N-methyl-D-aspartate receptor blockade differentially modifies regional cerebral metabolic responses to D1 and D2 dopamine agonists in rats with a unilateral 6-hydroxydopamine lesion

Prolactin and fMRI response to SKF38393 in the baboon

Background. This study's goal was to provide dose-response data for a dopamine agonist in the baboon using standard methods (replicate measurements at each dose, across a range of doses), as a standard against which to subsequently validate a novel pharmacological MRI (phMRI) method. Dependent variables were functional MRI (fMRI) data from brain regions selected a priori, and systemic prolactin release. Necessary first steps included estimating the magnitude and time course of prolactin response to anesthesia alone and to various doses of agonist. These first steps ("time course studies") were performed with three agonists, and the results were used to select promising agonists and to guide design details for the single-dose studies needed to generate dose-response curves. Methods. We studied 6 male baboons (Papio anubis) under low-dose isoflurane anesthesia after i.m. ketamine. Time course studies charted the changes in plasma prolactin levels over time after anesthesia alone or after an intravenous (i.v.) dose of the dopamine D 1-like agonists SKF82958 and SKF38393 or the D 2-like agonist pramipexole. In the single-dose dopamine agonist studies, one dose of SKF38393 (ranging from 0.0928-9.28 mg/kg, N = 5 animals) or pramipexole (0.00928-0.2 mg/kg, N = 1) was given i.v. during a 40-min blood oxygen level dependent (BOLD) fMRI session, to determine BOLD and plasma prolactin responses to different drug concentrations. BOLD response was quantified as the area under the time-signal curve for the first 15 min after the start of the drug infusion, compared to the linearly predicted signal from the baseline data before drug. The ED50 (estimated dose that produces 50% of the maximal possible response to drug) for SKF38393 was calculated for the serum prolactin response and for phMRI responses in hypothalamus, pituitary, striatum and midbrain. Results. Prolactin rose 2.4- to 12-fold with anesthesia alone, peaking around 50-90 min after ketamine administration and gradually tapering off but still remaining higher than baseline on isoflurane 3-5 h after ketamine. Baseline prolactin level increased with age. SKF82958 0.1 mg/kg i.v. produced no noticeable change in plasma prolactin concentration. SKF38393 produced a substantial increase in prolactin release that peaked at around 20-30 min and declined to pre-drug levels in about an hour. Pramipexole quickly reduced prolactin levels below baseline, reaching a nadir 2-3 h after infusion. SKF38393 produced clear, dose-responsive BOLD signal changes, and across the four regions, ED50 was estimated at 2.6-8.1 mg/kg. Conclusions. In the baboon, the dopamine D 1 receptor agonist SKF38393 produces clear plasma prolactin and phMRI dose-response curves. Variability in age and a modest sample size limit the precision of the conclusions.

Immunohistochemical detection of L-DOPA-derived dopamine within serotonergic fibers in the striatum and the substantia nigra pars reticulata in Parkinsonian model rats

On the basis of our previous studies in the normal rat [Arai, R., Karasawa, N., Geffard, M., Nagatsu, I., 1995. L-DOPA is converted to dopamine in serotonergic fibers of the striatum of the rat: a double-labeling immunofluorescence study. Neurosci. Lett. 195, 195-198; Arai, R., Karasawa, N., Nagatsu, I., 1996a. Aromatic L-amino acid decarboxylase is present in serotonergic fibers of the striatum of the rat. A double-labeling immunofluorescence study. Brain Res. 706, 177-179; Arai, R., Karasawa, N., Nagatsu, I., 1996b. Dopamine produced from L-DOPA is degraded by endogenous monoamine oxidase in neurons of the dorsal raphe nucleus of the rat: an immunohistochemical study. Brain Res. 722, 181-184] we have assumed that exogenously administered L-dihydroxyphenylalanine (L-DOPA) is converted into dopamine (DA) in serotonergic (5-HT) fibers within the striatum (ST) and the substantia nigra pars reticulata (SNR). In the present study, an attempt was made to confirm the assumptions in Parkinsonian rats, which were produced by unilateral injections of 6-hydroxydopamine (6-OHDA) into the substantia nigra pars compacta (SNC). The rats exhibiting more than 150 total controversial circles were regarded as satisfactory models of Parkinson disease (PD). Using a dual immunofluorescence histochemistry, we examined DA-immunoreactivity in the 5-HT fibers within the ST and the SNR of the PD model rats after L-DOPA was injected intraperitoneally. In experimental cases with the L-DOPA administration, DA-immunoreactivity was detected in 5-HT fibers in both the ST and the SNR on the 6-OHDA injection side; no DA-immunoreactivity was found in 5-HT fibers in the ST or the SNR in control cases without the L-DOPA administration. The results support the assumption that exogenously administered L-DOPA may be converted into DA within the 5-HT fibers in the ST and SNR of the PD model rats.

