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Molecular architecture of endocannabinoid signaling at nociceptive synapses mediating analgesia.

ABSTRACT: Cannabinoid administration suppresses pain by acting at spinal, supraspinal and peripheral levels. Intrinsic analgesic pathways also exploit endocannabinoids; however, the underlying neurobiological substrates of endocannabinoid-mediated analgesia have remained largely unknown. Compelling evidence shows that, upon exposure to a painful environmental stressor, an endocannabinoid molecule called 2-arachidonoylglycerol (2-AG) is mobilized in the lumbar spinal cord in temporal correlation with stress-induced antinociception. We therefore characterized the precise molecular architecture of 2-AG signaling and its involvement in nociception in the rodent spinal cord. Nonradioactive in situ hybridization revealed that dorsal horn neurons widely expressed the mRNA of diacylglycerol lipase-alpha (DGL-alpha), the synthesizing enzyme of 2-AG. Peroxidase-based immunocytochemistry demonstrated high levels of DGL-alpha protein and CB(1) cannabinoid receptor, a receptor for 2-AG, in the superficial dorsal horn, at the first site of modulation of the ascending pain pathway. High-resolution electron microscopy uncovered postsynaptic localization of DGL-alpha at nociceptive synapses formed by primary afferents, and revealed presynaptic positioning of CB(1) on excitatory axon terminals. Furthermore, DGL-alpha in postsynaptic elements receiving nociceptive input was colocalized with metabotropic glutamate receptor 5 (mGluR(5)), whose activation induces 2-AG biosynthesis. Finally, intrathecal activation of mGluR(5) at the lumbar level evoked endocannabinoid-mediated stress-induced analgesia through the DGL-2-AG-CB(1) pathway. Taken together, these findings suggest a key role for 2-AG-mediated retrograde suppression of nociceptive transmission at the spinal level. The striking positioning of the molecular players of 2-AG synthesis and action at nociceptive excitatory synapses suggests that pharmacological manipulation of spinal 2-AG levels may be an efficacious way to regulate pain sensation.

SUBMITTER: Nyilas R

PROVIDER: S-EPMC2692098 | biostudies-literature | 2009 May

REPOSITORIES: biostudies-literature

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Molecular architecture of endocannabinoid signaling at nociceptive synapses mediating analgesia.

Nyilas Rita R Gregg Laura C LC Mackie Ken K Watanabe Masahiko M Zimmer Andreas A Hohmann Andrea G AG Katona István I

The European journal of neuroscience 20090509 10

Cannabinoid administration suppresses pain by acting at spinal, supraspinal and peripheral levels. Intrinsic analgesic pathways also exploit endocannabinoids; however, the underlying neurobiological substrates of endocannabinoid-mediated analgesia have remained largely unknown. Compelling evidence shows that, upon exposure to a painful environmental stressor, an endocannabinoid molecule called 2-arachidonoylglycerol (2-AG) is mobilized in the lumbar spinal cord in temporal correlation with stres ...[more]

PMID: 19453631

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Project description:BackgroundAlthough a number of clinical and preclinical studies have demonstrated analgesic effects of cannabinoid treatments, there are also instances when cannabinoids have had no effect or even exacerbated pain. The observed pro-nociceptive effects appear to be due to cannabinoid-induced disinhibition of afferent synaptic input to nociceptive circuits. To better understand how cannabinoid-mediated plasticity can have both pro- and anti-nociceptive effects, we examined the possibility that cannabinoids differentially modulate nociceptive vs. non-nociceptive synapses onto a shared postsynaptic target. These experiments were carried out in the central nervous system (CNS) of the medicinal leech, in which it is possible to intracellularly record from presynaptic nociceptive (N-cell) or pressure-sensitive (P-cell) neurons and their shared postsynaptic targets.ResultsThe endocannabinoid 2-arachidonoyl glycerol (2AG) elicited significant long-lasting depression in nociceptive (N-cell) synapses. However, non-nociceptive (P-cell) synapses were potentiated following 2AG treatment. 2AG-induced potentiation of non-nociceptive synapses was blocked by the TRPV antagonist SB366791, suggesting involvement of the same TRPV-like receptor that has already been shown to mediate endocannabinoid-dependent depression in nociceptive inputs. Treatment with the GABA receptor antagonist bicuculline also blocked 2AG-induced potentiation, consistent with the idea that increased synaptic signaling was the result of endocannabinoid-mediated disinhibition. Interestingly, while bicuculline by itself increased non-nociceptive synaptic transmission, nociceptive synapses were depressed by this GABA receptor antagonist indicating that nociceptive synapses were actually excited by GABAergic input. Consistent with these observations, GABA application depolarized the nociceptive afferent and hyperpolarized the non-nociceptive afferent.ConclusionsThese findings show that endocannabinoids can differentially modulate nociceptive vs. non-nociceptive synapses and that GABAergic regulation of these synapses plays an important role in determining whether endocannabinoids have a potentiating or depressing effect.

Project description:We tested the hypotheses that steroidogenic factor (SF)-1 neurons in the hypothalamic ventromedial nucleus (VMN) provide sexually disparate, endocannabinoid (EC)- and diet-sensitive glutamatergic input onto proopiomelanocortin (POMC) neurons. Electrophysiological recordings were performed in hypothalamic slices from intact and castrated guinea pigs, along with in vitro optogenetic experiments in intact male as well as cycling and ovariectomized female NR5A1-Cre mice. In slices from castrated male and female guinea pigs, depolarized-induced suppression of excitation (DSE) time-dependently reduced the amplitude of evoked excitatory postsynaptic currents (eEPSCs) in POMC neurons generated by electrically stimulating the dorsomedial VMN. Androgen stimulation rapidly enhanced this DSE, which was also found in insulin-resistant, high-fat diet (HFD)-fed males. By contrast, retrograde signaling at VMN/ARC POMC synapses was markedly attenuated in periovulatory females. HFD potentiated central cannabinoid-induced hyperphagia in both males and females, but exerted differential influences on cannabinoid-induced increases in energy expenditure. In NR5A1-Cre mice, the reduction in light-evoked EPSC amplitude caused by postsynaptic depolarization in cycling females was modest in comparison to that seen in intact males. Estradiol attenuated the DSE in light-evoked EPSC amplitude in slices from ovariectomized females. Moreover, the retrograde inhibition of transmission was further accentuated in HFD-fed males. Chemogenetic activation of SF-1 neurons suppressed appetite and increased energy expenditure in males, effects which were attenuated by HFD. Conversely, energy expenditure was increased in estradiol- but not vehicle-treated ovariectomized females. Together with our previous studies indicating that DSE in POMC neurons is EC-mediated, these findings indicate that VMN SF-1/ARC POMC synapses represent a sexually differentiated, EC- and diet-sensitive anorexigenic component within the hypothalamic energy balance circuitry.

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Molecular architecture of endocannabinoid signaling at nociceptive synapses mediating analgesia.

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Molecular architecture of endocannabinoid signaling at nociceptive synapses mediating analgesia.

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