Decreased cerebrospinal fluid beta-endorphin and increased pain sensitivity in patients with functional abdominal pain

Enkephalinase inhibitors, potential therapeutics for the future treatment of diarrhea predominant functional gastrointestinal disorders

The endogenous opioid system (EOS) is considered being a crucial element involved in the pathophysiology of irritable bowel syndrome (IBS) as it regulates gastrointestinal (GI) homeostasis through modulation of motility and water and ion secretion/absorption. Along with opioid receptors (ORs), the following components of EOS can be distinguished: 1. endogenous opioid peptides (EOPs), namely enkephalins, endorphins, endomorphins and dynorphins, and 2. peptidases, which regulate the metabolism (synthesis and degradation) of EOPs. Enkephalins, which are δ-opioid receptors agonists, induce significant effects in the GI tract as they act as potent pro-absorptive neurotransmitters. The action of enkephalins and other EOPs is limited, since EOPs are easily and rapidly inactivated by a natural metalloendopeptidase (enkephalinase/neprilysin) and aminopeptidase N. Studies show that the activity of EOPs can be enhanced by inhibition of these enzymes. In this review, we discuss the antidiarrheal and antinociceptive potential of enkephalinase inhibitors. Furthermore, our review is to answer the question whether enkephalinase inhibitors may be helpful in the future treatment of diarrhea predominant functional GI disorders.

Involvement of neuropeptide systems in schizophrenia: human studies

Neuropeptides are heterogeneously distributed throughout the digestive, circulatory, and nervous systems and serve as neurotransmitters, neuromodulators, and hormones. Neuropeptides are phylogenetically conserved and have been demonstrated to regulate numerous behaviors. They have been hypothesized to be pathologically involved in several psychiatric disorders, including schizophrenia. On the basis of preclinical data, numerous studies have sought to examine the role of neuropeptide systems in schizophrenia. This chapter reviews the clinical data, linking alterations in neuropeptide systems to the etiology, pathophysiology, and treatment of schizophrenia. Data for the following neuropeptide systems are included: arginine-vasopressin, cholecystokinin (CCK), corticotropin-releasing factor (CRF), interleukins, neuregulin 1 (NRG1), neurotensin (NT), neuropeptide Y (NPY), opioids, secretin, somatostatin, tachykinins, thyrotropin-releasing hormone (TRH), and vasoactive intestinal peptide (VIP). Data from cerebrospinal fluid (CSF), postmortem and genetic studies, as well as clinical trials are described. Despite the inherent difficulties associated with human studies (including small sample size, variable duration of illness, medication status, the presence of comorbid psychiatric disorders, and diagnostic heterogeneity), several findings are noteworthy. Postmortem studies support disease-related alterations in several neuropeptide systems in the frontal and temporal cortices. The strongest genetic evidence supporting a role for neuropeptides in schizophrenia are those studies linking polymorphisms in NRG1 and the CCKA receptor with schizophrenia. Finally, the only compounds that act directly on neuropeptide systems that have demonstrated therapeutic efficacy in schizophrenia are neurokinin receptor antagonists. Clearly, additional investigation into the role of neuropeptide systems in the etiology, pathophysiology, and treatment of schizophrenia is warranted.

Preoperative concentration of beta-lipotropin immunoreactive material in cerebrospinal fluid: a predictor of postoperative pain?