Dopamine D1 and D2 receptors regulate extracellular lactate and glucose concentrations in the nucleus accumbens

Glucose and lactate have been shown to play a significant role in energy metabolism in the brain. In the present study, the relationship between extracellular glucose and lactate concentrations in the nucleus accumbens (NAC) was determined with in vivo microdialysis technique. We further evaluated the effect of dopamine (DA) receptor agonists on energy metabolism. Extracellular glucose levels were increased following inactivation of neurons by tetrodotoxin (TTX) perfusion, whereas neural activation by veratridine or K(+) perfusion decreased extracellular glucose concentrations. By contrast, lactate levels were increased by veratridine or K(+) perfusion, but were unaltered by TTX. Apomorphine (0.05 mg/kg), a mixed D1/D2 receptor agonist, did not alter the extracellular glucose and lactate concentrations, while a higher dose (0.5 mg/kg) increased them. Bromocriptine, a selective D2 receptor agonist, increased extracellular glucose, but not lactate concentrations. These results indicate that extracellular lactate levels may be a more suitable indicator of acute neural activation than glucose levels, and that simultaneous stimulation of D1 and D2 receptors enhances energy demands of DA neurons in the NAC.

Role of NMDA and AMPA glutamate receptors in the induction and the expression of dopamine-mediated sensitization in 6-hydroxydopamine-lesioned rats

Rats with unilateral 6-hydroxydopamine (6-OHDA) lesions exhibit behavioral sensitization following repeated treatment with dopamine agonists, a phenomenon called "priming." Priming has two distinct phases: induction and expression. Priming induction using three injections with D1/D2 agonist apomorphine (0.5 mg/kg) or D1 agonist SKF38393 (10 mg/kg) allows priming expression, robust contralateral rotational behavior and striatal Fos expression, following a challenge with the D2 agonist quinpirole (0.25 mg/kg). We examined the roles of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) glutamate receptors on dopamine agonist priming. Administration of the NMDA antagonist (+)5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate (MK801) (0.5 mg/kg) blocked apomorphine-priming of quinpirole-mediated responses, while MK801 dose-dependently attenuated SKF38393-priming of quinpirole-mediated striatal Fos expression and had no effect on SKF38393-priming of quinpirole-mediated rotational behavior. In contrast, administration of the AMPA antagonist 2,3-dihydroxy-6-nitro-7sulfamoyl-benzo[f]quinoxaline (NBQX) (5 or 10 mg/kg) potentiated apomorphine- and SKF38393-priming of quinpirole-mediated striatal Fos expression, but had no effect on their priming of quinpirole-mediated rotational behavior. In SKF38393-primed 6-OHDA rats, administration of MK801 (0.5 mg/kg) blocked the expression of quinpirole-mediated responses, while administration of NBQX (10 mg/kg) or the noncompetitive AMPA antagonist 4-(8-methyl-9H-1,3-dioxolo[4,5-h][2,3]benzodiazepin-5-yl)-benzenamine dihydrochloride (GYKI52466) (5 or 15 mg/kg) had no effect. These results suggest that NMDA and AMPA glutamate receptors have differing roles in dopamine agonist priming-with NMDA receptors required for D1/D2 priming induction and D2-mediated priming expression, and AMPA receptors inhibiting priming induction of D2-mediated immediate early gene expression in the striatum, but not affecting priming induction of D2-mediated rotational behavior or the expression of D2-mediated responses.

Blunted metabolic response to SKF 82958 in the mesolimbic system following preweaning cocaine treatment

This study examined glucose metabolic rates following dopamine D(1) agonist challenge in adult male rats pretreated with cocaine during postnatal days 11-20. Water-pretreated control rats showed a reliable decrease in glucose metabolism of rostral mesolimbic structures when challenged with SKF 82958 while cocaine-pretreated males did not. These data support the notion that cocaine exposure during the preweaning period dampens D(1) receptor-mediated function and that the mesolimbic system exhibits a selective vulnerability to early cocaine exposure.

Quetiapine attenuates levodopa-induced motor complications in rodent and primate parkinsonian models

The contribution of serotoninergic mechanisms to motor dysfunction in Parkinson's disease (PD) has yet to be fully elucidated. Recent clinical observations increasingly suggest that drugs able to block serotonin 5HT2A/C receptors can benefit patients with certain extrapyramidal movement disorders. To further explore the roles of these and other neurotransmitter receptors in the pathogenesis of parkinsonian signs and levodopa-induced dyskinesias; we evaluated the effects of quetiapine, an atypical antipsychotic with 5HT2A/C and D2/3 antagonistic activity, on motor behavior in 6-hydroxydopamine-lesioned rats and MPTP-lesioned nonhuman primates. In hemiparkinsonian rats, quetiapine (5 mg/kg, po) reversed the shortened motor response to levodopa challenge produced by 3 weeks of twice-daily levodopa treatment (P < 0.01). Quetiapine (5 mg/kg po) also normalized the shortened response to the acute injection of either a dopamine D1 receptor agonist (SKF 38392) or a D2 agonist (quinpirole) in rats that had received chronic levodopa treatment. Quetiapine had no effect on parkinsonian dysfunction when given alone or with levodopa to parkinsonian rats and monkeys. Quetiapine (4 mg/kg, po) did, however, substantially reduce levodopa-induced dyskinesias when coadministered with levodopa (P < 0.05). These results suggest that quetiapine could confer therapeutic benefits to patients with levodopa-induced motor complications. Moreover, our findings may indicate that 5HT2A/C receptor-mediated mechanisms, alone or in combination with other mechanisms, contribute to the pathogenesis of the altered motor responses associated with the treatment of PD.

