Immobilization and restraint effects on pain reactions in animals

The role of histamine H1 receptor in the anterior cingulate cortex on nociception level following acute restraint stress in male rats

Considering the importance of pain and stress, we decided to investigate the intra-anterior cingulate cortex (ACC) microinjection of histamine and mepyramine alone and concurrently on acute pain induced by hot plate following restraint stress in male rats. 24-gauge, 10 mm stainless steel guide cannula was implanted over the ACC in the incised scalp of 4 groups. Restraint stress in healthy rats produced a significant increase (p < .05) in the pain threshold. The simultaneous microinjection of 4 μg/side histamine and 8 μg/side mepyramine as a histaminergic system inverse agonist in healthy nonrestraint animals did not affect the pain threshold. Although Histamine decreased the threshold of pain meaningfully, mepyramine elevated it in a significant manner (p < .05). In the restrained animals, intra-ACC microinjection of histamine produced no significant impact on the pain threshold. However, intra-ACC microinjection of mepyramine before histamine, significantly (p < .01) altered the result and enhanced the threshold of pain. The results of our study demonstrated that histaminergic neurons have an important role in the processing of pain in the ACC following restraint stress.

The brain-body disconnect: A somatic sensory basis for trauma-related disorders

Although the manifestation of trauma in the body is a phenomenon well-endorsed by clinicians and traumatized individuals, the neurobiological underpinnings of this manifestation remain unclear. The notion of somatic sensory processing, which encompasses vestibular and somatosensory processing and relates to the sensory systems concerned with how the physical body exists in and relates to physical space, is introduced as a major contributor to overall regulatory, social-emotional, and self-referential functioning. From a phylogenetically and ontogenetically informed perspective, trauma-related symptomology is conceptualized to be grounded in brainstem-level somatic sensory processing dysfunction and its cascading influences on physiological arousal modulation, affect regulation, and higher-order capacities. Lastly, we introduce a novel hierarchical model bridging somatic sensory processes with limbic and neocortical mechanisms regulating an individual's emotional experience and sense of a relational, agentive self. This model provides a working framework for the neurobiologically informed assessment and treatment of trauma-related conditions from a somatic sensory processing perspective.

Gradual Restraint Habituation for Awake Functional Magnetic Resonance Imaging Combined With a Sparse Imaging Paradigm Reduces Motion Artifacts and Stress Levels in Rodents

Functional magnetic resonance imaging, as a non-invasive technique, offers unique opportunities to assess brain function and connectivity under a broad range of applications, ranging from passive sensory stimulation to high-level cognitive abilities, in awake animals. This approach is confounded, however, by the fact that physical restraint and loud unpredictable acoustic noise must inevitably accompany fMRI recordings. These factors induce marked stress in rodents, and stress-related elevations of corticosterone levels are known to alter information processing and cognition in the rodent. Here, we propose a habituation strategy that spans specific stages of adaptation to restraint, MRI noise, and confinement stress in awake rats and circumvents the need for surgical head restraint. This habituation protocol results in stress levels during awake fMRI that do not differ from pre-handling levels and enables stable image acquisition with very low motion artifacts. For this, rats were gradually trained over a period of three weeks and eighteen training sessions. Stress levels were assessed by analysis of fecal corticosterone metabolite levels and breathing rates. We observed significant drops in stress levels to below pre-handling levels at the end of the habituation procedure. During fMRI in awake rats, after the conclusion of habituation and using a non-invasive head-fixation device, breathing was stable and head motion artifacts were minimal. A task-based fMRI experiment, using acoustic stimulation, conducted 2 days after the end of habituation, resulted in precise whole brain mapping of BOLD signals in the brain, with clear delineation of the expected auditory-related structures. The active discrimination by the animals of the acoustic stimuli from the backdrop of scanner noise was corroborated by significant increases in BOLD signals in the thalamus and reticular formation. Taken together, these data show that effective habituation to awake fMRI can be achieved by gradual and incremental acclimatization to the experimental conditions. Subsequent BOLD recordings, even during superimposed acoustic stimulation, reflect low stress-levels, low motion and a corresponding high-quality image acquisition. Furthermore, BOLD signals obtained during fMRI indicate that effective habituation facilitates selective attention to sensory stimuli that can in turn support the discrimination of cognitive processes in the absence of stress confounds.

Dose-effect relationship and molecular mechanism by which BMSC-derived exosomes promote peripheral nerve regeneration after crush injury

Background

The development of new treatment strategies to improve peripheral nerve repair after injury, especially those that accelerate axonal nerve regeneration, is very important. The aim of this study is to elucidate the molecular mechanisms of how bone marrow stromal cell (BMSC)-derived exosomes (EXOs) participate in peripheral nerve regeneration and whether the regenerative effect of EXOs is correlated with dose.

Method

BMSCs were transfected with or without an siRNA targeting Ago2 (SiAgo2). EXOs extracted from the BMSCs were administered to dorsal root ganglion (DRG) neurons in vitro. After 48 h of culture, the neurite length was measured. Moreover, EXOs at four different doses were injected into the gastrocnemius muscles of rats with sciatic nerve crush injury. The sciatic nerve functional index (SFI) and latency of thermal pain (LTP) of the hind leg sciatic nerve were measured before the operation and at 7, 14, 21, and 28 days after the operation. Then, the number and diameter of the regenerated fibers in the injured distal sciatic nerve were quantified. Seven genes associated with nerve regeneration were investigated by qRT-PCR in DRG neurons extracted from rats 7 days after the sciatic nerve crush.

Results

We showed that after 48 h of culture, the mean number of neurites and the length of cultured DRG neurons in the SiAgo2-BMSC-EXO and SiAgo2-BMSC groups were smaller than that in the untreated and siRNA control groups. The average number and diameter of regenerated axons, LTP, and SFI in the group with 0.9 × 10¹⁰ particles/ml EXOs were better than those in other groups, while the group that received a minimum EXO dose (0.4 × 10¹⁰ particles/ml) was not significantly different from the PBS group. The expression of PMP22, VEGFA, NGFr, and S100b in DRGs from the EXO-treated group was significantly higher than that in the PBS control group. No significant difference was observed in the expression of HGF and Akt1 among the groups.

Conclusions

These results showed that BMSC-derived EXOs can promote the regeneration of peripheral nerves and that the mechanism may involve miRNA-mediated regulation of regeneration-related genes, such as VEGFA. Finally, a dose-effect relationship between EXO treatment and nerve regeneration was shown.

Factors affecting the reliability of behavioral assessments for rodent osteoarthritis models

The translational value of osteoarthritis (OA) models is often debated because numerous studies have shown that animal models frequently fail to predict the efficacy of therapies in humans. In part, this failing may be due to the paucity of preclinical studies that include behavioral assessments in their metrics. Behavioral assessments of animal OA models can provide valuable data on the pain and disability associated with disease-sequelae of significant clinical relevance. Clinical definitions of efficacy for OA therapeutics often center on their palliative effects. Thus, the widespread inclusion of behaviors indicative of pain and disability in preclinical animal studies may contribute to greater success identifying clinically relevant interventions. Unfortunately, studies that include behavioral assays still frequently encounter pitfalls in assay selection, protocol consistency, and data/methods transparency. Targeted selection of behavioral assays, with consideration of the array of clinical OA phenotypes and the limitations of individual behavioral assays, is necessary to identify clinically relevant outcomes in OA animal models appropriately. Furthermore, to facilitate accurate comparisons across research groups and studies, it is necessary to improve the transparency of methods. Finally, establishing agreed-upon and clear definitions of behavioral data will reduce the convolution of data both within and between studies. Improvement in these areas is critical to the continued benefit of preclinical animal studies as translationally relevant data in OA research. As such, this review highlights the current state of behavioral analyses in preclinical OA models.

