Primary afferents: Antidepressants enhance extracellular levels of adenosine in the rat hindpaw and inhibit adenosine uptake

Abstract

Peripheral antinociception by amitritpyline is inhibited by methylxanthine adenosine receptor antagonists suggesting involvement of endogenous adenosine in its action. This study examined the effects of antidepressants on in vivo extracellular levels of endogenous adenosine in the rat hindpaw, explored the mechanism of the increase, and determined in vitro effects of antidepressants on adenosine transport and enzymes involved in metabolism. Peripheral microdialysis of the hindpaw was used to determine extracellular tissue levels of adenosine. Adenosine uptake was determined using rat C6 glioma cells (contain rENT, transfected with DNA for rENT1); effects on enzymes were determined using standard assays. Amitriptyline (100-1000 nmol), injected locally into the rat hindpaw, enhances extracellular tissue levels of adenosine and augments release of adenosine by 1.5% formalin. Desipramine and fluoxetine produce similar effects. For amitriptyline, the adenosine originates from nucleotide, as inhibition of ecto-5'-nucleotidase (with α,β-methylene-ADP) inhibits the appearance of adenosine following exogenous administration of AMP and amitriptyline. Amitriptyline also greatly enhances the in vivo tissue recovery of adenosine when co-injected with exogenous adenosine. In uptake studies, all three antidepressants inhibit transport by rENT1 and rENT2 (>80% inhibition, IC50 0.24-1.02 mM). In enzyme assays for adenosine kinase and adenosine deaminase activity, minimal effects are observed. Antidepressants enhance extracellular tissue levels of adenosine in the rat hindpaw following local administration. For amitriptyline, the adenosine originates as nucleotide which is then converted to adenosine; it further involves inhibition of adenosine uptake from the extracellular space. This adenosine contributes to peripheral antinociception observed in behavioral studies. Supported by CIHR.