Dopa-induced blood flow responses in nonhuman primates

Initially, treatment with the dopamine precursor levodopa provides substantial symptomatic relief for patients with Parkinson's disease (PD). However, as the disease progresses, side effects such as involuntary movements or psychosis may accompany the response to medication. The mechanisms underlying these actions of levodopa remain unclear. To develop methodology for longitudinal studies of the effects of PD and levodopa treatment in living nonhuman primates, we first studied the effects of an acute dose of levodopa on regional brain activity in sedated baboons using positron emission tomography. We found that levodopa significantly decreased regional cerebral blood flow (rCBF) bilaterally in putamen and right cingulate and increased rCBF in right lateral temporal cortex and bilateral frontal cortex. We then performed similar studies on a nemestrina in awake and sedated states to determine whether these responses were affected by sedation. Interestingly, the directions of the rCBF responses in the putamen and temporal cortex were reversed depending on the presence or absence of sedation. Specifically, responses were decreased in sedated animals, but increased dose-dependently in the awake nemestrina. These findings have important implications for the interpretation of studies that use anesthesia. The responses in the awake nemestrina were most similar to those reported in humans and thus may be the most useful model system. Future imaging studies using selective dopaminergic agents in awake animals may permit the identification of relatively specific agonist-mediated pathways and may help separate the mechanisms that mediate levodopa's benefit from those that produce its unwanted side effects.

MK801 influences L-DOPA-induced dopamine release in intact and hemi-parkinson rats

In vivo microdialysis was used to investigate the influence of dizocilpine (MK801) on basal and levodopa (L-DOPA)-induced extracellular dopamine levels in striatum and substantia nigra of intact and 6-hydroxydopamine-lesioned rats. In lesioned rats, extracellular dopamine was decreased in striatum but not in substantia nigra. L-DOPA (25 mg/kg i.p. after benserazide 10 mg/kg i. p.) increased the dopamine levels in striatum and substantia nigra of intact and dopamine-depleted rats. This increase was significantly higher in dopamine-depleted compared to intact striatum. Pretreatment with MK801 (0.1 and 1.0 mg/kg i.p.) dose-dependently attenuated the L-DOPA-induced dopamine release in substantia nigra of intact rats. In dopamine-depleted striatum, MK801 enhanced L-DOPA-induced dopamine release. The present results indicate that the influence of MK801 on L-DOPA-induced dopamine release in striatum and substantia nigra depends on the integrity of the nigrostriatal pathway. In Parkinson's disease, NMDA receptor antagonists could be beneficial by enhancing the therapeutic efficacy of L-DOPA at the level of the striatum.

Dopamine D(1) agonist activates temporal lobe structures in primates

Changes in the function of dopamine D(1)-influenced neuronal pathways may be important to the pathophysiology of several human diseases. We recently developed methods for averaging functional imaging data across nonhuman primate subjects; in this study, we apply this method for the first time to map brain responses to experimental dopamine agonists in vivo. Here we report the use of positron emission tomography (PET) in seven normal baboons to measure the regional cerebral blood flow (rCBF) responses produced by an acute dose of the dopamine D(1) full agonist SKF82958. The most significant rCBF increases were in bilateral temporal lobe, including amygdala and superior temporal sulcus (6-17%, P < 0.001). Blood flow decreased in thalamus, pallidum, and pons (4-7%, P = 0.001). Furthermore the rCBF responses were dose-dependent and had a half-life of approximately 30 min, similar to that reported for the drug's antiparkinsonian effects. Absolute whole-brain blood flow did not change, suggesting that these local changes in rCBF reflect neuronal rather than direct vascular effects of the agonist. The prominent temporal lobe response to a D(1) agonist supports and extends our recent observations that levodopa produces prominent amygdala activation both in humans and in other primates. We speculate that levodopa may exert its known effects on mood in humans through increased amygdala activity, mediated in part by D(1) receptors.

Prior D1 dopamine receptor stimulation is required to prime D2-mediated striatal Fos expression in 6-hydroxydopamine-lesioned rats

