Primate model of Parkinson's disease: alterations in multiple opioid systems in the basal ganglia

Intranasal delivery of liposome encapsulated flavonoids ameliorates l-DOPA induced dyskinesia in hemiparkinsonian mice

In the present study, we explored the development of a novel noninvasive liposomal drug delivery material for use in intranasal drug delivery applications in human diseases. We used drug entrapment into liposomal nanoparticle assembly to efficiently deliver the drugs to the nasal mucosa to be delivered to the brain. The naturally occurring flavonoid 7,8-dihydroxyflavone (7,8-DHF) has previously been shown to have beneficial effects in ameliorating Parkinson's disease (PD). We used both naturally occurring 7,8-DHF and the chemically modified form of DHF, the DHF-ME, to be used as a drug candidate for the treatment of PD and l-DOPA induced dyskinesia (LID), which is the debilitating side effect of l-DOPA therapy in PD. The ligand-protein interaction behavior for 7,8-DHF and 6,7-DHF-ME was found to be more effective with molecular docking and molecular stimulation studies of flavonoid compounds with TrkB receptor. Our study showed that 7,8-DHF delivered via intranasal route using a liposomal formulation ameliorated LID in hemiparkinsonian mice model when these mice were chronically administered with l-DOPA, which is the only current medication for relieving the clinical symptoms of PD. The present study also demonstrated that apart from reducing the LID, 7,8-DHF delivery directly to the brain via the intranasal route also corrected some long-term signaling adaptations involving ΔFosB and α Synuclein in the brain of dopamine (DA) depleted animals.

Imaging mass spectrometry reveals elevated nigral levels of dynorphin neuropeptides in L-DOPA-induced dyskinesia in rat model of Parkinson's disease

L-DOPA-induced dyskinesia is a troublesome complication of L-DOPA pharmacotherapy of Parkinson's disease and has been associated with disturbed brain opioid transmission. However, so far the results of clinical and preclinical studies on the effects of opioids agonists and antagonists have been contradictory at best. Prodynorphin mRNA levels correlate well with the severity of dyskinesia in animal models of Parkinson's disease; however the identities of the actual neuroactive opioid effectors in their target basal ganglia output structures have not yet been determined. For the first time MALDI-TOF imaging mass spectrometry (IMS) was used for unbiased assessment and topographical elucidation of prodynorphin-derived peptides in the substantia nigra of a unilateral rat model of Parkinson's disease and L-DOPA induced dyskinesia. Nigral levels of dynorphin B and alpha-neoendorphin strongly correlated with the severity of dyskinesia. Even if dynorphin peptide levels were elevated in both the medial and lateral part of the substantia nigra, MALDI IMS analysis revealed that the most prominent changes were localized to the lateral part of the substantia nigra. MALDI IMS is advantageous compared with traditional molecular methods, such as radioimmunoassay, in that neither the molecular identity analyzed, nor the specific localization needs to be predetermined. Indeed, MALDI IMS revealed that the bioconverted metabolite leu-enkephalin-arg also correlated positively with severity of dyskinesia. Multiplexing DynB and leu-enkephalin-arg ion images revealed small (0.25 by 0.5 mm) nigral subregions with complementing ion intensities, indicating localized peptide release followed by bioconversion. The nigral dynorphins associated with L-DOPA-induced dyskinesia were not those with high affinity to kappa opioid receptors, but consisted of shorter peptides, mainly dynorphin B and alpha-neoendorphin that are known to bind and activate mu and delta opioid receptors. This suggests that mu and/or delta subtype-selective opioid receptor antagonists may be clinically relevant for reducing L-DOPA-induced dyskinesia in Parkinson's disease.

The 3R principle and the use of non-human primates in the study of neurodegenerative diseases: the case of Parkinson's disease

The aim of this paper is to offer an ethical perspective on the use of non-human primates in neurobiological studies, using the Parkinson's disease (PD) as an important case study. We refer, as theoretical framework, to the 3R principle, originally proposed by Russell and Burch [Russell, W.M.S., Burch, R.L., 1959. The Principles of Humane Experimental Technique. Universities Federation for Animal Welfare Wheathampstead, England (reprinted in 1992)]. Then, the use of non-human primates in the study of PD will be discussed in relation to the concepts of Replacement, Reduction, and Refinement. Replacement and Reduction result to be the more problematic concept to be applied, whereas Refinement offers relatively more opportunities of improvement. However, although in some cases the 3R principle shows its applicative limits, its value, as conceptual and inspirational tool remains extremely valuable. It suggests to the researchers a series of questions, both theoretical and methodological, which can have the results of improving the quality of life on the experimental models, the quality of the scientific data, and the public perception from the non-scientist community.

