Divergent projections of catecholaminergic neurons in the nucleus of the solitary tract to limbic forebrain and medullary autonomic brain regions

Abstract

The nucleus of the solitary tract (NTS) is a critical structure involved in coordinating autonomic and visceral activities. Previous independent studies have demonstrated efferent projections from the NTS to the nucleus paragigantocellularis (PGi) and the central nucleus of the amygdala (CNA) in rat brain. To further characterize the neural circuitry originating from the NTS with postsynaptic targets in the amygdala and medullary autonomic targets, distinct green or red fluorescent latex microspheres were injected into the PGi and the CNA, respectively, of the same rat. Thirty-micron thick tissue sections through the lower brainstem and forebrain were collected. Every fourth section through the NTS region was processed for immunocytochemical detection of tyrosine hydroxylase (TH), a marker of catecholaminergic neurons. Retrogradely labeled neurons from the PGi or CNA were distributed throughout the rostro-caudal segments of the NTS. However, the majority of neurons containing both retrograde tracers were distributed within the caudal third of the NTS. Cell counts revealed that approximately 27% of neurons projecting to the CNA in the NTS sent collateralized projections to the PGi while approximately 16% of neurons projecting to the PGi sent collateralized projections to the CNA. Interestingly, more than half of the PGi and CNA-projecting neurons in the NTS expressed TH immunoreactivity. These data indicate that catecholaminergic neurons in the NTS are poised to simultaneously coordinate activities in limbic and medullary autonomic brain regions.

Figure 1

Low-power magnification images of merged brightfield and immunofluorescence photomicrographs showing representative injections of green (B) and red (D) fluorescent latex microspheres into the PGi and the CNA, respectively, in rat brain. Schematic diagrams adapted from the rat brain atlas of Swanson (61) showing the anterior posterior level of the representative injection sites in the PGi (A) and the CNA (C). Asterisks in panels A and C show the injection sites presented in panels B and D, respectively. Arrows indicate dorsal (D) and medial (M) orientation of the sections. Gi, gigantocellular reticular nucleus; IA, intercalated nuclei amygdala; LH, lateral hypothalamic area; MEAd, medial nucleus amygdala, anterodorsal part; opt, optic tract; py, pyramidal tract. Scale bar, 100 μm.

Figure 2

Neurons within the NTS send collateralized projections to the PGi and the CNA. A–B. Photomicrographs of a coronal section through the NTS showing retrogradely labeled neurons from the PGi as detected using green fluorescent latex microspheres(A) and from the CNA using red fluorescent latex microspheres (B). Arrows indicate dorsal (D) and medial (M) orientation of the section. Inset shows a schematic illustration of the region shown in panels A–C adapted from the rat brain atlas of Swanson (1992). C. Merged image of panels A and B showing dual labeled neurons (double-headed arrows) and singly labeled neurons (arrowheads). AP, area postrema; cc, central canal; py, pyramidal tract; SPVC, spinal nucleus of the t rigeminal tract (caudal part); XII, hypoglossal nucleus. Scale bar, 100 μm.

Figure 3

Photomicrographs showing the distribution of retrogradely labeled neurons from the PGi and the CNA with respect to catecholaminergic neurons. A–C. Distribution of neurons projecting to the PGi (A), the CNA (B), and neurons immunoreactive to TH (C) in the NTS. D. Merged image of panels A, B and C showing singly labeled neurons (arrowheads) and dual-labeled neurons (double-headed arrows). Arrows indicate dorsal (D) and medial (M) orientation of the sections. Inset shows a schematic illustration of the region shown in panels A–D adapted from the rat brain atlas of Swanson (1992). AP, area postrema; cc, central canal; py, pyramidal tract; SPVC, spinal nucleus of the trigeminal tract (caudal part); hypoglossal nucleus. Scale bar = 100 μm.

Figure 4

A–B. Schematic drawings of two representative levels of the NTS showing location of retrogradely labeled neurons from the PGi (open circles) and the CNA (squares). Dual labeled neurons with and without tyrosine hydroxylase-immunoreactivity are illustrated as stars and filled circles, respectively. AP, area postrema; cc,central canal; py, pyramidal tract; pyd, pyramidal decussation; SPVC, spinal nucleus of trigeminal (caudal part); SPVI, spinal nucleus of trigeminal (interpolar part); XII, hypoglossal nucleus. C. Bar graph showing the number of singly and dual retrogradely labeled neurons in the NTS from the PGi and the CNA with (black) or without (white) tyrosine hydroxylase (TH)-immunoreactivity. All data are expressed as the mean ± standard error of the mean.

Figure 5

High magnification photomicrographs of coronal sections through the caudal NTS showing retrogradely labeled neurons from the PGi (A), the CNA (B), and TH-immunoreactive neurons (C). Merged image (D). Inset in B is adapted from the rat brain atlas of Swanson (1992) showing a low magnification schematic diagram of the region shown in panels A–D. Arrows in the inset indicate dorsal (D) and medial (M) orientation of the sections. Arrowheads point to singly labeled neurons while arrows indicate dual-retrogradely labeled neurons expressing TH-immunoreactivity. cc, central canal. Scale bar = 250 μm.