The Molecular Signatures of Acute-immobilization-induced Antinociception and Chronic-immobilization-induced Antinociceptive Tolerance

In the present study, the productions of antinociception induced by acute and chronic immobilization stress were compared in several animal pain models. In the acute immobilization stress model (up to 1 hr immobilization), the antinociception was produced in writhing, tail-flick, and formalin-induced pain models. In chronic immobilization stress experiment, the mouse was enforced into immobilization for 1 hr/day for 3, 7, or 14 days, then analgesic tests were performed. The antinociceptive effect was gradually reduced after 3, 7 and 14 days of immobilization stress. To delineate the molecular mechanism involved in the antinociceptive tolerance development in the chronic stress model, the expressions of some signal molecules in dorsal root ganglia (DRG), spinal cord, hippocampus, and the hypothalamus were observed in acute and chronic immobilization models. The COX-2 in DRG, p-JNK, p-AMPKα1, and p-mTOR in the spinal cord, p-P38 in the hippocampus, and p-AMPKα1 in the hypothalamus were elevated in acute immobilization stress, but were reduced gradually after 3, 7 and 14 days of immobilization stress. Our results suggest that the chronic immobilization stress causes development of tolerance to the antinociception induced by acute immobilization stress. In addition, the COX-2 in DRG, p-JNK, p-AMPKα1, and p-mTOR in the spinal cord, p-P38 in the hippocampus, and p-AMPKα1 in the hypothalamus may play important roles in the regulation of antinociception induced by acute immobilization stress and the tolerance development induced by chronic immobilization stress.

PGE2/EP4 receptor and TRPV1 channel are involved in repeated restraint stress-induced prolongation of sensitization pain evoked by subsequent PGE2 challenge

Prevalence of prior stressful experience is linked to high incidence of chronic pain. Stress, particularly repeated stress, is known to induce maladaptive neuroplasticity along peripheral and central pain transmission pathways. These maladaptive neuroplastic events facilitate sensitization of nociceptive neurons and transition from acute to chronic pain. Pro-inflammatory and pain mediators are involved in inducing neuroplasticity. Pain mediators such as prostaglandin E2 (PGE2), EP4 receptor and transient receptor potential vanilloid-1 (TRPV1) contribute to the genesis of chronic pain. In this study, we examined the role of PGE2/EP4 signaling and TRPV1 signaling in repeated restraint stress-induced prolongation of sensitization pain, a model for transition from acute to chronic pain, in both in vivo and in vitro models. We found that pre-exposure to single restraint stress induced analgesia that masked sensitization pain evoked by subsequent PGE2 challenge. However, pre-exposure to 3d consecutive restraint stress not only prolonged sensitization pain, but also increased stress hormone corticosterone (CORT) in serum, COX2 levels in paw skin, and EP4 and TRPV1 levels in dorsal root ganglion (DRG) and paw skin. Pre-exposure to CORT for 3d, not 1d, also prolonged sensitization pain evoked by PGE2. Co-injection of glucocorticoid receptor (GR) antagonist RU486, COX2 inhibitor NS-398, EP4 receptor antagonist L161,982 or TRPV1 antagonist capsazepine prevented 3d restraint stress prolonged sensitization pain evoked by PGE2. In DRG cultures, CORT increased EP4 and TRPV1 protein levels through GR activation. These data suggest that PGE2/EP4 signaling and TRPV1 signaling in peripheral pain pathway contribute to repeated stress-predisposed transition from acute to chronic pain.

The effects of trauma on brain and body: A unifying role for the midbrain periaqueductal gray

Post-traumatic stress disorder (PTSD), a diagnosis that may follow the experience of trauma, has multiple symptomatic phenotypes. Generally, individuals with PTSD display symptoms of hyperarousal and of hyperemotionality in the presence of fearful stimuli. A subset of individuals with PTSD; however, elicit dissociative symptomatology (i.e., depersonalization, derealization) in the wake of a perceived threat. This pattern of response characterizes the dissociative subtype of the disorder, which is often associated with emotional numbing and hypoarousal. Both symptomatic phenotypes exhibit attentional threat biases, where threat stimuli are processed preferentially leading to a hypervigilant state that is thought to promote defensive behaviors during threat processing. Accordingly, PTSD and its dissociative subtype are thought to differ in their proclivity to elicit active (i.e., fight, flight) versus passive (i.e., tonic immobility, emotional shutdown) defensive responses, which are characterized by the increased and the decreased expression of the sympathetic nervous system, respectively. Moreover, active and passive defenses are accompanied by primarily endocannabinoid- and opioid-mediated analgesics, respectively. Through critical review of the literature, we apply the defense cascade model to better understand the pathological presentation of defensive responses in PTSD with a focus on the functioning of lower-level midbrain and extended brainstem systems.

A refinement to the formalin test in mice

The constant refinement of tests used in animal research is crucial for the scientific community. This is particularly true for the field of pain research, where ethical standards are notably sensitive. The formalin test is widely used in pain research and some of its mechanisms resemble those underlying clinical pain in humans. Immediately upon injection, formalin triggers two waves (an early and a late phase) of strong, nociceptive behaviour, characterised by licking, biting, lifting and shaking the injected paw of the animal. Although well characterised at the behaviour level, since its proposal over four decades ago, there has not been any significant refinement to the formalin test, especially those combining minimisation of animal distress and preservation of behavioural outcomes of the test.  Here, we propose a modified and improved method for the formalin test. We show that anaesthetising the animal with the inhalable anaesthetic sevoflurane at the time of the injection can produce reliable, robust and reproducible results whilst animal distress during the initial phase is reduced. Importantly, our results were validated by pharmacological suppression of the behaviour during the late phase of the test with gabapentin, the anaesthetic showing no interference with the drug. In addition, we demonstrate that this is also a useful method to screen for changes in pain behaviour in response to formalin in transgenic lines.

A refinement to the formalin test in mice

The constant refinement of tests used in animal research is crucial for the scientific community. This is particularly true for the field of pain research, where ethical standards are notably sensitive. The formalin test is widely used in pain research and some of its mechanisms resemble those underlying clinical pain in humans. Immediately upon injection, formalin triggers two waves (an early and a late phase) of strong, nociceptive behaviour, characterised by licking, biting, lifting and shaking the injected paw of the animal. Although well characterised at the behaviour level, since its proposal over four decades ago, there has not been any significant refinement to the formalin test, especially those combining minimisation of animal distress and preservation of behavioural outcomes of the test.  Here, we propose a modified and improved method for the formalin test. We show that anaesthetising the animal with the inhalable anaesthetic sevoflurane at the time of the injection can produce reliable, robust and reproducible results whilst animal distress during the initial phase is reduced. Importantly, our results were validated by pharmacological suppression of the behaviour during the late phase of the test with gabapentin, the anaesthetic showing no interference with the drug. In addition, we demonstrate that this is also a useful method to screen for changes in pain behaviour in response to formalin in transgenic lines.

