Hypoalgesia and hyperalgesia with inherited hypertension in the rat

Impact of homeostatic body hydration status, evaluated by hemodynamic measures, on different pain sensitization paths to a chronic pain syndrome

Contrasting findings on the mechanisms of chronic pain and hypertension development render the current conventional evidence of a negative relationship between blood pressure (BP) and pain severity insufficient for developing personalized treatments. In this interdisciplinary study, patients with fibromyalgia (FM) exhibiting clinically normal or elevated BP, alongside healthy participants were assessed. Different pain sensitization responses were evaluated using a dynamic 'slowly repeated evoked pain' (SREP) measure, as well as static pain pressure threshold and tolerance measures. Cardiovascular responses to clino-orthostatic (lying-standing) challenges were also examined as acute re- and de-hydration events, challenging cardiovascular and cerebrovascular homeostasis. These challenges involve compensating effects from various cardiac preload or afterload mechanisms associated with different homeostatic body hydration statuses. Additionally, hair cortisol concentration was considered as a factor with an impact on chronic hydration statuses. Pain windup (SREP) and lower pain threshold in FM patients were found to be related to BP rise during clinostatic (lying) rehydration or orthostatic (standing) dehydration events, respectively. These events were determined by acute systemic vasoconstriction (i.e., cardiac afterload response) overcompensating for clinostatic or orthostatic cardiac preload under-responses (low cardiac output or stroke volume). Lower pain tolerance was associated with tonic blood pressure reduction, determined by permanent hypovolemia (low stroke volume) decompensated by permanent systemic vasodilation. In conclusion, the body hydration status profiles assessed by (re)activity of systemic vascular resistance and effective blood volume-related measures can help predict the risk and intensity of different pain sensitization components in chronic pain syndrome, facilitating a more personalized management approach.

Characterization of pain-, anxiety-, and cognition-related behaviors in the complete Freund's adjuvant model of chronic inflammatory pain in Wistar-Kyoto rats

Introduction

Chronic pain is often associated with comorbid anxiety and cognitive dysfunction, negatively affecting therapeutic outcomes. The influence of genetic background on such interactions is poorly understood. The stress-hyperresponsive Wistar-Kyoto (WKY) rat strain, which models aspects of anxiety and depression, displays enhanced sensitivity to noxious stimuli and impaired cognitive function, compared with Sprague-Dawley (SD) counterparts. However, pain- and anxiety-related behaviors and cognitive impairment following induction of a persistent inflammatory state have not been investigated simultaneously in the WKY rats. Here we compared the effects of complete Freund's adjuvant (CFA)-induced persistent inflammation on pain-, negative affect- and cognition-related behaviors in WKY vs. SD rats.

Methods

Male WKY and SD rats received intra-plantar injection of CFA or needle insertion (control) and, over the subsequent 4 weeks, underwent behavioral tests to assess mechanical and heat hypersensitivity, the aversive component of pain, and anxiety- and cognition-related behaviors.

Results

The CFA-injected WKY rats exhibited greater mechanical but similar heat hypersensitivity compared to SD counterparts. Neither strain displayed CFA-induced pain avoidance or anxiety-related behavior. No CFA-induced impairment was observed in social interaction or spatial memory in WKY or SD rats in the three-chamber sociability and T-maze tests, respectively, although strain differences were apparent. Reduced novel object exploration time was observed in CFA-injected SD, but not WKY, rats. However, CFA injection did not affect object recognition memory in either strain.

Conclusions

These data indicate exacerbated baseline and CFA-induced mechanical hypersensitivity, and impairments in novel object exploration, and social and spatial memory in WKY vs. SD rats.

Hypertension contributes to exacerbated osteoarthritis pathophysiology in rats in a sex-dependent manner

BACKGROUND

Hypertension is a common comorbidity of osteoarthritis (OA) with known autonomic dysregulation; thus, the autonomic nervous system may provide a shared underlying mechanism. The objective of this study was to examine the role of the autonomic nervous system in a preclinical model of OA and hypertension.

METHODS

Experiments were conducted in spontaneously hypertensive rats and a normotensive control strain, including male and female rats. OA was surgically induced via medial meniscus transection with skin incision used as a sham control (n = 7-8/strain/sex/surgery). Tactile sensitivity, anxiety-related behavior, and serum corticosterone were measured at baseline then bi-weekly across 8 weeks. At weeks 9-10, cardiovascular responses to a chemical vagal nerve agonist were determined to indirectly evaluate vagus nerve function. The joint structure was assessed via grading of histological sections.

