Selective modulation of epileptic tissue by an adenosine A3 receptor-activating drug

Background and purpose: Adenosine, through the A(1) receptor (A(1)R), is an endogenous anticonvulsant. The development of adenosine receptor agonists as antiseizure medications has been hampered by their cardiac side effects. A moderately A(1)R-selective agonist, MRS5474, has been reported to suppress seizures without considerable cardiac action. Hypothesizing that this drug could act through other than A(1)R 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, [H-3]GABA uptake and GABAergic currents were recorded from rodent or human hippocampal tissue. Alterations in adenosine A(3) receptor (A(3)R) density in human tissue were assessed by Western blot. Key results: MRS5474 (50-500 nM) was devoid of effect upon rodent excitatory synaptic signals in hippocampal slices, except when hyperexcitability was previously induced in vivo or ex vivo. MRS5474 inhibited GABA transporter type 1 (GAT-1)-mediated gamma-aminobutyric acid (GABA) uptake, an action not blocked by an A(1)R antagonist but blocked by an A(3)R antagonist and mimicked by an A(3)R agonist. A(3)R 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 A(3)R antagonist. Conclusion and implications: We identified a drug that activates A(3)R and has selective actions on epileptic hippocampal tissue. This underscores A(3)R as a promising target for the development of antiseizure medications.