Adrenal medullary enkephalin-like peptides may mediate opioid stress analgesia

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Resting blood pressure modulates chest pain intensity in patients with acute myocardial infarction

Introduction and Objectives

Animal models and human studies show that resting blood pressure (BP) is inversely associated with pain sensitivity. The phenomenon of "hypertension-associated hypoalgesia" was proposed as a possible explanation for the intervariability in pain perception. Given that a portion of patients with acute myocardial infarction (AMI) do not experience significant pain, we used the model of severe cardiac ischemia to explore whether BP affects the intensity of chest pain.

Methods

Patients with AMI admitted to the cardiac intensive care unit with coronary catheterization-proven completely occluded coronary artery were included (n = 67). Resting BP at admission and 5 days after AMI was obtained. Participants reported chest pain intensity and underwent psychophysical evaluation including pain ratings for pressure, heat, and pinprick stimuli as well as temporal summation and conditioned pain modulation paradigms.

Results

Patients with lower systolic BP (≤120 mm Hg) vs higher (≥140 mm Hg) reported higher chest pain scores at symptom onset (82.3 vs 61.7, P = 0.048) and during peak AMI (82.8 vs 57.5, P = 0.019). Higher pain ratings in response to pinprick stimulus were associated with lower BP at admission (analysis of variance P = 0.036). Patients with hypertension demonstrated lower pain sensitivity in response to pressure stimulation (531.7 ± 158.9 kPa/s vs 429.1 ± 197.4). No significant associations were observed between BP and the other assessed psychophysical measures.

Conclusion

Study findings reinforce the phenomenon of hypertension-associated hypoalgesia through characterization of the association between BP and clinical pain experiences at onset and during AMI in a model of acute clinical pain.

[Is morphine still the analgesic of choice in acute myocardial infarction?]

Chest pain is the most common symptom of patients who present with ischemic heart disease. Morphine has traditionally been the drug of choice for managing chest pain in acute coronary syndrome (ACS) due to its high analgesic potency, though its physiological effects are poorly understood. Routinely used for managing chest pain, morphine is recommended in the 2002 guidelines of the American College of Cardiology/American Heart Association. This recommendation, however, is not based on a high level of scientific evidence but on expert opinion. Studies have found both for and against the use of morphine in ACS, suggesting that its benefits are perhaps not altogether clear. This review examines the pathophysiological effects of morphine and their cardiac implications, with special attention to a possible negative effect on ACS. We reviewed articles in the MEDLINE database from 1982 to 2006.

The involvement of spinal bovine adrenal medulla 22-like peptide, the proenkephalin derivative, in modulation of nociceptive processing

Bovine adrenal medulla 22 (BAM22), one of the cleavage products of proenkephalin A, possesses high affinity for opioid receptors and sensory neuron-specific receptor (SNSR). The present study was designed to examine the expression of BAM22 in the spinal cord and dorsal root ganglion (DRG) of naive rats as well as in a model of inflammation. BAM22-like immunoreactivity (BAM22-IR) was expressed in fibers in the spinal cord, with high density seen in lamina I in naïve rats. The expression of BAM22-IR in the superficial laminae was greatly reduced following dorsal rhizotomy. BAM22-IR was also located in 19% of DRG cells, mainly in the small- and medium-sized subpopulations. Following injection of complete Freund's adjuvant (CFA) in the hindpaw, the expression of BAM22-IR in the superficial laminae of the spinal cord and small-sized DRG neurons on the ipsilateral side was markedly increased. Double labeling showed that the Fos-positive nucleus was surrounded by BAM22-IR cytoplasm in the spinal dorsal horn neurons or closely associated with BAM22-IR fibers in the superficial laminae. Furthermore, CFA-induced mechanical allodynia in the inflamed paw was potentiated by intrathecal administration of anti-BAM22 antibody. Together, these results demonstrate for the first time that BAM22-like peptide is mainly located in the superficial laminae of the spinal cord and mostly originates from nociceptive DRG neurons. BAM22 could thus act as a ligand for presynaptic opioid receptors and SNSR. Our study also provides evidence suggesting that BAM22 plays a role in the modulation of nociceptive processing at the spinal level under normal and inflammatory conditions.

Isometric exercise has opposite effects on central pain mechanisms in fibromyalgia patients compared to normal controls

Aerobic exercise has been shown to activate endogenous opioid and adrenergic systems and attenuate experimental pain in normal control subjects (NC). In contrast, fibromyalgia (FM) subjects' experimental pain ratings increase after aerobic exercise, suggestive of abnormal pain modulation. In order to determine whether central or peripheral mechanisms are predominantly involved in the abnormal pain modulation of FM patients, the effects of handgrip exercise on thermal (cutaneous) and mechanical (somatic) experimental pain was tested in local as well as remote body areas of FM and NC subjects. Supra-threshold thermal pain ratings and pressure pain thresholds over both forearms were obtained before and during 90 s of sustained 30% maximal voluntary contraction (MVC). This isometric exercise resulted in substantially decreased thermal pain ratings and increased mechanical thresholds in local as well as remote body areas in NC. Opposite effects were detected in FM patients. Thus, sustained local muscular contraction induced widespread pain inhibitory effects in NC. In contrast, the widespread hyperalgesic effects of exercise on FM patients clearly indicate altered central pain mechanisms. However, whether these exercise effects of FM patients result from abnormal descending inhibition or excessive activation of muscle nociceptive afferents needs to be addressed in future studies.

Thermal Pain Perception After Aerobic Exercise

OBJECTIVE

To examine thermal pain perception before, 5 minutes after, and 30 minutes after 30 minutes of treadmill exercise at 75% of maximal oxygen uptake (V o 2 max).

DESIGN

Repeated-measures.

SETTING

Sports science laboratory.

PARTICIPANTS

Convenience sample of 14 healthy male and female volunteers (mean age +/- standard deviation, 32+/-3y).