RESULTS

In males, OA resulted in thinner cartilage in both hypertensive (OA vs. non-OA p < 0.001) and normotensive (OA vs. non-OA p < 0.001). Only females with comorbid hypertension and OA displayed thinner cartilage (p = 0.013). Male hypertensive OA animals had increased calcified subchondral bone compared to normotensive OA animals (p = 0.043) while female hypertensive OA animals had increased calcified subchondral bone compared to hypertensive sham animals (p < 0.001). All MCLT+MMT groups developed low-grade synovitis; interestingly, hypertensive OA females had higher synovitis scores than normotensive OA females (p = 0.046). Additionally, hypertension led to larger drops in blood pressure with vagal activation in both OA (hypertensive vs. normotensive p = 0.018) and sham (hypertensive vs. normotensive p < 0.001) male animals. In females, this trend held true only in OA animals (normotensive vs. hypertensive p = 0.005).

CONCLUSION

These data provide preliminary evidence that hypertension influences OA progression and encourages further study into the autonomic nervous system as a possible mechanism.

Characterization of the Dahl salt-sensitive rat as a rodent model of inherited, widespread, persistent pain

Animal models are essential for studying the pathophysiology of chronic pain disorders and as screening tools for new therapies. However, most models available do not reproduce key characteristics of clinical persistent pain. This has limited their ability to accurately predict which new medicines will be clinically effective. Here, we characterize the Dahl salt-sensitive (SS) rat strain as the first rodent model of inherited widespread hyperalgesia. We show that this strain exhibits physiological phenotypes known to contribute to chronic pain, such as neuroinflammation, defective endogenous pain modulation, dysfunctional hypothalamic-pituitary-adrenal axis, increased oxidative stress and immune cell activation. When compared with Sprague Dawley and Brown Norway rats, SS rats have lower nociceptive thresholds due to increased inflammatory mediator concentrations, lower corticosterone levels, and high oxidative stress. Treatment with dexamethasone, the reactive oxygen species scavenger tempol, or the glial inhibitor minocycline attenuated the pain sensitivity in SS rats without affecting the other strains while indomethacin and gabapentin provided less robust pain relief. Moreover, SS rats presented impaired diffuse noxious inhibitory controls and an exacerbated response to the proalgesic mediator PGE₂, features of generalized pain conditions. These data establish this strain as a novel model of spontaneous, widespread hyperalgesia that can be used to identify biomarkers for chronic pain diagnosis and treatment.

Morphine Resistance in Spinal Cord Injury-Related Neuropathic Pain in Rats is Associated With Alterations in Dopamine and Dopamine-Related Metabolomics

Opioids are not universally effective for treating neuropathic pain following spinal cord injury (SCI), a finding that we previously demonstrated in a rat model of SCI. The aim of this study was to determine analgesic response of morphine-responsive and nonresponsive SCI rats to adjunct treatment with dopamine modulators and to establish if the animal groups expressed distinct metabolomic profiles. Thermal thresholds were tested in female Long Evans rats (N = 45) prior to contusion SCI, after SCI and following injection of morphine, morphine combined with dopamine modulators, or dopamine modulators alone. Spinal cord and striatum samples were processed for metabolomics and targeted mass spectrometry. Morphine provided analgesia in 1 of 3 of SCI animals. All animals showed improved analgesia with morphine + pramipexole (D3 receptor agonist). Only morphine nonresponsive animals showed improved analgesia with the addition of SCH 39166 (D1 receptor antagonist). Metabolomic analysis identified 3 distinct clusters related to the tyrosine pathway that corresponded to uninjured, SCI morphine-responsive and SCI morphine-nonresponsive groups. Mass spectrometry showed matching differences in dopamine levels in striatum and spinal cord between these groups. The data suggest an overall benefit of the D3 receptor system in improving analgesia, and an association between morphine responsiveness and metabolomic changes in the tyrosine/dopamine pathways in striatum and spinal cord. PERSPECTIVE: Spinal cord injury (SCI) leads to opioid-resistant neuropathic pain that is associated with changes in dopamine metabolomics in the spinal cord and striatum of rats. We present evidence that adjuvant targeting of the dopamine system may be a novel pain treatment approach to overcome opioid desensitization and tolerance after SCI.

