Srinivasa N. Raja - US grants

We determined in preliminary studies that the responses to cutaneous heat stimuli of nociceptive primary afferents in the monkey were substantially affected by the choice of anesthetic. These results challenge the commonly held assumption that general anesthetics do not after the response of primary afferents. The purpose of this proposed research is to characterize further the effect of anesthetics on the response properties of primary somatosensory afferents. A major objective will be to determine which anesthetic has the least effect on the responses of nociceptive afferents and thus can be used to model the awake animal in neurophysiological studies. Using single fiber recording techniques, we will examine the responses of nociceptive afferents not only to heat, but also to mechanical stimuli under several anesthetic conditions. We will determine if ancillary factors such as blood pressure, core and skin temperature, sympathetic tone, and acid-base balance contribute to the anesthetic effect. This research is fundamental to advancing knowledge of the somatosensory system, and in particular pain sensation. This research may also provide further clues as to the mechanism by which anesthetics affect the nervous system.

Description: (Adapted from investigator's abstract) Chronic pain is a disabling disease that affects a large proportion of the U.S. population. An intriguing pain syndrome that results from trauma, often with associated peripheral nerve injury, is sympathetically maintained pain (SMP). SMP is characterized by pain and hyperalgesia to mechanical and thermal stimuli that are dependent on sympathetic innervation of the painful region. Based on advances made during the last granting period, a model has been developed to explain the pathophysiology and pharmacology of SMP. The proposed studies will test a tenet of the model that a1 adrenoceptors develop in peripheral tissues such that the release of norepinephrine from the sympathetic terminals activates nociceptors and leads to pain. Parallel studies will be done in patients and in a recently developed nerve injury paradigm for SMP in rats. The proposed clinical and behavioral studies will address two questions: (1) Is a peripheral a-adrenergic receptor the culprit in SMP? To answer this question, the pain and hyperalgesia produced by intradermally administered alpha1-, alpha2- and beta-adrenergic agonists and a adrenergic blockers will be compared in normal subjects and in patients with SMP or with sympathetically independent pain (SIP). The adrenergic pharmacology of SMP will be further characterized with behavioral studies in rat using a partial sciatic nerve injury paradigm. Earlier studies have indicated that the pain behavior in this rat paradigm is sympathetically maintained. The proposed behavioral studies will also provide information about the site of interaction between the sympathetic efferents and the somatic afferent fibers. (2) Can alpha-adrenergic drugs be used in the diagnosis of SMP? During the past granting period, a diagnostic test for SMP was developed based on the pain relief obtained during intravenous administration of phentolamine. In the proposed studies, the relief of pain and hyperalgesia in SMP following topical administration of clonidine will be investigated as a less invasive diagnostic tool. The proposed studies will lead to a better understanding of the pathophysiology and pharmacology of SMP and to the development of more specific diagnostic tests and pharmacological therapies for alleviating the pain and hyperalgesia in this debilitating pain state.

The long range objective of this project is to develop better pharmacological strategies for treatment of neuropathic pain. Our specific goal here is to study the efficacy of opioid therapy for treatment of neuropathic pain. Because tricyclic antidepressant therapy (e.g. amitriptyline) is often considered the drug of choice for treatment of neuropathic pain, we wish to compare opioid therapy with tricyclic therapy. We have elected to focus in this study on post-herpetic neuralgia (PHN). We do this because PHN is common, PHN provides a homogeneous disease population, PHN has been studied in other clinical research trials, and present therapies are inadequate. Although tricyclic antidepressants have been used for the treatment of PHN, their effects on affective and cognitive functions have not been systematically assessed. In our preliminary studies open-label trials with opiates for treatment of PHN have yielded favorable outcomes. We propose to compare placebo, amitriptyline, and a long-acting preparation of morphine with regard to pain relief, and effects on affective and cognitive function. Effects of treatment, including complications, will be correlated with measures of lifetime and current psychiatric co-morbidity, particularly depression. Patients (N=120) with persistent pain for 3 months or longer will be enrolled in this randomized, blinded, placebo-controlled, cross- over study. After completing a drug-free baseline period, each patient will undergo 3 four-week treatment periods consisting of a placebo, amitriptyline, and morphine. Patients will be randomly assigned to one of the 6 possible orders in which the three treatments can be given. The treatment periods will be preceded by a drug wash-out period and a period for titration to maximal effect. The outcome measures will assess pain intensity, pain relief, psychophysical measures of hyperalgesia, cognition, mood and interference/impairment of function. Psychiatric screening will occur throughout the study, using structured diagnostic instruments. These measurements will be made before the first drug period, and during each of the three treatment periods. This study will contribute to our understanding of tricyclic and opioid therapy for treatment of neuropathic pain. Moreover, this study will help advance the field of clinical trials research as it applies to the study of pain. Lessons derived from this study will lead to further comparative studies of drugs that will impart patient care and cast light on the pathophysiological basis of neuropathic pain states.

