Stimulation of Baroresponsive Parts of the Nucleus of the Solitary Tract Produces Nitric Oxide-mediated Choroidal Vasodilation in Rat Eye

Abstract

Preganglionic parasympathetic neurons of the ventromedial part of the superior salivatory nucleus (SSN) mediate vasodilation of orbital and choroidal blood vessels, via their projection to the nitrergic pterygopalatine ganglion (PPG) neurons that innervate these vessels. We recently showed that the baroresponsive part of the nucleus of the solitary tract (NTS) innervates choroidal control parasympathetic preganglionic neurons of SSN in rats. As this projection provides a means by which blood pressure (BP) signals may modulate choroidal blood flow (ChBF), we investigated if activation of baroresponsive NTS evokes ChBF increases in rat eye, using Laser Doppler Flowmetry (LDF) to measure ChBF transclerally. We found that electrical activation of ipsilateral baroresponsive NTS and its efferent fiber pathway to choroidal SSN increased mean ChBF by about 40-80% above baseline, depending on current level. The ChBF responses obtained with stimulation of baroresponsive NTS were driven by increases in both choroidal blood volume (ChBVol; i.e., vasodilation) and choroidal blood velocity (ChBVel; possibly due to orbital vessel dilation). Stimulation of baroresponsive NTS, by contrast, yielded no significant mean increases in systemic arterial blood pressure (ABP). We further found that the increases in ChBF with NTS stimulation were significantly reduced by administration of the neuronal nitric oxide (NO) synthase inhibitor N^ω-propyl-l-arginine (NPA), thus implicating nitrergic PPG terminals in the NTS-elicited ChBF increases. Our results show that the NTS neurons projecting to choroidal SSN do mediate increase in ChBF, and thus suggest a role of baroresponsive NTS in the BP-dependent regulation of ChBF.

Keywords: autonomic; choroidal blood flow; nucleus of solitary tract; parasympathetic; superior salivatory nucleus.

Figure 1

Images showing the distribution of c-fos immunolabeling in: (1) nucleus of the solitary tract (NTS) just anterior to the area postrema (AP) in a sham hypotension rat (A) and a hypotensive rat (B); and (2) NTS at the level of AP in a sham hypotension rat (C) and a hypotensive rat (D). The NTS is outlined, as is the vagal motor nucleus (10), and the location of the solitary tract (sol) is indicated. Note that hypotension induces c-fos in any neurons around and medial to the solitary tract. All images are at the same magnification.

Figure 2

Schematic illustration showing: (1) the NTS subdivisions (row 1); (2) the distribution of the aortic depressor nerve (ADN) input to superior salivatory nucleus (SSN) according to published studies by others (row 2; Ciriello, ; Housley et al., ; Altschuler et al., 1989); (3) the distribution of c-fos immunolabeling in NTS induced by hypotension as indicated in the present study and as consistent with prior reports by others (row 3; Rogers et al., ; Mayne et al., 1998); (4) the distribution of neurons in NTS that project to choroidal SSN (cSSN) as revealed from our prior studies involving intrachoroidal injection of pseudorabies virus (row 4; Li et al., 2015); (5) the distribution of sites within NTS yielding a significant choroidal blood flow (ChBF) increase (green) or no significant ChBF increase (red) in response to 50 μA anodal current pulses (row 5); (6) the distribution of sites within NTS yielding a significant ChBF increase (green) or no significant ChBF increase (red) in response to 70–100 μA anodal current pulses (row 6); and (7) the distribution of sites within NTS yielding a significant ChBF increase (green) or no significant ChBF increase (red) in response to 20 μA cathodal current pulses (row 7). Note that hypotension responsive and cSSN neurons overlap one another, and both overlap but extend beyond the zone of ADN input. Thus, hypotension responsive and cSSN neurons either extend their dendrites into the ADN-receptive zone and/or receive input from AND-receptive neurons to account for the baroresponsiveness. The stimulation data indicate that ChBF increases can be driven by anodal stimulation in rostromedial NTS along the course of the axons traveling from NTS to cSSN and by cathodal stimulation within the region enriched in hypotension responsive and cSSN projecting neurons.

