Three novel neural pathways to the lacrimal glands of the cat: an investigation with cholera toxin B subunit as a retrograde tracer

Aquaporins in lacrimal glands and their role in dry eye disease

Aging is the most important known risk factor for dry eye is aging, which is associated with changes in the structure and function of the lacrimal gland (LG) and characterized by atrophy, duct blocking lymphocyte infiltration, and reduced protein secretion. Aquaporins (AQP) have been proposed as a potential producer of exocrine gland fluids since exocrine secretion depends on the mobility of water. Therefore, the main topics of this review will be the expression, localization, and function of AQPs in LG. In addition, we review the mechanisms of fluid transport in exocrine gland fluid secretion and discuss the potential role of AQPs in dry eye.

Vagus nerve stimulation modulates the cranial trigeminal autonomic reflex

OBJECTIVE

The trigeminal autonomic reflex plays an important role in primary headache syndromes. Noninvasive vagal nerve stimulation (nVNS) may be an effective modulator of this reflex.

METHODS

Twenty-two healthy volunteers underwent kinetic oscillation stimulation (KOS) of the left nostril as a reliable trigger of the trigeminal autonomic reflex. Previous to KOS, left cervical nVNS, sham simulation, or no stimulation was applied. Lacrimation was quantified using the standardized Schirmer ll test.

RESULTS

Treatment with cervical nVNS significantly reduced lacrimation between no stimulation and nVNS on the ipsilateral side (minute 5: p = 0.026, ηp² = 0.85, 95% confidence interval [CI] = 1.39-18.04; no stimulation: minute 5, 14.4 ± 9.3 mm; nVNS: minute 5, 4.7 ± 8.6 mm, mean ± standard deviation) as well as between sham stimulation and nVNS (minute 5: p = 0.030, ηp² = 0.85, 95% CI = 1.04-17.24; sham: minute 5, 13.9 ± 6.4 mm). On the contralateral side, no significant increase between baseline and KOS was observed for nVNS (minute 5: p = 0.614, d = 0.12, 95% CI = -7.09 to 4.31; minute 5, 1.4 ± 11.5 mm) compared to both sham stimulation (minute 5: p = 0.023, d = 0.57, 95% CI = -11.46 to -0.96; minute 5, 6.2 ± 10.9 mm) and no stimulation (minute 5: p < 0.030, d = 0.62, 95% CI = -13.45 to -0.81; minute 5, 7.1 ± 11.4 mm).

INTERPRETATION

Cervical nVNS resulted in a robust bilateral reduction of provoked lacrimation. This effect could be mediated either by direct bilateral activation of structures such as the nucleus of the solitary tract or by a top-down modulation via the hypothalamus. Ann Neurol 2018;84:886-892.

Distribution of pterygopalatine ganglion efferents to the lacrimal gland in man

To trace the path taken by the putative postganglionic secretomotor fibres to the lacrimal gland the contents of the orbital and pterygopalatine fossa were removed whole, cut coronally into slabs and embedded in resin. Thin sections were cut at varying intervals to reconstruct the pathway taken. One group of rami orbitales issuing from the pterygopalatine ganglion passed dorsally adjacent to the lateral wall of the orbit, joined the retro-orbital plexus at the apex, and 5-10 rami lacrimales advanced from the plexus to enter the gland. An accessory ophthalmic artery, a branch of the middle meningeal artery, entered the orbit through the superior fissure orbital joining the ophthalmic or lacrimal artery. Perivascular nerves of the artery continued to the gland as supplementary rami lacrimales and in some orbits others served the vasculature of the eye and orbit. The nerves are presumably derived from the middle meningeal supply and may include otic parasympathetic fibres. The route taken by parasympathetic nerves serving the human lacrimal gland is demonstrated here for the first time and apart from the perivascular meningeal artery source, it is similar to that described in monkeys. The traditional assumption that secretomotor nerves pass to the gland via the zygomatic and lacrimal nerves is therefore unlikely and clinical measures to reduce lacrimation based on that assumption and involving severance of ophthalmic branches is not indicated.

Up-regulation of methionine-enkephalin-like immunoreactivity by 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment in the forebrain of the Long-Evans rat

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is considered to be one of the most toxic environmental contaminants, named dioxin. Exposure to TCDD induces a plethora of intoxication symptoms, including anorexia and hypothermia, in several mammals and human. Enkephalin, an endogenous pentapeptide, is an important neuroregulator of autonomic functions, such as food intake and body temperature. In this study, we investigated the effects of TCDD gastric administration on methionine-enkephalin (MEK) immunoreactivity in the brain of the Long-Evans rat, the species strain considered to be the most TCDD-susceptible, using immunohistochemical staining. A single dose of TCDD (dissolved in olive oil, 50 microg/kg) or olive oil alone was administrated to the rats by gavage. Compared with the vehicle-treated rat, a marked increase in the density of MEK immunoreactive cell bodies, fibers and terminals was found 2 weeks after TCDD treatment in the forebrain of the TCDD-treated rat, i.e. the central amygdaloid nucleus, field CA3 of the hippocampus, paraventricular hypothalamic nucleus, medial preoptic nucleus, interstitial nucleus of the posterior limb of the anterior commissure, lateral globus pallidus, ventral pallidum and lateral division of the bed nucleus of the stria terminalis. These results demonstrated for the first time a site-specific increased enkephalinergic activity in certain brain regions of the Long-Evans rat. It is suggested that the increased MEK immunoreactivity may act as a compensatory adaptation for the pathophysiological alterations caused by TCDD exposure.

