Messenger RNAs encoding glutamate-decarboxylases are differentially affected by nigrostriatal lesions in subpopulations of striatal neurons

Adenosine A2A Receptor Gene Knockout Prevents l-3,4-Dihydroxyphenylalanine-Induced Dyskinesia by Downregulation of Striatal GAD67 in 6-OHDA-Lesioned Parkinson's Mice

l-3,4-Dihydroxyphenylalanine (l-DOPA) remains the primary pharmacological agent for the symptomatic treatment of Parkinson’s disease (PD). However, the development of l-DOPA-induced dyskinesia (LID) limits the long-term use of l-DOPA for PD patients. Some data have reported that adenosine A_2A receptor (A_2AR) antagonists prevented LID in animal model of PD. However, the mechanism in which adenosine A_2AR blockade alleviates the symptoms of LID has not been fully clarified. Here, we determined to knock out (KO) the gene of A_2AR and explored the possible underlying mechanisms implicated in development of LID in a mouse model of PD. A_2AR gene KO mice were unilaterally injected into the striatum with 6-hydroxydopamine (6-OHDA) in order to damage dopamine neurons on one side of the brain. 6-OHDA-lesioned mice were then injected once daily for 21 days with l-DOPA. Abnormal involuntary movements (AIMs) were evaluated on days 3, 8, 13, and 18 after l-DOPA administration, and real-time polymerase chain reaction and immunohistochemistry for glutamic acid decarboxylase (GAD) 65 and GAD67 were performed. We found that A_2AR gene KO was effective in reducing AIM scores and accompanied with decrease of striatal GAD67, rather than GAD65. These results demonstrated that the possible mechanism involved in alleviation of AIM symptoms by A_2AR gene KO might be through reducing the expression of striatal GAD67.

Altered postnatal maturation of striatal GABAergic interneurons in a phenotypic animal model of dystonia

GABAergic disinhibition has been suggested to play a critical role in the pathophysiology of several basal ganglia disorders, including dystonia, a common movement disorder. Previous studies have shown a deficit of striatal GABAergic interneurons (IN) in the dtsz mutant hamster, one of the few phenotypic animal models of dystonia. However, mechanisms underlying this deficit are largely unknown. In the present study, we investigated the migration and maturation of striatal IN during postnatal development (18days of age) and at age of highest severity of dystonia (33days of age) in this hamster model. In line with previous findings, the density of GAD67-positive IN and the level of parvalbumin mRNA, a marker for fast spiking GABAergic IN, were lower in the dtsz mutant than in control hamsters. However, an unaltered density of Nkx2.1 labeled cells and Nkx2.1 mRNA level suggested that the migration of GABAergic IN into the striatum was not retarded. Therefore, different factors that indicate maturation of GABAergic IN were determined. While mRNA of the KCC2 cation/chloride transporters and the cytosolic carboanhydrase VII, used as markers for the so called GABA switch, as well as BDNF were unaltered, we found a reduced number of IN expressing the alpha1 subunit of the GABAA-receptor (37.5%) in dtsz hamsters at an age of 33days, but not after spontaneous remission of dystonia at an age of 90days. Since IN shift expression from alpha2 to alpha1 subunits during postnatal maturation, this result together with a decreased parvalbumin mRNA expression suggest a delayed maturation of striatal GABAergic IN in this animal model, which might underlie abnormal neuronal activity and striatal plasticity.

Investigation of diazepam efficacy on anxiety-like behavior in hemiparkinsonian rats

There is growing recognition that anxiety disorders have a greater impact on quality of life in Parkinson's disease than motor symptoms. Yet, little is known about the pathophysiology underlying this non-motor symptom in Parkinson's disease which poses a considerable barrier in developing effective treatment strategies. Here, we administered diazepam to hemiparkinsonian and non-parkinsonian rats and assessed its efficacy in three anxiety behavioral tests. At present, no information about this exists in preclinical research with sparse data in the clinical literature. Moreover, diazepam is an acute anxiolytic which makes this drug a suitable research tool to unmask differences in anxiety-like behavior. Using the unilateral, medial forebrain bundle 6-hydroxydopamine rat model of Parkinson's disease, we noted that hemiparkinsonian rats had more baseline anxiety-like behavior with 60% of them exhibiting high anxiety (HA) behavior in the elevated plus maze. In contrast, 41% of sham-lesioned rats and 8% of naïve rats exhibited HA behavior. Next, we employed the elevated plus maze and noted that diazepam (1.5mg/kg) was anxiolytic in low anxiety (LA) sham-lesioned (p=0.006) and HA sham-lesioned rats (p=0.016). Interestingly, diazepam was anxiolytic for LA hemiparkinsonian rats (p=0.017), but not for HA hemiparkinsonian rats (p=0.174) despite both groups having similar motor impairment and parkinsonian phenotype. Overall, diazepam administration unmasked differences in anxiolytic efficacy between HA hemiparkinsonian rats, LA hemiparkinsonian rats and non-parkinsonian rats. Our data suggests that neuro-circuits involved in anxiety-like behavior may differ within these groups and posits that diazepam may have reduced efficacy in certain individuals with PD anxiety disorders.

Antisense gene silencing: therapy for neurodegenerative disorders?

Since the first reports that double-stranded RNAs can efficiently silence gene expression in C. elegans, the technology of RNA interference (RNAi) has been intensively exploited as an experimental tool to study gene function. With the subsequent discovery that RNAi could also be applied to mammalian cells, the technology of RNAi expanded from being a valuable experimental tool to being an applicable method for gene-specific therapeutic regulation, and much effort has been put into further refinement of the technique. This review will focus on how RNAi has developed over the years and how the technique is exploited in a pre-clinical and clinical perspective in relation to neurodegenerative disorders.

Knockdown of GAD67 protein levels normalizes neuronal activity in a rat model of Parkinson's disease

BACKGROUND

Dopamine depletion of the striatum is one of the hallmarks of Parkinson's disease. The loss of dopamine upregulates GAD67 expression in the striatal projection neurons and causes other changes in the activity of the basal ganglia circuit.

METHODS

To normalize the GAD67 expression in the striatum after dopamine depletion, we developed several lentiviral vectors that express RNA interference (RNAi) directed against GAD67 mitochondrial RNA. The vectors were injected into the striatum of hemiparkinsonian rats and the level of GAD67 protein as well as a marker of neuronal activity, mtCO1, was analyzed using western blots.

RESULTS

Unilateral lesions of the dopamine neurons in substantia nigra resulted in an increased level of GAD67 protein in the ipsilateral striatum. Furthermore, we detected significantly higher levels of mtCO1, after dopamine depletion in the striatum. Using a lentiviral vectors with a synthetic miRNA scaffold to deliver RNAi, we were able to normalize the GAD67 protein levels in the parkinsonian rat striatum. In addition, we were able to normalize the increased neural activity, which resulted from the loss of dopamine as measured by the marker mtCO1.

CONCLUSIONS

We conclude that RNAi directed against GAD67 may be a valid approach to correct the dysregulation of the basal ganglia circuit in a rat model of Parkinson's disease. The possibility to correct for a loss of dopamine using nondopamimetic tools is interesting because it may be more directed towards the casual mechanisms of the motor symptoms.

Basal Ganglia circuits underlying the pathophysiology of levodopa-induced dyskinesia

Involuntary movements or dyskinesia, represent a debilitating complication of levodopa therapy for Parkinson's disease. Dyskinesia is, ultimately, experienced by the vast majority of the patients. Despite the importance of this problem, little was known about the cause of dyskinesia, a situation that has dramatically evolved in the last few years with a focus upon the molecular and signaling changes induced by chronic levodopa treatment. Departing from this, we here review the progress made in functional anatomy and neuroimaging that have had a tremendous impact on our understanding of the anatomo-functional organization of the basal ganglia in Parkinsonism and dyskinetic states, notably the demonstration that dyskinesia are linked to a pathological processing of limbic and cognitive information.

