Regional distribution of LEU and MET enkephalin in rat brain

Supraspinal peroxynitrite modulates pain signaling by suppressing the endogenous opioid pathway

Peroxynitrite (PN, ONOO(-)) is a potent oxidant and nitrating agent that contributes to pain through peripheral and spinal mechanisms, but its supraspinal role is unknown. We present evidence here that PN in the rostral ventromedial medulla (RVM) is essential for descending nociceptive modulation in rats during inflammatory and neuropathic pain through PN-mediated suppression of opioid signaling. Carrageenan-induced thermal hyperalgesia was associated with increased 3-nitrotyrosine (NT), a PN biomarker, in the RVM. Furthermore, intra-RVM microinjections of the PN decomposition catalyst Fe(III)-5,10,15,20-tetrakis(N-methyl-pyridinium-4-yl)porphyrin (FeTMPyP(5+)) dose-dependently reversed this thermal hyperalgesia. These effects of FeTMPyP(5+) were abrogated by intra-RVM naloxone, implicating potential interplay between PN and opioids. In support, we identified NT colocalization with the endogenous opioid enkephalin (ENK) in the RVM during thermal hyperalgesia, suggesting potential in situ interactions. To address the functional significance of such interactions, we exposed methionine-enkephalin (MENK) to PN and identified the major metabolite, 3-nitrotyrosine-methionine-sulfoxide (NSO)-MENK, using liquid chromatography-mass spectrometry. Next, we isolated, purified, and tested NSO-MENK for opioid receptor binding affinity and analgesic effects. Compared to MENK, this NSO-MENK metabolite lacked appreciable binding affinity for δ, μ, and κ opioid receptors. Intrathecal injection of NSO-MENK in rats did not evoke antinociception, suggesting that PN-mediated chemical modifications of ENK suppress opioid signaling. When extended to chronic pain, intra-RVM FeTMPyP(5+) produced naloxone-sensitive reversal of mechanical allodynia in rats following chronic constriction injury of the sciatic nerve. Collectively, our data reveal the central role of PN in RVM descending facilitation during inflammatory and neuropathic pain potentially through anti-opioid activity.

Striatal alterations of secretogranin-1, somatostatin, prodynorphin, and cholecystokinin peptides in an experimental mouse model of Parkinson disease

The principal causative pathology of Parkinson disease is the progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta projecting to the striatum in the brain. The information regarding the expression of neuropeptides in parkinsonism is very limited. Here we have elucidated striatal neuropeptide mechanisms in experimental parkinsonism using the unilateral 6-hydroxydopamine model to degenerate dopamine neurons. A thoroughly controlled sample preparation technique together with a peptidomics approach and targeted neuropeptide sequence collections enabled sensitive detection, identification, and relative quantitation of a great number of endogenous neuropeptides. Previously not recognized alterations in neuropeptide levels were identified in the unilateral lesioned mice with or without subchronic 3,4-dihydroxy-L-phenylalanine administration, the conventional treatment of Parkinson disease. Several of these peptides originated from the same precursor such as secretogranin-1, somatostatin, prodynorphin, and cholecystokinin. Disease-related biotransformation of precursors into individual peptides was observed in the experimental model of Parkinson disease. Several previously unreported potentially biologically active peptides were also identified from the striatal samples. This study provides further evidence that neuropeptides take part in mediating the central nervous system failure associated with Parkinson disease.

Characterizing intercellular signaling peptides in drug addiction

Intercellular signaling peptides (SPs) coordinate the activity of cells and influence organism behavior. SPs, a chemically and structurally diverse group of compounds responsible for transferring information between neurons, are broadly involved in neural plasticity, learning and memory, as well as in drug addiction phenomena. Historically, SP discovery and characterization has tracked advances in measurement capabilities. Today, a suite of analytical technologies is available to investigate individual SPs, as well as entire intercellular signaling complements, in samples ranging from individual cells to entire organisms. Immunochemistry and in situ hybridization are commonly used for following preselected SPs. Discovery-type investigations targeting the transcriptome and proteome are accomplished using high-throughput characterization technologies such as microarrays and mass spectrometry. By integrating directed approaches with discovery approaches, multiplatform studies fill critical gaps in our knowledge of drug-induced alterations in intercellular signaling. Throughout the past 35 years, the National Institute on Drug Abuse has made significant resources available to scientists that study the mechanisms of drug addiction. The roles of SPs in the addiction process are highlighted, as are the analytical approaches used to detect and characterize them.

Reward deficiency syndrome: genetic aspects of behavioral disorders

The dopaminergic and opioidergic reward pathways of the brain are critical for survival since they provide the pleasure drives for eating, love and reproduction; these are called 'natural rewards' and involve the release of dopamine in the nucleus accumbens and frontal lobes. However, the same release of dopamine and production of sensations of pleasure can be produced by 'unnatural rewards' such as alcohol, cocaine, methamphetamine, heroin, nicotine, marijuana, and other drugs, and by compulsive activities such as gambling, eating, and sex, and by risk taking behaviors. Since only a minority of individuals become addicted to these compounds or behaviors, it is reasonable to ask what factors distinguish those who do become addicted from those who do not. It has usually been assumed that these behaviors are entirely voluntary and that environmental factors play the major role; however, since all of these behaviors have a significant genetic component, the presence of one or more variant genes presumably act as risk factors for these behaviors. Since the primary neurotransmitter of the reward pathway is dopamine, genes for dopamine synthesis, degradation, receptors, and transporters are reasonable candidates. However, serotonin, norepinephrine, GABA, opioid, and cannabinoid neurons all modify dopamine metabolism and dopamine neurons. We have proposed that defects in various combinations of the genes for these neurotransmitters result in a Reward Deficiency Syndrome (RDS) and that such individuals are at risk for abuse of the unnatural rewards. Because of its importance, the gene for the [figure: see text] dopamine D2 receptor was a major candidate gene. Studies in the past decade have shown that in various subject groups the Taq I A1 allele of the DRD2 gene is associated with alcoholism, drug abuse, smoking, obesity, compulsive gambling, and several personality traits. A range of other dopamine, opioid, cannabinoid, norepinephrine, and related genes have since been added to the list. Like other behavioral disorders, these are polygenically inherited and each gene accounts for only a small per cent of the variance. Techniques such as the Multivariate Analysis of Associations, which simultaneously examine the contribution of multiple genes, hold promise for understanding the genetic make up of polygenic disorders.