Hyporesponsivity to mu-opioid receptor agonism in the Wistar-Kyoto rat model of altered nociceptive responding associated with negative affective state

ABSTRACT

Chronic pain is often comorbid with anxiety and depression, altering the level of perceived pain, which negatively affects therapeutic outcomes. The role of the endogenous mu-opioid receptor (MOP) system in pain-negative affect interactions and the influence of genetic background thereon are poorly understood. The inbred Wistar-Kyoto (WKY) rat, which mimics aspects of anxiety and depression, displays increased sensitivity (hyperalgesia) to noxious stimuli, compared with Sprague-Dawley (SD) rats. Here, we report that WKY rats are hyporesponsive to the antinociceptive effects of systemically administered MOP agonist morphine in the hot plate and formalin tests, compared with SD counterparts. Equivalent plasma morphine levels in the 2 rat strains suggested that these differences in morphine sensitivity were unlikely to be due to strain-related differences in morphine pharmacokinetics. Although MOP expression in the ventrolateral periaqueductal gray (vlPAG) did not differ between WKY and SD rats, the vlPAG was identified as a key locus for the hyporesponsivity to MOP agonism in WKY rats in the formalin test. Moreover, morphine-induced effects on c-Fos (a marker of neuronal activity) in regions downstream of the vlPAG, namely, the rostral ventromedial medulla and lumbar spinal dorsal horn, were blunted in the WKY rats. Together, these findings suggest that a deficit in the MOP-induced recruitment of the descending inhibitory pain pathway may underlie hyperalgesia to noxious inflammatory pain in the WKY rat strain genetically predisposed to negative affect.

Angiotensin II inhibits DDAH1-nNOS signaling via AT1R and μOR dimerization to modulate blood pressure control in the central nervous system

G protein-coupled receptors (GPCRs) are important drug targets. Blocking angiotensin II (Ang II) type 1 receptor signaling alleviates hypertension and improves outcomes in patients with heart failure. Changes in structure and trafficking of GPCR, and desensitization of GPCR signaling induce pathophysiological processes. We investigated whether Ang II, via induction of AT1R and μ-opioid receptor (μOR) dimerization in the nucleus tractus solitarius (NTS), leads to progressive hypertension. Ang II signaling increased μOR and adrenergic receptor α2A (α2A-AR) heterodimer levels and decreased expression of extracellular signal-regulated kinases 1/2T202/Y204, ribosomal protein S6 kinaseT359/S363, and nNOSS1416 phosphorylation. Dimethylarginine dimethylaminohydrolase 1 (DDAH1) expression was abolished in the NTS of adult spontaneously hypertensive rats (SHRs). Endomorphin-2 was overexpressed in NTS of adult SHRs compared with that in 6-week-old Wistar-Kyoto rats (WKY). Administration of μOR agonist into the NTS of WKY increased blood pressure (BP), decreased nitric oxide (NO) production, and decreased DDAH1 activity. μOR agonist significantly reduced the activity of DDAH1 and decreased neuronal NO synthase (nNOS) phosphorylation. The AT1R II inhibitor, losartan, significantly decreased BP and abolished AT1R-induced formation of AT1R and μOR, and α2A-AR and μOR, heterodimers. Losartan also significantly increased the levels of nNOSS1416 phosphorylation and DDAH1 expression. These results show that Ang II may induce expression of endomorphin-2 and abolished DDAH1 activity by enhancing the formation of AT1R and μOR heterodimers in the NTS, leading to progressive hypertension.

Sex differences and the role of ovarian hormones in site-specific nociception of SHR

Accurate diagnosis and treatment of pain is dependent on knowledge of the variables that might alter this response. Some of these variables are the locality of the noxious stimulus, the sex of the individual, and the presence of chronic diseases. Among these chronic diseases, hypertension is considered a serious and silent disease that has been associated with hypoalgesia. The main goal of this study was to evaluate the potential nociceptive differences in spontaneously hypertensive rats (SHR) regarding the locality of the stimulus, i.e., the temporomandibular joint or paw, the sex, and the role of ovarian hormones in a model of mechanical nociception (Von Frey test) or formalin-induced inflammatory nociception. Our results indicate that SHR had lower orofacial mechanical nociception beyond the lower mechanical nociception in the paw compared with WKY rats. In a model of formalin-induced inflammatory nociception, SHR also had decreased nociception compared with normotensive rats. We also sought to evaluate the influence of sex and ovarian hormones on orofacial mechanical nociception in SHR. We observed that female SHR had higher mechanical nociception than male SHR only in the paw, but it had higher formalin-induced orofacial nociception than male SHR. Moreover, the absence of ovarian hormones caused an increase in mean arterial pressure and a decrease in paw nociception in female SHR.