Restraint training for awake functional brain scanning of rodents can cause long-lasting changes in pain and stress responses

With the increased interest in longitudinal brain imaging of awake rodents, it is important to understand both the short-term and long-term effects of restraint on sensory and emotional processing in the brain. To understand the effects of repeated restraint on pain behaviors and stress responses, we modeled a restraint protocol similar to those used to habituate rodents for magnetic resonance imaging scanning, and studied sensory sensitivity and stress hormone responses over 5 days. To uncover lasting effects of training, we also looked at responses to the formalin pain test 2 weeks later. We found that while restraint causes acute increases in the stress hormone corticosterone, it can also cause lasting reductions in nociceptive behavior in the formalin test, coupled with heightened corticosterone levels and increased activation of the "nociceptive" central nucleus of the amygdala, as seen by Fos protein expression. These results suggest that short-term repeated restraint, similar to that used to habituate rats for awake functional brain scanning, could potentially cause long-lasting changes in physiological and brain responses to pain stimuli that are stress-related, and therefore could potentially confound the functional activation patterns seen in awake rodents in response to pain stimuli.

Chronic restraint stress exacerbates nociception and inflammatory response induced by bee venom in rats: the role of the P2X7 receptors

OBJECTIVE

Chronic restraint stress exacerbates pain and inflammation. The present study was designed to evaluate the effect of chronic restraint stress on inflammatory pain induced by subcutaneous injection of bee venom (BV).

METHODS

First, we investigated: (1) the effect of two-week restraint stress with daily 2 or 8 h on the baseline paw withdrawal mechanical threshold (PWMT), paw withdrawal thermal latency (PWTL) and paw circumference (PC); (2) the effect of chronic stress on the spontaneous paw-flinching reflex (SPFR), decrease in PWM, PWTL and increase in PC of the injected paw induced by BV.

RESULTS

The results showed that (1) chronic restraint decreased significantly the PWMT and inhibited significantly the increase in PC, but had no effect on PWTL, compared with control group; (2) chronic restraint enhanced significantly BV-induced SPFR and inflammatory swelling of the injected paw. In a second series of experiments, the role of P2X7 receptor (P2X7R) in the enhancement of BV-induced inflammatory pain produced by chronic restraint stress was determined. Systemic pretreatment with P2X7R antagonist completely reversed the decrease in PWMT produced by chronic restraint, inhibited significantly the enhancement of BV-induced inflammatory pain produced by chronic restraint stress.

CONCLUSION

Taken together, our data indicate that chronic restraint stress-enhanced nociception and inflammation in the BV pain model, possibly involving the P2X7R.

Distinct pathways for norepinephrine- and opioid-triggered antinociception from the amygdala

BACKGROUND

The amygdala has an important role in pain and pain modulation. We showed previously in animal studies that α2 -adrenoreceptor activation in the central nucleus of the amygdala (CeA) mediates hypoalgesia produced by restraint stress, and that direct application of an α2 -agonist in this region produces analgesia.

AIMS

In the present animal experiments, we investigated the pathways through which α2 -sensitive systems in the CeA produce behavioural analgesia. The CeA has dense connections to a descending pain modulatory network, centred in the midbrain periaqueductal grey (PAG) and the rostral ventromedial medulla (RVM), which is implicated in various forms of stress-related hypoalgesia and which mediates the antinociceptive effect of morphine applied in the basolateral amygdala. We investigated whether this circuit mediates the hypoalgesic effects of α2 -adrenergic agonist administration into the CeA as well as the contribution of endogenous opioids and cannabinoids. We also tested the possibility that activation of α2 -receptors in the CeA produces antinociception by recruitment of noradrenergic pathways projecting to the spinal cord.

RESULTS

Hypoalgesia resulting from bilateral application of the α2 -adrenergic agonist clonidine in the CeA was not reversed by chemical inactivation of the RVM or by systemic injections of naloxone (μ-opioid antagonist) or rimonabant (CB1 antagonist). By contrast, spinal α2 -receptor blockade (intrathecal idazoxan) completely prevented the hypoalgesic effect of clonidine in the CeA, and unmasked a small but significant hyperalgesia.

CONCLUSION

In rats, adrenergic actions in the CeA mediating hypoalgesia require spinal adrenergic neurotransmission but not the PAG-RVM pain modulatory network, or opiate or cannabinoid systems.

Comparison of operant escape and reflex tests of nociceptive sensitivity

Testing of reflexes such as flexion/withdrawal or licking/guarding is well established as the standard for evaluating nociceptive sensitivity and its modulation in preclinical investigations of laboratory animals. Concerns about this approach have been dismissed for practical reasons - reflex testing requires no training of the animals; it is simple to instrument; and responses are characterized by observers as latencies or thresholds for evocation. In order to evaluate this method, the present review summarizes a series of experiments in which reflex and operant escape responding are compared in normal animals and following surgical models of neuropathic pain or pharmacological intervention for pain. Particular attention is paid to relationships between reflex and escape responding and information on the pain sensitivity of normal human subjects or patients with pain. Numerous disparities between results for reflex and operant escape measures are described, but the results of operant testing are consistent with evidence from humans. Objective reasons are given for experimenters to choose between these and other methods of evaluating the nociceptive sensitivity of laboratory animals.

Significance of neuronal cytochrome P450 activity in opioid-mediated stress-induced analgesia

Stressful environmental changes can suppress nociceptive transmission, a phenomenon known as "stress-induced analgesia". Depending on the stressor and the subject, opioid or non-opioid mechanisms are activated. Brain μ opioid receptors mediate analgesia evoked either by exogenous agents (e.g. morphine), or by the release of endogenous opioids following stressful procedures. Recent work with morphine and neuronal cytochrome P450 (P450)-deficient mice proposed a signal transduction role for P450 enzymes in µ analgesia. Since µ opioid receptors also mediate some forms of stress-induced analgesia, the present studies assessed the significance of brain P450 activity in opioid-mediated stress-induced analgesia. Two widely-used models of opioid stress-induced analgesia (restraint and warm water swim) were studied in both sexes of wild-type control and P450-deficient (Null) mice. In control mice, both stressors evoked moderate analgesic responses which were blocked by pretreatment with the opioid antagonist naltrexone, confirming the opioid nature of these responses. Consistent with literature, sex differences (control female>control male) were seen in swim-induced, but not restraint-induced, analgesia. Null mice showed differential responses to the two stress paradigms. As compared with control subjects, Null mice showed highly attenuated restraint-induced analgesia, showing a critical role for neuronal P450s in this response. However, warm water swim-induced analgesia was unchanged in Null vs. control mice. Additional control experiments confirmed the absence of morphine analgesia in Null mice. These results are the first to show that some forms of opioid-mediated stress-induced analgesia require brain neuronal P450 activity.

