Endogenous opioid peptides and pain mechanisms: a complex relationship

Electrophysiological changes in adult rat dorsal horn neurons after neonatal peripheral inflammation

Neonatal peripheral inflammation has been shown to produce profound anatomical changes in the dorsal horn of adult rats. In this study, we explored whether parallel physiological changes exist. Neonatal rats were injected with complete Freund's adjuvant (CFA) into the left hind paw. At 8-10 wk of age, single dorsal horn neurons were recorded in response to graded intensities of mechanical stimuli delivered to the receptive field. In addition, cord dorsum potentials, produced by electrical stimuli delivered to the left sciatic nerve at 2.5x threshold, were recorded bilaterally from L2 to S3. There were significant increases in background activity and responses to brush and pinch in neonatal rats that were treated with CFA, as compared with control rats. Further analysis showed similar significant changes when dorsal horn neurons were categorized into wide dynamic range (WDR), high-threshold (HT), and low-threshold (LT) groups. The receptive field was significantly larger in neonatally treated rats as compared with control rats. Additionally, there was a significant increase in the response to a 49 degrees C heat stimulus in neonatally treated rats as compared with control rats. There was also a trend for the amplitudes of N1, N2, and P waves of the cord dorsum potential to increase and latencies to decrease in neonatally treated rats, but no significant differences were detected between different levels of the spinal cord (L2 to S3). These data further support the notion that anatomical and physiological plasticity changes occurred in the spinal cord following early neonatal CFA treatment.

The brain-heart axis in the perception of cardiac pain: the elusive link between ischaemia and pain

Angina pectoris is a common symptom and one that can have profound implications for the patient. However, it correlates poorly with the extent of myocardial ischaemia and with prognosis. In order to understand more fully the heterogeneity of the experience of chest pain, we have adopted the technique of functional neuroimaging, where positron emission tomography is used to measure regional cerebral blood flow as an index of regional neuronal activation, during myocardial ischaemia in patients with coronary artery disease. We have been able to delineate those brain areas that are involved in the perception of angina: the hypothalamus, periaquaductal grey, thalami and bilaterally the prefrontal cortex and in the left the inferior anterocaudal cingulate cortex. By studying patients with silent myocardial ischaemia, we have established that the silence is not merely a matter of impaired afferent signalling resulting from autonomic neuropathy, but that it is associated with a failure of transmission of signals from the thalamus to the frontal cortex. At the other end of the spectrum, we have studied patients with syndrome X, a condition of chest pain with ischaemic-like stress electrocardiography (ECG) but entirely normal coronary angiogram; (on the basis of our own and other data we consider an ischaemic aetiology to be most unlikely in this condition). In syndrome X, distinct patterns of cerebral activation were found with characteristic activation of the right anterior insula at its junction with the frontal operculum. In conclusion, we present a unified view of the cerebral handling of afferent signals from the heart throughout this spectrum of experience of chest pain, a view that accounts for the clinical features of the patients studied.

RB 101, a purported pro drug inhibitor of enkephalin metabolism, is antinociceptive in pregnant mice

In an earlier study, we demonstrated the enhancement of pregnancy-induced analgesia with an inhibitor of endogenous enkephalin metabolism. The purpose of the present study was to evaluate the antinociceptive effect of another inhibitor of enkephalin metabolism, RB 101, on pregnant mice. Further, since other studies have shown RB 101 to be free of opioid side effects, we examined its effect on respiratory rate. Analgesia was assessed using the hot plate test, and respiratory rate was measured by recording the output from an end-tidal carbon dioxide detector. In pregnant mice, experiments were conducted on Day 17 or Day 18 of pregnancy; mice usually deliver on Day 19. For the hot plate test, animals were tested in the following groups: Group 1, RB 101 150 mg/kg (n = 15); Group 2, RB 101 50 mg/kg (n = 15); Group 3, RB 101 vehicle (n = 15); Group 4, morphine 5 mg/kg (n = 14); and Group 5, RB 101 150 mg/kg + naloxone 5 mg/kg (n = 10). The test was repeated on the second day after delivery in animals in Groups 1 and 3 (given RB 101 150 mg/kg and RB 101 vehicle, respectively). RB 101 150 mg/kg and morphine 5 mg/kg were significantly different (mean percentage of maximum possible effect 30.0 and 37.7, respectively, at 30 min and 41.6 and 32.6, respectively, at 60 min) in their antinociceptive effect in pregnant animals from all other groups. Naloxone, when coadministered with RB 101, prevented the development of antinociception. RB 101 150 mg/kg was not antinociceptive after delivery. Depression of respiratory rate was tested in a separate set of animals in the following groups: Group 1, RB 101 150 mg/kg (n = 16); Group 2, morphine 5 mg/kg (n = 16); Group 3, RB 101 vehicle (n = 15). Morphine 5 mg/kg produced significant depression of respiratory rate at 30 min postinjection when compared with RB 101 150 mg/kg and RB 101 vehicle (mean percent change in respiratory rate was 78.5% compared with 87.7% and 92.4%, respectively, where 100% = no change). These results suggest that drugs such as RB 101 may produce antinociception with minimal effects on respiration.

