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Characteristics of Rostral Solitary Tract Nucleus Neurons With Identified Afferent Connections That Project to the Parabrachial Nucleus in Rats

¹Department of Biologic and Materials Sciences, School of Dentistry and ²Department of Molecular and Integrative Physiology, Medical School, University of Michigan, Ann Arbor, Michigan

Submitted 28 October 2008; accepted in final form 5 May 2009

Afferent information derived from oral chemoreceptors is transmitted to second-order neurons in the rostral solitary tract nucleus (rNST) and then relayed to other CNS locations responsible for complex sensory and motor behaviors. Here we investigate the characteristics of rNST neurons sending information rostrally to the parabrachial nucleus (PBN). Afferent connections to these rNST-PBN projection neurons were identified by anterograde labeling of the chorda tympani (CT), glossopharyngeal (IX), and lingual (LV) nerves. We used voltage- and current-clamp recordings in brain slices to characterize the expression of both the transient A-type potassium current, I_KA and the hyperpolarization-activated inward current, I_h, important determinants of neuronal repetitive discharge characteristics. The majority of rNST-PBN neurons express I_KA, and these I_KA-expressing neurons predominate in CT and IX terminal fields but were expressed in approximately half of the neurons in the LV field. rNST-PBN neurons expressing I_h were evenly distributed among CT, IX and LV terminal fields. However, expression patterns of I_KA and I_h differed among CT, IX, and LV fields. I_KA-expressing neurons frequently coexpress I_h in CT and IX terminal fields, whereas neurons in LV terminal field often express only I_h. After GABA_A receptor block all rNST-PBN neurons responded to afferent stimulation with all-or-none excitatory synaptic responses. rNST-PBN neurons had either multipolar or elongate morphologies and were distributed throughout the rNST, but multipolar neurons were more often encountered in CT and IX terminal fields. No correlation was found between the biophysical and morphological characteristics of the rNST-PBN projection neurons in each terminal field.