Randomized Trial of Lower Extremity Splinting to Manage Neuropathic Pain and Sleep Disturbances in People Living with HIV/AIDS

Background/Aims:

Distal symmetrical peripheral neuropathy (DSPN) and sleep disturbances are among the most common complications reported in people living with the human immunodeficiency virus infection and acquired immunodeficiency syndrome (PLWHA). DSPN-pain is predominantly managed by using systemic agents with little evidence supporting their analgesic efficacy. The purpose of this study is to evaluate the effect of nighttime lower extremity splinting application on DSPN-related pain and sleep disturbances compared to a parallel splint liner application in PLWHA.

Methods:

Forty-six PLWHA and DSPN were randomized to nighttime wearing of bilateral lower extremity splints or the liners only. Pain and sleep outcomes were measured at baseline, week 3, and week 6. The pain was measured using the Neuropathic Pain Scale and sleep using the Pittsburgh Sleep Quality Index.

Results:

Pain and sleep scores improved in both groups over time. The median percentage pain reduction at week 6 was 8% in the liner group and 34% in the splint group. The change in pain scores in the splint group was found to be significant over time, P < .0005. The contrast between the splint and liner groups was underpowered (26%) and was not found to be significant, P > .05. Sleep scores improved 20% from baseline to the end of the study in both groups; all participants were classified as poor sleepers.

Conclusion:

The 6-week use of nighttime splints reduces DSPN-pain possibly by providing peripheral inhibition of external stimuli. Future studies are needed to validate this inhibitory intervention to manage DSPN in PLWHA and other neuropathic conditions.

Acute stress regulates nociception and inflammatory response induced by bee venom in rats: possible mechanisms

Restraint stress modulates pain and inflammation. The present study was designed to evaluate the effect of acute restraint stress on inflammatory pain induced by subcutaneous injection of bee venom (BV). First, we investigated the effect of 1 h restraint on the spontaneous paw-flinching reflex (SPFR), decrease in paw withdrawal mechanical threshold (PWMT) and increase in paw volume (PV) of the injected paw induced by BV. SPFR was measured immediately after BV injection, and PWMT and PV were measured 2 h before BV and 2-8 h after BV. The results showed that acute restraint inhibited significantly the SPFR but failed to affect mechanical hyperalgesia. In contrast, stress enhanced significantly inflammatory swelling of the injected paw. In a second series of experiments, the effects of pretreatment with capsaicin locally applied to the sciatic nerve, systemic 6-hydroxydopamine (6-OHDA), and systemic naloxone were examined on the antinociception and proinflammation produced by acute restraint stress. Local capsaicin pretreatment inhibited BV-induced nociception and inflammatory edema, and had additive effects with stress on nociception but reduced stress enhancement of edema. Systemic 6-OHDA treatment attenuated the proinflammatory effect of stress, but did not affect the antinociceptive effect. Systemic naloxone pretreatment eliminated the antinociceptive effect of stress, but did not affect proinflammation. Taken together, our data indicate that acute restraint stress contributes to antinociception via activating an endogenous opioid system, while sympathetic postganglionic fibers may contribute to enhanced inflammation in the BV pain model.

In vivo high-resolution localized (1) H MR spectroscopy in the awake rat brain at 7 T

In vivo localized high-resolution (1) H MR spectroscopy was performed in multiple brain regions without the use of anesthetic or paralytic agents in awake head-restrained rats that were previously trained in a simulated MRI environment using a 7T MR system. Spectra were obtained using a short echo time single-voxel point-resolved spectroscopy technique with voxel size ranging from 27 to 32.4 mm(3) in the regions of anterior cingulate cortex, somatosensory cortex, hippocampus, and thalamus. Quantifiable spectra, without the need for any additional postprocessing to correct for possible motion, were reliably detected including the metabolites of interest such as γ-aminobutyric acid, glutamine, glutamate, myo-inositol, N-acetylaspartate, taurine, glycerophosphorylcholine/phosphorylcholine, creatine/phosphocreatine, and N-acetylaspartate/N-acetylaspartylglutamate. The spectral quality was comparable to spectra from anesthetized animals with sufficient spectral dispersion to separate metabolites such as glutamine and glutamate. Results from this study suggest that reliable information on major metabolites can be obtained without the confounding effects of anesthesia or paralytic agents in rodents.

A reassessment of stress-induced "analgesia" in the rat using an unbiased method

An increased tail-flick latency to noxious heat during or after stress in the rodent is usually interpreted as a stress-induced reduction in pain sensitivity and often described as a form of stress-induced "analgesia." However, this measure is an indirect and flawed measure of the change in nociceptive threshold to noxious heat. A major confound of the latency measure is the initial temperature of the tail, which can drop down to room temperature during stress, the consequence of a marked sympathetically mediated vasoconstriction in the skin of the extremities. We addressed this issue with tail-flick tests during contextual fear using infrared thermography to monitor temperature changes and a CO2 laser to deliver the heat stimulus. The experiment revealed a 4.2°C increase of the nociceptive threshold, confirming a true antinociceptive effect. However, its contribution to the increased withdrawal latency was less than two-thirds (63.2%). Nearly one-third (32.2%) was due to the drop in tail temperature (4.4°C), which also slowed conduction along sensory fibers (2.2%, included in the 32.2%). The remaining 4.6% was due to an increase in decisional/motor latency. This new unbiased method establishes beyond doubt that a conditioned stress response is associated with true antinociception to noxious heat. It also confirms that stress-induced changes in skin temperature can be a major confound in tail-flick tests. The present study shows, for the first time, the exact contribution of these two components of the tail-flick latency for a stress response. Less than two-thirds of the increase in tail-flick latency to noxious heat, evoked by conditioned fear, reflects true antinociception. The remaining is due to skin vasoconstriction.

Pain and nociception in reptiles

The ability of reptiles to "feel" pain and the significance of pain or nociception on physiologic homeostasis is an exceedingly complex question requiring integration of both physiologic and behavioral evidence. Until further information is available, it would seem most ethical for veterinarians to assume that reptiles are capable of feeling pain, and to treat or manage pain when there is reasonable evidence that pain is present. With increased information available regarding analgesic use in reptiles and with the heightened awareness of the importance of analgesia for zoologic companion animals, it is likely that more veterinarians will provide pain relief to their reptile patients.

Transcriptome profiling analysis reveals region-distinctive changes of gene expression in the CNS in response to different moderate restraint stress

It is generally believed that temporary moderate stress to a living organism has protective and adaptive effects, but little is known about the responses of CNS to the moderate stresses at molecular level. This study aims to investigate the gene expression changes induced by moderate stress in CNS stress- and nociception-related regions of rats. Moderate restraint was applied to rats for 50 min and cDNA microarrays were used to detect the differential gene expression in different CNS regions. Transcriptome profiling analysis showed that at acute stage stress-related genes were up-regulated in arcuate nucleus; fight-or-flight behavior-related genes were up-regulated in periaqueductal gray, while nitric oxide and GABA signal transmission-related genes were up-regulated in spinal dorsal horn. In addition, immune-related genes were broadly regulated, especially at the late stage. These results suggested that specific genes of certain gene ontology categories were spatiotemporally regulated in specific CNS regions related to relevant functions under moderate external stimuli at acute stage, while immune response was broadly regulated at the late stage. The co-regulated genes among the three different CNS regions may play general roles in CNS when exposed to moderate stress. Furthermore, these results will help to elucidate the physiological processes involved in moderate stress in CNS.

