Kv3 subunits of voltage gated potassium channels are predominantly located within fast spiking neurones, facilitating brief action potentials (Rudy et al., 1999). Problems arise when investigating the functional properties of the Kv3 subunits in native tissue due to formation of heteromultimers. We have localised Kv3.4 immunoreactivity and an absence of immunoreactivity for the other Kv3 subunits within neurones of the dorsal vagal nucleus (DVN, Brooke et al., 2004). The aim of this study was to investigate a functional role for the Kv3.4 subunit within DVN neurones, utilising a peptide toxin, blood depressing substance, reported to selectively act on the Kv3.4 subunit (Diochot et al., 1998). For electrophysiology, male Wistar rats (15-21days) were humanely killed by anaesthetising with sodium pentobarbitone (120mg kg-1, I.P.) followed by transcardial perfusion with sucrose aCSF and subsequent decapitation. 300 μm coronal slices of the medulla oblongata were prepared. Whole cell patch clamp recordings were made from neurones within the dorsal vagal nucleus (DVN, n=10) at room temperature. Application of the Kv3.4 blocker BDS-II (50nM) increased the action potential duration (5.39±0.16ms to 7.04±0.17ms, mean±SEM, P<0.05, Student’s paired t test) and increased the afterhyperpolarisation amplitude (15.4±0.85mV to 18.6±0.63mV, P<0.05). This suggests that the Kv3.4 subunit contributes to the repolarisation phase of action potentials within the DVN. To investigate the presynaptic effect of BDS a stimulating electrode was placed in the solitary tract to evoke synaptic events within DVN neurones. BDS-II increased the amplitude of EPSPs (5.6±0.21mV to 7.3±0.21mV, P<0.05) and IPSPs (5.2±0.4 to 6.3±0.32, P<0.05) and decreased the paired pulse ratio (EPSPs 0.93 to 0.78, IPSPs 0.82 to 0.73, P<0.05), indicative of a pre-synaptic effect (n=8). Furthermore BDS-II significantly increased the frequency of spontaneous PSPs (0.227±0.06Hz to 0.373±0.04Hz, P<0.05) but did not significantly change the amplitude (n=4). In the presence of TTX (1mM) BDS-II did not alter the frequency or amplitude of mini PSPs (n=4) suggesting that BDS-II increases neurotransmitter release in an action potential dependent manner. These data show that neurones within the DVN are sensitive to BDS-II consistent with the presence of channels formed by the Kv3.4 subunit. In the DVN these Kv3.4 containing channels regulate action potential duration and transmitter release.