Repeated dopamine agonist administration to rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway potentiates behavioral and neuronal activation in response to subsequent dopamine agonist treatment. This response sensitization has been termed "priming" or "reverse-tolerance". Our prior work has shown that three pretreatment injections of the mixed D1/D2 agonist apomorphine (0.5 mg/kg) into 6-hydroxydopamine-lesioned rats permits a previously inactive dose of the D2 agonist quinpirole (0.25 mg/kg) to induce robust contralateral rotation and striatal Fos expression in striatoentopeduncular "direct" pathway neurons. These striatal neurons typically express D1 but not D2 receptors. Because apomorphine acts as an agonist at both D1 and D2 receptors, the present study sought to determine whether D1, D2, or concomitant D1/D2 receptor stimulation was required to prime D2-mediated contralateral rotation and striatal Fos expression. Twenty-one days following unilateral stereotaxic injection of 6-hydroxydopamine into the medial forebrain bundle, rats received three pretreatment injections, at three- to six-day intervals, with either: the mixed D1/D2 agonist apomorphine, the D1 agonist SKF38393, the D2 agonist quinpirole, or a combination of SKF38393 + quinpirole. Ten days following the third pretreatment injection, 6-hydroxydopamine-lesioned rats were challenged with the D2 agonist quinpirole (0.25 mg/kg). Pretreatment with SKF38393 (10 mg/kg), quinpirole (1 mg/kg) or SKF38393 (1 mg/kg) + quinpirole (0.25 mg/kg) permitted an otherwise inactive dose of quinpirole (0.25 mg/kg) to induce robust contralateral rotation which was similar in magnitude to that observed following apomorphine priming. However, only pretreatment with SKF38393 (10 mg/kg) or SKF38393 (1 mg/kg) + quinpirole (0.25 mg/kg) permitted the same dose of quinpirole (0.25 mg/kg) to induce striatal Fos expression. These results demonstrate that while prior stimulation of D1, D2 or D1/D2 receptors can effectively prime D2-mediated contralateral rotation, prior stimulation of D1 receptors is required to prime D2-mediated striatal Fos expression. This study demonstrates that priming of 6-hydroxydopamine-lesioned rats with a D1 agonist permits a subsequent challenge with a D2 agonist to produce robust rotational behavior that is accompanied by induction of immediate-early gene expression in neurons that comprise the "direct" striatal output pathway. These responses are equivalent to the changes observed in apomorphine-primed 6-hydroxydopamine-lesioned rats challenged with D2 agonist. In contrast, D2 agonist priming was not associated with D2-mediated induction of striatal immediate-early gene expression even though priming of D2-mediated rotational behavior was not different from that observed following priming with apomorphine or D1 agonist. Therefore, while priming-induced alterations in D2-mediated immediate early gene expression in the "direct" striatal output pathway may contribute to the enhanced motor behavior observed, such changes in striatal gene expression do not appear to be required for this potentiated motor response in dopamine-depleted rats.

Effect of dopamine denervation and dopamine agonist administration on serine phosphorylation of striatal NMDA receptor subunits

Sensitization of striatal N-methyl-d-aspartate (NMDA) receptors has been implicated in the pathogenesis of the response alterations associated with dopaminomimetic treatment of parkinsonian animals and patients. To determine whether serine phosphorylation of NMDA receptor subunits by activation of Ca2+/calmodulin-dependent protein-kinase II (CaMKII) contributes to this process, we examined the effects of unilateral nigrostriatal ablation with 6-hydroxydopamine and subsequent treatment with levodopa, SKF 38393 (D1-preferring dopamine agonist), or quinpirole (D2-preferring agonist) on motor responses and phosphorylation states. Three weeks of twice-daily levodopa administration to rats shortened the duration of their rotational response to levodopa or SKF 38393 challenge, but prolonged the duration of quinpirole-induced rotation. At the same time, levodopa treatment elevated serine phosphorylation of striatal NR2A (p<0.02), but not that of NR2B subunits, without associated changes in subunit protein levels. Chronic treatment with SKF 38393 increased NR2A (p<0.0001) but decreased NR2B (p<0.004) serine phosphorylation. In contrast, chronic quinpirole treatment had no effect on NR2A but increased NR2B phosphorylation (p<0.0001). The acute intrastriatal injection of the CaMKII inhibitor KN93 (1.0 micrograms) not only normalized the levodopa-induced motor response alterations but also attenuated the D1 and D2 receptor-mediated serine phosphorylation of NR2A and NR2B subunits, respectively (p<0.02). These results suggest that a CaMKII-mediated rise in serine phosphorylation of NMDA receptor subunits induced by intermittent stimulation of D1 or D2 dopaminergic receptors contributes to the apparent enhancement in striatal NMDA receptor sensitivity and thus to the dopaminergic response plasticity in levodopa-treated parkinsonian rats.

Enhanced tyrosine phosphorylation of striatal NMDA receptor subunits: effect of dopaminergic denervation and L-DOPA administration

Sensitization of striatal N-methyl-D-aspartate receptors (NMDAR) has been linked to events leading to the motor response changes associated with the administration of dopaminomimetics to parkinsonian animals and patients. To determine whether tyrosine phosphorylation of NMDAR subunits contributes to the apparent long-term enhancement in synaptic efficacy of these receptors, we examined the effect of unilateral nigrostriatal dopamine system ablation with 6-hydroxydopamine followed by twice-daily treatment with l-DOPA on the phosphorylation state of rat striatal NR2A and NR2B subunits. Three weeks of intermittent l-DOPA administration produced a shortening in the duration of the rotational response to dopaminergic challenge and other changes mimicking those occurring in patients with Parkinson's disease. Concurrently, tyrosine phosphorylation of NR2A and especially of NR2B subunits increased ipsilateral to the lesion (20+/-5% and 46+/-7% of intact striatum, respectively; p<0.01) without attendant changes in subunit protein levels. Selective blockade of NR2B subunits with ACEA 10-1244, but not of NR2A subunits with MDL 100,453, reversed the l-DOPA-induced response alterations. The intrastriatal injection of a tyrosine kinase inhibitor, genistein, at a dose (2.0 microg) that normalized the response shortening, attenuated the NR2A and NR2B phosphorylation increase by about 12% and 24%, respectively (p<0.01). Taken together, these results suggest that augmented tyrosine phosphorylation of NR2B subunits, alone or in combination with the smaller rise in NR2A subunit phosphorylation, contributes to the apparent enhancement in striatal NMDAR sensitivity and thus to the plastic alterations in dopaminergic responses in l-DOPA-treated parkinsonian rats.

