Cerebrospinal fluid dynorphin-converting enzyme activity is increased by voluntary exercise in the spontaneously hypertensive rat

Inhibitors of neuropeptide peptidases engaged in pain and drug dependence

Owing to a broad spectrum of functions performed by neuropeptides, this class of signaling molecules attracts an increasing interest. One of the key steps in the regulation of biological activity of neuropeptides is proteolytic conversion or degradation by proteinases that change or terminate biological activity of native peptides. These enzymes, in turn, are regulated by inhibitors, which play integral role in controlling many metabolic pathways. Thus, the search for selective inhibitors and detailed knowledge on the mechanisms of binding of these substances to enzymes, could be of importance for designing new pharmacological approaches. The aim of this review is to summarize the current knowledge on the inhibitors of enzymes that convert selected groups of neuropeptides, such as dynorphins, enkephalins, substance P and NPFF fragments. The importance of these substances in pathophysiological processes involved in pain and drug addiction, have been discussed. This article is part of the special issue on Neuropeptides.

Special feature for the Olympics: effects of exercise on the immune system: neuropeptides and their interaction with exercise and immune function

It is known today that the immune system is influenced by various types of psychological and physiological stressors, including physical activity. It is well known that physical activity can influence neuropeptide levels both in the central nervous system as well as in peripheral blood. The reported changes of immune function in response to exercise have been suggested to be partly regulated by the activation of different neuropeptides and the identification of receptors for neuropeptides and steroid hormones on cells of the immune system has created a new dimension in this endocrine-immune interaction. It has also been shown that immune cells are capable of producing neuropeptides, creating a bidirectional link between the nervous and immune systems. The most common neuropeptides mentioned in this context are the endogenous opioids. The activation of endogenous opioid peptides in response to physical exercise is well known in the literature, as well as the immunomodulation mediated by opioid peptides. The role of endogenous opioids in the exercise-induced modulation of immune function is less clear. The present paper will also discuss the role of other neuroendocrine factors, such as substance P, neuropeptide Y and vasoactive intestinal peptide, and pituitary hormones, including growth hormone, prolactin and adrenocorticotrophin, in exercise and their possible effects on immune function.

Special feature for the Olympics: effects of exercise on the immune system: neuropeptides and their interaction with exercise and immune function

It is known today that the immune system is influenced by various types of psychological and physiological stressors, including physical activity. It is well known that physical activity can influence neuropeptide levels both in the central nervous system as well as in peripheral blood. The reported changes of immune function in response to exercise have been suggested to be partly regulated by the activation of different neuropeptides and the identification of receptors for neuropeptides and steroid hormones on cells of the immune system has created a new dimension in this endocrine-immune interaction. It has also been shown that immune cells are capable of producing neuropeptides, creating a bidirectional link between the nervous and immune systems. The most common neuropeptides mentioned in this context are the endogenous opioids. The activation of endogenous opioid peptides in response to physical exercise is well known in the literature, as well as the immunomodulation mediated by opioid peptides. The role of endogenous opioids in the exercise-induced modulation of immune function is less clear. The present paper will also discuss the role of other neuroendocrine factors, such as substance P, neuropeptide Y and vasoactive intestinal peptide, and pituitary hormones, including growth hormone, prolactin and adrenocorticotrophin, in exercise and their possible effects on immune function.

Deprived of habitual running, rats downregulate BDNF and TrkB messages in the brain

To study possible effects of physical training on the expression of neurotrophic factors and their receptors in the brain, we used a rat strain (spontaneously hypertensive rat, SHR), known to spontaneously run up to 20 km/night. We show that such long-distance running affects the brain-derived neurotrophic factor (BDNF) and TrkB system in hippocampus, and in particular that abrupt deprivation of habitual running leads to long-lasting decreases of BDNF/TrkB expression in hippocampus. Quantitative in situ hybridization demonstrates that running increases the expression of mRNA coding for BDNF and its high affinity receptor TrkB in hippocampus in a running length dependent manner. In addition, we show that an abrupt interruption of prolonged spontaneous exercise decrease expression of mRNA encoding BDNF and TrkB in certain hippocampal areas and that this decrease lasts at least 10 days. This down-regulation was most prominent in medial cornu ammonis 3 (CA3M). Several other trophic factors and receptors were investigated, including NGF, NT3, GDNF, trkC and p75. For these other probes investigated, no robust changes in mRNA expression were noted. Areas examined included sensorimotor cortex and hippocampus. For RET, p75, NT3, TrkB and BDNF we also examined the spinal cord without detecting any robust changes. We conclude that spontaneous running as well as its abrupt termination, leads to area-specific and trophic factor-specific changes in hippocampus.

