Elevation of Met-enkephalin-like immunoreactivity in the rat striatum and globus pallidus following the focal injection of excitotoxins

Glutamate receptors participate in the nicotine-induced changes of met-enkephalin in striatum

A single dose of nicotine given to mice induces first a rapid decrease (presumed release/enhanced degradation) and then a rise (presumed synthesis/enhanced accumulation) of met-enkephalin (Met-Enk) in dorsal and ventral striatum observed at 30 and 60 min post-treatment, respectively. These studies investigated whether the nicotine effect on Met-Enk was mediated indirectly, in part, via other neurotransmitters known to be released by nicotine. Based on the ability of selective antagonists of dopamine (Sch 23390, D1; Sulpiride, D2), glutamate (CPP, competitive NMDA; dizocilpine, non-competitive NMDA; NBQX, AMPA) and GABA (bicuculline, GABA(A); Sch 50911, GABA(B)) receptors, to inhibit or enhance the response to nicotine, we conclude that nicotine alters striatal Met-Enk, in part, via glutamate NMDA and AMPA receptors. These findings further support the notion that glutamate might play a role in the pharmacology of nicotine.

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.

Modulation of striatal quinolinate neurotoxicity by elevation of endogenous brain kynurenic acid

1. Nicotinylalanine, an inhibitor of kynurenine metabolism, has been shown to elevate brain levels of endogenous kynurenic acid, an excitatory amino acid receptor antagonist. This study examined the potential of nicotinylalanine to influence excitotoxic damage to striatal NADPH diaphorase (NADPH-d) and gamma-aminobutyric acid (GABA)ergic neurones that are selectively lost in Huntington's disease. 2. A unilateral injection of the N-methyl-D-aspartate (NMDA) receptor agonist, quinolinic acid, into the rat striatum produced an 88% depletion of NADPH-d neurones. Intrastriatal infusion of quinolinic acid also produced a dose-dependent reduction in striatal GABA content. 3. Nicotinylalanine (2.3, 3.2, 4.6, 6.4 nmol 5 microl(-1), i.c.v.) administered with L-kynurenine (450 mg kg(-1)), a precursor of kynurenic acid, and probenecid (200 mg kg(-1)), an inhibitor of organic acid transport, 3 h before the injection of quinolinic acid (15 nmol) produced a dose-related attenuation of the quinolinic acid-induced loss of NADPH-d neurones. Nicotinylalanine (5.6 nmol 5 microl(-1)) in combination with L-kynurenine and probenecid also attenuated quinolinic acid-induced reductions in striatal GABA content. 4. Nicotinylalanine (4.6 nmol, i.c.v.), L-kynurenine alone or L-kynurenine administered with probenecid did not attenuate quinolinic acid-induced depletion of striatal NADPH-d neurones. However, combined administration of kynurenine and probenecid did prevent quinolinic acid-induced reductions in ipsilateral striatal GABA content. 5. Injection of nicotinylalanine, at doses (4.6 nmol and 5.6 nmol i.c.v.) which attenuated quinolinic acid-induced striatal neurotoxicity, when combined with L-kynurenine and probenecid produced increases in both whole brain and striatal kynurenic acid levels. Administration of L-kynurenine and probenecid without nicotinylalanine also elevated kynurenic acid, but to a lesser extent. 6. The results of this study demonstrate that nicotinylalanine has the potential to attenuate quinolinic acid-induced striatal neurotoxicity. It is suggested that nicotinylalanine exerts its effect by increasing levels of endogenous kynurenic acid in the brain. The results of this study suggest that agents which influence levels of endogenous excitatory amino acid antagonists such as kynurenic acid may be useful in preventing excitotoxic damage to neurones in the CNS.

NMDA receptor antagonist MK-801 down-regulates rat striatal proenkephalin and protachykinin mRNAs

Using quantitative in situ hybridization, a significant decrease in expression of proenkephalin (27.1%) and of protachykinin mRNAs (20.0%) is observed in the rat caudate-putamen 14 days after daily intraperitoneal administration of N-methyl-D-aspartate receptor antagonist MK-801, 2 mg/kg.

N-methyl-D-aspartate acutely increases proenkephalin mRNA in the rat striatum

Studies in which glutamate (GLU) neurotransmission has been reduced at striatal synapses have shown that GLU influences the biosynthesis of certain peptide cotransmitters by striatal neurons. The present experiment was designed to test the effects of direct activation of the NMDA or AMPA types of GLU receptor on the levels of two mRNAs that encode the peptide cotransmitters met5-enkephalin (ME) and substance P (SP). In situ hybridization histochemistry of forebrain tissue sections from rats 8 h after a single intracerebroventricular infusion of NMDA or AMPA revealed a significant and dose-dependent elevation (to a maximum of almost 50%) of striatal ME mRNA when compared to vehicle-injected controls. SP mRNA was not significantly affected. NMDA was more effective than AMPA over the dose range used. Pretreatment with a potent and highly specific AMPA antagonist (NBQX) predictably blocked the AMPA-mediated elevation, and was only slightly effective against the NMDA-induced response. In striking contrast, pretreatment with a potent and highly selective NMDA antagonist (CGP37849) fully opposed both the NMDA- and the AMPA-mediated elevation of ME mRNA. These data further implicate the NMDA receptor in the regulation of peptide cotransmitter gene transcription. They suggest also that the AMPA receptor may play an indirect, synergistic role in the genetic responses of striatal neurons to GLU transmission.