INTERVENTIONS

Sensory thresholds, pain thresholds, and pain ratings to hot and cold stimuli were measured before and after 30 minutes of treadmill exercise at 75% of V o 2 max. The hot and cold stimuli were delivered by using a thermode placed on the thenar eminence of the nondominant hand. Thermal sensory and pain thresholds were determined during continuous ramps in temperature of the thermode.

MAIN OUTCOME MEASURES

Pain ratings were measured on a visual analog scale at 10-second intervals over 2 minutes of thermal pain stimulation.

RESULTS

There were no significant changes in thermal sensitivity, pain thresholds, or pain ratings for either heat or cold after 30 minutes of exercise at 75% of V o 2 max.

CONCLUSIONS

Pain perception to thermal stimuli was unaltered after 30 minutes of exercise at 75% of V o 2 max, an intensity and duration of exercise previously shown to alter pain perception to electric and mechanical stimuli.

Intensity and duration threshold for aerobic exercise-induced analgesia to pressure pain

OBJECTIVE

To examine how exercise-induced analgesia is affected by the duration and intensity of aerobic exercise.

DESIGN

Repeated-measures design.

SETTING

Exercise science laboratory.

PARTICIPANTS

Convenience sample of 12 healthy male and female volunteers (mean age +/- standard deviation, 32+/-9 y).

INTERVENTIONS

Pain ratings were assessed before and at 5 and 30 minutes after treadmill exercise of 10 minutes at 75% maximal oxygen uptake (Vo(2)max), 30 minutes at 50% Vo(2)max, and 30 minutes at 75% Vo(2)max (randomized order and no less than 48 h between each bout).

MAIN OUTCOME MEASURES

Pain ratings were measured on a visual analog scale at 10-second intervals during a 2-minute pressure-pain stimulus to the nondominant index finger.

RESULTS

Pain ratings were significantly decreased (P<.05) from pre-exercise values 5 minutes after 30 minutes of exercise at 75% Vo(2)max but returned toward baseline by 30 minutes after exercise. There were no significant changes in pain ratings after 10 minutes of exercise or after exercise at 50% Vo(2)max.

CONCLUSIONS

There are thresholds for both the intensity (>50% Vo(2)max) and duration (>10 min) of exercise required to elicit exercise analgesia.

Segmental and plurisegmental modulation of pressure pain thresholds during static muscle contractions in healthy individuals

The purpose of this study was to assess possible segmental (uni- and/or bilateral) and plurisegmental changes in pressure pain thresholds (PPTs) during static muscle contractions. Twenty-four healthy subjects (12 female, 12 male) performed a standardised isometric contraction with the dominant m. quadriceps femoris (MQF) and m. infraspinatus (MI), respectively. PPTs were assessed using pressure algometry at the contracting muscle, at the contralateral (resting) muscle and at a distant resting muscle (MI during contraction of MQF and vice versa). The PPT assessments were performed before, during and 30min. following each contraction. The contractions were held until exhaustion or for a maximum of 10 PPT assessments/muscle. During contraction of MQF PPTs increased compared to baseline at the middle ( p<0.001) and the end (p<0.001) of the contraction period at all assessed sites alike. During contraction of MI PPTs increased compared to baseline at the middle (p<0.001) and the end (p<0.007) of the contraction period at all sites. The increase was more pronounced at the contracting muscle compared to the contralateral (p<0.002; p<0.01) and the distant (p<0.002; p<0.002) site. No statistically significant difference was seen in PPTs between the latter two. Following the contractions PPTs returned to baseline. Submaximal isometric contraction of MQF and MI gave rise to a statistically significant increase in PPTs at the contracting muscle, the resting homologous contralateral muscle and at the distant resting muscle indicating that generalised pain inhibitory mechanisms were activated. Contraction of MI, but not of MQF, gave rise to an additional activation of unilateral segmental antinociceptive effects.

Long-lasting delayed hyperalgesia after chronic restraint stress in rats-effect of morphine administration

Different effects upon the nociceptive response have been observed with exposure to acute and chronic stress in rats. In the present study we repeatedly submitted rats to restraint for 40 days, inducing hyperalgesia using the tail-flick test. A new session of acute stress was applied at the end of 40 days period, and the chronically-stressed animals demonstrated analgesia after forced swimming, but not after restraint. The effect of stress interruption for 14 or 28 days on the nociceptive threshold was then investigated. The basal tail-flick latency remained decreased for at least 28 days (hyperalgesic effect). Following the periods of suspension, the animals were submitted to new session of acute restraint, and stress-induced analgesia was observed only after 28 days of stress interruption. Thus, the mechanisms involved in the long-lasting hyperalgesia presented in this study are not exactly the same as those responsible for the analgesia induced by acute stressors. After 40 days of chronic stress treatment, morphine was injected i.p. (1.0, 5.0 mg/kg or saline). The repeatedly stressed rats displayed decreased morphine effects on nociception compared to unstressed controls. The tolerance of the response to morphine agrees with previous studies suggesting that chronic restraint stress could modify the activity of opioid systems.

Sex differences in chest pain in patients with documented coronary artery disease and exercise-induced ischemia: Results from the PIMI study

BACKGROUND

Sex differences in the pathophysiologic course of coronary artery disease (CAD) are widely recognized, yet accurate diagnosis of coronary artery disease in women remains challenging.

METHODS

To determine sex differences in the clinical manifestation of CAD, we studied chest pain reported during daily activities, exercise, and mental stress in 170 men and 26 women. All patients had documented CAD (>50% narrowing in at least 1 major coronary artery or prior myocardial infarction) and all had 1-mm ST-segment depression on treadmill exercise. We collected psychologic test results, serum samples (potassium, epinephrine, norepinephrine, cortisol, b-endorphin, and glucose), and cardiac function, sensory threshold, and autonomic function data at specified times before, during, or after exercise and mental stress tests to assess measures of depression, anxiety, and neurohormonal and thermal pain perception.

