Time dependence and role of N-methyl-D-aspartate glutamate receptors in the priming of D2-mediated rotational behavior and striatal Fos expression in 6-hydroxydopamine lesioned rats.

Administration of dopamine agonists to 6-hydroxydopamine (6-OHDA) lesioned rats enhances the rotational response to subsequent administration of dopamine agonist, an effect called 'priming'. Previously, we have shown that 6-OHDA rats primed with three injections of the D1/D2 dopamine agonist apomorphine (0.5 mg/kg) permitted a challenge with an otherwise inactive dose of the D2 agonist quinpirole (0.25 mg/kg) to elicit robust rotational behavior and to induce Fos expression in striatoentopeduncular neurons. In this study, the time-course and role of N-methyl-d-aspartate (NMDA) glutamate receptors on apomorphine-priming of these D2 responses were investigated. The enhanced rotational behavior and striatal Fos expression observed following challenge with quinpirole (0.25 mg/kg) peaked 1 day following the third apomorphine priming injection and persisted, in reduced form, for at least 4 months. Pretreatment with the NMDA antagonists MK-801 or 3-[(+)-2-carboxypiperazin-4-yl]-propyl-1-phosphonate (CPP) dose-dependently attenuated apomorphine-priming of quinpirole-mediated rotational behavior and striatal Fos induction compared to 6-OHDA rats primed with apomorphine alone. Taken together, these data suggest that priming of these D2-mediated responses in 6-OHDA rats develops rapidly, persists for several months, and is dependent on concomitant NMDA receptor stimulation. Since this priming effect resembles response fluctuations observed in patients with Parkinson's disease receiving long-term l-dihydroxyphenylalanine therapy, the results of the present study suggest that interventions that prevent the development of this enhanced response, such as NMDA antagonists, could prove useful in reducing the incidence these response fluctuations.     

NMDA glutamate receptor stimulation is required for the expression of D2 dopamine mediated responses in apomorphine primed 6-hydroxydopamine lesioned rats

Three priming injections with the D1/D2 dopamine agonist apomorphine permits a challenge with the D2 agonist quinpirole to elicit robust contralateral rotation and ipsilateral striatal Fos expression in 6-hydroxydopamine lesioned rats. Pretreatment with NMDA glutamate antagonists MK-801 or CPP dose-dependently attenuates these quinpirole-mediated responses. These findings suggest that concomitant NMDA receptor stimulation is required for the expression of D2-mediated responses in apomorphine primed dopamine-depleted rats.     

Role of NMDA glutamate receptors in regulating D2 dopamine-dependent Fos induction in the rat striatopallidal pathway

Acute administration of reserpine induces Fos expression in striatopallidal neurons, an effect blocked by pretreatment with the D2 dopamine agonist quinpirole. Pretreatment with the NMDA antagonists (+)MK-801 or CPP attenuated reserpine-mediated striatal Fos induction whereas pretreatment with ketamine or the inactive isomer (-)MK-801 did not. These results support a role of NMDA glutamate receptors in regulating the activity of the striatopallidal pathway.     

Quinpirole attenuates the striatal Fos expression induced by escape behavior

Several studies have shown that the D₂-like dopamine receptor agonist quinpirole is able to markedly potentiate the striatal Fos expression induced by D₁ agonists. The present study examined the effects of quinpirole on the striatal Fos-like immunoreactivity (FLI) induced by escape behavior. Male rats were pretreated with either saline or quinpirole (0.156, 0.625, 1.25 or 2.5 mg/kg) and 30 min later, placed in a shuttle box and required to crossover every 30 s in order to escape mild footshock. Animals were sacrificed 30 min following the completion of a 1-h block of escape trials and sections through the striatum were processed for FLI. Pretreatment with quinpirole produced a marked, dose-dependent, attenuation of escape-induced FLI in the striatum. These findings demonstrate that quinpirole affects the striatal Fos expression induced by shuttling in a very different fashion than it does that induced by D₁ agonists, and further support the view that dopaminergic mechanisms play an important role in behaviorally induced striatal Fos expression.     