Elevation of striatal and accumbal preproenkephalin, preprotachykinin and preprodynorphin mRNA abundance subsequent to N-methyl-D-aspartate receptor blockade with MK-801

The effect of N-methyl-D-aspartate (NMDA) receptor blockade on the expression of preproenkephalin (PPE), preprotachykinin (PPT) and preprodynorphin (PPD) mRNAs in the caudate-putamen and nucleus accumbens was assessed with the non-competitive NMDA receptor antagonist MK-801. Administration of MK-801 once daily for 7 consecutive days increased the abundance of all three neuropeptide mRNAs in the caudate-putamen (CPU) and nucleus accumbens (NAc). (1) PPE mRNA abundance was increased in the anterior CPU (26%) as well as dorsal and ventral CPU (46% and 39%, respectively) but was unaffected in the NAc. (2) PPT mRNA was increased in the NAc (33%), anterior CPU (27%), dorsal CPU (43%) and ventral CPU (67%). In the ventral CPU, PPT mRNA abundance doubled when the dose of MK-801 increased two-fold (from 67% to 119% above control). (3) PPD mRNA was elevated in dorsal and ventral regions of the CPU (49% and 24%, respectively) and in anterior CPU (50%). In the NAc PPD mRNA was increased only at the higher dose (0.1 mg/kg) of MK-801. Cellular analysis of the distribution of grains per cell shows that increases are due to increased accumulation of mRNA by previously expressing cells of the CPU and NAc. These observations demonstrate that NMDA receptor activity plays a significant role in the regulation of neuropeptide expression in the caudate-putamen and accumbens of the rat brain.

Expression of N-methyl-D-aspartate receptor subunit NR1 messenger RNA by identified striatal somatostatin cells

At present it is not clear whether N-methyl-D-aspartate and N-methyl-D-aspartate receptor agonists have a direct excitotoxic effect on somatostatin interneurons in rat striatum. The N-methyl-D-aspartate receptor comprises a multivariant complex encoded by a family of subunit complementary DNAs. Evidence suggests that expression of the N-methyl-D-aspartate receptor subunit NR1 (zeta 1) is essential for functional receptors. To investigate the expression of NR1 messenger RNA by striatal somatostatin cells, a dual in situ hybridization technique was applied to fresh frozen tissue sections. Cellular sites of NR1 and somatostatin gene expression were visualized in the same tissue section using [35S]NR1 and alkaline phosphatase-labelled somatostatin oligonucleotides. Only 8-18% of striatal somatostatin cells expressed a strong NR1 hybridization signal; most cells (> 80%) expressed a weak or undetectable signal. In contrast NR1 messenger RNA was enriched in neighbouring medium-sized non-somatostatin cells. These data suggest that while the NR1 gene is expressed in some striatal somatostatin cells most do not express a strong NR1 signal, a finding which may explain, in part, the preferential survival of somatostatin cells in Huntington's disease.

Met-enkephalin-Arg6-Phe7 in spinal cord and brain following traumatic injury to the spinal cord: influence of p-chlorophenylalanine. An experimental study in the rat using radioimmunoassay technique

The possibility that trauma to the dorsal horn may affect the release and distribution of enkephalin was examined using the opioid peptide Met-Enk-Arg6-Phe7 (MEAP) as a marker in a rat model. The peptide content of samples of spinal cord and whole brain was measured using a radioimmunoassay (RIA) technique. In addition, the possible functional relation between this peptide and serotonin was evaluated using a pharmacological approach that included depletion of endogenous serotonin. A focal trauma to the right dorsal horn in the T10-11 segments (2 mm deep and 5 mm long) markedly modified the content of MEAP of the adjacent rostral and caudal segments of the cord, as well as the content of MEAP of the brain. Depletion of serotonin with p-CPA (an inhibitor of the synthesis of serotonin) significantly elevated the content of MEAP in the whole brain without affecting the regions of the spinal cord (except T9 level which showed a 25% decrease from an intact control group). Trauma to the spinal cord in the serotonin-depleted animals did not alter the content of MEAP further, as compared to a p-CPA-treated but untraumatized group. These results indicate that enkephalin (i) participates in the pathophysiology of spinal cord trauma and (ii) suggest that the peptide is somehow functionally related with serotonin.