Localization and function of GABA transporters in the globus pallidus of parkinsonian monkeys

The GABA transporters GAT-1 and GAT-3 are abundant in the external and internal segments of the globus pallidus (GPe and GPi, respectively). We have shown that pharmacological blockade of either of these transporters results in decreased neuronal firing, and in elevated levels of extracellular GABA in normal monkeys. We now studied whether the electrophysiologic and biochemical effects of local intra-pallidal injections of GAT-1 and GAT-3 blockers, or the subcellular localization of these transporters, are altered in monkeys rendered parkinsonian by the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The subcellular localization of the transporters in GPe and GPi, studied with electron microscopy immunoperoxidase, was similar to that found in normal animals: i.e., GAT-3 immunoreactivity was mostly confined to glial processes, while GAT-1 labeling was expressed in unmyelinated axons and glial processes. A combined injection/recording device was used to record the extracellular activity of single neurons in GPe and GPi, before, during and after administration of small volumes (1microl) of either the GAT-1 inhibitor, SKF-89976A hydrochloride (720ng), or the GAT-3 inhibitor, (S)-SNAP-5114 (500ng). In GPe, the effects of GAT-1 or GAT-3 blockade were similar to those seen in normal monkeys. However, unlike the findings in the normal state, the firing of most neurons was not affected by blockade of either transporter in GPi. These results suggest that, after dopaminergic depletion, the functions of GABA transporters are altered in GPi; without major changes in their subcellular localization.

Time-course of SKF-81297-induced increase in glutamic acid decarboxylase 65 and 67 mRNA levels in striatonigral neurons and decrease in GABA(A) receptor alpha1 subunit mRNA levels in the substantia nigra, pars reticulata, in adult rats with a unilateral 6-hydroxydopamine lesion

Striatal projection neurons use GABA as their neurotransmitter and express the rate-limiting synthesizing enzyme glutamic acid decarboxylase (GAD) and the vesicular GABA transporter vGAT. The chronic systemic administration of an agonist of dopamine D1/D5-preferring receptors is known to alter GAD mRNA levels in striatonigral neurons in intact and dopamine-depleted rats. In the present study, the effects of a single or subchronic systemic administration of the dopamine D1/D5-preferring receptor agonist SKF-81297 on GAD65, GAD67, PPD and vGAT mRNA levels in the striatum and GABA(A) receptor alpha1 subunit mRNA levels in the substantia nigra, pars reticulata, were measured in rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion. After a single injection of SKF-81297, striatal GAD65 mRNA levels were significantly increased at 3 but not 72 h. In contrast, striatal GAD67 mRNA levels were increased and nigral alpha1 mRNA levels were decreased at 72 but not 3 h. Single cell analysis on double-labeled sections indicated that increased GAD or vGAT mRNA levels after acute SKF-81297 occurred in striatonigral neurons identified by their lack of preproenkephalin expression. Subchronic SKF-81297 induced significant increases in striatal GAD67, GAD65, preprodynorphin and vGAT mRNA levels and decreases in nigral alpha1 mRNA levels. In the striatum contralateral to the 6-OHDA lesion, subchronic but not acute SKF-81297 induced a significant increase in GAD65 mRNA levels. The other mRNA levels were not significantly altered. Finally, striatal GAD67 mRNA levels were negatively correlated with nigral alpha1 mRNA levels in the dopamine-depleted but not dopamine-intact side. The results suggest that different signaling pathways are involved in the modulation by dopamine D1/D5 receptors of GAD65 and GAD67 mRNA levels in striatonigral neurons. They also suggest that the down-regulation of nigral GABA(A) receptors is linked to the increase in striatal GAD67 mRNA levels in the dopamine-depleted striatum.

Decreased GAD67 mRNA levels in cerebellar Purkinje cells in autism: pathophysiological implications

The recent identification of decreased protein levels of glutamate decarboxylase (GAD) 65 and 67 isoforms in the autistic cerebellar tissue raises the possibility that abnormal regulation of GABA production in individual neurons may contribute to the clinical features of autism. Reductions in Purkinje cell number have been widely reported in autism. It is not known whether the GAD changes also occur in Purkinje cells at the level of transcription. Using a novel approach, the present study quantified GAD67 mRNA, the most abundant isoform in Purkinje cells, using in situ hybridization in adult autistic and control cases. The results indicate that GAD67 mRNA level was reduced by 40% in the autistic group (P < 0.0001; two-tailed t test), suggesting that reduced Purkinje cell GABA input to the cerebellar nuclei potentially disrupts cerebellar output to higher association cortices affecting motor and/or cognitive function. These findings may also contribute to the understanding of previous reports of alterations in the GABAergic system in limbic and cerebro-cortical areas contributing to a more widespread pathophysiology in autistic brains.

Toxic influence of subchronic paraquat administration on dopaminergic neurons in rats

Paraquat is a toxin suggested to contribute to pathogenesis of Parkinson's disease. The aim of the present study was to examine toxic influence of subchronic treatment with this pesticide (5 days, one injection per day, 2-3 days of withdrawal) on dopaminergic, serotonergic, noradrenergic and GABAergic neurons. Paraquat decreased the number of tyrosine hydroxylase-immunoreactive (TH-ir) neurons in the substantia nigra by 22% (measured 3 days after withdrawal). Two days after withdrawal the levels of the dopamine metabolites and dopamine turnover in the caudate-putamen, substantia nigra and prefrontal cortex were reduced by ca. 20-60%, and the binding of [(3)H]GBR 12,935 to dopamine transporter dropped by 25-40% in the caudate-putamen. Three days after paraquat withdrawal, the level of dopamine in the caudate-putamen was significantly increased, and earlier decreases in DOPAC and HVA in the substantia nigra, as well as [(3)H]GBR 12,935 binding in the caudate-putamen were reversed. Moreover, an increase in serotonin turnover in the caudate-putamen and prefrontal cortex, and noradrenaline level in the former structure was observed 2-3 days after paraquat withdrawal. Three days after the last paraquat injection 24-35% decreases in the proenkephalin mRNA levels and 5-7% reduction in glutamic acid decarboxylase (GAD)67 mRNA were found in the caudate-putamen. The present study suggests that subchronic paraquat administration triggers processes characteristic of early stages of dopaminergic neuron degeneration, and activates compensatory mechanisms involving dopaminergic, noradrenergic, serotonergic and GABAergic transmissions.

Unilateral 6-hydroxydopamine lesion of dopamine neurons and subchronic L-DOPA administration in the adult rat alters the expression of the vesicular GABA transporter in different subsets of striatal neurons and in the substantia nigra, pars reticulata

The loss of dopamine neurons combined or not with the subsequent administration of L-DOPA in patients with Parkinson's disease or in experimental models of the disease results in altered GABAergic signaling throughout the basal ganglia, including the striatum and the substantia nigra, pars reticulata. However, the molecular mechanisms involved in altered GABA neurotransmission remain poorly understood. In order to be released from synaptic vesicles, newly synthesized GABA is transported from the cytosol into synaptic vesicles by a vesicular GABA transporter. The objective of this study was to examine the hypothesis that expression of the vesicular GABA transporter (vGAT) is altered in the unilateral 6-hydroxydopamine model of Parkinson's disease. Our results provide evidence that a unilateral 6-hydroxydopamine lesion results in increased and decreased vGAT mRNA levels in striatopallidal and striatonigral neurons, respectively. These two subsets of neurons were identified by the co-expression or lack of co-expression of preproenkephalin, a marker of striatopallidal neurons, using double-labeling in situ hybridization histochemistry. Such changes occurred in the striatum ipsilateral to the 6-hydroxydopamine lesion and were paralleled by decreased vGAT protein levels in the substantia nigra, pars reticulate (SNr). On the other hand, the subchronic systemic administration of L-DOPA increased vGAT mRNA levels in preproenkephalin-negative neurons on the side ipsilateral and, to a lesser extent, the side contralateral to the 6-hydroxydopamine lesion. Systemic L-DOPA also increased vGAT protein levels in the ipsi- and contralateral SNr. As a whole, the results provide original evidence that vGAT expression is altered in the 6-hydroxydopamine model of Parkinson's disease. They also suggest that the behavioral effects induced by a subchronic administration of L-DOPA to 6-hydroxydopamine-lesioned rats involve an increase in the vesicular release of GABA by striatonigral neurons.