Altered thalamic response to levodopa in Parkinson's patients with dopa-induced dyskinesias

Parkinson's disease (PD) is a progressive neurologic condition characterized by tremor, slowness, stiffness, and unstable posture. Degeneration of dopamine-producing neurons in the substantia nigra causes PD. Treatment with levodopa, a precursor of dopamine, initially ameliorates the clinical manifestations of PD. However, chronic levodopa treatment can produce severe involuntary movements (so-called dopa-induced dyskinesias or DID), limiting treatment. Pallidotomy, placement of a surgical lesion in the internal segment of the globus pallidus, reduces DID. Because this result is inconsistent with current theories of both basal ganglia function and DID, it prompted us to investigate the brain's response to levodopa. We measured regional cerebral blood flow response to levodopa with positron-emission tomography in 6 PD patients with DID, 10 chronically treated PD patients without DID, 17 dopa-naïve PD patients, and 11 normals. The dose of levodopa was chosen to produce clinical benefit without inducing DID. This strategy allowed us to examine the brain response to levodopa across groups without the confounding effect of differences in motor behavior. We found that the DID group had a significantly greater response in ventrolateral thalamus than the other groups. This was associated with decreased activity in primary motor cortex. These findings are consistent with increased inhibitory output from the internal segment of the globus pallidus to thalamus after levodopa administration. They provide a physiological explanation for the clinical efficacy of pallidotomy and new insights into the physiology of the basal ganglia.

Antiparkinsonian actions of glutamate antagonists--alone and with L-DOPA: a review of evidence and suggestions for possible mechanisms

There has been much speculation of late as to whether antagonists of glutamate receptors can be used to combat the motor difficulties of Parkinson's disease, either as monotherapy, or as polytherapy to boost the effects of conventional L-DOPA treatment. The latter seems to be the more practical approach and the therapeutic implications of such treatment have been discussed in some detail. However, the mechanisms by which glutamate antagonists potentiate the antiparkinsonian actions of L-DOPA, remain cryptic. In this review we have explored the evidence and considered the practicality of using NMDA and non-NMDA receptor blockers to treat parkinsonism, as well as focusing on the ways in which the behavioural synergy between dopamine and glutamate systems could conceivably arise at the cellular level. Particular attention has been paid to the differential interaction between glutamate antagonists and postsynaptic dopamine D1 and D2 receptory mechanisms, since these are currently believed to reflect the activity of the two major basal ganglia output circuits: the so-called direct pathway to the substantia nigra and the indirect pathway to the globus pallidus. Finally, we have considered the new proposal, that inhibiting glutamate transmission in the basal ganglia accelerates the enzymic conversion of L-DOPA to dopamine at presynaptic sites.

PET measurement of dopamine D2 receptor-mediated changes in striatopallidal function

This study was designed to validate an in vivo measurement of the functional sensitivity of basal ganglia neuronal circuits containing dopamine D2 receptors. We hypothesized that a D2 agonist would decrease striatopallidal neuronal activity, and hence regional cerebral blood flow (rCBF) over the axon terminals in the globus pallidus. Quantitative pallidal blood flow was measured using positron emission tomography (PET) with bolus injections of H215O and arterial sampling in six baboons before and after intravenous administration of the selective D2 agonist U91356a. We also tested whether the response to U91356a was modified by previous acute administration of various antagonists. Another baboon had serial measurements of blood flow under identical conditions, but received no dopaminergic drugs. In all animals that received U91356a, pallidal flow decreased in a dose-related manner. Global CBF had a similar response, but the decline in pallidal flow was greater in magnitude and remained significant after accounting for the global effect. A D2 antagonist, but not antagonists of D1, serotonin-2, or peripheral D2 receptors, prevented this decrease. This work demonstrates and validates an in vivo measure of the sensitivity of D2-mediated basal ganglia pathways. It also supports the hypothesis that activation of the indirect striatopallidal pathway, previously demonstrated using nonselective D2-like agonists, can be mediated specifically by D2 receptors. We speculate that the U91356a-PET technique may prove useful in detecting functional abnormalities of D2-mediated dopaminergic function in diseases such as parkinsonism, dystonia, Tourette syndrome, or schizophrenia.

Dopamine depletion in the rostral nucleus accumbens alters the cerebral metabolic response to cocaine in the rat

The functional consequences of dopamine depletion in the rostral nucleus accumbens were examined using the quantitative 2-[14C]deoxyglucose method for determining rates of local cerebral glucose utilization. Cerebral metabolism was determined in 35 brain structures of Sprague-Dawley rats with unilateral 6-hydroxydopamine or sham lesions of the rostral accumbens. The effect of the lesion was assessed in cocaine-naive animals treated systemically with cocaine or saline. In saline-treated animals, the lesion increased cerebral metabolism in typical basal ganglia regions, such as the globus pallidus and entopeduncular nucleus, as well as portions of the extended amygdala that included the bed nucleus of the stria terminalis and the hypothalamic preoptic area. Cerebral metabolism was affected bilaterally in a subset of all affected structures which demonstrated that the functional consequences of the lesion extended beyond the primary monosynaptic output zones of the rostral accumbens. The lesion also changed the topography of the normal cocaine response such that cocaine effects were blunted in the shell of the nucleus accumbens, globus pallidus and the medial ventral pallidum. Thus, the present study describes functional evidence of the link between the rostral accumbens and the extended amygdala and demonstrates that dopamine in the rostral accumbens plays an important role in the central response to cocaine.

