Abstract
Purpose::
The parasympathetic nervous system mediates increases in choroidal blood flow (ChBF) via the superior salivatory nucleus (SSN) and pterygopalatine ganglion (PPG). As part of a larger effort to understand the neural circuitry that regulates ChBF, a collection of neuroanatomical and physiological approaches have been used to identify brain regions that project to the SSN. Herein, we define an important role for the paramedian pontine raphe cell groups (PPR; magnus, pallidus and obscurus) in SSN-mediated choroidal vasodilation.
Methods::
SSN afferent structures were localized in adult Sprague-Dawley rats via retrograde transneuronal transport of pseudo-rabies virus (PRV) from the choroid. After 65-87 hour post-injection survival intervals, perfusion-fixed brains were processed for immunocytochemical localization of PRV or serotonin. In separate experiments, an anterograde tracer, biotinylated dextran amine 10K (BDA 10K), was injected into the paraventricular nucleus (PVN) of the hypothalamus, and the effects of PPR stimulation on ChBF were recorded transsclerally using a laser Doppler.
Results::
Following PRV injection into rat choroid, transneuronal labeling was observed in the PPG, SSN and PPR. Immunolabeling for serotonin revealed that neurons resembling the PRV+ neurons of PPR contained serotonin, and consistent with a projection of these neurons to the choroidal part of SSN, this part of SSN was rich in serotonergic fibers. Further consistent with a projection of PPR to choroidal SSN, electrical stimulation of the PPR region yielded vasodilation in the ispilateral choroid. The BDA10K experiments showed that the PVN projects to both the choroidal SSN and the PPR, indicating that the PVN can control ChBF directly via a projection to SSN as well as by a projection to PPR, an afferent cell group of the choroidal SSN.
Conclusions::
The results reported herein suggest that serotonergic neurons of PPR are part of the central parasympathetic circuit regulating choroidal vasodilation via the SSN. Furthermore, in addition to its direct projection to the SSN, the PVN may control the choroidal SSN by way of the PPR. Given the well-established PVN projection to preganglionic sympathetic neurons in the intermediolateral cell column of the thoracolumbar spinal cord, our findings suggest that the PVN, PPR and SSN function in coordinated fashion to produce systemic vasoconstriction and choroidal vasodilation in response to systemic hypotension.
Keywords: choroid • innervation: neural regulation • neurotransmitters/neurotransmitter systems