Development of striatal fast-spiking GABAergic interneurons

Fast-spiking GABAergic interneurons represent a very small portion of striatal neurons, yet they play a critical role in modulating cortical input and mediating inhibition of striatal medium-sized spiny projection neurons. Considering their pivotal role in the adult striatum, it is of importance to determine when during development these neurons acquire their characteristic properties and function. In this review we describe recent work from our laboratories indicating that fast-spiking GABAergic interneurons are under stronger cortical control than efferent neurons at postnatal day 12 but mature considerably between postnatal days 12-19 in the rat striatum. During this time period, their molecular development is under the control of GABAergic and cholinergic mechanisms. Thus, fast-spiking interneurons are poised to influence striatal function and perhaps development during the postnatal period in rats, and their properties could be influenced by commonly used pharmacological agents during a protracted developmental window. These findings point to the need for future research to better understand the functional maturation of this critical population of striatal GABAergic neurons, and the consequences of abnormal maturation of these cells.

New therapies for the treatment of Parkinson's disease: Adenosine A2A receptor antagonists

The development of non-dopaminergic therapies for the treatment of Parkinson's disease (PD) has attracted much interest in recent years. Among new different classes of drugs, adenosine A2A receptor antagonists have emerged as best candidates. The present review will provide an updated summary of the results reported in literature concerning the effects of adenosine A2A antagonists in rodent and primate models of PD. These results show that A2A receptor antagonists improve motor deficits without inducing dyskinesia and counteract parkinsonian tremor. In progress clinical trials have shown that a low dose of L-DOPA plus KW-6002 produced symptomatic relief no different from that produced by an optimal dose of L-DOPA alone, whereas dyskinesias were reduced rendering this class of compounds particularly attractive.

Expression of GAD65 and GAD67 immunoreactivity in MPTP-treated monkeys with or without l-DOPA administration

This study investigated the consequences of levodopa treatment on the expression of the 65- and 67-kDa isoforms of glutamate decarboxylase (GAD65 and GAD67) immunoreactivity in the basal ganglia and cortex of monkeys rendered Parkinsonian by systemic MPTP administration. All MPTP-treated monkeys showed Parkinsonian impairment and selective loss of tyrosine hydroxylase (TH) with sparing of GAD immunoreactive (-ir) fibers and terminals in basal ganglia. The distribution of GAD65- and GAD67-ir in the cortex, caudate, and putamen was not significantly different in MPTP vs. naïve monkeys nor as a function of L-DOPA treatment. In comparison, the expression of GAD67- but not GAD65-ir was augmented in the globus pallidus in MPTP-treated monkeys. Quantification revealed significant increases in number of GAD67-ir neurons in the external and internal segments of the globus pallidus while no significant difference in the number of GAD65-ir neurons was observed. L-DOPA treatment did not significantly change the number of GAD65- or GAD67-ir pallidal neurons following MPTP. These results support and extend the findings that transcriptional elevation of GAD67 occurs in the globus pallidus and demonstrate that GAD65 and GAD67 are differentially altered following lesion. The finding of elevated GAD67 expression in the pallidum is consistent with alterations in inhibitory neurocircuitry playing a key role in the pathophysiology of motor disturbances in Parkinson's disease.

Comparative effects of acute or chronic administration of levodopa to 6-hydroxydopamine-lesioned rats on the expression of glutamic acid decarboxylase in the neostriatum and GABAA receptors subunits in the substantia nigra, pars reticulata

Current evidence suggests that behavioral sensitization to the chronic administration of levodopa (L-DOPA) to dopamine-depleted animals involves a plasticity of GABA-mediated signaling in output regions of the basal ganglia. The purpose of this study was to compare in adult rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion the effects of an acute or chronic (for 3 or 7 days) injection of L-DOPA on mRNA levels encoding for glutamic acid decarboxylase (GAD65 and GAD67) in the striatum and GABA(A) receptor alpha1, beta2 and gamma2 subunits in the substantia nigra, pars reticulata (SNr), by in situ hybridization histochemistry. In addition, immunostaining levels for the alpha1 subunit were examined in the SNr. In agreement with previous studies, we found that L-DOPA administration increased GAD mRNA levels in the striatum of 6-OHDA-lesioned rats. However, the magnitude of this effect increased with the number of injections of L-DOPA. On the other hand, we found that 6-OHDA lesions resulted in increases in alpha1, beta2 and gamma2 mRNA levels in the ipsilateral SNr, which were normalized or decreased compared with the contralateral side by the acute or chronic administration of L-DOPA. In addition, alpha1 immunostaining in the SNr was significantly decreased in rats injected for 7 days but not for 3 days or acutely with L-DOPA. Our results demonstrate that a chronic administration of L-DOPA results in a progressive increase in GAD and decrease in GABA(A) receptor expression in the striatum and SNr, respectively. They provide further evidence that behavioral sensitization and dyskinesia induced by a chronic administration of L-DOPA in an experimental model of Parkinson's disease is paralleled by a plasticity of GABA-mediated signaling in the SNr.

Dopamine manipulation alters immediate-early gene response of striatal parvalbumin interneurons to cortical stimulation

Cortical projections provide the major excitatory inputs to the striatum. In addition to innervating medium spiny cells, these axons contact striatal interneurons that are parvalbumin-immunoreactive (PV-ir). PV-ir interneurons make synaptic connections with many medium spiny cells, and thus can modulate striatal output. The striatum also receives dopaminergic projections from the substantia nigra, but it has been challenging to study the impact of dopamine (DA) cell injury on corticostriatal activity in vivo due to limitations in the methods used to induce cortical activity. Using epidural application of the GABA(A) antagonist picrotoxin, which produces a topographically restricted region of striatal immediate-early gene expression, we have investigated the effect of DA cell injury or DA receptor antagonism on immediate-early gene (IEG) expression in striatal medium spiny cells and PV-ir interneurons. Epidural application of picrotoxin to the rat's M1 motor cortex induced Fos in ipsilateral dorsolateral striatum. Animals previously given 6-hydroxydopamine (6-OHDA) injections into the ascending DA pathways had greater total numbers of cortical stimulation-induced striatal Fos-ir cells but fewer Fos-ir/PV-ir cells, compared to sham-operates. In a separate experiment, rats given cortical stimulation and treated with the DA D2-class antagonist eticlopride (0.10 mg/kg) exhibited fewer Fos-ir/PV-ir cells than did vehicle-treated rats. Taken together, these results indicate that DA may importantly control striatal output via influences on PV-ir interneurons. Possible mechanisms for these influences are discussed.

Response of the GABAergic and dopaminergic mesostriatal projections to the lesion of the contralateral dopaminergic mesostriatal pathway in the rat

Although dopamine is the main neurotransmitter in the mesostriatal system, recent studies indicate the existence of two nigrostriatal GABAergic projections: one arising from neurons immunoreactive for GABA, glutamic acid decarboxylase (GAD67), and parvalbumin (PV) lying in the substantia nigra pars reticulata (nigrostriatal GABA cells) and the other arising from a subpopulation of dopaminergic neurons lying in the substantia nigra pars compacta and ventral tegmental area, which under normal conditions, contains mRNA for GAD65 (one of the two isoforms of glutamic acid decarboxylase), but which is not immunoreactive for GABA and GAD65 (nigrostriatal dopaminergic [DA]/GABA cells). With the aim of improving our knowledge about the interaction between the nigrostriatal system of both brain hemispheres, we have studied the response of these three components of the mesostriatal system (GABA, DA/GABA, and DA) to the lesion of the contralateral mesostriatal DA pathway, by using morphological and neurophysiological techniques. Our findings show that, in the side contralateral to the lesion, (1) the number of nigrostriatal GABA cells increases from 6% to 17% with respect to the total number of nigrostriatal cells, (2) the soma of DA/GABA cells becomes immunoreactive for GABA and GAD65, and (3) there is an increase in the firing rate and burst activity of DA-neurons, except in those projecting to the striatum, which may be under the action of the GABA hyperactivity. Taken together, our results suggest that the GABAergic components of the mesostriatal projection play a regulatory role on the DA component, activated or upregulated after contralateral DA lesion and are probably addressed to restoring the functional symmetry in basal ganglia and to slowing down the contralateral progression of DA-cell degeneration in Parkinson's disease.

