The P2X₇ receptor, a member of the P2X receptor family of ATP-gated ion channels, is unique, since in addition to ion channel formation, it can also regulate the release of IL-1β via regulation of caspase-1 and induce membrane blebbing, leading to apoptosis. We have previously demonstrated increased expression of P2X₇ receptor mRNA on day 4 after nephrotoxic serum injection in glomeruli of rodent models of glomerulonephritis (GN) (1). This coincides with the onset of proteinuria and macrophage infiltration which are maximal on day 4 in this model. Male Wistar Kyoto (WKY) rats weighing 200-250 g were injected with 2.5–10 mg of rabbit anti-rat glomerular basement membrane (GBM) globulin. At 2, 4 and 7 days after the nephrotoxic serum injection (n=6 for each time point) glomeruli were isolated from the renal cortex using a differential sieving technique and RNA was extracted for real-time PCR with gene-specific primers for the P2X₇ receptor p53, bax and bcl-2. Mouse renal tissue obtained previously (2) was used for the immunohistochemical localisation of P2X7 receptor protein and TUNEL staining as a marker of apoptosis. To compare sets of data, one-way ANOVA, followed by the Tukey-Kramer multiple comparisons post-hoc test, was used (Graphpad Instat version 3.06). Differences were considered statistically significant at P<0.05. Expression levels were calculated as a ratio to the housekeeping gene hypoxanthine phosphoribosyl transferase (HPRT) and control values were normalized to 1.0. P2X₇ receptor mRNA expression was unchanged on day 2 after nephrotoxic serum injection (control 1.0 ± 0.16, day 2 0.85 ± 0.20), but increased maximally on day 4 (2.66 ± 0.44 P <0.01); on day 7 there was no significant increase (1.41 ± 0.21). Expression of the pro-apoptotic p53 and bax genes was increased on day 2 (p53 control 1.0 ± 0.22, day 2 2.50 ± 0.55 n.s. and bax control 1.0 ± 0.52, day 2 6.35 ± 1.58 P = 0.01) and maximally increased on day 4 (p53 4 4.81 ± 1.12 P <0.01 and bax 8.81 ± 2.24 P <0.01). By day 7, p53 and bax expression was still increased, but to a lesser degree (p53 2.6 ± 0.49, P = 0.01, and bax 2.06 ± 0.60 n.s.). Expression of the anti-apoptotic gene bcl-2 was unchanged at each time point. Results are expressed as mean ± SEM. In summary, the pro-apoptotic P2X₇ receptor, as well as the p53 and bax genes, is maximally increased on day 4. These results suggest that glomerular expression of the P2X₇ receptor could be important in the pathogenesis of GN, perhaps through cell loss by apoptosis.