BLOCKADE BY DENDROTOXIN HOMOLOGS OF VOLTAGE-DEPENDENT K+ CURRENTS IN CULTURED SENSORY NEURONS FROM NEONATAL RATS

1 Homologues of dendrotoxin (Dtx) were isolated from the crude venom of Green and Black Mamba snakes and examined for K+ channel blocking activity in neonatal rat dorsal root ganglion cells (DRGs) by whole-cell patch clamp recording. 2 Outward potassium current activated by depolarization was composed of two major components: a slowly inactivating current (SIC, tau(decay)approximate to 50 ms, 200 ms and 2 s), and a non-inactivating current (NIC, tau(decay) > 2 min). Tail current analysis revealed two time constants of deactivation of total outward current, 3-12 ms and 50-150 ms (at - 80 mV) which corresponded to SIC and NIC, respectively. 3 All the homologues (alpha-, beta-, gamma- and delta-Dtx and toxins I and K) blocked outward current activated by depolarization in a dose-dependent manner. The most potent in blocking total outward current was delta-Dtx (EC(50) of 0.5 +/- 0.2 nM), although there were no statistically significant differences in potency between any of the homologues. 4 Qualitative differences in the nature of the block were noted between homologues. In particular, the block by delta-Dtx was time-dependent, whereas that by alpha-Dtx was not. 5 alpha-Dtx was a much better blocker of SIC (EC(50) = 1.0 +/- 0.4 nM) than was delta-Dtx (EC(50) = 17.6 +/- 5.8 nM). Furthermore, delta-Dtx was selective for NIC (EC(50) +/- 0.24 +/- 0.03 nM) over SIC and reduced the slow component of tail currents (NIC), preferentially. On the other hand, alpha-Dtx did not significantly distinguish between SIC and NIC although tail current analysis showed that alpha-Dtx preferentially reduced the fast component of tail currents (SIC). 6 The results confirm, using direct electrophysiological methods, that homologues of dendrotoxins from Mamba snake venom block K+ channels in rat sensory neurones. Furthermore, alpha-Dtx and delta-Dtx distinguish between sub-types of K+ channels in these cells and may thus be useful pharmacological tools in other neuronal K+ channel studies.