Compartmentally specific effects of quinpirole on the striatal Fos expression induced by stimulation of D1-dopamine receptors in intact rats

Injections of the full D₁-agonist A-77636 (1.45 mg/kg) were found to induce clear Fos-like immunoreactivity (FLI) in the striatum of neurologically intact rats. Pretreatment with the D₂-like agonist quinpirole (3 mg/kg) potentiated staining in the lateral striatum, but actually decreased the number of immunoreactive cells observed in the medial portion of the rostral striatum. Comparison with adjacent sections processed for the calcium binding protein calbindin, indicated that quinpirole pretreatment specifically suppressed staining in the matrix compartment of the striatum while tending to potentiate it in the striosomes, resulting in an extremely patchy pattern of labeling. These results suggest that exogenous stimulation of D₂-receptors, although not essential for the induction of FLI, may play an important role in the compartmental patterning of neuronal activity within the striatum.     

Modulation of dopamine D1-mediated turning behavior and striatal c-fos expression by the substantia nigra

In order to study the possible contribution of the substantia nigra (SN) in the positive interaction between dopamine D₁ receptor agonists and glutamate antagonists in unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, the effect of the D₁ agonist, SKF 38393, was studied in combination with intranigral infusions of glutamate antagonists of the NMDA (MK 801, CPP) or AMPA (NBQX) type of receptor. Local infusion into the SN of the 6-OHDA lesioned side of MK 801, CPP or NBQX at doses inducing no or minimal behavioral effects significantly increased the turning behavior and the expression of c-fos induced, in the lesioned caudate-putamen (CPu), by a parenteral administration of SKF 38393. The same result was obtained after intra-SN infusion of the GABA agonist, muscimol. High doses of MK 801, CPP or muscimol infused into the SN produced intense contralateral turning per se and induced a sparse c-fos expression in the lesioned CPu which was antagonized by parenteral administration of MK 801. The results indicate that a depression of SN pars reticulata efferent neurons potentiates D₁-mediated responses and suggest that this area may play a role in the positive interaction between glutamate antagonists and D₁ receptor agonists. © 1995 Wiley-Liss, Inc.     

Involvement of N-methyl- -aspartate receptor in the delayed transneuronal regression of substantia nigra neurons in rats

The substantia nigra pars reticulata (SNr) receives both inhibitory GABAergic and excitatory glutamatergic afferents from diverse origins. Ischemic injury to the striatum and/or the globus pallidus causes delayed transneuronal death of the SNr neurons, in the course of which neuronal disinhibition induced by loss of GABAergic inputs is supposed to trigger a lethal hypermetabolic process. In the in vivo experiment presented herein, we clarified the role of glutamatergic action via the N-methyl- -aspartate receptor in this cell death process. Continuous intraventricular infusion (0.5 μl/h) of the N-methyl- -aspartate receptor antagonist MK-801 (1000 μg/ml), or of saline (control group) was initiated 24 h after 2 h of transient middle cerebral artery (MCA) occlusion in rats, by which massive ischemic injury was produced in the striatopallidal regions. The measured rectal temperature was not significantly altered in the MK-801-infused and in the control rats throughout the time period examined. The rats were killed at 15 days after MCA occlusion. The volume of the focal ischemic infarction of the MK-801-infused group did not significantly differ from that of controls. Also, MK-801-infusion did not significantly ameliorate the nigral atrophy subsequent to MCA occlusion. In association with a marked depletion of GABAergic afferent fibers, neuronal cell number in the ipsilateral SNr was significantly decreased in the control group. In contrast, the neuronal cell loss in the nucleus was completely prevented in the MK-801-infusion group. The data suggested that withdrawal of GABAergic inputs may cause a severe imbalance between excitation and inhibition of the SNr neurons and may eventually result in neurotoxicity mediated by the N-methyl- -aspartate receptor. Suppression of glutamatergic excitatory effects by suitable drugs may be a reasonable therapy for the transneuronal death of the SNr neurons.     

