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Abstract :
[en] K channels are small conductance calcium-activated potassium channels which trigger an outward current generating an afterhyperpolarization (AHP). This AHP follows a single or a train of action potential, and therefore is important in the regulation of the firing frequency and/or pattern of many types of neurons. Serotonergic (5-HT) neurons from the raphe nuclei express SK channels and exhibit a significant AHP which can be efficiently blocked in vitro by apamin and N-methyl laudanosine (NML) (Scuvée-Moreau et al, 2004). In the later study, we found that some but not all neurons (50%) had a significantly increase in their firing rate when positive current was injected after SK channel blockade.
In order to determine the physiological relevance of these channels in vivo, single unit extracellular recordings were carried out in anesthetized rats and combined with iontophoresis of the specific non-peptidic SK channel blocker, UCL1684. 5-HT neurons were tested for their inhibitory response to locally applied 5-HT and histological analysis confirmed the localization of the recording site. UCL 1684 was used at a concentration of 200 µM. Out of 11 neurons recorded, 6 showed a significant increase in the production of doublets, with no effect on their mean firing rate as compared to the control condition. The other neurons were completely unaffected. These results suggest that the responsiveness of presumed 5-HT neurons to SK channel block is variable. Although the use of 200 µM UCL allow us to be sure of a sufficient SK blockade at the recording site (Waroux et al, 2005), we can not rule out the possibility that SK channels present at the dendritic level were not completely blocked. In conclusion, SK channels in vivo might play a role in controlling the firing pattern of a subgroup of 5-HT neurons.