RESULTS

Women reported chest pain more often than men during daily activities (P =.04) and during laboratory mental stressors (P =.01) but not during exercise. Men had lower scores than women on measures of depression, trait anxiety, harm avoidance, and reward dependence (P <.05 for all). Women had significantly lower plasma b-endorphin levels at rest (4.2 +/- 3.9 vs 5.0 +/- 2.5 pmol/L for men, P =.005) and at maximal mental stress (6.4 +/- 5.1 vs 7.4 +/- 3.5 pmol/L for men, P <.01). A higher proportion of women than men had marked pain sensitivity to graded heat stimuli applied to skin (hot pain threshold <41 degrees C, 33% vs 10%, P =.001).

CONCLUSIONS

Our results reflect sex differences in the affective and discriminative aspects of pain perception and may help explain sex-related differences in clinical presentations.

Adrenalectomy-induced potentiation of morphine action in guinea-pig ileum: possible decrease in the release of endogenous opioids from opioidergic neurones

1. The effects of adrenalectomy on exogenous and endogenous opioid actions in guinea-pig isolated myenteric plexus-longitudinal muscle (MPLM) were investigated. 2. A decrease in serum cortisol level to about 37% of the level in the sham-operated group was obtained in adrenalectomized animals (sham: 53.5 +/- 7.2 microg 100 ml(-1); adrenalectomized: 20.0 +/- 3.6 microg 100 ml(-1)). 3. The concentration-response curve of twitch inhibition, which was induced by electrical field stimulation (0.1 Hz, 0.5 ms pulse width, maximum intensity), caused by a low concentration of morphine (5 x 10(-9)-5 x 10(-7) M) was not affected, but at high concentration (10(-6)-10(-5) M) there was an upward shift in the adrenalectomized group compared with the sham-operated control, although the basal twitch contraction was not changed by adrenalectomy. 4. The twitch inhibition induced by a high concentration of morphine (10(-6) M) in the adrenalectomized group was antagonized to the same level as that in sham-operated controls by naloxone (NLX) (3 x 10(-7) M). 5 Post-tetanic twitch inhibition, an indicator of endogenous opioid release, induced by tetanic stimulation (10 Hz, 0.5 ms pulse width, maximum intensity, for 1 min) was inhibited in the adrenalectomized group compared with the sham-operated controls. The antagonism of inhibition in both groups was equivalent to that exerted by NLX (10(-7) M). 6. Acetylcholine-evoked contraction of the muscle was not influenced by adrenalectomy. 7. These results suggested a possible mechanism for the increase in sensitivity of the opioid receptors to morphine by adrenalectomy, resulting from a decrease in the release of endogenous opioids from the opioidergic neurones in the ileum.

A peripheral, intracerebral, or intrathecal administration of an opioid receptor antagonist blocks illness-induced hyperalgesia in the rat

We used the tail-flick response of rats to study the role of opioid receptors in illness-induced hyperalgesia. An intraperitoneal injection of lithium chloride (LiCl) produced hyperalgesia that was blocked in a dose-dependent manner by subcutaneous injection of the opioid antagonist naloxone. Neither hyperalgesia nor its blockade by naloxone were due to variations in tail-skin temperature induced by LiCl. Hyperalgesia was also blocked when opioid receptor antagonism was restricted to (a) the periphery, by intraperitoneal administration of the quaternary opioid receptor antagonist naloxone methiodide; (b) the brain, by intracerebroventricular microinjection of naloxone; or (c) the spinal cord, by intrathecal microinjection of naloxone. These results document a pain facilitatory role of opioid receptors in both the peripheral and central nervous systems and are discussed with reference to their analgesic and motivational functions.

Augmenting effect of methionine-enkephalin on interleukin-6 production by cytokine-stimulated murine macrophages

The effects of methionine-enkephalin on the production of interleukin-6 by activated peritoneal murine macrophages were studied. Macrophage were activated with interleukin-1beta or interferon-gamma in the presence or absence of graded concentrations of methionine-enkephalin. Methionine-enkephalin combined with interleukin-1beta or interferon-gamma caused an increase in IL-6 release from cultured macrophages. The opioid receptor antagonist naloxone did not change the stimulatory effect of methionine-enkephalin on IL-6 production by stimulated macrophages. Methionine-enkephalin added to the culture medium of resting macrophages increased IL-6 release from macrophages which were later induced with interleukin-1beta or interferon-gamma. The results of this study suggest that methionine-enkephalin can modulate the proinflammatory cytokine response by controlling, via non-opioid receptor mechanism, the production of IL-6.

Blockade of tolerance to stress-induced analgesia by MK-801 in mice

The N-methyl-D-aspartate (NMDA) receptor has been implicated in mechanisms of tolerance to morphine-induced analgesia. The present study examined the role of the NMDA receptor in the development of tolerance to stress-induced analgesia (SIA). In the first experiment, mice were exposed to a stressor (a 3-min forced swim in water maintained at 32 degrees C) once daily for 15 consecutive days. Analgesia was measured 2 min after stress on the first and last day using the hot-plate test. To examine the role of the NMDA receptor in the development of tolerance to SIA mice were treated daily with the non-competitive NMDA receptor antagonist, MK-801, 15 min before swimming. Pretreatment with MK-801 was found to block both analgesia and tolerance. In a second experiment, to examine whether SIA and tolerance to SIA are mediated by similar or different mechanisms, mice were injected daily with MK-801 after analgesia had dissipated (1 h following swim). Tolerance to SIA was blocked by delayed injections of MK-801. These results suggest that the NMDA receptor is involved in mechanisms of tolerance to SIA, independent of its role in analgesia.