Differential gene expression in the striatum of mice with very low expression of the vesicular monoamine transporter type 2 gene

The vesicular monoamine transporter type 2 (VMAT2) packages pre-synaptic monoamines into vesicles. Previously, we generated mice hypomorphic for the VMAT2 gene (Slc18a2), which results in a approximately 95% reduction in VMAT2 protein, disrupted vesicular storage, severe depletion of striatal dopamine and mice with moderate motor behaviour deficits. Dopamine released from mid-brain dopamine neurons acts on post-synaptic type 1 (D1) and 2 (D2) receptors located on striatal medium spiny neurons to initiate a signalling cascade that leads to altered transcription factor activity, gene expression and neuronal activity. We investigated striatal gene expression changes in VMAT2hypo mice by quantitative real-time PCR and in situ hybridisation. Despite unaltered expression of D1 and D2 dopamine receptors, there were dramatic alterations in striatal mRNAs encoding the neuropeptides substance P, dynorphin, enkephalin and cholecystokinin. The promoters of these genes are regulated by a combination of transcription factors that includes cAMP responsive element binding protein-1 (CREB) and c-Fos. Indeed, the changes in peptide mRNAs were associated with elevated expression of Creb1 and c-Fos. These data indicate that striatal dopamine depletion, as a consequence of deficient vesicular storage in this mouse, triggers a complex program of gene expression, consistent with this mouse being an excellent model of Parkinson's disease.

Presynaptic modulation of synaptic transmission by opioid receptor in rat subthalamic nucleus in vitro

Presynaptic modulation of synaptic transmission in rat subthalamic nucleus (STN) neurons was investigated using whole-cell patch-clamp recordings in brain slices. Evoked GABAergic inhibitory postsynaptic currents (IPSCs) were reversibly reduced by methionine enkephalin (ME) with an IC(50) value of 1.1 +/- 0.3 microM. The action of ME was mimicked by the mu-selective agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO), and was partially blocked by the mu-selective antagonists naloxonazine and D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP). Evoked GABA(A) IPSCs were also inhibited by the delta-selective agonist [D-Pen(2,5)]-enkephalin (DPDPE), but not by the kappa-selective agonist (+)-(5 alpha,7 alpha,8 beta)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzeneacetamide (U-69593) and the orphan receptor agonist orphanin FQ/nociceptin (OFQ). DPDPE-induced inhibition was completely blocked by the delta-selective antagonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174,864). ME, DAMGO and DPDPE increased the paired-pulse ratio of IPSCs. Evoked excitatory postsynaptic currents (EPSCs) were reversibly reduced by ME with an IC(50) value of 0.35 +/- 0.14 microM. Inhibition by ME was associated with an increase in the paired-pulse ratio of EPSCs. The action of ME was mimicked by DAMGO, and blocked by naloxonazine. DPDPE had little effect on evoked EPSCs. Neither U-69593 nor OFQ had any effect. ME significantly decreased the frequency of spontaneous miniature EPSCs (mEPSCs) without change in their amplitude. The action of ME was mimicked by DAMGO. DPDPE had no effect. The presynaptic voltage-dependent potassium conductance blocker 4-aminopyridine (4-AP, 100 microM) abolished the inhibitory effects of ME on evoked IPSCs and EPSCs. In contrast, 4-AP only partially blocked the actions of baclofen. These results suggest that opioids inhibit inhibitory synaptic transmission in the STN through the activation of presynaptic mu- and delta- receptors. In contrast, inhibition of excitatory synaptic inputs to the STN occurs through the activation of only mu-receptors. Both inhibitions may be mediated by blockade of voltage-dependent potassium conductance.