SCH 32615, an enkephalinase inhibitor, enhances pregnancy-induced analgesia in mice

Increased tolerance to noxious stimuli during pregnancy has been demonstrated. The purpose of this study was to examine the effect of SCH 32615, an inhibitor of one of the enzymes (enkephalinase) responsible for the degradation of endogenous enkephalins, on pregnancy-induced analgesia in mice. Analgesia was tested using the hot-plate and tail-flick tests. For the hot-plate test, animals were tested in late pregnancy (Day 17 or Day 18 of pregnancy; mice deliver on Day 19) and in the postpartum period (Days 2 and 8 after delivery) in the following groups: i) no treatment (n = 15); ii) vehicle only (n = 15); iii) SCH 32615 250 mg/kg (n = 20), 150 mg/kg (n = 15), 50 mg/kg (n = 14); iv) naloxone 5 mg/kg (n = 15); v) naloxone 5 mg/kg+SCH 32615 150 mg/kg (n = 10); vi) nonpregnant control given SCH 32615 150 mg/kg (n = 14). All drugs were given subcutaneously. Hot-plate latency (HPL) was significantly higher in pregnant mice (mean hot-plate latency 17.5 s) than postpartum mice (mean hot-plate latency 11 s on Day 2 and 8.5 s on Day 8). SCH 32615 250 mg/kg and 150 mg/kg significantly enhanced this analgesia in pregnant mice (mean percent of maximum possible effect 24.2 and 29.9, respectively) but not SCH 32615 50 mg/kg or the vehicle alone (mean percent of maximum possible effect 12.4 and 0.5, respectively). Naloxone significantly lowered HPL in pregnant mice (19.8 s-16.2 s) and antagonized the effect of SCH 32615.(ABSTRACT TRUNCATED AT 250 WORDS)

CSF neuropeptides in cancer pain: effects of spinal opioid therapy

The cerebrospinal fluid (CSF) levels of the opioid peptides met-enkephalin (ME), beta-endorphin (BE) and dynorphin (DYN) as well as the putative sensory neuropeptides substance P (SP), somatostatin (SOM), calcitonin gene related peptide (CGRP) and vasoactive intestinal polypeptide (VIP) were determined in 10 patients with severe nociceptive pain due to malignancy, before and after initiation of spinal opioid therapy, and in 10 control patients. Pain intensity, evaluated by means of a 100-mm visual analog scale (VAS), was reduced from 39 +/- 9 to 18 +/- 10 for continuous pain and from 70 +/- 10 to 10 +/- 8 for intermittent pain (means +/- s.e.mean). Lumbar CSF immunoreactive ME and DYN concentrations were significantly increased (P = 0.05) and BE and VIP were significantly decreased (P < or = 0.05) in the pain patients. A slight, but non-significant (P = 0.06) decrease in SP-like immunoreactivity was found after initiation of spinal opioid therapy. Visceral pain seemed to be associated with low immunoreactive SP and ME concentrations compared to somatic pain. A highly significant correlation was found between SP and ME (P < 0.001) and to a lesser extent also between other peptides. We conclude that the concentrations of the endogenous opioids were more affected by nociceptive pain states than the non-opioid peptides. The origin of pain may also influence the results. The postulated inhibition of peptide release by spinal opioid application seemed to be present for SP, but could otherwise not be confirmed.