Distribution of glutamate and preproenkephalin messenger RNAs following transient focal cerebral ischemia

Middle cerebral artery occlusion may result in increased activation of N-methyl-D-aspartate- or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type receptors by glutamate and lead to neuronal cell death. To characterize molecular events that precede cell death following transient focal ischemia, in situ hybridization histochemistry was used to measure levels of glutamate receptor subunit 1 (GluR1), GluR2, GluR3, N-methyl-D-aspartate receptor subunit 1 (NR1) and preproenkephalin messenger RNAs in adult rats at various recirculation times (1.5, 3 and 24 h) following a 90-min period of middle cerebral artery occlusion. At 1.5 and 3 h recirculation, autoradiography showed pronounced but differential decreases in AMPA, NR1 and preproenkephalin messenger RNA expression throughout the infarcted ipsilateral striatum. Non-uniform patterns of in situ hybridization grains emerged such that many striatal neurons were depleted of AMPA and preproenkephalin messenger RNAs, while others retained control levels. In cortical regions destined to undergo infarction, GluR2 and NR1 messenger RNAs were preferentially reduced relative to the contralateral side (to 75+/-8.5% and 66+/-4.5%, respectively); GluR1, GluR3 and preproenkephalin messenger RNAs were unaltered. At 24 h recirculation, depletion of striatal and cortical messenger RNAs became less selective. GluR3 and preproenkephalin messenger RNAs were up-regulated in ipsilateral spared regions of the striatum, and GluR1 and GluR2 messenger RNAs increased bilaterally in the cingulate cortex and in selective nuclei of the amygdala. Histological cell death or neurodegeneration was not detected in areas of reduced glutamate and preproenkephalin messenger RNA expression in either the ipsilateral striatum or cortex before 24 h. These findings suggest that complex and long-lasting decreases in messenger RNA expression occur prior to significant cell loss in regions destined to undergo infarction. Increased formation of Ca2+-permeable AMPA receptor assemblies may occur in "unspared" and "spared" regions via different mechanisms and contribute to alterations in post-ischemic synaptic activity. The possibility arises that there may be altered relationships between glutamatergic and enkephalin synapses, since the dorsolateral striatum, where preproenkephalin messenger RNA expression is acutely reduced, receives innervation by the affected ipsilateral cortical region.

Antihyperalgesic effects of infection with a preproenkephalin-encoding herpes virus

To test the utility of gene therapeutic approaches for the treatment of pain, a recombinant herpes simplex virus, type 1, has been engineered to contain the cDNA for an opioid peptide precursor, human preproenkephalin, under control of the human cytomegalovirus promoter. This virus and a similar recombinant containing the Escherichia coli lacZ gene were applied to the abraded skin of the dorsal hindpaw of mice. After infection, the presence of beta-galactosidase in neuronal cell bodies of the relevant spinal ganglia (lacZ-containing virus) and of human proenkephalin (preproenkephalin-encoding virus) in the central terminals of these neurons indicated appropriate gene delivery and expression. Baseline foot withdrawal responses to noxious radiant heat mediated by Adelta and C fibers were similar in animals infected with proenkephalin-encoding and beta-galactosidase-encoding viruses. Sensitization of the foot withdrawal response after application of capsaicin (C fibers) or dimethyl sulfoxide (Adelta fibers) observed in control animals was reduced or eliminated in animals infected with the proenkephalin-encoding virus for at least 7 weeks postinfection. Hence, preproenkephalin cDNA delivery selectively blocked hyperalgesia without disrupting baseline sensory neurotransmission. This blockade of sensitization was reversed by administration of the opioid antagonist naloxone, apparently acting in the spinal cord. The results demonstrate that the function of sensory neurons can be selectively altered by viral delivery of a transgene. Because hyperalgesic mechanisms may be important in establishing and maintaining neuropathic and other chronic pain states, this approach may be useful for treatment of chronic pain and hyperalgesia in humans.

Opioid peptides in response to mental stress in asymptomatic dilated cardiomyopathy

Fourteen asymptomatic dilated cardiomyopathy patients showing normal plasma levels of beta-endorphin, Met-enkephalin, dynorphin B, norepinephrine and endothelin-1 but elevated atrial natriuretic factor (ANF) levels underwent two Mental Arithmetic Tests (MAT), with placebo and naloxone hydrochloride infusion, respectively. MAT significantly (p < 0.01) increased blood pressure, heart rate, opioid peptides, norepinephrine, ANF, but not endothelin-1. Naloxone infusion significantly (p < 0.05) attenuated the increments produced by MAT in all measured parameters during placebo infusion. These results indicate that in asymptomatic dilated cardiomyopathy the endogenous opioid system, activated by stress-induced sympathoadrenergic hyperactivity, may further increase the sympathetic tone in a positive feedback that is interrupted by naloxone.

Developmental switch in phenotypic expression of preproenkephalin mRNA and 45Ca2+ accumulation following kainate-induced status epilepticus

Kainic acid-induced status epilepticus results in delayed degeneration of CA3 hippocampal neurons in the mature but not immature rat hippocampus. In adult rats, the putative opioid precursor, preproenkephalin (PPE) mRNA increases in the dentate gyrus (DG), a region resistant to damage, following continuous limbic seizures. To explore why the immature brain is resistant to seizure-induced damage, the regional distribution of PPE mRNA expression and 45Ca2+ accumulation were compared in postnatal day 14 (P14) pup, and adult hippocampus at 5-6 h after kainate-induced status epilepticus. Inverted patterns of PPE expression and Ca2+ uptake were observed at the two ages. In P14 pups, PPE mRNA expression increased in DG and escalated in CA3, regions where 45Ca2+ accumulations were absent. In adult rats, PPE mRNA expression increased only in DG; 45Ca2+ labeling was predominant in CA3a,c and absent in DG. Pronounced increases in enkephalin neuropeptide synthesis in immature CA3 neurons may reduce glutamate release presynaptically and also prevent voltage-gated Ca2+ uptake into these neurons despite recurrent seizure activity. Opioid-mediated inhibition may provide an explanation for the resistance of the immature CA3 region to seizure-induced damage.

