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Effects of Intrathecal Injection of the Adenosine Receptor Agonists R-Phenylisopropyl-Adenosine and N-Ethylcarboxamide-Adenosine on Nociception and Motor Function in the Rat

Received from the Department of Anesthesiology and the Hospital Pharmacy, University Hospital, Uppsala, Sweden; Astra Pain Control, Södertälje, Sweden; and the Department of Pharmacology, Uppsala University, Uppsala, Sweden.

Abstract

R-phenylisopropyl-adenosine, which has an affinity for the adenosine A1 receptor higher than that for the A2 receptor, and N-ethylcarboxamide-adenosine, which has near equal affinity for the A1 and A2 receptors, were injected intrathecally into rats to evaluate differences in antinociceptive effect and motor impairment. Using the tail-immersion test, both compounds had antinociceptive effects. Motor function was evaluated during spontaneous movement in a free space. N-ethylcarboxamide-adenosine rapidly impaired motor function even after low intrathecal doses. R-phenylisopropyl-adenosine also induced motor impairment, but only after high intrathecal doses, and onset was much slower. These results suggest that the receptor selectivity of R-phenylisopropyl-adenosine is diminished at higher doses and that the motor impairment is an A2-receptor-mediated effect. A selective A1 receptor agonist, e.g., R-phenylisopropyl-adenosine, which produces a good antinociceptive effect without motor impairment, is more promising as a drug of possible use for the future treatment of clinical pain.

Key Words: RECEPTORS, ADENOSINE—spinal. • ANALGESICS, ADENOSINE RECEPTOR AGONISTS. • PAIN, ADENOSINE RECEPTOR AGONISTS.

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