Antiparkinsonian effect of a new selective adenosine A2A receptor antagonist in MPTP-treated monkeys

BACKGROUND

Chronic treatment with L-3,4-dihydroxyphenylalanine (L-dopa) is often associated with motor side effects in PD patients. The search for new therapeutic approaches has led to study the role of other neuromodulators including adenosine. Among the four adenosine receptors characterized so far, the A2A subtype is distinctively present on striatopallidal output neurons containing enkephalin and mainly bearing dopamine (DA) D2 receptors (indirect pathway). Studies in DA-denervated rats suggest that blockade of adenosine A2A receptors might be used in PD.

OBJECTIVE

To evaluate the antiparkinsonian effect of a new selective adenosine A2A receptor antagonist, KW-6002, in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys.

METHODS

In the present study, we used six MPTP-exposed cynomolgus monkeys already primed and exhibiting L-dopa-induced dyskinesias to evaluate both the antiparkinsonian and dyskinetic effect upon challenge with two oral doses (60 and 90 mg/kg) of KW-6002 administered alone or in combination with L-dopa/benserazide (50/12.5 mg).

RESULTS

KW-6002 administered alone produced a dose-dependent antiparkinsonian response that reached the level of efficacy of L-dopa/benserazide but was less likely to reproduce dyskinesias in these animals. When co-administered, KW-6002 potentiated the effects of L-dopa/benserazide on motor activity (up to 30%) without affecting the dyskinetic response.

CONCLUSION

Adenosine A2A receptor antagonists have antiparkinsonian effects of their own with a reduced propensity to elicit dyskinesias. They might therefore be useful agents in the treatment of PD.

Chronic alterations in dopaminergic neurotransmission produce a persistent elevation of deltaFosB-like protein(s) in both the rodent and primate striatum

Using an antibody that recognizes the products of all known members of the fos family of immediate early genes, it was demonstrated that destruction of the nigrostriatal pathway by 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle produces a prolonged (>3 months) elevation of Fos-like immunoreactivity in the striatum. Using retrograde tract tracing techniques, we have previously shown that this increase in Fos-like immunoreactivity is located predominantly in striatal neurons that project to the globus pallidus. In the present study, Western blots were performed on nuclear extracts from the intact and denervated striatum of 6-OHDA-lesioned rats to determine the nature of Fos-immunoreactive protein(s) responsible for this increase. Approximately 6 weeks after the 6-OHDA lesion, expression of two Fos-related antigens with apparent molecular masses of 43 and 45 kDa was enhanced in the denervated striatum. Chronic haloperidol administration also selectively elevated expression of these Fos-related antigens, suggesting that their induction after dopaminergic denervation is mediated by reduced activation of D2-like dopamine receptors. Western blot immunostaining using an antibody which recognizes the N-terminus of FosB indicated that the 43 and 45 kDa Fos-related antigens induced by dopaminergic denervation and chronic haloperidol administration may be related to a truncated form of FosB known as deltaFosB. Consistent with this proposal, retrograde tracing experiments confirmed that deltaFosB-like immunoreactivity in the deafferented striatum was located predominantly in striatopallidal neurons. Gel shift experiments demonstrated that elevated AP-1 binding activity in denervated striata contained FosB-like protein(s), suggesting that enhanced deltaFosB levels may mediate some of the effects of prolonged dopamine depletion on AP-1-regulated genes in striatopallidal neurons. In contrast, chronic administration of the D1-like receptor agonist CY 208243 to 6-OHDA-lesioned rats dramatically enhanced deltaFosB-like immunoreactivity in striatal neurons projecting to the substantia nigra. Western blot immunostaining revealed that deltaFosB and, to a lesser extent, FosB are elevated by chronic D1-like agonist administration. Both the quantitative reverse transcriptase-polymerase chain reaction and the ribonuclease protection assay demonstrated that deltafosB mRNA levels were substantially enhanced in the denervated striatum by chronic D1-like agonist administration. Lastly, we examined the effects of chronic administration ofD1-like and D2-like dopamine receptor agonists on striatal deltaFosB expression in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) primate model of Parkinson's disease. In monkeys rendered Parkinsonian by MPTP, there was a modest increase in deltaFosB-like protein(s), while the development of dyskinesia produced by chronic D1-like agonist administration was accompanied by large increases in DeltaFosB-like protein(s). In contrast, administration of the long-acting D2-like agonist cabergoline, which alleviated Parkinsonian symptoms without producing dyskinesia reduced deltaFosB levels to near normal. Taken together, these results demonstrate that chronic alterations in dopaminergic neurotransmission produce a persistent elevation of deltaFosB-like protein(s) in both the rodent and primate striatum.