Effect of dopaminergic drugs on processing and degradative neuropeptidase mRNA in rat frontal cortex and caudate-putamen

Drugs which act upon central dopamine receptors alter the level, mRNA expression and in vitro degradation of neuropeptides associated with dopamine neuron regulation. Changes in the degradation of certain neuropeptides are correlated with significant alterations in the activity of specific neuropeptidases, namely aminopeptidase N (APN) and neutral endopeptidase 24.11 (NEP 24.11). In the present study, we sought to examine the molecular mechanism of neuropeptidase activity changes in response to dopaminergic drug treatment. The effects of dopaminergic drugs on the mRNA level of APN and NEP 24.11 were determined by RNase protection assays of RNA extracted from rat frontal cortex and caudate-putamen. Additionally, the effects of dopaminergic drugs on the mRNA expression for the neuropeptide processing enzymes, prohormone convertase 1 (PC1) and PC2, were determined. After 7-day administration of the dopamine receptor antagonist, haloperidol (1 mg/kg), no effect on the mRNA expression of APN, NEP 24.11, PC1 or PC2 was observed in either of the rat brain regions studied. Administration of the dopamine receptor agonist, apomorphine (5 mg/kg, bid), altered only the expression of APN mRNA in rat caudate-putamen, where the greatest effect on APN activity has been previously observed. These results suggest that alterations in other post-transcriptional events, such as mRNA translation or insertion of neuropeptidase protein into the membrane, likely play a larger role than changes in mRNA expression in the modulation of neuropeptidase activity.

Pressor effects of endogenous opioid system during acute episodes of blood pressure increases in hypertensive patients

To investigate the involvement of endogenous opioids in acute increases in blood pressure and their functional relationship with atrial natriuretic factor and endothelin-1, we assessed plasma levels of beta-endorphin, met-enkephalin, dynorphin B, catecholamines, atrial natriuretic factor, and endothelin-1 before and after administration of the opioid antagonist naloxone hydrochloride (8 mg i.v.) in 28 hypertensive patients with a stress-induced acute increase in blood pressure. Ten patients with established mild or moderate essential hypertension and 10 normotensive subjects served as control groups. Opioids, atrial natriuretic factor, and endothelin-I were radioimmunoassayed after chromatographic preextraction; catecholamines were determined by high-performance liquid chromatography with electrochemical detection. Patients with an acute increase in blood pressure (systolic, 203.2 +/- 2.2 mm Hg; diastolic, 108.4 +/- 1.3) had plasma opioid, catecholamine, and atrial natriuretic factor levels significantly (P < .01) higher than hypertensive control patients (systolic pressure, 176.4 +/- 1.0 mm Hg; diastolic, 100.0 +/- 1.4), who had a hormonal pattern similar to that of normotensive subjects (systolic pressure, 123.2 +/- 1.5 mm Hg; diastolic, 75.0 +/- 2.0). Endothelin-1 did not differ in any group. In patients with an acute increase in blood pressure, naloxone significantly (P < .01) reduced blood pressure, heart rate, opioids, catecholamines, and atrial natriuretic factor 10 minutes after administration. Naloxone effects on blood pressure, heart rate, opioids, and catecholamines wore off within 20 minutes. In control groups, naloxone failed to modify any of the considered parameters. Our findings suggest that pressor effects of opioid peptides mediated by the autonomic nervous system during stress-induced acute episodes of blood pressure increase in hypertensive patients.

Opioid peptide modulation of circulatory and endocrine response to mental stress in humans

Healthy subjects were classified according to their percent increase in systolic blood pressure (SBP) after mental arithmetic test (MAT) as low (delta SBP 9.3-15.1%, n = 15) and high (delta SBP 35.1-45.4%, n = 15) responders. During MAT, low responders showed significantly (p < 0.01) increased plasma levels of beta-endorphin, cortisol, catecholamines, and atrial natriuretic factor (ANF) and decreased levels of endothelin-1, whereas high responders showed increased (p < 0.01) levels of Metenkephalin, dynorphin B, and catecholamines. Pretreatment with naloxone hydrochloride enhanced (p < 0.01) SBP, heart rate, noradrenaline, cortisol, and endothelin-1 levels, and reduced (p < 0.01) ANF in low responders in response to MAT, whereas it decreased (p < 0.01) hemodynamic parameters, noradrenaline, and endothelin-1 in high responders. The individual differences in hemodynamic and endocrine responses to MAT may depend on a different activation of the endogenous opioid system.

Dopamine, serotonin and tachykinin in self-injurious behavior

The neurobiologic basis of self-injurious behavior (SIB) in Lesch-Nyhan syndrome and in other neuropsychiatric conditions remains unclear. The purpose of this review is to summarize recent data concerning SIB induced by the dopamine (DA) uptake inhibitor, GBR-12909 (GBR) and to compare the neurochemical data that have accumulated over the years on SIB in neonatal 6-hydroxydopamine (6OHDA) lesioned rats. The DA uptake inhibitor, GBR, upon repeated administration to adult rats elicits SIB that is temporally associated with a reduction of striatal DA (approximately 30%), increased turnover of serotonin and a robust induction of tachykinin transcription resulting in enhanced biosynthesis and presumably release of tachykinins (substance P and neurokinin A). GBR-induced SIB could be blocked by dopaminergic lesions or by D1 or D2 antagonists. Neonatal dopaminergic lesions result in a high degree of DA loss (> 90%) and elevated levels of serotonin. In this model, SIB is precipitated by DA agonists via activation of D1 DA receptors which are in turn linked to an induction of tachykinin biosynthesis and release. The data taken together suggest that (a) a substantial reduction of DA accompanied by an increase in serotonin turnover may be essential conditions that are conducive to the occurrence of SIB, and (b) this phase is either superimposed with, or followed by a D1 and/or D2 DA receptor-linked activation of striatonigral tachykinin neurons resulting in enhanced tachykinin biosynthesis and release that may sustain the SIB. Thus, a dynamic interplay between DA, serotonin and tachykinin neuronal systems of the basal ganglia appear to influence the genesis and/or expression of SIB.

