Dopaminergic regulation of striatal proenkephalin mRNA and prodynorphin mRNA: contrasting effects of D1 and D2 antagonists

Impaired working memory, cognitive flexibility and reward processing in mice genetically lacking Gpr88: Evidence for a key role for Gpr88 in multiple cortico-striatal-thalamic circuits

The GPR88 orphan G protein-coupled receptor is expressed throughout the striatum, being preferentially localised in medium spiny neurons. It is also present in lower densities in frontal cortex and thalamus. Rare mutations in humans suggest a role in cognition and motor function, while common variants are associated with psychosis. Here we evaluate the influence of genetic deletion of GPR88 upon performance in translational tasks interrogating motivation, reward evaluation and cognitive function. In an automated radial arm maze 'N-back' working memory task, Gpr88 KO mice showed impaired correct responding, suggesting a role for GPR88 receptors in working memory circuitry. Associative learning performance was similar to wild-type controls in a touchscreen task but performance was impaired at the reversal learning stage, suggesting cognitive inflexibility. Gpr88 KO mice showed higher breakpoints, reduced latencies and lengthened session time in a progressive ratio task consistent with enhanced motivation. Simultaneously, locomotor hyperactivity was apparent in this task, supporting previous findings of actions of GPR88 in a cortico-striatal-thalamic motor loop. Evidence for a role of GPR88 in reward processing was demonstrated in a touchscreen-based equivalent of the Iowa gambling task. Although both Gpr88 KO and wild-type mice showed a preference for an optimum contingency choice, Gpr88 KO mice selected more risky choices at the expense of more advantageous lower risk options. Together these novel data suggest that striatal GPR88 receptors influence activity in a range of procedures integrated by prefrontal, orbitofrontal and anterior cingulate cortico-striatal-thalamic loops leading to altered cognitive, motivational and reward evaluation processes.

Increased expression of proenkephalin and prodynorphin mRNAs in the nucleus accumbens of compulsive methamphetamine taking rats

Addiction is associated with neuroadaptive changes in the brain. In the present paper, we used a model of methamphetamine self-administration during which we used footshocks to divide rats into animals that continue to press a lever to get methamphetamine (shock-resistant) and those that significantly reduce pressing the lever (shock-sensitive) despite the shocks. We trained male Sprague-Dawley rats to self-administer methamphetamine (0.1 mg/kg/infusion) for 9 hours daily for 20 days. Control group self-administered saline. Subsequently, methamphetamine self-administration rats were punished by mild electric footshocks for 10 days with gradual increases in shock intensity. Two hours after stopping behavioral experiments, we euthanized rats and isolated nucleus accumbens (NAc) samples. Affymetrix Array experiments revealed 24 differentially expressed genes between the shock-resistant and shock-sensitive rats, with 15 up- and 9 downregulated transcripts. Ingenuity pathway analysis showed that these transcripts belong to classes of genes involved in nervous system function, behavior, and disorders of the basal ganglia. These genes included prodynorphin (PDYN) and proenkephalin (PENK), among others. Because PDYN and PENK are expressed in dopamine D1- and D2-containing NAc neurons, respectively, these findings suggest that mechanisms, which impact both cell types may play a role in the regulation of compulsive methamphetamine taking by rats.

Chronic suppression of phosphodiesterase 10A alters striatal expression of genes responsible for neurotransmitter synthesis, neurotransmission, and signaling pathways implicated in Huntington's disease

Inhibition of phosphodiesterase 10A (PDE10A) promotes cyclic nucleotide signaling, increases striatal activation, and decreases behavioral activity. Enhanced cyclic nucleotide signaling is a well established route to producing changes in gene expression. We hypothesized that chronic suppression of PDE10A activity would have significant effects on gene expression in the striatum. A comparison of the expression profile of PDE10A knockout (KO) mice and wild-type mice after chronic PDE10A inhibition revealed altered expression of 19 overlapping genes with few significant changes outside the striatum or after administration of a PDE10A inhibitor to KO animals. Chronic inhibition of PDE10A produced up-regulation of mRNAs encoding genes that included prodynorphin, synaptotagmin10, phosphodiesterase 1C, glutamate decarboxylase 1, and diacylglycerol O-acyltransferase and a down-regulation of mRNAs encoding choline acetyltransferase and Kv1.6, suggesting long-term suppression of the PDE10A enzyme is consistent with altered striatal excitability and potential utility as a antipsychotic therapy. In addition, up-regulation of mRNAs encoding histone 3 (H3) and down-regulation of histone deacetylase 4, follistatin, and claspin mRNAs suggests activation of molecular cascades capable of neuroprotection. We used lentiviral delivery of cAMP response element (CRE)-luciferase reporter constructs into the striatum and live animal imaging of 2-{4-[-pyridin-4-yl-1-(2,2,2-trifluoro-ethyl)-1H-pyrazol-3-yl]-phenoxymethyl}-quinoline succinic acid (TP-10)-induced luciferase activity to further demonstrate PDE10 inhibition results in CRE-mediated transcription. Consistent with potential neuroprotective cascades, we also demonstrate phosphorylation of mitogen- and stress-activated kinase 1 and H3 in vivo after TP-10 treatment. The observed changes in signaling and gene expression are predicted to provide neuroprotective effects in models of Huntington's disease.

Brain iron deficiency and excess; cognitive impairment and neurodegeneration with involvement of striatum and hippocampus

While iron deficiency is not perceived as a life threatening disorder, it is the most prevalent nutritional abnormality in the world, and a better understanding of modes and sites of action, can help devise better treatment programs for those who suffer from it. Nowhere is this more important than in infants and children that make up the bulk of iron deficiency in society. Although the effects of iron deficiency have been extensively studied in systemic organs, until very recently little attention was paid to its effects on brain function. The studies of Oski at Johns Hopkin Medical School in 1974, demonstrating the impairment of learning in young school children with iron deficiency, prompted us to study its relevance to brain biochemistry and function in an animal model of iron deficiency. Indeed, rats made iron deficient have lowered brain iron and impaired behaviours including learning. This can become irreversible especially in newborns, even after long-term iron supplementation. We have shown that in this condition it is the brain striatal dopaminergic-opiate system which becomes defective, resulting in alterations in circadian behaviours, cognitive impairment and neurochemical changes closely associated with them. More recently we have extended these studies and have established that cognitive impairment may be closely associated with neuroanatomical damage and zinc metabolism in the hippocampus due to iron deficiency, and which may result from abnormal cholinergic function. The hippocampus is the focus of many studies today, since this brain structure has high zinc concentration and is highly involved in many forms of cognitive deficits as a consequence of cholinergic deficiency and has achieved prominence because of dementia in ageing and Alzheimer's disease. Thus, it is now apparent that cognitive impairment may not be attributed to a single neurotransmitter, but rather, alterations and interactions of several systems in different brain regions. In animal models of iron deficiency it is apparent that dopaminergic interaction with the opiate system and cholinergic neurotransmission may be defective.

Prenatal exposure to valproic acid disturbs the enkephalinergic system functioning, basal hedonic tone, and emotional responses in an animal model of autism

RATIONALE

It has been suggested that behavioral aberrations observed in autism could be the result of dysfunction of the neuroregulatory role performed by the endogenous opioid peptides. Many of those aberrations have been recently modeled in rats exposed to valproic acid (VPA) on the 12th day of gestation (VPA rats).

OBJECTIVES

The aim of the present study was to elucidate functioning of the enkephalinergic system, one of the endogenous opioid peptide systems strongly involved in emotional responses, in VPA rats using both biochemical and behavioral methods.

MATERIALS AND METHODS

In situ hybridization was used to measure proenkephalin mRNA expression in adult VPA rats' central nucleus of the amygdala, the dorsal striatum, and the nucleus accumbens. Additional groups of animals were examined in a conditioned place aversion to naloxone, the elevated plus maze, and object recognition tests to assess their basal hedonic tone, anxiety, learning and memory, respectively.

RESULTS

Prenatal exposure to VPA decreased proenkephalin mRNA expression in the dorsal striatum and the nucleus accumbens but not in the central nucleus of the amygdala. It also increased anxiety and attenuated conditioned place aversion to naloxone but had no impact on learning and memory.

CONCLUSIONS

The present results suggest that prenatal exposure to VPA may lead to the decreased activity of the striatal enkephalinergic system and in consequence to increased anxiety and disregulated basal hedonic tone observed in VPA rats. Presented results are discussed in light of interactions between enkephalinergic, GABAergic, and dopaminergic systems in the striatum and mesolimbic areas of the brain.

