Persistent secondary hyperalgesia after gastrocnemius incision in the rat

A new long-acting analgesic formulation for postoperative pain management

Local anesthetics (LA), as part of multimodal analgesia, have garnered significant interest for their role in delaying the initiation of opioid therapy, reducing postoperative opioid usage, and mitigating both hospitalization duration and related expenses. Despite numerous endeavors to extend the duration of local anesthetic effects, achieving truly satisfactory long-acting analgesia remains elusive. Drawing upon prior investigations, vesicular phospholipid gels (VPGs) emerge as promising candidates for extended-release modalities in small-molecule drug delivery systems. Therefore, we tried to use the amphiphilicity of phospholipids to co-encapsulate levobupivacaine hydrochloride and meloxicam, two drugs with different hydrophilicity, to obtain a long-term synergistic analgesic effect. Initially, the physicochemical attributes of the formulation were characterized, followed by an examination of its in vitro release kinetics, substantiating the viability of extending the release duration of the dual drugs. Sequentially, in vivo investigations encompassing pharmacokinetic profiling and assessment of analgesic efficacy were undertaken, revealing a prolonged release duration of up to 120 h and attainment of optimal postoperative analgesia. Subsequently, inquiries into the mechanism underlying synergistic analgesic effects and safety evaluations pertinent to the delivery strategy were pursued. In summation, we successfully developed a promising formulation to achieve long-acting analgesia.

Local anesthetic delivery systems for the management of postoperative pain

Postoperative pain (POP) is a major clinical challenge. Local anesthetics (LAs), including amide-type LAs, ester-type LAs, and other potential ion-channel blockers, are emerging as drugs for POP management because of their effectiveness and affordability. However, LAs typically exhibit short durations of action and prolonging the duration by increasing their dosage or concentration may increase the risk of motor block or systemic local anesthetic toxicity. In addition, techniques using LAs, such as intrathecal infusion, require professional operation and are prone to catheter displacement, dislodgement, infection, and nerve damage. With the development of materials science and nanotechnology, various LAs delivery systems have been developed to compensate for these disadvantages. Numerous delivery systems have been designed to continuously release a safe dose in a single administration to ensure minimal systemic toxicity and prolong pain relief. LAs delivery systems can also be designed to control the duration and intensity of analgesia according to changes in the external trigger conditions, achieve on-demand analgesia, and significantly improve pain relief and patient satisfaction. In this review, we summarize POP pathways, animal models and methods for POP testing, and highlight LAs delivery systems for POP management. STATEMENT OF SIGNIFICANCE: Postoperative pain (POP) is a major clinical challenge. Local anesthetics (LAs) are emerging as drugs for POP management because of their effectiveness and affordability. However, they exhibit short durations and toxicity. Various LAs delivery systems have been developed to compensate for these disadvantages. They have been designed to continuously release a safe dose in a single administration to ensure minimal toxicity and prolong pain relief. LAs delivery systems can also be designed to control the duration and intensity of analgesia to achieve on-demand analgesia, and significantly improve pain relief and patient satisfaction. In this paper, we summarize POP pathways, animal models, and methods for POP testing and highlight LAs delivery systems for POP management.

The mechanisms and management of persistent postsurgical pain

An estimated 10%-50% of patients undergoing a surgical intervention will develop persistent postsurgical pain (PPP) lasting more than 3 months despite adequate acute pain management and the availability of minimally invasive procedures. The link between early and late pain outcomes for surgical procedures remains unclear-some patients improve while others develop persistent pain. The elective nature of a surgical procedure offers a unique opportunity for prophylactic or early intervention to prevent the development of PPP and improve our understanding of its associated risk factors, such as pre-operative anxiety and the duration of severe acute postoperative pain. Current perioperative pain management strategies often include opioids, but long-term consumption can lead to tolerance, addiction, opioid-induced hyperalgesia, and death. Pre-clinical models provide the opportunity to dissect mechanisms underpinning the transition from acute to chronic, or persistent, postsurgical pain. This review highlights putative mechanisms of PPP, including sensitisation of peripheral sensory neurons, neuroplasticity in the central nervous system and nociceptive signalling along the neuro-immune axis.

Pain condition and sex differences in the descending noradrenergic system following lateral hypothalamic stimulation

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LH stimulation produced pronociceptive and antinociceptive effects from alpha-adrenoceptors in naïve male and female rats.

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LH stimulation produced pronociceptive and antinociceptive effects from alpha-adrenoceptors in male CCI rats.

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LH stimulation produced alpha-adrenoceptor-mediated pronociception, but not antinociception in female CCI rats.

The lateral hypothalamus (LH) is known to modulate nociception via the descending noradrenergic system in acute nociception, but less is known about its role in neuropathic pain states. In naïve females, LH stimulation produces opposing effects of α-adrenoceptors, with α₂-adrenoceptors mediating antinociception, while pronociceptive α₁-adrenoceptors attenuate the effect. Whether this opposing response is seen in neuropathic conditions or in naïve males is unknown. We used a mixed factorial design to compare male and female rats with chronic constriction injury (CCI) to naïve rats, measured by Total Paw Withdrawal (TPW) responses to a thermal stimulus. Rats received one of three doses of carbachol to stimulate the LH followed by intrathecal injection of either an α₁- or an α₂-adrenoceptor antagonist (WB4101 or yohimbine, resp.) or saline for control. Overall, naïve rats showed a more pronounced opposing alpha-adrenergic response than CCI rats (p < 0.04). Naïve male and female rats demonstrated antinociception following α₁-adrenoceptor blockade and hyperalgesia following α₂-adrenoceptor blockade. Male CCI rats also showed dose dependent effects from either WB4101 or yohimbine (p < 0.05), while female CCI rats had significant antinociception from WB4101 (p < 0.05), but no effect from yohimbine. These results support the idea that peripheral nerve damage differentially alters the descending noradrenergic modulatory system in male and female rats, and notably, that female CCI rats do not show antinociception from descending noradrenergic input. These findings are suggestive that clinical therapies that recruit the descending noradrenergic system may require a different approach based on patient gender.

