Abstract
Endocannabinoid signaling in the dorsal raphe (DR) has recently been implicated in the regulation of anxiety and depression. However, the cellular mechanisms by which endocannabinoids (eCBs) regulate the excitability of DR 5-hydroxytryptamine (serotonin; 5-HT) neurons remain poorly understood. In the present study, using whole-cell recording from DR 5-HT neurons, we examined the effects of eCBs on glutamatergic synapses in the DR. We found that the eCB anandamide decreased the amplitude of evoked excitatory postsynaptic currents (eEPSCs). This effect was blocked by CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM 251) and mimicked by (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN 55,212-2), a CB1 receptor agonist. The inhibition of eEPSC amplitude was associated with an increase in the paired-pulse ratio and coefficient of variance. Activation of CB1 receptors also reduced the frequency, but not the amplitude, of miniature excitatory postsynaptic currents, indicating that eCBs inhibit glutamate release in the DR. In addition, we found that depolarization of DR 5-HT neurons induced a transient inhibition of the amplitude of eEPSCs, termed depolarization-induced suppression of excitation (DSE). The induction of DSE required an increase in postsynaptic intracellular calcium and was due to a decrease in glutamate release. Furthermore, pharmacological studies showed that blockade of CB1 receptors with AM 251 abolished the DSE. In contrast, activation of CB1 receptors with WIN 55,212-2 mimicked and occluded the DSE, indicating that depolarization of DR 5-HT neurons triggers eCB release, which in turn mediates the DSE. Together, these results indicate that eCBs play a role in modulating glutamatergic synaptic transmission to DR 5-HT neurons.
- eCB, endocannabinoid
- 5-HT, 5-hydroxytryptamine (serotonin)
- DR, dorsal raphe
- ACSF, artificial cerebrospinal fluid
- QX314, 2-((2,6-dimethylphenyl)amino)-N,N,N-triethyl-2-oxoethanaminium
- BAPTA, 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid
- EPSC, excitatory postsynaptic current
- DSE, depolarization-induced suppression of excitation
- eEPSC, evoked excitatory postsynaptic current
- WIN 55,212-2, (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate
- CV, coefficient of variance
- mEPSC, miniature excitatory postsynaptic current
- K-S, Kolmogorov-Smirnov
- GYKI 52466, 4-(8-methyl-9H-1,3-dioxolo[4,5-h][2,3]benzodiazepine-5-yl)-benzenamine hydrochloride
- AM 251, N-(piperidin-1-yl)-5-(4-iodophenyl)-4-methyl-1H-pyrazole-3-carboxamide
- O-1602, 5-methyl-4-[(1R,6R)-3-methyl-6-(1-cyclohexen-1-]-1,3-benzenediol
- d-AP5, d-(−)-2-amino-5-phosphonopentanoic acid
- AMPA, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid
- DPE, depolarization-induced potentiation of excitation
- PPR, paired pulse ratio
- NMDA, N-methyl-d-aspartate
Footnotes
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This work was supported by the National Institutes of Health National Institute of Mental Health [Grant MH078009] (to S.H.D.); and the Research Foundation of the State University of New York. R.-Y.S. was supported by the National Institutes of Health National Institute of Alcohol Abuse and Alcoholism [Grant AA12435].
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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ABBREVIATIONS:
- Received March 25, 2009.
- Accepted July 9, 2009.
- © 2009 by the American Society for Pharmacology and Experimental Therapeutics
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