Chronic 3,4-dihydroxyphenylalanine treatment induces dyskinesia in aphakia mice, a novel genetic model of Parkinson's disease

L-DOPA-induced dyskinesia (LID) is one of the main limitations of long term L-DOPA use in Parkinson's disease (PD) patients. We show that chronic L-DOPA treatment induces novel dyskinetic behaviors in aphakia mouse with selective nigrostriatal deficit mimicking PD. The stereotypical abnormal involuntary movements were induced by dopamine receptor agonists and attenuated by antidyskinetic agents. The development of LID was accompanied by preprodynorphin and preproenkephalin expression changes in the denervated dorsal striatum. Increased FosB-expression was also noted in the dorsal striatum. In addition, FosB expression was noted in the pedunculopontine nucleus and the zona incerta, structures previously not examined in the setting of LID. The aphakia mouse is a novel genetic model with behavioral and biochemical characteristics consistent with those of PD dyskinesia and provides a more consistent, convenient, and physiologic model than toxic lesion models to study the mechanism of LID and to test therapeutic approaches for LID.

The influence of group III metabotropic glutamate receptor stimulation by (1S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid on the parkinsonian-like akinesia and striatal proenkephalin and prodynorphin mRNA expression in rats

Group III metabotropic glutamate receptors (mGluRs) are widely distributed in the basal ganglia, especially on the terminals of pathways which seem to be overactive in Parkinson's disease. The aim of the present study was to determine whether (1S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid (ACPT-1), an agonist of group III mGluRs, injected bilaterally into the globus pallidus (GP), striatum or substantia nigra pars reticulata (SNr), can attenuate the haloperidol-induced catalepsy in rats, and whether that effect was related to modulation of proenkephalin (PENK) or prodynorphin (PDYN) mRNA expression in the striatum. Administration of ACPT-1 (0.05-1.6 microg/0.5 microl/side) caused a dose-and-structure-dependent decrease in the haloperidol (0.5 mg/kg i.p. or 1.5 mg/kg s.c.)-induced catalepsy whose order was as follows: GP>striatum>SNr. ACPT-1, given alone to any of those structures, induced no catalepsy in rats. Haloperidol (3 x 1.5 mg/kg s.c.) significantly increased PENK mRNA expression in the striatum, while PDYN mRNA levels were not affected by that treatment. ACPT-1 (3 x 1.6 microg/0.5 microl/side) injected into the striatum significantly attenuated the haloperidol-increased PENK mRNA expression, whereas administration of that compound into the GP or SNr did not influence the haloperidol-increased striatal PENK mRNA levels. Our results demonstrate that stimulation of group III mGluRs in the striatum, GP or SNr exerts antiparkinsonian-like effects in rats. The anticataleptic effect of intrastriatally injected ACPT-1 seems to correlate with diminished striatal PENK mRNA expression. However, since the anticataleptic effect produced by intrapallidal and intranigral injection of ACPT-1 is not related to a simultaneous decrease in striatal PENK mRNA levels, it is likely that a decrease in enkephalin biosynthesis is not a necessary condition to obtain an antiparkinsonian effect.

Distribution of prodynorphin mRNA and its interaction with the NPY system in the mouse brain

Using radioactive in situ hybridisation, the distribution of prodynorphin mRNA in the brains of C57Bl/6 mice was systemically investigated, and double-labelling in situ hybridisation was used to determine the extent to which neuropeptide Y (NPY) and prodynorphin mRNAs were co-expressed. Our results demonstrate that prodynorphin mRNA expression in the mouse brain is localised at specific subregions of the olfactory bulb, cortex, hippocampus, amygdala, basal ganglia, thalamus, hypothalamus, mesencephalon and myelencephalon. Among the regions displaying the most intense labelling were the olfactory tubercle, lateral septum (LS), caudate putamen (Cpu), central amygdaloid nucleus (Ce), paraventricular hypothalamic nucleus (PVN), supraoptic nucleus (SO), lateral hypothalamic area (LHA), ventromedial hypothalamic nucleus (VMH), lateral reticular nucleus (LRt) and solitary tract nucleus (NTS). In the arcuate nucleus of the hypothalamus (Arc), double-labelling in situ hybridisation revealed that prodynorphin expressing neurons also contained NPY mRNA, with a co-localisation rate of approximately 88% in the lateral part of the Arc, and 79% in the dorsal part of the Arc, respectively, suggesting potential overlapping functions of these two neurotransmitters in feeding type behaviour.

Dopamine-D1 and -D2 receptors differentially regulate synapsin II expression in the rat brain

We previously demonstrated that chronic treatment with the dopamine-D2 receptor antagonist, haloperidol, increases mRNA and protein content of the phosphoprotein, synapsin II, in the rat striatum. Since dopamine-D2 receptor antagonism and dopamine-D1 receptor blockade can have opposing effects on gene expression, the present investigation compared the effects of haloperidol with those of the dopamine-D1 receptor antagonist, R-[+]-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH23390), on the expression of synapsin II protein. Haloperidol and SCH23390 respectively elevated and reduced concentrations of the molecule in mouse primary midbrain cell cultures. Additional experiments revealed that the dopamine-D1 receptor agonist, R-[+]-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzapezine-7,8-diol (SKF38393), upregulated the phosphoprotein in these cells. Furthermore, in vivo rat studies demonstrated that chronic haloperidol treatment increases synapsin II protein expression in the medial prefrontal cortex and nucleus accumbens, as was observed in the striatum. In contrast, chronic SCH23390 administration reduced concentrations of this protein in all of these regions, although the reductions seen in the medial prefrontal cortex were insignificant. Neither haloperidol nor the dopamine-D1 receptor antagonist affected synapsin I protein expression in any of the studied brain areas. Based on these findings, we propose dopamine receptors may specifically regulate synapsin II expression through a cyclic AMP-dependent pathway. Since synapsin II is involved in neurotransmitter release and synaptogenesis, and changes in synaptic efficacy and structure are suggested in schizophrenia as well as in haloperidol treatment, our findings offer insight into the mechanistic actions of the antipsychotic agent at the synaptic level.

Combined treatment of ascorbic acid or alpha-tocopherol with dopamine receptor antagonist or nitric oxide synthase inhibitor potentiates cataleptic effect in mice

RATIONALE

Drugs like haloperidol (Hal) that decrease dopamine (DA) neurotransmission in the striatum induce catalepsy in rodents and Parkinson disease-like symptoms in humans. Nitric oxide synthase (NOS) inhibitors interfere with motor activity, disrupting rodent exploratory behavior and inducing catalepsy. Catalepsy induced by NOS inhibitors probably involves striatal DA-mediated neurotransmission. Antioxidants such as ascorbic acid (vitamin C) and alpha-tocopherol (vitamin E) have also been shown to interfere with movement modulation and the DA system.

OBJECTIVE

The objective of the study is to investigate if the antioxidants vitamins C and E would influence the catalepsy produced by Hal and NOS inhibitors.

METHODS

The effects of the following treatments on catalepsy were examined using the hanging-bar test on male Swiss mice (25-30 g): (1) vitamin C (30-1,000 mg/kg)xHal (1 mg/kg); (2) vitamin C (90-1,000 mg/kg)xN (G)-nitro-L: -arginine (LNOARG, 10 and 40 mg/kg); (3) vitamin C (300 mg/kg)xN (G)-nitro-L: -arginine methylester (LNAME, 20-80 mg/kg); (4) vitamin C (300 mg/kg) x 7-nitroindazole (7NI, 3-50 mg/kg); (5) vitamin C (90 mg/kg i.p.) x LNOARG [40 mg/kg twice a day during 4 days (subchronic treatment)]; (7) vitamin E (3-100 mg/kg) x Hal (1 mg/kg); and (6) vitamin E (3-100 mg/kg) x LNOARG (40 mg/kg).

RESULTS

Vitamin C enhanced the catalepsy produced by NOS inhibitors and Hal. Treatment with vitamin C did not affect tolerance to LNOARG cataleptic effect induced by subchronic treatment. Vitamin E potentiated the catalepsy induced by LNOARG at all doses tested; in contrast, catalepsy induced by Hal was enhanced only by the dose of 100 mg/kg.

CONCLUSIONS

Results support an involvement of dopaminergic and nitrergic systems in motor behavior control and provide compelling evidence that combined administration of the antioxidants vitamins C and E with either Hal or NOS inhibitors exacerbates extrapyramidal effects. Further studies are needed to assess possible clinical implications of these findings.