Hypertension and vulnerability to hemorrhagic shock in a rat model

Trauma mortality may be increased in the presence of preexisting diseases such as chronic hypertension. We hypothesized that systemic and microvascular alterations accompanying chronic hypertension would increase the vulnerability to hemorrhage relative to normotensive controls in a rat model of hemorrhagic shock. We present a novel comparative hemorrhage model of shock vulnerability, quantified by "vulnerability curves" expressing physiological response to hemorrhage as a function of three matched shock metrics: cumulative blood volume, mean arterial pressure (MAP), and oxygen delivery (Do2). Responses were central hemodynamics and respiratory and muscle oxygenation obtained for one hypertensive (spontaneously hypertensive [SHR]) and two normotensive (Sprague-Dawley, Wistar-Kyoto) rat strains. Hemorrhagic shock was induced by incremental (0.5 mL) hemorrhage to cardiovascular collapse in anesthetized and mechanically ventilated animals. Shock vulnerability of SHR rats was primarily pressure-driven; in general, SHR exhibited the expected patterns of more rapid deterioration in MAP and Vo2 over smaller ranges of blood loss and Do2. Sternotomy-related depression of CO and thus Do2 in SHR meant that we could not test hypotheses related to the role of Do2 and contribution to perfusion differences between normotensive and hypertensive subjects. Insensitivity of lactate to strain effects suggests that lactate may be a reliable biomarker of shock status. Unexpected similarities between Wistar-Kyoto and SHR suggest strain-related effects other than those related to hypertension per se contribute to hemorrhage response; body size effects and genetic relationships could not be ruled out. Future studies should incorporate phylogenetically based methods to examine the role of hypertension and physiological response to hemorrhage across multiple strains.

Peripheral gene expression profile of mechanical hyperalgesia induced by repeated cold stress in SHRSP5/Dmcr rats

Repeated cold stress (RCS) is known to transiently induce functional disorders associated with hypotension and hyperalgesia. In this study, we investigated the effects of RCS (24 and 4 °C alternately at 30-min intervals during the day and 4 °C at night for 2 days, followed by 4 °C on the next 2 consecutive nights) on the thresholds for cutaneous mechanical pain responses and on peripheral expression of "pain-related genes" in SHRSP5/Dmcr rats, which are derived from stroke-prone spontaneously hypertensive rats. To define genes peripherally regulated by RCS, we detected changes in the expression of pain-related genes in dorsal root ganglion cells by PCR-based cDNA subtraction analysis or DNA microarray analysis, and confirmed the changes by RT-PCR. We found significantly changed expression in eight pain-related genes (upregulated: Fyn, St8sia1, and Tac 1; downregulated: Ctsb, Fstl1, Itpr1, Npy, S100a10). At least some of these genes may play key roles in hyperalgesia induced by RCS.

GPCR dimerization in brainstem nuclei contributes to the development of hypertension

BACKGROUND AND PURPOSE

μ-Opioid receptors, pro-opiomelanocortin and pro-enkephalin are highly expressed in the nucleus tractus solitarii (NTS) and μ receptor agonists given to the NTS dose-dependently increased BP. However, the molecular mechanisms of this process remain unclear. In vitro, μ receptors heterodimerize with α2A -adrenoceptors. We hypothesized that α2A -adrenoceptor agonists would lose their depressor effects when their receptors heterodimerize in the NTS with μ receptors.

EXPERIMENTAL APPROACH

We microinjected μ-opioid agonists and antagonists into the NTS of rats and measured changes in BP. Formation of μ receptor/α2A -adrenoceptor heterodimers was assessed with immunofluorescence and co-immunoprecipitation methods, along with proximity ligation assays.

KEY RESULTS

Immunofluorescence staining revealed colocalization of α2A -adrenoceptors and μ receptors in NTS neurons. Co-immunoprecipitation revealed interactions between α2A -adrenoceptors and μ receptors. In situ proximity ligation assays confirmed the presence of μ receptor/α2A -adrenoceptor heterodimers in the NTS. Higher levels of endogenous endomorphin-1 and μ receptor/α2A -adrenoceptor heterodimers were found in the NTS of hypertensive rats, than in normotensive rats. Microinjection of the μ receptor agonist [D-Ala(2) , MePhe(4) , Gly(5) -ol]-enkephalin (DAMGO), but not that of the α2A -adrenoceptor agonist guanfacine, into the NTS of normotensive rats increased μ receptor/α2A -adrenoceptor heterodimer formation and BP elevation. The NO-dependent BP-lowering effect of α2A -adrenoceptor agonists was blunted following increased formation of μ receptor/α2A -adrenoceptor heterodimers in the NTS of hypertensive rats and DAMGO-treated normotensive rats.

