Lack of presynaptic interaction between glucocorticoid and CB1 cannabinoid receptors in GABA- and glutamatergic terminals in the frontal cortex of laboratory rodents

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 [(3)H]GABA and [(14)C]glutamate in superfused synaptosomes, isolated from both rats and mice. In contrast, the synthetic cannabinoid, WIN55212-2 (1 microM) diminished the release of both [(3)H]GABA and [(14)C]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.

Cannabis: A Treasure Trove or Pandora's Box?

“…Notwithstanding, a recent characterization of O-2050 revealed some unexpected in vitro partial agonist activity in cyclic AMP assay, and a lack of antagonism on in vivo effects of Δ 9 -THC and WIN55212-2 [393]. In our hands, O-2050 (1 µM) had no effect on the depolarizationinduced release of GABA, glutamate and serotonin in the frontal cortex [396,397], and prevented the inhibitory action of WIN55212-2. In that sense, it mimicked the effect of the genetic deletion of the CB 1 R and we concluded that O-2050 is a useful in vitro CB 1 R antagonist up to the concentration of 1 µM.…”

“…Steroid hormones interact with the endocannabinoid system, which has significant therapeutic potential. One important feature of glucocorticoid receptors (GcRs) is that they co-localize with DAGLα in post-synaptic neural compartments apposing CB 1 R-positive nerve terminals in the brain [397]. GcR activation in the prefrontal cortex triggers endocannabinoid release [966], which in turn downregulates the hypothalamo-pituitary-adrenal (HPA) axis to decrease circulating corticosteroid levels [967,968], facilitate the extinction and impair the retrieval of aversive memories [965,967,[969][970][971].…”

Endocannabinoid Signaling and the Hypothalamic‐Pituitary‐Adrenal Axis

“…2). Like in the hypothalamus, recent data indicate that the GR in the frontal cortex are expressed on postsynaptic profiles (3). Excitatory projections from these brain regions accelerate the poststress decline in circulating glucocorticoids likely through activation of inhibitory inputs from the bed nucleus of the stria terminalis to the PVN (31, 35).…”

Voltage Dependence of a Neuromodulator-Activated Ionic Current

“…Neuromodulators enhance the flexibility of neural networks by multiple mechanisms. These include the regulation of intrinsic properties (i.e., pre-existing conductances; Benson and Levitan, 1983 ; Kiehn and Harris-Warrick, 1992a , b ; Harris-Warrick et al, 1995 ; Galbavy et al, 2013 ), modulating synaptic properties ( Johnson et al, 1993a , b,1995 ; Zhao et al, 2011 ; Bitencourt et al, 2015 ; Böhm et al, 2015 ), reconfiguring participating neurons within the network ( Hooper and Moulins, 1989 ; Fénelon et al, 1998 ; Lieske et al, 2000 ), and modulating plasticity and even modulation itself ( Mesce, 2002 ; McLean and Sillar, 2004 ; Zhou et al, 2007 ; Pawlak et al, 2010 ; Lawrence et al, 2015 ). Another important mechanism of the regulation of neuronal excitability is the modulation of leak currents ( Bayliss et al, 1992 ; Erxleben et al, 1995 ; Talley et al, 2000 ; Cymbalyuk et al, 2002 ; Xu et al, 2009 ), and of the ratio of leak current to pacemaking current amplitude (and not just the pacemaking current amplitude), as is the case in the regulation of pacemaking activity in the pre-Bötzinger complex ( Del Negro et al, 2002 ).…”