Effect of Thoracic Epidural Anesthesia in a Rat Model of Phrenic Motor Inhibition after Upper Abdominal Surgery

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: One important example of impaired motor function after surgery is diaphragmatic dysfunction after upper abdominal surgery. In this study, the authors directly recorded efferent phrenic nerve activity and determined the effect of the upper abdominal incision. The authors hypothesized that phrenic motor output would be decreased after the upper abdominal incision; it was also hypothesized that blocking sensory input from the incision using thoracic epidural anesthesia would diminish this incision-induced change in phrenic motor activity.

METHODS

Efferent phrenic activity was recorded 1 h to 10 days after upper abdominal incision in urethane-anesthetized rats. Ventilatory parameters were measured in unanesthetized rats using whole-body plethysmography at multiple time points after incision. The authors then determined the effect of thoracic epidural anesthesia on phrenic nerve activity and ventilatory parameters after incision.

RESULTS

Phrenic motor output remained reduced by approximately 40% 1 h and 1 day after incision, but was not different from the sham group by postoperative day 10. One day after incision (n = 9), compared to sham-operated animals (n = 7), there was a significant decrease in spike frequency area-under-the-curve (median [interquartile range]: 54.0 [48.7 to 84.4] vs. 97.8 [88.7 to 130.3]; P = 0.0184), central respiratory rate (0.71 [0.63 to 0.79] vs. 0.86 [0.82 to 0.93]/s; P = 0.0460), and inspiratory-to-expiratory duration ratio (0.46 [0.44 to 0.55] vs. 0.78 [0.72 to 0.93]; P = 0.0023). Unlike humans, a decrease, not an increase, in breathing frequency has been observed after the abdominal incision in whole-body plethysmography. Thoracic epidural anesthesia attenuated the incision-induced changes in phrenic motor output and ventilatory parameters.

CONCLUSIONS

Upper abdominal incision decreased phrenic motor output and ventilatory parameters, and this incision-induced impairment was attenuated by thoracic epidural anesthesia. The authors' results provide direct evidence that afferent inputs from the upper abdominal incision induce reflex inhibition of phrenic motor activity.

Analgesic effects and pharmacologic mechanisms of the Gelsemium alkaloid koumine on a rat model of postoperative pain

Postoperative pain (POP) of various durations is a common complication of surgical procedures. POP is caused by nerve damage and inflammatory responses that are difficult to treat. The neuroinflammation-glia-steroid network is known to be important in POP. It has been reported that the Gelsemium alkaloid koumine possesses analgesic, anti-inflammatory and neurosteroid modulating activities. This study was undertaken to test the analgesic effects of koumine against POP and explore the underlying pharmacologic mechanisms. Our results showed that microglia and astroglia were activated in the spinal dorsal horn post-incision, along with an increase of proinflammatory cytokines (interleukin 1β, interleukin 6, and tumor necrosis factor α). Both subcutaneous and intrathecal (i.t.) koumine treatment after incision significantly prevented mechanical allodynia and thermal hyperalgesia, inhibited microglial and astroglial activation, and suppressed expression of proinflammatory cytokines. Moreover, the analgesic effects of koumine were antagonized by i.t. administration of translocator protein (18 kDa) (TSPO) antagonist PK11195 and GABAA receptor antagonist bicuculline. Together, koumine prevented mechanical allodynia and thermal hyperalgesia caused by POP. The pharmacologic mechanism of koumine-mediated analgesia might involve inhibition of spinal neuroinflammation and activation of TSPO. These data suggested that koumine might be a potential pharmacotherapy for the management of POP.

Impact of different analgesic depths and abdominal trauma of different severities on stress and recovery of rats undergoing total intravenous anesthesia

A number of animal models have been developed to examine the pathophysiological consequences of surgical procedures, but anesthetic methods, monitoring, and management measures in these models are very different from those used in humans. This study was designed to create a rat model of abdominal surgery using anesthetic methods and perioperative treatment similar to those used in the clinic and to investigate the effects of different injury severities and depths of anesthesia and analgesia on surgical stress and postoperative recovery. Abdominal skin/muscle incision was compared with exploratory laparotomy in rats under propofol intravenous anesthesia, accompanied by perioperative measures such as oxygen inhalation, fluid infusion, warmth, blood gas analysis, and infection prevention. Stress indices (mean arterial pressure, heart rate, blood glucose, and plasma corticosterone) were monitored during anesthesia and surgery, and recovery indicators (body weight, food consumption, and pain) were measured after surgery. In addition, animals undergoing laparotomy were subjected to low and high dosages of propofol and sufentanil, in order to examine the relationship between anesthetic and analgesic depth and stress on recovery. Exploratory laparotomy induced a greater stress response and caused slower postoperative recovery as measured than somatic injury. High-dose sufentanil downregulated plasma corticosterone and improved postoperative recovery more effectively than high-dose propofol (P<0.05). Taken together, a rat model of abdominal surgery using anesthetic methods and perioperative treatment similar to those used in the clinic was successfully developed. It showed a positive correlation between severity of surgical trauma and stress response and postoperative recovery and a significant role of adequate analgesia in reducing surgical stress and improving postoperative recovery.

Postoperative pain-from mechanisms to treatment

INTRODUCTION

Pain management after surgery continues to be suboptimal; there are several reasons including lack of translation of results from basic science studies and scientific clinical evidence into clinical praxis.

OBJECTIVES

This review presents and discusses basic science findings and scientific evidence generated within the last 2 decades in the field of acute postoperative pain.

METHODS

In the first part of the review, we give an overview about studies that have investigated the pathophysiology of postoperative pain by using rodent models of incisional pain up to July 2016. The second focus of the review lies on treatment recommendations based on guidelines and clinical evidence, eg, by using the fourth edition of the "Acute Pain Management: Scientific Evidence" of the Australian and New Zealand College of Anaesthetists and Faculty of Pain Medicine.

RESULTS

Preclinical studies in rodent models characterized responses of primary afferent nociceptors and dorsal horn neurons as one neural basis for pain behavior including resting pain, hyperalgesia, movement-evoked pain or anxiety- and depression-like behaviors after surgery. Furthermore, the role of certain receptors, mediators, and neurotransmitters involved in peripheral and central sensitization after incision were identified; many of these are very specific, relate to some modalities only, and are unique for incisional pain. Future treatment should focus on these targets to develop therapeutic agents that are effective for the treatment of postoperative pain as well as have few side effects. Furthermore, basic science findings translate well into results from clinical studies. Scientific evidence is able to point towards useful (and less useful) elements of multimodal analgesia able to reduce opioid consumption, improve pain management, and enhance recovery.