The neuroanatomy, neurophysiology, and neurochemistry of pain, stress, and analgesia in newborns and children

Beginning with a brief description of mature anatomic pathways and neurotransmitters in the "pain system," this article details their development in the human fetus, neonate, and child. Special emphasis is given to the basic mechanisms and physiologic effects of opioid analgesia. The clinical implications of these data are described, particularly with regard to the maintenance of cardiovascular stability and hormonal-metabolic homeostasis in newborns and children undergoing surgery or other forms of stress.

Plasma levels of beta-endorphin in depressed patients with and without pain

We studied 27 patients who complained of pain, for which no organic basis could be found, and 11 patients without pain. All patients were depressed according to clinical diagnosis and Beck Depression Inventory (BDI) scores. Plasma beta-endorphin levels were found not to differ between the two groups or across other variables. The patients who complained of pain had significantly higher scores on the BDI.

The effect of naloxone on sexual behavior in female rats depends on the site of injection

Intraperitoneal injections of the opiate receptor antagonist naloxone (0.1, 1, 5 or 10 mg) had no effects on sexual behavior in ovariectomized female rats made either partially or fully receptive by injections of ovarian hormones. Intracerebroventricular (i.c.v.) injections of naloxone (10, 50 or 100 micrograms) had no effects on the behavior of partially receptive rats and in fully receptive rats an i.c.v. injection of a high dose of naloxone (100 micrograms) inhibited the behavior. Intrathecal (i.t.) injections of naloxone (50 or 100 micrograms) facilitated sexual behavior in partially receptive rats while i.t. injections of even very high doses of naloxone (500 micrograms) had no behavioral effects in fully receptive rats. Thus, the effects of naloxone on female sexual behavior depend on the site of injection.

Peripartum concentrations of beta endorphin and cortisol and maternal mood states

Nineteen women were studied before, during and after labour by assessment of their mood using a variety of psychological tests and by measurement of their plasma concentrations of beta-endorphin and cortisol. Beta-endorphin and cortisol concentrations rose markedly during labour and were influenced by the type of analgesia used. A deterioration in cognitive performance between days 2 and 4 postpartum correlated positively with the fall in beta-endorphin concentrations from those in labour to those on the fourth day postpartum. The women were more anxious and depressed at 38 weeks' gestation than on days 1-4 postpartum and the elevation of mood on day 2 postpartum correlated with a measure of depression 8 weeks later. It is postulated that the phenomenon of postpartum blues is a reaction to the euphoria of delivery which in turn is a response to endorphin release during labour. Whilst these changes may have a role in promoting maternal-infant attachment it is at the expense of maternal depression some weeks later.

beta-Endorphins (beta-EP) in amphibians: higher beta-EP levels during regenerating stages of anuran life cycle and immunocytochemical localization of beta-EP in regeneration blastemata

Plasma beta-endorphin (beta-EP) levels were measured at different stages of metamorphosis in anuran species, namely Rana catesbeiana, R. pipiens, and Xenopus laevis. The mean levels of beta-EP in tadpoles and postmetamorphic juveniles were significantly higher than adult levels in all three species examined. The pattern of high tadpole levels and low adult levels corresponds with the limb regeneration ability of the tadpole and restriction in the adult. We have also reported plasma beta-EP levels in two urodele species, Ambystoma tigrinum and A. jeffersonianum. Localization of beta-EP by immunocytochemistry shows bright fluorescence to beta-EP antiserum in the wound epithelium of the regeneration blastema. The mesoderm and nuclei of all blastema cells remain nonreactive. It is suggested that receptors to beta-EP reside in the epidermis.

Pain reactions in pregnant women

Reactions to radiant heat stimuli were measured in pregnant women and in a control group of non-pregnant women matched in age. Receiver operating characteristic curve parameters were computed for each of the stages of pregnancy from the 6th month until the postpartum period. When painful stimuli were used, a group by stage of pregnancy interaction was found in the analysis on the response criterion for reporting pain. Pregnant women were found to be significantly more willing to label radiant heat stimuli as painful during the last 2 weeks of pregnancy than during any of the other stages of pregnancy. The control women tested during the same time sequence as the pregnant women showed stable response criteria across the 6 month testing period. There were no group or stage of pregnancy effects evident when the discrimination accuracy measures were analyzed; nor were any effects found with responses to thermal stimuli. The results suggest that the changes in pain reaction associated with pregnancy are based on non-sensory factors that influence the subject's willingness to report pain.