The transcriptional regulation of the preproenkephalin gene

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Opioid modulation of amphetamine-stimulated dopamine release and concentration in rat striatal slices

The effects of morphine and naltrexone on amphetamine-stimulated release and total concentration of dopamine (DA) from rat striatal slices in vitro were examined in this study. Adult male Sprague-Dawley rats were sacrificed and the striata were dissected, sliced, and then incubated in buffer solution at 37 degrees C with amphetamine in the presence or absence of various concentrations of morphine, naltrexone (or both), or the dihydroxyphenylalanine (DOPA) decarboxylase inhibitor (NSD-1015). The concentrations of DOPA, DA, and dihydroxyphenylacetic acid (DOPAC) in the tissue slices and buffer media were measured by HPLC/EC. Amphetamine enhanced DA release and also increased total DA concentrations. However, neither morphine nor naltrexone alone altered DA concentration in the media or tissue slices relative to control (no drug added). Moreover, neither morphine nor naltrexone at 1, 10, or 100 microM altered amphetamine-stimulated DA release. However, morphine (1 or 10 microM) inhibited the amphetamine-stimulated increase in total concentration of DA. This effect of morphine was blocked by naltrexone. NSD-1015 alone or in combination with morphine did not alter amphetamine-stimulated DA release, but significantly reduced DA concentration in striatal slices. NSD-1015 alone also increased DOPA accumulation in both media and tissue slices, and this effect was inhibited by the addition of morphine. These results indicate that morphine inhibits the amphetamine-stimulated increase in total DA content, but not the amphetamine-stimulated release of DA.

Alterations in delta opioid receptor levels in discrete areas of the neocortex and in the globus pallidus of the aging guinea pig: a quantitative autoradiographic study

The effect of aging on delta opioid receptors was examined in the brains of guinea pigs aged 1, 6, 24 and 36 months. Quantitative autoradiography was used to monitor the concentrations of delta receptors in various anatomical regions at five rostro-caudal levels. delta opioid receptor populations were found to be remarkably stable throughout the life span of this species. We have, however, discovered anatomical areas which offer striking exceptions. In the globus pallidus, progressive age-related losses of delta receptors reached 50% in the senescent animal. In contrast, laminae I, II of the lateral agranular frontal cortex and laminae I, II and III, IV of the primary somatosensory cortex demonstrated age-related increases in the concentrations of delta receptors ranging from 30 to 45%. These changes are discussed with the view to their being functionally related components of motor circuitry involving pyramidal and extrapyramidal elements.

Tetrahydropapaveroline in brain regions of rats after acute ethanol administration

Tetrahydropapaveroline (THP), the condensation product of dopamine, and its aldehyde, dopaldehyde, have been detected in brain regions of rats after acute ethanol administration. THP levels were determined in eight brain regions of animals that received ethanol (3.0 g/kg) by intraperitoneal injection 100 or 120 minutes before decapitation. The levels of THP in two brain regions, i.e., the midbrain and striatum, were determined at time intervals ranging from 50 to 120 minutes after ethanol administration. THP was not found in brain regions of untreated animals. However, significant levels of THP were found in pooled midbrains (0.50 pmol/g tissue) and pooled hypothalami (0.20 pmol/g tissue) of animals that received ethanol 120 minutes before decapitation. Most brain regions had detectable levels of THP 100 minutes after the animals received ethanol and the striatum contained the highest concentration of the alkaloid. The concentration of THP in striata tissue of rats at 50, 70, 90, or 100 minutes after ethanol administration were 0.33, 0.38, 0.33, and 0.33 pmol/g tissue, respectively. These results demonstrate that THP can be detected in specific brain regions of the rat after acute ethanol administration.

Preproenkephalin RNA increases in the hypothalamus of rats stressed by social deprivation

1. Pharmacological evidence indicates that stress induced by brief (14 to 20-day) social deprivation in the rat is associated with an activation of the central preproenkephalin (ENK) opioid system. This study examines the neurochemical evidence that substantiates such an activation. 2. Using a specific ENK complementary DNA probe, ENK RNA levels were measured by dot blot and Northern blot analyses in different brain areas of socially deprived rats. Immunoreactivity to met-enkephalin-derived peptides was also evaluated by radioimmunoassay in the same brain regions. 3. Brief social deprivation increased the levels of ENK RNA and enkephalin immunoreactivity in whole hypothalamus. 4. Our data suggest that this type of stress appears to be associated to an induction of ENK gene transcription in hypothalamus.

Analgesia induced by morphine injected into the pallidum

Bilateral microinjections of morphine hydrochloride (10; 20; 30 micrograms/0.5 microliter/side) or saline were aimed at three different regions of the rat globus pallidus: dorsal, medial, ventral. Before and at various intervals after intrapallidal morphine (15; 30; 60; 90; 180 min), estimation of pain threshold was made by the hot plate procedure. Dose-dependent morphine analgesia was elicited from all three regions injected. Differences between the pallidal areas as to the intensity and duration of the drug's effect were noticed. Pretreatment with subcutaneous naloxone (1 mg/kg, s.c.) inhibited the morphine (20 micrograms) analgesia elicited from the medial and dorsal pallidum; it decreased and delayed the effect of morphine injected into the ventral pallidum. The results suggest that the three pallidal areas tested are involved to a different degree (medial/dorsal greater than ventral) in the morphine analgesia mediated by opiate receptors.

Morphine and methadone dependence in the rat: withdrawal and brain met-enkephalin levels

Opioids were administered to female Long Evans rats in their drinking water. Maintenance doses of 0.8 and 0.4 mg/ml for morphine and methadone, respectively, were achieved using an ascending dosage schedule. Rats were decapitated 0, 20, or 60 min after naloxone (10 mg/kg, IP) or saline. Brain met-enkephalin-like immunoreactivity (ME-LI) was determined by radioimmunoassay (RIA). In morphine-drinking animals, ME-LI in all regions of the brain was unaltered following saline administration; however, 20 min after naloxone injection ME-LI had increased in the striatum, hypothalamus, midbrain, and pituitary. By 60 min, ME-LI was no longer elevated. In both methadone- and water-drinking rats, ME-LI did not deviate from normal. These elevated levels of ME-LI, 20 min after naloxone-precipitated withdrawal in morphine-dependent rats, coincided with the peak of behavioural signs in the precipitated withdrawal syndrome. The milder behavioural disturbances observed in the withdrawal of methadone-drinking rats were consistent with the unaltered ME-LI in these animals.