CONCLUSION

Understanding basic mechanisms of postoperative pain to identify effective treatment strategies may improve patients' outcome after surgery.

Muscle Reactive Oxygen Species (ROS) Contribute to Post-Incisional Guarding via the TRPA1 Receptor

BACKGROUND

Deep tissues and their afferents have unique responses to various stimuli and respond to injury distinctively. However, the types of receptors and endogenous ligands that have a key role in pain after deep tissue incision are unknown. TRPA1 has been shown to mediate pain-related responses in inflammation- and nerve injury-induced pain models. We hypothesized that TRPA1 has an important role in pain behaviors after deep tissue incision.

METHODS

The effect of various doses of intraperitoneal (i.p.) TRPA1 antagonist, HC-030031, on pain behaviors after skin + deep tissue incision of the rat hind paw was measured. In vivo reactive oxygen species (ROS)-imaging and hydrogen peroxide (H2O2) levels after incision were also evaluated. Separate groups of rats were examined for H2O2-evoked pain-related behaviors after injections into the deep tissue or the subcutaneous tissue.

RESULTS

Guarding pain behavior after skin + deep tissue incision was decreased by i.p. HC-030031. However, HC-030031 did not affect mechanical or heat responses after incision. Treatment either before or after incision was effective against incision-induced guarding behavior. ROS increased after skin + deep tissue incision in both the incised muscle and the skin. Tissue H2O2 also increased in both skin and muscle after incision. H2O2 injection produced pain behaviors when injected into muscle but not after subcutaneous injection.

CONCLUSIONS

This study demonstrates that TRPA1 antagonist HC-030031 reduced spontaneous guarding pain behavior after skin + deep tissue incision. These data indicate that TRPA1 receptors on nociceptors are active in incised fascia and muscle but this is not evident in incised skin. Even though endogenous TRPA1 agonists like ROS and H2O2 were increased in both incised skin and muscle, those in skin do not contribute to nociceptive behaviors. This study suggests that endogenous TRPA1 ligands and the TRPA1 receptor are important targets for acute pain from deep tissue injury.

Cutaneous tissue damage induces long-lasting nociceptive sensitization and regulation of cellular stress- and nerve injury-associated genes in sensory neurons

Tissue damage is one of the major etiological factors in the emergence of chronic/persistent pain, although mechanisms remain enigmatic. Using incision of the back skin of adult rats as a model for tissue damage, we observed sensitization in a nociceptive reflex enduring to 28days post-incision (DPI). To determine if the enduring behavioral changes corresponded with a long-term impact of tissue damage on sensory neurons, we examined the temporal expression profile of injury-regulated genes and the electrophysiological properties of traced dorsal root ganglion (DRG) sensory neurons. The mRNA for the injury/stress-hub gene Activating Transcription Factor 3 (ATF3) was upregulated and peaked within 4 DPI, after which levels declined but remained significantly elevated out to 28 DPI, a time when the initial incision appears healed and tissue-inflammation largely resolved. Accordingly, stereological image analysis indicated that some neurons expressed ATF3 only transiently (mostly medium-large neurons), while in others it was sustained (mostly small neurons), suggesting cell-type-specific responses. In retrogradely-traced ATF3-expressing neurons, Calcium/calmodulin-dependent protein kinase type IV (CAMK4) protein levels and isolectin-B4 (IB4)-binding were suppressed whereas Growth Associated Protein-43 (GAP-43) and Neuropeptide Y (NPY) protein levels were enhanced. Electrophysiological recordings from DiI-traced sensory neurons 28 DPI showed a significant sensitization limited to ATF3-expressing neurons. Thus, ATF3 expression is revealed as a strong predictor of single cells displaying enduring pain-related electrophysiological properties. The cellular injury/stress response induced in sensory neurons by tissue damage and indicated by ATF3 expression is positioned to contribute to pain which can occur after tissue damage.

Differences in carbachol dose, pain condition, and sex following lateral hypothalamic stimulation

Lateral hypothalamic (LH) stimulation produces antinociception in female rats in acute, nociceptive pain. Whether this effect occurs in neuropathic pain or whether male-female sex differences exist is unknown. We examined the effect of LH stimulation in male and female rats using conditions of nociceptive and neuropathic pain. Neuropathic groups received chronic constriction injury (CCI) to induce thermal hyperalgesia, a sign of neuropathic pain. Nociceptive rats were naive for CCI, but received the same thermal stimulus following LH stimulation. To demonstrate that CCI ligation produced thermal hyperalgesia, males and females received either ligation or sham surgery for control. Both males and females demonstrated significant thermal hyperalgesia following CCI ligation (p<0.05), but male sham surgery rats also showed a significant left-right difference not present in female sham rats. In the second experiment, rats randomly assigned to CCI or nociceptive groups were given one of three doses of the cholinergic agonist carbachol (125, 250, or 500 nmol) or normal saline for control, microinjected into the left LH. Paw withdrawal from a thermal stimulus (paw withdrawal latency; PWL) was measured every 5 min for 45 min. Linear mixed models analysis showed that males and females in both pain conditions demonstrated significant antinociception, with the 500-nmol dose producing the greatest effect across groups compared with controls for the left paw (p<0.05). Female CCI rats showed equivalent responses to the three doses, while male CCI rats showed more variability for dose. However, nociceptive females responded only to the 500-nmol dose, while nociceptive males responded to all doses (p<0.05). For right PWL, only nociceptive males showed a significant carbachol dose response. These findings are suggestive that LH stimulation produces antinociception in male and female rats in both nociceptive and neuropathic pain, but dose response differences exist based on sex and pain condition.

An overview of animal models of pain: disease models and outcome measures

Pain is ultimately a perceptual phenomenon. It is built from information gathered by specialized pain receptors in tissue, modified by spinal and supraspinal mechanisms, and integrated into a discrete sensory experience with an emotional valence in the brain. Because of this, studying intact animals allows the multidimensional nature of pain to be examined. A number of animal models have been developed, reflecting observations that pain phenotypes are mediated by distinct mechanisms. Animal models of pain are designed to mimic distinct clinical diseases to better evaluate underlying mechanisms and potential treatments. Outcome measures are designed to measure multiple parts of the pain experience, including reflexive hyperalgesia measures, sensory and affective dimensions of pain, and impact of pain on function and quality of life. In this review, we discuss the common methods used for inducing each of the pain phenotypes related to clinical pain syndromes as well as the main behavioral tests for assessing pain in each model.