Glutamic acid decarboxylase 67 mRNA regulation in two globus pallidus neuron populations by dopamine and the subthalamic nucleus

The globus pallidus (GP) consists of two neuron populations, distinguished according to their immunoreactivity for parvalbumin (PV). The PV-immunoreactive (PV+) neurons project preferentially to "downstream" targets such as the subthalamic and entopeduncular nuclei, whereas neurons lacking PV (PV- neurons) project preferentially to the striatum, suggesting a role for PV- cells in feedback to striatal neurons. Although dopamine D2 antagonist administration induces immediate early gene expression preferentially in PV- GP neurons, little is known about long-term regulation of PV- versus PV+ GP neurons. Nigral 6-hydroxydopamine (6-OHDA) lesions or repeated D2-class antagonist injections have been shown to increase pallidal expression of glutamate decarboxylase (GAD(67) isoform) mRNA. This increase in GAD(67) is believed to be secondary to activation of excitatory subthalamopallidal projections. The current study examined the effects of subthalamic nucleus (STN) lesion on 6-OHDA- or repeated D2 antagonist-induced changes in GP GAD(67) mRNA expression in PV+ and PV- neurons. Five or 21 d after nigral 6-OHDA injections or after 3, 7, or 21 d of D2 antagonist administration, GAD(67) mRNA increased in both the PV- and PV+ GP neurons, but the magnitude of the increase was significantly greater in PV- neurons. By contrast, STN lesion resulted in declines in GAD(67) mRNA in both cell populations, with the decreases in PV+ neurons exceeding those in PV- neurons. Furthermore, STN lesion completely blocked 6-OHDA- or D2 antagonist-induced GAD(67) mRNA increases in PV+ cells but only partly offset the GAD(67) mRNA increase in PV- pallidal neurons. Thus, the PV+ and PV- neurons are influenced in qualitatively similar ways by dopamine and the STN, but these cell types exhibit contrasting degrees of regulation by the dopaminergic and STN perturbations. This pattern of results has implications for pallidal control of striatal versus downstream basal ganglia nuclei.

Late degeneration of nigro-striatal neurons in ATM-/- mice

The generation of an Atm -/- mouse model of the human ataxia-telangiectasia (AT) opened new avenues toward a better understanding of the molecular and cellular basis of AT. We have recently reported that 5-month-old Atm-/- mice exhibit severe loss of tyrosine hydroxylase-positive, dopaminergic nigro-striatal neurons, down to 26% of age-matched controls. In the present study we analyzed development of the dopaminergic cell loss in the context of the nigro-striatal system. We found that dopaminergic neurons are formed normally in the Atm-/- mouse, and degenerate during the first few months of life; there was no difference between 1-month-old Atm-/- and control mice in the number of dopaminergic cells that were retrogradely labeled by an injection of fluorescent tracer into the striatum. On the other hand, a dramatic reduction in the number of labeled cells was found in 5-month-old Atm-/- mice. This cell loss was significant in areas A9 and A10 but not in area A9-I. These findings indicate that midbrain dopaminergic neurons in Atm-/- mice initially send normal axons to the striatum, only to degenerate later in life. In addition, an age-dependent as well as topographic, medial-to-lateral loss of GAD, met-enkephaline and substance-P immunopositive cells was found in the striatum of the Atm-/- mice. This phenomenon was significant only in the 5-month-old Atm-/- mice (3 months after the beginning of detectable dopaminergic cell loss). In both the striatum and the substantia nigra, the apparent cell loss was accompanied by gliosis. In addition, alpha-synuclein immunopositive bodies were observed in the cortex, striatum and substantia nigra of these mice. The present data indicate that Atm-/- mice exhibit a progressive, age-dependent, reduction in dopaminergic cells of the substantia nigra, followed by a reduction in projection neurons of the striatum. Thus, the Atm-/- mouse may model the extrapyramidal motor deficits seen in AT patients.

Selective modifications in GAD67 mRNA levels in striatonigral and striatopallidal pathways correlate to dopamine agonist priming in 6-hydroxydopamine-lesioned rats

The present study investigated long-term alterations in striatal gene expression after single exposure of unilaterally 6-hydroxydopamine-lesioned rats to different dopamine agonists (priming). Rats were primed with the D1 agonist SKF38393 (10 mg/kg), the D2/D3 agonist quinpirole (0.2 mg/kg), the dopamine precursor L-DOPA (50 mg/kg) or with vehicle (drug-naive), and GAD67, dynorphin and enkephalin mRNAs were evaluated in the striatum by in situ hybridization, 3 days after priming. To evaluate GAD67 mRNA in striatonigral and striatopallidal neurons, identified as enkephalin (-) and (+) neurons, double-labelling in situ hybridization was used. Drug-naive lesioned rats showed an increase in GAD67 mRNA in enkephalin (-) and (+) neurons, an increase in enkephalin and a decrease in dynorphin mRNAs. Priming with either SKF38393 or quinpirole further increased GAD67 mRNA in enkephalin (-) and (+) neurons, however, while SKF38393 produced a high and unbalanced activation toward enkephalin (-) neurons, after quinpirole the increase was of low intensity and similar in the two pathways. Dynorphin mRNA was increased by SKF38393 but not by quinpirole, whereas enkephalin mRNA was not changed by either priming. L-DOPA produced a high and similar increase in GAD67 mRNA in enkephalin (-) and (+) neurons. Priming differentially affected peptides and GAD67 mRNA in striatopallidal and striatonigral neurons depending on the dopamine agonist used. The degree of enduring overactivity of the striatopallidal and striatonigral pathways may be related to the ability of L-DOPA and D1 or D2/D3 receptor agonists to prime motor behavioural responses and to produce dyskinetic side-effects.

EEG modifications in the cortex and striatum after dopaminergic priming in the 6-hydroxydopamine rat model of Parkinson's disease

In rats bearing a unilateral 6-hydroxydopamine (6-OHDA) lesion of the medial forebrain bundle, a single administration of a dopamine receptor agonist (priming) sensitizes the behavioral motor responses to a dopaminergic agonist, administered 3 days after priming. In this study, changes in the electroencephalogram (EEG) frequency spectra were evaluated during priming in unilaterally 6-OHDA-lesioned rats, implanted bilaterally with electrodes both in the somatosensory cortex and striatum. Two weeks after 6-OHDA lesion, rats were primed with apomorphine (0.2 mg/kg) and received a challenge with the D(1) agonist SKF 38393 (3 mg/kg) 3 days later. 6-OHDA lesion modified the EEG pattern mainly in the beta(1) frequency band, in both cortex and striatum. Apomorphine priming produced a power decrease in the beta(1) frequency band, more pronounced in the cortex than in the striatum, as compared to saline-treated rats. Antagonism of NMDA receptor with MK-801, a treatment known to block the development of priming, increased apomorphine inhibitory effect mainly in the striatum, producing the same degree of inhibition in the two structures. Administration of SKF 38393, 3 days after priming, caused a power decrease in beta(1) frequency band of the cortex and striatum, which was more pronounced in apomorphine-primed as compared to drug-naive rats. The inhibitory effect of SKF 38393 was enhanced in rats primed with MK-801 plus apomorphine, particularly in the striatum. The results of this study suggest that long-term changes in the electrical activity of cortex and striatum after priming, might contribute to the development of the behavioral sensitization observed after priming. Development of priming might be related to the degree and cortical/striatal ratio of EEG power inhibition produced by dopamine agonists.

Glutamate NMDA receptor subunit R1 and GAD mRNA expression in human temporal lobe epilepsy

1. Molecular mechanisms underlying increased hippocampal excitability in human temporal lobe epilepsy (TLE) are largely unknown. A disturbance of the imbalance between excitatory and inhibitory neurotransmission pathways in the epileptic hippocampus may contribute substantially to a decreased seizure threshold. 2. We have extended the investigation whether TLE is associated with changes in the expression of GAD67 and NMDAR1 by assessing the relative amounts of the mRNAs in human hippocampal samples by means of semiquantitative RT-PCR. The samples included 16 hippocampal slices obtained at surgery from intractable TLE (HS, n = 14; non-HS, n = 2) and 3 postmortem control hippocampi. 3. The ratio for the GAD/NMDAR1 transcripts was significantly higher in TLE cases when compared to the nonepileptic samples. Such findings are mainly a consequence of the increased amounts of GAD mRNA detected in the epileptic hippocampus. Compared with nonepileptic samples, and without correction for neuron losses, the amounts of NMDAR1 mRNA in HS are slightly reduced, and in the non-HS samples they are significantly increased, which is consistent with an increase of NMDAR1 in the hippocampal remaining neurons, as previously reported. 4. Our results also contribute to the indication of GAD67 mRNA upregulation in human TLE. A possible functional implication for the increased GAD mRNA levels could be a mechanism to reduce neuronal hyperexcitability, synchronization, and/or the spread of seizure.