A novel automated method for measuring the effect of analgesics on formalin-evoked licking behavior in rats

The behavioral assessment of pain is essential for the analysis of pain mechanisms and the evaluation of analgesic drugs. The formalin test is one of such methods widely used as a model of injury-induced pain in rodents. This test is manually demanding and the recording of results is left to the subjectivity of the experimenters. Thus we developed a novel automated method to estimate the pharmacological response in formalin-induced licking behavior in rats using a multicolor detection technique. Two color markers were preliminarily applied to rats-yellow dye on the mouth and fluorescent green tape on the right hind paw. Behaviors of the animals were recorded from both above and below the subject, by a dual-view digital video camera system. After injection with formalin into the hind paw, rats exhibited a biphasic display of licking behavior. Licking time was measured by the sum of frames where the distance between these markers was less than an appropriate threshold of distance (TD). The split-plot analysis of variance demonstrated that the sum of squares of differences in licking time between manual and automated measurement was minimized when TD = 20mm. In addition, frames in which moving velocity of these markers is less than 2.5mm/s was neglected for calculation in order to eliminate sedative effect on the recorded data. On these conditions, subcutaneous administration of morphine in rats dose-dependently decreased formalin-elicited nociceptive responses. These results suggest that under optimal conditions the automated technique when applied to pharmacological studies are more reliable and efficient than if they are manually recorded.

Opioid modulation of reflex versus operant responses following stress in the rat

In pre-clinical models intended to evaluate nociceptive processing, acute stress suppresses reflex responses to thermal stimulation, an effect previously described as stress-induced "analgesia." Suggestions that endogenous opioids mediate this effect are based on demonstrations that stress-induced hyporeflexia is enhanced by high dose morphine (>5 mg/kg) and is reversed by naloxone. However, reflexes and pain sensations can be modulated differentially. Therefore, in the present study direct comparisons were made of opioid agonist and antagonist actions, independently and in combination with acute restraint stress in Long Evans rats, on reflex lick-guard (L/G) and operant escape responses to nociceptive thermal stimulation (44.5 degrees C). A high dose of morphine (>8 mg/kg) was required to reduce reflex responding, but a moderate dose of morphine (1 mg/kg) significantly reduced escape responding. The same moderate dose (and also 5 mg/kg) of morphine significantly enhanced reflex responding. Naloxone (3 mg/kg) significantly enhanced escape responding but did not affect L/G responding. Restraint stress significantly suppressed L/G reflexes (hyporeflexia) but enhanced escape responses (hyperalgesia). Stress-induced hyperalgesia was significantly reduced by morphine and enhanced by naloxone. In contrast, stress-induced hyporeflexia was blocked by both naloxone and 1 mg/kg of morphine. Thus, stress-induced hyperalgesia was opposed by endogenous opioid release and by administration of morphine. Stress-induced hyporeflexia was dependent upon endogenous opioid release but was counteracted by a moderate dose of morphine. These data demonstrate a differential modulation of reflex and operant outcome measures by stress and by separate or combined opioid antagonism or administration of morphine.

Effects of social isolation and restraint on adrenocortical responses and hypoalgesia in loose-housed dairy cows1

Effects of social isolation or restraint, applied outside the home pen, on adrenocortical and nociceptive responses were examined in 28 loose-housed dairy cows. Treatments lasted 15 min and consisted of social isolation in novel surroundings or restraint by the head in a test pen. A control treatment was applied in the test pen as well. Each cow was exposed to all treatments in a balanced order, with 3 to 4 d between treatments. Compared with the control treatment, social isolation in novel surroundings led to increased plasma concentration of cortisol (P < 0.001) as well as to indications of hypoalgesia [posttreatment lack of decrease in latency to respond toward nociceptive laser stimulation, a tendency for decreased frequency of kicking in the pauses between laser stimulations (P = 0.06), and an increased proportion of leg moving (least possible active response) after treatment (P = 0.04)]. Indications of hypoalgesia were also observed after restraint (reduced kicking in response to laser stimulation, P = 0.04); however, the indications were to a lesser extent than after social isolation, and restraint treatment did not lead to increased plasma concentration of cortisol. For control and restraint treatment, an initial increase (P < 0.02) in plasma concentration of cortisol was found, suggesting effects of pretreatment factors such as handling. No correlations between adrenocortical and nociceptive responses toward social isolation were found. The results confirm earlier reports stating that nociceptive changes induced by environmental challenges can be shown in dairy cows, even when they are kept in groups and removed from the home pen during the study of stress responses. However, testing outside the home pen seemed to affect the nociceptive and adrenocortical responses, thereby suggesting that care should be taken to avoid effects of pretreatment situational factors.

Anxiety stress and nociceptive responses in mice

Nociception in laboratory animals appears to be influenced by physical or emotional stressors. Nevertheless, the reported data are not univocal. Discrepancies seem to be caused by some kind of stress model and/or by the timing of stressor application. The aim of the present work is to study the influence of chronic application of a well-controlled and defined anxiety stress paradigm (rotational stress) on the behavioral formalin pain responses in mice maintained in a low-stress environment. The results indicate that emotional chronic stress increases specific pain responses in the late inflammatory phase and, correspondingly, decreases self-grooming. Locomotor activity appears influenced by pain presence only. The hormonal and neural mechanisms that could be involved in the observed nonspecific and specific nociceptive responses to stress are discussed.

Sex differences in tail-flick latency of non-stressed and stressed rats

The present study was conducted to assess whether there are sex differences between male and female subjects in their response to noxious stimuli under non-stressed and stressed conditions. Tail-flick latency assay was used as an experimental tool on 12 adult male and 12 adult female Sprague-Dawley rats before immobilization (i.e., non-stressed condition) and after 30, 120, 240, and 360 min of continuous immobilization (i.e., stressed conditions). It was found that the non-stressed female rats exhibited significantly longer response latency to noxious thermal stimuli than the non-stressed male rats. Stressed conditions caused by immobilization of the animal inside a ventilated restrainer significantly prolonged the tail-flick latency thresholds of both sexes. Female and male rats did not develop any adaptation after 120, 240, and 360 min of immobilization compared to their 30 min of immobilization, as demonstrated by the tail-flick assay. Moreover, the difference in the tail-flick latency between male and female rats was reduced as the duration of immobilization in the restrainer lengthened. At 30 min of immobilization, the significant difference between the sexes reduced to p < .05 compared to p < .01 before immobilization, and to statistically non-significant differences after 120, 240, and 360 min of immobilization. Results from this study suggested that female rats had longer tail-flick latency than male rats in non-stressed environment, and the difference in tail-flick latency after immobilization between the sexes became less significant with time. In addition, 360 min of immobilization was not enough to adapt to the restrainer, as shown by the tail-flick assay.

