ABSTRACT
Title
Interactions between adenosine A2A and cannabinoid CB1 receptors in the striatum
Authors
V.Chiodi, M. T. Tebano, A. Martire, M. R. Domenici, P. Popoli
Dept. of Therapeutic Research and Medicines Evaluation, Istituto Superiore di Sanità, Roma, Italy
Dept. of Therapeutic Research and Medicines Evaluation, Istituto Superiore di Sanità, Roma, Italy
Abstract
Adenosine A2A (A2ARs) and cannabinoid CB1 (CB1Rs) receptors form heteromers and functionally interact in the striatum. We recently identified two different ways by which A2ARs can modulate CB1-mediated effects. Indeed, besides the CB1 antagonism elicited by A2AR blockade (Tebano et al., 2009), also A2AR activation by CGS21680 abrogated CB1-induced effects (Martire et al., 2011). The former effect seemed mainly related to the blockade of A2ARs forming heteromers with CB1Rs or, possibly, to indirect mechanisms such as the A2A/mGlu5 receptor interaction. Conversely, the antagonism of CB1 effects induced by activation of A2ARs seemed to occur presynaptically and to involve, at least in part, the A2A/A1 receptor interaction. To further investigate the complex A2ARs/CB1 relationship, we used two models in which the expression and the functions of A2Aand/or CB1 receptors are altered: namely transgenic (TGR) A2AR overexpressing rats (generously supplied by Michael Bader) and transgenic R6/2 mice (a model of Huntington’s disease).
We first explored the effect of CB1 receptor agonist in WT and TGR (NSEhA2a) rat, a transgenic rat strain overexpressing adenosine A2ARs mainly in the cerebral cortex, the hippocampus and the cerebellum (Gimenez-LLort et al., 2007). Although the increase in A2ARs is less marked in the striatum, it is still significant and, interestingly, on the basis of neurochemical and functional evidences, it seems that in this model the overexpression of A2ARs occurs more in the corticostriatal terminals than in striatal neurons. In extracellular recordings from rat corticostriatal slices (300 µm), the CB1 receptor agonist WIN 55,212-2 (2 μM applied for 40 minutes) induced a marked depression of synaptic transmission both in WT and in TGR rat, as demonstrated by the reduction of FP amplitude. However, in line with our previous findings of an inhibitory influence of presynaptic A2ARs, the ability of WIN55,212-2 to reduce synaptic transmission was significantly attenuated in TGR rat in comparison to WT (63.24 ± 3.3% and 30.40 ± 3.4% of the basal values, respectively, p<0.0001; n=8 Mann Whitney U-test). Co-application of the selective A2AR agonist, CGS21680 (100 nM) prevented the inhibitory action of WIN55,212-2 both in WT (61.83 ± 11% CGS plus WIN, p<0.05 vs WIN alone; n=6) and in TGR rat (81.8 ± 6.6% CGS plus WIN, p<0.05 vs WIN alone, Mann Whitney U-test; n=3). Conversely, the pharmacological blockade of A2ARs by ZM241385 (100 nM) prevented CB1-mediated effects in WT (62,68 ± 9.9% ZM plus WIN, p<0.005 vs WIN alone, Mann Whitney U-test; n=3) but not in TGR rats (61 ± 7% ZM plus WIN; n=4).
As previously reported in mouse corticostriatal slices (Tebano et al. 2009), slice perfusion with the CB1 receptor agonist WIN 55,212-2 (2 μM) induced a marked depression of synaptic transmission both in WT and in R6/2 mice, as demonstrated by the reduction of FP amplitude. However, WIN-induced reduction of FP amplitude was significantly more pronounced in R6/2 than in WT mice (30.6 ± 8.2% and 54.37 ± 5.1% of the basal values, respectively, p<0.05 Mann Whitney U-test). Such an effect of CB1R stimulation, which was accompanied by an increase in the magnitude of paired pulse facilitation (an index of reduced presynaptic neurotransmitter release) and by a reduction of evoked glutamate efflux from striatal synaptosomes, was prevented by the CB1R antagonist AM 251 and by the A2AR agonist CGS21680 (100 nM) but not by the antagonist ZM241385 (100 nM).