Description: (Adapted from investigator's abstract) Chronic pain is a disabling disease that affects a large proportion of the U.S. population. An intriguing pain syndrome that results from trauma, often with associated peripheral nerve injury, is sympathetically maintained pain (SMP). SMP is characterized by pain and hyperalgesia to mechanical and thermal stimuli that are dependent on sympathetic innervation of the painful region. Based on advances made during the last granting period, a model has been developed to explain the pathophysiology and pharmacology of SMP. The proposed studies will test a tenet of the model that a1 adrenoceptors develop in peripheral tissues such that the release of norepinephrine from the sympathetic terminals activates nociceptors and leads to pain. Parallel studies will be done in patients and in a recently developed nerve injury paradigm for SMP in rats. The proposed clinical and behavioral studies will address two questions: (1) Is a peripheral a-adrenergic receptor the culprit in SMP? To answer this question, the pain and hyperalgesia produced by intradermally administered alpha1-, alpha2- and beta-adrenergic agonists and a adrenergic blockers will be compared in normal subjects and in patients with SMP or with sympathetically independent pain (SIP). The adrenergic pharmacology of SMP will be further characterized with behavioral studies in rat using a partial sciatic nerve injury paradigm. Earlier studies have indicated that the pain behavior in this rat paradigm is sympathetically maintained. The proposed behavioral studies will also provide information about the site of interaction between the sympathetic efferents and the somatic afferent fibers. (2) Can alpha-adrenergic drugs be used in the diagnosis of SMP? During the past granting period, a diagnostic test for SMP was developed based on the pain relief obtained during intravenous administration of phentolamine. In the proposed studies, the relief of pain and hyperalgesia in SMP following topical administration of clonidine will be investigated as a less invasive diagnostic tool. The proposed studies will lead to a better understanding of the pathophysiology and pharmacology of SMP and to the development of more specific diagnostic tests and pharmacological therapies for alleviating the pain and hyperalgesia in this debilitating pain state.

The purpose of this research is to determine the effectiveness of two medications, tricyclic antidepressants (TCA) and opioids, compared to a placebo in the treatment of postherpetic neuralgia. The specific aims are to compare the effects of opioids and TCA against placebo on pain, altered skin sensitivity, affective, and cognitive function. In addition, the study will determine if the presence of psychiatric co- morbidity, particularly depression, predicts the outcome of treatment with opiods, TCA and placebo. We have studied the relationships between ongoing pain, alterations in thermal sensibility, and allodynia to mechanical and thermal stimuli of patients with PHN to determine the role of peripheral and/or central mechanisms in PHN. Ongoing pain ratings were obtained using a verbal score (0-10). Sensory tests were performed within the affected site, and the corresponding, contralateral, normal site. The area of mechanical allodynia was mapped with a cotton swab and pain evoked by mechanical stimuli (soft hair brush, brass probe, von Frey hairs) was rated on a verbal scale of 0-10. Thermal thresholds to warm, cold, heat pain, and cold pain were determined using a Peltier device and a modified Marstock technique. To date, thirty-two patients (19F, 13M) with PHN of 3-216 months duration (M=39m) have been studied. The average rating of ongoing pain was 7.1 plus or minus 2(M plus or minus SD). The majority of patients (23/32) had allodynia to dynamic (hair brush), statis ( brass probe) or punctate (von Frey) mechanical stimuli (Z equal to or greater than 3.82, p equal to or less than .0001). No significant correlaation was observed between the intensity of ongoing pain and mechanical allodynia. Most patients (22/32) had an increase in heat pain threshold, i.e. hypalgesia (3.4 plus or minus 2.8oC greater than control) despite the presence of mechanical allodynia. The severity of ongoing pain was positively correlated with deficits in heat pain thresholds (r=.39, p=.028). Hence, the role of primary afferent input in the mechanism of PHN is uncertain. The presence of allodynia/hyperalgesia to mechanical stimuli, but the absence of hyperalgesia to thermal stimuli in most patients in this group suggests that central sensitization in PHN is not generalized for all modalities of sensation. It has been observed that patients with chronic pain have high rates of psychiatric conditions. Controversy has existed as to whether these conditions are uniquely related to chronic pain or simply the result of ongoing suffering from a chronic physical symptom. The results demonstrate a high rate of depression in PHN patients consistent with the existing literature. PHN patients compared to patients with chronic and distressing but non-pain physical symptoms also reported higher numbers of depression symptoms as well as other medically unexplained physical symptoms.