Pallidal substrate of morphine-induced locomotion

Bilateral microinjections of morphine hydrochloride (5.0; 7.5; 10.0 micrograms/0.5 microliters/side) or saline were infused into 3 different regions (dorsal, medial, ventral) of the rat globus pallidus, to examine their effects on locomotor activity. Locomotor activity of each rat was measured 45 min before and 90 min after saline or morphine pallidal microinjections. Morphine induced a dose-dependent increase in locomotion. This increase in locomotion was also significantly different between the 3 pallidal regions. Pretreatment with naloxone (1 mg/kg, sc) inhibited the morphine (7.5 micrograms) hyperlocomotion elicited from all three pallidal areas. The results suggest that the entire pallidum serves as substrate of morphine hyperlocomotion mediated by opiate receptors.

Characterization of delta-sleep-inducing peptide-evoked release of Met-enkephalin from brain synaptosomes in rats

Delta-sleep-inducing peptide (DSIP) stimulates the release of Met-enkephalin (Met-ENK) from superfused slices of the rodent lower brainstem in vitro. In our present study, DSIP (10(-10)-10(-9) M) induced a significant release of Met-ENK from medullary synaptosomes of rats. This DSIP-evoked release of Met-ENK was Ca2+ dependent and tetrodotoxin (TTX) insensitive. Furthermore, DSIP (10(-11)-10(-9) M) significantly increased 45Ca2+ uptake in medullary synaptosomes. These results demonstrate that DSIP acts directly on the nerve endings of Met-ENK-containing neurons to release this pentapeptide by generating a Ca2+ influx into these neurons. Effects of DSIP on Met-ENK release in other discrete brain regions were also studied. Significant DSIP-evoked Met-ENK release from synaptosomes was observed in the cortex, hypothalamus, and midbrain (at concentrations of 10(-10) and 10(-9) M) and in the hippocampus and thalamus (only at 10(-9) M), but not in the striatum. In the hypothalamus, the release of Leu-enkephalin from its synaptosomes was slightly, but not significantly, enhanced by DSIP (10(-10)-10(-8) M). Our findings demonstrate that DSIP triggered a Ca2+ influx in nerve endings to induce a subsequent release of Met-ENK from neurons in only certain brain regions.

Differential secretion of enkephalin-like peptides from bovine adrenal chromaffin cells

Stimulation of chromaffin cells in culture with 1,1-dimethyl-4-phenylpiperazinium (DMPP) or depolarizing concentrations of K+ resulted in a significant secretion of high and low molecular weight enkephalin-like peptides (ELPs) into the culture medium. BioGel P-10 column chromatography was used to characterize the ELPs in chromaffin cell extracts and in culture media before and after stimulation with either DMPP or K+. DMPP (50 microM) stimulation produced a significant secretion of primarily low molecular weight (less than 3 kDa) ELPs whereas 56 mM K+ caused a secretion of both high and low molecular weights ELPs. The expected decrease in cellular content of low molecular weight peptides was not observed regardless of stimulation type. Our results support the hypothesis that the precursor/product ratio of secreted ELPs is dependent upon the nature of the chromaffin cell stimulus. Moreover the cellular content of low molecular weight ELPs is not depleted with either type of stimulation.

The regional distribution of thyrotropin releasing hormone, leu-enkephalin, met-enkephalin, substance P, somatostatin and cholecystokinin in the rat brain and pituitary

There was no apparent difference in the regional distribution of neuropeptides in the brain of male and female rats. The highest levels of immunoreactive leu-enkephalin, TRH, substance P and somatostatin were found in the hypothalamus, while the striatum and the cerebral cortex had the highest concentrations of met-enkephalin and cholecystokinin respectively. The lowest concentrations of these were found in the cerebellum. Enkephalins (cerebral cortex), substance P (cerebral cortex and brain stem), and somatostatin (brain stem and striatum) showed higher level in the female while enkephalin and substance P contents in the anterior pituitary were higher in the male.

Enkephalinergic involvement in periaqueductal gray control of hypothalamically elicited predatory attack in the cat

The effects of central infusion of naloxone into the midbrain periaqueductal gray (PAG) upon predatory attack behavior in the cat were studied in 12 cats. Initially, quiet biting attack was elicited by electrical stimulation of sites within the lateral hypothalamus using monopolar electrodes. Then cannula-electrodes were implanted into sites within the PAG from which electrical stimulation facilitated or suppressed the attack response. Following identification of modulatory sites within the PAG, naloxone (1.0 micrograms/0.5 microliter) was microinjected into those sites and the effects upon hypothalamically elicited attack were assessed. At nine of twelve sites in the PAG where suppression was obtained, administration of naloxone served to block those effects. Similarly, at six of eight facilitatory sites within the PAG, naloxone also blocked the modulatory effects of PAG stimulation. However, vehicle (isotonic saline) alone did not alter the modulatory effects of PAG stimulation. Administration of DAME (250 ng/0.3 microliter) into PAG modulatory sites in four cats, two which facilitated and two that suppressed the attack response, reversed the effects of naloxone at these sites. These results demonstrate that opioid peptides within the PAG play a complex role in the expression of predatory attack behavior in the cat.

Distribution of substance P, GnRH, Met-enkephalin in the central nervous system of the pig

The distribution of substance P (SP), gonadotropin releasing hormone (GnRH) and Met-enkephalin in the brain and spinal cord of the domestic pig is described for the first time. The levels of SP, GnRH and Met-enkephalin were measured by specific radioimmunoassays in various regions of the brain and spinal cord of the pig. Substance P and Met-enkephalin are widely distributed within the central nervous system of the pig. High levels of SP were found in the preoptic area (POA), suprachiasmatic area (SCA), medial basal hypothalamus (MBH) and brain stem while moderate amounts of SP were found in olfactory bulb (OB). High levels of Met-enkephalin were found in POA, SCA and MBH, and moderate levels of Met-enkephalin in OB and brain stem. Both SP and Met-enkephalin levels were higher in the dorsal spinal cord in comparison with the levels of these peptides in the ventral spinal cord. This finding is in agreement with the predominant role played by these neural systems in primary afferent mediation of nociceptive impulses. The POA and SCA contained only low levels of GnRH while the MBH contained high levels of GnRH. Finally, some differences in the quantitative distribution of these peptides in the pig and rat are discussed.