The role of hypophyseal and adrenal mechanisms in the hypoalgesic response to non-contingent food delivery in the rat

Exposure to non-contingent food delivery has been shown to elicit an increase in nociceptive thresholds in rats. This hypoalgesia has previously been shown to be reversible by the opiate antagonist naloxone. Given that most opioid forms of SIA are found to rely on an intact pituitary-adrenal axis, this research examined the possibility that the hypoalgesic response to non-contingent food delivery is also hormonally mediated. Hypophysectomy (Expt. 1) but not bilateral adrenalectomy (Expt. 2) was found to completely attenuate the hypoalgesic response to non-contingent food delivery. Preliminary data from ongoing research is presented that points to the possible involvement of the hypophyseal peptides beta-endorphin and adrenocorticotropin hormone (ACTH).

Modulation of pressure pain thresholds during and following isometric contraction in patients with fibromyalgia and in healthy controls

This study aimed at evaluating the influence of submaximal isometric contraction on pressure pain thresholds (PPTs) in 14 fibromyalgia (FM) patients and 14 healthy volunteers, before and after skin hypoesthesia. PPTs were determined with pressure algometry over m. quadriceps femoris before, during and following an isometric contraction. Maximum voluntary contraction (MVC) was assessed using a computerized dynamometer. A contraction of 22% MVC on average was held until exhaustion (max. 5 min) and PPTs were assessed every 30 sec. A local anesthetic cream and a control cream were applied following a double-blind design and PPTs were reassessed. In healthy volunteers PPTs increased during contraction (P < 0.001), then decreased after the end of contraction (P < 0.001) but remained above precontraction values during the 5 min of post-contraction assessments (P < 0.001). In FM patients PPTs decreased in the middle of the contraction period (P < 0.05) and remained below precontraction levels during the rest of the contraction period (P < 0.05) and during the 5 min of post-contraction assessment (immediately post-contraction NS; 2.5 min post-contraction P < 0.01; 5 min post-contraction P < 0.05). The normalized PPTs were significantly lower in patients than in controls during contraction (start P < 0.01; middle P < 0.001; end P < 0.001) and at all times during post-contraction assessments (P < 0.001). Anesthetic cream raised PPTs at rest in controls (P < 0.01) but not in FM patients, and did not influence contraction or post-contraction PPTs in either group. Therefore, the increased pressure pain sensibility in FM patients is more pronounced deep to the skin. The observed decrease of PPTs during isometric contraction in FM patients could be due to sensitization of mechanonociceptors caused by muscle ischemia and/or dysfunction in pain modulation during muscle contraction.

Modulation of pressure pain thresholds during and following isometric contraction

This study aimed at evaluating the influence of submaximal isometric contraction on pressure pain thresholds (PPTs) in 14 healthy volunteers before and after skin hypoesthesia. PPTs were determined with pressure algometry over m. quadriceps femoris before, during, and following an isometric contraction. Maximum voluntary contraction (MVC) was assessed using a computerized dynamometer. A contraction of 21% MVC was held until exhaustion (max: 5 min) and PPTs were assessed every 30 sec. A local anesthetic cream and a control cream were applied following a double-blind design and PPTs were reassessed. PPTs increased significantly at the start of contraction and continued to increase until the middle of the contraction period, then remaining at this level. After contraction PPTs decreased significantly but for 5 min remained slightly above precontraction levels. Anesthetic cream raised PPT at rest but not during and following contraction. The relative increase in PPTs during and immediately following isometric contraction was lower with anesthetic cream. Isometric contraction of m. quadriceps femoris increase PPTs during and following contraction. The results suggest that input from cutaneous and deeper tissues interacts with nociceptive activity set up by the pressure stimulus. Determining the degree of sensory modulation in muscle and skin in different chronic pain syndromes could become a functional method of patient assessment important for differential diagnosis, treatment evaluation, and follow-up.

Adrenalectomy potentiates the morphine--but not cocaine-induced place preference in rats

The conditioned place preference paradigm is commonly used to study the reinforcing properties of various drugs. In the present study, the effect of adrenalectomy (ADX) on the morphine-induced place preference was examined in rats. Morphine produced a significant preference for the drug-associated place in sham-operated (sham) and ADX rats. In sham rats, only the highest dose of morphine (8 mg/kg, i.p.) produced a significant preference, while in ADX rats, lower doses of morphine (1 and 2 mg/kg, i.p.) produced a significant preference for the drug-associated place. Furthermore, the morphine-induced place preference was blocked by the dopamine D1 antagonist SCH23390 in both sham and ADX rats. On the other hand, the cocaine-induced place preference was not affected by ADX. In the present study, we found that ADX potentiates the reinforcing effect induced by morphine, but not that induced by cocaine, which suggests that the enhancement by ADX may be due to a change in opioid receptors, morphine metabolism and/or some other cause, but not a change in dopamine receptors.

Localization of neuropeptide Y Y1 receptors in the rat nervous system with special reference to somatic receptors on small dorsal root ganglion neurons

Immunohistochemical staining with an antiserum against the neuropeptide Y (NPY)-Y1 receptor (Y1-R) protein was shown in rat small dorsal root ganglion (DRG) neurons, which also were Y1-R mRNA-positive and calcitonin gene-related peptide (CGRP)-positive. The Y1-R-like immunoreactivity was almost exclusively located in the somatic plasmalemma and in the perinuclear region. Intracellular recording showed that the Y1 agonist [Leu31,Pro34]NPY evoked an outward current in small DRG neurons, suggesting a functional somatic Y1-R. No evidence for axonal transport of Y1-R protein was obtained after analysis of the dorsal horn for double staining with CGRP, after dorsal rhizotomy, or after compression of dorsal roots and the sciatic nerve. It is proposed that blood-borne NPY released from sympathetic nerves and adrenal medulla is the endogenous ligand for the Y1 receptors on the small DRG neurons. Y1-R-positive neurons were also seen in the dorsal horn of spinal cord, the hypothalamic arcuate nucleus, pyramidal cells in the cerebral cortex, Purkinje and basket cells in the cerebellar cortex, and in many other brain regions.