Independent time courses of supraspinal nociceptive activity and spinally mediated behavior during tonic pain

The behavioral response to acute tissue injury is usually characterized by different phases, but the brain mechanisms underlying changes in pain-related behavior over time are still poorly understood. We aimed to analyze time-dependent changes in metabolic activity levels of 49 forebrain structures in the formalin pain model, using the autoradiographic 2-deoxyglucose method in unanesthetized, freely moving rats. We examined rats during the first phase of pain-related reactions ('early' groups), or during the third recovery phase, 60 min later, when the supraspinally mediated behavioral responses were reduced ('late' group). In the early groups, metabolic rates were bilaterally increased over control values in the periaqueductal gray, zona incerta and in several thalamic nuclei (anteroventral, centrolateral, lateral dorsal, parafascicular, posteromedial, submedius, ventromedial, and ventrobasal complex), as well as in the habenulae and in the parietal, cingulate, antero-dorsal insular, and anterior piriform cortex. A contralateral, somatotopically specific activation was found in the putative hindlimb representation area of the somatosensory cortex. In the late group, noxious-induced activation declined in most structures. However, metabolic rates were higher than controls in the periaqueductal gray and zona incerta and in two other structures not previously active: the prerubral area/field of Forel and the arcuate hypothalamic nucleus. These findings provide a time-dependent functional map of nociceptive and anti-nociceptive forebrain circuits during tonic pain. The parallel decrease in licking behavior and forebrain activity, at times when spinally mediated limb flexion responses were still present, suggests that endogenous antinociceptive systems may differently modulate spinal and supraspinal nociceptive networks following acute tissue injury.

Oxytocin mediates stress-induced analgesia in adult mice

As a neurohormone and as a neurotransmitter, oxytocin has been implicated in the stress response. Descending oxytocin-containing fibres project to the dorsal horn of the spinal cord, an area important for processing nociceptive inputs. Here we tested the hypothesis that oxytocin plays a role in stress-induced analgesia and modulates spinal sensory transmission. Mice lacking oxytocin exhibited significantly reduced stress-induced antinociception following both cold-swim (10 degrees C, 3 min) and restraint stress (30 min). In contrast, the mice exhibited normal behavioural responses to thermal and mechanical noxious stimuli and morphine-induced antinociception. In wild-type mice, intrathecal injection of the oxytocin antagonist dOVT (200 microM in 5 microl) significantly attenuated antinociception induced by cold-swim. Immunocytochemical staining revealed that, in the mouse, oxytocin-containing neurones in the paraventricular nucleus of the hypothalamus are activated by stress. Furthermore, oxytocin-containing fibres were present in the dorsal horn of the spinal cord. To test whether descending oxytocin-containing fibres could alter nociceptive transmission, we performed intracellular recordings of dorsal horn neurones in spinal slices from adult mice. Bath application of oxytocin (1 and 10 microM) inhibited excitatory postsynaptic potentials (EPSPs) evoked by dorsal root stimulation. This effect was reversed by the oxytocin antagonist dOVT (1 microM). Whole-cell recordings of dorsal horn neurones in postnatal rat slices revealed that the effect of oxytocin could be blocked by the addition of GTP-gamma-S to the recording pipette, suggesting activation of postsynaptic oxytocin receptors. We conclude that oxytocin is important for both cold-swim and restraint stress-induced antinociception, acting by inhibiting glutamatergic spinal sensory transmission.

Measuring pain in the (knockout) mouse: big challenges in a small mammal

The increasing popularity of the mouse as a subject in basic science studies of pain can largely be attributed to the development of transgenic "knockout" technology in this species only. To take advantage of this biological technique, many investigators are rushing to adapt to the mouse experimental protocols that were designed for the rat. However, the myriad physiological and behavioral differences between these two rodent species render such adaptations non-trivial and in many cases seriously problematic. In this article we review the basic nociceptive assays used in behavioral pain research (thermal, mechanical, electrical and chemical), and highlight how species differences affect their proper application. In addition, some of the issues specifically pertaining to the interpretation of such data in knockout studies are addressed.

[Critical analysis of animal models of acute pain. II]

OBJECTIVE

To analyse models of acute pain in experimental animals.

DATA SOURCES

References were obtained from computerised bibliographic data banks (Medline and others) and the authors' personal documents.

DATA SYNTHESIS

The majority of tests permit only a measurement of threshold, whereas clinical pain is almost always prolonged. The relationships between tests of acute pain and motor activity are reviewed from a number of standpoints; in particular we consider the influence, which postural adjustments of the animal may exert on motor responses in the limbs and the significance of flexor and extensor reflexes. In analysing the problem of the sensitivity of tests, we raise the following questions: 1) what type(s) of fibres underlie the observed responses and might these be different depending on whether one is stimulating a healthy or an inflamed tissue; 2) what significance do measurements of "latency" have when a stimulus is increasing; 3) how valid are the methods of analysing the results? The influence of species and the genetic line used in tests and the specificity and predictivity of tests are considered. Finally, we review those factors, which may distort behavioural measurements in animals, notably--pharmacokinetics, interactions between heterotopic stimuli, environmental factors and related psychophysiological/psychological considerations (subjective "undesirable" phenomena, learning phenomena). We pay particular attention to related physiological functions (thermoregulation, vasomotricity, blood pressure). These considerations lead us to re-position nociception within a much larger homeostatic framework which in addition to pain, includes phenomena such as anxiety and vegetative functions. They also suggest that we should define an "effective stimulus" as one, which activates nociceptive nerve terminals after a physical stimulus, has passed through a "peripheral lens" which regulates its intensity for reasons, which are physical, albeit of biological origin. Finally they remind us that the "system of pain" forms part of a whole set of subsystems--sensory, motor, vegetative, emotional, motivational--which scientific method, being reductionist by nature, cannot study in its entirety. However one must consider results of nociceptive tests within this general context.

CONCLUSION

It is only by taking the approach described in this review, that fundamental and clinical research can interact usefully.

The effect of genotype on sensitivity to electroacupuncture analgesia

Individual differences in sensitivity to pain and analgesia are well appreciated, and increasing evidence has pointed towards a role of inherited genetic factors in explaining some proportion of such variability. It has long been known by practitioners of acupuncture, an ancient modality of analgesia, that some patients are 'responders' and others 'non-responders.' The present research was aimed at defining the inherited genetic influence on acupuncture analgesia in the mouse, using 10 common inbred strains. Two pairs of metallic needles were inserted into acupoints ST 36 and SP 6, fixed in situ and then connected to the output channel of an electric pulse generator. Electroacupuncture (EA) parameters were set as constant current output (intensity: 1.0-1.5-2.0 mA, 10 min each; frequency: 2 or 100 Hz) with alteration of a positive and negative square wave, 0.3 ms in pulse width. Tail-flick latencies evoked by radiant heat were measured before, during and after EA stimulation. Narrow-sense heritability estimates of 2 and 100 Hz EA were 0.37 and 0.16, respectively. We found that the C57BL/10 strain was the most sensitive, and the SM strain was the least sensitive to both 2 and 100 Hz EA. However, the relative sensitivities of other strains to these two EA frequencies suggested some genetic dissociation between them as well. These results demonstrate a role of inherited genetic factors in EA sensitivity in the mouse, although the low-to-moderate heritability estimates suggest that environmental factors may be of greater importance in predicting who will benefit from this analgesic modality.