Role of nitric oxide on motor behavior

The present review paper describes results indicating the influence of nitric oxide (NO) on motor control. Our last studies showed that systemic injections of low doses of inhibitors of NO synthase (NOS), the enzyme responsible for NO formation, induce anxiolytic effects in the elevated plus maze whereas higher doses decrease maze exploration. Also, NOS inhibitors decrease locomotion and rearing in an open field arena. These results may involve motor effects of this compounds, since inhibitors of NOS, NG-nitro-L-arginine (L-NOARG), N(G)-nitro-L-arginine methylester (L-NAME), N(G)-monomethyl-L-arginine (L-NMMA), and 7-Nitroindazole (7-NIO), induced catalepsy in mice. This effect was also found in rats after systemic, intracebroventricular or intrastriatal administration. Acute administration of L-NOARG has an additive cataleptic effect with haloperidol, a dopamine D2 antagonist. The catalepsy is also potentiated by WAY 100135 (5-HT1a receptor antagonist), ketanserin (5HT2a and alfal adrenergic receptor antagonist), and ritanserin (5-HT2a and 5HT2c receptor antagonist). Atropine sulfate and biperiden, antimuscarinic drugs, block L-NOARG-induced catalepsy in mice. L-NOARG subchronic administration in mice induces rapid tolerance (3 days) to its cataleptic effects. It also produces cross-tolerance to haloperidol-induced catalepsy. After subchronic L-NOARG treatment there is an increase in the density NADPH-d positive neurons in the dorsal part of nucleus caudate-putamen, nucleus accumbens, and tegmental pedunculupontinus nucleus. In contrast, this treatment decreases NADPH-d neuronal number in the substantia nigra compacta. Considering these results we suggest that (i) NO may modulate motor behavior, probably by interfering with dopaminergic, serotonergic, and cholinergic neurotransmission in the striatum; (ii) Subchronic NO synthesis inhibition induces plastic changes in NO-producing neurons in brain areas related to motor control and causes cross-tolerance to the cataleptic effect of haloperidol, raising the possibility that such treatments could decrease motor side effects associated with antipsychotic medications. Finally, recent studies using experimental Parkinson's disease models suggest an interaction between NO system and neurodegenerative processes in the nigrostriatal pathway. It provides evidence of a protective role of NO. Together, our results indicate that NO may be a key participant on physiological and pathophysiological processes in the nigrostriatal system.

Preproenkephalin mRNA expression in rat dorsal striatum induced by selective activation of metabotropic glutamate receptor subtype-5

Group I metabotropic glutamate receptors (mGluR1 and mGluR5 subtypes) are positively coupled to phosphoinositide hydrolysis through G-proteins and are densely expressed in medium-sized projection neurons of striatum. Selective activation of Group I mGluRs upregulates preproenkephalin (PPE) mRNA expression in the rat dorsal striatum. This study investigated the role of one subtype of Group I receptors, mGluR5, in the regulation of PPE mRNA expression in the rat dorsal striatum using quantitative in situ hybridization. Unilateral injection of the mGluR5 selective agonist (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG) into the dorsal striatum (caudoputamen) of chronically cannulated rats at doses of 50 and 200 nmol elevated basal levels of PPE mRNA in the injected dorsal striatum. The induction of PPE mRNA was evident at 1 h, remained at 3 h, and returned to normal level 6 h after CHPG injection. Pretreatment with an mGluR5 selective antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP) at a dose of 10 mg/kg (i.p.) blocked CHPG-stimulated PPE expression. MPEP also attenuated PPE expression induced by dopamine D(2) receptor blockade with eticlopride (0.5 mg/kg, i.p.). Administration of MPEP alone had no significant effects on basal levels of PPE mRNA in the striatum. The results from the present study demonstrate that glutamatergic tone on mGluR5 possesses the ability to positively regulate PPE gene expression in striatal neurons in vivo. Moreover, activation of mGluR5 participates in the mediation of D(2) antagonist-induced PPE expression.

Dopaminergic system modulation of nociceptive response in long-term diabetic rats

The present study examines the effects of dopaminergic system modulation on nociceptive response time in male diabetic rats. In this study, diabetes was induced by streptozotocin (STZ, 45 mg/kg) in adult male Sprague-Dawley rats. Insulin replacement therapy was initiated 6 weeks after the induction of diabetes for one-half of the diabetic group (1.5-2.5 IU/12 h/rat) and was continued throughout the duration of the study (up to 14 weeks). After 6 weeks of daily insulin replacement therapy, eight rats from each experimental group (STZ-diabetic, STZ-diabetic+insulin and nondiabetic control) were injected with either bromocriptine (BROM, 3 mg/kg/12 h), haloperidol (HALO, 1.5 mg/kg/12 h) or vehicle. Nociceptive response was measured by the hot plate (HP) latency test before the induction of diabetes (baseline), every 3 weeks for the first 12 weeks and then on days 5, 9 and 14 of treatment with dopaminergic agents. Animals were sacrificed 3 or 4 days after the last HP test and the brain, blood, spinal cord (SC), pituitary and adrenal glands (AD) were dissected for Met-enkephalin (ME) assay. The results show that nociceptive response of untreated diabetic animals increased gradually and significantly over the duration of this study. Administration of BROM and HALO significantly decreased and increased the nociceptive response, respectively, in all groups. However, the response of the diabetic group was more pronounced than that of the other two groups, especially for those treated with BROM. Daily insulin administration normalized nociceptive response to that of the nondiabetic controls. Diabetic animals receiving insulin replacement+BROM also showed normalized nociceptive response while the diabetic animals+HALO did not. Moreover, the administration of HALO and BROM resulted in an increase and decrease ME concentrations, respectively, in most tissues and brain regions examined. The effect of these dopaminergic agents on ME levels was greater in brain regions and tissues of the diabetic rats than in the diabetic groups receiving vehicle or in the nondiabetic control receiving these two agents. These data suggest that diabetes alters the sensitivity of the dopaminergic receptors and that altered response of the dopaminergic system could be indirectly involved in the modulation of nociception in diabetic rats possibly through the enhancement and/or deactivation of the endogenous Met-enkephalinergic system.

Ciproxifan, a histamine H3-receptor antagonist/inverse agonist, potentiates neurochemical and behavioral effects of haloperidol in the rat

By using double in situ hybridization performed with proenkephalin and H3-receptor riboprobes on the same sections from rat brain, we show that histamine H3 receptors are expressed within striatopallidal neurons of the indirect movement pathway. The majority ( approximately 70%) of striatal enkephalin neurons express H3-receptor mRNAs. This important degree of coexpression of proenkephalin and H3-receptor mRNAs prompted us to explore the effect of H3-receptor ligands on the regulation of enkephalin mRNA expression in the striatum. Acute administration of ciproxifan, a H3-receptor antagonist/inverse agonist, did not modify the expression of the neuropeptide by itself but strongly increased the upregulation of its expression induced by haloperidol. This potentiation (1) was suppressed by the administration of (R)-alpha-methylhistamine, a H3-receptor agonist, (2) occurred both in the caudate-putamen and nucleus accumbens, and (3) was also observed with a similar pattern on c-fos and neurotensin mRNA expression. Similarly, whereas it was devoid of any motor effect when used alone, ciproxifan strongly potentiated haloperidol-induced locomotor hypoactivity and catalepsy, two behaviors in which striatal neurons are involved. The strong H3-receptor mRNA expression in enkephalin neurons suggests that the synergistic neurochemical and motor effects of ciproxifan and haloperidol result from direct H3/D2-receptor interactions, leading to an enhanced activation of striatopallidal neurons of the indirect movement pathway. The potentiation of the effects of haloperidol by ciproxifan strengthens the potential interest of H3-receptor antagonists/inverse agonists to improve the symptomatic treatment of schizophrenia.

Role of Dopaminergic D2 Receptors in the Regulation of Glutamic Acid Decarboxylase Messenger RNA in the Striatum of the Rat

Levels of messenger RNA (mRNA) encoding glutamic acid decarboxylase (GAD) and preproenkephalin (PPE) were measured by Northern blot and in situ hybridization analyses in the striatum of the rat, after chronic injections of two neuroleptics, sulpiride and haloperidol. The Northern blot analysis showed that the chronic injection of sulpiride at high doses (80 mg/kg, twice a day, 14 days) increased striatal GAD and PPE mRNA levels by 120% and 78% respectively, when compared to vehicle-injected rats. Haloperidol injections at relatively low doses (1 mg/kg, once a day, 14 days) produced parallel increases in GAD (40%) and PPE (52%) mRNA levels. After in situ hybridization densitometric measurements were performed on autoradiograms from rats treated with sulpiride, haloperidol or vehicle. The distribution of GAD and PPE mRNA signals in control rats was homogeneous along the rostrocaudal extension of the striatum. A similar increase was found along this axis after sulpiride (20%) and haloperidol (30%) treatments. The cellular observation of hybridization signals showed that grain density for GAD mRNA was increased in a majority of striatal cells after both treatments. By contrast, the PPE mRNA hybridization signal only increased in a subpopulation of neurons. The effects of such treatments were also analysed by measuring GAD activity in the striatum and in its output structures, the globus pallidus and the substantia nigra. After the administration of sulpiride, GAD activity was not modified in the striatum but increased in the globus pallidus (by 17%). After haloperidol treatment, GAD activity was increased in the globus pallidus (20%) and the substantia nigra (17%). It is concluded that the interruption of dopaminergic transmission, more precisely the D2 receptor blockade, promotes in striatopallidal neurons an increase in GAD mRNA accompanied by an increase in GAD activity and PPE mRNA. A possible regulation of GAD mRNA and GAD activity in striatonigral neurons is also discussed.