Postnatal development of preproenkephalin mRNA containing neurons in the rat lower brainstem

Postnatal developmental changes of preproenkephalin (PPE) gene expression in rat brainstem neurons were studied by in situ hybridization histochemistry. On the basis of PPE mRNA expression, brainstem neurons were categorized into three types: 1) type I neurons were characterized by constant or increasing expression of PPE mRNA during postnatal development; 2) type II neurons started to express PPE mRNA several days after birth and continued to do so thereafter; and 3) type III neurons showed transient expression of PPE mRNA or stopped expressing the mRNA during early postnatal development. Type I PPE neurons were observed in diverse brainstem structures including the mesencephalic and pontine central gray matter, various reticular and raphe nuclei, the ventral tegmental area of Tsai, the interpeduncular nucleus, the nucleus of the brachium of the inferior colliculus, the ventral and dorsal tegmental nuclei of Gudden, the sphenoid nucleus, the laterodorsal tegmental nucleus, Barrington's nucleus, the parabrachial region, the lateral lemniscus and its related nuclei, the trapezoid nucleus, the rostral and ventromedial periolivary nuclei, the mesencephalic trigeminal and principal sensory trigeminal nuclei, the locus coeruleus, the subcoeruleus nucleus, the medial and spinal vestibular nuclei, the dorsal and ventral cochlear nuclei, the medial and lateral cerebellar nuclei, the Roller nucleus, and the intermedius nucleus of the medulla. Type II PPE neurons were found in the superior colliculus, the inferior colliculus, the central part of the dorsal tegmental nucleus, and as Golgi neurons in the granular layer of the cerebellum. Type III PPE neurons were located in the substantia nigra, the red nucleus, the superior olive, the motor trigeminal nucleus, the facial nucleus, the inferior olive, the dorsal motor nucleus of the vagus, and the hypoglossal nucleus. Such region-specific expression of the PPE gene during postnatal ontogeny suggests that rat brainstem PPE neurons may be involved in a variety of developmental events, such as cell proliferation, differentiation, and migration.

Developmental and regional alteration of methionine enkephalin-like immunoreactivity in seizure-susceptible E1 mouse brain

Methionine enkephalin-like immunoreactivity (ME-LI) in the brain of El mice (seizure-susceptible strain) was measured by radioimmunoassay (RIA) to elucidate the relation between seizures and the opioid system. The lyophilized supernatants of tissue extracts were subjected to ME RIA. The concentration of ME-LI in 25-day-old El mice that had no seizures was significantly decreased in the hippocampus. At the age of 50 days when El mice displayed abortive seizures, the levels of ME-LI in both El(+) and nonstimulated El(o) mice were also significantly reduced in the hippocampus and septal area. It was further shown that the ME-LI concentrations in both 150-day-old adult El(+) during interictal periods and El(o) mice were markedly decreased in the cerebral cortex, septal area, and striatum, as compared with the corresponding regions in ddY mice (seizure-nonsusceptible strain; the mother strain of El). The decrease of ME-LI in the El mouse brain was generally compatible with our previous findings concerning the up-regulation of opioid delta receptors in this species. These results suggest that the reduction of ME-LI in the El mouse brain is not due to convulsions, but could be associated with the pathogenesis of seizure diathesis and seizure manifestations in the El mouse.

Measurement of methionine enkephalin and leucine enkephalin in rat brain regions by high-performance liquid chromatography with coulometric electrochemical detection

A method is described for the determination of two pentapeptides, methionine enkephalin (H-Tyr-Gly-Gly-Phe-Met-OH) (ME) and leucine enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) (LE) in discrete rat brain regions. Separation and quantitation were performed by reversed-phase high-performance liquid chromatography with coulometric detection. Perchloric acid extracts of the tissue after enzyme inactivation by heat treatment were passed through a normal-phase solid-phase extraction diol (COHCOH) column, and endogenous ME and LE were subsequently eluted with methanol. The mobile phase was 1-propanol-phosphate buffer (pH 5.5) (9:91). Eluted samples were detected electrochemically using dual coulometric electrodes operated in screen mode. Each of these enkephalins gave a linear response over the range 40-160 ng/ml cerebellar homogenate (0.8-3.2 ng absolute amount on column). Analytical recoveries of synthetic ME and LE, added to the homogenates, were 70 +/- 3 and 70 +/- 10%, respectively, when compared with enkephalins dissolved in water. The mean between-assay coefficients of variation for synthetic ME and LE were lower than 10.7 and 7.4%, respectively, over the concentration range studied. The within-assay coefficients of variation for synthetic ME and LE were 11.4 and 9.5%, respectively, at the lowest concentration. The present method has been applied to a study determining the levels of endogenous ME and LE in discrete rat brain regions.

Use of methionine-enkephalin sulfoxide and leucine-enkephalin radio-immunoassays for the measurement of enkephalins in the rat brain

1. A radio-immunoassay (RIA) for methionine-enkephalin sulfoxide was developed and was used to measure methionine-enkephalin, after oxidation, in the rat brain. 2. The RIA was performed together with assays that use commercially available met- and leu-enkephalin antisera. 3. Evidence is presented here indicating that antisera raised against the native met-enkephalin peptide underestimate met-enkephalin content due to loss of immunoreactivity after oxidation of peptide. 4. These results show that a more accurate measurement of met-enkephalin may be obtained by using the met-enkephalin sulfoxide RIA. The cross-reactivity of met-enkephalin with the leu-enkephalin RIA was diminished following oxidation of peptide.

Opioid peptides within the midbrain periaqueductal gray suppress affective defense behavior in the cat

The effects of the methionine-enkephalin analog [D-Ala2-Met5]-enkephalinamide (DAME) upon the threshold for affective defense behavior were determined following microinjections placed into midbrain periaqueductal gray sites from which this response was elicited. Affective defense behavior was elicited by electrical stimulation through a cannula electrode situated in the dorsal aspect of the midbrain periaqueductal gray. Dose-response curves characterizing the effects of DAME upon affective defense behavior were determined utilizing the following doses: 0.25, 0.5 and 1.0 microgram in 0.5 microliter saline, pH = 7.4 or vehicle control (saline). Response thresholds were tested 10-30, 30-60, 60-90, 120-150, 180-210, 1440-1470 and 2880-2910 min postinjection. The results obtained indicated that injections of DAME at a dose of 1.0 microgram/0.5 microliter produced significant, long duration elevations in affective defense thresholds, lasting up to 1440-1470 min postinjection. Lower doses of DAME (0.25 and 0.5 microgram/0.5 microliter) also resulted in significant increases in affective defense thresholds, but these effects were of shorter durations (60-90 and 120-150 min) postinjection, respectively. The suppressive effects of DAME were blocked when animals were pretreated with naloxone (10 micrograms/0.5 microliter) microinjected into the same midbrain periaqueductal gray site into which 0.25 microgram DAME was injected and affective defense behavior was elicited.