Automated behavioural analysis in animal pain studies

The assessment of the effectiveness of analgesics is strongly based on observational data from behavioural tests. These tests are interesting and give a quantification of the effect of the drugs on the whole animal but their use is subject to several difficulties: (i) many results are difficult to analyse as they only correspond to the evaluation of a reflex response; (ii) the tests dealing with more integrated responses are also more difficult to use and closely depend on the experimenter's subjectivity. If automation is widely used in a lot of research fields, this is not the case in behavioural pharmacology. Yet, it can contribute to optimize the tests. The use of signal processing devices allows the automated (and thus objective) measurement of behavioural reactions to nociceptive stimulation (amplitude of a reflex, vocal emission intensity). Mechanical devices based on a computer-driven dynamic force detector allows the recording of some pain behaviours. Video image analysis allows the quantification of more complex behaviours (nociception-induced specific motor behaviours) as well as meaningful information during the same experimentation (exploratory behaviour, total motor activity, feeding behaviour). Moreover, these methods make it possible to obtain a more objective measurement, to reduce animal-experimenter interactions, to ease system use, and to improve effectiveness. The prospects to work in this field are multiple: continuation of the attempts at an automation of the behaviours specifically induced by chronic pain; development of real animal pain monitoring based on analysis of specific and non-specific behavioural modifications induced by pain. In this context, the automation of the behavioural analysis is likely to make possible real ethical progress thanks to an increase in the test's effectiveness and a real taking into account of animal's pain. Nevertheless, there are some limits due to characteristics of the behavioural expression of nociception and technological problems.

Role of tachykinin NK2 receptors in normal and altered rectal sensitivity in rats

Irritable bowel syndrome is characterized by visceral hyperalgesia commonly associated with stress and inflammatory processes. We investigated the role of tachykinin NK2 receptors in the ability of trinitrobenzenesulphonic acid (TNBS) and stress to enhance the sensitivity of the rat rectum to distension using a selective tachykinin NK2 receptor antagonist (MEN 11420). Rats were fitted with electrodes implanted in the striated muscles of the abdomen. Rectal distension (RD) was performed with a balloon inflated by steps of 0.4 ml from 0 to 1.6 ml. Five groups were submitted to RD performed 3 days before and after intrarectal instillation of TNBS. Fifteen minutes before RD, rats were treated with saline or MEN 11420 (5 - 100 microg kg(-1) i.v.). Two other groups, submitted to 2 h restraint or sham stress sessions were randomly treated i.v. with saline or MEN 11420 (10 - 200 microg kg(-1)) prior to RD applied 20 min later. The basal response to RD was characterized by a significant increase in the number of abdominal contractions. This response occurred with a threshold volume of 0.8 ml and was dose-dependently reduced by MEN 11420 (5 - 100 microg kg(-1) i.v.). Rectal inflammation lowered the volume of distension producing abdominal contractions to 0.4 ml (allodynia). This effect was either reduced or suppressed by MEN 11420. A similar allodynia was observed after a stress session and this effect was reduced (49%) or suppressed by MEN 11420 at 200 and 100 microg kg(-1), respectively. Tachykinin NK2 receptors are involved in rectal hypersensitivity associated with inflammation and stress. British Journal of Pharmacology (2000) 129, 193 - 199

The development of the DEGR(R): A scale to assess pain in young children with cancer

The Gustave Roussy Child Pain Scale (Douleur Enfant Gustave Roussy, DEGR(R)Scale) is a scale for grading prolonged pain in children aged 2-6 years with cancer. The scale comprised six behaviours specific to pain items, five psychomotor inertia items, and four anxiety items, with a total score ranging from 0 to 60. This work was designed to confirm the scale structure and to study its construct validity and convergent validity.Our work was composed of two parts. In the first part of the study, 152 children with progressive cancer were scored by two nurses using the DEGR(R)scale, in a cross-sectional design. And in the second part, 53 of these 152 children were video-recorded. The tapes were assessed both by a panel of four pain specialists using a 0 to 7 Likert scale and by a nurse using DEGR(R)scale.As for the 152 children, the mean of the total scores derived from the DEGR(R)is 20.2 (SD = 6.2). Both the degree of agreement between the nurses (the weighted kappa coefficient) and the internal consistency of the scale (Cronbach alpha coefficient = 0.90) were high, providing evidence of good reliability. Multivariate factor analyses showed a first factor of intensity of pain (51% of the total variance) and a second factor (14% of the total variance) which distinguishes the psychomotor inertia items from the items concerning voluntary expression of pain. Also, the results showed that psychomotor inertia items contribute to both factors and that it is an important sign of prolonged pain. Construct validity was strengthened by the absence of correlation between DEGR(R)scores and variables not related to pain, including fever, neutropenia and anaemia (indicative of poor medical condition) and the absence of parents' visits (indicative of psychological distress).Concerning the 53 video-recorded children, the nurses' DEGR(R)ratings were strongly correlated with the specialists panel scores indicating a fairly good case for convergent validity. Copyright 1999 European Federation of Chapters of the International Association for the Study of Pain.

Ginsenosides that produce differential antinociception in mice

Ginsenoside Rc, Rd, and Re induced antinociception in writhing and formalin tests among five representative ginsenosides: Rb1, Rc, Rd, Re, and Rg1. However, these ginsenosides had no effect in the tail-flick test. The antinociceptive effects induced by three ginsenosides were dose dependent. ED50 was 20.5 (7.3-57.4 mg/kg) for Rc, 17 (11.0-27.6 mg/kg) for Rd, and 3.5 (1-12 mg/ kg) for Re in the writhing test and 62 (42-90 mg/kg) for Rc, 45 (20.5-99.0 mg/kg) for Rd, and 82 (48-139 mg/kg) for Re in the second phase of the formalin test. The antinociceptive effects were not blocked by the opioid receptor antagonist naloxone in the writhing and formalin tests. These three ginsenosides did not affect motor function. Ginsenoside Rc and Rd induced hypothermia for 30 to 60 min, and ginsenoside Rc induced hyperthemia after 150 min of treatment at doses of 100 mg/kg. These results suggest that ginsenosides such as Rc, Rd, or Re inhibit mainly chemogenic pain rather than thermal pain by the nonopioid system in mice.

Presence of a reduced opioid response in interleukin-6 knock out mice

Cytokines are known to influence neuronal functions. The purpose of this study was to investigate the putative role of the cytokine interleukin-6 (IL-6) in the pathways involved in opioid-mediated responses, by using IL-6-deficient mice. We reported that with a thermal stimulus IL-6-knock-out (IL-6KO) mice presented nociceptive thresholds similar to those measured in their controls. However, they showed a reduced analgesic response both to the restraint stress and to the administration of low doses of morphine. Hypothalamic levels of the opioid peptide beta-endorphin were significantly higher in IL-6KO mice than they were in their controls. The development of tolerance to the analgesic effect of morphine was more rapid in IL-6-deficient mice than in wild-type controls. Binding experiments showed that the number of opioid receptors in the midbrain, but not in the hypothalamus, decreased in IL-6KO mice. Autoradiographic binding analysis revealed that the density of mu receptors diminished while the delta-opioid receptors did not. These results suggest that IL-6 is necessary for a correct development of neuronal mechanisms involved in the response to both endogenous and exogenous opiates.