A permissive role of corticosterone in an opioid form of stress-induced analgesia: blockade of opiate analgesia is not due to stress-induced hormone release

The 100 inescapable tail-shock paradigm produces three sequential analgesic states as the number of shocks increases: an early opioid analgesia (after 2 shocks) that is attenuated by systemic naltrexone, a middle analgesia (after 5-40 shocks) that is unaffected by systemic naltrexone, and a late opioid analgesia (after 80-100 shocks) that is attenuated by systemic naltrexone. In order to determine whether the absence of adrenal hormones would affect any of these analgesias, we tested adrenalectomized (ADX) versus sham-operated control rats 2 weeks post-surgery. Pain threshold was assessed using the tail-flick (TF) test. ADX attenuated both the early (2 shock) and late (80-100 shock) opiate analgesias and failed to reduce the naltrexone-insensitive analgesia after 5-40 shocks. We demonstrated that a loss of adrenomedullary catecholamines does not underlie the ADX-induced attenuation of opioid analgesia since sympathetic blockade using systemic chlorisondamine (6 mg/kg) failed to reduce analgesia at any point in the shock session. It was further shown that stress levels of adrenal hormones are not critical since (a) analgesia was unaffected when animals were tested 48 h after ADX, (b) 2 shocks do not produce a surge in corticosterone (CORT) over and above levels observed in animals restrained and TF tested in preparation for shock, and (c) basal CORT replacement in drinking water fully restored analgesia in ADX rats. These experiments demonstrate that basal CORT, rather than adrenomedullary substances, is critical to the expression of analgesia. The function of CORT here is not linked to a shock-induced surge of the steroid. CORT appears to play a permissive role in the expression of analgesia. Potential effects of the absence of corticosteroids on neurotransmitter biosynthesis important in analgesia production are discussed.

Increased concentration of beta-endorphin in sera of patients with psoriasis and other inflammatory dermatoses

Serum beta-endorphin was quantified by radioimmunoassay in 71 patients with psoriasis vulgaris, other chronic inflammatory skin diseases with T-cell infiltrates [atopic dermatitis (n = 25), and systemic sclerosis (n = 34)], and 100 healthy subjects. The neuropeptide was found to be markedly (P < 0.001) increased in patients with psoriasis (14.4 pg/ml), atopic dermatitis (9.2 pg/ml) and systemic sclerosis (9.8 pg/ml) compared with normal controls (6.1 pg/ml). The highest values of beta-endorphin were found in patients with actively spreading plaque psoriasis (17.3 pg/ml), whereas lesion-free patients showed a reduction in neuropeptide concentration (10.2 pg/ml). The levels were much higher in patients with widespread psoriatic lesions (> 60% body surface; 16.2 pg/ml), which lasted longer than 3 months (15.8 pg/ml), whereas neither the presence of stress nor itching correlated with the serum peptide concentration. Our data suggest that beta-endorphin is produced in psoriatic lesions by inflammatory cells, rather than the increased levels being the result of activation of the pituitary-adrenal axis by chronic stress. The generation of neuropeptide in psoriatic lesions and its antinociceptive effect on the peripheral sensory nerves might explain why pruritus is a relatively rare phenomenon in psoriasis.

Behavioral effects of lipopolysaccharide in rats: involvement of endogenous opioids

Activation of the immune system in response to either infection or lipopolysaccharide (LPS) produces neurophysiological, neuroendocrine and behavioral changes. Some of the physiological consequences of LPS are mediated by endogenous opioid peptides. The following studies were designed to characterize the effects of LPS in several behavioral paradigms, and to determine the role of opioids in mediating these effects. The effects of LPS on locomotor and self-care activity were assessed in the open field test. Rats were injected with either saline or a dose of LPS (25, 50, 100, or 1000 micrograms/kg). 4 h later, the animals were placed in an open field and the numbers of line crossings, rearings and grooming episodes were counted. LPS significantly suppressed the three open field behaviors in a dose-related manner. The effect of LPS on sensitivity to pain was determined using the hot-plate and tail-flick tests. Administration of LPS (200 micrograms/kg) increased pain sensitivity in the hot plate test 30 min after drug administration, but produced a significant analgesic response 4 h after drug administration in both tests. Further characterization of LPS-induced analgesia demonstrated that it began about 2 h after and disappeared 30 h after the administration of LPS. Administration of naltrexone completely blocked the analgesic effects of LPS 4 h after its administration, but had no effect on LPS-induced suppression of activity in the open field. The effect of LPS on body temperature was biphasic, producing hypothermia at 2 h and hyperthermia at 8-30 h after its administration. Naltrexone had no effect on the body temperature changes induced by LPS.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptides in psoriasis: possible role of beta-endorphin in the pathomechanism of the disease

BACKGROUND

An increased concentration of neuropeptides in psoriatic lesional skin may be responsible for alterations in the neurogenic erythematous response and transmission of stimuli through sensory nerve fibers (sensation of pruritus).

METHODS

Increasing doses of capsaicin from 0.125 to 4 micrograms/cm2 were applied to nonlesional psoriatic skin to establish the minimal dose that induced the substance P-mediated neurogenic response in 30 patients with psoriasis. Plasma beta-endorphin was quantitated in 71 psoriatics by radioimmunoassay using NEN 1251-RIA kit.

RESULTS

The mean beta-endorphin concentration was increased about 2-fold compared to normals, whereas doses of capsaicin needed to induce erythema were higher (1-4 micrograms/cm2) in psoriatics (mainly in patients with type II psoriasis) than in healthy subjects (0.125-0.25 microgram/cm2).