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.

Dopamine D1 receptor-mediated facilitation of GABAergic neurotransmission in the rat strioentopenduncular pathway and its modulation by adenosine A1 receptor-mediated mechanisms

By using in vivo microdialysis it was found that one of the main functions of striatal dopamine D1 receptors is to selectively facilitate GABAergic neurotransmission in the 'direct' strioentopeduncular pathway. D1 receptors localized in the entopeduncular nucleus were also found to facilitate GABA release. However, results obtained from in vivo microdialysis, in vivo electrochemistry, immunohistochemistry and confocal laser microscopy suggested that entopeduncular D1 receptors could only be activated under pharmacological conditions. Adenosine A1 receptors were found to antagonistically modulate the D1-mediated regulation of the strioentopeduncular pathway. Furthermore, using in situ hybridization D1 and A1 receptors were shown to be colocalized in medium-sized striatal neurons. These results show that the strioentopeduncular neuron is a main locus for adenosine-dopamine interactions in the brain.

MK-801 differentially affects dopamine D1 and D2 receptor agonist-induced behavioral tolerance and desensitization

In this study we explored the effects of repeated MK-801 (0.10 mg/kg) treatment on rotation in rats with unilateral forebrain dopamine depletions. Daily injections of MK-801 across a 13-day period produced mild ipsilateral rotation which did not change significantly across days compared to daily injections of vehicle. Rats given repeated cotreatment of MK-801 with the selective D1 receptor agonist, A-85653 (0.06 mg/kg), developed response sensitization rather than the behavioral tolerance that was seen in rats given repeated vehicle+A-85653 injections. However, MK-801+A-85653 treated rats did demonstrate behavioral tolerance after an acute vehicle+A-85653 challenge, and the behavioral subsensitivity of rats given repeated vehicle+A-85653 injections reverted to normal in response to an acute MK-801+A-85653 challenge. Thus, MK-801 blocked the expression but not the development of D1-agonist induced behavioral tolerance. MK-801 treatment also enhanced striatal c-fos expression produced by A-85653 but only if MK-801 were given in combination with A-85653 2 h prior to sacrifice; prior daily treatment with MK-801 had no carry-over effect. In contrast to its effects on D1 agonist induced rotation, MK-801 cotreatment inhibited the robust contralateral rotation produced by repeated treatment with the D2/D3 agonist, quinpirole (0.15 mg/kg), and blocked both the development and expression of behavioral supersensitivity compared to rats treated with quinpirole alone. These results demonstrate differential effects of repeated MK-801 treatment on the development and expression of D1 and D2/D3 agonist induced response tolerance and sensitization, respectively.

Unilateral 6-hydroxydopamine lesions of meso-striatal dopamine neurons and their physiological sequelae

One of the primary approaches in experimental brain research is to investigate the effects of specific destruction of its parts. Here, several neurotoxins are available which can be used to eliminate neurons of a certain neurochemical type or family. With respect to the study of dopamine neurons in the brain, especially within the basal ganglia, the neurotoxin 6-hydroxydopamine (6-OHDA) provides an important tool. The most common version of lesion induced with this toxin is the unilateral lesion placed in the area of mesencephalic dopamine somata or their ascending fibers, which leads to a lateralized loss of striatal dopamine. This approach has contributed to neuroscientific knowledge at the basic and clinical levels, since it has been used to clarify the neuroanatomy, neurochemistry, and electrophysiology of mesencephalic dopamine neurons and their relationships with the basal ganglia. Furthermore, unilateral 6-OHDA lesions have been used to investigate the role of these dopamine neurons with respect to behavior, and to examine the brain's capacity to recover from or compensate for specific neurochemical depletions. Finally, in clinically-oriented research, the lesion has been used to model aspects of Parkinson's disease, a human neurodegenerative disease which is neuronally characterized by a severe loss of the meso-striatal dopamine neurons. In the present review, which is the first of two, the lesion's effects on physiological parameters are being dealt with, including histological manifestations, effects on dopaminergic measures, other neurotransmitters (e.g. GABA, acetylcholine, glutamate), neuromodulators (e.g. neuropeptides, neurotrophins), electrophysiological activity, and measures of energy consumption. The findings are being discussed especially in relation to time after lesion and in relation to lesion severeness, that is, the differential role of total versus partial depletions of dopamine and the possible mechanisms of compensation. Finally, the advantages and possible drawbacks of such a lateralized lesion model are discussed.

Antiparkinsonian effect of flupirtine in monoamine-depleted rats

Excitatory amino acid receptor antagonists lead to marked suppression of parkinsonian-like symptoms in rodent and primate models of Parkinson's disease and are able to potentiate the ability of L-DOPA to reverse akinesia and ameliorate muscular rigidity displayed in these animal models. Flupirtine, which is clinically used as a non-opioid analgesic agent, has some N-methyl-D-aspartate (NMDA) antagonistic properties in several in vivo and in vitro experiments. We now report that in monoamine depleted rats (pretreated with reserpine, 5 mg/kg, and alpha-methyl-para-tyrosine, 250 mg/ kg i.p.) flupirtine dose-dependently (1-20 mg/kg i.p.) suppressed rigidity, measured as tonic EMG activity in the gastrocnemius muscle, but had no effect on akinesia, measured as locomotor activity. In addition, it potentiated the antiparkinsonian effect of L-DOPA on akinesia and rigidity in this rodent model of Parkinson's disease. These effects of flupirtine are of particular clinical relevance, since flupirtine is devoid of the typical side effects of NMDA-receptor antagonists.