PERSPECTIVE

Understanding animal models and outcome measures in animals will assist in translating data from basic science to the clinic.

Wound hypoxia in deep tissue after incision in rats

Our previous studies using rat models of incisional pain have shown that tissue lactate levels increase and pH decreases for several days after incision, suggesting the presence of an ischemic-like condition. The purpose of this study was to evaluate the time course and the extent of tissue hypoxia that develops in incised muscle and skin. We directly measured oxygen tension at several time points after incisions of the gastrocnemius muscle, the paraspinal skin, and the plantar hindpaw in anesthetized rats using an oxygen-sensitive microelectrode. In vivo hypoxia of the incised tissues was also evaluated immunohistochemically using a hypoxia marker, pimonidazole hydrochloride. To minimize intersubject variability, unincised contralateral tissues were used as a control. Tissue oxygen tension was decreased in both skeletal muscle and skin compared with control, for several days after incision. When measured directly, oxygen tension decreased immediately and remained low for several days after incisions. Pimonidazole immunostaining revealed hypoxic areas in incised muscle and skin for several days. By postoperative day 10, tissue oxygen tension recovered to that of control tissue. These results support the evidence that a hypoxic condition is present in deep tissue after incisions and that an ischemic-like mechanism may contribute to postoperative pain.

The pathophysiology of acute pain: animal models

PURPOSE OF REVIEW

Trauma, surgery, and burns are three common clinical scenarios that are associated with significant acute pain. This review describes the pathophysiology of acute pain utilizing three preclinical models: surgery, burn, and fracture.

RECENT FINDINGS

In general, there is greater interest directed toward peripheral mediators of acute pain. Studies indicate that treatment against nerve growth factor, interleukins, and ischemic-like mediators may provide valuable avenues for treatment of acute pain. By targeting the periphery, analgesic therapies may have reduced side-effects.

SUMMARY

Peripheral mediators of acute pain can vary depending upon the type of injury. Treatment aimed toward those mediators specific to the injury may improve acute pain management in the future. It will be important to translate these findings into clinical trials in the future.

Acid-sensing ion channels in postoperative pain

Iatrogenic pain consecutive to a large number of surgical procedures has become a growing health concern. The etiology and pathophysiology of postoperative pain are still poorly understood, but hydrogen ions appear to be important in this process. We have investigated the role of peripheral acid-sensing ion channels (ASICs), which form depolarizing channels activated by extracellular protons, in a rat model of postoperative pain (i.e., hindpaw skin/muscle incision). We report high levels of ASIC-type currents (∼ 77%) in sensory neurons innervating the hindpaw muscles, with a prevalence of ASIC3-like currents. The ASIC3 protein is largely expressed in lumbar DRG neurons innervating the plantar muscle, and its mRNA and protein levels are increased by plantar incision 24 h after surgery. Pharmacological inhibition of ASIC3 channels with the specific toxin APETx2 or in vivo knockdown of ASIC3 subunit by small interfering RNA led to a significant reduction of postoperative spontaneous, thermal, and postural pain behaviors (spontaneous flinching, heat hyperalgesia, and weight bearing). ASIC3 appears to have an important role in deep tissue but also affects prolonged pain evoked by skin incision alone. The specific homomeric ASIC1a blocker PcTx1 has no effect on spontaneous flinching, when applied peripherally. Together, these data demonstrate a significant role for peripheral ASIC3-containing channels in postoperative pain.

[Predictors of chronic pain following surgery. What do we know?]

Chronic postoperative pain is known to be a significant clinical and economic problem. The estimated mean incidence is high and varies between 10 and 50%, with variations mostly related to procedure-specific conditions. High-risk types of surgeries are e.g. thoracotomy, breast or inguinal hernia surgery and amputations. Although there is increasing knowledge about the incidence of chronic postoperative pain after certain types of surgical procedures, there are only limited data related to the mechanisms and pathophysiology leading to chronic pain after surgery. Neuropathic pain components have been discussed, especially following operations with a high incidence of nerve damage (for example axillary lymphadenectomy). Besides surgical factors it seems that there are a number of other factors which likely increase the risk of chronic postoperative pain. These predictors for the development of chronic postoperative pain are multiple and include individual genetic factors, age and sex of the individual patient, preoperative chronic pain, psychosocial factors, neurophysiological factors, intraoperative nerve and muscle damage, postoperative complications and acute pain in the early postoperative period. Quantitative sensory testing including tests of inhibitory circuits like DNIC might help to predict the risk of individual patients even before surgery has started. The perioperative identification of patients who are at high risk for developing chronic pain after surgery is therefore a major goal for the future. This may help to develop preventive treatment strategies and avoid treatments with side effects for patients who are not at risk for developing chronic pain after surgery. Due to a lack of appropriate data for sufficient preventive approaches an effective postoperative acute pain management and a nerve-conserving surgical technique are the major keys in the prophylaxis of chronic postoperative pain.

Botulinum neurotoxin for pain management: insights from animal models

The action of botulinum neurotoxins (BoNTs) at the neuromuscular junction has been extensively investigated and knowledge gained in this field laid the foundation for the use of BoNTs in human pathologies characterized by excessive muscle contractions. Although much more is known about the action of BoNTs on the peripheral system, growing evidence has demonstrated several effects also at the central level. Pain conditions, with special regard to neuropathic and intractable pain, are some of the pathological states that have been recently treated with BoNTs with beneficial effects. The knowledge of the action and potentiality of BoNTs utilization against pain, with emphasis for its possible use in modulation and alleviation of chronic pain, still represents an outstanding challenge for experimental research. This review highlights recent findings on the effects of BoNTs in animal pain models.