Stress-induced increase in extracellular dopamine in striatum: role of glutamatergic action via N-methyl- -aspartate receptors in substantia nigra

There is considerable support for an influence of excitatory amino acids released from corticofugal neurons on dopaminergic activity in the basal ganglia. However, the relative importance of cortico-striatal and cortico-mesencephalic projections remains unclear, particularly with respect to the nigro-neostriatal pathway. We have therefore examined the influence of endogenous excitatory amino acids in substantia nigra on stress-induced dopaminergic activity in neostriatum. Microdialysis probes were implanted unilaterally into substantia nigra and ipsilateral neostriatum, and dopamine release in neostriatum was monitored by measuring changes in extracellular dopamine. In separate animals, neostriatal dopamine synthesis was assessed by measuring extracellular DOPA in the presence of 3-hydroxylbenzylhydrazine (NSD-1015; 100 μM), an inhibitor of aromatic amino acid decarboxylase. Thirty minutes of intermittent foot shock increased both dopamine release (+41%) and synthesis (+37%) in neostriatum. Infusion of 2-amino-5-phosphonovalerate (APV; 100 μM), an inhibitor of N-methyl- -aspartate (NMDA) receptors, into substantia nigra greatly attenuated the stress-induced increase in neostriatal dopamine release, while having no effect on the apparent increase in stress-induced dopamine synthesis. These data suggest that excitatory amino acids such as glutamate act on NMDA receptors in substantia nigra to increase striatal dopamine release produced by exposure to stress, but that the increase in dopamine synthesis is mediated through a separate mechanism.     

Stimulatory effects of centrally injected kainate and N-methyl- -aspartate on gastric acid secretion in anesthetized rats

The effects of N-methyl- -aspartate (NMDA), kainate and (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), ionotropic glutamate agonists, on gastric acid secretion were investigated in the continuously perfused stomach of anesthetized rats. The lateral ventricular (LV) injection of kainate (0.01–1 μg) or NMDA (0.3–3 μg) dose-dependently stimulated gastric acid secretion. AMPA (3–10 μg) also stimulated gastric acid secretion but the effect was very weak. Repeated injections of kainate (0.1 μg) or NMDA (1 μg), at least twice, stimulated gastric acid secretion to a similar degree. The effect of kainate (0.1 μg) was blocked by the kainate receptor antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione disodium (3 μg, LV) and -γ-glutamylaminomethanesulfonic acid (30 μg, LV), but not by NMDA receptor antagonists. The effect of NMDA (10 μg) was blocked by (±)-3-(2-carboxypiperazin-4-yl)-1-propylphosphonic acid (10 μg, LV), a competitive NMDA receptor antagonist, and (+)-5-methyl-10,11-dihydro-5H-dibenzocyclo-hepten-5,10-imine hydrogen maleate (10 μg, LV), a non-competitive NMDA receptor antagonist, but not by kainate receptor antagonists. Moreover, the gastric acid secretion stimulated by kainate and NMDA were completely blocked by systemic atropine injection (1 mg/kg, i.v.) and vagotomy. These findings suggest that kainate and NMDA receptor mechanisms are independently involved in the central nervous system to control gastric acid secretion through vagus cholinergic activation.     

Conditioned taste aversion and c-Fos expression in cholestatic rats

Rats in which a ligation of the bile duct (BDL) was paired with a saccharin taste developed a persistent conditioned taste aversion in both preference and taste reactivity tests. All BDL animals regardless of pairing had increased c-Fos-like immunoreactivity (FLI) in the area postrema and the nucleus of the solitary tract. This FLI may reflect the illness associated with BDL, but there was no evidence of conditioned FLI.     

Interactive effects of stimulation of D1 and D2 dopamine receptors on Fos expression in the lateral habenula

We have previously shown that systemic administration of non-selective dopamine agonists results in a pronounced expression of the proto-oncoprotein Fos within the lateral habenula. In the current study we examined the effects of selective D1 and D2 dopamine receptor agonists on habenular Fos expression. Rats were injected with various doses of the selective D2 agonist quinpirole (0, 0.62 or 2.5 mg/kg) either alone or in combination with various doses of the selective full D1 agonist A-77636 (0, 0.75 or 3.0 mg/kg). The selective agonists, by themselves, induced only small increases in Fos-like immunoreactivity within the lateral habenula, but combinations of the two drugs resulted in a very robust response. These findings indicate that D1 and D2 receptor agonists interact to induce Fos expression within the habenula and that the nature of this interaction differs from that reported in the striatum and the globus pallidus.     

c-fos Protooncogene expression in rat hippocampus and entorhinal cortex following tetanic stimulation of the perforant path

The elevated expression of the c-fos protooncogene has been proposed to be a marker of cell activation leading to a long term cellular response. In this communication we compared the c-fos mRNA accumulation in the hippocampus (i.e. postsynaptic cells) and entorhinal cortex (i.e. presynaptic cells) following high (tetanic) and low frequency electrical stimulation of the perforant path. Using Northern blot analysis we have found that high frequency stimulation elevates c-fos expression in both hippocampus and entorhinal cortex, and the increase of c-fos mRNA levels in the entorhinal cortex is less pronounced, but longer lasting, than in the hippocampus. Slight increase of c-fos mRNA levels has been also observed in low frequency treated animals in the entorhinal cortex, but not in the hippocampus. These findings raise the question about differences in mechanisms involved in c-fos activation in both parts of the brain after stimulation which evokes long term potentiation (LTP) of synaptic efficacy.     