Differential regulation of GAD67, enkephalin and dynorphin mRNAs by chronic-intermittent L-dopa and A2A receptor blockade plus L-dopa in dopamine-denervated rats

Adenosine A2A receptor antagonists have been proposed as an effective therapy in the treatment of Parkinson's disease. In the present study, we compared the modifications on striatal glutamate decarboxylase (GAD67), enkephalin, and dynorphin mRNA levels produced by a chronic-intermittent administration of L-3,4-dihydroxyphenyl-alanine (L-dopa) (6 mg/kg) with those produced by the adenosine A2A receptor antagonist SCH 58261 (5 mg/kg) plus L-dopa (3 mg/kg) in unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats. As previously reported, L-dopa (6 mg/kg) and SCH 58261 (5 mg/kg) plus L-dopa (3 mg/kg) produced the same degree of turning behavior after the first administration. However, while L-dopa (6 mg/kg) induced a sensitized turning behavior response during the course of the treatment, which indicated a dyskinetic potential, SCH 58261 (5 mg/kg) plus L-dopa (3 mg/kg) produced a stable turning behavior response, which was predictive of absence of dyskinetic side effects. Unilateral 6-OHDA lesion produced an elevation in striatal GAD67 and enkephalin mRNA levels and to a decrease in dynorphin mRNA levels. Chronic-intermittent L-dopa (6 mg/kg) treatment increased the striatal levels of GAD67, dynorphin, and enkephalin mRNA in the lesioned side as compared to the vehicle treatment. Chronic-intermittent SCH 58261 (5 mg/kg) plus L-dopa (3 mg/kg) as well as L-dopa (3 mg/kg) or SCH 58261 (5 mg/kg) alone did not produce any significant modification in GAD67, dynorphin, or enkephalin mRNA levels in the lesioned striatum as compared to the striatum of vehicle-treated rats. The results show that combined SCH 58261 plus L-dopa did not produce long-term changes in markers of striatal efferent neurons activity and suggest that the lack of modifications in GAD67 and dynorphin mRNA after SCH 58261 plus L-dopa might correlate with the lack of turning behavior sensitization which predicts drug dyskinetic potential.

Regulation of glutamic acid decarboxylase 65 and 67 gene expression by ovarian steroids: identification of two functionally distinct populations of GABA neurones in the preoptic area

GABA neurones in the preoptic area (POA) are critical for oestradiol (E2)-dependent surge release of luteinizing hormone (LH); however, it is not clear which population(s) of POA GABA neurones is involved. The goals of the present studies were: (i) to determine whether E2 regulates GABA neurones similarly in two subdivisions of the POA that play a role in LH surge release, the rostral POA region that contains the organum vasculosum of the lamina terminalis (rPOA/OVLT), and the region containing the anteroventral periventricular nucleus (AVPV) and medial preoptic nucleus (MPN) and (ii) to determine whether GABA neurones in either or both regions exhibit temporal changes consistent with a role in the regulation of LH surge release. To accomplish these goals, we measured glutamic acid decarboxylase (GAD) 65 and 67 mRNA levels at several time points in ovariectomized (OVX), E2-treated OVX rats exhibiting LH surge release, and in E2-treated OVX rats in which LH surge release was blocked by prior administration of progesterone (P4). Our findings demonstrate that, despite their close proximity, GABA neurones in the AVPV/MPN region are regulated differently from those in the rPOA/OVLT. Only neurones in the AVPV/MPN region show temporal changes in GAD 67 mRNA expression that appear to be linked to positive-feedback effects of E2 on luteinizing hormone-releasing hormone (LHRH) and LH release. Our findings also indicate that a morning rise and an afternoon fall in GAD 67 mRNA levels marks two E2-dependent signals required for LHRH and LH surge release. Finally, our results suggest that there are distinct E2-induced signals to the rPOA/OVLT and AVPV/MPN regions and that these signals differentially regulate GAD 65 and 67 gene expression.

The role of the globus pallidus D2 subfamily of dopamine receptors in pallidal immediate early gene expression

The globus pallidus plays an important role in basal ganglia circuitry, representing the first relay nucleus of the 'indirect pathway' of striatal efferents. In contrast to the well-characterized actions of dopamine on striatal neurons, the functional role of the dopamine innervation of globus pallidus is less well understood. Previous research showed that systemic administration of either a dopamine D2 receptor antagonist or combined dopamine D1 and D2 receptor agonists induces Fos, the protein product of the immediate early gene c-fos, in neurons of globus pallidus [Ruskin and Marshall (1997) Neuroscience 81, 79-92]. To determine whether the ability of the D2 receptor antagonist, sulpiride, to induce Fos in rat pallidal neurons is mediated by D2-like receptors in striatum or globus pallidus, intrastriatal or intrapallidal sulpiride infusions were conducted. The diffusion of intrastriatal sulpiride was estimated by measuring this antagonist's competition for N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)-induced D2 receptor inactivation. The phenotype of the striatal neurons expressing Fos after intrastriatal infusion was assessed by combining Fos immunocytochemistry with D2 receptor mRNA in situ hybridization. Intrastriatal infusions of (-)-sulpiride (10-200 ng) dose-dependently increased the number of striatal cells expressing Fos; and the Fos-immunoreactive striatal cells were D2 receptor mRNA-expressing, the same population in which systemic D2 receptor antagonists induce Fos. Intrastriatal infusions of high (5 microg), but not low (10-200 ng), (-)-sulpiride doses also induced Fos in globus pallidus cells but the sulpiride appeared to spread to the globus pallidus. Direct intrapallidal infusions of (-)-sulpiride (50-100 ng) dose-dependently induced Fos in globus pallidus with minimal influence on striatum or other basal ganglia structures. Using sensitive in situ hybridization conditions, prominent labeling of D2 receptor mRNA was evident in globus pallidus. D2 receptor mRNA was densest in a lateral 200 microm wide band that follows the curvature of the pallidal/striatal boundary. Cellular analysis revealed silver clusters associated with D2 receptor mRNA labeling over globus pallidus neurons that were immunoreactive for neuron-specific nuclear protein. These results strongly suggest that the dopaminergic innervation of globus pallidus, acting through D2-like receptors internal to this structure, can control gene expression in pallidal neurons.

The temporal alteration of GAD67/GAD65 ratio in the gerbil hippocampal complex following seizure

In the present study, the distribution of glutamic acid decarboxylase (GAD) isoforms in the hippocampus of the Mongolian gerbil and its association with different sequelae of spontaneous seizure were investigated to identify the roles of balance of GAD isoforms in the epileptogenesis and the recovery mechanisms in these animals. The GAD67/GAD65 ratio in the hippocampus of pre-seizure seizure sensitive (SS) gerbil was approximately 3.5-fold higher as compared to seizure resistant (SR) gerbil. Following seizure, this ratio shifted to the level of SR gerbils up to 12 h postical. Therefore, the mismatched GAD67/GAD65 ratio (imbalance of GAD isoform expressions) in the hippocampus of SS gerbil implies that GABAergic neurons may be highly activated in order to regulate the increased neuronal excitability. In addition, the alteration in this ratio after seizure may be the compensatory response for reduction of epileptic activity in this animal.

Acute and chronic caffeine administration differentially alters striatal gene expression in wild-type and adenosine A(2A) receptor-deficient mice

In order to assess for the respective involvement of adenosine A(1) and A(2A) receptors (A(2A)-R) in the consequences of short- and long-term caffeine exposure on gene expression, the effects of acute caffeine administration on striatal, cortical, and hippocampal expression of immediate early genes (IEG), zif-268 and arc, and the effects of long-term caffeine or 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) exposure (once daily for 15 days) on striatal gene expression of substance P, enkephalin, and glutamic acid decarboxylase isoforms, GAD65 and GAD67, were evaluated in wild-type and A(2A)-R-deficient (A(2A)-R(-/-)) mice. In situ hybridization histochemistry was performed using oligonucleotides followed by quantitative image analysis. Our results demonstrated that a biphasic response of IEG expression to acute caffeine observed in the wild-type striatum was resumed in a monophasic response in the mutant striatum. In the cerebral cortex and hippocampus, the effect of caffeine was weak in wild-type, whereas in mutant mice it induced a 2-3-fold increase in the IEG expression to restore a level similar to the wild-type basal expression. Chronic caffeine and DPCPX-mediated regulation in neuropeptide and GADs striatal gene expression typically showed the mimicking of alterations resulting from the A(2A)-R genetic deficiency in 25 mg/kg caffeine-treated wild-type mice as well as the dose-dependent normalization of substance P and enkephalin expression in A(2A)-R(-/-) mice. These results indicate that, depending on the dose, the blockade of A(2A)-R or A(1) receptors by caffeine is preferentially revealed leading to highly differential alterations in striatal gene expression and they also suggested the central role of these two receptors on the control of dopaminergic functions.

