Implicit in purinergic signalling is the presence of specific
receptors for purines. Two families of purine receptors were recognised
in 1978, P1 receptors for adenosine and P2 receptors for ATP.
Two families of P2 receptors were proposed in 1985 and the molecular
structure and second messenger agents involved discovered in
the 1990s. Seven P2X ligand-gated ion channel receptor subtypes
and eight P2Y G protein-coupled receptor subtypes are currently
established (see
receptors were cloned in 1995 and shown to be located almost
exclusively on sensory nerve endings
These receptors have been shown with immunohistochemistry
to be located on nerve endings in the skin and are particularly
abundant in the tongue also used for acupuncture
recording from an isolated tongue-nerve preparation showed
increased activity in the lingual nerves supplying the tongue
during mechanical stimulation of the tongue that was mimicked
by ATP and attenuated with P2X
distension of the ureter led to release of substantial amounts
of ATP and evoked a discharge in the suburothelial sensory nerves
that was mimicked by ATP and reduced by 2
ATP, a potent P2X
(see