Intrinsic GABA neurons inhibit proenkephalin gene expression in slice cultures of rat neostriatum

In the neostriatum, the proenkephalin gene is expressed in medium spiny GABA neurons, which project to the globus pallidus. The expression is activated by glutamatergic projections from the neocortex via NMDA receptors. In these experiments we have used slice cultures of rat neostriatum to study the role of GABA in proenkephalin gene expression. Our results show that GABA is released from neostriatal neurons and negatively regulates the proenkephalin gene expression induced by NMDA receptor stimulation. The GABAA receptors involved seem to be colocalized with NMDA receptors on the projection neurons, which express the proenkephalin gene. In further experiments, we have found that the proenkephalin gene expression is not only activated by neocortical projection neurons but also by intrinsic striatal neurons as well as by projections from the thalamus. All these glutamatergic afferents enhance the proenkephalin gene expression via NMDA receptors. Their efficacy is regulated by endogenous GABA.

The effect of a paracetamol and morphine combination on dynorphin A levels in the rat brain

The purpose of this study was to find out whether the combination of inactive doses of paracetamol (PARA) and morphine was able to change dynorphin (DYN) A levels, evaluated by radioimmunoassay, and whether naloxone or [(-)-2-(3 furylmethyl)-normetazocine] (MR 2266), a kappa-opioid antagonist, modifies or prevents the activity of this combination on nociception and on DYN levels. The work was suggested by our previous findings which demonstrated that inactive doses of PARA and morphine, when given in combination, share an antinociceptive effect, and that PARA, at antinociceptive doses, decreases DYN levels in the frontal cortex, thus indicating a selective action within the CNS. Our present results demonstrate that the combination of inactive doses of PARA (100 mg/kg) and morphine (3 mg/kg) is just as effective in decreasing the levels of DYN A as full antinociceptive doses of PARA or morphine alone in the frontal cortex of the rat. The values, expressed in pmol/g tissue, were: control = 2.83 +/- 0.20; paracetamol (100) = 2.60 +/- 0.23; morphine (3) = 2.73 +/- 0.24; paracetamol + morphine = 1.34 + 0.16 (P < 0.05). The decrease was partially antagonised by MR 2266, but not by naloxone, suggesting that the activity of PARA and morphine in combination on DYN A levels could be mediated, at least in part, through kappa-receptors, although other systems may be involved. On the other hand, both naloxone and MR 2266 prevented the antinociceptive effect of the combination in the hot plate test. All our experimental data suggest that PARA and morphine in combination exert their antinociceptive effect through the opioidergic system, which in turn may cause a decrease in DYN levels in the CNS of the rat.

The effect of paracetamol on nociception and dynorphin A levels in the rat brain

Male Wistar rats were administered with naloxone (1 mg/kg i.p.) or MR 2266 (5 mg/kg i.p) 15 min before paracetamol (400 mg/kg i.p.) treatment and the pain threshold was evaluated. Rats were subjected to the hot-plate and formalin tests and immunoreactive dynorphin A (ir-dynorphin A) levels were measured in the hypothalamus, hippocampus, striatum, brainstem, frontal and parietal-temporal cortex by radioimmunoassay. Pretreatment with naloxone abolished paracetamol antinociceptive activity both in hot-plate and in the first phase, but not in the second phase of the formalin test, while MR 2266 pretreatment was able to antagonise paracetamol effect either in the hot-plate test or in both phases of the formalin test. Among different brain areas investigated paracetamol significantly decreased ir-dynorphin A levels only in the frontal cortex. MR 2266 but not naloxone reversed the decrease in ir-dynorphin A levels elicited by paracetamol. Paracetamol seems to exert its antinociceptive effect also through the opioidergic system modulating dynorphin release in the central nervous system (CNS) of the rat, as suggested by the decrease in the peptide levels.

Fenfluramine-induced increase in preproenkephalin mRNA levels in the striatum: interaction between the serotonergic, glutamatergic, and dopaminergic systems

Fenfluramine (FE) is a halogenated amphetamine derivative that has been used in the treatment of obesity. It has been suggested that the effects of FE on the striatum are mediated by serotonergic mechanisms. However, several major afferent systems may be involved, and administration of FE may be useful to study interactions between these systems. In this work, the effects of FE on striatopallidal neurons and the possible involvement of the major striatal afferent systems were studied in rats by determination of FE-induced changes in striatal levels of preproenkephalin (PPE) mRNA using in situ hybridization. Injection of FE induced a significant increase (60%) in striatal levels of PPE mRNA. This increase was blocked by pretreatment with the D(1) dopamine receptor antagonist SCH-23390 or with the NMDA glutamate receptor antagonist MK-801, or by lesion of the serotonergic system with 5,7-dihydroxytryptamine or p-chlorophenylalanine. In 6-hydroxydopamine lesioned rats, the lesion-induced increase in PPE mRNA levels was not affected by injection of FE, but was reduced by simultaneous serotonergic deafferentation. The results suggest that the serotonergic, glutamatergic, and dopaminergic system interact to increase striatal PPE mRNA levels after FE administration.

Differential regulation of striatal dopamine D(1) and D(2) receptors in acute and chronic parkinsonian monkeys

The contribution of the duration of the striatal dopamine (DA) depletion and the expression of parkinsonian signs to changes in D(1) and D(2) receptor number was investigated in the present study. Some animals (N=4) received large doses of 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) over short periods of time and were symptomatic for a short period of time (1-3 months; acute parkinsonian group). Other animals (N8 months; chronic parkinsonian group). Despite similar symptomatology and similar degrees of striatal DA denervation, only acute parkinsonian animals had significantly increased numbers of D(1) receptors in most striatal regions. Striatal D(2) receptor binding was elevated in acute parkinsonian monkeys but only in some lateral striatal subregions at mid and caudal levels. These findings further suggest that the duration of parkinsonism is a critical factor in modulating changes in striatal neurochemistry.

Dopamine-opiate interaction in the regulation of neostriatal and pallidal neuronal activity as assessed by opioid precursor peptides and glutamate decarboxylase messenger RNA expression

Neostriatal GABAergic neurons projecting to the globus pallidus synthesize the opioid peptide enkephalin, while those innervating the substantia nigra pars reticulata and the entopeduncular nucleus synthesize dynorphin. The differential control exerted by dopamine on the activity of these two efferent projections concerns also the biosynthesis of these opioid peptides. Using in situ hybridization histochemistry, we investigated the role of opioid co-transmission in the regulation of neostriatal and pallidal activity. The expression of the messenger RNAs encoding glutamate decarboxylase-the biosynthetic enzyme of GABA-and the precursor peptides of enkephalin (preproenkephalin) and dynorphin (preprodynorphin) were measured in rats after a sustained blockade of opioid receptors by naloxone (s.c. implanted osmotic minipump, eight days, 3 mg/kg per h), and/or a subchronic blockade of D2 dopamine receptors by haloperidol (one week, 1.25 mg/kg s.c. twice a day). The density of mu opioid receptors in the neostriatum and globus pallidus was determined by autoradiography. Naloxone treatment resulted in a strong up-regulation of neostriatal and pallidal mu opioid receptors that was not affected by the concurrent administration of haloperidol. Haloperidol alone produced a moderate down-regulation of neostriatal and pallidal micro opioid receptors. Haloperidol strongly stimulated the expression of neostriatal preproenkephalin and preprodynorphin messenger RNAs. This effect was partially attenuated by naloxone, which alone produced moderate increases in preproenkephalin and preprodynorphin messenger RNA levels. In the neostriatum, naloxone did not affect either basal or haloperidol-stimulated glutamate decarboxylase messenger RNA expression. A strong reduction of glutamate decarboxylase messenger RNA expression was detected over pallidal neurons following either naloxone or haloperidol treatment, but concurrent administration of the two antagonists did not result in a further decrease. The amplitude of the variations of mu opioid receptor density and of preproenkephalin and preprodynorphin messenger RNA levels suggests that the regulation of neostriatal and pallidal micro opioid receptors is more susceptible to a direct opioid antagonism, while the biosynthesis of opioid peptides in the neostriatum is more dependent on the dopaminergic transmission. The down-regulation of mu opioid receptors following haloperidol represents probably an adaptive change to increased enkephalin biosynthesis and release. The haloperidol-induced increase in neostriatal preprodynorphin messenger RNA expression might result from an indirect, intermittent stimulation of neostriatal D1 receptors. The haloperidol-induced decrease of pallidal glutamate decarboxylase messenger RNA expression suggests, in keeping with the current functional model of the basal ganglia, that the activation of the striatopallidal projection produced by the interruption of neostriatal dopaminergic transmission reduces the GABAergic output of the globus pallidus. The reduction of pallidal glutamate decarboxylase messenger RNA expression following opioid receptor blockade indicates an indirect, excitatory influence of enkephalin upon globus pallidus neurons and, consequently, a functional antagonism between the two neuroactive substances (GABA and enkephalin) of the striatopallidal projection in the control of globus pallidus output. Through this antagonism enkephalin could partly attenuate the GABA-mediated effects of a dopaminergic denervation on pallidal neuronal activity.