Transplantation of embryonic Raphe cells regulates the modifications of the gabaergic phenotype occurring in the injured spinal cord

Transection of the spinal cord yields a permanent deficit due to the interruption of descending and ascending tracts which subserve the supraspinal control of spinal cord functions. We have shown previously that transplantation below the level of the section of embryonic monoaminergic neurons can promote the recovery of some segmental functions via a local serotonergic and noradrenergic reinnervation. Moreover, the up-regulation of the corresponding receptors resulting from the section was corrected by the transplants. The aim of the present work was to determine whether such a graft could also influence non-monoaminergic local neurons, the GABAergic interneurons of the spinal cord. Following spinal cord transection, the number of cells which express glutamate decarboxylase (mol. wt 67,000) messenger RNA--a marker of GABA synthesis--increased significantly below the lesion compared with the intact animal. In contrast, in lesioned animals which had been grafted one week later with raphe neuroblasts, this number was close to control level. These post-grafting modifications were further associated with increased GABA immunoreactivity in the host tissue. These data suggest that the graft of embryonic raphe cells which compensates the deficit of serotonin in the distal segment also regulates the expression of the GABAergic phenotype in the host spinal cord. This regulation could be mediated by the re-establishment of a local functional innervation by both serotonin and GABAergic transplanted neurons and/or by trophic factors released from the embryonic cells. It appears then that grafted cells influence the host tissue in a complex manner, through the release and/or regulation of several neurotransmitter systems.

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.

Gene expression of the GAD67 and GAD65 isoforms of glutamate decarboxylase is differentially altered in subpopulations of striatal neurons in adult rats lesioned with 6-OHDA as neonates

The levels of mRNAs encoding for the two isoforms of glutamate decarboxylase, GAD65 and GAD67, were measured in subpopulations of striatal neurons in adult rats depleted of dopamine as neonates with 6-OHDA and chronically injected with vehicle or with the dopamine receptor agonists apomorphine or SKF-38393. In adult rats depleted of dopamine as neonates, an increase of GAD65 and GAD67 mRNA levels was measured in the striatum. These changes were paralleled by an increase in preproenkephalin (PPE) and a decrease in preprodynorphin (PPD) mRNA levels. Quantitative analysis at the cellular level indicated that GAD67 mRNA levels were increased in PPE-labeled neurons, whereas GAD65 mRNA levels were increased in PPE-unlabeled neurons. Chronic and systemic injections of apomorphine or SKF-38393 induced further increases in striatal GAD65 and GAD67 mRNA levels. These increases were only detected in the subpopulation of PPE-unlabeled neurons and were paralleled by an increase in PPD mRNA levels. The increases in GAD67, GAD65, and PPD mRNA levels induced by SKF-38393 were abolished by the administration of the D1 receptor antagonist SCH-23390. The present results provide further evidence that GAD67 and GAD65 gene expression is differentially regulated in the two subpopulations of efferent striatal neurons. They also suggest that neonatal depletions in dopamine levels induce alterations of GABA-mediated signaling in the two subpopulations of striatal efferent neurons. We speculate that these alterations are involved in the behavioral particularities exhibited by rats depleted of dopamine as neonates.

Increased striatal expression of glutamate decarboxylase 67 after priming of 6-hydroxydopamine-lesioned rats

Previous single exposure (priming) to a dopamine receptor agonist greatly enhances the contralateral turning behaviour elicited by dopamine D1 receptor agonists in unilaterally 6-hydroxydopamine lesioned rats. In the present study we have investigated the levels of glutamate decarboxylase 67 and glutamate decarboxylase 65 messenger RNA in the striatum of 6-hydroxydopamine-lesioned rats primed with L-3,4-dihydroxyphenylalanine (L-DOPA) and challenged with the D1 receptor agonist SKF 38393, three days thereafter. As previously reported, levels of glutamate decarboxylase 67 messenger RNA increased in the striatum denervated by the 6-hydroxydopamine lesion as compared with the intact one. Striatal glutamate decarboxylase 67 messenger RNA levels, measured three days after priming with L-DOPA (50 mg/kg), further increased in the lesioned striatum while were not modified in the intact one. Administration of SKF 38393 (3 mg/kg) elicited a more intense contralateral turning behaviour in primed than in drug-naive 6-hydroxydopamine-lesioned rats but did not induce any change in striatal glutamate decarboxylase 67 messenger RNA. In contrast, striatal levels of glutamate decarboxylase 65 messenger RNA were not modified by either 6-hydroxydopamine lesions or priming with L-DOPA. The results show that priming with L-DOPA induces long-lasting changes in GABAergic neurons of the 6-hydroxydopamine-lesioned striatum. These changes might play a role in the increased behavioural response of striatal D1 receptors induced by priming.

Intra-accumbens infusions of antisense oligodeoxynucleotides to one isoform of glutamic acid decarboxylase mRNA, GAD65, but not to GAD67 mRNA, impairs sustained attention performance in the rat

The effects of bilateral infusions of antisense oligodeoxynucleotides (ODNs) for the two isoforms of glutamic acid decarboxylase (GAD65; GAD67) into the nucleus accumbens on the performance of intact rats in a task designed to assess sustained attention were tested. The task required the animals to discriminate between signal and non-signal events. Signals and non-signals were presented randomly and unpredictably. The task generated all four response types of a sustained attention task, i.e., hits, misses, correct rejections, false alarms. Infusions of the scrambled sequence ODNs did not affect performance. Likewise, infusions of the GAD67 ODNs failed to produce any effect. However, infusions of the GAD65 ODNs into the nucleus accumbens resulted in a robust and reliable decrease in the relative number of hits. Similarly, the combined infusion of GAD65+67 ODNs impaired the hit rate but did not affect the animals' ability to reject non-signals. Following each treatment series, performance rapidly returned to baseline, further indicating the specificity and reversibility of the effects of the infusions of the ODNs. While these data suggest that translation arrest of specifically the GAD65 isoform of the enzyme in the nucleus accumbens impairs attentional performance, the neuronal mechanisms mediating these effects remain unsettled.

Changes in GAD- and GABA- immunoreactivity in the spinal dorsal horn after peripheral nerve injury and promotion of recovery by lumbar transplant of immortalized serotonergic precursors

We have utilized RN46A cells, an immortalized neuronal cell line derived from E13 brainstem raphe, as a model for transplant of bioengineered serotonergic cells. RN46A cells require brain-derived neurotrophic factor (BDNF) for increased survival and serotonin (5HT) synthesis in vitro and in vivo. RN46A cells were transfected with the rat BDNF gene, and the 46A-B14 cell line was subcloned. These cells survive longer than 7 weeks after transplantation into the subarachnoid space of the lumbar spinal cord and synthesize 5HT and BDNF. Chronic constriction injury (CCI) of the sciatic nerve was used to induce chronic neuropathic pain in the affected hindpaw in rats. Transplants of 46A-B14 cells placed 1 week after CCI alleviated chronic neuropathic pain, while transplants of 46A-V1 control cells, negative for 5HT and without the BDNF gene, had no effect on the induction of thermal and tactile nociception. When endogenous cells of the dorsal horn which contain the neurotransmitter gamma-aminobutyric acid (GABA) and its synthetic enzyme glutamate decarboxylase (GAD) were immunohistochemically quantified in the lumbar spinal cord 3 days and 1-8 weeks after CCI, the number of GABA- and GAD-immunoreactive (ir) cells decreased bilateral to the nerve injury as soon as 3 days after CCI. At 1 week after CCI, the number of GABA-ir cells continued to significantly decline bilaterally, returning to near normal numbers on the side contralateral to the nerve injury by 8 weeks after the nerve injury. The number of GAD-ir cells began to increase bilaterally to the nerve injury at 1 week after CCI and continued to significantly increase in numbers over normal values by 8 weeks after the nerve injury. When examined 2 and 8 weeks after CCI plus cell transplants, the transplants of 46A-B14 cells reversed the increase in GAD-ir cell numbers and the decrease in GABA-ir cells by 1 week after transplantation, while 46A-V1 control cell transplants after CCI had no effect on the changes in numbers of GAD-ir or GABA-ir cells. Collectively, these data suggest that altered 5HT levels, and perhaps BDNF secretion, related to the transplants ameliorate chronic pain and reverse the induction and maintenance of an endogenous pain mechanism in the dorsal horn. This induction mechanism is likely dependent on altered GAD regulation and GABA synthesis, initiated by CCI.