These data indicate that, under particular conditions, the inhibitory influence of A2ARs on CB1 Rs may prevail over the enabling one. These findings strengthen the complex regulatory role exerted by A2ARs on CB1-mediated effects and suggest that such an interaction may contribute to the pathogenesis of striatal dysfunctions.
Tebano et al.(2009) J Neurochem., 110:1921.
Martire et al. (2011). J Neurochem 116:273.
Gimènez-Llort et al. (2007) Neurobiol Learn Mem 87:42.
We first explored the effect of CB1 receptor agonist in WT and TGR (NSEhA2a) rat, a transgenic rat strain overexpressing adenosine A2ARs mainly in the cerebral cortex, the hippocampus and the cerebellum (Gimenez-LLort et al., 2007). Although the increase in A2ARs is less marked in the striatum, it is still significant and, interestingly, on the basis of neurochemical and functional evidences, it seems that in this model the overexpression of A2ARs occurs more in the corticostriatal terminals than in striatal neurons. In extracellular recordings from rat corticostriatal slices (300 µm), the CB1 receptor agonist WIN 55,212-2 (2 μM applied for 40 minutes) induced a marked depression of synaptic transmission both in WT and in TGR rat, as demonstrated by the reduction of FP amplitude. However, in line with our previous findings of an inhibitory influence of presynaptic A2ARs, the ability of WIN55,212-2 to reduce synaptic transmission was significantly attenuated in TGR rat in comparison to WT (63.24 ± 3.3% and 30.40 ± 3.4% of the basal values, respectively, p<0.0001; n=8 Mann Whitney U-test). Co-application of the selective A2AR agonist, CGS21680 (100 nM) prevented the inhibitory action of WIN55,212-2 both in WT (61.83 ± 11% CGS plus WIN, p<0.05 vs WIN alone; n=6) and in TGR rat (81.8 ± 6.6% CGS plus WIN, p<0.05 vs WIN alone, Mann Whitney U-test; n=3). Conversely, the pharmacological blockade of A2ARs by ZM241385 (100 nM) prevented CB1-mediated effects in WT (62,68 ± 9.9% ZM plus WIN, p<0.005 vs WIN alone, Mann Whitney U-test; n=3) but not in TGR rats (61 ± 7% ZM plus WIN; n=4).
As previously reported in mouse corticostriatal slices (Tebano et al. 2009), slice perfusion with the CB1 receptor agonist WIN 55,212-2 (2 μM) induced a marked depression of synaptic transmission both in WT and in R6/2 mice, as demonstrated by the reduction of FP amplitude. However, WIN-induced reduction of FP amplitude was significantly more pronounced in R6/2 than in WT mice (30.6 ± 8.2% and 54.37 ± 5.1% of the basal values, respectively, p<0.05 Mann Whitney U-test). Such an effect of CB1R stimulation, which was accompanied by an increase in the magnitude of paired pulse facilitation (an index of reduced presynaptic neurotransmitter release) and by a reduction of evoked glutamate efflux from striatal synaptosomes, was prevented by the CB1R antagonist AM 251 and by the A2AR agonist CGS21680 (100 nM) but not by the antagonist ZM241385 (100 nM).
These data indicate that, under particular conditions, the inhibitory influence of A2ARs on CB1 Rs may prevail over the enabling one. These findings strengthen the complex regulatory role exerted by A2ARs on CB1-mediated effects and suggest that such an interaction may contribute to the pathogenesis of striatal dysfunctions.
Tebano et al.(2009) J Neurochem., 110:1921.
Martire et al. (2011). J Neurochem 116:273.
Gimènez-Llort et al. (2007) Neurobiol Learn Mem 87:42.