Comparison of the ability of adenosine kinase inhibitors and adenosine receptor agonists to attenuate thermal hyperalgesia and reduce motor performance in rats

Abstract

Inhibitors of adenosine kinase (AK) enhance extracellular concentrations of the inhibitory neuromodulator adenosine (ADO) at sites of tissue hyperexcitability and produce antinociceptive effects in animal models of pain and inflammation. The present study compared the ability of several novel and selective AK inhibitors and ADO receptor-selective agonists to attenuate carrageenan-induced thermal hyperalgesia and to impair motor performance as measured by effects on exploratory motor activity and rotorod performance. The prototypical nucleoside AK inhibitor, 5'deoxy-5-iodotubercidin (5'd-5IT), dose-dependently blocked thermal hyperalgesia (ED(50)=0.2 micromol/kg ip) and was 4- and 75-fold less potent in reducing exploratory motor activity and rotorod performance, respectively. The antihyperalgesic effects of 5'd-5IT were fully blocked by the A(1) antagonist, cyclopentyltheophylline (CPT) and the A(2A) antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX). Novel nucleoside and non-nucleoside AK inhibitors (A-134974, A-286501 and ABT-702) also potently (ED(50)=0.7-2 micromol/kg ip) blocked carrageenan-induced thermal hyperalgesia and were significantly less potent than 5'd-5IT in impairing motor performance. The systemic administration of N(6)-cyclopentyladenosine (CPA), an A(1) receptor-selective agonist, CGS 21680, an A(2A) receptor-selective agonist, and N(6)-ethylcarboxamidoadenosine (NECA), a nonselective ADO receptor agonist potently reduced (ED(50)=0.3-1.0 micromol/kg ip) thermal hyperalgesia. Unlike the AK inhibitors, however, these ADO receptor agonists produced significant antinociception only at doses that also decreased motor performance. These data demonstrate that AK inhibitors produce specific antihyperalgesic effects via an interaction with ADO A(1) and A(2A) receptors at doses that lack detectable effects on exploratory motor activity and rotorod performance and offer an improved separation between antinociceptive and motor impairing effects as compared to ADO receptor agonists.