Responses of primary afferents and spinal dorsal horn neurons to thermal and mechanical stimuli before and during zymosan-induced inflammation of the rat hindpaw

Abstract

Intraplantar administration of zymosan produces inflammation and results in behavioral evidence of hyperalgesia to mechanical and thermal stimuli in the rat. In the present studies, responses of primary afferents and spinal dorsal horn neurons to mechanical and thermal stimuli were examined before and during zymosan-induced inflammation of the hindpaw. In tests of responses of primary afferents to mechanical stimuli, group mean mechanical response thresholds of C-mechanonociceptor (CMN) units significantly decreased after zymosan administration. The group mean mechanical response thresholds of low threshold mechanoreceptor (LTM) units, A-mechanoheat (AMH) units, high threshold mechanoreceptor (HTM) units, and C-mechanoheat (CMH) units showed either no change or were increased significantly by intraplantar administration of zymosan. The group mean total discharges evoked during the 10 s mechanical stimulus were significantly increased after zymosan administration in CMN units. The group mean total discharges were either significantly decreased or unchanged in LTM, AMH, HTM, and CMH units. In tests of responses of spinal dorsal horn neurons to mechanical stimuli, the group mean mechanical response threshold of nociceptive specific (NS) units decreased significantly 1 h following administration of zymosan, whereas no significant changes occurred in the mechanical response thresholds of wide dynamic range (WDR) neurons in zymosan-injected rats, WDR neurons in saline-injected rats, or NS neurons in saline-injected rats. The group mean total discharges of only NS neurons were significantly increased during the 10 s mechanical stimulus 3 and 4 h after zymosan administration. In tests of responses of primary afferents to thermal stimuli, intraplantar administration of zymosan resulted in significant decreases in group mean response thresholds of CMH units and significant increases in group mean response thresholds of AMH units. The group mean total discharges of CMH units was either unchanged or significantly increased during thermal stimuli depending on both the time of testing and the temperature of the test stimulus. The group mean total number of discharges of AMH units was significantly decreased during tests of all thermal stimuli. In tests of responses of spinal dorsal horn neurons to thermal stimuli, intraplantar administration of zymosan resulted in significant decreases in thermal response thresholds of both WDR and NS units of zymosan-injected rats, but no changes in WDR and NS units of saline-injected rats. The group mean total discharges evoked by the 15 s thermal stimuli also increased significantly in both WDR and NS units after zymosan administration. Zymosan administration resulted in increased background activity only in CMH units. These increases occurred immediately following the injection and dissipated by the first hourly test period. Significant changes in background discharges of both WDR and NS units occurred at some hourly test intervals following administration of zymosan, but these changes were not consistent with respect to either unit type or modality of the test stimulus. These data suggest that the zymosan-induced hyperalgesia to mechanical stimuli observed in behavioral studies reflects decreases in response thresholds of peripheral CMN units and spinal NS neurons. Hyperalgesia to thermal stimuli reflects decreases in response thresholds of peripheral CMH units, spinal WDR neurons, and spinal NS neurons. These data support the view that different physiological substrates mediate hyperalgesia to either thermal or mechanical stimuli following intraplantar administration of zymosan.

Introduction

Inflammation is most often associated with either tissue damage or nerve injury. Common signs of inflammation include redness, swelling, and primary hyperalgesia. Primary hyperalgesia is characterized by a lowering of response threshold and/or an increased response to supra-threshold stimuli at the site of injury. This is typically manifested to both mechanical and thermal stimuli, and it is generally held that alterations in both peripheral and central nervous system processes are critical for the production of primary hyperalgesia (cf., 6, 8, 30).

Primary hyperalgesia can result from sensitization of primary nociceptors, or an increase in the excitability of peripheral nociceptors to mechanical and thermal stimuli at the inflammatory site. Sensitization often depends on the release, synthesis, or attraction of inflammatory mediators to the site of injury 7, 20, 22, 28. However, the precise mechanisms by which peripheral nociceptors are sensitized, the specific types of peripheral nociceptors that are affected, and the relationship of these peripheral changes to events occurring in the central nervous system during inflammation are not well characterized.

It also has been suggested that either the initial discharge or sensitization of primary nociceptors can lead to some form of central plasticity of spinal dorsal horn neurons. This may either augment or unmask input from primary afferents/nociceptors. This view is applied most often to secondary hyperalgesia, which typically affects the response of uninjured tissue surrounding the site of injury, and primarily is manifested to mechanical stimuli 12, 22, 27, 29. However, this hypothesis applies equally well to issues of primary hyperalgesia. Recent data suggests that central plasticity may depend on the release of excitatory amino acids (EAA) and/or tachykinins, but there remains considerable controversy about the receptor subtypes and intracellular cascades which are involved in this central plasticity 6, 14.

A variety of animal models have been developed to study hyperalgesia resulting from inflammation. The present study examined one model which involves intraplantar injection of zymosan into the rat hindpaw. Behavioral studies have shown that zymosan administration produces a time- and dose-dependent cutaneous hyperalgesia to both mechanical and thermal stimuli 15, 16, 17, 18. This model has many desirable features for performing electrophysiological analyses of neuronal changes occurring during hyperalgesia resulting from inflammation. First, the hyperalgesia develops rapidly and enables analyses of activity changes within a neuron over time rather than relying on less-desirable population studies. Second, it is stable over relatively long periods of time, thereby enabling the necessary time frames in which to perform important, stimulus-response function (SRF) analyses. Third, pharmacological data obtained in behavioral studies suggest that different mechanisms may mediate the development of hyperalgesia to either mechanical or thermal stimuli 14, 16, 17. An understanding of these differences may provide unique insights into the mechanisms responsible for inflammation-induced changes in pain sensitivity.

Since zymosan-induced inflammation is being examined in behavioral studies of both cutaneous hyperalgesia and, more recently, visceral primary hyperalgesia 3, 4, 5, the goal of the present studies was to determine how primary afferents and spinal dorsal horn neurons were affected by intraplantar administration of zymosan in the rat. These single unit, time-course studies determined response thresholds to mechanical and thermal stimuli, total discharges during sustained mechanical and thermal stimuli, and changes in background activity before and during zymosan-induced inflammation. Preliminary reports of these data have been previously published as abstracts 23, 24.