Activation of vagal afferents after intravenous injection of interleukin-1beta: role of endogenous prostaglandins

Abstract

Intravenous administration of interleukin-1 (IL-1) activates central autonomic neuronal circuitries originating in the nucleus of the solitary tract (NTS). The mechanism(s) by which blood-borne IL-1 regulates brain functions, whether by operating across the blood-brain barrier and/or by activating peripheral sensory afferents, remains to be characterized. It has been proposed that vagal afferents originating in the periphery may monitor circulating IL-1 levels, because neurons within the NTS are primary recipients of sensory information from the vagus nerve and also exhibit exquisite sensitivity to blood-borne IL-1. In this study, we present evidence that viscerosensory afferents of the vagus nerve respond to intravenously administered IL-1beta. Specific labeling for mRNAs encoding the type 1 IL-1 receptor and the EP3 subtype of the prostaglandin E2 receptor was detected in situ over neuronal cell bodies in the rat nodose ganglion. Moreover, intravenously applied IL-1 increased the number of sensory neurons in the nodose ganglion that express the cellular activation marker c-Fos, which was matched by an increase in discharge activity of vagal afferents arising from gastric compartments. This response to IL-1 administration was attenuated in animals pretreated with the cyclooxygenase inhibitor indomethacin, suggesting partial mediation by prostaglandins. In conclusion, these results demonstrate that somata and/or fibers of sensory neurons of the vagus nerve express receptors to IL-1 and prostaglandin E2 and that circulating IL-1 stimulates vagal sensory activity via both prostaglandin-dependent and -independent mechanisms.

Fig. 1.

Sensory neurons in the nodose ganglion express receptors for IL-1 and PGE2. An antisense (A) or sense (B) cRNA probe encoding the IL-1Rt1 was hybridized in situ to nodose ganglion sections from a rat that had previously received an injection with vehicle only 60 min before perfusion fixation. Specific hybridization with the antisense IL-1 receptor probe resulted in the accumulation of silver grains over cells throughout the nodose ganglion (A), whereas no specific labeling was obtained in parallel sections after hybridization with the sense control probe (B). The arrows in the bright-field micrograph inA indicate cells typical of those expressing the IL-1 receptor in the nodose ganglion. Cells expressing mRNA encoding the EP3 receptor were identified in sections throughout the nodose ganglion after hybridization with a specific antisense cRNA probe (C; dark-field micrograph), whereas parallel tissue sections hybridized with a sense probe showed no labeling above background levels (D). Arrows inE identify specific labeling for EP3 receptor mRNA over neuron-like cells (bright-field micrograph). These receptor-expressing cells in the nodose ganglion send projections to the vagus nerve, as was revealed by their labeling with the retrograde tracer true blue (F), which was injected into the vagal fiber trunk ∼1 cm distal to the caudal pole of the nodose ganglion. These rats showed extensive and specific in situ labeling with the EP3R antisense probe in retrogradely filled neurons (F). Arrows depict double-labeled cells. Scale bars: A, B,E, 30 μm; C, D, 300 μm; F, 120 μm.

Fig. 2.

Sensory neurons in the nodose ganglion are responsive to blood-borne IL-1. Photomicrographs in the top left panels show in situ hybridization analysis with a ³³P-labeled cRNA antisense probe encoding the cellular activation marker c-Fos to 12 μm sections through the nodose ganglia of rats injected with 2 μg/kg human recombinant IL-1β (top photomicrograph) or vehicle alone (bottom photomicrograph) 60 min before perfusion fixation. Note the accumulation of black silver grains, corresponding to specific labeling for c-Fos mRNA, over neuron-like cells in IL-1- but not in vehicle-injected rats. Parallel sections hybridized with a c-Fos cRNA sense probe displayed background levels of silver grains throughout the ganglion (data not shown). Thick black or open arrows, respectively, indicate neurons that either do or do not expressc-Fos mRNA. Thin black arrowsindicate specific labeling for c-Fos mRNA over non-neuronal cells. Scale bar (in top photomicrograph), 25 μm. Top right panel shows results from the quantitative evaluation of neurons specifically expressingc-Fos mRNA in the nodose ganglia of vehicle/vehicle-, vehicle/IL-1-, indomethacin/IL-1-, or indomethacin/vehicle-injected rats. Silver grains were visually counted over neurons plotted with a camera lucida. The data are displayed in box plots in which the horizontal lines in each boxcorrespond to the 25th percentile, the median, and the 75th percentile, and the range for each group is indicated by the extent of thevertical lines. *p = 0.0134; **p = 0.0093; *** p < 0.0001. The bottom panel is a schematic drawing showing the distribution of c-Fos-expressing cells within the nodose ganglion of IL-1- or vehicle-injected rats. Cells having a neuronal morphology and a clearly delineated nucleus were plotted throughout the ganglion using a camera lucida. Sensory neurons were defined as being specifically labeled or unlabeled with the c-Fosantisense mRNA probe and are represented by filled oropen circles, respectively. The outline of each ganglion is shown with the caudal pole of the ganglion positioned to theright.

Fig. 3.

Afferent activity of the gastric vagal nerve responds to intravenous administration of IL-1β (2 μg/kg).A and B show sample recordings from individual rats injected with vehicle/IL-1 or indomethacin/IL-1, respectively. C summarizes the responses from 19 rats. The ordinate indicates the magnitude of the response (>1 min), which is expressed as the percentage of the preadministration control value (taken 1 min immediately before injection of IL-1β or vehicle). The abscissa shows time in minutes in which 0 indicates the onset of injection. Circles show the data from vehicle/IL-1β-injected rats (n = 5), andtriangles show the data from indomethacin/IL-1β-injected rats (n = 5). Eachpoint and vertical bar indicate the mean ± SEM. *p < 0.05; **p < 0.01, comparing the vehicle/IL-1β-injected with the vehicle/vehicle-injected groups. a,p < 0.05; b, p < 0.01, comparing the indomethacin/IL-1β with the vehicle/IL-1β groups.