Expression of P2X 5 receptors in the rat, cat, mouse and guinea pig dorsal root ganglion

P2X receptors are ATP-gated cationic channels composed of seven cloned subunits (P2X 1 –7 ). P2X 3 homomultimer and P2X 2/3 heteromultimer receptors expressed by primary afferent dorsal root ganglion (DRG) neurons are involved in pain processing. The aim of the study was to investigate the expression of the P2X 5 receptor subunit in DRG in different species including mouse, rat, cat and guinea pig. Immunohistochemistry showed that P2X 5 receptors exhibited low levels of immunostaining in rat DRG, but high levels in mouse and guinea pig. Only a few neurons were immunoreactive for P2X 5 receptors in cat. In mouse DRG, the P2X 5 receptor was expressed largely by medium-diameter neurons (42.9 %), less in small (29.3 %) and large (27.8 %) neurons. In contrast, in the guinea pig DRG, P2X 5 receptor expression was greatest in small-diameter (42.6 %), less in medium- (36.3 %) and large-diameter (21.1 %) neurons. Colocalization experiments revealed that, in mouse DRG, 65.5, 10.9 and 27.1 % of P2X 5 receptors were immunoreactive for NF-200, CGRP and calbindin, while only a few P2X 5 -immunoreactive (IR) neurons were coexpressed with IB4 or with NOS. In guinea pig DRG, a total of 60.5 and 40.5 % of P2X 5 -IR neurons were coexpressed with IB4 or with CGRP, while 20.3 and 24.5 % of P2X 5 receptors were coexpressed with NF-200 or with NOS. Only a few P2X 5 -IR neurons were coexpressed with calbindin in guinea pig DRG. It will be of great interest to clarify the relative physiological and pathophysiological roles of P2X 5 receptors.