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

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

Associative and limbic regions of monkey striatum express high levels of dopamine D3 receptors: effects of MPTP and dopamine agonist replacement therapies

The role of the dopamine D3 receptor subtype in the central nervous system is still not well understood. It has a distinct and restricted distribution, mostly associated with limbic territories of the striatum (olfactory tubercle and the shell of nucleus accumbens) in rat brain. Dopaminergic denervation induced by a 6-hydroxydopamine lesion of the nigrostriatal system in rat down-regulates the expression of the D3 receptor. In the present study, we investigated the functional neuroanatomy of the dopamine D3 receptor subtype in the monkey (Macaca fascicularis) basal ganglia. We also studied the effect of administration of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and chronic D1-like (SKF 82958) or D2-like (cabergoline) agonist treatments on dopamine D3 receptor levels using receptor autoradiography. Our results clearly show that the distribution of D3 receptors in the monkey is more closely related to associative and limbic components of the striatum (caudate-putamen), as compared with its sensorimotor counterpart. Hence, D3 receptors may be more specifically involved in cognitive and motivational aspects of striatal functions, which are elaborated in prefrontal, temporal, parietal, cingulate and limbic cortices. Moreover, MPTP administration significantly decreased levels of D3 receptors and this effect was reversed or compensated by a chronic treatment with a D1-like, but not a D2-like, receptor agonist. The D3 receptor may represent an important target for adjunct or direct therapy designed to improve cognitive deficits observed in patients with Parkinson's disease, schizophrenia and other illnesses with frontal lobe cognitive disturbances.

Glial cell line-derived neurotrophic factor (GDNF): a drug candidate for the treatment of Parkinson's disease

Considerable effort has been devoted to the search for molecules that might exert trophic influences on midbrain dopamine neurons, and potentially be of therapeutic value in the treatment of Parkinson's disease. One such candidate is glial cell line-derived neurotrophic factor (GDNF). GNDF is distantly related to the transforming growth factor-beta superfamily and is widely expressed in many neuronal and non-neuronal tissues. GDNF uses a multisubunit receptor system in which GFRalpha-1 and Ret function as the ligand-binding and signalling components, respectively. In addition to its effects on cultured fetal midbrain dopamine neurons, GDNF promotes recovery of the injured nigrostriatal dopamine system and improves motor functions in rodent and nonhuman primate models of Parkinson's disease. Intraventricular, intrastriatal and intranigral routes of administration are efficacious in both models. In parkinsonian nonhuman primates, GDNF treatment improves bradykinesia, rigidity and postural instability. In this model, adult midbrain dopamine neurons stimulated by GDNF show increased cell size, neuritic extent, and expression of phenotypic markers. The neurorestorative effects of a single administration of GDNF last for at least a month and can be maintained in rhesus monkeys by monthly injections. GDNF also induces neuroprotective changes in dopamine neurons, which are active within hours following trophic factor administration in rodents. The powerful neuroprotective and neurorestorative properties of GDNF seen in preclinical studies suggest that trophic factors may play an important role in treating Parkinson's disease.

Motor slowing and parkinsonian signs in aging rhesus monkeys mirror human aging

Motor slowing is a universal feature of human aging, and parkinsonian signs are commonly expressed in human senescence. In the present study, age-associated declines in motor functions in 31 female rhesus monkeys were quantified by activity monitors and an automated test panel, and the incidence of parkinsonian signs was scored using a movement dysfunction assessment scale. Activity levels in middle-aged monkeys (12-17 years old) were less than half that of young animals (5-8 years old) and were further depressed in aged monkeys (21-27 years old). Movement dysfunction scores increased significantly with increasing age. Two or more parkinsonian signs were exhibited by 20% of the middle-aged monkeys and 36% of the aged monkeys. Slowing performance times on fine-motor hand tasks correlated significantly with increasing age. Motor learning was seen in all age groups, but improved faster in the young monkeys. The data suggest that aging rhesus monkeys provide an appropriate model to analyze the biological processes leading to motor slowing and the expression of parkinsonian signs in human senescence.

