Nephi Stella

A second endogenous cannabinoid that modulates long-term potentiation

Cannabinoid receptors are molecular targets for marijuana and hashish, the widespread drugs of abuse. These receptors are expressed in areas of the central nervous system that contribute in important ways to the control of memory, cognition, movement and pain perception. Indeed, such functions can be strongly influenced by cannabinoid drugs, with consequences that include euphoria, analgesia, sedation and memory impairment. Although the pharmacology of cannabinoid drugs is now beginning to be understood, we still lack essential information on the endogenous signalling system(s) by which cannabinoid receptors are normally engaged. An endogenous ligand for cannabinoid receptors, anandamide, has been described. Here we report that sn-2 arachidonylglycerol (2-AG), a cannabinoid ligand isolated from intestinal tissue, is present in brain in amounts 170 times greater than anandamide. 2-AG is produced in hippocampal slices by stimulation of the Schaffer collaterals, an excitatory fibre tract that projects from CA3 to CA1 neurons. Formation of 2-AG is calcium dependent and is mediated by the enzymes phospholipase C and diacylglycerol lipase. 2-AG activates neuronal cannabinoid receptors as a full agonist, and prevents the induction of long-term potentiation at CA3–CA1 synapses. Our results indicate that 2-AG is a second endogenous cannabinoid ligand in the central nervous system.

Evidence supporting the existence of an activity-dependent astrocyte-neuron lactate shuttle

Mounting evidence from in vitro experiments indicates that lactate is an efficient energy substrate for neurons and that it may significantly contribute to maintain synaptic transmission, particularly during periods of intense activity. Since lactate does not cross the blood-brain barrier easily, blood-borne lactate cannot be a significant source. In vitro studies by several laboratories indicate that astrocytes release large amounts of lactate. In 1994, we proposed a mechanism whereby lactate could be produced by astrocytes in an activity-dependent, glutamate-mediated manner. Over the last 2 years we have obtained further evidence supporting the notion that a transfer of lactate from astrocytes to neurons might indeed take place. In this article, we first review data showing the presence of mRNA encoding for two monocarboxylate transporters, MCT1 and MCT2, in the adult mouse brain. Second, by using monoclonal antibodies selectively directed against the two distinct lactate dehydrogenase isoforms, LDH1 and LDH5, a specific cellular distribution between neurons and astrocytes is revealed which suggests that a population of astrocytes is a lactate ‘source’ while neurons may be a lactate ‘sink’. Third, we provide biochemical evidence that lactate is interchangeable with glucose to support oxidative metabolism in cortical neurons. This set of data is consistent with the existence of an activity-dependent astrocyte-neuron lactate shuttle for the supply of energy substrates to neurons.

The serine hydrolase ABHD6 controls the accumulation and efficacy of 2-AG at cannabinoid receptors

The endocannabinoid 2-arachidonoylglycerol (2-AG) regulates neurotransmission and neuroinflammation by activating CB1 cannabinoid receptors on neurons and CB2 cannabinoid receptors on microglia. Enzymes that hydrolyze 2-AG, such as monoacylglycerol lipase, regulate the accumulation and efficacy of 2-AG at cannabinoid receptors. We found that the recently described serine hydrolase α-β-hydrolase domain 6 (ABHD6) also controls the accumulation and efficacy of 2-AG at cannabinoid receptors. In cells from the BV-2 microglia cell line, ABHD6 knockdown reduced hydrolysis of 2-AG and increased the efficacy with which 2-AG can stimulate CB2-mediated cell migration. ABHD6 was expressed by neurons in primary culture and its inhibition led to activity-dependent accumulation of 2-AG. In adult mouse cortex, ABHD6 was located postsynaptically and its selective inhibition allowed the induction of CB1-dependent long-term depression by otherwise subthreshold stimulation. Our results indicate that ABHD6 is a rate-limiting step of 2-AG signaling and is therefore a bona fide member of the endocannabinoid signaling system.

Cannabinoids and neuroinflammation

Growing evidence suggests that a major physiological function of the cannabinoid signaling system is to modulate neuroinflammation. This review discusses the anti‐inflammatory properties of cannabinoid compounds at molecular, cellular and whole animal levels, first by examining the evidence for anti‐inflammatory effects of cannabinoids obtained using in vivo animal models of clinical neuroinflammatory conditions, specifically rodent models of multiple sclerosis, and second by describing the endogenous cannabinoid (endocannabinoid) system components in immune cells. Our aim is to identify immune functions modulated by cannabinoids that could account for their anti‐inflammatory effects in these animal models.

The endocannabinoid system drives neural progenitor proliferation

The discovery of multipotent neural progenitor (NP) cells has provided strong support for the existence of neurogenesis in the adult brain. However, the signals controlling NP proliferation remain elusive. Endocannabinoids, the endogenous counterparts of marijuana‐derived cannabinoids, act as neuromodulators via presynaptic CB1 receptors and also control neural cell death and survival. Here we show that progenitor cells express a functional endocannabinoid system that actively regulates cell proliferation both in vitro and in vivo. Specifically, NPs produce endocannabinoids and express the CB1 receptor and the endocannabinoid‐inactivating enzyme fatty acid amide hydrolase (FAAH). CB1 receptor activation promotes cell proliferation and neurosphere generation, an action that is abrogated in CB1‐deficient NPs. Accordingly, proliferation of hippocampal NPs is increased in FAAH‐deficient mice. Our results demonstrate that endocannabinoids constitute a new group of signaling cues that regulate NP proliferation and thus open novel therapeutic avenues for manipulation of NP cell fate in the adult brain.

