Nociception- and anxiety-like behavior in rats submitted to different periods of restraint stress
Introduction
An extensive literature has shown that acute exposure to a variety of stressors produces an immediate analgesia in several pain tests [1], [2], [3], [4]. Prolonged stress can also evoke analgesia [5]. However, some studies have reported that, under some experimental conditions, both acute and chronic stress can elicit hyperalgesia instead of analgesia. For example, rats exposed to acute and chronic restraint stress exhibit elevation and reduction of tail flick latencies, respectively [6]. Similarly, acute restraint stress reduced the duration of lick/guard responses to nociceptive input (analgesic effect), while the same acute stress for the same animals increased sensitivity to thermal stimulation, as assessed by learned escape responses (hyperalgesic effect) [7]. Taken together, these results reveal that the types of stressor, its intensity, duration, as well as the type of the nociceptive model used, affect not only the potency of analgesic or hyperalgesic effect but also the neuronal mechanisms responsible for them. The literature suggests that the stress-regulatory circuit activated by a particular stressor is crucially dependent on stimulus attributes [for review, see [8]].
One factor that is particularly important is the emotional state induced by stress. For example, anxiety can produce hypervigilance which should increase attention to pain in human subjects, thereby amplifying its perceived intensity [9]. Recent work has shown that temporomandibular disorders (TMD) patients show increased stress, depression, anxiety and somatization compared with healthy controls [10], [11]. Many of the current treatments for these diseases utilize drugs that increase the levels or activity of the biogenic amine (e.g. serotonin, norepinephrine, dopamine) class of neurotransmitters. For example, fluoxetine, a specific serotonin-reuptake inhibitor (SSRI) that blocks the activity of serotonin transporter and increases the levels of 5-HT in the synaptic cleft, can be an effective treatment for depression and anxiety [12]. Although recent studies have investigated the role of psychological factors in TMD, the mechanisms responsible for nociceptive changes induced by stress are not established. The existence of multiple pain-modulatory systems is used to clarify the bewildering profile of clinical observation resulting from various pain treatments. A major component of these systems is the intrinsic opioid systems, which are activated in stress situations and can diminish pain sensation. For example, Maixner et al. [13] have showed that ischemic pain induced in the left arm was able to reduce pain sensation in patients suffering from acute dental pain. One important question is if these endogenous inhibitory systems are functional in patients suffering from chronic facial pain. It is possible that chronic orofacial pain associated with TMD results from inhibitory systems diminished in the central nervous systems. The absence of novelty-induced antinociception, which has been attributed to opioid activation [14], [15], in chronic stressed animals supports this theory. Thus, considering that the nociceptive behavioral responses elicited by the injection of formalin into the TMJ represent a valid and reliable model of orofacial deep pain [16], one of the aims of the present work was to evaluate the effects of different stress protocols on the nociceptive responses induced by TMJ formalin test. The role of serotoninergic and opioid systems in nociceptive changes induced by stress was also reported.
Section snippets
Animals
Male Wistar rats (weighing 200–230 g at the beginning of experiment) obtained from Centro Multidisciplinar de Investigação Biológica-Cemib, Unicamp, Campinas, Brazil were used in this study. The rats were housed in groups of five and maintained in a temperature-controlled room (23 ± 1 °C) with a 12/12 light–dark cycle (lights on at 7:00 am) and food and water were available ad libitum. Rats were adapted to the testing apparatus and handled prior to behavioral testing. Procedures were performed
Effects of stress procedures on plasma corticosterone and ACTH levels
This experiment was carried out to define the efficacy of restraint in inducing stress-like hormonal modifications. There was a significant increase in plasma corticosterone levels after the various stress protocols used (Fig. 1; Mann–Whitney test, p < 0.05). This increase was lower after sub-chronic and chronic stress than after acute stress for 30 min (Fig. 1; Kruskal–Wallis, p < 0.05). The increase in plasma ACTH levels was statistically significant for all acute groups tested (Fig. 2;
Discussion
In both clinical and experimental settings, anxiety and the experience of pain are sometimes found to be positively related. It has been hypothesized that anxiety increases pain through the release of catecholamines, peripherally sensitizing or even stimulating nociceptors [29]. This idea is supported by research on sympathetically maintained pain (SMP), a chronic pain state that can be alleviated by sympathetic block or sympathectomy [30]. This is in contrast with Bolles and Fanselow (1980)
Acknowledgments
The authors thank Gláucia M. B. Ambrosano for statistical analyses. Thanks are due to Adriana Rossi and José Roberto da Silva for technical assistance. This work was supported by CNPq and FAPESP, Brazil.
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