Levels of beta-endorphin immunoreactive material (IRM) in cerebrospinal fluid (CSF) have been reported to correlate inversely with postoperative morphine requirement. Considering proopiomelanocortin (POMC) derivatives as predictors for sensitivity to postoperative pain, we determined authentic beta-endorphin (beta-endorphin(1-31)), beta-lipotropin IRM, N-acetyl-beta-endorphin IRM and ACTH in CSF of 17 patients undergoing hip or knee arthroplasty, before surgery (t(A)), immediately after termination of propofol infusion and still under spinal anesthesia (t(B)), under postoperative pain (t(C)) and one day after surgery (t(D)); patients rated their severity of pain on a visual analogue scale (VAS) at those four times. In all patients CSF concentrations of N-acetyl-beta-endorphin IRM and beta-lipotropin IRM were found to be increased after terminating the propofol infusion with spinal anesthesia still effective at t(B). Patients did not feel pain at times t(A), t(B) or t(D); however, they reported moderate to considerable pain at t(C). There were no correlations of postoperative pain severity at t(C) with ACTH, beta-endorphin(1-31) or N-acetyl-beta-endorphin IRM concentrations in CSF. In contrast, we observed significant inverse correlations (Spearman's rank correlation coefficients between -0.83 and -0.85, p<0.01) for postoperative pain severity with beta-lipotropin IRM concentrations in CSF at t(C), and, in addition, at t(A), t(B) and t(D); thus, postoperative pain severity appeared to be dependent on a central system controlling sensitivity to pain, linked to a POMC system releasing beta-lipotropin IRM into CSF and already active at times t(A) and t(B). We conclude that beta-lipotropin IRM in CSF might be considered to serve as a predictor of sensitivity to postoperative pain.

The relationship of sex and clinical pain to experimental pain responses

Considerable research indicates increased experience of clinical pain among females relative to males, and females also demonstrate enhanced responses to experimentally-induced pain. However, previous research has not investigated the relationship between clinical and experimental pain responses in healthy females and males. This experiment examined recent clinical pain as well as thermal pain thresholds and tolerances in 209 (117 female, 92 male) healthy young adults. All subjects completed questionnaires concerning pain-related symptoms over the previous month and subsequently underwent thermal pain assessment. Females reported a larger number of pain sites and greater health care utilization over the month preceding the experimental session, and females also exhibited greater sensitivity to thermal stimuli. In addition, females above the median on the number of pain episodes demonstrated greater thermal pain sensitivity compared to females below the median, but thermal pain responses did not differ as a function of clinical pain among males. The differences remained significant after correcting for psychological variables including hypervigilance and sex role expectancies. These results indicate that experimental pain responses may be more clinically relevant for females than males. Potential explanations and implications for this pattern of results are discussed.

Beta-endorphin in the brain. A role in nociception

We have known the endogenous opioid peptide beta-endorphin for 20 years. Surprisingly, our knowledge of the physiological role of this peptide and its receptors in modulation of pain perception is still fragmentary. Whereas most studies have tried to elucidate the physiological role of beta-endorphin by reversing evoked responses by the opioid antagonist naloxone, this review focuses on quantification of release of beta-endorphin in the brain as the approach to define physiological and pathophysiological roles of beta-endorphin in relation to nociception. Using a lateral ventricle-cisterna magna perfusion model in the anesthetized rat, it was shown that depolarization of neurons in the arcuate nucleus of the hypothalamus, where beta-endorphin in produced, was followed by release of beta-endorphin to the cerebrospinal fluid compartment. Intense activation of spinal nociceptive pathways by intrathecal capsaicin injections also led to beta-endorphin release. It is concluded that there may still be good reason to quantify beta-endorphin in human cerebrospinal fluid to elucidate the role of beta-endorphin in pain perception.

Increased cerebrospinal fluid Met-enkephalin immunoreactivity in patients with chronic tension-type headache

Cerebrospinal fluid (CSF) concentration of Met-enkephalin immunoreactivity (Met-enkephalin-ir) was determined by radioimmunoassay in 47 patients with chronic tension-type headache and in 47 headache-free control subjects. Thirty-nine of the controls were patients receiving spinal analgesia before surgery for diseases not associated with pain; 8 were healthy paid volunteers. Patients reporting migraine more than 1 day per month were excluded. Pericranial tenderness, nociceptive flexion reflex and thermal pain thresholds were determined in the majority of the patients. The median level of CSF Met-enkephalin-ir was significantly higher (115 pmol/l) (quartiles (107-134) pmol/l) in the headache patients than in the controls (median 79 pmol/l) (quartiles (73-87) pmol/l) (Mann-Whitney, P < 0.001). No indication of sex-difference or correlation with age with respect to CSF Met-enkephalin-ir was found. No correlation was found between CSF Met-enkephalin-ir and either pericranial tenderness, nociceptive flexion-reflex threshold, or thermal pain threshold. There was no indication of correlation between consumption of mild analgesics and CSF Met-enkephalin-ir. The higher levels of CSF Met-enkephalin-ir in the headache patients may be indicate activation of the enkephalinergic antinociceptive system at the spinal/trigeminal level, whereas the beta-endorphinergic system appears normal. This enkephalinergic activation may be caused by increased activity in the primary nociceptive afferents, or may be compensatory to decreased activity in other endogenous antinociceptive systems than the opioid.

