The dorsomedial hypothalamus mediates stress-induced hyperalgesia and is the source of the pronociceptive peptide cholecystokinin in the rostral ventromedial medulla
Highlights
► Air-puff stress with restraint produces mechanical hyperalgesia in behaving animals. ► Stress-induced hyperalgesia is blocked by inactivation of the DMH or RVM. ► The DMH is the supraspinal source of the pronociceptive peptide cholecystokinin in the RVM. ► A connection between the DMH and RVM allows stress to facilitate pain.
Introduction
Pain is a normal indication of acute tissue injury, but the intensity of pain experienced often fails to reflect the actual extent of damage (Beecher, 1959). Perception of an injury can be altered by cognitive and emotional factors, including anxiety and stress. Stress can either suppress or enhance pain, depending on the intensity of the stressor and the level of associated arousal (Rhudy and Meagher, 2000, Meagher et al., 2001). Extreme stress can inhibit pain, producing “stress-induced analgesia,” whereas mild or prolonged stress often heightens pain, giving rise to “stress-induced hyperalgesia” (SIH).
The rostral ventromedial medulla (RVM) is a brainstem region shown to be critical for some forms of stress-induced analgesia (Watkins et al., 1983, Butler and Finn, 2009), but has also been implicated in SIH (Imbe et al., 2006, Reynolds et al., 2011). The RVM integrates somatic information with influences from higher structures, and can facilitate nociception via activation of the pronociceptive “ON-cells” (Porreca et al., 2002, Heinricher and Ingram, 2008). Blocking ON-cell activity prevents behavioral hyperalgesia in acute injury, and also reverses hyperalgesia following inflammation and nerve injury (Heinricher and Ingram, 2008, Heinricher et al., 2009).
RVM-mediated hyperalgesia can occur through the selective activation of ON-cells by the pronociceptive neuropeptide cholecystokinin (CCK; Kovelowski et al., 2000, Heinricher and Neubert, 2004). Conversely, blocking CCK receptors in the RVM reverses behavioral hypersensitivity in these and other abnormal pain states (Kovelowski et al., 2000, Xie et al., 2005, Edelmayer et al., 2009, Marshall et al., 2012). Levels of endogenous CCK are increased in the RVM following nerve injury and in opioid-induced hyperalgesia. Although the physiology of CCK in the RVM is well described, the origin of CCK-containing projections to the RVM has not been determined. Here, we used retrograde tract tracing combined with CCK immunohistochemistry to identify the dorsomedial hypothalamus (DMH), a site with abundant endogenous CCK (Innis et al., 1979), as the most prominent source of this pronociceptive neuropeptide in the RVM.
Functional and anatomical connections tie the DMH to the RVM, and link both regions to physiological responses to stress. Stress-activated neurons in the DMH project to the RVM, and stimulating the DMH mimics the physiological effects of external stressors, evoking tachycardia, tachypnea, and hyperthermia (DiMicco et al., 2002, Samuels et al., 2004, Sarkar et al., 2007, Martenson et al., 2009). Conversely, blocking the DMH interferes with these responses (DiMicco et al., 2006, Dampney et al., 2008, Fontes et al., 2011). Given the contribution of the DMH to stress-induced physiological responses, as well as the anatomical link between the DMH and RVM, the present study explored how a functional connection between these regions could contribute to SIH. In our experiments, we used an air-puff stress protocol that has been previously employed in rodents as a paradigm of mild stress (DiMicco et al., 2002). We found mechanical hypersensitivity in awake, behaving animals that was dependent on both the DMH and the RVM. These results demonstrate a top-down pathway through which stress can facilitate the perception of pain.
Section snippets
Experimental procedures
All surgeries and animal procedures followed the guidelines of the National Institutes of Health Guide for the Care and Use of Laboratory Animals and the Committee for Research and Ethical Issues of the International Association for the Study of Pain, and were approved by the Institutional Animal Care and Use Committee at Oregon Health & Science University.
Adult male, Sprague–Dawley rats (250–350 g, Charles River Laboratories) were maintained in a 12-h light/dark cycle with ad libitum access to
Identification of CCK-containing projections to the RVM
Brains in which retrograde injections were limited to the RVM were used for a general survey to identify sites of double labeling. Three brains (CCK1, CCK2, and CCK3) had retrograde tracer injections restricted to the RVM (including the nucleus raphe magnus and nucleus raphe pallidus at the level of the facial nucleus, Fig. 2B). The tissue was sliced at 40-μm thickness and surveyed for CTB-LI and CCK-LI neurons from -1 to -9 mm relative to the bregma (CCK1) or from −1 to −15 mm relative to the
Discussion
Exogenous application of the neuropeptide CCK in the RVM increases sensitivity to noxious stimuli, and endogenous CCK release in this region is implicated in hyperalgesia seen in some models of chronic pain (Kovelowski et al., 2000, Xie et al., 2005, Edelmayer et al., 2009, Ambriz-Tututi et al., 2011). However, the source of CCK-releasing inputs to the RVM had not previously been identified. We showed here that neurons in the dorsal aspect of the DMH are the only consistent supraspinal source
Conclusion
Given previous evidence that stimulation of the DMH results in thermal and mechanical hyperalgesia mediated by the RVM ON-cells (Martenson et al., 2009), the present results demonstrate that a defined anatomical and functional neural circuit between the DMH and the RVM is vital for SIH. Thus, although the pathways of descending facilitation converge with those of descending inhibition in the RVM, they involve different RVM cell populations, and are separable at higher levels of the nervous
Author Contributions
K.M.W. performed behavioral experiments, analyzed data, and contributed to the writing of the manuscript. Z.R. and K.D. performed immunohistochemical staining and contributed to the writing of the manuscript. A.V.B. advised on immunohistochemical methods and contributed to the writing of the manuscript. M.M.H. and D.R.C. designed experiments, analyzed data, and contributed to the writing of the manuscript.
Competing Financial Interests
None.
Acknowledgments
NIH: NINDS NS066159, NS070374. D.R.C. was supported by the Neurobiology of Disease Fellowship from the OHSU Brain Institute. K.D. was supported by a fellowship from the N.L. Tartar Trust.
We thank Shaun Morrison for the contribution of one brain for the retrograde tracing study.
Confocal microscopy was carried out at the Advanced Light Microscopy Core at the Jungers Center for Neuroscience Research, and was supported by NINDS (P30 NS061800).
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2020, Brain ResearchCitation Excerpt :We have previously demonstrated that repeated restraint stress for 3 weeks induces SIH and impacts ERK and glial activations in the descending pain modulation system (Imbe et al., 2004, 2012, 2013). Moreover, several studies have shown that the descending pain modulation system contributed to the establishment of SIH (Imbe et al., 2010; Rivat et al., 2010; Reynolds et al., 2011; Wagner et al., 2013). In the present study we examined the expression of pCREB and the acetylation of histone H3 in the subcortical and cortical areas after repeated restraint stress for 3 weeks to reveal changes in the subcortical and cortical areas that affect the function of descending pain modulatory system in the rats with SIH.