Dopamine depletion reorganizes projections from the nucleus accumbens and ventral pallidum that mediate opioid-induced motor activity

Motor activity elicited pharmacologically from the nucleus accumbens by the mu-opioid receptor agonist D-Ala-Tyr-Gly-NMePhe-Gly-OH (DAMGO) is augmented in rats sustaining dopamine depletions. GABAergic projections from the nucleus accumbens to ventral pallidum and ventral tegmental area (VTA) are involved because stimulation of GABAB receptors in the VTA (by baclofen) or GABAA receptors in the ventral pallidum (by muscimol) inhibit the motor response induced by the microinjection of DAMGO into the nucleus accumbens. The present study was done to determine which of these projections is mediating the augmented DAMGO-induced motor activity that follows 6-hydroxydopamine lesions of the nucleus accumbens. The inhibition of DAMGO-induced activation by pallidal injections of muscimol was markedly attenuated in lesioned animals, whereas the inhibition by VTA injections with baclofen was greatly enhanced. A similar switch in emphasis from pallidal to mesencephalic efferents was not observed for dopamine-induced motor activity, because muscimol microinjections inhibited the response elicited by dopamine microinjection into the nucleus accumbens in all subjects. The stimulation of mu-opioid receptors in the ventral pallidum also elicits motor activation, and this is blocked by baclofen microinjection into the VTA. However, after dopamine depletion in the nucleus accumbens, baclofen in the VTA was ineffective in blocking the motor response by DAMGO in the ventral pallidum. These data reveal that dopamine depletion in the nucleus accumbens produces a lesion-induced plasticity that alters the effect of mu-opioid receptor stimulation on efferent projections from the nucleus accumbens and ventral pallidum.

Region-specific regulation of glutamic acid decarboxylase (GAD) mRNA expression in central stress circuits

Neurocircuit inhibition of hypothalamic paraventricular nucleus (PVN) neurons controlling hypothalamo-pituitary-adrenocortical (HPA) activity prominently involves GABAergic cell groups of the hypothalamus and basal forebrain. In the present study, stress responsiveness of GABAergic regions implicated in HPA inhibition was assessed by in situ hybridization, using probes recognizing the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD65 and GAD67 isoforms). Acute restraint preferentially increased GAD67 mRNA expression in several stress-relevant brain regions, including the arcuate nucleus, dorsomedial hypothalamic nucleus, medial preoptic area, bed nucleus of the stria terminalis (BST) and hippocampus (CA1 and dentate gyrus). In all cases GAD67 mRNA peaked at 1 hr after stress and returned to unstimulated levels by 2 hr. GAD65 mRNA upregulation was only observed in the BST and dentate gyrus. In contrast, chronic intermittent stress increased GAD65 mRNA in the anterior hypothalamic area, dorsomedial nucleus, medial preoptic area, suprachiasmatic nucleus, anterior BST, perifornical nucleus, and periparaventricular nucleus region. GAD67 mRNA increases were only observed in the medial preoptic area, anterior BST, and hippocampus. Acute and chronic stress did not affect GAD65 or GAD67 mRNA expression in the caudate nucleus, reticular thalamus, or parietal cortex. Overall, the results indicate preferential upregulation of GAD in central circuitry responsible for direct (hypothalamus, BST) or multisynaptic (hippocampus) control of HPA activity. The distinct patterns of GAD65 and GAD67 by acute versus chronic stress suggest stimulus duration-dependent control of GAD biosynthesis. Chronic stress-induced increases in GAD65 mRNA expression predict enhanced availability of GAD65 apoenzyme after prolonged stimulation, whereas acute stress-specific GAD67 upregulation is consistent with de novo synthesis of active enzyme by discrete stressful stimuli.

Thalamo-striatal deafferentation affects preproenkephalin but not preprotachykinin gene expression in the rat striatum

This study examined the effects of thalamo-striatal deafferentation on preprotachykinin and preproenkephalin mRNA expression in the rat neostriatum, using quantitative in situ hybridization histochemistry. Unilateral ibotenate-induced intralaminar thalamic lesion produced a significant decrease in preproenkephalin mRNA levels (-27%) restricted to the ipsilateral striatum at 5 days post-lesion. At 12 days post-lesion, significant decreases in striatal preproenkephalin mRNA expression were found on both brain sides. This post-lesional response was more pronounced in the ipsilateral (-32%) than contralateral (-18%) striatum. All these changes were homogeneously distributed between the dorsolateral and ventromedial parts of the striatum. In parallel, no significant change in preprotachykinin mRNA expression was found at either 5 or 12 days after thalamic lesion, when considering the striatum as a whole. However, at 5 days post-lesion, the regional analysis revealed a slight decrease (-17%) in preprotachykinin mRNA expression, confined to the dorsolateral part of the ipsilateral striatum. These results show that thalamic lesion preferentially affects preproenkephalin vs. preprotachykinin gene expression in the striatum, suggesting, at the first site, a predominant influence of thalamo-striatal inputs on the enkephalin-containing striato-pallidal pathway. However, given that the thalamo-striatal projection is strictly ipsilateral, the bilateralization of the down-regulation of preproenkephalin mRNA expression at 12 days post-lesion suggests an involvement of interhemispheric adaptive mechanisms via cortical networks.

Differential expression of glutamate decarboxylase messenger RNA in cerebellar Purkinje cells and deep cerebellar nuclei of the genetically dystonic rat

The genetically dystonic rat exhibits a motor syndrome that closely resembles the human disease, generalized idiopathic dystonia. Although in humans dystonia is often the result of pathology in the basal ganglia, previous studies have revealed electrophysiological abnormalities and alterations in glutamate decarboxylase, the synthetic enzyme for GABA, in the cerebellum of dystonic rats. In this study, we further characterized the alterations in cerebellar GABAergic transmission in these mutants by examining the expression of the messenger RNA encoding glutamate decarboxylase (67000 mol. wt) with in situ hybridization histochemistry at the single cell level in Purkinje cells and neurons of the deep cerebellar nuclei. Glutamate decarboxylase (67000 mol. wt) messenger RNA levels were increased in the Purkinje cells and decreased in the deep cerebellar nuclei of dystonic rats compared to control littermates, suggesting opposite changes in GABAergic transmission in Purkinje cells and in their target neurons in the deep cerebellar nuclei. In contrast, levels of glutamate decarboxylase (67000 mol. wt) messenger RNA in the pallidum, and of enkephalin messenger RNA in the striatum, were unaffected in dystonic rats. The data indicate that both the Purkinje cells and GABAergic neurons of the deep cerebellar nuclei are the site of significant functional abnormality in the dystonic rat.

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.

The parafascicular thalamic nucleus modulates messenger RNA encoding glutamate decarboxylase 67 in rat striatum

We investigated whether the parafascicular thalamostriatal pathway, one of the major excitatory inputs to the striatum, regulates the expression in rat striatum of messenger RNA encoding two isoforms of glutamate decarboxylase (mol. wt 67,000: glutamate decarboxylase 67 and mol. wt 65,000: glutamate decarboxylase 65). Acute (one day) and chronic (14 days) electrolytic lesions of the parafascicular nucleus resulted in 58% and 23% decreases in glutamate decarboxylase 67 messenger RNA expression, respectively, as determined by northern blot analysis. Glutamate decarboxylase 65 messenger RNA was not modified by either lesion. Sections of sham- and acute-lesioned striata were processed for in situ hybridization histochemistry at the single cell level with an RNA probe for glutamate decarboxylase 67. Labelling of glutamate decarboxylase 67 messenger RNA was decreased in both types of cells known to be present in the striatum, i.e. the lightly and the very densely-labelled neurons. The frequency distribution of glutamate decarboxylase 67 labelling per neuron in the lesioned striata, in fact, was shifted to the left and its median was lower than in the sham-lesioned striata. In view of the excitatory nature of the thalamostriatal pathway, we examined the subtype of glutamate receptors modulating the glutamate decarboxylase 67 gene expression. The N-methyl-D-aspartate-type receptor antagonist, dizocilpine, at 0.1-0.5 mg/kg i.p., produced a marked and persistent reduction in striatal glutamate decarboxylase 67 messenger RNA. The non-N-methyl-D-aspartate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (12 nmol/side, i.c.v.) had no such effect. The results provide evidence that excitatory thalamostriatal afferents selectively modulate the gene expression of glutamate decarboxylase 67, probably through the N-methyl-D-aspartate subtype of glutamate receptors.

