Expression of P2X sub(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 sub(1) sub(-7)). P2X sub(3) homomultimer and P2X sub(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 sub(5) receptor subunit in DRG in different species including mouse, rat, cat and guinea pig. Immunohistochemistry showed that P2X sub(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 sub(5) receptors in cat. In mouse DRG, the P2X sub(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 sub(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 sub(5) receptors were immunoreactive for NF-200, CGRP and calbindin, while only a few P2X sub(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 sub(5)-IR neurons were coexpressed with IB4 or with CGRP, while 20.3 and 24.5 % of P2X sub(5) receptors were coexpressed with NF-200 or with NOS. Only a few P2X sub(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 sub(5) receptors.