Disinhibition of neurons of the nucleus of solitary tract that project to the superior salivatory nucleus causes choroidal vasodilation: Implications for mechanisms underlying choroidal baroregulation

Abstract

Preganglionic neurons in the superior salivatory nucleus (SSN) that mediate parasympathetic vasodilation of choroidal blood vessels receive a major excitatory input from the baroresponsive part of the nucleus of the solitary tract (NTS). This input appears likely to mediate choroidal vasodilation during systemic hypotension, which prevents decreases in choroidal blood flow (ChBF) due to reduced perfusion pressure. It is uncertain, however, how low blood pressure signals to NTS from the aortic depressor nerve (ADN), which fires at a low rate during systemic hypotension, could yield increased firing in the NTS output to SSN. The simplest hypothesis is that SSN-projecting NTS neurons are under the inhibitory control of ADN-receptive GABAergic NTS neurons. As part of evaluating this hypothesis, we assessed if SSN-projecting NTS neurons, in fact, receive prominent inhibitory input and if blocking GABAergic modulation of them increases ChBF. We found that SSN-projecting NTS neuronal perikarya identified by retrograde labeling are densely coated with GABAergic terminals, but lightly coated with excitatory terminals. We also found that, infusion of the GABA-A receptor antagonist GABAzine into NTS increased ChBF. Our results are consistent with the possibility that low blood pressure signals from the ADN produce vasodilation in choroid by causing diminished activity in ADN-receptive NTS neurons that tonically suppress SSN-projecting NTS neurons.

Keywords: Autonomic; Choroidal blood flow; Disinhibition; GABA; Nucleus of solitary tract; Parasympathetic; Superior salivatory nucleus.

Figure 1

Schematic illustrating the hypothetical circuit by which baroreceptive input to the NTS may mediate disinhibitory control of choroidal vasodilation via the SSN.

Figure 2

Images showing the greater abundance of GAD65+ inhibitory terminals on SSN-projecting NTS neuron perikarya than VGLUT2+ excitatory terminals. The SSN-projecting neurons shown in the A–B and C–D image pairs were retrogradely labeled from SSN and visualized using streptavidin Alexa-488, while GAD65 was visualized with Alexa-594 and VGLUT was visualized with Alexa-647. Images were captured with a CLSM, and are displayed in these image pairs with the GAD65 labeling (red) shown together with the labeling of SSN-projecting NTS neurons (green) in images A and C, and the VGLUT2 labeling (displayed here as red) shown together with the labeling of SSN-projecting NTS neurons (green) in images B and D.

Figure 3

Bar graph illustrating that GAD65+ terminals covered significantly more of the soma of the BDA3K+ NTS neurons than did contacts by VGLUT2+ terminals, but GAD65+ terminals and VGLUT2+ terminals were relatively comparable in their areal coverage of the proximal dendrites of BDA3K+ NTS neurons. Asterisk indicates a significant difference between GAD65+ and VGLUT2+ terminals.

Figure 4

Chart recordings showing that iontophoretic GABAzine infusion into NTS (A) produced a sustained increase in ChBF, but not ABP (C). The response had a latency of about 600 seconds from stimulus onset (arrow) and then lasted about 1500 seconds beyond stimulus offset (arrow). In contrast to iontophoretic GABAzine infusion with 7 second on – 7 second off 5 µA cathodal pulses, 7 second trains of 0.5 msec 100 Hz 20 µA cathodal pulses produced rapid and brief increases in ChBF (B). Note that in both B and C, the x-axis shows seconds. The site of GABAzine infusion is shown by the asterisk at the center of the c-fos positive labeling of nuclei within NTS, whose borders are outlined (A). The c-fos nuclear labeling reflects neurons activated by the GABAzine infusion.

Figure 5

Bar graph illustrating that GABAzine infusion into NTS produced increases in ChBF, but not ABP. Asterisk indicates a significant increase above baseline after GABAzine administration