Regulation of Rostral Nucleus of the Solitary Tract Responses to Afferent Input by A-type K+ Current.

Responses in the rostral (gustatory) nucleus of the solitary tract (rNST) are modified by synaptic interactions within the nucleus and the constitutive membrane properties of the neurons themselves. The potassium current I is one potential source of modulation. In the caudal NST, projection neurons with I show lower fidelity to afferent stimulation compared to cells without. We explored the role of an A-type K+ current (I) in modulating the response to afferent stimulation and GABA-mediated inhibition in the rNST using whole cell patch clamp recording in transgenic mice that expressed channelrhodopsin (ChR2 H134R) in GABAergic neurons. The presence of I was determined in current clamp and the response to electrical stimulation of afferent fibers in the solitary tract was assessed before and after treatment with the specific Kv4 channel blocker AmmTX3. Blocking I significantly increased the response to afferent stimulation by 53%. Using dynamic clamp to create a synthetic I conductance, we demonstrated a significant 14% decrease in responsiveness to afferent stimulation in cells lacking I. Because I reduced excitability and is hyperpolarization-sensitive, we examined whether I contributed to the inhibition resulting from optogenetic release of GABA. Although blocking I decreased the percent suppression induced by GABA, this effect was attributable to the increased responsiveness resulting from AmmTX3, not to a change in the absolute magnitude of suppression. We conclude that rNST responses to afferent input are regulated independently by I and GABA.