Noradrenoceptor antagonism with idazoxan improves L-dopa-induced dyskinesias in MPTP monkeys

Treatment of Parkinson's disease with L-dopa is plagued in a majority of patients by dyskinesias. Noradrenaline/dopamine interactions are proposed on behavioral, biochemical, physiological and anatomical grounds. The aim of the study was to test the potential antidyskinetic effect of the alpha2-adrenoceptor antagonist, idazoxan, in a primate model of Parkinson's disease. Six female cynomolgus monkeys previously rendered parkinsonian by the toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and presenting an unchanged syndrome for several months were used. All responded readily to L-dopa but had developed dyskinesias which were manifested with each dose. In the first part of the study, seven doses of idazoxan (ranging from 0.25 mg/kg to 10 mg/kg, p.o.) were administered together with the vehicle or in combination with a fixed dose of L-dopa/benserazide (100/25 mg, p.o.). In the second part of the study, a fixed dose of idazoxan (7.5 mg/kg) was administered daily for 10 days and L-dopa was added to idazoxan on days 1, 4, 7 and 10. Vehicle (empty capsule) was used as control. Idazoxan, by itself (ranging from 5 mg/kg to 10 mg/kg), increased locomotor activity and improved the disability score with virtually no dyskinesias in three animals. In combination with L-dopa, idazoxan did not impair the antiparkinsonian response but significantly reduced dyskinesias in all six animals up to 65% at doses of 7.5 mg/kg and 10 mg/kg and delayed their onset, so that the "ON" state without dyskinesias was prolonged. The antidyskinetic effect of idazoxan was maintained when repeatedly administered for 10 days. On day 10, the locomotor response to L-dopa was significantly potentiated by chronic administration of idazoxan. Our results indicate that idazoxan has some antiparkinsonian effect of its own and may constitute a useful adjunct to L-dopa as it can reduce dyskinesias without impairing the relief of symptoms, this effect being maintained over time in this model.

Dopamine-receptor stimulation: biobehavioral and biochemical consequences

The MPTP monkey is a well-characterized animal model of parkinsonism and provides an exceptional tool for the study of dyskinesias induced by dopamine-like agents. Several such agents have been tested during the past 15 years, and it has been found that the duration of action of these compounds is the most reliable variable with which to predict their dyskinesiogenic profile. It is proposed that L-dopa-induced dyskinesias represent a form of pathological learning caused by chronic pulsatile (nonphysiological) stimulation of dopamine receptors, which activates a cascade of molecular and biochemical events. These events include defective regulation of Fos proteins that belong to the deltaFosB family, increased expression of neuropeptides, and defective GABA- and glutamate-mediated neurotransmission in the output structures of the basal ganglia.

Dyskinesia and wearing-off following dopamine D1 agonist treatment in drug-naive 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned primates

The motor effects of the short-acting, full D1 agonist SKF 82958 were studied in three drug-naive, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned, parkinsonian monkeys treated for 4 weeks. D1 receptor stimulation with SKF 82958 effectively relieved parkinsonism but induced choreic dyskinesia (n = 2) and a shorter duration of motor benefit (n = 3) over time. Isolated, short-lived D1 receptor activation would not appear to confer advantage over levodopa for dyskinesia prevention. Our data also support the involvement of postsynaptic dopamine receptor mechanisms in the wearing-off phenomenon seen in levodopa-treated parkinsonian patients.

D1 receptor blockade improves L-dopa-induced dyskinesia but worsens parkinsonism in MPTP monkeys

OBJECTIVE

To determine whether dopamine (DA) D1 or DA D2 receptors are associated predominantly with the antiparkinsonian versus the dyskinetic effect of levodopa.

METHODS

The authors used four L-dopa-primed, dyskinetic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-exposed monkeys to test whether acute and selective blockade of the DA D1 receptor subtype, using SCH 23390 and NNC 01-112, could reduce L-dopa-induced dyskinesias without altering the relief of symptoms. Blockade of DA receptors using sulpiride (D2) and clozapine (D1-D2-like) was studied for comparison.

RESULTS

With the notable exception of the lowest dose of clozapine tested, coadministration of DA D1 or D2 antidopaminergic agents with L-dopa reduced the L-dopa-induced dyskinesias but also caused a return of parkinsonian disability. Prolonged latencies from intake of a single oral dose of L-dopa to turning "on," decreased duration of the "on" state, and a complete failure to induce benefit was also observed.

CONCLUSION

Low-dose clozapine could be an effective adjunct to reduce L-dopa-induced dyskinesias without altering the relief of parkinsonian symptoms. Interactions with many neurotransmitter systems may explain the better pharmacologic profile of clozapine, including DA D4 (rather than D1), serotonin, acetylcholine, and noradrenaline. Neither dyskinesias nor antiparkinsonian effects can be ascribed solely to the D2 or D1 receptor. Thus, some cooperation between the two receptors appears necessary for these behavioral effects.