Locomotor-activity-induced changes in striatal levels of preprotachykinin and preproenkephalin mRNA. Regulation by the dopaminergic and glutamatergic systems

The mechanisms by which dopaminergic and glutamatergic inputs interact to regulate striatal neuropeptide expression during physiological motor activity are poorly understood. In this work, striatal expression of preprotachykinin (PPT) and preproenkephalin (PPE) mRNA was studied by in situ hybridization in rats killed 2 h after treadmill running (36 m/min for 20 min). Treadmill running induced a significant increase in the levels of both PPT (60% increase) and PPE (90% increase) mRNA in the striatum of normal rats. The increase in the level of PPT mRNA was blocked in rats previously subjected to nigrostriatal deafferentation (i.e., 6-hydroxydopamine lesion) or pretreated with D1-receptor antagonist SCH-23390 (0.1 mg/kg), the D2-receptor antagonist eticlopride (0.5 mg/kg), or the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801 (0.1 mg/kg). The running-induced increase in the level of PPE mRNA was blocked in rats pretreated with SCH-23390 or MK-801. Rats subjected to nigrostriatal deafferentation or pretreated with eticlopride showed an increase in PPE mRNA levels (around 150% and 40% increase, respectively), that was enhanced by running (around 230% and 160% increase, respectively). These results suggest that locomotor activity increases, in a NMDA receptor dependent fashion, the excitatory influence of the corticostriatal glutamatergic system on the two populations of striatal projection neurons, as reflected by increases in the levels of PPT and PPE mRNA. The results obtained after dopamine depletion or injection of dopamine receptor antagonists suggest that a concomitant increase in dopamine release may enhance PPT mRNA level in striatonigral neurons via D1 receptors, and reduce PPE mRNA level in striatopallidal neurons via D2 receptors. Additionally, levels of dopamine and glutamate may be regulated by other complex indirect mechanisms.

Prodynorphin and proenkephalin mRNAs are increased in rat brain after acute and chronic administration of gamma-hydroxybutyrate

The effects of gamma-hydroxybutyrate (GHB) on prodynorphin (PD) and proenkephalin (PE) mRNA expression were examined using in situ hybridization histochemistry in discrete rat brain structures rich in GHB receptors. A single dose of GHB (500 mg/kg i.p.) increased striatal PE mRNA levels (+60%) between 15 and 90 min after injection. An increase in PD mRNA expression was observed in the frontal cortex (+90%) 6 h after GHB administration. Chronic exposure to GHB (500 mg/kg i.p. twice a day) for 10 days induced significant increases in both PE and PD mRNA levels in different brain regions examined, suggesting that PD and PE mRNA expressions are modulated by the endogenous GHBergic system.

Differential regulation of striatal preproenkephalin and preprotachykinin mRNA levels in MPTP-lesioned monkeys chronically treated with dopamine D1 or D2 receptor agonists

Studies in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys and in parkinsonian patients show elevated preproenkephalin (PPE) mRNA levels, unaltered by chronic L-DOPA therapy, whereas preprotachykinin (PPT) mRNA levels are decreased by the lesion and corrected by L-DOPA. The relative contributions of the dopamine D1 and D2 receptors for PPE mRNA regulation were investigated in the present study and compared with those for PPT mRNA. In situ hybridization was used to measure peptide mRNA levels in the striatum of MPTP cynomolgus monkeys after chronic 1-month treatment with the D1 agonist SKF-82958, administered subcutaneously in pulsatile or continuous mode, compared with the long-acting D2 agonist cabergoline. Normal as well as untreated MPTP animals were also studied. PPE mRNA levels were elevated in the caudate nucleus and putamen of untreated MPTP monkeys compared with control animals with a more pronounced increase in the lateral as compared with the medial part of both structures. PPT mRNA levels showed a rostrocaudal gradient, with higher values in the middle of the caudate-putamen and more so in the medial versus the lateral parts. PPT mRNA levels were decreased in the caudate and putamen of untreated MPTP monkeys compared with control animals, and this was observed in the middle and posterior parts of these brain areas. Elevated PPE and decreased PPT mRNA levels observed after MPTP exposure were corrected after treatment with cabergoline (0.25 mg/kg, every other day), a dose that had antiparkinsonian effects and did not give sustained dyskinesia. In contrast, elevated PPE mRNA levels observed in untreated MPTP monkeys were markedly increased by pulsatile administration of SKF-82958 (1 mg/kg, three times daily) in two monkeys in which the parkinsonian symptoms were improved and dyskinesias developed, whereas it remained close to control values in a third one that did not display dyskinesias despite a sustained improvement in disability; a shorter duration of motor benefit (wearing off) over time was observed in these three animals. By contrast, pulsatile administration of SKF-82958 corrected the decreased PPT level observed in untreated MPTP monkeys. Continuous treatment with SKF-82958 (equivalent daily dose) produced no clear antiparkinsonian and dyskinetic responses and did not alter the denervation-induced elevation of PPE or decrease of PPT mRNA levels. The present data suggest an opposite contribution of the dopamine D1 receptors (stimulatory) as compared with the dopamine D2 receptors (inhibitory) on PPE mRNA, whereas a similar stimulatory contribution of D1 or D2 receptors is observed for PPT mRNA. An increase in PPE expression could be involved in the induction of dyskinesias and wearing off, whereas our data do not support this link for PPT. The antiparkinsonian response was associated with a correction of the lesion-induced decrease of PPT.

Effect of nicotine use and withdrawal on brain preproenkephalin A mRNA

Although the effect of nicotine on brain neurotransmitters and behavior has been studied, the mechanism(s) by which nicotine contributes to tobacco use remains unclear. One transmitter that may relate to long-term nicotine use and its withdrawal is enkephalin, a five-amino acid opioid peptide derived from the proenkephalin A family. In the present study we determined the effect of acute and chronic nicotine treatment and its withdrawal on preproenkephalin A mRNA levels (PPE mRNA) in specific rat brain regions using Northern blot analysis. Acute treatment with nicotine produced a significant increase in PPE mRNA in striatum and hippocampus. Chronic treatment with nicotine caused a significant decrease in PPE mRNA in these brain regions. In both striatum and hippocampus there was a rebound increase in PPE mRNA 24 h after nicotine cessation which approached the saline level 7 days later. Nicotine withdrawal 24 h following nicotine cessation, caused a significant increase in PPE mRNA in both brain regions. These effects of nicotine were blocked by pretreating rats with the nicotinic antagonist, mecamylamine. These data strongly suggest that brain opioid system(s) are involved in mediating nicotinic responses and its withdrawal and may have clinical implications in treating nicotine addiction.

Enhanced sensitivity of the nucleus accumbens proenkephalin system to alcohol in rats selectively bred for alcohol preference

Evidence suggests that alcohol-induced activation of the endogenous opioid system is part of a neurobiological mechanism that may be functionally involved in alcohol reinforcement and high alcohol drinking behavior. We postulate that a genetic predisposition toward alcohol drinking is accompanied by increased responsiveness of the opioid system to alcohol. To test this hypothesis, the present study compared the effect of an acute alcohol challenge on enkephalin gene expression in discrete brain regions which are high in preproenkephalin (PPENK) mRNA content and/or are important in mediating alcohol reward in rats selectively bred for alcohol preference (P) or nonpreference (NP). PPENK mRNA content was measured by in situ hybridization performed with a 36 base oligonucleotide probe for PPENK mRNA and was quantified using a computerized image-analysis system. Blood alcohol concentration (BAC) and rate of alcohol elimination following alcohol infusion were similar in P and NP rats. P and NP rats did not differ in basal content of PPENK mRNA in any of the brain areas examined prior to onset of infusion. An intragastric (I.G.) infusion of alcohol (2.5 g/kg b.wt) produced a significant increase in PPENK mRNA in the nucleus accumbens (both shell and core) of P but not NP rats at 1 h after the onset of infusion which coincided with the time at which peak BAC was attained. In contrast, at 8 h after the onset of the alcohol infusion, when BAC was falling toward baseline, PPENK mRNA was decreased in the nucleus accumbens of both P and NP rats and in the anterior striatum and amygdala of NP rats. The results suggest that enhanced responsiveness of the enkephalinergic system to alcohol is associated with, and may be functionally involved in, mediating high alcohol drinking behavior.