Increase of preproenkephalin mRNA levels in the putamen of Parkinson disease patients with levodopa-induced dyskinesias

The expression of preproenkephalin messenger RNA was studied in the brain of Parkinson disease (PD) patients using in situ hybridization. All these patients were treated with levodopa (LD) and the development of motor complications was recorded. Eleven normal controls and 14 PD patients were used, of which 4 developed dyskinesias, 3 developed wearing-off, 3 developed both dyskinesias and wearing-off, and 4 developed no adverse effect following dopaminomimetic therapy. Nigrostriatal denervation was similar between the subgroups of PD patients as assessed using 125I-RTI-specific binding to the dopamine transporter and measures of catecholamine concentrations by HPLC. A significant increase of preproenkephalin messenger RNA levels was observed in the lateral putamen of dyskinetic patients in comparison to controls (+210%; p < 0.01) and in comparison to nondyskinetic patients (+112%; p < 0.05). No change was observed in medial parts of the putamen or in the caudate nucleus. No relationship between preproenkephalin messenger RNA levels and other clinical variables such as development of wearing-off, age of death, duration of disease, or duration of LD therapy was found. These findings suggest that increase synthesis of preproenkephalin in the medium spiny output neurons of the striatopallidal pathway play a role in the development of dyskinesias following long-term LD therapy in Parkinson disease.

Opiate receptor avidity is reduced bilaterally in rhesus monkeys unilaterally lesioned with MPTP

Opiate receptor avidity (unoccupied receptor density / the receptor dissociation constant), was measured in four animals with unilateral parkinsonian symptoms following MPTP (1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine) infusions into the internal carotid of one side, and nine normal controls with positron emission tomography (PET) and 6-deoxy-6-beta-[(18)F]fluoronaltrexone (cyclofoxy, CF), a mu- and kappa-opiate receptor antagonist. PET studies of 6-[(18)F]-L-fluoro-L-3,4-dihydroxyphenylalanine ([(18)F]-DOPA) in these parkinsonian animals, although documenting the primarily unilateral nature of the lesion, also demonstrated a milder loss of dopaminergic on the side opposite the infusion. Opiate receptor avidity was found to be reduced by 20-34% in the caudate, anterior putamen, thalamus, and amygdala of these primarily unilaterally MPTP-exposed animals, bilaterally with no statistically significant differences between the two sides. The affected regions are the same as those previously demonstrated to have a 30-35% loss in clinically recovered bilaterally MPTP-lesioned animals. These findings confirm that the opiate pathway can change in response to modest decreases in basal ganglia dopamine innervation. Thus, opiate pathway adaptation is likely to contribute to the dynamic changes in basal ganglia circuits that forestall the initial clinical manifestations of Parkinson's disease. In addition, opiate pathway(s) may contribute to the treatment responsiveness and progression of the disease either directly through effects on basal ganglia function or indirectly through effects on basal ganglia plasticity.

Opiate receptor avidity is reduced in non-motor impaired MPTP-lesioned rhesus monkeys

Opiate receptor avidity, roughly equivalent to the ratio of unoccupied receptor density to the receptor dissociation constant (B'max/KD), was measured in four MPTP (1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine)-lesioned rhesus monkeys and nine normal controls with positron emission tomography (PET) and 6-deoxy-6-beta-[18F]fluoronaltrexone (cyclofoxy, CF), a mu- and kappa-opiate receptor antagonist. Although the MPTP-lesioned monkeys were dopamine deficient as measured with [18F]-L-fluorodopa ([18F]-DOPA) and PET [Doudet et al., 6-[18F]-L-DOPA imaging of the dopamine neostriatal system in normal and clinically normal-MPTP-treated rhesus monkeys, Exp. Brain Res. 78 (1989) 69-80], they had clinically recovered from the acute motor effects of MPTP exposure. Opiate receptor avidity was found to be reduced by 30-35% in the opiate-receptor rich areas of caudate, anterior putamen, thalamus, and amygdala of the MPTP-lesioned animals. The results suggest that opiate pathways make a significant contribution to the adjustment of cortico-striatal-thalamic pathway activity and thereby to behavior in rhesus monkeys following dopamine loss.