CONCLUSIONS AND IMPLICATIONS

Increases in endogenous μ receptor agonists in the NTS induced μ receptor/α2A -adrenoceptor heterodimer formation and reduced the NO-dependent depressor effect of α2A -adrenoceptor agonists. This process could contribute to the pathogenesis of hypertension.

"On-" and "off-" cells in the rostral ventromedial medulla of rats held in thermoneutral conditions: are they involved in thermoregulation?

In thermal neutral condition, rats display cyclic variations of the vasomotion of the tail and paws, synchronized with fluctuations of blood pressure, heart rate, and core body temperature. "On-" and "off-" cells located in the rostral ventromedial medulla, a cerebral structure implicated in somatic sympathetic drive, 1) exhibit similar spontaneous cyclic activities in antiphase and 2) are activated and inhibited by thermal nociceptive stimuli, respectively. We aimed at evaluating the implication of such neurons in autonomic regulation by establishing correlations between their firing and blood pressure, heart rate, and skin and core body temperature variations. When, during a cycle, a relative high core body temperature was reached, the on-cells were activated and within half a minute, the off-cells and blood pressure were depressed, followed by heart rate depression within a further minute; vasodilatation of the tail followed invariably within ∼3 min, often completed with vasodilatation of hind paws. The outcome was an increased heat loss that lessened the core body temperature. When the decrease of core body temperature achieved a few tenths of degrees, sympathetic activation switches off and converse variations occurred, providing cycles of three to seven periods/h. On- and off-cell activities were correlated with inhibition and activation of the sympathetic system, respectively. The temporal sequence of events was as follows: core body temperature → on-cell → off-cell ∼ blood pressure → heart rate → skin temperature → core body temperature. The function of on- and off-cells in nociception should be reexamined, taking into account their correlation with autonomic regulations.

The influence of a continuous rate infusion of dexmedetomidine on the nociceptive withdrawal reflex and temporal summation during isoflurane anaesthesia in dogs

OBJECTIVE

To examine the influence of a low dose dexmedetomidine infusion on the nociceptive withdrawal reflex and temporal summation in dogs during isoflurane anaesthesia.

STUDY DESIGN

Prospective experimental blinded cross-over study.

ANIMALS

Eight healthy mixed breed dogs, body weight Mean ± SD 26.5 ± 8.4 kg and age 25 ± 16 months.

METHODS

Anaesthesia was induced with propofol and maintained with isoflurane (Fe'ISO 1.3%) delivered in oxygen and air. After stabilization, baseline recordings (time 0) were obtained, then a dexmedetomidine bolus (1 μg kg(-1) IV) followed by a continuous rate infusion (1 μg kg(-1) hour(-1)) or saline placebo were administered. At times 10, 30 and 60 minutes after the initial bolus, electrical stimulations of increasing intensity were applied over the lateral plantar digital nerve, and administered both as single and as repeated stimuli. The resulting reflex responses were recorded using electromyography. Data were analysed using a multivariable linear regression model and a Kruskal Wallis test for single stimulation data, and repeated measures anova and paired t-test for repeated stimulation data.

RESULTS

The AUC for the stimulus-response curves after single stimulation were similar for both treatments at time 0. At times 10, 30 and 60 the AUCs for the stimulus-response curves were significantly lower with dexmedetomidine treatment than with placebo. Temporal summation was evident in both treatments at times 0, 10, 30 and 60 starting from a stimulation intensity of 10 mA. The magnitude of temporal summation was smaller in dexmedetomidine than in placebo treated dogs at time 10, 30 and 60, but not at time 0.

CONCLUSIONS

During isoflurane anaesthesia, low dose dexmedetomidine suppresses the nociceptive reflex responses after single and repeated electrical stimulation.

CLINICAL RELEVANCE

This experimental study confirms previous reports on its peri-operative efficacy under clinical conditions, and further indicates that dexmedetomidine might reduce the risk of post-operative chronic pain development.

