Astrocytic calcium (Ca2+) signaling is required for the long-lasting facilitatory action of endocannabinoids (eCB) upon transmitter release from pyramidal hippocampal neurons, a form of long-term potentiation (LTP) known as eCB-LTP, which does not involve NMDA receptors (NMDAR). Astrocytic type one cannabinoid receptors (CB1R) are also required for “classical” forms of LTP but whether astrocytic Ca2+ signaling is a requisite this action of CB1R is poorly known. Adenosine A1 receptor (A1R) modulates CB1R activity in neurons, but the consequences of the astrocytic Ca2+ signaling for the A1R-CB1R crosstalk and its impact in LTP are also unknown. Thus, we aimed at evaluating whether CB1R-mediated astrocytic Ca2+ signaling impacts theta-burst-induced LTP at hippocampal CA3-CA1 synapses and how it is affected by A1R. We used IP3R2-KO mice, which lack IP3R2-mediated astrocytic Ca2+ signaling, in comparison with wild-type littermates (IP3R2-WT). We show that while the absence of IP3R2-mediated astrocytic Ca2+ signaling does not impact LTP, it does reduce the protein levels of NMDAR-NR2B subunit. Exogenous activation or blockade of CB1R reduced LTP in IP3R2-WT mice, while in IP3R2-KO mice LTP levels were not significantly affected by CB1R activation. Blocking A1R reduced LTP in both IP3R2-WT and IP3R2-KO mice, and prevented the inhibition caused by exogenous CB1R activation in IP3R2-WT mice but not in IP3R2-KO mice. Our data demonstrates that the inhibition of hippocampal LTP by exogenous CB1R activation requires astrocytic calcium signaling, which also play a role in the CB1R/A1R interaction. This work thus adds novel partners to synaptic plasticity modulation by cannabinoids.

Background and Purpose Adenosine, through the A1 receptor (A1R), is an endogenous anticonvulsant. Development of adenosine receptor agonists as antiseizure medications has been hampered by their cardiac side effects. A moderately A1R-selective agonist, MRS5474, has been reported to suppress seizures without considerable cardiac action. Hypothesizing that this drug could act through other than A1R and/or through a disease specific mechanism, we assessed the effect of MRS5474 on the hippocampus. Experimental Approach Excitatory synaptic currents, field potentials, spontaneous activity, [3H]GABA uptake and GABAergic currents were recorded from rodent or human hippocampal tissue. Alterations in adenosine A3 receptor (A3R) density in human tissue were assessed by Western Blot. Key Results MRS5474 (50-500nM) was devoid of effect upon rodent excitatory synaptic signals in hippocampal slices, except when hyperexcitability was previously induced in vivo or ex vivo. This contrasted with the effect of other A1R agonists. MRS5474 inhibited GAT-1 mediated GABA uptake, an action not blocked by an A1R antagonist but blocked by an A3R antagonist and mimicked by an A3R agonist. A3R was overexpressed in human hippocampal tissue samples from patients with epilepsy that had focal resection from surgery. MRS5474 induced a concentration-dependent potentiation of GABA-evoked currents in oocytes micro-transplanted with human hippocampal membranes prepared from epileptic hippocampal tissue but not from non-epileptic tissue, an action blocked by an A3R antagonist. Conclusion and Implications We identified a drug that activates A3R and has selective actions on epileptic hippocampal tissue. This underscores A3R as a promising target for the development of antiseizure medications.