Serotonin receptors are involved in the spinal mediation of descending facilitation of surgical incision-induced increase of Fos-like immunoreactivity in rats

Background

Descending pronociceptive pathways may be implicated in states of persistent pain. Paw skin incision is a well-established postoperative pain model that causes behavioral nociceptive responses and enhanced excitability of spinal dorsal horn neurons. The number of spinal c-Fos positive neurons of rats treated intrathecally with serotonin, noradrenaline or acetylcholine antagonists where evaluated to study the descending pathways activated by a surgical paw incision.

Results

The number of c-Fos positive neurons in laminae I/II ipsilateral, lamina V bilateral to the incised paw, and in lamina X significantly increased after the incision. These changes: remained unchanged in phenoxybenzamine-treated rats; were increased in the contralateral lamina V of atropine-treated rats; were inhibited in the ipsilateral lamina I/II by 5-HT_1/2B/2C (methysergide), 5-HT_2A (ketanserin) or 5-HT_{1/2A/2C/5/6/7} (methiothepin) receptors antagonists, in the ipsilateral lamina V by methysergide or methiothepin, in the contralateral lamina V by all the serotonergic antagonists and in the lamina X by LY 278,584, ketanserin or methiothepin.

Conclusions

We conclude: (1) muscarinic cholinergic mechanisms reduce incision-induced response of spinal neurons inputs from the contralateral paw; (2) 5-HT_1/2A/2C/3 receptors-mediate mechanisms increase the activity of descending pathways that facilitates the response of spinal neurons to noxious inputs from the contralateral paw; (3) 5-HT_1/2A/2C and 5-HT_1/2C receptors increases the descending facilitation mechanisms induced by incision in the ipsilateral paw; (4) 5-HT_2A/3 receptors contribute to descending pronociceptive pathways conveyed by lamina X spinal neurons; (5) α-adrenergic receptors are unlikely to participate in the incision-induced facilitation of the spinal neurons.

Animal models of acute surgical pain

Animal models of tissue injury have been used to investigate the mechanisms of pain. Here, we describe a variety of animal models that have been used to mimic acute surgical pain in human subjects, which include the plantar, tail, and gastrocnemius incision models. We also provide discussion on animal models of laparotomy, thoracotomy, visceral pain, and bone injury. Preclinical studies using these models have provided insights into the mechanisms and causes of acute surgical pain as well as the treatment options to control postsurgical pain.

Lasting reduction of postsurgical hyperalgesia after single injection of botulinum toxin type A in rat

A single injection of low doses of botulinum toxin type A (3.5 U/kg) completely abolished secondary mechanical hyperalgesia throughout its duration in a model of post surgical pain after gastrocnemius incision in rat.

Chemosensitivity and mechanosensitivity of nociceptors from incised rat hindpaw skin

BACKGROUND

The authors have demonstrated a decrease in pH in the incisional wound environment, suggesting a possible contribution of low pH to postsurgical pain. In this study, the authors characterized the acid-responsiveness of nociceptors innervating the plantar aspect of the rat hind paw 1 day after plantar incision and compared this to plantar skin from unincised control rats.

METHODS

Using the rat glabrous in vitro skin-tibial nerve preparation, afferent nerve activities from single mechanosensitive nociceptors were recorded. Differences in mechanosensitivity, spontaneous activity, and chemosensitivity of units were evaluated. For chemosensitivity, acid-responsiveness of nociceptors to lactic acid (pH 5.5 to 6.5) was studied.

RESULTS

C-fibers showed dose-dependent, sustained responses to lactic acid. A greater proportion of C-fibers from 2 mm or less from the incision was activated by pH 6.0 lactic acid (52.9%) compared to control (14.3%). Total evoked potentials during acid exposure were greater in C-fibers innervating 2 mm or less from the incision compared to those in unincised skin. The prevalence of acid responses and total evoked potentials during acid exposure in C-fibers innervating more than 2 mm from the incision were not different from control. Few A-fibers responded to lactic acid, with a range of pH 5.5 to 6.5 in both incision and control groups. Increased spontaneous activity and mechanosensitivity were also evident.

CONCLUSIONS

C-fibers in the vicinity of the incision showed qualitatively and quantitatively greater chemosensitivity to pH 6.0 lactic acid compared to control. This change was localized to 2 mm or less from the incision, suggesting increased chemosensitivity of nociceptive C-fibers 1 day after plantar incision.

Comparison of skin incision vs. skin plus deep tissue incision on ongoing pain and spontaneous activity in dorsal horn neurons

Surgery injures both skin and deep tissue causing pain at rest and evoked pain with activities. In this study, we examined the extent of injury by incision and dorsal horn neuron (DHN) spontaneous activity (SA) in rats that underwent a sham operation, skin incision or skin plus deep tissue incision. Pain behaviors were measured 1 day later followed by DHN recordings in the same rats. On postoperative day (POD) 1, guarding pain, assessed with an abbreviated pain score, was increased in the skin plus deep tissue incision group (7.0+/-0.7 vs. 0.1+/-0.6 in control, P<0.001), but not in the skin incision group (1.8+/-1.0); yet, mechanical and heat hyperalgesia were similar in both incised groups. In the rats that underwent skin plus deep tissue incision, more DHNs expressed SA (78.1% vs. 35.7% in control, P<0.01) and SA rate also tended to be greater (13.8+/-2.9 vs. 5.6+/-2.0 imp/s). Bupivacaine infiltration into the incision decreased SA in both skin incision and skin plus deep tissue incision (POD1) groups to the same level as in the sham-operated rats. In a separate group of rats that underwent skin plus deep tissue incision, guarding pain was not present (0.1+/-0.6) on POD7 and the percentage and rate of DHN SA were the same as in the sham control. These data demonstrate that incised deep tissue rather than skin is critical for the development of guarding pain and increased SA of DHNs. Skin incision alone is sufficient for primary mechanical and heat hyperalgesia.

New advances in musculoskeletal pain

Non-malignant musculoskeletal pain is the most common clinical symptom that causes patients to seek medical attention and is a major cause of disability in the world. Musculoskeletal pain can arise from a variety of common conditions including osteoarthritis, rheumatoid arthritis, osteoporosis, surgery, low back pain and bone fracture. A major problem in designing new therapies to treat musculoskeletal pain is that the underlying mechanisms driving musculoskeletal pain are not well understood. This lack of knowledge is largely due to the scarcity of animal models that closely mirror the human condition which would allow the development of a mechanistic understanding and novel therapies to treat this pain. To begin to develop a mechanism-based understanding of the factors involved in generating musculoskeletal pain, in this review we present recent advances in preclinical models of osteoarthritis, post-surgical pain and bone fracture pain. The models discussed appear to offer an attractive platform for understanding the factors that drive this pain and the preclinical screening of novel therapies to treat musculoskeletal pain. Developing both an understanding of the mechanisms that drive persistent musculoskeletal pain and novel mechanism-based therapies to treat these unique pain states would address a major unmet clinical need and have significant clinical, economic and societal benefits.