Sumatriptan alleviates nitroglycerin-induced mechanical and thermal allodynia in mice

The association between the clinical use of nitroglycerin (NTG) and headache has led to the examination of NTG as a model trigger for migraine and related headache disorders, both in humans and laboratory animals. In this study in mice, we hypothesized that NTG could trigger behavioural and physiological responses that resemble a common manifestation of migraine in humans. We report that animals exhibit a dose-dependent and prolonged NTG-induced thermal and mechanical allodynia, starting 30-60 min after intraperitoneal injection of NTG at 5-10 mg/kg. NTG administration also induced Fos expression, an anatomical marker of neuronal activity in neurons of the trigeminal nucleus caudalis and cervical spinal cord dorsal horn, suggesting that enhanced nociceptive processing within the spinal cord contributes to the increased nociceptive behaviour. Moreover, sumatriptan, a drug with relative specificity for migraine, alleviated the NTG-induced allodynia. We also tested whether NTG reduces the threshold for cortical spreading depression (CSD), an event considered to be the physiological substrate of the migraine aura. We found that the threshold of CSD was unaffected by NTG, suggesting that NTG stimulates migraine mechanisms that are independent of the regulation of cortical excitability.

Does chronic pain alter the normal interaction between cardiovascular and pain regulatory systems? Pain modulation in the hypertensive-monoarthritic rat

Hypertension-associated hypoalgesia is widely recognized in acute pain conditions. In chronic pain states, however, the relationship between blood pressure and pain sensitivity is still ill-defined, with different authors reporting negative, positive, or even no relationship at all. This work addresses this issue, using complete Freund's adjuvant (CFA)-induced monoarthritis in different models of hypertension: Spontaneous (spontaneously hypertensive rats, SHR), induced by infusion of angiotensin II (ANG) or 1,3-dipropyl-8-sulfophenylxanthine (DPSPX, an adenosine receptors' antagonist), and renal artery ligation (RAL). Nociceptive responses associated with monoarthritis were evaluated by different behavioral tests (von Frey, ankle-bend and CatWalk) and by quantification of Fos expression at the dorsal horn upon noxious stimulation. In all hypertension models, higher von Frey thresholds and lower Fos expression were detected in hypertensive rats with chronic inflammatory pain, as compared to normotensive monoarthritic rats. In SHR and DPSPX, but not ANG or RAL models, hypertensive animals displayed lower inflammation than normotensives. Ankle-bend and CatWalk results indicated lower pain sensitivity in hypertensive rats only in SHR and DPSPX models. The present study shows the importance of using multiple models of hypertension, and evaluating pain responses by various methods, to better understand the complexity of the interactions between pain and cardiovascular regulatory systems.

PERSPECTIVE

This study used different models of hypertension to investigate whether chronic pain alters the normal integration of cardiovascular and pain regulatory systems. A complete understanding of the mechanisms underlying the complex interactions between these systems may disclose future therapeutic approaches to treat hypertension/chronic pain comorbidity states.

Baroreceptor reflex is suppressed in rats that develop hyperalgesia behavior after nerve injury

The baroreceptor reflex buffers autonomic changes by decreasing sympathetic activity and increasing vagal activity in response to blood pressure elevations, and by the reverse actions when the blood pressure falls. Because of the many bidirectional interactions of pain and autonomic function, we investigated the effect of painful nerve injury by spinal nerve ligation (SNL) on heart rate (HR), blood pressure (BP) and their regulation by the baroreceptor reflex. Rats receiving SNL were separated into either a hyperalgesic group that developed sustained lifting, shaking and grooming of the foot after plantar punctate nociceptive stimulation by pin touch or a group of animals that failed to show this hyperalgesic behavior after SNL. SNL produced no effect on resting BP recorded telemetrically in unrestrained rats compared to control rats receiving either skin incision or sham SNL. However, two tests of baroreceptor gain showed depression only in animals that developed sustained hyperalgesia after SNL. The animals that failed to develop hyperalgesia after SNL were found to have elevations in HR both before and for the first 4 days after SNL, and HR variability analysis gave indications of decreased vagal control of resting HR and elevated sympatho-vagal balance at these same time intervals. In human patients, other research has shown that blunted baroreceptor reflex sensitivity predicts poor outcome during conditions such as hypertension, congestive heart failure, myocardial infarction, and stroke. If baroreceptor reflex suppression is also found in human subjects during chronic neuropathic pain, this may adversely affect survival.

Neuroleptic drugs revert the contextual fear conditioning deficit presented by spontaneously hypertensive rats: a potential animal model of emotional context processing in schizophrenia?

