Central changes of beta-endorphin-like immunoreactivity during rat tonic pain differ from those of purified beta-endorphin

Possible involvement of β-endorphin in docosahexaenoic acid-induced antinociception

We have previously demonstrated that the n-3 polyunsaturated fatty acid docosahexaenoic acid (DHA) has an antinociceptive effect on various pain stimuli in a naloxone-reversible manner. In the present study, the role of the endogenous opioid peptide β-endorphin in DHA-induced antinociception was examined. DHA-induced antinociception was abolished when mice were pretreated with the μ-opioid receptor antagonist β-funaltrexamine (β-FNA) and the δ-opioid receptor antagonist naltrindole, but not by the κ-opioid receptor antagonist nor-binaltorphimine (nor-BNI) in the acetic acid-induced writhing test. In the radioligand binding assay, DHA itself did not have affinity for μ- , δ- or κ-opioid receptors. On the other hand, the pretreatment of anti-β-endorphin antiserum inhibited DHA-induced antinociception. Furthermore, the intracerebroventricular injection of DHA dose-dependently reduced writhing behavior, and this effect was inhibited by d-Phe-Cys-Tyr-Orn-Thr-Pen-Thr-NH(2) (CTOP) and naltrindole, but not nor-BNI. β-endorphin-induced antinociception was inhibited by the pretreatment of β-FNA, but not naltrindole or nor-BNI, and its levels in plasma were increased by DHA treatment. These findings suggest that the induction of antinociception by DHA may partially involve the μ-opioid receptor via the release of β-endorphin.

Involvement of endogenous beta-endorphin in antinociception in the arcuate nucleus of hypothalamus in rats with inflammation

Although exogenous administration of beta-endorphin to the arcuate nucleus of hypothalamus (ARC) had been shown to produce antinociception, the role of endogenous beta-endorphin of the ARC in nociceptive processing has not been studied directly. The aim of the present study was to investigate the effect of endogenous beta-endorphin in the ARC on nociception in rats with carrageenan-induced inflammation. The hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. Intra-ARC injection of naloxone had no significant influence on the HWL to thermal and mechanical stimulation in intact rats. The HWL decreased significantly after intra-ARC injection of 1 or 10 microg of naloxone in rats with inflammation, but not with 0.1 microg of naloxone. Furthermore, intra-ARC administration of the selective mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) decreased the nociceptive response latencies to both stimulation in a dose-dependent manner in rats with inflammation, while intra-ARC administration of the selective delta-opioid receptor antagonist naltrindole or the selective kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) showed no influences on the nociceptive response latency. The antiserum against beta-endorphin, administered to the ARC, also dose-dependently reduced the HWL in rats with inflammation. The results indicate that endogenous beta-endorphin in the ARC plays an important role in the endogenous antinociceptive system in rats with inflammation, and that its effect is predominantly mediated by the mu-opioid receptor.

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

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

Persistent inflammatory nociception increases levels of dynorphin1-17 in the spinal cord, but not in supraspinal nuclei involved in pain modulation

It is well established that nerve injury or inflammatory injury results in a time-dependent increase in the expression of dynorphin in the spinal cord. However, little is known about the effects of persistent pain on the expression of this endogenous opioid peptide by supraspinal nuclei implicated in the modulation of pain sensitivity. This study used enzyme-linked immunosorbent assay to measure the levels of dynorphin(1-17) in the spinal cord as well as in brainstem nuclei 4 hours, 4 days, or 2 weeks after intraplantar injection of saline or complete Freund's adjuvant in the left hind paw. As previously reported, complete Freund adjuvant produced a time-dependent increase in dynorphin that was confined to the ipsilateral dorsal horn. In contrast, levels of dynorphin(1-17) in the nucleus raphe magnus, nucleus reticularis gigantocellularis pars alpha, parabrachial nuclei, microcellular tegmentum, pontine periaqueductal gray, and midbrain periaqueductal gray were not affected at any time after injection of complete Freund adjuvant. These data suggest that alterations in levels of dynorphin do not mediate the up-regulation of activity in bulbospinal pain inhibitory or pain facilitatory pathways that occurs during persistent pain.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Roles of endogenous opioid peptides in modulation of nocifensive response to formalin

