Is ATP a central synaptic mediator for certain primary afferent fibers from mammalian skin?

The possibility that ATP acts as a synaptic mediator at the central terminals of primary afferent fibers was examined by applying it iontophoretically to neurons of the outer layers of the cat spinal cord in vivo. ATP proved to be selectively excitatory for a limited subset of spinal neurons. Those units consistently excited by ATP iontophoresis with very small currents (2-15 nA) responded to gentle mechanical stimulation of the skin and usually evidenced excitatory input from unmyelinated primary afferent fibers. Most units excited by ATP were specifically mechanoreceptive; a few neurons receiving excitatory input from both low-threshold mechanoreceptors and nociceptors also responded to ATP. Selectively nocireceptive neurons were unresponsive. Generally, the mechanoreceptive neurons excited by ATP were located in the deeper substantia gelatinosa or in the immediately adjacent nucleus proprius of the dorsal horn. The results suggest the presence of a purinergic excitatory receptor on central neurons receiving excitatory projection from tactile mechanoreceptors with fine-diameter afferent fibers and are consistent with the possibility that an ATP-like agent may mediate central synaptic excitation for this set of sense organs.