CONCLUSIONS

The data indicate that increased beta-endorphin in psoriatic skin might affect both substance P-mediated neurogenic inflammation and transmission of sensory stimuli due to local antinociceptive effects of this opioid. The differences in the neurogenic response in type I and II psoriasis may be related to the degradation of substance P and beta-endorphin by neutral proteinases in the lesional skin.

Pharmacological characterization of an opioid receptor in the ciliate Tetrahymena

A pharmacological characterization has been performed of the opioid receptor involved in modulation of phagocytosis in the protozoan ciliate Tetrahymena. Studies on inhibition of phagocytosis by mammalian prototypic opioid agonists revealed that morphine and beta-endorphin have the highest intrinsic activity, whereas all the other opioids tested can only be considered partial agonists. However, morphine (a mu-receptor agonist) is twice as potent as beta-endorphin (a delta-receptor agonist). Furthermore, the sensitivity for the opioid antagonist naloxone, determined in the presence of morphine and beta-endorphin, is very similar to the sensitivity exhibited by mammalian tissues rich in mu-opioid receptors. We suggest that the opioid receptor coupled to phagocytosis in Tetrahymena is mu-like in some of its pharmacological characteristics and may serve as a model system for studies on opioid receptor function and evolution.

The antinociceptive effect of non-noxious sensory stimulation is mediated partly through oxytocinergic mechanisms

The objective of the present study was to investigate whether oxytocinergic mechanisms may contribute to the antinociceptive effect of non-noxious, sensory stimulation. To test this hypothesis, oxytocin levels in plasma and cerebrospinal fluid (CSF) were measured in control rats as well as in rats exposed for 30 min to electro-acupuncture (2 Hz), thermal stimulation (40 degrees C) or vibration (100 Hz). All modes of stimulation induced significant elevations of oxytocin levels in plasma and/or in CSF, 30 or 90 min after the end of stimulation. Secondly, the antinociceptive effects of these treatments were investigated in the tail-flick test with and without prior administration of the oxytocin antagonist 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-oxytocin (1 mg kg-1 i.p.). All three modes of stimulation caused a significant delay of the tail-flick latency to the same degree as that caused by injection of oxytocin 1 mg kg-1 i.p. (electro-acupuncture P < 0.01, thermal stimulation and vibration P < 0.05). In all cases, the delay was reversed by administration of the oxytocin antagonist (1 mg kg-1 i.p.). These findings suggest that analgesic effects induced by non-noxious sensory stimulation may, in part, be mediated through activation of oxytocinergic mechanisms.

Beta-endorphin response to exercise and mental stress in patients with ischemic heart disease

We compared symptomatic, hemodynamic and opioid responses of heart disease patients to exercise testing and a stressful public speaking task. Plasma beta-endorphins were measured at rest and immediately post stress. Nineteen of 50 patients had angina during exercise; 31 had asymptomatic ischemia. No patient had angina during the speech, but two had ECG changes and 39% had radionuclide changes indicating ischemia. Patients with asymptomatic ischemia on exercise had a significantly greater beta-endorphin response than those with angina. Public speaking elicited a significantly larger beta-endorphin increase relative to change in double product (an index of stress) than did exercise.

CONCLUSIONS

(1) Patients with silent vs painful ischemia experience a greater beta-endorphin response to exercise. (2) beta-endorphin response to a speech stressor is greater than to exercise when controlled for an index of stress. (3) Increased beta-endorphin response to a speech stressor may partially explain the predominance of silent ischemia during psychological stress.

Borderline disorder and post-traumatic stress disorder: an equivalence?

This paper draws attention to various similarities between Borderline Personality Disorder and Post-traumatic Stress Disorder. It is argued that the former may be considered to be an equivalent of the latter, with the difference that the trauma has either undergone repression or, having been suffered before the establishment of speech, has not been registered in verbal form. The criteria for Borderline Personality Disorder are presented in terms of symptoms of failed attachment consistent with early trauma, and various research papers linking these concepts are considered. Other research in infant attachment suggests intergenerational links between disorganised attachment patterns in infancy and parents with unresolved problems with their own parents. Research concerning the biochemical underpinning of emotional responses is quoted to link these conditions more securely, and may offer a conceptual framework in which to understand the need of these patients for a therapeutic milieu in which early developmental needs may be understood.

Characterization of pituitary mediation of stress-induced antinociception in rats

Antinociception, induced by continuous cold-water swims (CCWS) and certain parameters of inescapable foot shock, is reduced in hypophysectomized rats receiving supplements of corticosterone and l-thyroxine. To assess which lobe of the pituitary gland is involved in this effect, the first experiment compared the effects of total hypophysectomy and posterior lobectomy in supplemented rats upon CCWS antinociception on the tail-flick and jump tests and upon continuous inescapable foot shock antinociception on the tail-flick test. Total hypophysectomy, but not posterior lobectomy, significantly reduced CCWS antinociception on both tests in supplemented rats relative to sham surgery. Both total and posterior hypophysectomy either reduced or potentiated foot shock antinociception as functions of shock intensity or duration of exposure in supplemented rats. To assess whether hormonal supplementation is necessary for the observed effects, the second experiment examined CCWS antinociception in sham-operated and hypophysectomized rats that received either no hormonal supplements or corticosterone and/or l-thyroxine. These regimens failed to alter CCWS antinociception in sham-operated rats. Treatment of hypophysectomized rats with corticosterone and l-thyroxine either separately or together significantly reduced CCWS antinociception. In contrast, if hypophysectomized rats did not receive supplements, CCWS antinociception was significantly potentiated relative to sham-operated controls. These effects could not be attributed to treatment-induced changes in either body weight or CCWS hypothermia. These data suggest that the anterior lobe of the pituitary gland and adrenal cortex are involved in the mediation and/or maintenance of CCWS antinociception.