Glutamate/dopamine D1/D2 balance in the basal ganglia and its relevance to Parkinson's disease

The recent availability of selective ligands for NMDA and AMPA receptors has enabled neuroscientists to test the hypothesis that Parkinson's disease is a glutamate hyperactivity disorder and hence treatable with glutamate antagonists. This review takes a critical look at the motor characteristics of this new class of drugs in rodent and primate models of parkinsonism and assesses the clinical potential and pitfalls of this radical new approach. Monotherapy of Parkinson's disease with glutamate antagonists appears impractical at the present time, due to their low efficacy and unacceptable side effects, but polypharmacy with L-DOPA and a glutamate antagonist as adjuvant is a more realistic prospect. This review will focus on the ways in which glutamate receptor blockade facilitates motor recovery with L-DOPA and will examine whether the basis for this beneficial effect can be traced to a specific interaction with dopamine at D1 or D2 receptors, and therefore to discrete motor pathways within the basal ganglia.

Effect of coadministration of glutamate receptor antagonists and dopaminergic agonists on locomotion in monoamine-depleted rats

Combinations of dopaminergic agonists with glutamate receptor antagonists have been suggested to be a possible alternative treatment of Parkinson's disease. To gain further insights into this possibility, the antagonist of the competitive AMPA-type glutamate receptor NBQX and the ion-channel blocker of the NMDA glutamate receptor (+)-MK-801 in combination with the dopamine D1 receptor agonists: SKF 38393, SKF 82958 and dihydrexidine; the dopamine D2 receptor agonist bromocriptine and the dopamine-precursor L-DOPA were tested in rats pretreated with reserpine and alpha-methyl-p-tyrosine. MK-801 on its own induced locomotor behaviour and potentiated the antiakinetic effects of dihydrexidine and L-DOPA but not of the other dopamine agonists tested. NBQX neither on its own nor coadministered with the dopamine agonists tested had an antiakinetic effect. These results indicate that agents, blocking the ion-channel of the NMDA receptor, might be useful adjuvants to some but not all dopaminomimetics in therapy of Parkinson's disease. The same does not seem to be true for the AMPA-antagonist NBQX.

A role for non-NMDA excitatory amino acid receptors in regulating the basal activity of rat globus pallidus neurons and their activation by the subthalamic nucleus

We have investigated the hypothesis that excitatory amino acid (EAA) receptors in the globus pallidus (GP) play a significant role in maintaining the firing rates of GP neurons under basal conditions and following activation of the subthalamic nucleus (STN). Drugs were infused directly into the GP and/or STN while the extracellular single unit activity of Type II GP neurons was recorded in ketamine-anesthetized rats. Local infusions of the EAA agonists NMDA (30-300 pmol/200 nl) or AMPA (0.1-1 pmol/200 nl) elicited increases in the firing rate of GP neurons in a dose-dependent fashion. Infusion of the GABAA receptor antagonist bicuculline methiodide (1-10 pmol/100 nl) into the STN also elicited dose-related increases in the firing rate of GP neurons. Intrapallidal infusion of the non-NMDA (AMPA/kainate) receptor antagonist NBQX (0.1-1.0 nmol) reduced the basal firing rate of GP neurons by 40%. In contrast, the NMDA antagonist MK-801 (0.01-0.1 nmol) produced no significant effect on basal firing rate. Intrapallidal infusion of the non-selective EAA receptor antagonist kynurenic acid or NBQX reversed or blocked the increase in firing rate of GP neurons following bicuculline-induced activation of the STN. Similar treatment with MK-801, however, had no significant effect on this response. These results indicate that tonic stimulation of non-NMDA receptors plays an important role in maintaining the basal activity of GP neurons and in mediating the effects of increased excitatory input from subthalamic afferent neurons.

Antiparkinsonian effects of remacemide hydrochloride, a glutamate antagonist, in rodent and primate models of Parkinson's disease

Loss of dopaminergic innervation of the striatum results in overactivity of the glutamatergic pathways from the subthalamic nucleus to the internal segment of the globus pallidus and the substantia nigra pars reticulata, the output nuclei of the basal ganglia. Previous work has shown that local blockade of glutamate receptors in the internal segment of the globus pallidus or substantia nigra pars reticulata leads to marked suppression of parkinsonian signs. We have now examined whether systemic administration of a glutamate receptor antagonist has antiparkinsonian effects in rodent and primate models of Parkinson's disease. Remacemide hydrochloride is an anticonvulsant, neuroprotective compound with antagonist activity at the N-methyl-D-aspartate receptor ion channel. In normal rats and monoamine-depleted rats, remacemide hydrochloride did not cause locomotor hyperactivity, unlike MK-801. When monoamine-depleted rats were treated with a subthreshold dose of levodopa methylester, remacemide hydrochloride (5-40 mg/kg, orally) caused a dose-dependent increase in locomotor activity. Moreover, remacemide hydrochloride (10 mg/kg, orally) potentiated the effects of each suprathreshold dose of levodopa methylester tested (100-200 mg/kg, intraperitoneally). Parkinsonian rhesus monkeys were tested with oral doses of vehicle plus vehicle, vehicle plus levodopa-carbidopa, and remacemide hydrochloride (5 mg/kg) plus levodopa-carbidopa. Blinded clinical scoring of videotapes revealed that treatment with remacemide hydrochloride plus levodopa-carbidopa was substantially better than levodopa-carbidopa plus vehicle or vehicle plus vehicle. The effects of remacemide hydrochloride lasted at least 5 hours. We conclude that certain N-methyl-D-aspartate receptor antagonists have antiparkinsonian actions and low potential for side effects. Clinical trials of remacemide hydrochloride in patients with Parkinson's disease may be warranted.