Effects of risperidone and haloperidol on tachykinin and opioid precursor peptide mRNA levels in the caudate-putamen and nucleus accumbens of the rat

We investigated whether the two output pathways of the striatum are differently affected by the novel atypical drug risperidone and the conventional typical antipsychotic drug haloperidol. To this end, changes in mRNA levels of preproenkephalin-A, preproenkephalin-B, and preprotachykinin were determined in the rat striatum following chronic drug treatment for 14 days, using quantitative in situ hybridization. Furthermore, we studied the contribution of the dopamine D2 and serotonin 5-HT2A antagonist components of risperidone in establishing its effects on neuropeptide mRNA levels in the striatum. The results showed that both risperidone and haloperidol had major effects on the preproenkephalin-A mRNA and thus on the indirect striatal output route, whereas they had minor effects on preproenkephalin-B and preprotachykinin mRNA, contained by the direct output route. When both drugs were administered in the same dose, preproenkephalin-A mRNA was much more elevated by haloperidol than by risperidone. However, when doses of risperidone and haloperidol were modified to attain comparable dopamine D2 receptor occupancy, the drugs had comparable effects on preproenkephalin-A mRNA levels. It was further found that 5-HT2A/C receptor blockade with ritanserin had only modest effects on preproenkephalin-B and preprotachykinin mRNA levels and did not affect preproenkephalin-A mRNA levels. We conclude that risperidone and haloperidol, administered in the same dose, differently affect the striatal output routes. Furthermore, the results suggest that the effects of risperidone on neuropeptide mRNA levels are fully accounted for by its D2 antagonism and that no indication exists for a role of 5-HT2A receptor blockade in this action.

Topographical organization of opioid peptide precursor gene expression following repeated apomorphine treatment in the 6-hydroxydopamine-lesioned rat

Many studies have previously described changes in preproenkephalin-A (PPE-A) and preproenkephalin-B (PPE-B) gene expression in the striatum of the 6-hydroxydopamine (6-OHDA)-lesioned rat model of Parkinson's disease (both with or without dopamine replacement treatment). To date, these studies have either taken the striatum as a whole or have focused on a single subregion of the striatum. However, the striatum is organized into anatomically discrete parallel circuits serving different functions (motor, associative, and limbic). We have therefore employed in situ hybridization to examine the detailed topography of changes in opioid precursor expression following dopamine depletion and subsequent treatment with apomorphine (5 mg/kg twice daily for 10 days). In the untreated 6-OHDA-lesioned striatum PPE-A expression was elevated only in the dorsal (sensorimotor) caudate-putamen. Following apomorphine treatment PPE-A mRNA levels were further raised in the sensorimotor striatum (</=77%) and approximately doubled and tripled in the ventral caudate-putamen (associative) and nucleus accumbens (limbic), respectively. These subsequent elevations were mostly restricted to rostral portions of the striatum. Although unchanged following vehicle treatment, PPE-B gene expression in the lesioned caudate-putamen (sensorimotor and associative) was elevated some 30-fold by apomorphine treatment. A smaller rise (fivefold) was seen in rostral regions of the lesioned nucleus accumbens. Thus, differential regulation of opioid peptide transmission exists in motor, limbic, and associative regions of the striatum and may contribute to the generation of motor and cognitive disturbances following long-term treatment of the dopamine-depleted striatum.

Evidence for Fos involvement in the regulation of proenkephalin and prodynorphin gene expression in the rat hippocampus

For a long time Fos has been proposed to play some role in regulation of the proenkephalin (PENK) and prodynorphin (PDYN) gene expression. In recent years, however, evidence has accumulated that the transcription of both genes in several brain regions in vivo is transactivated by the transcription factor CREB rather than by Fos. In the present study, involvement of Fos in the mechanism of the PENK and PDYN gene induction in the hippocampal dentate gyrus during seizures elicited by kainic acid was studied using a knock-down technique. Pretreatment with an antisense oligonucleotide complementary to c-fos mRNA did not influence the kainic acid-elicited convulsions. It inhibited, by about 50%, the induction of Fos protein in the dentate gyrus during seizures. The subsequent induction of PENK and PDYN mRNAs was reduced by more than 60% by the c-fos antisense oligonucleotide, while constitutive expression of three other genes (alpha-tubulin, NMDA receptor-1, and GS protein alpha-subunit) was not affected. The obtained results support the view that Fos may be involved in regulation of the PENK and PDYN gene expression in the dentate gyrus during seizures, which further suggests that the mechanisms triggering the up-regulation of both these genes in the dentate gyrus may differ from these working in other brain regions, such as the striatum and hypothalamus.

Dynamic changes in NADPH-diaphorase staining reflect activity of nitric oxide synthase: evidence for a dopaminergic regulation of striatal nitric oxide release

In fixed tissue, neuronal NADPH-diaphorase staining results from nitric oxide synthase (NOS) activity. Neuronal NOS only synthesizes nitric oxide once activated by the binding of Ca2+/calmodulin. We show here that neuronal NADPH-diaphorase staining is also dependent on Ca2+/calmodulin, implying that only activated NOS is detected. In addition, in bovine pulmonary endothelial cells, carbachol and bradykinin dramatically and rapidly increase the intensity of NADPH-diaphorase staining. Furthermore, administration of MK801, an NMDA antagonist, decreases neuronal NADPH-diaphorase staining. This suggests that the intensity of the NADPH-diaphorase staining is related to the level of enzyme activation at the moment of tissue fixation. The potential of exploiting this observation to detect cellular activation of NOS is illustrated by the observations that the intensity of NADPH-diaphorase staining in rat striatal neurones is decreased following systemic treatment with the D1-like dopamine receptor antagonist SCH23390, and increased by the D2-like antagonist eticlopride. These results therefore provide strong evidence that the NADPH-diaphorase reaction can be used to monitor NOS activity at a cellular level of resolution, and reveal a dopaminergic regulation of NOS activity in the striatum mediated by D1-like and D2-like dopamine receptors.

Alterations in behavior and opioid gene expression induced by the novel tropane analog WF-31

The effects of the acute administration of the serotonin-selective tropane analog, [2beta-propanoyl-3beta-(4-isopropylphenyl)-tropane, WF-31, on spontaneous locomotor activity were measured and compared to those of the highly selective serotonin uptake inhibitor, fluoxetine and cocaine, a non-selective re-uptake inhibitor of dopamine and serotonin. WF-31 (1, 10 and 30 mg/kg)-elicited increases in locomotor behaviors when compared to vehicle-treated rats. This increased activity was blocked by pre-treatment with the dopaminergic antagonist, flupenthixol, suggesting that these effects may be mediated by dopaminergic mechanisms. Cocaine, but not fluoxetine, also elicited increases in behaviors. In addition, the effects of these three compounds on opioid peptide gene expression were also assessed using in situ hybridization histochemistry in the same animals. The acute administration of both WF-31 and cocaine increased the expression of preprodynorphin mRNA in the dorsal striatum whereas fluoxetine had no effect. Expression of striatal preproenkephalin mRNA was augmented by all three compounds. Within the nucleus accumbens, PPD mRNA levels were affected only by treatment with WF-31, an effect that was blocked by pre-treatment with flupenthixol. In contrast, the acute administration of both WF-31 and fluoxetine, but not cocaine, increased the expression of preproenkephalin mRNA. These increases, however, were not reversed by pre-treatment with flupenthixol. Despite its profile in vitro as a relatively selective serotonin re-uptake inhibitor, some of the in vivo actions of WF-31 appear to be mediated by dopaminergic mechanisms. These data further suggest that the mechanisms underlying expression of the opioid peptides in the nucleus accumbens may vary from those in the dorsal striatum.