Pharmacological MRI mapping of age-associated changes in basal ganglia circuitry of awake rhesus monkeys

While the pathophysiological changes induced by the loss of dopamine innervation in the basal ganglia by Parkinson's disease (PD) are well studied, little is known about functional changes in the neural circuitry of this area during normal aging. Here we report the first survey of age-associated changes in the basal ganglia of behaviorally characterized, awake rhesus monkeys, using pharmacological MRI to map responses to dopaminergic stimulation. Apomorphine, a mixed D(1)/D(2) dopamine receptor agonist, evoked little change in the substantia nigra (SN) of aged animals while significantly reducing activation in young adult monkeys. Compared to young animals, both apomorphine and d-amphetamine (which increases synaptic dopamine levels) significantly increased activation of the aged rhesus globus pallidus externa (GPe). In addition, the aged animals showed decreased activity in the putamen in response to d-amphetamine administration. Although the responses in the SN and putamen of the aged monkeys differed from those in animal models of PD, the apomorphine-evoked activation of their GPe corresponded with apomorphine-induced increases in neuronal activity seen in Parkinson's patients and animal models. Given the major role of the GPe in regulating motor behavior, the altered responses in the aged GPe may contribute significantly to the motor slowing and movement dysfunctions characterizing advanced age.

Dyskinesias and tolerance induced by chronic treatment with a D1 agonist administered in pulsatile or continuous mode do not correlate with changes of putaminal D1 receptors in drug-naive MPTP monkeys

Nine monkeys (Macaca fascicularis) were rendered parkinsonian after intravenous administration of the toxin MPTP. Three of these animals received pulsatile administration of the D1 receptor agonist SKF 82958 (1 mg/kg, three times daily) while three were treated by continuous infusion via an osmotic mini-pump with SKF 82958 (at an equivalent amount daily) for 29 days. Untreated MPTP as well as healthy control animals were also studied. Relief of parkinsonian symptoms was observed in the three animals of the pulsatile group. However, dyskinesia occurred in two monkeys which had striatal dopamine depletion of > 99% compared to the non-dyskinetic animal slightly less denervated (94%). Monkeys receiving continuous SKF 82958 showed no anti-parkinsonian effect and no dyskinesia. All monkeys from the pulsatile and continuous group had measurable amount of plasma SKF 82958 as assayed by HPLC with electrochemical detection. In the putamen of all SKF 82958-treated monkeys, Bmax of D1 receptors labeled with [3H]SCH 23390 were increased versus untreated MPTP-monkeys with no change in Kd. In contrast, a decrease D1 receptor density was observed in the nucleus accumbens of untreated MPTP monkeys versus controls and this was not corrected with either pulsatile or continuous SKF 82958 treatments. D2 receptor density measured with [3H]spiperone binding was increased in the posterior putamen of SKF 82958-treated monkeys whereas no change was observed in the accumbens compared to control animals. Hence, tolerance with the continuous administration of a D1 agonist is not associated with a decrease of putaminal D1 or D2 receptor densities and dyskinesia could not be specifically associated with an increase of putaminal D1 receptors.

Chronic D1 and D2 dopaminomimetic treatment of MPTP-denervated monkeys: effects on basal ganglia GABA(A)/benzodiazepine receptor complex and GABA content

The effect of various chronic dopaminergic treatments in 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys on the brain gamma-aminobutyric acid type A (GABA(A)) /benzodiazepine receptor complex and GABA content was investigated in order to assess the GABAergic involvement in dopaminomimetic-induced dyskinesia. Three MPTP monkeys received for one month pulsatile administrations of the D1 dopamine (DA) receptor agonist SKF 82958 whereas three others received the same dose of SKF 82958 by continuous infusion. A long acting D2 DA receptor agonist, cabergoline, was given to another three animals. Untreated MPTP as well as naive control animals were also included. Pulsatile SKF 82958 relieved parkinsonian symptoms but was also associated with dyskinesia in two of the three animals whereas animals treated continuously with SKF 82958 remained as untreated MPTP monkeys. Chronic cabergoline administration improved motor response with no persistent dyskinesia. MPTP treatment induced a decrease of 3H-flunitrazepam binding in the medial anterior part of caudate-putamen and an increase in the internal segment of globus pallidus (GPi) which was in general unchanged by pulsatile or continuous SKF 82958 administration. Throughout the striatum, binding of 3H-flunitrazepam remained reduced in MPTP monkeys treated with cabergoline but was not significantly lower than untreated MPTP monkeys. Moreover, cabergoline treatment reversed the MPTP-induced increase in 3H-flunitrazepam binding in the GPi. GABA concentrations remained unchanged in the striatum, external segment of globus pallidus and GPi following MPTP denervation. Pulsatile but not continuous SKF 82958 administration decreased putamen GABA content whereas cabergoline treatment decreased caudate GABA. No alteration in GABA levels were observed in the GPe and GPi following the experimental treatments. These results suggest that: (1) D2-like receptor stimulation with cabergoline modulates GABA(A) receptor density in striatal subregions anatomically related to associative cortical afferent and (2) the absence of dyskinesia in dopaminomimetic-treated monkeys might be associated with the reversal of the MPTP-induced upregulation of the GABA(A)/benzodiazepine receptor complex in the Gpi.