Excitatory amino acid regulation of the enkephalin phenotype in mouse embryonic spinal cord cultures

Expression of the preproenkephalin gene in developing spinal cord-dorsal root ganglia (SC-DRG) cultures was determined by Northern analysis following treatments with different agonists and antagonists of the glutamate receptor. Cultures (10-12 days old) were treated with various concentrations (10(-7)-10(-3) M) of N-methyl-D-aspartate (NMDA), quisqualate, kainic acid (KA), 2-amino-5-phosphonovaleric acid (APV) and 5-methyl-10,11-dihydro-5H-dibenzo[a, d]cyclohepten-5,10-imine maleate (MK801) either with or without blocking spontaneous electrical activity with 1 microM tetrodotoxin (TTX). In electrically active cultures, treatments with NMDA and KA increased preproenkephalin transcripts (mRNAppENK), showing maximum effects at 1 microM (4-fold and 2-fold, respectively), while treatments with quisqualate and MK801 caused concentration-dependent down-regulation in mRNAppENK. The most effective concentrations of NMDA (1 microM) and quisqualate (10 microM) altered mRNAppENK levels within 4 h of treatment and peaked after 24 h for NMDA and 48 h for quisqualate treatment. Co-treatment with APV completely blocked the NMDA-induced rise of mRNAppENK. During electrical blockade, none of the concentrations of NMDA tested showed any effect on enkephalin expression, neither could NMDA pre-treatment prevent the TTX-induced down-regulation of mRNAppENK. Our results indicate that the activity-dependent establishment of the enkephalin phenotype is modulated through the selective activation of the NMDA-glutamate receptor.

Paradoxical increase in striatal neuropeptide gene expression following ischemic lesions of the cerebral cortex

Ischemic lesions of the cerebral cortex occur frequently in humans as a result of stroke. One major consequence of the death of cortical neurons is the loss of excitatory cortical projections to subcortical regions. Little is known, however, about the transsynaptic effect of such lesions on neurotransmitter expression in subcortical structures. We have examined the effects of ischemic cortical lesions on the peptidergic neurotransmitters enkephalin and tachykinins in the striatum, a brain region massively innervated by glutamatergic cortical inputs. The levels of enkephalin and tachykinin mRNAs increased in the striatum of adult rats after thermocoagulation of pial vessels. The effects were more pronounced in the striatal region most heavily innervated by the lesioned cortex but were also observed in other striatal regions and on the contralateral side. Increased gene expression was accompanied by increased immunoreactivity for the two peptides. Elevated levels of enkephalin mRNA were observed up to 3 months after surgery in the ipsilateral striatum. Whereas results of previous studies of acute cortical ablations suggested that excitatory corticostriatal neurons were necessary to maintain normal peptide levels in striatal efferent neurons, the present data indicate that lesions of the same corticostriatal neurons secondary to local ischemia result in a paradoxical transsynaptic activation of neuropeptide synthesis in subcortical structures. This effect may play a role in the functional consequences of cortical strokes and progressive cortical atrophy in humans and may have critical bearing for their treatment and prognosis.

Endogenous opiates: 1990

This paper, an examination of works published during 1990, is thirteenth in a series of our annual reviews of the research involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence, eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, pregnancy, development, and aging; immunological responses; and other behavior.

Quinolinic acid elevates striatal and pallidal Met-enkephalin levels: the role of enkephalin synthesis and release

Huntington's chorea (HC) is characterized, in part, by a substantial deficit in the striatal and pallidal enkephalin levels. Recently, an attempt was made to replicate this deficit by focally injecting quinolinic acid (QUIN), an excitotoxin, into the rat striatum. However, at 7 days post-injection, QUIN produced a dose-related and bilateral increase in the striatal and pallidal levels of met-enkephalin-like immunoreactivity (ME-i.r.), an effect which was attenuated in the presence of excitatory amino acid (EAA) receptor antagonists. In the present study, the action of QUIN was investigated further. To determine whether the QUIN (72 nmol)-induced elevations in ME-i.r. reflected the enhanced synthesis of the peptide, the striatal levels of proenkephalin mRNA were assayed 7 days following a unilateral injection of QUIN into the rat striatum. QUIN significantly depleted (50%) of the proenkephalin mRNA level in the injected, but not the contralateral striatum when compared to that in the saline-injected animals. To determine whether the QUIN-induced increases in ME-i.r. were due to an impaired release of the peptide, the release of ME-i.r. from the striatal or pallidal slices obtained from animals 7 days after a saline- or QUIN-injection, was measured. The 30 mM K(+)-stimulated ME-i.r. release from the saline-injected and contralateral striatum represented an 8-fold increase above the spontaneous release level, while this stimulus induced a 6-fold increase in the ME-i.r. release from both the QUIN-injected and contralateral striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of precipitated withdrawal from spinal morphine: changes in [Met5]enkephalin levels

This investigation was carried out to study the development of physical dependence on spinally administered morphine, and it was determined if this phenomenon is associated with altered levels of [Met5]enkephalin. Morphine was infused continuously into the intrathecal space of rats for three or six days. In morphine-dependent animals, an intrathecal naloxone challenge produced increased reaction to nociceptive stimuli, hypertension, hyperthermia, decreased urinary output, and loss of body weight. Chronic spinal infusion of morphine alone failed to alter levels of [Met5]enkephalin in sacral and lumbar spinal cord. However, 24 h after the naloxone challenge, there was a significant increase in spinal enkephalin levels in morphine-dependent animals. It is concluded that spinal morphine treatment leads to the development of physical dependence. Certain characteristics of this phenomenon, as reflected in the naloxone-precipitated withdrawal signs, differ from those associated with dependence on systemic morphine.