Figure 3

Choroidal vasodilation with NTS stimulation from an illustrative case (NTSBF2) with 100 μA anodal stimulation (A,C), and an illustrative case (NTSBF19) with 20 μA cathodal stimulation (B,D). (A) Shows recording traces for arterial blood pressure (ABP), ChBF, volume and velocity in response to 7 s anodal current pulse trains for NTSBF2 at the rostromedial NTS site shown by the arrow in (C). During each NTS stimulation, ChBF, ChBVol, and choroidal blood velocity (ChBVel) increased for NTSBF2, but ABP did not. The effective stimulation site shown in this case is at the medial edge of the fiber tract traveling within NTS to cSSN. (B) Shows recording traces for ABP, ChBF, volume and velocity in response to 7 s cathodal current pulse trains for NTSBF19 at the more caudocentral NTS site shown by the arrow in (D). During each NTS stimulation, ChBF, ChBVol, and ChBVel increased for NTSBF19, but ABP did not. The effective stimulation site shown in this case is within a part of NTS enriched in neurons projecting to cSSN. The magnification is the same in (A,B).

Figure 4

Histogram showing the mean ChBF, ChBVol, ChBVel, and ABP responses to NTS stimulation at effective 50 μA anodal, 70–100 μA anodal, and 20 μA cathodal NTS stimulation sites, expressed as a percent of basal ChBF, ChBVol and ChBVel responses and ABP. ChBF, ChBVol and ChBVel were significantly increased, but ABP was not at these sites. Asterisks indicate significantly different from baseline (100%). Error bars represent the SEM.

Figure 5

Images showing the course of the axons from baroreceptive NTS coursing to prechoroidal SSN, as visualized in a previously described case (Li et al., 2010, 2015) in which biotinylated dextran amine was injected into NTS just anterior to the AP. Images (A,B,D) show increasingly rostral levels through NTS, and the NTS axons are indicated by an arrow in (A,B). Higher magnification view of the axons shown in image (B) is shown in image (C; asterisk). In image (D), two-color DAB labeling has been performed to simultaneously detect the neurons (brown) of choroidal SSN for their enrichment in neuronal nitric oxide (NO) synthase, and the axons (black) coursing from NTS to the SSN. The magnification is the same in (A,B,D).

Figure 6

Histogram showing the mean and peak ChBF, ChBVol and ChBVel responses and ABP responses to NTS stimulation at effective 100 μA anodal stimulation sites, expressed as a percent of basal ChBF, ChBVol and ChBVel responses and ABP, prior to N^ω-propyl-l-arginine (NPA) administration (A) and after NPA administration (B). Note that NTS stimulation prior to NPA yielded significant ChBF, ChBVol and ChBVel increases but had no effect on ABP, and that NPA attenuated the ChBF, ChBVol and ChBVel increases but had no effect on ABP. Asterisks indicate significantly different from baseline, and ampersands indicate significantly less than pre-NPA. Error bars represent the SEM.

Figure 7

Graph showing the time course of the mean ChBF, ChBVol, ChBVel and ABP responses to stimulation at effective anodal NTS sites (n = x). The blue bar marks the stimulation period. Each data point is the mean for a 333 ms interval, and ChBF, ChBVol, ChBVel and ABP responses are all expressed as percent of basal. The rapid ChBF increases are driven by rapid increases in both ChBVel and volume.

Figure 8

Graphs showing the mean ChBF (A) and ABP (B) responses to stimulation at effective NTS sites prior to and after NPA administration (n = x). The blue bar marks the stimulation period. Each data point is the mean for 333 ms. ChBF is expressed in relative blood flow units (BFU) and ABP in mm/Hg. Note that NPA significantly reduces the ChBF response to NTS activation, and has a consistent but insignificant depressive effect on ABP.

Figure 9

Graphs showing the mean ChBVol (A) and ChBVel (B) responses to stimulation at effective NTS sites prior to and after NPA administration (n = x). The blue bar marks the stimulation period. Each data point is the mean for 333 ms. ChBVol and ChBVel are expressed in relative units. Note that NPA significantly reduces both ChBVol and ChBVel responses to NTS activation.