Roles of endogenous opioid peptides and their receptors in modulation of the nocifensive responses to formalin in mice were studied. Mice were pretreated i.c.v. or intrathecally (i.t.) with selective opioid receptor antagonists or intrathecally with antisera against endogenous opioid peptides and the nocifensive licking responses to intraplantar injection of formalin (0.5%, 25 microl) were then observed. Pretreatment with the epsilon-opioid receptor antagonist beta-endorphin(1-27) or the selective mu-opioid receptor antagonist D-Phe-Cys-Tyr-Orn-Thr-Pen-Thr-NH(2) (CTOP) given i.c.v. dose dependently enhanced the second, but not the first phase of the nocifensive response. However, i.c.v. pretreatment with the selective delta-receptor antagonist naltrindole or kappa-receptor antagonist nor-binaltrophimine did not affect the nocifensive responses. Intrathecal pretreatment with selective delta(1)-opioid antagonist 7-benzylidene naltrexamine significantly enhanced both the first and second phases of nocifension. Intrathecal pretreatment with CTOP also increased the second but not the first phase of the nocifension. However, i.t. pretreatment with the selective delta(2)-receptor antagonist naltriben or nor-binaltrophimine did not affect the second phase of the nocifension. Intrathecal pretreatment with antiserum against Leu-enkephalin, Met-enkephalin, or dynorphin A(1-17), but not beta-endorphin, enhanced only the second phase of nocifensive response to formalin. It is concluded that the blockade of epsilon- and mu-receptors, but not delta- or kappa-receptors, at the supraspinal sites enhanced the second phase of formalin-induced nocifension. In the spinal cord, Leu-enkephalin, and to a lesser extent, Met-enkephalin and dynorphin A(1-17) and mu- and delta(1)-opioid receptors, but not delta(2)- or kappa-opioid receptors, are involved in modulating the feedback inhibition of the second phase of formalin-induced nocifension.

Antisera against endogenous opioids increase the nocifensive response to formalin: demonstration of inhibitory beta-endorphinergic control

The roles of endogenous opioid peptides in the brain in the modulation of nocifensive responses to formalin in ICR mice were studied. Mice were pretreated intracerebroventricularly (i.c.v.) with rabbit antiserum against beta-endorphin, [Leu5]enkephalin, [Met5]enkephalin or dynorphin A-(1-17) 1 h prior to intraplantar injection of formalin (0.5%, 25 microl) and the nocifensive licking responses were then observed. Pretreatment of mice with antiserum against beta-endorphin enhanced the second phase, but not the first phase of the nocifensive responses to formalin. Pretreatment with antiserum against [Leu5]enkephalin also caused a small but statistically significant enhancement of the second phase, but not the first phase of nocifensive responses to formalin. On the other hand, pretreatment with antiserum against [Met5]enkephalin or dynorphin A-(1-17) did not affect the nocifensive response to formalin. Our results indicate that beta-endorphinergic, and to a lesser extent, [Leu5]enkephalinergic systems are activated at the supraspinal sites to attenuate the nocifensive responses to formalin stimulation.

Effect of VMH lesion on sucrose-fed analgesia in formalin pain

The ingestion of sucrose (ad libitum) produces an immediate analgesic response to phasic noxious stimuli. The underlying mechanism for the analgesic effect of sucrose is attributed to its palatability, which mediates analgesia probably by the release of beta-endorphin in the hypothalamus. The present study was designed to explore the role of ventromedial hypothalamus in the mediation of sucrose-fed analgesia. Adult male albino rats each received (20%) sucrose solution orally through a separate bottle until they had ingested 4-5 ml. Their behavioral responses to tonic noxious stimulus in a formalin test were studied in pre- and postsucrose-fed rats of control and in the VMH lesion groups. The average pain rating of a 60-min session significantly (p < 0.01) decreased after sucrose feeding in control rats, from 1.94 +/- 0.13 to 1.45 +/- 0.14, but sucrose feeding by the VMH lesion rats did not alter their tonic nociceptive response from a 1.70 +/- 0.07 presucrose-fed state to a 1.71 +/- 0.08 postsucrose-fed state. VMH lesion per se did not alter the nociceptive response in comparison with controls. The results suggest that sucrose feeding produces analgesia to tonic noxious stimulus, which is abolished by lesion of the VMH, thereby indicating a significant role of VMH in sucrose-fed analgesia.

Effect of VMH lesion on sucrose-Fed nociceptive responses

An initial analgesia followed by hyperalgesia to phasic noxious stimuli occurs after ingestion of sucrose ad libitum. However, the mechanism underlying hyperalgesia is not known. The present study was designed to explore the role of VMH in the mediation of the hyperalgesic effect of sucrose ingestion. Adult male albino rats received sucrose solution (20% p.o.) in addition to laboratory food pellets and tap water ad libitum. Their behavioural responses to various phasic and tonic noxious stimuli were recorded after 6, 12 and 48 h during pre and post-sucrose fed states in both the control and VMH lesion groups of rats. Sucrose feeding to control rats significantly reduced the tail flick latency (TFL) and threshold of vocalization during stimulus (SV) and after discharge (VA) indicating hyperalgesia, while the threshold of tail flick remained unaffected. The average pain rating during the formalin test (tonic pain) decreased significantly indicating analgesia. VMH lesion decreased the latency (mean +/- SD) for tail flick (11.26 +/- 4.65 from 15.61 +/- 5.12 s), threshold (median) for tail flick (0.04 from 0.08 mA), vocalization during stimulus (0.05 from 0.1 mA) and vocalization after discharge (0.15 from 0.2 mA), while the tonic pain rating increased, thereby suggesting a hyperalgesic state. However, sucrose feeding to lesioned rats neither potentiated nor attenuated their hyperalgesia. The results suggest that sucrose feeding for 6-48 h ad libitum produces hyperalgesia to phasic noxious and analgesia to tonic noxious stimuli, while VMH lesion produces hyperalgesia to both phasic and tonic noxious stimuli. Secondly, sucrose ingestion by VMH lesion rats does not affect their responses to pain, suggesting the possible role of VMH in the mediation of sucrose-fed nociceptive responses.

