Responsiveness to nicotine of neurons of the caudal nucleus of the solitary tract correlates with the neuronal projection target

Abstract

The caudal nucleus of the solitary tract (NTS) is the key integrating center of visceral sensory-motor signaling supporting autonomic homeostasis. Two key projections of this nucleus are the parabrachial nucleus (PbN) and the dorsal motor nucleus of the vagus (DMV). The PbN integrates and relays viscerosensory information primarily to the forebrain, supporting behavioral, emotional, and endocrine responses to visceral events, while the DMV contains parasympathetic preganglionic cholinergic motoneurons that support primarily gastrointestinal reflexes. Subsets of caudal NTS neurons express presynaptic and somatodendritic nicotinic acetylcholine receptors (nAChRs). However, the anatomical identification of nicotine-responsive caudal NTS neurons has not been determined. This study used in vivo and ex vivo fluorescent tracing and slice patch-clamp electrophysiological recordings from anatomically identified caudal NTS neurons to test the hypothesis that the responsiveness of these cells to nicotine correlates with the target of their axonal projections. The results demonstrate that the majority of glutamatergic terminals that synapse on PbN-projecting caudal NTS neurons are unaffected by nicotine. Moreover, only a fraction of these cells express somatodendritic nAChRs. In contrast, the majority of DMV-projecting caudal NTS neurons exhibit robust presynaptic and somatodendritic responsiveness to nicotine. However, PbN-projecting neurons also exhibit significantly lower background frequencies of glutamatergic miniature postsynaptic currents than DMV-projecting neurons. Therefore, presynaptic unresponsiveness to nicotine may result from deficient glutamatergic innervation of PbN-projecting neurons. Nevertheless, the caudal NTS contains function-specific subsets of cells with target-specific responsiveness to nicotine. These results may support development of therapeutic strategies for selective targeting of specific autonomic pathways and impaired autonomic homeostasis.