Nitrergic modulation of voltage-gated calcium channels and neuronal excitability
thesisposted on 28.06.2013, 14:19 by Adam J.B. Tozer
Nitric oxide (NO) is a diffusible messenger utilised body‐wide for cellular signalling. In principal neurons of the medial nucleus of the trapezoid body (MNTB) NO is produced by the neuronal isoform of the nitric oxide synthase enzymes, nNOS. NO is known to inhibit K[subscript V]3.1 channels in these cells and to also reduce mini EPSC amplitudes (Steinert et al. 2008). In light of these known effects, NO‐dependent modulation of the native voltage‐gated calcium channels (Ca[subscript V]s) in the principal neurons of the MNTB and the downstream superparaolivary nucleus (SPN) was investigated, and the functional relevance of the NO‐dependent increase in Ca[superscript 2+]influx in the MNTB explored. Whole cell voltage clamp and pharmacological isolation of the high‐voltage‐activated (HVA) Ca[subscript V] subtypes in the MNTB revealed an NO‐dependent potentiation of whole cell I[subscript Ca], underlied by differential modulation of the channels, such that L‐type channels were potentiated via a direct pathway, whereas P/Q were potentiated via the canonical NO‐cGMP pathway. N‐type channel activation was left‐shifted by the NOcGMP dependent pathway, and R‐type channels were unaffected by NO. Whole cell voltage clamp of SPN neurons revealed a low‐voltage‐activated (LVA) Ttype current, which was sensitive to mibefradilMibefradil (2μM), and inhibited by NO (~50%). The T‐type current contributed to the rebound firing phenotype of these cells following relief of glycinergic inhibition by the MNTB, and T‐type blockers and NO exposure reduced the number of rebound action potentials (APs) generated. Whole cell voltage clamp of MNTB principal neurons revealed an apaminApamin sensitive current which was potentiated by NO, suggesting the presence of SK channels in the MNTB. Further investigations using current clamp and synaptic stimulation revealed presynaptic expression of SK channels, which restrict synaptic release, as SK block increased evoked EPSC amplitude. This research demonstrates that nitric oxide differentially modulates Ca[subscript V] channels expressed in two communicating nuclei of the auditory brainstem. This work also reveals the presence of SK channels both pre and postsynaptically at the calyx of Held synapse, and suggests a role for these channels in facilitating high‐fidelity transmission at this auditory synapse.