The purpose of this research is to determine the effectiveness of two medications, tricyclic antitidepressants (TCA) and opioids, compared to a placebo in the treatment of postherpetic neuralgia. The specific aims are to compare the effects of opioids and TCA against placebo on pain, altered skin sensitivity, and affective and cognitive function. In addition, the study will determine if the presence of psychiatric co- morbidity, particularly depression, predicts the outcome of treatment with opioids, TCA, and placebo. We have studied the relationships between ongoing pain, alterations in thermal sensibility, and allodynia to mechanical and thermal stimuli of patients with, PHN to determine the role of peripheral and/or central mechanisms, in PHN. Ongoing pain ratings were obtained using a verbal score (0-10). Sensory tests were performed within the affected site, and the corresponding, contralateral, normal site. The area of mechanical allodynia was mapped with a cotton swab and pain, evoked by mechanical stimuli (soft hair brush, brass probe, von Frey hairs) was rated on a verbal scale of 1-10. Thermal thresholds to warm, cold, heat pain, and cold pain were determined using a Peltier device and a modified Marstock technique. To date, fifty-nine patients (33 F, 26 M) with PHN of 3-216 months duration (median = 19 months) have been studied. The average rating of ongoing pain was 7.3 + 2 (M + SD). The majority of patients (80%) had allodynia to dynamic (hair brush), static (brass probe) or punctate (von Frey) mechanical stimuli (Z > 5.01, p<.0001). No significant correlation was observed between the intensity of ongoing pain and mechanical allodynia. As a group, the patients demonstrated hypoesthesia to cold and warm detection, and hypoalgesia to heat pain and cold pain. There were no significant correlations between ongoing pain intensity and the degree of sensory alterations (between the unaffected and affected sites) in warm, cold, heat pain, and cold pain detection thresholds. Hence, the role of primary afferent input in tlie mechanism of PHN is uncertain. The presence of allodynia/hyperalgesia to mechanical stimuli, but the relative absence of hyperalgesia to thermal stimuli in most patients in this group suggests that central sensitization in PHN is not generalized for all modalities of sensation. It has been observed that patients with chronic pain have high rates of psychiatric conditions. Controversy has existed as to whether these conditions are uniquely related to chronic pain or simply the result of ongoing suffering from a chronic physical symptom. Patients completed a structured interview (DIS-III-A) for Major Depression, Dysthymic Disorder, Generalized Anxiety Disorder and Somatization Disorder and completed the Somatization and Anxiety subscales of the SCL-90-R. Our results in PHN patients are consistent with the existing literature. PHN patients compared to patients with chronic and distressing but non- pain physical symptoms also reported higher numbers of depression symptoms as well as other medically unexplained physical symptoms. These findings suggest that depression and higher levels of somatic focus are more than secondary complications of experiencing chronic symptoms and are uniquely related to chronic pain.

The Pilot patient was admitted and enrolled into the study 7/28/97 and discharged 7/30/97. From this pilot, some revisions took place in order to ensure a smoother running protocol. Issues that were worked on included the addition of an active placebo, as the pilot patient had no difficulty in identifying the placebo phase of the study. This double- blinded cross-over design was an improvement and was approved by the JCCI. Also, arrangements were made with the Investigational Drug Service to provide the study medications pre-mixed as to ensure the blinding of our study physician and to streamline the protocol. The Investigational Drug Service provided a block randomization schedule, as well. Four infusion patients have followed the pilot patient in participating in this study (a total of 4 prior to November, a total of 8 have been completed to date). Patients continue to be enrolled into the Post Amputation Pain Infusion Study.