Characterization of a model of persistent postoperative pain evoked by skin/muscle incision and retraction (SMIR)

Various surgical procedures, e.g. thoracotomy and inguinal hernia repair, frequently evoke persistent pain lasting for many months following the initial surgery. The essential prolonged tissue retraction required during such surgeries may account for the persistence and high incidence of postoperative pain in these patient populations. This study describes a new rat model of persistent postoperative pain evoked by skin/muscle incision and retraction (SMIR), akin to a clinical procedure. Under anaesthesia, skin and superficial muscle of the medial thigh were incised and a small pair of retractors inserted. This tissue was retracted for 1h causing potential stretch of the saphenous nerve. SMIR surgery evoked persistent significant mechanical hypersensitivity to von Frey stimulation of the plantar ipsilateral hindpaw, compared to either pre-surgery responses or concurrent responses of sham-operated rats. SMIR-evoked mechanical hypersensitivity was observed by postoperative day 3, most prominent between postoperative days 10 and 13, persisted until at least postoperative day 22 and had dissipated by postoperative day 32. Overall, mechanical sensitivity of the SMIR contralateral paw and the sham ipsilateral paw did not significantly change from pre-surgery responses. SMIR did not evoke significant heat hyperalgesia or cold allodynia. Light microscopy of saphenous nerve sections did not show degeneration or oedema in the saphenous nerve at, or proximally or distally to, the surgical site. In addition, very little to no degeneration was detected with ATF3 staining in DRG from SMIR-operated rats. These data suggest that prolonged retraction of superficial tissue evokes a persistent pain syndrome that is not driven by neuronal damage.

Perioperative pain

PURPOSE OF REVIEW

Incisional pain remains underevaluated and undermanaged while evidence is growing that perioperative treatments strongly influence patients' outcome. The present review examines the recent developments in mechanisms underlying perioperative pain and questions current understanding of incisional pain features observed in patients.

RECENT FINDINGS

Experimental models of incisional pain have highlighted specific mechanisms underlying perioperative pain plasticity, i.e. sensitization of sensory processing, clinically expressed by the development of hyperalgesia. In patients, however, the long-term impact of sensory system sensitization, e.g. development of residual pain, as well as the effects of perioperative pain management on sensitization, are still poorly understood, partly because most of the trials assess only the subjective experience of pain (by evaluation of either pain intensity or analgesic needs) instead of directly measuring objective changes (i.e. hyperalgesia) with quantitative sensory testing.

SUMMARY

Experimental studies and recent clinical trials using objective measures of sensory processing sensitization induced by surgical incision have shown the importance of hyperalgesia in perioperative pain. Effective perioperative block of nociceptive inputs from the wound as well as use of antihyperalgesic and analgesic drugs in combination seem the best way to control postoperative pain and specifically to prevent central sensitization.

Lactate Concentrations in Incisions Indicate Ischemic-like Conditions May Contribute to Postoperative Pain

The substances in wounds that cause incisional pain and hyperalgesia after surgery are poorly understood. We have developed and characterized rat models for incision-induced pain behaviors and measured increased tissue hydrogen ion concentration. Because lactate may facilitate nociceptor responses to low pH and contribute to ischemic pain mechanisms, we measured tissue lactate after incision of the plantar region of the hindpaw, gastrocnemius muscle, and paraspinal region in halothane anesthetized rats using in vivo microdialysis. Incisions were performed at 1 site (plantar, gastrocnemius, or paraspinal incision) in each rat. The corresponding contralateral side was used as the control. In anesthetized rats, a microdialysis fiber was passed into the incision and the control side. L-Lactate was measured using the lactate oxidase method. Tissue concentration was determined from postoperative day 0 to postoperative day 14 using the no net flux method. Lactate was increased on the day of hindpaw incision to 3.6 +/- 1.6 mmol/L compared with control (2.1 +/- .6 mmol/L) and remained increased through 7 days. In the gastrocnemius muscle, lactate was increased the day after incision (4.2 +/- 1.2 mmol/L vs 1.7 +/- .5 mmol/L) until postoperative day 7. On the day of the paraspinal incision, lactate was 3.4 +/- 1.1 mmol/L on the operated side and 2.2 +/- .6 mmol/L in the control side. Lactate remained increased through postoperative day 8 at the paraspinal incision. These experiments demonstrate that incision of the plantar hindpaw, the gastrocnemius muscle, and the paraspinal region increased tissue lactate concentration. The wound environment contains increased lactate at the same time that pH is decreased; lactate could potentially facilitate nociceptor activation by low pH and contribute to pain after surgery.

PERSPECTIVE

This study demonstrates that lactate is increased in wounds when pain behaviors and acid are increased. Lactate and low pH are present in incisions and indicate an ischemic pain mechanism that may contribute to postsurgical pain.

Spinale Glutamatrezeptorantagonisten: Differenzierung von primärer und sekundärer mechanischer Hyperalgesie nach operativer Schnittinzision im Tierexperiment

Secondary mechanical hyperalgesia has been demonstrated in postoperative patients indicating that central sensitization occurs after surgery. However, the underlying mechanisms are unknown. Here, we studied the role of spinal AMPA/kainate receptors for pain behaviors indicating secondary hyperalgesia caused by gastrocnemius incision in the rat. These were reduced by NBQX, a selective antagonist of AMPA/kainate receptors. However, administration of NMDA receptor antagonists caused no or only a modest decrease in behaviors for secondary hyperalgesia but produced associated motor deficits and supraspinal side effects. We further determined that only secondary mechanical hyperalgesia was reversed by JSTX, a selective antagonist of calcium-permeable AMPA receptor; primary mechanical hyperalgesia and guarding behavior were unchanged. These findings indicate that JSTX influenced a spinal amplification process that leads to secondary hyperalgesia but does not contribute to primary hyperalgesia and guarding after incision. This amplification process likely requires Ca(2) influx through spinal AMPA/KA (but not NMDA) receptors. Behaviors for secondary mechanical hyperalgesia after incision can be inhibited without affecting primary mechanical hyperalgesia and guarding. Mechanisms for central sensitization causing secondary hyperalgesia in postoperative patients may therefore be separated from spontaneous pain and hyperalgesia that arises adjacent to the area of the incision.