Electrospray ionization mass spectrometric and liquid chromatographic-mass spectrometric studies on the metabolism of synthetic dynorphin A peptides in brain tissue in vitro and in vivo

Metabolic stability of synthetic dynorphins [N-terminal fragments of dynorphin A (Dyn A)] were evaluated in vitro and in vivo. These peptides were applied at concentrations 100-1000 times higher than those of the endogenous dynorphins. Degradation kinetics of these peptides were studied in rat brain homogenate by using microbore gradient RP-LC assay, and limited information on their metabolism was obtained by electrospray ionization mass spectrometry (ESI-MS) of the isolated metabolites. In vivo cerebral microdialysis, in which the peptides were introduced via the probe placed in striatum region of the brain of the experimental animals, was used to circumvent contamination arising from autoproteolysis of brain during incubation of the samples in vitro. Metabolites of Dyn A (1-13) and Dyn A (1-11) were identified from electrospray ionization mass spectra of the microdialysates without chromatographic separation; the identification of peptides in the mixtures were supported by medium resolution ESI Fourier-transform ion cyclotron resonance MS. LC-MS was used to fully characterize the complex peptide mixture obtained after the striatal perfusion of Dyn A (1-12).

Neural control in human muscle fatigue: changes in muscle afferents, motoneurones and motor cortical drive [corrected]

To understand the neural factors which contribute to fatigue, it is not satisfactory to regard fatigue as occurring only when a task can no longer be performed. Changes in muscle afferent feedback, motoneuronal discharge, motor cortical output, and perceived effort develop well before an endurance limit in limb muscles. During sustained maximal contractions the discharge of motoneurones declines, commonly to below the level required to produce maximal force from the muscle whose contractile speed is usually slowed. Thus, some 'central' fatigue develops. Recent findings using transcranial stimulation have revealed that the motor cortex is one site at which suboptimal output develops during human muscle fatigue. There is a need to study the reflex effects on motoneurones and the excitability of the motor cortex in experimental animals, as well as to apply rigorous methods to assess these processes in voluntary exercise in human subjects [corrected].

Elevated cerebrospinal fluid substance P-like immunoreactivity in patients with painful osteoarthritis, but not in patients with rhizopatic pain from a herniated lumbar disc

Cerebrospinal fluid (CSF) levels of substance P like immunoreactivity (SPLI) were determined in 11 patients with painful osteoarthritis in hip or knee, 9 patients with rhizopatic pain from a herniated lumbar disc, and in 9 healthy volunteers without pain. The patients with osteoarthritis had increased levels of SPLI in CSF (p < 0.001) compared to the controls. A positive correlation was also seen between the CSF SPLI and the degree of pain. At a second lumbar puncture 5 months after operation, SPLI had decreased, but was still significantly higher than in the controls. No difference in CSF SPLI was seen in the patients with herniated lumbar disc compared to the controls, neither before treatment, nor at follow up CSF postoperatively. The results suggest that nociceptive joint pain is consistent with increased SPLI in CSF. Differences in SPLI in CSF may be useful to differentiate pain from various origin, and may also increase our understanding of different pain mechanisms.

Reversed-phase high-performance liquid chromatography combined with tandem mass spectrometry in studies of a substance P-converting enzyme from human cerebrospinal fluid

The application of reversed-phase HPLC in combination with micro-electrospray mass spectrometry to study a substance P (SP)-hydrolysing endoprotease in human cerebrospinal fluid (hCSF) is reported. The enzyme was partially purified from the hCSF specimens by ion-exchange chromatography and molecular sieving. During the purification procedure the enzyme activity was monitored by measuring the formation of the SP-fragment 1-7 from SP by radioimmunoassay. Regarding its behaviour upon molecular sieve chromatography, the enzyme was suggested to be associated with an apparent molecular mass of around 100 x 10(3). In subsequent experiments using the partially purified endopeptidase, the hydrolysis of SP was demonstrated by HPLC. The reaction product mixture was resolved in several components including the N-terminal fragments 1-8, 1-7 and 1-6 and the C-terminal fragment 8-11. The identity of these fragments were confirmed by tandem mass spectrometry. It was concluded that the present SP-degrading enzyme is different from those previously identified and purified from hCSF. The applied techniques were proven to be highly efficient for the recovery and identification of the released peptide products.

