The role of purines in nociception

The preceding review indicates that there is convincing evidence for the presence of adenosine in and release of adenosine from capsaicin-sensitive small diameter primary afferent neurons in the spinal cord (Fig. 1). Within the dorsal spinal cord, adenosine inhibits the transmission of nociceptive information, although details of mechanisms involved in this action remain to be established. In view of the antinociceptive actions of adenosine analogues, there has been some interest in the possibility of developing adenosine analogues as analgesic agents. However, this goal may be frustrated by this concomitant suppression of motor function, as well as the production of other side effects due to the diverse nature of pharmacological effects seen with adenosine analogues. Release of adenosine from small diameter primary afferent nerve terminals and subsequent activation of extracellular adenosine receptors in the dorsal horn of the spinal cord appears to contribute significantly to the spinal action of opioids. An understanding of spinal mechanisms of actions of adenosine therefore is an important prerequisite for our understanding of the action of this clinically important group of drugs. ATP may be a sensory neurotransmitter released from non-nociceptive large diameter primary afferent neurons (Fig. 1). The subsequent extracellular conversion of released ATP to adenosine may produce suppression of the transmission of noxious sensory information via small diameter primary afferent fibres, and contribute to the phenomenon of vibration induced analgesia. Clearly, the role of purines on spinal cord processing of nociceptive information merits considerable attention.