Dopamine D1 receptor desensitization profile in MPTP-lesioned primates

The motor effects of dopamine D1 receptor activation and the optimal way to stimulate these receptors were studied in a primate model of parkinsonism induced by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), using 2 selective full dopamine D1 receptor agonists: A-77636 ([1 R,3S] 3-(1'-adamantyl)-1-aminomethyl-3,4-dihydro-5,6-dihydroxy-1 H-2-benzopyran hydrochloride), and SKF 82958 (6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine hydrobromide). A-77636 was administered to one group of primed monkeys (N = 4) previously treated with levodopa and other dopamine receptor agonists, while SKF 82958 was given to another group of drug-naive monkeys (N = 3). These drugs have different durations of efficacy, lasting > 20 h and approximately 1 h, respectively, and were administered once daily (A-77636) or thrice daily (SKF 82958) for 7 days. Both drugs demonstrated excellent antiparkinsonian efficacy and locomotor stimulation. However, a rapid, functionally important, homologous (selective for D1 receptor agonists) desensitization process took place as early as on the second day with the longer-acting drug and a dose escalation of A-77636 failed to restore the initial benefit. Thrice daily dosing at a 4-h interval with the short-acting agent SKF 82958 maintained the maximal antiparkinsonian response but some shortening in the duration of response was observed after several days. These behavioral results show that dopamine D1 receptors are susceptible to desensitization after prolonged occupancy and can be desensitized profoundly and independently of dopamine D2 receptors in vivo in this model. Potent dopamine D1 receptor agonists with an intermediate half-life may prove to be better adjuncts in the treatment of Parkinson's disease. Clinical entities with pathologically enhanced dopamine D1 receptor-linked neural transmission might eventually also benefit from such desensitization.

Intraputamenal Infusion of Exogenous Neurturin Protein Restores Motor and Dopaminergic Function in the Globus Pallidus of MPTP-Lesioned Rhesus Monkeys

The neurorestorative effects of exogenous neurturin (NTN) delivered directly into the putamen via multiport catheters were studied in 10 MPTP-lesioned rhesus monkeys expressing stable parkinsonism. The parkinsonian animals were blindly assigned to receive coded solutions containing either vehicle (n = 5) or NTN (n = 5, 30 μg/day). Both solutions were coinfused with heparin using convection-enhanced delivery for 3 months. The NTN recipients showed a significant and sustained behavioral improvement in their parkinsonian features during the treatment period, an effect not seen in the vehicle-treated animals. At study termination, locomotor activity levels were increased by 50% in the NTN versus vehicle recipients. Also, DOPAC levels were significantly increased by 150% ipsilateral (right) to NTN infusion in the globus pallidus, while HVA levels were elevated bilaterally in the NTN-treated animals by 10% on the left and 67% on the right hemisphere. No significant changes in DA function were seen in the putamen. Volumetric analysis of putamenal NTN labeling showed between-subject variation, with tissue distribution ranging from 214 to 744 mm3, approximately equivalent to 27–93% of area coverage. Our results support the concept that intraparenchymal delivery of NTN protein may be effective for the treatment of PD. More studies are needed to determine strategies that would enhance tissue distribution of exogenous NTN protein, which could contribute to optimize its trophic effects in the parkinsonian brain.

Cabergoline, a long-acting dopamine D2-like receptor agonist, produces a sustained antiparkinsonian effect with transient dyskinesias in parkinsonian drug-naive primates

