Lack of presynaptic interaction between glucocorticoid and CB1 cannabinoid receptors in GABA- and glutamatergic terminals in the frontal cortex of laboratory rodents
CB1 and glucocorticoid (GcRs) receptors are present in rat frontocortical synapses.
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CB1Rs are mostly presynaptic, while GcRs are mostly post-synaptic.
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Lack of presynaptic corticosteroid effect on frontocortical GABA/glutamate release.
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Lack of direct presynaptic interaction between corticosteroids and CB1Rs.
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Mifepristone strongly inhibits resting and evoked release of GABA and glutamate.
Abstract
Corticosteroid and endocannabinoid actions converge on prefrontocortical circuits associated with neuropsychiatric illnesses. Corticosteroids can also modulate forebrain synapses by using endocannabinoid effector systems. Here, we determined whether corticosteroids can modulate transmitter release directly in the frontal cortex and, in doing so, whether they affect presynaptic CB1 cannabinoid receptor- (CB1R) mediated neuromodulation. By Western blotting of purified subcellular fractions of the rat frontal cortex, we found glucocorticoid receptors (GcRs) and CB1Rs enriched in isolated frontocortical nerve terminals (synaptosomes). CB1Rs were predominantly presynaptically located while GcRs showed preference for the post-synaptic fraction. Additional confocal microscopy analysis of cortical and hippocampal regions revealed vesicular GABA transporter-positive and vesicular glutamate transporter 1-positive nerve terminals endowed with CB1R immunoreactivity, apposing GcR-positive post-synaptic compartments. In functional transmitter release assay, corticosteroids, corticosterone (0.1–10 microM) and dexamethasone (0.1–10 microM) did not significantly affect the evoked release of [3H]GABA and [14C]glutamate in superfused synaptosomes, isolated from both rats and mice. In contrast, the synthetic cannabinoid, WIN55212-2 (1 microM) diminished the release of both [3H]GABA and [14C]glutamate, evoked with various depolarization paradigms. This effect of WIN55212-2 was abolished by the CB1R neutral antagonist, O-2050 (1 microM), and was absent in the CB1R KO mice. CB2R-selective agonists did not affect the release of either neurotransmitter. The lack of robust presynaptic neuromodulation by corticosteroids was unchanged upon either CB1R activation or genetic inactivation. Altogether, corticosteroids are unlikely to exert direct non-genomic presynaptic neuromodulation in the frontal cortex, but they may do so indirectly, via the stimulation of trans-synaptic endocannabinoid signaling.
Keywords
CB1 cannabinoid receptor
Frontal cortex
GABA
Glucocorticoid receptor
Glutamate
Presynaptic
Abbreviations
3Rs
Replacement, Refinement and Reduction of Animals in Research
4-AP
4-aminopyridine
ARRIVE
Animals in Research: Reporting In Vivo Experiments
CB1R(s), CB2R(s)
cannabinoid CB1 and CB2 receptor(s)
corti
corticosterone
DAGLα
diacylglycerol lipase α
dexa
dexamethasone
DMSO
dimethylsulfoxide
DPM
disintegration per minute
FELASA
Federation for Laboratory Animal Science Associations
Present address: Dept. Pharmacy, University of Western Santa Catarina – UNOESC, Campus Videira, Rua Paese, N 198, Bairro Universitário, 89560-000 Videira, SC, Brazil.