Direct projections from the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons: a link to cerebrovascular regulation

Peripheral or central interruption of the baroreflex or the parasympathetic innervation of cerebral vessels leads to similar changes in regulation of cerebral blood flow. Therefore, we sought to test the hypothesis that the cardiovascular nucleus tractus solitarii, the site of termination of arterial baroreceptor nerves, projects to pontine preganglionic neurons whose stimulation elicits cerebral vasodilatation. The current study utilized both light and electron microscopic techniques to analyze anterograde tracing from the cardiovascular nucleus tractus solitarii to preganglionic parasympathetic neurons in the pons. We further used retrograde tracing from that same pontine region to the cardiovascular nucleus tractus solitarii and evaluated the confluence of tracing from the cardiovascular nucleus tractus solitarii to pontine preganglionic neurons labeled retrogradely from the pterygopalatine ganglia. The cardiovascular nucleus tractus solitarii projected to pontine preganglionic parasympathetic neurons, but more rostral and caudal regions of nucleus tractus solitarii did not. In contrast, all three regions of nucleus tractus solitarii projected to the nucleus ambiguus and dorsal motor nucleus of the vagus. Although not projecting to pontine preganglionic parasympathetic neurons, regions lateral, rostral, and caudal to cardiovascular nucleus tractus solitarii sent projections through the pons medial to the preganglionics. The study establishes the presence of a direct monosynaptic pathway from neurons in the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons that project to the pterygopalatine ganglia, the source of nitroxidergic vasodilatory innervation of cerebral blood vessels. It provides evidence that activation of those preganglionic neurons can cause cerebral vasodilatation and increased cerebral blood flow. Finally, it demonstrates differential innervation of medullary and pontine preganglionic parasympathetic neurons by different regions of the nucleus tractus solitarii.

Presence of neuronal nitric oxide synthase in autonomic and sensory ganglion neurons innervating the lacrimal glands of the cat: an immunofluorescent and retrograde tracer double-labeling study

It is generally considered that parasympathetic postganglionic nerve fibers innervating the lacrimal gland (LG) arise from the pterygopalatine ganglion (PPG), while sympathetic and sensory innervations arise from the superior cervical ganglion (SCG) and trigeminal ganglion (TG), respectively. Recently, we reported for the first time that the parasympathetic innervation of the cat LG was also provided by the otic ganglion (OG) and ciliary ganglion (CG), and that the sensory innervation was also provided by the superior vagal ganglion (SVG) and superior glossopharyngeal ganglion (SGG). To determine if nitric oxide (NO) is a neurotransmitter of the autonomic and sensory neurons innervating the LG, we injected the cholera toxin B subunit (CTB) as a retrograde tracer into the cat LG, and used double-labeling fluorescent immunohistochemistry for CTB and nitric oxide synthase (NOS). We found that NOS-/CTB-immunofluorescent double-labeled perikarya were localized in the PPG, OG, TG, SVG and SGG, but not in the CG and SCG. The highest numbers of NOS-/CTB-immunofluorescent double-labeled neurons were found in the PPG and TG. In addition, we examined the presence of nitrergic nerve fibers in the LG using NADPH-d histochemistry and found that a large amount of NADPH-d-stained nerve fibers were distributed around the glandular acini and in the walls of glandular ducts and blood vessels. This study provides the first direct evidence showing that NO may act as a neurotransmitter or modulator involved in the parasympathetic and sensory regulation of lacrimal secretion and blood circulation, but may not be implicated in the sympathetic control of LG activities, and that nitrergic nerve fibers in the LG arise mainly from parasympathetic postganglionic neurons in the PPG and sensory neurons in the TG. The present results suggest that NO plays an important role in the regulation of LG activities.

Morphological distinction between vasodilator and secretomotor neurons in the pterygopalatine ganglion of the cat

We investigated whether vasodilator and secretomotor ganglion neurons are morphologically distinguishable from each other in the parasympathetic ganglion of the cat. When Cholera toxin B subunit, a retrograde tracer, was injected into the palatine gland, both large and small ganglion neurons were retrogradely labeled in the pterygopalatine ganglion. On the other hand, when the tracer was injected into gland-free areas (the upper gingiva or epidural space), all neurons labeled in the ganglion were small in size. Thus, it was assumed that small and large neurons labeled in the ganglion represented, respectively, vasomotor and secretomotor and neurons [corrected].