A model of incisional pain: the effects of dermal tail incision on pain behaviours of Sprague Dawley rats

Hyperalgesia, a component of post-operative pain, is an enhanced responsiveness to painful challenges after the tissue damage caused by an incision. It is important to understand the mechanisms involved in the development of incisional pain, in order to treat the condition appropriately. The aim of this study was to develop a model of post-operative pain using the rat's tail. Under halothane-induced anaesthesia, female Sprague Dawley rats underwent 10mm longitudinal incisions through skin and fascia (n = 10) or 20 mm incisions through skin, fascia and muscle (n = 10) of the mid-portion of the tail. Control rats were only anaesthetised (n = 14). Withdrawal latencies to noxious mechanical and thermal challenges were recorded daily. A bar algometer was placed onto and 15 mm proximal to the incision with a force of 4N and the tail was immersed in 49 degrees C water. Daily withdrawal latencies were compared to pre-incision values using one way analysis of variance (ANOVA) with Dunnett's post-hoc test. Primary mechanical hyperalgesia lasted for 6 days after the 10 mm incisions (P < 0.0001) and for 7 days after the 20 mm incisions (P < 0.0001). Secondary mechanical hyperalgesia persisted for 1 day after the 10 mm incisions (P = 0.0013) and for 2 days after the 20 mm incisions (P = 0.0028). Thermal hyperalgesia was not elicited. This model is suitable to examine the mechanisms involved in post-operative pain.

Heat hyperalgesia after incision requires TRPV1 and is distinct from pure inflammatory pain

Postoperative pain significantly impacts patient recovery. However, postoperative pain management remains suboptimal, perhaps because treatment strategies are based mainly on studies using inflammatory pain models. We used a recently developed mouse model of incisional pain to investigate peripheral and spinal mechanisms contributing to heat hyperalgesia after incision. Behavioral experiments involving TRPV1 KO mice demonstrate that, as previously observed in inflammatory models, TRPV1 is necessary for heat (but not mechanical) hyperalgesia after incision. However, in WT mice, neither the proportion of TRPV1 immunoreactive neurons in the DRG nor the intensity of TRPV1 staining in the sciatic nerve was different from that in controls up to 4 days after incision. This result was corroborated by immunoblot analysis of sciatic nerve in rats subjected to an incision, and is distinct from that following inflammation of the rat hind paw, a situation in which TRPV1 expression levels in sciatic nerve increases. In the absence of heat exposure, spinal c-Fos staining was similar between incised TRPV1 KO and WT mice. However, differences in c-Fos staining between heat exposed TRPV1 KO and WT mice after incision suggest that the incision-mediated enhancement of heat-evoked signaling to the spinal cord involves a TRPV1-dependent mechanism. Finally, heat hyperalgesia after incision was reversed by antagonism of spinal non-NMDA receptors, unlike inflammatory hyperalgesia, which is mediated via NMDA receptors . Thus, TRPV1 is important for the generation of thermal hyperalgesia after incision. Our observations suggest that all experimental pain models may not be equally appropriate to guide the development of postoperative pain therapies.

Spinal administration of MK-801 and NBQX demonstrates NMDA-independent dorsal horn sensitization in incisional pain

Surgery commonly causes pain and neural plasticity that are unique compared to other persistent pain problems. To more precisely study central sensitization and plasticity, we examined the role of ionotropic EAA receptors in dorsal horn neuron sensitization early after incision. Sensitization, in the form of increased background activity, increased mechanosensitivity or pinch receptive field expansion, was induced by plantar incision 1 h later in 30 neurons. (+)-5-Methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine (MK-801) or 1 mM 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo[f]quinoxaline-7-sulfonamide (NBQX) was administered through a microdialysis fiber to block NMDA and nonNMDA EAA receptors, respectively. Dorsal horn neuron sensitization was reexamined 1 h later. Spinal administration of NBQX blocked AMPA-induced excitation but did not affect excitation by NMDA. NBQX decreased background activity in the neurons that developed sustained increased activity after incision. The median decrease caused by NBQX was from 2.3 to 0.0 imp/s. Spinal administration of 5 mM MK-801 blocked NMDA-induced excitation but did not affect excitation by AMPA. The median change (from 2.6 to 1.1 imp/s) in background activity increased by incision was not significantly affected by MK-801. The responses to mechanical stimuli were enhanced after incision in wide dynamic range (WDR) neurons. NBQX eliminated these responses but MK-801 had no effect. The pinch receptive field (RF) expansion into uninjured areas of the paw and hindquarters occurred after incision. Only 1 of 13 neurons exhibited RF expansion after spinal NBQX administration; 9 of 12 neurons had RF expansion remaining after MK-801. Thus, nonNMDA receptors are critical and NMDA-independent factors influence the increased responsiveness of dorsal horn neurons that occur early after incision.

Neurokinin-1 receptor gene expression in the mouse dorsal horn increases with neuropathic pain

Peripheral nerve injury is associated with hyperesthesia and increased neurokinin-1 receptor (NK-1) expression in the dorsal horn of the spinal cord. To test the hypothesis that NK-1 gene expression underlies these responses, we used solution hybridization-nuclease protection assays to quantify NK-1 mRNA levels in dorsal quadrants of the mouse lumbar dorsal horn. Partial sciatic nerve ligation was associated with mechanical allodynia, thermal hyperalgesia, and an increase in NK-1 mRNA on the ipsilateral, but not contralateral, side. Regression analysis showed that NK-1 mRNA was significantly correlated with thermal paw withdrawal latency but not mechanical threshold. Our results support the idea that substance P is an important mediator of thermal hypersensitivity in the setting of nerve injury and suggest that increased NK-1 receptor transcription precedes increased NK-1 receptor density, ultimately leading to behavioral hypersensitivity to peripheral thermal stimulation.

