Differential Ascending Projections From the Male Rat Caudal Nucleus of the Tractus Solitarius: An Interface Between Local Microcircuits and Global Macrocircuits.

Yoshinori Kawai

doi:10.3389/fnana.2018.00063

Abstract

To integrate and broadcast neural information, local microcircuits and global macrocircuits interact within certain specific nuclei of the central nervous system. The structural and functional architecture of this interaction was determined for the caudal nucleus of the tractus solitarius (NTS) at the level of the area postrema (AP), a relay station of peripheral viscerosensory information that is processed and conveyed to brain regions concerned with autonomic-affective and other interoceptive reflexive functions. Axon collaterals of most small NTS cells (soma <150 μm2) establish excitatory or inhibitory local microcircuits likely to control the activity of nearby NTS cells and to transfer peripheral signals to efferent projection neurons. At least two types of cells that constitute efferent pathways from the caudal NTS (cNTS) were distinguished: (1) a greater numbers of small cells, seemingly forming local excitatory microcircuits via recurrent axon collaterals, that project specifically and unidirectionally to the lateral parabrachial nucleus; and (2) a much smaller numbers of cells likely to establish multiple global connections, mostly via the medial forebrain bundle (MFB) or the dorsal longitudinal fascicle (DLF), with a wide range of brain regions, including the ventrolateral medulla (VLM), hypothalamus, central nucleus of the amygdala (ACe), bed nucleus of the stria terminalis (BNST), spinal cord dorsal horn, brainstem reticular formation, locus coeruleus (LC), periaqueductal gray (PAG) and periventricular diencephalon (including the epithalamus). The evidence presented here suggests that distinct cNTS cell types distinguished by projection pattern and related structural and functional features participate differentially in the computation of viscerosensory information and coordination of global macro-networks in a highly organized manner.

Highlights

The caudal nucleus of the tractus solitarius (NTS; Bregma −14.6 to −13.3 mm, Paxinos and Watson, 1998, −1.0 to 0.3 mm from the calamus scriptorius) receives peripheral viscerosensory afferents from the cardiovascular, respiratory, and digestive systems via glossopharyngeal and vagal nerves (Housley et al, 1987; Altschuler et al, 1989; Chan et al, 2000)
Cases in which the maximum labeled extent of the injection site was within the medial portion of the NTS at the level of the area postrema (AP) were analyzed in the present study (n = 16; Table 1)
The results demonstrate that 61% of parabrachial nucleus (PBL), 89% of periaqueductal gray (PAG), and more than 98% of paraventricular hypothalamic nucleus (PVH), amygdaloid central nucleus (ACe), bed nucleus of the stria terminalis (BNST)-projecting cells of small and larger sizes were localized in the ventral caudal NTS (cNTS) (Figure 8, Table 2)

Summary

Introduction

The caudal nucleus of the tractus solitarius (NTS; Bregma −14.6 to −13.3 mm, Paxinos and Watson, 1998, −1.0 to 0.3 mm from the calamus scriptorius) receives peripheral viscerosensory afferents from the cardiovascular, respiratory, and digestive systems via glossopharyngeal and vagal nerves (Housley et al, 1987; Altschuler et al, 1989; Chan et al, 2000). The soma size of cNTS neurons is an excellent marker for other morphological and functional features: (1) small cells (soma 150 μm2), which lack local axon collaterals, receive robust inhibitory synaptic inputs and are concentrated in the ventral subregion (Kawai and Senba, 1996, 1999; Okada et al, 2006, 2008; Yoshioka et al, 2006; Negishi and Kawai, 2011). The purpose of the present study was to assess differences in projection targets or pathways of projection axons Such local structure/function correlates of NTS neurons are essential for understanding the role of the cNTS as a relay station for peripheral visceral sensory inputs

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