Elsevier

Brain Research

Volume 963, Issues 1–2, 14 February 2003, Pages 190-196
Brain Research

Research report
Local inflammation increases vanilloid receptor 1 expression within distinct subgroups of DRG neurons

https://doi.org/10.1016/S0006-8993(02)03972-0Get rights and content

Abstract

Vanilloid receptor 1 (VR1) is essential to the development of inflammatory hyperalgesia. We investigated whether inflammation can increase in VR1 positive neuronal profiles in rat DRG neurons using histochemical methods. We also used size frequency analysis and double staining with several neuronal markers to investigate whether or not inflammation alters VR1 expression. Inflammation induced a 1.5-fold increase in percentage of VR1-like immunoreactivity (LI) positive profiles per total neuronal profiles, suggesting that the number of heat and pH sensitive neurons increase during inflammation. Area frequency histograms showed that VR1 expression increased in small and medium-sized neurons after inflammation. Double labeling of VR1 with NF200 showed that VR1 positive neurons with NF200 positive profiles significantly increased, indicating that the medium-sized VR1 positive neurons were neurons with myelinated A-fibers. Local inflammation thus increases in VR1 protein level within distinct subgroups of DRG neurons that may participate in the development and maintenance of inflammatory hyperalgesia.

Introduction

Painful stimuli applied to the skin activate specific primary sensory neurons called nociceptors. Neurons with myelinated A-delta and unmyelinated C fibers are considered to be nociceptive neurons in mammals. Peripheral inflammation induced by plantar injection of a chemical irritant such as Freund’s complete adjuvant (CFA) can elicit acute hyperalgesia at the injection site and adjacent area. Electrophysiological studies show that inflammation increases the number of neurons with A-delta and C fibers that respond to ATP injection [16] and receptive field stimulation [14]. Therefore, the number of activated neurons must increase as well as the electrical excitability of individual neurons to cause behavioral hyperalgesia during inflammation.

Vanilloid receptor 1 (VR1) is a non-selective cation channel gated by noxious heat, proton and capsaicin [8], [28], that has recently been cloned [9], [12]. This receptor is essential for the development of thermal hyperalgesia, as mice lacking this receptor lack thermal hyperalgesia after inflammation [10]. Studies in vitro have shown that substances produced by local inflammation such as ATP [29], bradykinin [22] and NGF [11] can increase the capsaicin-evoked current in VR1 expressing cells. These functional modulations of VR1 channels, however, cannot explain why the number of active neurons increases after inflammation; novel recruitment of VR1 expression would be necessary.

In the present study, we postulated that inflammation increases VR1 positive neuronal profiles, which would cause the ratio of electrically activated neurons to increase. We therefore performed histochemical analyses to compare VR1 expression in DRG neurons between control and inflamed animals. We also double-stained VR1 with several neuronal markers to investigate whether or not inflammation can alter the expression of VR1 within heterogeneous DRG neurons. We found that peripheral inflammation can increase in VR1 positive neuronal profiles within specific subsets of DRG neurons.

Section snippets

Animals

All experiments were approved by the Kyoto Prefectural University of Medicine Animal Care Committee and were in accordance with National Institutes of Health Guide for the Care and Use of Laboratory Animals and guidelines of the International Association for the Study of Pain. Peripheral inflammation was generated by an intraplantar injection of Freund’s complete adjuvant (CFA; Sigma; 50 μl/each) into the left hind paws of eight male Sprague–Dawley rats, each weighing 200–250 g. A total of

VR1 expression in lumbar DRG

VR1-like immunoreactivity (LI) was detected within DRG neurons of both control and inflamed animals. In control animals, a total of 3300 DRG neurons were analyzed. Among them, 879 neurons were positive for VR-1. In inflamed animals, on the other hand, 1352 neurons were positive for VR1 among 3031 total neurons. Of DRG neurons, 27% were positive for VR1-LI in control animals. Compared with controls, these neurons were more frequently detected after inflammation (Fig. 1A), since the ratio of

Inflammation causes VR1 expression to increase

We detected DRG neurons that were positive for VR1-LI more frequently after local inflammation. The ratio of neurons expressing VR1 reached 1.5-fold the basal level, showing up-regulation of VR1 expression in DRG neurons in which VR1 was previously undetectable during inflammatory hyperalgesia. Recent studies show that levels of VR1-LI increase within nerve fibers at the site of inflammation in human and rodents [7], [34]. These and our results indicate that increased expression of VR1 in the

Acknowledgements

This work was supported by the NOVARTIS Foundation (Japan) for the Promotion of Science

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