Continuous dopaminergic receptor stimulation is now considered as an interesting approach for the control of motor complications often seen in parkinsonian patients treated chronically with levodopa. Cabergoline, which is a long-acting dopamine D2-like receptor agonist, has been tried recently with good results as an adjunct in patients already on levodopa-therapy. Thus, the present study was designed to test the effects of repeated s.c. administration of cabergoline as sole therapeutic agent during a month in 3 drug-naive MPTP parkinsonian monkeys to see whether or not cabergoline, given every other day at 0.25 mg/kg, would have a sustained antiparkinsonian effect and would induce dyskinesias. The animals were rated to quantify the antiparkinsonian as well as the dyskinetic response and gross locomotor activity was monitored by photocells. The averaged locomotor response, initially greatly increased (approximately 9 times higher than after saline treatment in the same animals), decreased by approximately 50% after 2 weeks but was thereafter maintained at this level until the end of the study. The parkinsonian features were improved in a sustained manner in all monkeys and transient dyskinesias (week 1 and 2) were present in 2 of 3 monkeys. After sacrifice receptor binding assays were performed on striatal and pallidal tissues homogenates with tritiated selective ligands and compared with those of 3 normal and 3 MPTP-exposed monkeys otherwise untreated. A significant decrease in dopamine D2-like receptor density in the putamen (-36% on average vs. untreated MPTP-exposed monkeys) may be involved in the behavioral partial tolerance to antiparkinsonian effect of cabergoline and the disappearance of dyskinesias. A reversal of the supersensitivity of GABAA receptor in the internal segment of the globus pallidus (-15% on average vs. untreated MPTP-exposed monkeys) may also be implicated in this latter behavioral effect.

125I-CGP 64213 binding to GABA(B) receptors in the brain of monkeys: effect of MPTP and dopaminomimetic treatments

Much evidence indicates that abnormal GABA neurotransmission may be implicated in the pathophysiology of Parkinson's disease (PD) and dopaminomimetic-induced dyskinesias (DID). In this study, autoradiography using (125)I-CGP 64213 was performed to investigate GABA(B) receptor density in the brain of control monkeys as well as monkeys with MPTP-induced nigrostriatal depletion. Three MPTP monkeys received pulsatile administrations of the D1 dopamine (DA) receptor agonist (SKF 82958) whereas a long-acting D2 DA receptor agonist (cabergoline) was given to another three animals. SKF 82958 treatment relieved parkinsonian symptoms but two of three animals developed DID. Cabergoline induced a comparable motor benefit effect without persistent DID. (125)I-CGP 64213 binding to GABA(B) receptors was heterogeneous throughout the brain with the highest levels in the medial habenula of the thalamus. MPTP induced a decrease (-40%) of (125)I-CGP 64213 binding to GABA(B) receptors in the substantia nigra pars compacta (SNpc) and an increase (+29%) in the internal segment of the globus pallidus (GPi). This increase in the GPi was not affected by SKF 82958 but partly reversed by cabergoline. No change was seen in the striatum, the thalamus, the external segment of the globus pallidus, and the substantia nigra pars reticulata following MPTP and dopaminomimetic treatments. The changes of GABA(B) receptors observed in the SNpc and in the GPi suggest that alteration of GABA(B) receptors may play a role in the pathophysiology of PD and DID.

Potential therapeutic use of the selective dopamine D1 receptor agonist, A-86929: an acute study in parkinsonian levodopa-primed monkeys

The clinical utility of dopamine (DA) D1 receptor agonists in the treatment of Parkinson's disease (PD) is still unclear. The therapeutic use of selective DA D1 receptor agonists such as SKF-82958 (6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzaze pine hydrobromide) and A-77636 ([1R, 3S] 3-[1'-admantyl]-1-aminomethyl-3,4-dihydro-5,6-dihydroxy-1H-2-benzo pyran hydrochloride) seems limited because of their duration of action, which is too short for SKF-82958 (< 1 hr) and too long for A-77636 (> 20 hr, leading to behavioral tolerance). We therefore conducted the present acute dose-response study in four 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-exposed cynomolgus monkeys primed to exhibit levodopa-induced dyskinesias to evaluate the locomotor and dyskinetic effects on challenge with four doses (from 0.03 to 1.0 mg/kg) of A-86929 ([-]-[5aR,11bS]-4,5,5a,6,7,11b-hexahydro-2-propyl-3-thia-5-+ ++azacyclopent-1- ena[c]phenathrene-9-10-diol), a selective and full DA D1-like receptor agonist with an intermediate duration of action. Levodopa and the DA D2-like receptor agonist, LY-171555 ([4aR-trans]-4,4a,5,6,7,8,8a,9-o-dihydro-5n-propyl-2H-pyrazo lo-3-4-quinoline hydrochloride) were also used for comparison. Acute administration of A-86929 was as efficacious in alleviating MPTP-induced parkinsonism as levodopa and LY-171555, but was less likely to reproduce the levodopa-induced dyskinesias in these animals than with either LY-171555 or subsequent challenge of levodopa. Selective stimulation of the DA D1 receptor may provide better integration of neural inputs transmitted to the internal segment of the globus pallidus (referred to as the basal ganglia output) compared with levodopa and selective DA D2 receptor agonist. Potent DA D1 receptor agents with an intermediate duration of efficacy such as A-86929 (approximately 4 hr at higher doses tested) are potential therapeutic tools in PD and merit further attention.