Modulation of basal ganglia neurotransmission by the classical antipsychotic fluphenazine is due in part to the blockade of dopamine D1-receptors

Classical antipsychotics, such as fluphenazine, influence neurotransmission by blocking both dopamine D1- and D2-receptors which in turn results in widespread adaptive changes in the neurochemistry of the basal ganglia. The purpose of the present study was to determine the role of D1-receptors in mediating some of these neurochemical events, including changes in D1- and D2-receptor binding, and the expression of preproenkephalin and glutamic acid decarboxylase mRNAs. For these experiments, rats were given a depot injection of fluphenazine decanoate or injected twice daily for 21 days with the D1-receptor antagonist SCH-23390. An additional group received both fluphenazine and SCH-23390 and controls were given saline. Fluphenazine administration decreased D2-receptor binding throughout the basal ganglia while SCH-23390 was without effect. In contrast to the uniform reduction in D2-receptor binding, fluphenazine altered D1-receptor binding in a region-dependent manner. Region-dependent changes were also observed in animals given SCH-23390 which increased binding in the entopeduncular nucleus and posterior caudate-putamen without affecting other brain regions. Both fluphenazine and SCH-23390 significantly enhanced preproenkephalin and glutamic acid decarboxylase (GAD) mRNA expression in the anterior striatum. Fluphenazine also increased GAD mRNA levels in the entopeduncular nucleus. Together, these results indicate that the attenuation of D1-receptor-mediated neurotransmission modulates a number of clinically relevant neurochemical processes in the basal ganglia.

Sensitization to the behavioral effects of cocaine: modulation by dynorphin and kappa-opioid receptor agonists

Several lines of evidence suggest an involvement of the mesolimbic dopamine (DA) system in the mediation of psychostimulant-induced sensitization. It is also apparent that endogenous opioid peptide systems can modulate the activity of this same DA system. Psychostimulant-induced alterations in opioid peptide gene expression have also been reported. In this review, evidence will be presented that demonstrates that the administration of kappa-opioid agonists can prevent the initiation of behavioral sensitization to cocaine and that such treatment is also effective in preventing alterations in mesolimbic DA neurotransmission that occur as a consequence of repeated cocaine administration. The putative role of opioid-DA interactions in the modulation of psychostimulant-induced sensitization will also be discussed.

Dynorphin and epilepsy

Studies on dynorphin involvement in epilepsy are summarised in this review. Electrophysiological, biochemical and pharmacological data support the hypothesis that dynorphin is implicated in specific types of seizures. There is clear evidence that this is true for complex partial (limbic) seizures, i.e. those characteristic of temporal lobe epilepsy, because; (1) dynorphin is highly expressed in various parts of the limbic system, and particularly in the granule cells of the hippocampus; (2) dynorphin appears to be released in the hippocampus (and in other brain areas) during complex partial seizures; (3) released dynorphin inhibits excitatory neurotransmission at multiple synapses in the hippocampus via activation of kappa opioid receptors; (4) kappa opioid receptor agonists are highly effective against limbic seizures. Data on generalised tonic-clonic seizures are less straightforward. Dynorphin release appears to occur after ECS seizures and kappa agonists exert a clear anticonvulsant effect in this model. However, more uncertain biochemical data and lack of efficacy of kappa agonists in other generalised tonic-clonic seizure models argue that the involvement of dynorphin in this seizure type may not be paramount. Finally, an involvement of dynorphin in generalised absence seizures appears unlikely on the basis of available data. This may not be surprising, given the presumed origin of absence seizures in alterations of the thalamo-cortical circuit and the low representation of dynorphin in the thalamus. In conclusion, it may be suggested that dynorphin plays a role as an endogenous anticonvulsant in complex partial seizures and in some cases of tonic-clonic seizures, but most likely not in generalised absence. This pattern of effects may coincide with the antiseizure spectrum of selective kappa agonists.

Nitric oxide synthase inhibitor L-NAME prevents amphetamine-induced prodynorphin gene expression in the rat

1. The effect of nitric oxide synthase inhibitor on amphetamine-induced behavioral changes and prodynorphin gene expression in the nucleus accumbens and striatum of the rat has been studied. 2. L-NAME attenuated behavioral stereotypic activity evoked by amphetamine administration. 3. Acute amphetamine administration (5mg/kg i.p.) increased the prodynorphin mRNA level in the nucleus accumbens and striatum after 3 hr. 4. A nitric oxide synthase inhibitor L-nitro-arginine methyl ester (L-NAME, 10, 50 mg/kg i.p.) had no effect on the level of prodynorphin mRNA, but prevented dose-dependently the amphetamine-induced increase in prodynorphin gene expression in both the nucleus accumbens and the striatum. 5. These data indicate that inhibition of the nitric oxide pathway attenuates biochemical and behavioral effects of amphetamine.

Dopamine and opioid regulation of the memory retrieval recovery in mice

The reactivation effects of the delta-opioid receptor blockade and D2 dopamine receptor activation on the detention-induced memory deficit in mice were investigated, in order to study possible interactions between opioid and dopamine systems in memory retrieval. Animals were trained in a one-trial passive-avoidance task. Pretesting treatment with ICI 174,864 (1, 3 or 5 mg/kg, i.p.) or quinpirole (0.5, 1 or 2 mg/kg, i.p.) facilitated retrieval of memory trace in saline-pretreated mice. Pretraining injection of the dopamine autoreceptor agonist, (+)-3PPP (2 mg/kg), having no effect alone in learning, prevented the ability of ICI 174,864 to produce the memory-enhancing effect. It is suggested that the normal functioning of the dopamine system was critical for the facilitation of retrieval by delta-antagonist. Quinpirole-induced reactivátion of memory retrieval was enhanced by pretreatment with Leu-enkephalin (0.2 mg/kg), inducing increased retention. We discuss these results in the context of an important interactions between D2 dopamine and delta-opioid receptors.

Effect of long-term haloperidol treatment on striatal neuropeptides: relation to stereotyped behavior

Behavioral and biochemical responses to D1 and D2 dopamine (DA) agonists were used to evaluate the participation of striatal peptidergic mechanisms in the motor function alterations that attend chronic neuroleptic treatment. Rats, given haloperidol (1 mg/kg, i.c.) for 21 consecutive days, were randomly allocated to one of the following treatments: the D1 agonist SKF 38393, the D2 agonist quinpirole, their combination or saline. Stereotyped behavior and neuropeptide levels were evaluated after 5 days treatment and 4 days washout. Haloperidol increased most oral behaviors including licking, chewing and biting as well as striatal enkephalin and somatostatin levels. Subsequent treatment with SKF 38393 diminished the haloperidol-induced increase in licking and chewing; quinpirole reduced chewing behavior. The administration of both agonists together decreased chewing and biting. Neither DA agonist alone, nor their combination, reduced the haloperidol-induced increase in enkephalin levels. Both SKF 38393 and quinpirole, when given alone, tended to decrease the haloperidol-induced increase in somatostatin levels; when both D1 and D2 agonists were administered together, somatostatin levels declined significantly. These results suggest that somatostatin- but not enkephalin-containing striatal neurons contribute to the expression of haloperidol-induced stereotypies.

Differential messenger RNA expression of prodynorphin and proenkephalin in the human brain

The opiate system is involved in a wide variety of neural functions including pain perception, neuroendocrine regulation, memory, drug reward, and tolerance. Such functions imply that endogenous opioid peptides should have anatomical interactions with limbic brain structures believed to be involved in the experience and expression of emotion. Using in situ hybridization histochemistry, the messenger RNA expression of the opioid precursors, prodynorphin and proenkephalin, was studied in whole hemisphere human brain tissue. Different components of the limbic system were found to be characterized by a high gene expression of either prodynorphin or proenkephalin messenger RNA. Brain regions traditionally included within the limbic system (e.g. amygdala, hippocampus, entorhinal cortex and cingulate cortex) as well as limbic-associated regions including the ventromedial prefrontal cortex and patch compartment of the neostriatum showed high prodynorphin messenger RNA expression. In contrast, high levels of proenkephalin messenger RNA were more widely expressed in the hypothalamus, periaqueductal gray, various mesencephalic nuclei, bed nucleus of the stria terminalis, and ventral pallidum; brain regions associated with endocrine-reticular-motor continuum of the limbic system. The marked anatomical dissociation between the expression of these two opioid peptide genes, seen clearly in whole hemisphere sections, indicates that distinct functions must be subserved by the prodynorphin and proenkephalin systems in the human brain.

[3H]naloxone binding in the human brain: alcoholism and the TaqI A D2 dopamine receptor polymorphism

[3H]Naloxone binding was measured in frontal gray cortex, caudate nucleus, amygdala, hippocampus and cerebellar cortex obtained post mortem from human alcoholic and nonalcoholic subjects. Binding was found to be higher in alcoholics than in nonalcoholics for all of the brain regions examined, with a significant difference in the frontal cortex. When subjects were grouped by the presence or absence of the A1 (minor) allele of the D2 dopamine receptor gene, [3H]naloxone binding was lower in all brain regions examined of subjects with the A1 allele than in those without this allele, with a significant difference in the caudate nucleus. These findings suggest that one of the consequences of chronic alcohol exposure in humans is an enhancement of the brain opiate receptor system. However, the decreased [3H]naloxone binding observed in subjects with the A1 allele may be a compensatory response to their decreased dopaminergic modulation of opiate receptor activity.