Enkephalins, brain and immunity: modulation of immune responses by methionine-enkephalin injected into the cerebral cavity

There is a large number of interactions at molecular and cellular levels between the nervous system and the immune system. It has been demonstrated that the opioid neuropentapeptide methionine-enkephalin (Met-Enk) is involved in humoral and cell-mediated immune reactions. Met-Enk injected peripherally produces a dual and dose-dependent immunomodulatory effect: high doses suppress, whereas low doses potentiate the immune reactivity. The present mini-review concerns the immunological activity of Met-Enk after its administration into the lateral ventricles of the rat brain, and describes the extraordinary capacity of centrally applied Met-Enk to regulate/modulate the immune function. This survey is composed of sections dealing with (a) the role of opioid peptides in the central nervous system (CNS); (b) the activity of opioid peptides in the immune system; (c) the application of Met-Enk into the cerebral cavity; (d) the influence of centrally administered Met-Enk on nonspecific local inflammatory reaction; (e) the effect of Met-Enk injected intracerebroventricularly (i.c.v.) on specific delayed hypersensitivity skin reaction, experimental allergic encephalomyelitis, anaphylactic shock, plaque-forming cell response, and hemagglutinin production; (f) the central antagonizing action of quaternary naltrexone, an opioid antagonist that does not cross the brain-blood barrier, on Met-Enk-induced immunomodulation; (g) the alteration of immune responsiveness by i.c.v. injection of enkephalinase-degrading enzymes; (h) the participation of the brain-blood/blood-brain barrier in the CNS-immune system interaction; and (i) the role of opioid receptors in immunological activity of Met-Enk. A hypothesis has been advanced for the reaction of Met-Enk and opioid receptor sitting on the cell membrane. This concept suggests that the constellation of chemical residues of enkephalin and receptor in the microenvironment determines the binding between the opioid partners. The plurality of conformational structures of enkephalins and receptors makes possible their involvement in a variety of processes which occur in different physiological systems, including the nervous system and the immune system, and intercommunications between the two systems.

Effects of various stressors on milk release in the rat

This study investigated the effect of four stimuli on milk release (MR), namely, sound, nociception, novelty, and restraint. The role of the ensuing adrenocortical response in the suppression of MR was also evaluated. Plasma corticosterone (CORT) levels were measured at 0 (basal), 15, 30, and 60 min during the suckling sessions to determine whether elevated CORT normally associated with stress could be inhibitory to MR. Compared to nonstressed lactators, dams exposed to sound demonstrated no suppression in MR, but a significant increase in plasma CORT. Pain did not alter milk yield and elevated CORT only at the end of the first hour of exposure. During novelty, MR was suppressed and again CORT was only elevated at the end of the sampling period. Restraint decreased milk yield and increased CORT. During novelty, MR appeared to be regulated by an adrenal factor, which remains to be identified. The peripheral opiates seem to be partially involved during restraint. In conclusion, not all types of aversive stimuli interfere with MR. Of those which do, different mechanisms seem to be implicated depending upon the nature of the stressor. Furthermore, reduced MR during stress is not a direct consequence of increased CORT.

Possible participation of endogenous opioid peptides on the mechanism involved in analgesia induced by vouacapan

The involvement of opioid peptides in the mechanism of action of vouacapan, a new experimental compound extracted from seeds of Pterodon poligalaeflorus Benth, was investigated both in mice utilizing acetic acid writhing response and in rats utilizing inflammatory hyperalgesia induced by carrageenan and modified Randall-Selitto method. Vouacapan, in both models, caused a dose-dependent analgesia when injected p.o., s.c. and i.p. The analgesic effect was partially blocked by naloxone, nalorphine and n-methyl-nalorphine. Significant tolerance to analgesic effect was observed following repeated administration of vouacapan or morphine. On the last day of treatment, cross administration revealed symmetrical and asymmetrical cross-tolerance between vouacapan and morphine, in rats and mice, respectively. We conclude that a release of endorphins could be involved in the analgesic mechanism of vouacapan in both models tudied.

Abnormal adrenal gland metabolism in opioid addicts: implications for clinical treatment

Adrenal gland metabolism is markedly altered in heroin addicts. During daytime hours, the addict may suffer corticoid deficiency of the addisonian type, and in the evening, an excess of the cushingoid type. The high plasma levels of cortisol that are found in the evening in addicts antagonize endogenous opioids in a manner similar to naloxone. In the present study, 72% of the heroin addicts who sought treatment demonstrated reduced adrenal cortisol reserve. Effective immune and stress responses are dependent on adrenal cortisol reserve. This finding provides an explanation for the heroin addict's vulnerability to AIDS and other infectious diseases. One of methadone's greatest attributes is that it helps normalize adrenal metabolism. Clinical methods to at least partially correct adrenal metabolism may enhance current opioid addiction treatment modalities.

Lappaconitine and N-deacetyllappaconitine potentiate footshock-induced analgesia in rats

The effects of lappaconitine (LA) and N-deacetyllappaconitine (DLA) on footshock-induced analgesia (FSIA) were studied by the rat tail flick test. Rats subjected to 90 s nonescaping footshock had a significant increase in tail flick latency. Naloxone (4 micrograms, i.c.v.) partially antagonized the FSIA. After 5 consecutive exposures to footshock, rats developed a complete tolerance to the FSIA. The rats tolerant to FSIA showed a cross-tolerance to morphine- but not LA- and DLA-induced analgesia. Administrations of subanalgesic doses of LA and DLA potentiated the FSIA in both intact and adrenalectomized rats.

Altered diurnal pattern of arousal following naloxone administration in opioid-naive rats

Heightened arousal occurring in response to physical or psychological stressors in associated with increased levels of endogenous opioid peptides in peripheral circulation and at binding sites in the central nervous system. When administered as a series of bolus injections during active periods, the opioid antagonist naloxone increased delta wave activity and total spectral power in the EEG of opioid-naive rats. A single 1.0 mg/kg injection of naloxone had a similar effect when given at the onset of night-time active periods, but not if administered during the day when rats are normally inactive. These results are consistent with a diminished level of arousal following blockade of endogenous opioid activity, and suggest an excitatory effect of opioid peptides in certain behavioral settings.