Effect of MPTP-induced denervation on basal ganglia GABA(B) receptors: correlation with dopamine concentrations and dopamine transporter

We investigated the effect of MPTP-induced lesion of the substantia nigra pars compacta (SNpc) dopaminergic neurons on GABA(B) receptors in the basal ganglia of mice and monkeys using receptor autoradiography and in situ hybridization. The extent of the lesion was measured with striatal catecholamine content, striatal binding of (125)I-RTI-121 to dopamine transporter (DAT), and DAT expression in the SNpc. GABA(B) receptors in mice brain were evaluated using (3)H-CGP54626 and its expression was measured with oligonucleotides probes targeting the mRNAs of GABA(B(1a+b)), GABA(B(1a)), GABA(B(1b)), GABA(B(2)) subunits. In monkeys, (125)I-CGP64213 and selective probes for GABA(B(1a+b)) and GABA(B(2)) mRNAs were used. In mice, dopamine content, (125)I-RTI-121 binding, and DAT expression were reduced by 44%, 40%, and 39% after a dose of 40 mg/kg of MPTP and 74%, 70%, and 34% after 120 mg/kg of MPTP, respectively. In monkeys, dopamine content and DAT expression were decreased by more than 90% and 80%, respectively. In the striatum and the subthalamic nucleus, GABA(B) receptors were unchanged following MPTP in both species. In the SNpc of mice, MPTP (120 mg/kg) induced a significant decrease of (3)H-CGP54626 binding (-10%) and of the expression of GABA(B(1a+b)) mRNA (-13%). The decrease of the expression of GABA(B(1a+b)) mRNA was correlated with dopamine content, (125)I-RTI-121 binding and DAT expression. In MPTP-treated monkeys, (125)I-CGP64213 binding (-40%), GABA(B(1a+b)) mRNA (-69%) and GABA(B(2)) mRNA (-66%) were also significantly decreased in the SNpc. Our results suggest that MPTP-induced denervation is associated with a decrease of GABA(B) receptors restricted to the SNpc. These observations may be relevant to the pathophysiology of motor disorders involving dysfunction of the basal ganglia such as Parkinson disease.

Neurotensin receptors and dopamine transporters: effects of MPTP lesioning and chronic dopaminergic treatments in monkeys

The effect of denervation with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) of the dopamine (DA) nigrostriatal pathway on neurotensin (NT) receptor and DA transporter (DAT) in basal ganglia of monkeys (Macaca fascicularis) was investigated. The MPTP lesion induced a marked depletion of DA (90% or more vs. control) in the caudate nucleus and putamen. The densities of NT agonist binding sites labeled with [125I]NT and the NT antagonist binding sites labeled with [3H]SR142948A decreased by half in the caudate-putamen of MPTP-monkeys. In addition, the densities of [125I]NT and [3H]SR142948A binding sites markedly decreased (-77 and -63%, respectively) in the substantia nigra of MPTP-monkeys. Levocabastine did not compete with high affinity for [125I]NT binding in the monkey cingulate cortex, suggesting that only one class of NT receptors was labelled in the monkey brain. An extensive decrease of [3H]GBR12935 DAT binding sites (-92% vs. Control) was observed in the striatum of MPTP-monkeys and an important loss of DAT mRNA(-86% vs. Control) was observed in substantia nigra. Treatments for 1 month with either the D1 agonist SKF-82958 (3 mg/kg/day) or the D2 agonist cabergoline (0.25 mg/kg/day) had no effect on the lesion-induced decrease in NT and DAT binding sites or DAT mRNA levels. The decrease of striatal NT binding sites was less than expected from the decrease of DA content in this nucleus, suggesting only partial localization of NT receptors on nigrostriatal DAergic projections. These data also suggest that under severe DA denervation, treatment with D1 or D2 DA agonists does not modulate NT receptors and DAT density.

Dopamine D1 receptor mRNA and receptor levels in the striatum of MPTP monkeys chronically treated with SKF-82958

The density of dopamine D1 receptor antagonist sites was measured by autoradiography and dopamine D1 receptor mRNA levels were measured by in situ hybridization in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-exposed monkeys chronically treated with the dopamine D1 receptor agonist 6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benza zep ine hydrobromide (SKF-82958) administered in intermittent or continuous mode for a month. Normal and MPTP-exposed but otherwise untreated animals were used for comparison. Intermittent treatment with SKF-82958 relieved parkinsonian features and induced dyskinesias whereas given continuously this drug induced behavioral tolerance without dyskinesias. On the one hand, MPTP treatment tended to increase dopamine D1 receptor density in the putamen whereas treatment of MPTP monkeys with SKF-82958, intermittent or continuous, produced a significant increase compared to control animals. On the other hand, dopamine D1 receptor mRNA levels in the putamen appeared to decrease after MPTP lesion and agonist treatment as compared to dopamine D1 receptor density. In contrast, an apparent decrease in dopamine D1 receptor density and mRNA levels was observed in the nucleus accumbens of untreated MPTP monkeys whereas treatment of MPTP monkeys with SKF-82958, intermittent or continuous, produced a significant decrease compared to control animals. Thus, neither dyskinesias nor tolerance can be exclusively related to an increase or decrease in striatal dopamine D1 receptors, respectively.