Schizophrenia, bipolar disorder, and attention deficit/hyperactivity disorder (ADHD) present abnormalities in emotion processing. A previous study showed that the spontaneously hypertensive rats (SHR), a putative animal model of ADHD, present reduced contextual fear conditioning (CFC). The aim of the present study was to characterize the deficit in CFC presented by SHR. Adult male normotensive Wistar rats and SHR were submitted to the CFC task. Sensitivity of the animals to the shock and the CFC performance after repeated exposure to the task were investigated. Pharmacological characterization consisted in the evaluation of the effects of the following drugs administered previously to the acquisition of the CFC: pentylenetetrazole (anxiogenic) and chlordiazepoxide (anxiolytic); methylphenidate and amphetamine (used for ADHD); lamotrigine, carbamazepine, and valproic acid (mood stabilizers); haloperidol, ziprasidone, risperidone, amisulpride, and clozapine (neuroleptic drugs); metoclopramide and SCH 23390 (dopamine antagonists without antipsychotic properties); and ketamine (a psychotomimmetic). The effects of paradoxical sleep deprivation (that worsens psychotic symptoms) and the performance in a latent inhibition protocol (an animal model of schizophrenia) were also verified. No differences in the sensitivity to the shock were observed. The repeated exposure to the CFC task did not modify the deficit in CFC presented by SHR. Considering pharmacological treatments, only the neuroleptic drugs reversed this deficit. This deficit was potentiated by proschizophrenia manipulations. Finally, a deficit in latent inhibition was also presented by SHR. These findings suggest that the deficit in CFC presented by SHR could be a useful animal model to study abnormalities in emotional context processing related to schizophrenia.

Microinjection of angiotensin II in the caudal ventrolateral medulla induces hyperalgesia

Nociceptive transmission from the spinal cord is controlled by supraspinal pain modulating systems that include the caudal ventrolateral medulla (CVLM). The neuropeptide angiotensin II (Ang II) has multiple effects in the CNS and at the medulla oblongata. Here we evaluated the expression of angiotensin type 1 (AT(1)) receptors in spinally-projecting CVLM neurons, and tested the effect of direct application of exogenous Ang II in the CVLM on nociceptive behaviors. Although AT(1)-immunoreactive neurons occurred in the CVLM, only 3% of AT(1)-positive neurons were found to project to the dorsal horn, using double-immunodetection of the retrograde tracer cholera toxin subunit B. In behavioral studies, administration of Ang II (100 pmol) in the CVLM gave rise to hyperalgesia in both the tail-flick and formalin tests. This hyperalgesia was significantly attenuated by local administration of the AT(1) antagonist losartan. The present study demonstrates that Ang II can act on AT(1) receptors in the CVLM to modulate nociception. The effect on spinal nociceptive processing is likely indirect, since few AT(1)-expressing CVLM neurons were found to project to the spinal cord. The renin-angiotensin system may also play a role in other supraspinal areas implicated in pain modulation.

Increased sensitivity to cocaine-induced analgesia in Spontaneously Hypertensive Rats (SHR)

This study examined the analgesic effect of cocaine in Spontaneously Hypertensive Rats (SHR), which are considered a suitable model for the study of attention deficit hyperactivity disorder (ADHD), and in Wistar (WIS) rats of both sexes using the hot-plate test. In addition, we tested whether habituation to the unheated hot-plate apparatus, that "normalizes" the basal hypoalgesic phenotype of SHR, alters the subsequent cocaine-induced analgesia (CIA) in this strain. SHR of both sexes were hypoalgesic compared to WIS rats in the hot-plate test and showed higher sensitivity to CIA. Habituation to the unheated hot-plate reduced the basal nociceptive latency of SHR, suggesting cognitive/emotional modulation of pain in this strain, but did not alter the magnitude of CIA. The present study shows increased sensitivity to CIA in SHR, which may be related to abnormalities in the mesocorticolimbic dopaminergic system. Further studies using SHR strain may reveal new information on the neurobiological mechanisms underlying ADHD and its co-morbidity with drug addiction.