CNS pattern of metabolic activity during tonic pain: evidence for modulation by beta-endorphin

CNS correlates of acute prolonged pain, and the effects of partial blockade of the central beta-endorphin system, were investigated by the quantitative 2-deoxyglucose technique in unanaesthetized, freely moving rats. Experiments were performed during the second, tonic phase of the behavioural response to a prolonged chemical noxious stimulus (s.c. injection of dilute formalin into a forepaw), or after minor tissue injury (s.c. saline injection). During formalin-induced pain, local glucose utilization rates in the CNS were bilaterally increased in the grey matter of the cervical spinal cord, in spinal white matter tracts and in several supraspinal structures, including portions of the medullary reticular formation, locus coeruleus, lateral parabrachial region, anterior pretectal nucleus, the medial, lateral and posterior thalamic regions, basal ganglia, and the parietal, cingulate, frontal, insular and orbital cortical areas. Pretreatment with anti-beta-endorphin antibodies, injected i.c.v., led to increased metabolism in the tegmental nuclei, locus coeruleus, hypothalamic and thalamic structures, putamen, nucleus accumbens, diagonal band nuclei and dentate gyrus, and in portions of the parietal, cingulate, insular, frontal and orbital cortex. In formalin-injected rats, pretreated with anti-beta-endorphin, behavioural changes indicative of hyperalgesia (increased licking response) were found, which were paralleled by a significant enhancement of functional activity in the anterior pretectal nucleus and in thalamo-cortical systems. A positive correlation was found between the duration of the licking response and metabolic activity of several forebrain regions. These results provide a map of the CNS pattern of metabolic activity during tonic somatic pain, and demonstrate a modulatory role for beta-endorphin in central networks that process somatosensory inputs.

The effect of handling and immobilization on the response to acute pain in newborn infants

BACKGROUND

Previous reports have suggested that healthy, full-term newborn infants who are in more aroused behavioral states tend to respond more robustly to painful events. Others have shown that acutely ill premature and full-term infants who undergo significant handling and immobilization as part of required nursery procedures respond less robustly to concurrent painful events.

PURPOSE

To investigate, using an experimental manipulation, the effect of arousal associated with handling and immobilization on response to acute pain in generally healthy, premature and full-term newborn infants.

METHODS

Infants were randomly assigned to a group that underwent a series of handling and immobilization procedures before a heelstick or to a group that underwent the heelstick without previous handling and immobilization. Heart rate, behavioral state, and facial activity were compared between the handled (n = 21) and nonhandled (n = 27) infants during an undisturbed baseline, a preparatory, and a standard heelstick procedure.

RESULTS

In the handled group, heart rate increased over baseline levels in response to the handling but promptly returned to prehandling levels. There were no significant differences between handled and nonhandled groups in mean heart rate, behavioral state, or facial activity during the baseline or preparatory periods before the heelstick. However, in response to the heelstick, handled infants had a higher mean heart rate, greater behavioral arousal, and displayed more facial activity as compared with nonhandled infants.

CONCLUSIONS

Healthy premature and full-term newborn infants who undergo common nursery experiences such as handling and immobilization as part of their routine care can exhibit greater physiologic and behavioral reactivity to subsequent painful procedures. The effects of the previous handling may be undetectable, using conventional indices of reactivity, until the painful event. These findings emphasize the importance of identifying reliable markers of previous stress, particularly for newborn infants who may become clinically compromised as a result of the physiologic instability associated with response to pain.

Influence of trimebutine on inflammation- and stress-induced hyperalgesia to rectal distension in rats

The effects of trimebutine and its major metabolite, N-desmethyltrimebutine on inflammation- and stress-induced rectal hyperalgesia have been evaluated in rats fitted with electrodes implanted in the longitudinal striated muscle of the abdomen. Intermittent rectal distension was performed before and 3 days after induction of rectal inflammation by local infusion of trinitrobenzenesulphonic acid (in ethanol). Stress consisted of 2h partial restraint and rectal distension was performed before and 30min after the end of the partial restraint session. The animals were treated intraperitoneally with trimebutine or desmethyltrimebutine (5, 10 or 20mgkg(-1)) or vehicle 15min before rectal distension. Naloxone (1mgkg(-1)) or saline was injected subcutaneously before trimebutine and desmethyltrimebutine. Before treatment trimebutine at the highest dose (20mgkg(-1)) reduced the abdominal response to rectal distension for the highest volume of distension (1.6mL) whereas desmethyltrimebutine was inactive. After rectocolitis the abdominal response to rectal distension was enhanced and trimebutine at 5mgkg(-1) reduced and at 10 mgkg(-1) suppressed inflammation-induced hyperalgesia, an effect reversed by naloxone. Desmethyltrimebutine was inactive. Stress-induced hypersensitivity was attenuated or suppressed, or both, by trimebutine and desmethyltrimebutine at doses of 5, 10 or 20mgkg(-l); greater efficacy was observed for desmethyltrimebutine and the effects were not reversed by naloxone. It was concluded that trimebutine and desmethyltrimebutine are active against inflammation- and stress-induced rectal hyperalgesia but act differently. The effect of trimebutine on inflammation-induced hyperalgesia is mediated through opioid receptors.

Parasitized female mice display reduced aversive responses to the odours of infected males

The present study showed that parasites influence both the responses of uninfected females to males and the responses of female hosts to infected males. In female laboratory mice one of the consequences of exposure to the olfactory cues associated with an infected male was a reduction of the reactivity to a thermal surface, i.e. pain inhibition or analgaesia. Uninfected oestrous and non-oestrous female mice displayed marked analgaesic responses after exposure to the odours of males infected with either the enteric single-host nematode parasite, Heligmosomoides polygyrus, or the protozoan parasite, Eimeria vermiformis. The uninfected oestrous females distinguished between infected and physically stressed males, displaying a greater analgaesic response to the odours of infected males. These analgaesic responses and their anxiety/ fearfulness-associated behavioural correlates could elicit either a reduced interest in, or avoidance of, parasitized males by females. Oestrous female mice infected with H. polygyrus displayed a reduced analgaesic response to the odours of the infected males and differentially responded to the odours of males infected with either the same (H. polygyrus) or a different parasite (E. vermiformis). An exposure time of 1 min elicited minimal responses to the odours of males infected with the same parasite, H. polygyrus, and an attenuated, though significant, non-opioid peptide-mediated analgaesic response to males infected with E. vermiformis. An exposure time of 30 min elicited similar markedly reduced endogenous opioid peptide-mediated analgaesic responses to the odours of both of the categories of infected males. The responses to the odours of a stressed male were, however, unaffected by the parasitic infection. The reduced analgaesic responses of the parasitized females to the odours of infected males may involve either enhanced odour familiarity and responses to group odour templates and/or neuromodulatory shifts resulting in reduced fearfulness and potentially greater interest in the infected males.