Chronic intracerebroventricular administration of beta-endorphin augments natural killer cell cytotoxicity in rats

We have studied the effect of chronic intracerebroventricular (i.c.v.) infusion of different opioid peptides on natural killer (NK) cell mediated cytotoxicity in vivo in the spontaneously hypertensive rat (SHR). The in vivo NK cell activity was measured as the clearance of 51Cr-labelled YAC-l lymphoma cells from the lung tissues. Further, the phenotype of lymphocytes in spleen and peripheral blood was analysed by flow cytometry (FACS). All opioid drugs were administered i.c.v. for 6 days with osmotic minipumps releasing 1.0 microliter/h. beta-Endorphin (10 or 20 micrograms/rat per day) significantly increased NK cell cytotoxicity in vivo. The opioid receptor antagonist naloxone (10 mg/kg, i.p.) given immediately before the injection of YAC-lymphoma cells, completely abolished the effects of i.c.v. administered beta-endorphin. Corresponding doses of beta-endorphin administered subcutaneously (s.c.) with minipumps for 6 days did not significantly affect NK cell cytotoxicity. Neither Leu- or Met-enkephalin (20 micrograms/rat per day) nor dynorphin (20 micrograms/rat per day) administered i.c.v. had any significant effects on NK cell activity. In beta-endorphin treated SHR, the percentage of cells with NK cell phenotype (OX52+/CD5-) in peripheral blood was not significantly different from that of controls, while the percentage of cells with T cell phenotype (CD5+/OX52-) was significantly decreased. The percentage of splenic NK cells (OX52+/CD5-) and T cells (CD5+/OX52-) was also unchanged by beta-endorphin treatment i.c.v. These results suggest that of the opioid peptides administered i.c.v., only beta-endorphin augments in vivo NK cell mediated cytotoxicity. We thus conclude that these effects most probably are centrally and opioid receptor mediated effects, since beta-endorphin in the same dose administered peripherally does not influence in vivo NK cell cytotoxicity.

Purification and characterization of a secreted arginine-specific dibasic cleaving enzyme from EL-4 cells

A secreted dibasic cleaving peptidase capable of converting dynorphins into Leu-enkephalin-Arg6 was purified from the medium of EL-4 mouse thymoma cells. The enzyme is a novel metalloendopeptidase with a neutral pH optimum (6.9) and a molecular weight of approximately 130 000. The dibasic cleaving enzyme was completely inhibited in the presence of 20-50 mM amine buffers, 0.1 mM EDTA, 0.5 mM 1,10-phenanthroline, 0.5 mM N-ethylmaleimide, and 1mM DTNB. Unlike the Kex2 family of proteases, Ca2+ did not activate the endopeptidase, but high concentrations (1 mM) of metal ions such as Cu2+, Ni2+, Zn2+, and Co2+ completely inhibited the enzyme. Inhibition was not seen with 0.2 mM TLCK, 1 mM DTT, and 1 mM PMSF. The enzyme will cleave Arg-Arg and Arg-Lys bonds, but not Lys-Arg or Lys-Lys bonds in identical environments, and no aminopeptidase or carboxypeptidase activity was seen. The size of the substrate does not seem to be a determining factor, since dynorphin A(1-12) is cleaved at a rate similar to prodynorphin B(228-256) containing 29 amino acids. The identity of the residues on either side of the cleavage site influences the rate of processing, as noted by different rates of cleavage for the same size peptides dynorphin A(1-13) vs dynorphin A(1-9) vs beta-neoendorphin. The presence of proline in the P3' (alpha-neoendorphin), P4' (dynorphin A(1-11)), or P5' (bovine adrenal medulla dodecapeptide) position does not prevent cleavage, but neurotensin and its (1-11) fragment containing both P2 and P2' proline residues are not cleaved.

Effects of long-lasting voluntary running on the cerebral levels of dopamine, serotonin and their metabolites in the spontaneously hypertensive rat

The brain regional dopamine (DA) and serotonin (5-HT) levels and turnover were studied in the spontaneously hypertensive rat (SHR), following voluntary, long-lasting (7 weeks) wheel-running exercise. Groups of rats were sacrificed 1-2 h, 23-24 h or 47-48 h after termination of the last running session, and the cerebral tissue levels of 5-HT, 5-HIAA, DA and DOPAC were determined and compared to corresponding levels obtained in sedentary controls. In the exercised animals, there was a selective decrease in the limbic forebrain levels of DOPAC in the immediate post-exercise period (1-2 h), while the DA turnover (DOPAC/DA ratio) was not altered. In addition, the 5-HT and 5-HIAA levels in the serotoninergic nerve terminal limbic forebrain and the 5-HT turnover (5-HIAA/5-HT ratio) in the cell body-containing brain stem areas were decreased in the immediate post-exercise period. No significant changes in the DA, DOPAC, 5-HT or 5-HIAA levels, nor in the DA and 5-HT turnover, were obtained in the remainder of the brain regions analyzed, regardless of time after termination of running. Taken together, the present study indicates that voluntary exercise in the SHR gives rise to differentiated regional effects on brain DA and 5-HT levels and turnover, thus supporting the view that changes in central monoaminergic activity are involved in the functional effects of long-term exercise. Interestingly, the psychomotor-associated limbic forebrain areas appeared particularly susceptible.

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.