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

Adenosine, through the A₁ receptor (A₁R), is an endogenous anticonvulsant. The 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.

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.

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 γ-aminobutyric acid (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.

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.