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

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

The effect of an acute ethanol exposure on the rat brain POMC opiopeptide system

Endogenous peptides with a high affinity for opiate receptors located in the central nervous system have been postulated to play a role in the etiology of alcoholism or in other addictive diseases. Effects of different doses of ethanol (EtOH) at different time points post-EtOH administration on hypothalamic and hindbrain beta-endorphin-like peptide (beta-EPLP) content in male rats were measured by radioimmunoassay (RIA). A single EtOH exposure resulted in an increase in hypothalamic and a decrease in the hindbrain beta-EPLP levels. In another set of experiments, proopiomelanocortin (POMC) synthesis in the hypothalamus was measured by assaying both POMC primary transcript and mature mRNA. No changes in primary transcript or mature POMC hypothalamic mRNA were detected. Based on the results from this experiment we conclude that an acute EtOH exposure affects the rat hypothalamic POMC opiopeptide system by increasing levels of beta-EPLP and this increase in levels is not related to an increase in synthesis.

Sex-related effects on behaviour and beta-endorphin of different intensities of formalin pain in rats

The effects of two intensities of formalin pain on behaviour and beta-Endorphin (beta-EP) concentration in the brain and pituitary were studied in male and female rats. The animals were familiarized with the Hole-Board apparatus for 3 days, and then, after a subcutaneous injection of formalin (50 microliter, 0.1 or 10%) or Sham-injection (Control) in the hindpaw, they were tested in the Hole-Board for 60 min. Licking, Flexing and Paw-Jerk of the injected limb were recorded. beta-EP concentration was determined in the hypothalamus (HYP), the periaqueductal gray matter (PAG), the anterior pituitary (AP) and the neurointermediate lobe (NIL). Licking and Flexing durations were greater in females than males only with formalin 10%. Sex differences in beta-EP concentration between the Control groups were found in all tissues except the HYP; beta-EP levels were higher in females in the PAG and NIL, but greater in the AP in males. beta-EP concentration increased in males in the HYP and NIL with formalin 10%; in females, a decrease was found in the HYP with formalin 0.1%. The present results suggest that: (a) there are differences between males and females in the responses to formalin pain, and the nature (pattern and duration) of the sex differences varies according to the pain intensity; (b) there are differences in beta-EP concentration between the two sexes in control animals, and male and female rats also exhibit differences in the modifications of beta-EP in response to formalin-induced pain.

Cerebrospinal fluid beta-endorphin in models of hyperalgesia in the rat

Cerebrospinal fluid (CSF) obtained by acute percutaneous puncture of the cisternal membrane of the halothane anesthetized rat has low but measurable concentrations of beta-endorphin-like immunoreactivity (beta-EPir: 32.8 +/- 3.0 pmol/l). Chromatographic separation of beta-EPir showed that authentic beta-endorphin1-31 was the main component of beta-EPir in cisternal CSF. Subcutaneous injection of 5% formalin in the hind paws did not increase beta-EPir in cisternal CSF. Rats with tactile paw hyperalgesia evoked by unilateral ligation of the L5/6 nerve roots 2 weeks earlier had beta-EPir concentrations that did not differ from sham operated or unoperated control animals. In contrast, capsaicin injected in the hindpaws increased the mean beta-EPir concentration compared to saline injections (P = 0.006) 45 min after emerging from anesthesia following injection. These results show that acute activation of C fibers (by capsaicin) will evoke the release of beta-endorphin into the CSF, suggesting activation of the beta-endorphin terminal systems in the brain/midbrain. The failure of formalin injections to release beta-EPir to CSF may be due to specificity of the afferent stimulus evoking beta-EPir release, a lower stimulus intensity, and/or the duration of the stimulus generated by formalin. The normal concentrations of beta-EPir found in the hyperalgesic state following nerve injury suggest that the supraspinal beta-endorphin system does not display tonic changes under such conditions.

Noxious mechanical stimulation of the hind paws of the anaesthetized rat fails to elicit release of immunoreactive beta-endorphin in the periaqueductal grey matter

As a test of the hypothesis that an animal responds to a severe peripheral painful stimulus by a central release of beta-endorphin, antibody microprobes were inserted stereotactically into the midbrain of urethane anesthetized rats. These microprobes bore antibodies to beta-endorphin immobilized to their outer surfaces. While microprobes were in the brain for periods of 10 to 30 min either no stimulus was delivered or alligator clamps were applied to both hind paws. Microprobes were then incubated with 125I-beta-endorphin. Quantitative image analysis of microprobe autoradiographs showed no differences between the no-stimulus and noxious-stimulus groups. Thus these experiments found no evidence for beta-endorphin release following a severe peripheral painful stimulus.