PERSPECTIVE

The therapeutic efficacy of NK-1 receptor antagonists is unclear. The current data suggest that peripheral nerve injury increases the expression of substance P (NK-1) receptors in the spinal cord dorsal horn; this is correlated with heat hypersensitivity. The analgesic effects of NK-1 antagonists might become apparent if tested against heat-evoked pain in nerve injury patients.

A new rodent model of hind limb penetrating wound injury characterized by continuous primary and secondary hyperalgesia

This article reports the development of a new hind limb pain model in which an incisional stab wound is placed on the front and back of the calf, causing both superficial and deep tissue injury. The injury causes primary mechanical hyperalgesia on the calf and secondary hind paw hyperalgesia, which served as the focus of the present study. Animals with unilateral stab wounds showed a significant increase in percent paw withdrawal (secondary mechanical hyperalgesia, reversed by morphine administration) from 2 to 48 hours after surgery, but no evidence of thermal hyperalgesia. In contrast, animals with bilateral leg injuries showed bilateral secondary mechanical and thermal hyperalgesia. Rats with unilateral leg incisional stab wounds showed a significant decrease in cage activity in both the horizontal and vertical directions, monitored by using a novel activity box approach, as compared to their 24-hour baseline levels or to the activity of naïve animals. Analysis of spinal cord Fos labeling demonstrated that calf injury significantly increased Fos expression in laminae I to VI of the L3-L5 cord segments. The data indicate that this model might be useful for evaluation of the mechanisms underlying penetrating injury-induced primary and secondary hyperalgesia or for testing the effect of analgesics on hyperalgesia induced by such injury.

PERSPECTIVE

Stab wounds and other types of penetrating wounds routinely encountered in emergency rooms and clinics are accompanied by pain associated with superficial and deep tissue injury. Here we present a rodent stab wound model that affords an opportunity to study the mechanisms of pain associated with traumatic injury.

Spinal glutamate receptor antagonists differentiate primary and secondary mechanical hyperalgesia caused by incision

Secondary mechanical hyperalgesia has been demonstrated in postoperative patients indicating that central sensitization occurs after surgery. However, the underlying mechanisms are unknown. Here, we studied the role of spinal N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)/kainate receptors for pain behaviors indicating secondary hyperalgesia caused by gastrocnemius incision in the rat. We further determined if Ca(2+) permeable AMPA/kainate receptors are important for secondary hyperalgesia after gastrocnemius incision and for pain behaviors indicating primary hyperalgesia and guarding behavior after plantar incision. Withdrawal thresholds (WTs) to punctate mechanical stimuli were assessed by applying calibrated monofilaments to the plantar hind paw before gastrocnemius incision. WTs were tested again 2 h after gastrocnemius incision and again after intrathecal (IT) injection of either dizocilpine maleate (MK-801), 2-amino-5-phosphonovaleric acid (AP5), 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo[f]quinoxaline-7-sulfonamide (NBQX), or Joro spider toxin (JSTX). The doses used were: MK-801 (vehicle, 15, 30, 40 nmol), AP5 (vehicle, 10, 30 nmol), NBQX (vehicle, 5, 10 nmol), and JSTX (vehicle, 2, 5, 9 nmol). In the same rats, WTs were tested on postoperative day 2 before and after the same drugs were injected again. In other rats, WTs to monofilaments and response frequencies to a non-punctate mechanical stimulus or guarding behaviors were determined before, 1 h after plantar incision was made, and assessed again after JSTX (9 nmol or vehicle) was administered IT. Secondary mechanical hyperalgesia after gastrocnemius incision was dose-dependently blocked by NBQX but was only marginally affected by AP5 or MK-801. Only secondary mechanical hyperalgesia was reversed by JSTX; primary mechanical hyperalgesia and guarding behavior were unchanged. These results indicate that spinal sensitization contributing to behaviors for secondary hyperalgesia after incision requires Ca(2+) permeable AMPA/kainate receptors. The data further demonstrate that behaviors for secondary mechanical hyperalgesia after incision can be inhibited without affecting behaviors for primary mechanical hyperalgesia and guarding. Mechanisms for central sensitization causing secondary hyperalgesia in postoperative patients may therefore be separated from spontaneous pain and hyperalgesia that arises adjacent to the area of the incision.

Pharmacological characterisation of a rat model of incisional pain

1. Both clinical and preclinical models of postsurgical pain are being used more frequently in the early evaluation of new chemical entities. In order to assess the validity and reliability of a rat model of postincisional pain, the effects of different classes of clinically effective analgesic drugs were evaluated against multiple behavioural end points. 2. Following surgical incision, under general anaesthesia, of the plantar surface of the rat hind paw, we determined the time course of mechanical hyperalgesia, tactile allodynia and hind limb weight bearing using the Randall-Selitto (paw pressure) assay, electronic von Frey and dual channel weight averager, respectively. Behavioural evaluations began 24 h following surgery, and were continued for 9-14 days. 3. Mechanical hyperalgesia, tactile allodynia and a decrease in weight bearing were present on the affected limb within 1 day of surgery with maximum sensitivity 1-3 days postsurgery. Accordingly, we examined the effect of nonsteroidal antiinflammatory drugs (NSAIDs), morphine and gabapentin, on established hyperalgesia and allodynia, 1 day following plantar incision.4. In accordance with previous reports, both systemic morphine and gabapentin administration reversed mechanical hyperalgesia and tactile allodynia in the incised rat hind paw. Both drugs were more potent against mechanical hyperalgesia than tactile allodynia. 5. All of the NSAIDs tested, including cyclooxygenase 2 selective inhibitors, reversed mechanical hyperalgesia and tactile allodynia in the incised rat hind paw. The rank order of potency for both hyperalgesia and allodynia was indomethacin > celecoxib > etoricoxib > naproxen. 6. We have investigated the potency and efficacy of different classes of analgesic drugs in a rat model of postincisional pain. The rank order of potency for these drugs reflects their utility in treating postoperative pain in the clinic. As these compounds showed reliable efficacy across two different behavioural end points, the Randall-Selitto (paw pressure) assay and electronic von Frey, these methods may prove useful in the study of postsurgical pain and the assessment of novel treatments.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).