurons in the hippocampus, with which they share not only a GABA-containing phenotype, but also the ability to discharge high-frequency bursts of action potentials that can inhibit firing in large assemblies of projection cells. Does locally released anandamide gain access to these interneurons? Or does it primarily act on medium spiny cells and their cortical afferents? We don’t know yet. But these unanswered questions do not diminish the significance of striatal endocannabinoid signalling, which is further highlighted by the effectiveness of cannabinoid agonists in the symptomatic treatment of LEVODOPA-INDUCED DYSKINESIAS and TOURETTE’S SYNDROME, two disorders with strong striatal underpinnings.Beside their actions in the amygdala, cannabinoid agonists can influence the central processing of pain by interacting with CB1 receptors in the periaqueductal grey, rostral ventromedial medulla128 and spinal trigeminal nucleus. At each of these sites, CB1 activation depresses GABA release through a presynaptic mechanism, without causing significant changes in somatic membrane conductances. In the trigeminal nucleus, glycinergic transmission also is inhibited. Painful stimuli elicit anandamide release in the rat periaqueductal grey, and systemic administration of CB1 antagonists produces HYPERALGESIA in rats and mice. So, noxious stimuli can engage a central analgesic circuit operated by the endocannabinoids, which, working in combination with a parallel mechanism in the periphery, could underlie the analgesic properties of cannabinoid drugs.