Synergistic interaction between an adenosine antagonist and a D1 dopamine agonist on rotational behavior and striatal c-Fos induction in 6-hydroxydopamine-lesioned rats

The interaction between adenosine and D₁ dopamine systems in regulating motor behavior and striatal c-Fos expression was examined in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions. These results were compared to the synergistic interaction between D₁ and D₂ dopamine systems in 6-OHDA rats. Coadministration of the adenosine antagonist 3,7-dimethyl-1-propargylxanthine (DMPX: 10 mg/kg) and the D₁ dopamine agonist SKF38393 (0.5 mg/kg) to 6-OHDA-lesioned rats produced significant contralateral rotation and c-Fos expression in the ipsilateral striatum compared to 6-OHDA rats treated with either drug alone. However, the regional pattern of striatal c-Fos activation following treatment of 6-OHDA rats with SKF38393 and DMPX was different from the dorsolateral pattern of striatal c-Fos induction observed after coadministration of D₁ and D₂ dopamine agonists (SKF38393: 0.5 mg/kg+quinpirole: 0.05 mg/kg). These data are consistent with a functional interaction between D₁ dopamine and adenosine systems in the striatum, but suggest that activation of different subsets of striatal neurons underlie the behavioral synergy observed following combined adenosine antagonist-D₁ dopamine agonist and combined D₁ dopamine agonist–D₂ dopamine agonist treatment.     

Characterization of N-methyl-d-aspartate receptors in the hyperammonemic Sparse fur mouse

The N-methyl- -aspartate (NMDA) receptor, a glutamate receptor subtype, is a ligand-gated ion channel. Overstimulation of NMDA receptors may increase intracellular Ca²⁺ concentrations to lethal levels in neurodegenerative disorders affecting the basal ganglia. Such excitotoxicity may also contribute to the loss of medium spiny neurons in the striata of the hyperammonemic sparse fur (spf/Y) mouse, a model of the X-linked disorder of the urea cycle, ornithine carbamoyltransferase deficiency (OCTD). Levels of quinolinic acid (QA), a potent NMDA agonist, are elevated in the brains of spf/Y mice. Further, direct injection of QA into the striatum produces selective degeneration of medium spiny neurons. Microglia, an endogenous source of QA in the brain, are abundant in spf/Y mice during the period of neuronal degeneration. The location and density of NMDA receptors was visualized by gold labelled immunocytochemistry with a polyclonal antibody to the NMDAR1 receptor subtype and their distribution quantified. A 58% reduction was found in the median density value in the layer V pyramidal neurons in fronto-parietal cortex (p<0.001), but no significant change was observed in the striatum. NMDA receptor binding was examined using [ ]dizocilpine ([ ]MK-801). Receptor density (B_max) in the striata of clinically stable spf/Y mice and +/Y littermates was unchanged, but was decreased 15% (p<0.01) in the fronto-parietal cortices in clinically stable spf/Y mice compared with +/Y littermate controls.     

Decreased striatal D2 dopamine receptors in obese Zucker rats: changes during aging

The specific binding of [³H]YM-09151-2 was used to investigate the possible differences in age-associated changes in striatal D₂ dopamine (DA) receptor properties in genetically obese (fa/fa) Zucker rats and their lean3(Fa/?) littermates. The maximal binding sites (B_max) of D₂ DA receptors was found to decline with age in both obese and lean rats: the rate of decline in receptor B_max was slightly higher in lean than obese rats. However. the B_max of D₂ DA receptor in 6-, 12- and 18-month-old obese rats was significantly lower compared to the age-matched lean rats. These data indicate that obesity decreases the number of striatal D₂ DA receptors without affecting the rate at which receptor number decreases with age.     