Regulation by chronic treatment with cabergoline of dopamine D1 and D2 receptor levels and their expression in the striatum of Parkinsonian-monkeys

1. Chronic treatment for one month with the long-acting dopamine D2-like agonist cabergoline (0.25 mg/kg s.c. every 48 hours), had despite partial tolerance, sustained antiparkinsonian activity in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) Parkinsonian monkeys (Macaca fascicularis). 2. Cabergoline treatment decreased by half striatal D2 receptor binding density measured by [3H]spiperone autoradiography versus untreated MPTP monkeys. No change in D2 mRNA measured by in situ hybridization and D2 receptor immunostaining was observed. 3. No change in either D1 receptor binding density or D1 receptor mRNA levels was observed in cabergoline-treated MPTP-monkeys compared to untreated MPTP-monkeys, suggesting receptor subfamily specificity of cabergoline. 4. The present results suggest that the cabergoline-induced behavioral partial tolerance is accompanied by a decrease in D2 receptor binding but not due to alterations in the steady state of D2 mRNA levels.

Chronic Intracerebral Delivery of Trophic Factors via a Programmable Pump as a Treatment for Parkinsonism

The most common treatment for Parkinson's disease (PD) aims at pharmacologically augmenting striatal dopamine (DA) using the DA precursor levodopa. Such treatment provides symptomatic relief, but does not slow or halt continued degeneration of nigral dopaminergic neurons. Considerable effort has been devoted to the search for neurotrophic factors with survival-promoting activities on dopaminergic neurons that could potentially be of therapeutic value in the treatment of PD. One such candidate is glial cell line-derived neurotrophic factor (GDNF).

[Pathophysiology and therapy of L-Dopa-induced dyskinesia]

Involuntary movements, or dyskinesias, 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. The present review presents: 1) the current understanding of dyskinesia pathophysiology and 2) the therapeutic modalities, mainly non-dopaminergic, available or in development. We here show that the questions raised by the dyskinesia may have a clinically-driven pharmacological answer: the symptomatic treatment of dyskinesia, the prevention of the priming and the de-priming of the neural networks.

Validations of apomorphine-induced BOLD activation correlations in hemiparkinsonian rhesus macaques

Identification of Parkinson's disease at the earliest possible stage of the disease may provide the best opportunity for the use of disease modifying treatments. However, diagnosing the disease during the pre-symptomatic period remains an unmet goal. To that end, we used pharmacological MRI (phMRI) to assess the function of the cortico-basal ganglia circuit in a non-human primate model of dopamine deficiency to determine the possible relationships between phMRI signals with behavioral, neurochemical, and histological measurements. Animals with unilateral treatments with the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), that expressed stable, long-term hemiparkinsonism were challenged with the dopaminergic receptor agonist, apomorphine, and structure-specific phMRI blood oxygen level-dependent (BOLD) activation responses were measured. Behavioral, histopathological, and neurochemical measurements were obtained and correlated with phMRI activation of structures of the cortico-basal ganglia system. Greater phMRI activations in the basal ganglia and cortex were associated with slower movement speed, decreased daytime activity, or more pronounced parkinsonian features. Animals showed decreased stimulus-evoked dopamine release in the putamen and substantia nigra pars compacta and lower basal glutamate levels in the motor cortex on the MPTP-lesioned hemisphere compared to the contralateral hemisphere. The altered neurochemistry was significantly correlated with phMRI signals in the motor cortex and putamen. Finally, greater phMRI activations in the caudate nucleus correlated with fewer tyrosine hydroxylase-positive (TH⁺) nigral cells and decreased TH⁺ fiber density in the putamen. These results reveal the correlation of phMRI signals with the severity of the motor deficits and pathophysiological changes in the cortico-basal ganglia circuit.

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Apomorphine in hemiparkinsonian animals can evoke changes in functional MRI signals.

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Cortico-basal ganglia activation correlates to behavior, neurochemistry, histology

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Pharmacological MRI has potential to be biomarker for Parkinson's disease.