Strain and sex differences in the expression of nociceptive behavior and stress-induced analgesia in rats

Evidence indicates that genetic, gender, and emotional/attentional aspects modulate the pain sensation. The present study examined the effect of swim-stress on nociceptive responses in Lewis (LEW) and spontaneously hypertensive (SHR) inbred rats (contrasting for anxiety-related behaviors), as well as in Wistar (WIS) rats of both sexes. Furthermore, we explored possible neurochemical mechanisms involved. In addition, we investigated whether habituation in the hot-plate apparatus could modify the hypoalgesic phenotype of SHR. Male and female LEW, SHR, and WIS rats were tested immediately before and 2 min after a 3-min swim in 15 degrees C water. The swim-stress induced analgesia in LEW and WIS, but not in SHR male rats. The same stressor induced analgesia in females of all three strains. In WIS female rats, the stress-induced analgesia (SIA) seems to involve, at least partially, a nonopioid N-methyl-d-aspartate (NMDA) analgesic system. Moreover, five brief exposures (90 s; 10-min intertrial interval) to the unheated hot-plate apparatus completely abolished the differences in basal hot-plate latencies observed in SHR compared with LEW and WIS strains. The present results demonstrate genetic and gender differences in nociceptive sensitivity and in the activation of endogenous analgesic systems in rats and highlight the influence of emotional reactivity. The SHR's hypoalgesic phenotype seems to involve central cognitive processes. Therefore, the LEW and SHR inbred strains may provide an important tool for study of the molecular bases underlying nociception and its modulation and the relationship with emotional/attentional processes.

Interactions between the cardiovascular and pain regulatory systems: an updated review of mechanisms and possible alterations in chronic pain

Endogenous pain regulatory system dysfunction appears to play a role in the maintenance of chronic pain. An important component of the pain regulatory process is the functional interaction between the cardiovascular and pain regulatory systems, which results in an association between elevated resting blood pressure (BP) and diminished acute pain sensitivity. This BP/pain sensitivity relationship is proposed to reflect a homeostatic feedback loop helping restore arousal levels in the presence of painful stimuli. Evidence is emerging that this normally adaptive BP/pain sensitivity relationship is significantly altered in chronic pain conditions, affecting responsiveness to both acute and chronic pain stimuli. Several mechanisms that may underlie this adaptive relationship in healthy individuals are overviewed, including endogenous opioid, noradrenergic, and baroreceptor-related mechanisms. Theoretical models are presented regarding how chronic pain-related alterations in the mechanisms above and increased pain facilatory system activity (central sensitization) may contribute to altered BP/pain sensitivity interactions in chronic pain. Clinical implications are discussed.

Morphine-Induced Analgesia, Hypotension, and Bradycardia Are Enhanced in Hypertensive Rats

Several studies have emphasized an opioidergic link between the central regulation of cardiovascular function and acute noninflammatory pain. By contrast, relatively few studies have investigated the relationships between opioids, hypertension, and inflammatory pain. We used the formalin model of acute inflammatory pain to compare morphine antinociception among spontaneously hypertensive (SHR) rats, their genetic normotensive controls, Wistar-Kyoto (WKY) rats, and Sprague-Dawley (SD) rats. Measures of nociception included both behavioral and cardiovascular end-points (increased mean arterial blood pressure and heart rate). Morphine (3.0 mg/kg subcutaneously) produced greater hypotension and bradycardia in SHR than in WKY or SD rats. We next administered formalin (5%; 50 microL) and observed greater nociception during both Phase 1 and Phase 2 in SHR controls than in WKY controls. The morphine-treated groups did not differ, suggesting that morphine attenuates hypersensitivity to formalin pain in the SHR. Morphine inhibited edema but not paw hyperthermia to a greater degree in SHR, whereas Phase 1 remifentanil produced a relatively shorter delay in the onset of Phase 2 in SHR. We suggest that the presentation of essential hypertension be considered when opioid regimens are planned both during surgery (to minimize cardiovascular complications) and during the postoperative period (to optimize analgesic effects).

IMPLICATIONS

Presentation of essential hypertension should be considered when opioid regimens are planned both during surgery (to minimize cardiovascular complications) and during the postoperative period (to optimize analgesic effects).

Intrathecal neuropeptide Y inhibits behavioral and cardiovascular responses to noxious inflammatory stimuli in awake rats

To test the hypothesis that administration of neuropeptide Y (NPY) to the spinal cord reduces inflammatory pain, we evaluated the effects of intrathecal NPY on behavioral and cardiovascular markers of the nociception associated with intraplantar formalin injection in rats. Before the administration of formalin, NPY dose dependently increased blood pressure, an effect that could be prevented with the coadministration of the Y2 antagonist, BIIE0246. This effect lasted only 20 min, and thus was over before initiation of the formalin test. NPY dose dependently inhibited the flinching, licking, pressor, and tachycardia responses associated with formalin injection. The Y1 receptor antagonist BIBO 3304 partially reversed the antinociceptive effect of NPY at a dose that did not by itself have an effect (3 microg). We conclude that intrathecal NPY acts in part via Y1 receptors to inhibit ongoing inflammatory nociception.