Distribution of enkephalin-immunoreactive nerve fibres and terminals in the region of the nucleus basalis magnocellularis of the rat: a light and electron microscopic study

This investigation was carried out on the distribution of enkephalin-containing nerve fibres and terminals in the region of the nucleus basalis magnocellularis (NBM) of the rat. At the light microscope (LM) level, enkephalin-immunoreactive sites and endogenous choline acetyltransferase (ChAT) were demonstrated by employing the two-colour immunoperoxidase staining technique, using highly specific monoclonal antibodies against enkephalin and ChAT. A pharmacohistochemical procedure to reveal acetylcholinesterase (AChE)-synthesizing neurons combined with the peroxidase-antiperoxidase (PAP) immunocytochemical technique to detect endogenous enkephalins, provided ultrastructural data on the relationships of neuronal elements containing AChE and enkephalins in the region of the NBM. At the LM level, cholinergic neurons of the NBM were surrounded by a dense network of enkephalin-immunoreactive nerve fibres. Electron microscopic (EM) observations of histochemically characterized structures, that were first identified in the LM, revealed that intensely AChE-stained structures in the region of the NBM received sparse synaptic inputs from enkephalin immunoreactive terminals. Synaptic inputs of immunoreactive terminals onto intensely AChE-stained neuron cell bodies were not detected. Synaptic contacts onto proximal AChE-positive dendrites were sparse, but the density increased on more distal regions of the dendrites. All immunoreactive boutons studied established symmetrical synaptic contacts with AChE-positive post-synaptic structures. The pattern of the synaptic input to these cells differs strikingly from that onto typical globus pallidus neurons. The perikarya and dendrites of the latter neurons were characteristically ensheathed in immunoreactive synaptic boutons. Results are consistent with the view that enkephalin-like substances in the rat might be synaptic transmitters or neuromodulators in the area of the NBM and that cholinergic neurons of the NBM (Ch4) are integrated into the circuitry of the basal ganglia. Enkephalins may play an important role regulating the extrinsic cholinergic innervation of the neocortex.

Developmental expression of proenkephalin mRNA in rat striatum and in striatal cultures

Proenkephalin mRNA shows a biphasic developmental profile in rat striatum, with an initial peak at postnatal day 2, a decline to embryonic levels by day 7, and a second increase to adult levels over the course of the second to 4th week after birth. The same 4-fold increase is seen in cultured striatal neurons, over the same time course but without a biphasic response. Cultured fetal glia also contain proenkephalin mRNA.

Nicotine-induced alterations in brain regional concentrations of native and cryptic Met- and Leu-enkephalin

The distribution of cryptic forms (larger enkephalin-containing peptides) in neostriatum, hypothalamus, spinal cord T3-L1 and neurointermediate lobe of pituitary were determined by radioimmunoassay. Optimal conditions for enzymic hydrolysis of the cryptic enkephalins by trypsin and carboxypeptidase B were established. The proportion of total Met- and Leu-enkephalin represented by native pentapeptide varied markedly among these central nervous system regions. Also, the distributions of native and cryptic Met-enkephalin were distinct from that of Leu-enkephalin. Chromatographic separation by HPLC of immunoreactive Met-enkephalin peptides revealed only two peaks corresponding to Met-enkephalin and Met-enkephalin sulfoxide in rather equal amounts. Hydrolysis of cryptic Met-enkephalin also produced only two HPLC-separable peaks of immunoreactive Met-enkephalin, again corresponding to Met-enkephalin and Met-enkephalin sulfoxide. Bioactivity of cryptic striatal Met-enkephalin after hydrolysis was demonstrated by antinociception and catalepsy in rats following its intracerebroventricular injection. Repeated short-term administration of nicotine, 0.1 mg/kg IP six times at 30 min intervals, produced significant increases in native and cryptic Met-enkephalin in striatum, consistent with an increase in neuronal release of Met-enkephalin together with increases in synthesis and processing of proenkephalin A in this brain region. This regimen of nicotine also decreased levels of native Met-enkephalin and of both native and cryptic Leu-enkephalin in neurointermediate lobe, consistent with nicotine-induced release of both proenkephalin A- and prodynorphin-derived peptides from neurointermediate lobe.

On-line post-column fluorescence detection for N-terminal tyrosine-containing peptides in high-performance liquid chromatography

A detection system based on on-line post-column fluorescence derivatization is described for the determination of N-terminal tyrosine-containing peptides by reversed-phase high-performance liquid chromatography. The peptides are automatically converted into fluorescent derivatives by reaction with hydroxylamine, cobalt (II) and borate after peptide separation on a reversed-phase column (TSKgel ODS-120T) followed by passage through an ultraviolet absorbance detector. The reaction system permits the fluorescence detection at 435 nm (emission) with excitation at 335 nm for N-terminal tyrosine-containing synthetic peptides in as little as picomole amounts. The facile fluorescence detection of N-terminal tyrosine-containing fragments produced from methionine-enkephalin by enzymatic degradation using a rat brain homogenate was achieved by comparison with the ultraviolet absorption detection at 215 nm.

Systemic methionine-enkephalin evokes cardiostimulatory responses in the human

The cardiovascular effects of bolus doses of methionine-enkephalin (Met5-ENK) (1 to 100 micrograms/kg) were studied in 9 subjects in whom, at cardiac catheterization for evaluation of chest pain, patent coronary arteries were found. Met5-ENK produced a simultaneous increase in blood pressure and heart rate beginning within 20 sec, reaching maximal values between 30 and 40 sec, and then terminating by 60 sec. Heart rate, systolic, diastolic, and mean arterial blood pressures increased significantly (p less than 0.0005); pulse pressure remained unchanged. Positive dose-effect relationships were observed for heart rate (p less than 0.002), systolic, diastolic, and mean arterial blood pressures (p less than 0.05). Naloxone (0.5 mg/kg), given to 4 subjects, prevented the heart rate and blood pressure changes associated with Met5-ENK administration, demonstrating that the cardiovascular changes were mediated by opiate receptors. Subjects also described cutaneous paresthesias which were not prevented by naloxone pretreatment. These data suggest a role for peripheral enkephalins in cardiovascular regulation.

