A family of four voltage-gated sodium channels have been partially cloned in the medicinal leech, Hirudo medicinalis and are named LeNa1-4 (Blackshaw et al., 2003). We have devised an isoform specific quantitative real-time RT-PCR technique to study the expression of the four voltage-gated sodium channel transcripts in the leech central nervous system following experimental nerve injury. The levels of LeNa transcripts were measured in identified segmental ganglia, 1 day and 7 days after bilateral transection of their nerve roots and compared to the levels in control ganglia (n=6 for all groups). The levels of three housekeeping genes, elongation factor 1-alpha, cyclophilin-A and 18S rRNA were assayed in parallel and used to normalise LeNa expression. There was a statistically significant reduction in the level of non-normalised LeNa1 and LeNa3 in ganglia 1 and 7 days after injury relative to control ganglia (Dunnett’s t test P<0.05). There was no significant change in the level of non-normalised LeNa2 or LeNa4. A similar pattern of results was obtained when LeNa expression was normalised to each of the housekeeping genes but there were some inconsistencies in the statistical significance of the data. In order to confirm the dynamic expression of voltage-gated sodium channel isoforms independently of housekeeping gene expression, we analysed the ratios of the LeNa isoforms to each other. There was a highly significant reduction in the ratio of LeNa1/LeNa4 (mean ratio calibrated to control group which is set as 1±SEM, control = 1±0.03, 7 days = 0.65±0.04) and LeNa1/LeNa2 (control = 1±0.04, 7 days = 0.59±0.02) at 7 days after injury relative to control (Dunnett's t test P<0.001). There was also a significant reduction in the ratio of LeNa3/LeNa2 (control = 1±0.12, 7 days = 0.56±0.06) and LeNa3/LeNa4 (control = 1±0.14, 7 days = 0.61±0.06) in ganglia at 7 days after injury relative to control (Dunnett's t test P<0.05). There was no significant difference in the ratio of LeNa2/LeNa4 and LeNa1/LeNa3 after injury relative to control ganglia. The ratio data supports the conclusion that LeNa1 and LeNa3 are specifically down-regulated in ganglia 7 days after injury in comparison to LeNa2 and LeNa4 which are expressed at control levels. Current experiments are aimed at analysing the expression of LeNa transcripts in identified neuronal types after axotomy.