Cannabinoid receptors and endocannabinoids: evidence for new players.

It is now well established that the psychoactive effects ofCannabis sativa are primarily mediated through neuronal CB1 receptors, while its therapeutic immune properties are primarily mediated through CB2 receptors. Two endocannabinoids, arachidonoylethanolamide and 2-arachidonoylglycerol, have been identified, their action on CB1 and CB2 thoroughly characterized, and their production and inactivation elucidated. However, many significant exceptions to these rules exist. Here we review the evidence suggesting that cannabinoids can modulate synaptic transmission, the cardiovascular system, and the immune system through receptors distinct from CB1 and CB2, and that an additional “independent” endocannabinoid signaling system that involves palmitoylethanolamide may exist.

Receptor-dependent formation of endogenous cannabinoids in cortical neurons.

We investigated the transduction mechanisms mediating formation of the endogenous cannabinoid (endocannabinoid) lipids, anandamide (arachidonylethanolamide) and 2-arachidonylglycerol, in primary cultures of rat cortical neurons. Unstimulated neurons contained 0.3 +/- 0.1 pmol of anandamide and 16.5 +/- 3.3 pmol of 2-arachidonylglycerol per mg of protein, as determined by gas chromatography/mass spectrometry. Ca(2+) entry into the neurons via activated glutamate N-methyl-D-aspartate (NMDA) receptors increased 2-arachidonylglycerol levels approximately three times, but had no effect on anandamide levels. By contrast, anandamide formation was stimulated five times by simultaneous activation of NMDA and acetylcholine receptors. Alone, acetylcholine receptor activation had no effect on anandamide or 2-arachidonylglycerol levels. The formation of fatty acid ethanolamides that do not activate cannabinoid receptors, including palmitylethanolamide and oleylethanolamide, was stimulated by coactivation of NMDA and acetylcholine receptors. Pharmacological experiments suggest that the cholinergic contribution to anandamide formation was mediated by alpha7 nicotinic receptors (antagonized by methyllycaconitine), whereas the contribution to palmitylethanolamide and oleylethanolamide formation was mediated by muscarinic receptors (antagonized by atropine). These findings indicate that cortical neurons produce anandamide and 2-arachidonylglycerol in a receptor-dependent manner, and that brain neurons may generate different endocannabinoid lipids depending on their complement of neurotransmitter receptors.

Chronic microsensors for longitudinal, subsecond dopamine detection in behaving animals.

Neurotransmission operates on a millisecond timescale but is changed by normal experience or neuropathology over days to months. Despite the importance of long-term neurotransmitter dynamics, no technique exists to track these changes in a subject from day to day over extended periods of time. Here we describe and characterize a microsensor that can detect the neurotransmitter dopamine with subsecond temporal resolution over months in vivo in rats and mice.

Cannabinoid signaling in glial cells

The cannabinoid signaling system is composed of cannabinoid (CB) receptors, their endogenous ligands, the endocannabinoids, and the enzymes that produce and inactivate them. It is well known that neurons communicate between each other through this signaling system. Δ9‐tetrahydrocannabinol, the main psychoactive compound of marijuana, interacts with CB receptors, impinging on this communication and inducing profound behavioral effects such as memory impairment and analgesia. Recent evidence suggests that glial cells also express components of the cannabinoid signaling system and marijuana‐derived compounds act at CB receptors expressed by glial cells, affecting their functions. This review summarizes this evidence, discusses how glial cells might use the cannabinoid signaling system to communicate with neighboring cells, and argues that nonpsychotropic cannabinoids, both marijuana‐derived and synthetic, likely constitute lead compounds for therapy aimed at reducing acute and chronic neuroinflammation, such as occurs in multiple sclerosis.

The Endocannabinoid System Promotes Astroglial Differentiation by Acting on Neural Progenitor Cells

Endocannabinoids exert an important neuromodulatory role via presynaptic cannabinoid CB1 receptors and may also participate in the control of neural cell death and survival. The function of the endocannabinoid system has been extensively studied in differentiated neurons, but its potential role in neural progenitor cells remains to be elucidated. Here we show that the CB1 receptor and the endocannabinoid-inactivating enzyme fatty acid amide hydrolase are expressed, both in vitro and in vivo, in postnatal radial glia (RC2+ cells) and in adult nestin type I (nestin+GFAP+) neural progenitor cells. Cell culture experiments show that CB1 receptor activation increases progenitor proliferation and differentiation into astroglial cells in vitro. In vivo analysis evidences that, in postnatal CB1−/− mouse brain, progenitor proliferation and astrogliogenesis are impaired. Likewise, in adult CB1-deficient mice, neural progenitor proliferation is decreased but is increased in fatty acid amide hydrolase-deficient mice. In addition, endocannabinoid signaling controls neural progenitor differentiation in the adult brain by promoting astroglial differentiation of newly born cells. These results show a novel physiological role of endocannabinoids, which constitute a new family of signaling cues involved in the regulation of neural progenitor cell function.