Glutamate decarboxylase (GAD67 and GAD65) gene expression is increased in a subpopulation of neurons in the putamen of Parkinsonian monkeys

The cellular distribution of the mRNAs encoding for the two isoforms of glutamate decarboxylase, GAD67 and GAD65, was analyzed by in situ hybridization histochemistry in the caudate nucleus and putamen of control and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian squirrel monkeys. On brain sections processed with a radioactive and a digoxigenin-labeled cRNA probe, the GAD67 and GAD65 mRNAs were colocalized in virtually all labeled neurons of the caudate nucleus and putamen, in both control and MPTP-treated monkeys. Furthermore, neurons labeled with the GAD cRNAs constituted at least 90% of all striatal neurons, as estimated on adjacent Nissl-stained sections. In the two groups of monkeys, double-labeling experiments using a combination of radioactive GAD67 or GAD65 and digoxigenin-labeled preproenkephalin (PPE) cRNA probes showed that roughly half of all neurons labeled with the GAD cRNAs were also labeled with the PPE cRNA probe. When compared to controls, GAD67 and GAD65 mRNA levels were higher in the putamen, and to a lesser extent in the caudate nucleus, of MPTP-treated monkeys. Further analysis of labeling at the cellular level in a dorsolateral sector of the putamen revealed that GAD67 and GAD65 mRNA levels in MPTP-treated monkeys were increased in PPE-labeled (presumed striato-pallidal) neurons but not in PPE-unlabeled (presumed striato-nigral) neurons. Our results demonstrate that most neurons in the caudate nucleus and putamen of squirrel monkeys contain the mRNAs encoding for the two GAD isoforms. In addition, the selective increase in GAD mRNA levels in PPE-labeled neurons provides further evidence that striato-pallidal GABAergic neurons are hyperactive in MPTP-treated parkinsonian monkeys.

Glutamate decarboxylase (GAD65) gene expression is increased by dopamine receptor agonists in a subpopulation of rat striatal neurons

The mRNA levels encoding for the two isoforms of glutamate decarboxylase (GAD65 and GAD67) were measured in the adult rat striatum following systemic administration of dopamine receptor agonists. Double-labeling in situ hybridization histochemistry was used to measure GAD65 or GAD67 mRNA levels in neurons labeled or not with a preproenkephalin (PPE) cRNA probe. Chronic treatment with the D1/D2 dopamine receptor agonist apomorphine or with the D1 dopamine receptor agonist SKF-38393 induced an increase in GAD65 but not GAD67 mRNA levels in different sectors of the striatum. These effects were abolished by pre-administration of the D1 dopamine receptor antagonist SCH-23390. On double-labeled sections, GAD65 mRNA labeling was distributed in neurons labeled and unlabeled with the PPE cRNA probe. About half of all neuronal profiles labeled with the GAD65 cRNA probe were also labeled with the PPE cRNA probe. Quantification of labeling at cellular level demonstrated a significant increase of GAD65 mRNA levels in PPE-unlabeled neurons. On the other hand, no significant changes of GAD65 mRNA levels were detected in PPE-labeled neurons. Our results demonstrate a differential effect of dopamine receptor agonists on striatal GAD65 and GAD67 gene expression. In particular, we show that GAD65 mRNA levels are selectively increased in presumed striato-nigral neurons following treatments with dopamine receptor agonists. These data provide evidence that the GAD65 isoform is preferentially involved in the regulation of GABAergic neurotransmission in striato-nigral neurons.

Glutamic acid decarboxylase gene expression in the dopamine-denervated striatum: effects of intrastriatal fetal nigral transplants or chronic apomorphine treatment

Glutamic acid decarboxylase mRNA expression was studied in the striatum of rats subjected to a unilateral 6-hydroxydopamine lesion of the nigrostriatal dopamine projection, followed by either intrastriatal transplants of fetal nigral neurons or chronic apomorphine treatment. For in situ hybridization histochemistry, a 35S-labelled cRNA probe selective for the mRNA encoding the 67-kDa isoform of the enzyme (GAD67) was used. The results show that fetal dopaminergic grafts in the dopamine-denervated striatum restore an inhibitory control on GAD67 gene expression in the host neurons and also counteract the up-regulation of GAD67 mRNA levels induced by chronic apomorphine treatment.

Increased expression of trkB mRNA in rat caudate--putamen following 6-OHDA lesions of the nigrostriatal pathway

The tyrosine kinase receptors trkB and trkC are essential components of the high-affinity receptors for members of the neurotrophin family, including brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3). Both neurotrophin receptor mRNAs are broadly distributed throughout the caudate-putamen. In animal models of Parkinson's disease, loss of the ventral mesencephalic dopamine projection to the striatum has been shown to alter the expression of several striatal peptides, neurotransmitter-synthesizing enzymes and receptors. To determine if expression of trkB and/or trkC striatal mRNAs is also regulated by the integrity of the dopaminergic afferents, adult rats were given unilateral injections of 6-hydroxydopamine (6-OHDA), a selective catecholamine neurotoxin, or vehicle into the right ascending medial forebrain bundle. Following a 2 week survival period, in situ hybridization with 35S-labelled cRNA probes for the kinase-specific, full-length form of trkB mRNA and all forms of trkC mRNA was performed in striatal sections. A significant increase in the hybridization density for trkB mRNA was observed in the caudate-putamen ipsilateral to the 6-OHDA injection, compared with the uninjected control side (P < 0.001). In contrast, no alteration in the hybridization density for trkC mRNA was observed in the striatum of 6-OHDA-treated rats. No alterations in trkB or trkC mRNA levels were observed in the striata of vehicle-treated animals. These data suggest that midbrain dopaminergic afferents regulate the expression of trkB mRNA in the caudate-putamen. Alternatively, since dopaminergic neurons of the ventral mesencephalon express BDNF mRNA, the up-regulation of striatal trkB mRNA may reflect a compensatory response by striatal neurons due to a loss of anterogradely and/or retrogradely derived trophic support from the ventral midbrain.

Repeated injections of dizocilpine maleate (MK-801) do not suppress the effects of nigrostriatal dopamine deafferentation on glutamate decarboxylase (GAD67) mRNA expression in the adult rat striatum

The present study examined the effects of glutamate transmission blockade through N-methyl-D-aspartate (NMDA) receptor subtype by repeated administration of dizocilpine maleate (0.2 mg/kg. i.p., twice a day for eight days) alone or in combination with unilateral 6-hydroxydopamine-induced lesion of the nigrostriatal dopaminergic pathway on GABAergic neurons in the adult rat striatum. For this purpose, the expression of the messenger RNA encoding for the 67 kDa isoform of the GABA synthesizing enzyme, glutamate decarboxylase (GAD67 mRNA), was studied in the various conditions by quantitative in situ hybridization. The dizocilpine maleate treatment alone did not induce significant change of GAD67 mRNA levels in the striatum, indicating that NMDA receptors may not have a major role in the transcriptional regulation of GAD67 in the adult rat striatum. As reported previously, the unilateral dopaminergic lesion resulted in marked increases in GAD67 mRNA levels in the ipsilateral striatum. This up-regulation was not significantly affected by the treatment with dizocilpine maleate started 12 days after the unilateral intranigral 6-hydroxydopamine injection. Therefore, NMDA receptors are unlikely to contribute to the dopamine lesion-induced GAD67 mRNA up-regulation in striatal projection neurons. This result is of major interest in comparison with our previous finding that NMDA receptor activation is necessary to maintain the up-regulation of enkephalin expression in the striatum after dopamine lesion.