Early adoption modifies the effects of prenatal stress on dopamine and glutamate receptors in adult rat brain

Stressful stimuli during pregnancy induce complex effects that influence the development of offspring. These effects can be prevented by environmental manipulations during the early postnatal period. Repeated restraint during the last week of pregnancy was used as a model of prenatal stress, and adoption at birth was used to change the postnatal environment. No differences were found in various physical landmarks, except for testis descent, for which all prenatally stressed pups showed a 1-day delay in comparison with control rats, regardless of the postnatal adoption procedure. Levels of dopamine (DA) D(2) and glutamate (Glu) N-methyl-D-aspartate (NMDA) receptors were differentially regulated in different forebrain regions of cross-fostered adult offspring. Increased concentrations of cortical D(2) receptors detected in stressed pups, raised by a gestationally stressed biological mother, were not detected when the pups were raised by a control mother. Control pups raised by a foster mother whether gestationally stressed or not had higher levels of NMDA receptors in cortical areas. These findings suggest that the normal expression of DA and Glu receptors is influenced by in utero experience and by lactation. The complex pattern of receptor changes reflects the high vulnerability of DA and Glu systems to variations both in prenatal and in postnatal environment, particularly for cortical D(2) receptors and NMDA receptors in cerebral cortex and nucleus accumbens. In contrast, testis descent appears to be more susceptible to prenatal than to postnatal environmental events.     

Adaptations of NMDA and dopamine D2, but not of muscarinic receptors following 14 days administration of uncompetitive NMDA receptor antagonists

Summary. Behavioral changes have previously been reported following administrations of uncompetitive NMDA receptor antagonists memantine, amantadine and MK-801 for 14 days, at the doses that produce plasma levels comparable to those seen in patients (20, 100 and 0.31 mg/kg/day respectively). Using the same doses, the effect on receptor binding (autoradiography) was studied in rats. [³H]MK-801 binding was increased in the dentate gyrus and CA3 region of the hippocampus (35.2 and 24.3% respectively) following 3 days S.C. infusion of memantine by ALZET minipumps. One daily injection of memantine for 14 days, increased [³H]MK-801 binding in the frontal cortex by 40.3%. The same treatment with amantadine did increase [³H]raclopride binding to dopamine D₂ receptors by 13.5%. None of these treatments changed the expression of muscarinic receptors. It is concluded that subchronic blockade of the NMDA receptor by uncompetitive antagonists at moderate (therapeutically-relevant) doses induced only minor changes in NMDA and dopamine D₂ receptor expression. Received September 18, 1998; accepted November 16, 1998     

Pharmacological MRI responses of raclopride in rats: The relationship with D2 receptor occupancy and cataleptic behavior

Pharmacological magnetic resonance imaging (phMRI) allows the visualization of brain pharmacological effects of drugs using functional MRI (fMRI). phMRI can help us facilitate central nervous system (CNS) drug development. However, there have been few studies demonstrating the dose relationship of the fMRI response induced by CNS drugs to underlying target engagement or behavioral efficacy. To clarify these relationships, we examined receptor occupancy measurements using positron emission tomography (PET) (n = 3~5), fMRI (n = 5~8) and a cataleptic behavior (n = 6) with raclopride, a dopamine D2 receptor antagonist (8, 20, and 200 μg/kg) on Wistar rats. Dopamine D2 receptor occupancy was increased dose dependently by raclopride (41.8 ± 2.7%, 8 μg/kg; 64.9 ± 2.8%, 20 μg/kg; 83.1 ± 3.0%, 200 μg/kg). phMRI study revealed significant positive responses to raclopride at 200 μg/kg specifically in the striatum and nucleus accumbens, related to dopaminergic system. Slight fMRI responses were observed at 20 μg/kg in some areas corresponding to the striatum and nucleus accumbens. There were no noticeable fMRI responses at 8 μg/kg raclopride administration. Raclopride at 200 μg/kg significantly increased the cataleptic score, although, at 8 and 20 μg/kg, raclopride had no significant effects. These findings showed that raclopride-induced fMRI responses were observed at doses inducing cataleptic behavior and high D2 receptor occupancy, suggesting that phMRI can be useful for dose selection in clinical trial as an evaluation method of brain activity, which reflects behavioral responses induced by target engagements.