Characterization of Na+ currents regulating intrinsic excitability of optic tectal neurons

Developing neurons adapt their intrinsic excitability to maintain stable output despite changing synaptic input. The mechanisms behind this process remain unclear. In this study, we examined Xenopus optic tectal neurons and found that the expressions of Nav1.1 and Nav1.6 voltage-gated Na+ channels are regulated during changes in intrinsic excitability, both during development and becsuse of changes in visual experience. Using whole-cell electrophysiology, we demonstrate the existence of distinct, fast, persistent, and resurgent Na+ currents in the tectum, and show that these Na+ currents are co-regulated with changes in Nav channel expression. Using antisense RNA to suppress the expression of specific Nav subunits, we found that up-regulation of Nav1.6 expression, but not Nav1.1, was necessary for experience-dependent increases in Na+ currents and intrinsic excitability. Furthermore, this regulation was also necessary for normal development of sensory guided behaviors. These data suggest that the regulation of Na+ currents through the modulation of Nav1.6 expression, and to a lesser extent Nav1.1, plays a crucial role in controlling the intrinsic excitability of tectal neurons and guiding normal development of the tectal circuitry.

Cell-attached single-channel recordings in intact prefrontal cortex pyramidal neurons reveal compartmentalized D1/D5 receptor modulation of the persistent sodium current

The persistent Na⁺ current (I_Nap) is believed to be an important target of dopamine modulation in prefrontal cortex (PFC) neurons. While past studies have tested the effects of dopamine on I_Nap, the results have been contradictory largely because of difficulties in measuring I_Nap using somatic whole-cell recordings. To circumvent these confounds we used the cell-attached patch-clamp technique to record single Na⁺ channels from the soma, proximal dendrite (PD) or proximal axon (PA) of intact prefrontal layer V pyramidal neurons. Under baseline conditions, numerous well resolved Na⁺ channel openings were recorded that exhibited an extrapolated reversal potential of 73 mV, a slope conductance of 14–19 pS and were blocked by tetrodotoxin (TTX). While similar in most respects, the propensity to exhibit prolonged bursts lasting >40 ms was many fold greater in the axon than the soma or dendrite. Bath application of the D1/D5 receptor agonist SKF81297 shifted the ensemble current activation curve leftward and increased the number of late events recorded from the PD but not the soma or PA. However, the greatest effect was on prolonged bursting where the D1/D5 receptor agonist increased their occurrence 3 fold in the PD and nearly 7 fold in the soma, but not at all in the PA. As a result, D1/D5 receptor activation equalized the probability of prolonged burst occurrence across the proximal axosomatodendritic region. Therefore, D1/D5 receptor modulation appears to be targeted mainly to Na⁺ channels in the PD/soma and not the PA. By circumventing the pitfalls of previous attempts to study the D1/D5 receptor modulation of I_Nap, we demonstrate conclusively that D1/D5 receptor activation can increase the I_Nap generated proximally, however questions still remain as to how D1/D5 receptor modulates Na⁺ currents in the more distal initial segment where most of the I_Nap is normally generated.