Morphological changes in met(5)-enkephalin-immunoreactive synaptic boutons in the rat neostriatum after haloperidol decanoate treatment

The morphological plasticity of an identified population of synaptic boutons in the rat neostriatum was investigated 24 h (short-term treatment) or 14 days (long-term treatment) after administration of the depot neuroleptic, haloperidol decanoate. Specific methionine(5)-enkephalin antiserum was used to label bouton profiles in the dorsal neostriatum. The size and shape of these boutons was subsequently analysed with quantitative methods at the ultrastructural level. Immunoreactive synaptic bouton profiles were found to have a larger cross-sectional area, to be less circular in shape and to have a longer maximum diameter after long-term neuroleptic treatment. These parameters were not significantly affected by short-term neuroleptic treatment. The morphological parameters indicate that methionine(5)-enkephalin-immunoreactive boutons become enlarged, probably by elongating. This suggests that boutons containing methionine(5)-enkephalin increase their potential synaptic efficacy in the long term after neuroleptic treatment.

D1 and D2 receptor regulation of preproenkephalin and preprodynorphin mRNA in rat striatum following acute injection of amphetamine or methamphetamine

Our previous work has demonstrated a dose-dependent induction of striatal preprodynorphin (PPD) in response to a single injection of the psychostimulants amphetamine (AMPH) or methamphetamine (METH). In the present study, dose-response effects of acute administration of these stimulants on preproenkephalin (PPE) mRNA expression in the rat striatum were investigated with quantitative in situ hybridization histochemistry 3 h after injection. Acute AMPH or METH at equimolar doses (3.75, 7.5, 15, and 30 mumol/kg) significantly increased PPE mRNA expression in dorsal (caudoputamen), but not ventral (nucleus accumbens), striatum in a dose-dependent fashion. In addition, the role of D1 and D2 dopamine receptors in mediating AMPH- and METH-stimulated PPE and PPD expression was also evaluated by using subtype-specific antagonists. Pretreatment of rats with SCH 23390 (0.1 mg/kg, i.p.), a selective D1 receptor antagonist, completely blocked acute AMPH (21 mumol/kg, i.p.)- or METH (21 mumol/kg, i.p.)-induced PPE as well as PPD mRNA expression in the caudoputamen. Pretreatment with eticlopride (0.5 mg/kg, i.p.), a selective D2 receptor antagonist, also blocked PPD induction by the two stimulants, but PPE induction was unaffected. Furthermore, SCH 23390 decreased, and eticlopride elevated, constitutive PPE mRNA levels in the caudoputamen. Neither antagonist had a significant effect on the basal level of PPE or PPD mRNA in the nucleus accumbens. These results demonstrate a clear dose-related responsiveness of PPE gene expression in striatal neurons in response to acute administration of amphetamines, although the intensity of the response is far less than that for striatal PPD. Furthermore, both D1 and D2 subtypes of dopamine receptors mediate AMPH- and METH-stimulated striatal PPD mRNA expression, whereas D1 receptor activation alone mediates amphetamine-stimulated PPE mRNA expression in the rat striatum.

Fos is not involved in the regulation of the proenkephalin gene by haloperidol in the mouse striatum

The aim of the study was to determine whether the haloperidol-produced induction of the c-fos gene in the mouse striatum is the cause of the increased expression of the striatal proenkephalin (PENK) gene after repeated haloperidol administration. Mice were treated with haloperidol (1 mg/kg, i.p., once daily), MK-801 (1 mg/kg, i.p., twice daily), or with both those drugs for 9 days. Pretreatment with MK-801 prevented the haloperidol-produced induction of the striatal c-fos mRNA. In animals injected with haloperidol for 9 days, levels of the striatal PENK mRNA were increased by 100%. Coadministration of MK-801 did not reduce that increase. These results suggest that Fos is not necessary for activation of the PENK gene expression, produced by chronic haloperidol application, in the striatum.

Differential regulation of mRNA levels encoding for the two isoforms of glutamate decarboxylase (GAD65 and GAD67) by dopamine receptors in the rat striatum

The effects of in vivo administration of dopamine receptor agonists or antagonists on the mRNA levels encoding for the two isoforms of glutamate decarboxylase, GAD65 and GAD67, and for preproenkephalin were studied in regions of the rat dorsal striatum by radioactive in situ hybridization histochemistry. Changes in striatal mRNA levels after drug treatment were quantified by computerized densitometry on X-ray films. Chronic administration of the dopamine receptor agonist apomorphine or the D1 dopamine receptor agonist SKF-38393 resulted in increased GAD65 mRNA levels in the dorsomedial, ventromedial, dorsolateral and ventrolateral sectors of the striatum. Apomorphine or SKF-38393 treatment did not induce significant effects on GAD67 and preproenkephalin mRNA levels in striatum. On the other hand, chronic administration of the D2 dopamine receptor agonist quinpirole significantly decreased GAD67 in the dorsolateral and ventrolateral and preproenkephalin in the ventrolateral sectors of the striatum. Quinpirole treatment did not induce significant changes in GAD65 mRNA levels. Chronic administration of the dopamine receptor antagonist haloperidol resulted in a significant increase in GAD67 and preproenkephalin mRNA levels in the dorsomedial, dorsolateral and ventrolateral striatal sectors. Chronic treatment with the D2/D3 dopamine receptor antagonist sulpiride resulted in a significant increase in GAD67 in the ventromedial and ventrolateral and PPE in the dorsomedial and ventrolateral striatal sectors. Haloperidol or sulpiride did not induce significant changes in striatal GAD65 mRNA levels. Chronic administration of the D1 dopamine receptor antagonist SCH-23390 had no significant effect on GAD67, GAD65 or preproenkephalin mRNA levels. In the present experimental conditions, stimulation of dopamine receptors with apomorphine or SKF-38393 resulted in increased GAD65 mRNA levels whereas blockade of dopamine receptors with haloperidol or sulpiride resulted in increased GAD67 mRNA levels. These results indicate that striatal GAD65 and GAD67 mRNA levels are differentially regulated by dopamine receptor subtypes.

Adaptive changes in the proenkephalin and D2 dopamine receptor mRNA expression after chronic cocaine in the nucleus accumbens and striatum of the rat

The effects of single and repeated cocaine administration on proenkephalin (PENK) and D2 dopamine receptor mRNA expression in the nucleus accumbens and striatum of the rat were studied. Acute cocaine administration increased PENK expression of mRNA in the striatum and decreased it in both those structures after 24 and 48 h. D2 receptor expression of mRNA fell after 3 h, returned to the control value after 24 h and rose after 48 h in both those brain regions following single cocaine injection. Repeated cocaine increased PENK expression of mRNA after 3 h, but after 24 and 48 h depletion of mRNA expression was observed. On the other hand, a decrease in the D2 expression of mRNA was found after 3 h in those structures, but no changes were found after 24 and 48 h following withdrawal. The obtained results suggest that in the nucleus accumbens and striatum there is an opposite regulation between PENK and D2 receptor gene expression a short time after single and chronic cocaine administration. Hence, these data provide further evidence for the significance of the PENK and dopamine systems in the neurochemical mechanism of cocaine.

Changes in methionine-enkephalin levels in specific rat brain regions following repeated treatment with selective dopaminergic agonists and antagonists

The present study was conducted to investigate the effects of repeated treatment with selective dopaminergic agents on the level of methionine-enkephalin (Met-Enk) in rat brain cortex (CTX), hypothalamus, (HYPO), hippocampus (HIPP) and midbrain (MID). Male Sprague-Dawley rats were kept under controlled conditions for at least one week. After adaptation period, rats were randomly assigned into nine groups (7-9 rats per group) for intraperitoneal treatment with dopaminergic agents. Group 1 served as control, while, groups 2, 3 and 4 were treated with either SKF-81297, SCH 23390 or their combination, respectively. Groups 5, 6 and 7 received either LY 171555, (-)-sulpiride or their combination, whereas groups 8 and 9 were treated with nomifensine or selegiline, respectively. One hour after the last injection, rats were sacrificed, brains were removed and dissected into different regions, then extracted and their Met-Enk levels determined by radioimmunoassay (RIA). Administration of SKF-81297 or SCH 23390 significantly elevated Met-Enk levels in all brain regions examined, while their combination elevated Met-Enk levels in HYPO and HIPP only. On the other hand, treatment with LY 171555 or (-)-sulpiride, but not their combination, markedly increased Met-ENK levels in all brain regions investigated, whereas, treatment with nomifensine increased Met-Enk levels in all brain regions investigated, whereas, treatment with selegiline significantly elevated Met-Enk in HYPO, HIPP and MID but not in CTX. These findings clearly indicate that dopaminergic agonists and antagonists alter Met-Enk levels in specific rat brain regions.