The glossopharyngeal nerve as a novel pathway in immune-to-brain communication: relevance to neuroimmune surveillance of the oral cavity

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Abstract

Glossopharyngeal afferents may be the neural channel by which immune challenge of the posterior oral cavity conveys information to the brain. If this is the case, then bilateral transection of the glossopharyngeal nerves (GLOx) should disrupt this communication. Injection of lipopolysaccharide (LPS) or interleukin (IL)-1β into the soft palate (ISP) of sham-operated rats induced a dose-related febrile response. GLOx significantly attenuated the febrile response induced by ISP injection of both LPS and IL-1β. In contrast, GLOx did not affect the febrile response when LPS or IL-1β were injected intraperitoneally, indicating that the effect of GLOx is not systemic. These results provide experimental evidence for a novel neural pathway for immune-to-brain communication.

Introduction

Infection and inflammation characteristically elicit fever and a series of systemic and behavioral responses that collectively are known as the acute-phase response. Fever represents a primary host defense response against the invasion of bacterial and viral pathogens Blatteis and Sehic, 1997, Kluger, 1991. Cytokines, released from antigen-activated immune cells, play a key role as regulators of the host defense response by providing bi-directional signals to the immune system and to the central nervous system (CNS) (Besedovsky and Del Rey, 1996). In fact, the nervous system, alerted by peripherally released cytokines, reacts by producing changes in neural activity that lead to physiological and behavioral responses Besedovsky and Del Rey, 1996, Rothwell and Hopkins, 1995, Schobitz et al., 1994.

Proinflammatory peripheral cytokines can reach and/or stimulate the brain through several pathways: (a) by a specific carrier-mediated transport mechanism (Banks et al., 1995); (b) by binding to cytokine receptors in the endothelial cells of the brain vasculature, where they induce the release of secondary messengers, such as nitric oxide Licinio et al., 1999, Van Dam et al., 1993; (c) acting directly at the circumventricular organs, and other brain areas lacking the blood–brain barrier Blatteis, 1992, Licinio and Wong, 1997, Saper and Breder, 1994; and (d) from immune structures innervated by the peripheral and autonomic nervous systems Felten et al., 1985, Weihe et al., 1999 which transmit afferent information directly to the brain. The latter route may be of particular importance given that its activation can be achieved by amounts of lipopolysaccharide (LPS) or cytokines that produce virtually undetectable levels of blood cytokines Kluger, 1991, Miller et al., 1997.

Over the last few years, experimental evidence has accumulated to support the role of the vagus nerve (X cranial nerve) as the neural pathway transmitting peripheral immune challenges to the brain Ek et al., 1998, Goehler et al., 1997, Goehler et al., 1999, Maier et al., 1998, Niijima, 1996, Simons et al., 1998, Watkins et al., 1995. The vagus nerve contains a high percentage of afferent fibers Berthound et al., 1992, Berthound and Powley, 1993 and bilateral subdiaphragmatic vagotomy, a surgical procedure that disrupts both afferent and efferent innervation of the abdominal cavity, has been employed as an experimental paradigm to examine the role of vagal transmission of peripheral immune signals to the brain. In several recent studies, subdiaphragmatic vagotomy has been shown to block or at least attenuate brain-mediated responses to peripheral immune challenge induced by intraperitoneal (i.p.) or intravenous (i.v.) injections of lipopolysacharide (LPS) or interleukin (IL)-1β Bluthe et al., 1994, Fleshner et al., 1998, Gaykema et al., 1995, Hansen and Krueger, 1997, Hansen et al., 1998, Laye et al., 1995, Opp and Toth, 1998, Romanovsky et al., 1997, Simons et al., 1998, Watkins et al., 1995. However, it should be noted that the effects of subdiaphragmatic vagotomy may be restricted to the abdominal cavity. Sectioning the vagus at its upper levels in order to disrupt the afferent input of the entire nerve would be preferable but transection of the vagus above thoracic levels is fatal. Despite the critical role of the vagus in immune-to-brain communication, other neural routes linking peripheral cytokines and brain are conceivable. For instance, subcutaneous injections of IL-1β into the plantar skin of the hindpaw has been found to induce spontaneous activation of cutaneous nerves (Fukuoka et al., 1994), while tumor necrosis factor-α (TNF-α) directly applied to the sciatic nerve induces ectopic activity (Sorkin et al., 1997). In addition, a low dose of LPS, injected into a subcutaneous chamber after anesthetic pretreatment, attenuated the fever response in guinea pigs, suggesting a role for cutaneous nerves in immune-to-brain communication (Ross et al., 1999).

A neural route that has been overlooked and therefore has not been investigated as another pathway in immune-to-brain communication is the glossopharyngeal (IX cranial nerve). The rat glossopharyngeal is a mixed nerve that mainly comprises afferent (sensory) fibers and provides innervation for the parotid gland, posterior one-third of the tongue, soft palate and the pharynx (Altschuler et al., 1989). Indeed, the posterior oral cavity, with its glossopharyngeal-innervated mucosal-associated lymphoid tissues (Weihe et al., 1999), is located in the front line of defense against potential infections.

Our hypothesis is that the glossopharyngeal nerve plays a key role in immune-to-brain communication from the posterior oral cavity. We anticipate that bilateral transection of the glossopharyngeal nerves will disrupt this communication by blocking or attenuating the acute-phase response induced by local immune challenges of the posterior oral cavity. Fever, as the most manifest sign of the acute-phase response to infection and inflammation, is observable following administration of LPS and IL-1β. Thus, in order to test our hypothesis, we investigated the effects of a bilateral surgical transection of the glossopharyngeal nerve on the LPS- and IL-1β-induced febrile response recorded with biotelemetric measurements of body temperature.

Section snippets

Subjects

Male Sprague–Dawley rats from Charles River Breeding Labs (Portage, MI), weighing 320–350 g at the start of the experiments, were used in all the studies. Upon arrival, rats were acclimated for 1 week to our standard temperature and light conditions (22±1°C, 14/10 h light cycle: 0600–2000 h). Food (pellet rat chow, Harlan, WI) and water were provided ad libitum. All experimental procedures were approved by the UCLA and VA institutional animal care and use committees.

Substances

Purified LPS (Escherichia

Experiment 1: febrile response to varying doses of LPS injected ISP in intact rats

LPS injections into the soft palate of intact rats began to produce fever at 120 min after injection, and fever subsided around 480 min post-injection (Fig. 1). Repeated measures ANOVA of the differences between temperatures recorded at comparable time-points after LPS and saline for the hyperthermic period (120–480 min) indicated a significant effect of dose [F(2,14)=4.08, p=0.04]. The mean±S.E.M. average total responses for this period to 25, 50 and 100 μg/kg were: −0.064±0.109°C,

Discussion

In the present study, we have provided experimental support for our hypothesis that postulates the glossopharyngeal nerve as a novel neural route for immune-to-brain communication. Our demonstration that bilateral surgical transection of the glossopharyngeal nerve attenuates the LPS- or IL-1β-induced febrile component of the acute-phase response is evidence for disruption of immune-to-brain communication by this route. The soft palate was selected as the anatomical site for experimental immune

Acknowledgements

The helpful discussions on electrophysiological aspects of the glossopharyngeal nerve with Dr. Oscar U. Scremin and on statistical analyses with Dr. Jeffrey A. Gornbein are gratefully acknowledged. We also thank Terrill T.-L. Tang for technical assistance. Supported by grants from the NIH/NIAAA (AA09850), the Department of Veterans Affairs Medical Research Service, and the Norman Cousins Center for Psychoneuroimmunology in the Neuropsychiatric Institute at UCLA.

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