Adrenalectomy-induced potentiation of morphine analgesia: reversal by prednisolone

Effects of adrenalectomy (ADX) on analgesic potency and morphine (MOR) content after SC administration of 3.5 or 7 mg/kg of MOR, and effects of prednisolone (PRED) on the ADX-induced effects were studied. ADX significantly potentiated MOR analgesia at both MOR doses, and PRED reversed the ADX-induced potentiation of MOR analgesia, ADX did not affect MOR content in brain and plasma after 3.5 mg/kg MOR, but significantly increased MOR content in brain and plasma after 7 mg/kg MOR, and PRED reversed the ADX-induced increase in the MOR content. Although the analgesic potency of 3.5 mg/kg MOR in ADX group was equipotent with those of 7 mg/kg MOR in sham-operated and PRED-treated ADX groups, MOR content in the former group was significantly lower than those in the latter two groups. These results suggest that ADX potentiates MOR analgesia through both mechanisms of the increased MOR content and the increased sensitivity to MOR, and that the lack of glucocorticoids participates in both of these ADX-induced effects.

Stressors and pain sensitivity in CFW mice. Role of opioid peptides

Effects of several environmental situations on pain threshold were studied in CFW male mice. Immobilization induced significant and naloxone reversible analgesia. Isolation produced analgesia which was partially reversed by naloxone. One minute swimming in + 4 degrees C or + 42 degrees C water increased naloxone reversible analgesia. Isolation produced analgesia which was partially reversed by naloxone. One minute swimming in 4 degrees C or + 42 degrees C water increased naloxone irreversible pain threshold. Other situations: drinking 2% NaCl solution, disturbance of light-dark cycle or social aggregation did not produce analgesia. The role of these situations as stress-inducers, as well as the role of endogenous opioid peptides in stress-induced analgesia, were discussed.

Is spinal cord dorsolateral funiculus involved in hypoalgesia induced by counter-irritation?

Several previous studies have demonstrated that, depending upon the behavioral test used, counter-irritation (i.e. the pain-relieving effects of pain elicited from heterotopic body areas) can produce hypoalgesia. In the present study, behavioral responses were elicited in the rat by increasing calibrated pressure applied to a hindpaw (Randall-Selitto test; 'passive' stimulus) and were studied before and after a subcutaneous formalin injection ('active' stimulus). The vocalization threshold to the pressure was clearly increased after injection of the algogenic solution either in the forepaw or in the cheek. Using this vocalization threshold, the counter-irritation-produced hypoalgesia was generally unchanged by unilateral dorsolateral funiculus (DLF) lesions. Following bilateral DLF lesions, hypoalgesia was decreased when formalin was injected in the forepaw, but was unaffected when the algogen was injected in the cheek. The present results partly contrast with previous papers from our group, where it has been assumed that the DLF is mainly involved in the neural circuitry subserving diffuse noxious inhibitory controls (DNIC), which have been considered as one possible neurophysiological basis for counter-irritation phenomena. They are discussed with reference to various hypotheses, including DNIC, as explanations for counter-irritation.

Hypoalgesia induced by counter-irritation is not affected by pCPA pretreatment

In a previous work (4), it has been described that a noxious visceral stimulation through the intraperitoneal injection of acetic acid (ipAA) induced a transient and low magnitude increase in tail-flick latencies, but a marked increase in the threshold for vocalization and hot-plate latencies. In the present work, this phenomenon of hypoalgesia through counter-irritation was investigated in intact rats with or without pretreatment with the potent serotonin depletor parachlorophenylalanine (pCPA). Three behavioural tests were performed. In two tests (tail flick, vocalization induced by transcutaneous electrical stimulation of the tail), pCPA pretreatment induced an increase of baseline levels, before IP injection of the allogenic agent (ipAA). In the third test, pCPA pretreatment had no effects on jump latencies. Parachlorophenylalanine pretreatment had no effect upon hypoalgesic actions of IP injected AA in all three tests. These results are discussed in terms of pCPA's differential effects upon basal nociception and analgesia induced by various heterotopic nociceptive stimulations.

Antagonism of morphine analgesia by nonopioid cold-water swim analgesia: direct evidence for collateral inhibition

The demonstrated existence of opioid and nonopioid forms of pain control has raised questions as to how they interact. Previous indirect evidence suggests that activation of one system inhibited the activation of the other. The present study assessed this directly using morphine as an opiate form of analgesia and continuous cold-water swims (CCWS, 4 degrees C, 2 min) as the nonopioid form. A significant reduction in morphine (8 mg/kg, SC) analgesia on the tail-flick test was observed if rats were acutely exposed to CCWS immediately prior to morphine administration. The inability of naloxone (10 mg/kg, SC) to reduce CCWS analgesia verified its nonopioid nature. The antagonism of morphine (3 mg/kg, SC) analgesia was greater following preexposure to 2 min of CCWS than 1 min of CCWS. CCWS was also more effective in antagonizing analgesia induced by the 3 mg/kg than the 8 mg/kg dose of morphine. The antagonism of morphine analgesia by CCWS was dependent upon the temporal patterning of stimulus presentation: exposure to CCWS 20 or 60 min prior to morphine failed to alter subsequent morphine analgesia. A significant reduction in analgesia induced by intraperitoneal administration of morphine (10 mg/kg) was also observed when CCWS was presented immediately prior to injection, suggesting that pharmacokinetic factors such as altered drug absorbance by CCWS-induced vasoconstriction do not appear to explain these effects. These data provide direct support for the existence of collateral inhibitory mechanisms activated by CCWS and morphine, and suggests that these opioid and nonopioid forms of analgesia do not function synergistically, but instead involve some form of hierarchical order.