Antagonism of adenosine receptors by caffeine and caffeine metabolites in equine forebrain tissues.

This research paper explores the presence of adenosine receptor subtypes A1 and A2a in horse forebrain tissues and describes how caffeine and its metabolites interact with these receptors. The findings confirm that these receptors exist in the equine brain and also reveal that caffeine and its metabolites can counteract the function of adenosine in the horse’s central nervous system. This interaction could possibly affect the horse’s locomotion and racing performance.

Methods

• Brain tissue samples were collected post-mortem from five adult male research horses.
• The researchers focused on the membrane-enriched homogenates from the cerebral cortex and striatum of the horse’s brain.
• Radioligand binding assays using A1-selective ligand [3H]DPCPX and A2a-selective ligand [3H]ZM241385 were carried out to measure the presence and number of adenosine receptors.
• A nucleotide exchange assay was done to determine the functional response of the adenosine receptors to agonists and antagonists using [35S]-guanosine 5′-(gamma-thio) triphosphate ([35S]GTPgammaS).

Results

• [3H]DPCPX binding (A1) sites were found in cerebral cortex and striatum, proving the presence of adenosine A1 receptors.
• [3H]ZM241385 (A2a) binding sites were found only in striatum, validating the presence of adenosine A2a receptors.
• The caffeine-based compounds studied had comparable binding affinities at both A1 and A2a sites.
• The rank orders of drug binding affinities (theophylline > paraxanthine > or = caffeine >> theobromine) were found to be similar to that in other species.
• The nucleotide exchange study revealed that caffeine and its metabolites function purely as adenosine receptor antagonists at concentrations matching the receptor binding affinities.

Conclusions

• The research confirmed the presence of high-affinity A1 and A2a adenosine receptors linked to guanine nucleotide-binding proteins in equine forebrain tissues.
• Caffeine and several of its metabolites were found to function as antagonists, counteracting the function of adenosine in the equine central nervous system.
• The research provides evidence that caffeine and its metabolites may influence the motor functioning and racing performance of horses by antagonizing the function of adenosine in the central nervous system.