Neuroanatomical boundaries of the reward-relevant opiate-receptor field in the ventral tegmental area as mapped by the conditioned place preference method in rats

The conditioned place preference produced by morphine microinjected into the ventral tegmental area was studied in rats. Cannula placements were varied along the rostrocaudal plane to determine the approximate anatomical focus of morphine's rewarding effect. Microinjections within a 1.4-mm range produced a significant change in place preference suggesting that morphine injected into this zone is rewarding. Injection sites rostral and caudal to this zone were ineffective as were injections ventral to this region. The approximate anatomical boundaries of the reward-relevant opiate-receptor field within the ventral tegmental area correspond well with the distribution of the A10 dopamine-containing cell bodies.

On-line post-column fluorescence derivatization of arginine-containing peptides in high-performance liquid chromatography

A selective detection system based on on-line post-column fluorescence derivatization is described for the analysis of arginine-containing peptides by high-performance liquid chromatography. The peptides are automatically converted into fluorescent derivatives with benzoin, a fluorogenic reagent for guanidino compounds, after separation on a reversed-phase column (TSKgel ODS-120T) and detection in an ultraviolet absorption detector. The system permits fluorescence detection at 435 nm emission with irradiation at 325 nm for arginine-containing peptides in as little as picomole amounts. The chromatogram obtained with fluorescence detection only shows peaks corresponding to arginine-containing peptides. The facile detection of arginine-containing fragments in the tryptic digest of beta-melanocyte stimulating hormone as a model compound could be achieved by comparison with a chromatogram obtained with ultraviolet absorption detection at 215 nm.

Opposite effects of mu and kappa opiates on the firing-rate of dopamine cells in the substantia nigra of the rat

Single unit extracellular recording was carried out in rats to compare the actions of a mu receptor agonist (morphine) to a kappa receptor agonist (U50,488) in the substantia nigra pars compacta (SNC). Intravenously administered morphine and U50,488 exhibited opposite effects: morphine excited SNC dopamine cells, and U50,488 inhibited them. Both effects were blocked by naloxone, although significantly more naloxone was necessary to block the effect of the kappa agonist. Local administration of U50,488 in the caudate also inhibited the firing of SNC dopamine cells. These results are discussed in terms of the natural functions of the prodynorphinergic straiatonigral projection.

Acute and repeated administration of sulpiride alters Met-and Leu-enkephalin content of rat brain

Following acute administration of sulpiride (100 mg/kg, i.p.), Met-enkephalin levels were increased in striatum but decreased in substantia nigra and nucleus accumbens. No effect was observed in hypothalamus or frontal cortex. In contrast, Leu-enkephalin content was decreased in the nucleus accumbens but not elsewhere. Administration of sulpiride (10 or 100 mg/kg, i.p.) for 21 days, followed by 2 days drug withdrawal, caused a dose-dependent increase in Met-enkephalin in nucleus accumbens but decreases in hypothalamus and frontal cortex. No change was observed in the striatum or substantia nigra. Repeated sulpiride administration caused a decrease in Leu-enkephalin content of the hypothalamus and cortex but not elsewhere.

Diurnal rhythm of the in vivo release of enkephalin from the globus pallidus of the rat

The in vivo spontaneous release of enkephalin in the globus pallidus of the rat increases from noon to evening by 100%; during this period the local release of exogenous gamma-aminobutyric acid (GABA) decreases by 60%. These diurnal rhythms are more marked in the K+-stimulated release: enkephalin-induced output increases 6-fold while GABA decreases 10-fold during the afternoon and evening hours. Since pallidal enkephalin and GABA are involved in the control of locomotor activity we suggest that these rhythms may be linked to the circadian changes of activity in the rat.

Autoradiographic localization of mu- and delta-opiate receptors in the forebrain of the rat

The autoradiographic distributions of mu opiate receptors, labeled in vitro by [125I]D-Ala2-MePhe4-Met(o)5-ol-enkephalin (FK), and delta-opiate receptors, labeled by [3H]D-Ala2-D-Leu5-enkephalin (DADLE) in the presence of oxymorphone to block high affinity binding to the mu site, were examined and compared in the forebrain of the rat. The mu- and delta-receptors were differentially distributed in most structures. mu Binding sites were found in nearly all gray matter structures and showed heterogeneous patterns of density that were correlated with cytoarchitecture and neuronal connections. Laminar density profiles were seen in laminated structures such as olfactory bulb, cerebral cortex and hippocampus. Highest mu binding densities were in striatal patches and the habenular streak. delta Sites had distinct laminar patterns in the main olfactory bulb and cortex which differed from the mu patterns. The external plexiform layer of the main olfactory bulb had the greatest density of delta binding sites; cortex and striatum were also densely labeled. The septum, globus pallidus, preoptic area and hypothalamus were lightly labeled by both ligands. The magnocellular hypothalamic nuclei had negligible mu and delta labeling. The thalamus had dense mu but sparse delta sites. mu And delta binding sites were both present in the amygdala but had different distributions. Two fiber tracts--optic tract and fasciculus retroflexus--had FK labeling. In contrast, a portion of the corpus callosum was labeled by DADLE and not by FK. The results suggest an association of mu-opiate receptors with sensory, especially olfactory, and limbic projections in the forebrain, and delta-opiate receptors with intrinsic and commissural forebrain pathways.

Met-enkephalin selectively increases the spontaneous release of amino acid neurotransmitters from rat striatal slices

Met-enkephalin (1 microM) increased the spontaneous release of endogenous glutamate (+155%), taurine (+80%) and glycine (+50%) from rat striatal slices, but was without effect in the cerebral cortex. This effect was calcium-dependent and significantly reduced in the presence of naloxone (1 microM). Naloxone alone had no effect on release of any substance. Release of aspartate, gamma-aminobutyric acid and [3H]acetylcholine was not significantly affected. Met-enkephalin did not affect potassium (35 mM)-evoked neurotransmitter release, nor did it affect the uptake of D-[3H]aspartate, [3H]taurine or [14C]glycine. The data indicate that opioid receptor activation selectively increases the spontaneous calcium-dependent release of putative amino acid neurotransmitters in the rat striatum.