Potentiation of formalin-evoked adenosine release by an adenosine kinase inhibitor and an adenosine deaminase inhibitor in the rat hind paw: a microdialysis study

Abstract

The present study examined the effects of local subcutaneous administration of formalin on adenosine release from the rat hind paw, and the effects of inhibitors of adenosine metabolism on such release. Microdialysis probes were inserted into the subcutaneous tissue of the plantar surface of rat hind paws. Samples were collected every 10 min at a perfusion rate of 2 μl/min and high performance liquid chromatography was used to measure adenosine levels. At lower concentrations of formalin (0.5–2.5%), a significant increase in adenosine levels was observed in the first 10 min after formalin injection, while at the highest concentration of formalin (5%), the increase in adenosine release was observed over 60 min. Co-administration of the adenosine kinase inhibitor 5′-amino-5′-deoxyadenosine (100 nmol) with formalin, significantly increased adenosine release evoked by 0.5–1.5% formalin, but did not produce a further enhancement of release evoked by 5% formalin. The adenosine deaminase inhibitor 2′-deoxycoformycin (100 nmol) significantly increased adenosine levels at 5% formalin but had no effect at lower concentrations of formalin. In confirmation of previous studies, subcutaneous injection of formalin (0.5–5%) produced a characteristic biphasic concentration-related expression of nociceptive behaviours and an increase in paw volume. This study directly demonstrates that formalin can evoke a concentration-dependent local release of adenosine from the rat hind paw. The ability of an adenosine kinase inhibitor and an adenosine deaminase inhibitor to modulate this release is dependent on substrate adenosine concentrations.