Release into ventriculo-cisternal perfusate of beta-endorphin- and Met-enkephalin-immunoreactivity: effects of electrical stimulation in the arcuate nucleus and periaqueductal gray of the rat

To examine the resting and evoked release of the endogenous opioid peptides beta-endorphin and Met-enkephalin from brain, we examined the levels of the respective immunoreactivities in the lateral ventricle-cisterna magna perfusate of the halothane-anesthetized rat. Ten Hz but not 100 Hz stimulation in the arcuate nucleus (ARC) of the hypothalamus released beta-endorphin immunoreactivity (beta-EPir) to the perfusate, whereas 100 Hz but not 10 Hz stimulation in the periaqueductal gray (PAG) of the mid brain released Met-enkephalin immunoreactivity (MEir). MEir was not released by stimulation in ARC and beta-EPir was not released by stimulation in PAG. Characterization of the released beta-EPir and MEir by high performance liquid chromatography showed that authentic beta-endorphin and Met-enkephalin were the major constituents of beta-EPir and MEir, respectively. Systemic administration of the dopaminergic antagonist haloperidol increased plasma, but not perfusate levels of beta-EPir. Both the opioid antagonist naloxone and the NMDA antagonist MK-801 failed to affect beta-EPir or MEir release. ARC and PAG stimulated inhibited a nociceptive reflex (tail-dip in 52.5 degrees C water), and naloxone did not reliably reverse this inhibition. These data support the previously suggested possibility of opioid mediation of stimulation induced analgesia, although we were unable to confirm the theory by naloxone reversibility in this study. Furthermore, the data support the assumption that measurement of opioid peptides in cerebrospinal fluid is a relevant approach in research aimed at elucidating the physiological and pathophysiological roles of endogenous opioid peptides.

Irritable bowel syndrome. Managing the patient with abdominal pain and altered bowel habits

Irritable bowel syndrome is a common complex of syndromes thought to be generated by a motility or sensory disturbance of the gastrointestinal tract. It is a frequent cause of chronic abdominal pain and altered bowel habits. Patients who seek medical attention for irritable bowel syndrome often do so because of psychosocial factors. Therapy remains largely empirical, directed toward the relief of symptoms in the context of a supportive physician-patient relationship.

Effect of sulpiride or paroxetine on cerebrospinal fluid neuropeptide concentrations in patients with chronic tension-type headache

In lumbar cerebrospinal fluid (CSF) obtained from patients with chronic tension-type headache (CTH), the concentrations of beta-endorphin, met-enkephalin, dynorphin, cholecystokinin (CCK), calcitonin gene-related peptide (CGRP), and somatostatin were measured before and after 8 weeks of treatment with sulpiride or paroxetine. We previously reported higher than normal met-enkephalin concentrations in CTH. The present study reveals normal basal concentrations of CCK, CGRP and somatostatin and slightly decreased dynorphin in the same patients. Treatment with sulpiride or paroxetine did not change the concentration of any of the neuropeptides measured. These data suggest central changes in opioid systems but not in other peptide systems (CCK, CGRP, somatostatin) involved in nociceptive processing at the level of the spinal cord dorsal horn/nucleus caudalis of the trigeminal nerve in CTH. Such central changes might be pathophysiologically important or merely secondary to other more important occurrences. The lack of changes in neuropeptide concentrations during drug treatment makes planning of studies involving CSF analysis easier, but also limits the probability of obtaining information on specific neuropeptide systems through CSF analysis.