Motor activation in short- and long-term reserpinized mice: role of N-methyl-D-aspartate, dopamine D1 and dopamine D2 receptors

The effects of dopamine D1 and dopamine D2 receptor agonists and of subconvulsant doses of N-methyl-D-aspartate (NMDA) and the non-competitive NMDA receptor antagonist, dizocilpine (MK-801), alone and in combination, on the motor activity of short- and long-term reserpinized mice (mice pretreated with 5 mg/kg reserpine 4 h or 20 h before, respectively) were analyzed. With short-term reserpinization, the dopamine D2 receptor agonist, quinpirole (1.5 mg/kg), but not the dopamine D1 receptor agonist, SKF-38393 (15 mg/kg), increased motor activity. The effect of quinpirole in short-term reserpinized mice was potentiated by the simultaneous administration of SKF-38393 (15 mg/kg) and was counteracted by the previous administration of the dopamine D2 receptor antagonist, raclopride (1 mg/kg), or by the simultaneous administration of NMDA (25 mg/kg) or MK-801 (0.5 mg/kg). Neither NMDA (25-100 mg/kg) nor MK-801 (0.5-3 mg/kg) induced motor activation in short-term reserpinized mice. With long-term reserpinization, either quinpirole (1.5 mg/kg) or SKF-38393 (15 mg/kg) increased motor activity. The effect of quinpirole in long-term reserpinized mice was not potentiated by the concurrent administration of SKF-38393 (15 mg/kg), was inhibited by the simultaneous administration of MK-801 (0.5 mg/kg) and was not modified by NMDA (25 mg/kg). The effect of SKF-38393 (15 mg/kg) in long-term reserpinized mice was inhibited by the concomitant administration of MK-801 (0.5 mg/kg) and was slightly antagonized by NMDA (25 mg/kg). NMDA induced motor activation in long-term reserpinized mice at doses which were similar to those causing motor activation in non-reserpinized mice (75 and 100 mg/kg), while MK-801 induced motor activation at a dose which was associated with motor depression in non-reserpinized mice (2 mg/kg). The NMDA-induced motor activation in long-term reserpinized mice was counteracted by the previous administration of a low dose of MK-801 (0.5 mg/kg) and was still present when a stronger dopamine-depleting pretreatment was used. These results are interpreted on the basis of changes in sensitivity of the direct striatal efferent pathway after long-term reserpinization.

Excitatory amino acid receptor antagonists modify regional cerebral metabolic responses to levodopa in 6-hydroxydopamine-lesioned rats

Excitatory amino acid receptor antagonists have been proposed as novel therapeutic agents to be used with levopoda in the treatment of Parkinson's disease. We examined the neural substrates for the interaction between levodopa and antagonists of either the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid or N-methyl-D-aspartate type of excitatory amino acid receptor using 2-deoxyglucose autoradiography. Thus, we compared the effects of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (10 mg/kg, i.v.) and the N-methyl-D-aspartate antagonist MK-801 (0.1 mg/kg, i.v.) on cerebral metabolic responses to levodopa (25 mg/kg, i.v., with 12.5 mg/kg benserazide) in rats with a unilateral nigrostriatal pathway lesion. Levodopa increased glucose utilization ipsilateral to the lesion in substantia nigra pars reticula (up to 104%), entopeduncular nucleus (up 90%) and subthalamic nucleus (up 30%), indicating that levodopa alters striatal output through the striatonigral, striatoentopeduncular and striatopallidal pathways. Levodopa also decreased metabolic rate in lateral habenula (down 39%), a target of projections from entopeduncular nucleus, implying a reduction in basal ganglia output. 2,3-Dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline and MK-801 by themselves did not affect glucose utilization in any of these regions. Pretreatment with 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline reduced the effect of levodopa in substantia nigra pars reticulata but not in entopeduncular nucleus or subthalamic nucleus, while MK-801 attenuated the effect of levodopa in all three of these structures; neither 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline nor MK-801 altered the effect of levodopa in lateral habenula. When given at the same doses to a separate group of lesioned animals, neither 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline nor MK-801 affected rotational behavior elicited by levodopa. These findings indicate that alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and N-methyl-D-aspartate receptor antagonists differentially modify dopamine receptor-mediated striatal output. alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor blockade may preferentially attenuate the effect of dopamine receptor activation on the striatonigral pathway, while N-methyl-D-aspartate blockade appears to reduce the actions of dopamine on the striatonigral, striatoentopeduncular and striatopallidal pathways.(ABSTRACT TRUNCATED AT 400 WORDS)