The mechanisms underlying neuropathic pain remain incompletely understood. This grant will examine changes that occur in both the peripheral and central nervous system after a peripheral nerve injury. Psychophysical studies in patients with neuropathic pain will be complemented by behavioral and electrophysiological studies in a well-defined model of neuropathic pain that involves ligation of the L5 spinal nerve in rats, and in mice mutant for specific proteins. The first two specific aims focus on the adrenergic sensitivity that develops in the skin of a subset of patients with neuropathic pain that is maintained by activity in sympathetic efferent fibers (sympathetically maintained pain, SMP). During the previous grant period, intradermal injections of norepinephrine were found to produce a dose-dependent pain in patients with SMP, suggesting that cutaneous nociceptors develop a sensitivity to adrenaline in this disease. During the coming grant period, norepinephrine iontophoresis in patients with and without SMP will be tested as a new, non-invasive, diagnostic test for SMP (Sp. Aim 1). In addition, the adrenoceptor pharmacology of SMP will be investigated by measuring the alterations in pain following the cutaneous iontophoresis of selective adrenergic agonists and antagonists (Sp. Aim 2). Patients with neuropathic pain are particularly distressed by the pain evoked by normally innocuous mechanical stimuli. A series of experiments will focus on determining the differential role of nociceptive afferent fibers and low-threshold mechanoreceptors (LTMs) in signaling the hyperalgesia in neuropathic animals. The effects of selective loss of nociceptor function will be investigated using the neurotoxin, capsaicin (Sp. Aim 3), and selective depletion of LTMs will be examined by use of mice mutant for specific neurotrophic factors (Sp. Aim 4). The final two specific aims focus on the mechanisms of increased excitability of the spinal dorsal horn that develops following a nerve injury. This dorsal horn sensitization could be due to either an increase in excitatory mechanisms or a decrease in inhibitory mechanisms. During the coming grant period, changes in excitatory mechanisms will be explored by investigating the modulatory effects of neurokinin (NK) receptor systems on nociceptive transmission in the spinal cord (Sp. Aim 5). Changes in inhibitory mechanisms will be explored by investigating the modulatory effects of opioid receptor systems (Sp. Aim 6). Pharmacological and neurophysiological studies with selective NK antagonists will be used to determine if the NK receptors are upregulated in neuropathic pain. In addition, pain behavior and dorsal horn physiology will be examined in NK-1 receptor and mu-opioid receptor knockout mice. The results of these studies should provide new insights into the peripheral and central mechanisms of neuropathic pain and may lead to novel diagnostic and therapeutic strategies.

human therapy evaluation; amputation; pain; local anesthetics; combination chemotherapy; morphine; opiate alkaloid; lidocaine; analgesia; disease /therapy duration; prognosis; clinical trials; diagnostic tests; diagnosis design /evaluation; intravenous administration; slow release drug; chronic pain; oral administration; patient oriented research; clinical research; human subject;

DESCRIPTION (provided by applicant): The ability to treat neuropathic pain (NP) remains unsatisfactory, as many patients experience pain despite taking prescribed medications. Although opioids are effective against NP, their clinical usefulness is limited by their central nervous system effects (i.e., sedation, cognitive dysfunction, and tolerance). Using the L5 spinal nerve ligation model in rodents, we found that loperamide, a peripherally acting opioid, alleviates behavioral signs of NP. In addition, we reported that enhancing <-opioid receptor (MOR) expression in primary afferents, using HSV viral vectors, results in a leftward shift in the analgesic dose-response curve of loperamide. Thus, opioid receptors in the peripheral nervous system present a valuable target for the treatment of NP in humans. Surprisingly, repetitive administration of loperamide induces peripheral opioid tolerance. In this proposal, we aim to modulate the interactions of MORs and -opioid receptors (DORs) in the peripheral nervous system to enhance peripheral opioid analgesia and to attenuate peripheral opioid tolerance. We hypothesize that after nerve injury, the functional interactions between MORs and DORs are crucial for both the observed peripheral opioid antihyperalgesic effects and the development of peripheral opioid tolerance. Specifically, we hypothesize that: 1) DOR agonists enhance the analgesic effects of peripheral MOR agonists in opioid-naove animals with NP, 2) chronic administration of a MOR agonist switches DORs from an enhancing to an inhibitory role, thereby contributing to peripheral opioid tolerance, and 3) the development of tolerance is due to the formation of MOR-DOR heterodimers, and peripheral opioid tolerance in NP can be prevented or reversed by inhibiting heterodimerization. A broad range of strategies will be used to test these hypotheses, including behavioral, electrophysiological, Ca2+ imaging, and molecular biological methods. We will test whether concurrent activation of peripheral MORs and DORs will result in enhanced antihyperalgesia in opioid-naove rodents with NP (Aim 1) and whether repeated administration of peripheral MOR agonist switches the DOR into a negative modulator of MOR function, resulting in attenuation of peripheral opioid antihyperalgesia (Aim 2). To study the role of DOR and DOR-MOR heterodimerization in opioid antihyperalgesia and peripheral opioid tolerance, innovative virus-mediated gene transfer strategies will be used to suppress DOR expression and to inhibit DOR-MOR heterodimerization by expressing a dimer-deficient DOR (Aim 3). In addition, a novel small peptide that blocks DOR-MOR heterodimerization will be studied for its effect on opioid tolerance. The results of these studies will provide new insights into the mechanisms that underlie the peripheral pain-reducing effects of opioids in NP and the mechanisms that underlie peripheral opioid tolerance. Furthermore, the use of virus mediated gene transfer and small cell permeable peptide inhibitors of heterodimerization are viable pre-clinical approaches to develop novel therapeutic strategies for the treatment of NP and opioid tolerance.