**3.5. Purinergic P2X receptors inhibitors**

One pharmacological target in the area of analgesics and anti-inflammatory agents is purinergic P2X receptors (P2XR), which are important receptors in the modulation of inflammation and pain. In 1978, Burnstock [73] mentioned the existence of two classes of purinergic receptors, known as receptors P1 (adenosine) and P2 (adenosine 5'-triphosphate, ATP) [50]. Markedly, mammalian ATP-gated nonselective cation channels (P2XRs) consist of seven potential subunits; denoted P2X1 to P2X7 [74].

In 1995, a significant advance was made when the P2X3 ionotropic ion channel purinergic receptor was cloned and presented to be mainly localized on small nociceptive sensory neurons in dorsal root ganglia (DRG) [75]. Shortly, Burnstock [76] suggested a unifying purinergic hypothesis for the initiation of pain that ATP released as a co-transmitter with noradrenaline (NA) and neuropeptide Y from sympathetic nerve terminal varicosities probably that is involved in activating these receptors in three different pain conditions: as a co-transmitter from sympathetic nerves in sympathetic pain as causalgia and reflex sympathetic dystrophy; from endothelial cells in vascular pain, including migraine and angina; and from tumour cells in cancer.

Likewise, purinergic mechano-sensory transduction has been implicated for visceral pain. Meanwhile, ATP released from urothelial cells and epithelial cells lining intestine during the distension acts on P2X3 and P2X2/3, and perhaps P2Y, receptors on subepithelial sensory nerve fibres to initiate impulses in sensory pathways to the pain centres in the brain as well as triggering local reflexes. Besides, P1, P2X and P2Y receptors are possibly implicated in nociceptive neural pathways in the spinal cord, while P2X4 receptors on the spinal microglia are involved in allodynia [77].

Of the seven subtypes of P2XR, the types that are most related to the progression or control of pain status are the P2X3R, the heteromericeP2X2/3R, P2X4R, and the P2X7R [78].

An example of natural inhibitor of purinergic receptors is a product known as puerarin. Puerarin is an isoflavone isolated from a traditional Chinese herb (*Radix puerariae*). It was found to have an inhibitory effect on burn pain hyperalgesia through inhibiting the upregulation of the P2X3R protein expression in the dorsal root ganglion neurons [79].

Also, purotoxin, a peptide isolated from the venom of the Asian spider *Geolycosa*sp, also showed a potent and selective antagonist effect on P2X3R, inhibiting the ionic current in rat neurons and showing an analgesic effect on inflammatory pain [78].