Preproenkephalin mRNA expression in the caudate-putamen of MPTP monkeys after chronic treatment with the D2 agonist U91356A in continuous or intermittent mode of administration: comparison with L-DOPA therapy

The effect of chronic treatment with the D2 dopamine agonist U91356A or L-DOPA therapy on the regulation of preproenkephalin (PPE) mRNA was investigated in the caudate-putamen of previously drug-naive cynomolgus monkeys Macaca fascicularis rendered parkinsonian by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In MPTP monkeys, pulsatile treatment with either L-DOPA or U91356A relieved parkinsonian symptoms but caused progressive sensitization to treatment and, as expected, induced choreic dyskinesias. In contrast, U91356A given in a continuous mode led to partial behavioral tolerance without appearance of dyskinesias. Using in situ hybridization histochemistry, lesioning was shown to produce elevation of PPE mRNA levels in the lateral and medial parts of the putamen and in the lateral part of the caudate nucleus compared to control animals at the three rostrocaudal regions analyzed. In general, no change of PPE mRNA levels were observed in the medial caudate after MPTP lesioning with or without L-DOPA or U91356A treatments in the three rostrocaudal regions measured except for an increase in the caudal part of L-DOPA-treated MPTP monkeys. In the putamen and lateral caudate nucleus, elevated PPE mRNA expression by MPTP generally was not corrected (or only partially corrected) by chronic L-DOPA treatment except for the rostral medial putamen where correction to control values was observed. In general, pulsatile administration of U91356A partially corrected the lesion-induced elevation of PPE mRNA levels in the putamen and lateral caudate nucleus whereas the correction was more pronounced and widespread when MPTP monkeys received the continuous administration of this drug. These results indicate that the mode of administration of a D2 dopamine receptor agonist, such as U91356A, although at a roughly equivalent dosage influences the extent of inhibition of the expression of PPE in the denervated striatum of monkeys. In addition, the general lack of correction of the MPTP-induced increase of PPE mRNA in the striatum of L-DOPA-treated monkeys compared to the decreases observed with the D2 agonist treatments suggest that the D1 agonist component of L-DOPA therapy opposes the D2 agonist activity. Hence, D1 receptor agonist activity would stimulate PPE mRNA expression whereas D2 receptor agonists inhibit the expression of this peptide. Increases in PPE expression in the striatum may be implicated in the induction of dyskinesias since both groups of treated MPTP monkeys displaying dyskinesias had elevated striatal PPE mRNA levels whereas the MPTP monkeys with the lowest striatal PPE mRNA levels developed tolerance without dyskinesias.

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

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

Effects of unilateral 6-hydroxydopamine lesions on neuropeptide immunoreactivity in the basal ganglia of the common marmoset, Callithrix jacchus, a quantitative immunohistochemical analysis

Previous immunocytochemical studies in rats have indicated that striatal dopamine depletion leads to an increase in enkephalin-immunoreactivity and a decrease in substance P-immunoreactivity in the striatum. Similar studies in primates have lead to contradictory results. In the present study changes in tyrosine hydroxylase-, met-enkephalin- and substance P-immunoreactivity were determined in the basal ganglia of 6 common marmosets Callithrix jacchus following dopamine depletion by unilateral intracerebral 6-hydroxydopamine (6-OHDA) injections using three different survival times. The non-lesioned side served as an intra-individual control. Tyrosine hydroxylase immunoreactivity was strongly reduced in the entire ipsilateral striatum. Enkephalin-immunoreactivity was increased throughout the striatum. Substance P-immunoreactivity was significantly increased in only one case in the caudate nucleus and in two cases in the putamen, while in other cases either a non-significant increase or decrease was found. Therefore, the results of the present study indicate that in marmosets dopamine has a inhibiting effect on the levels of striatal enkephalin, while its effect on substance P (SP) appears to be absent.

L-DOPA reverses altered gene expression of substance P but not enkephalin in the caudate-putamen of common marmosets treated with MPTP

The mRNA levels encoding neuropeptides were measured in the caudate nucleus, putamen and nucleus accumbens of common marmosets exposed to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine pyridine (MPTP). Motor deficits induced by MPTP treatment were characterized by akinesia, postural abnormalities and rigidity. Seven days after MPTP treatment, there was a marked increase in levels of enkephalin mRNA in the caudate nucleus and putamen. In contrast, the hybridization signal for substance P mRNA was reduced. Alterations in the mRNA encoding neuropeptides were similar but less extensive in marmosets at 18-50 months following MPTP treatment. No significant changes in enkephalin or substance P mRNA in the nucleus accumbens were observed at either time. Treatment with L-DOPA plus carbidopa for 4 weeks reversed MPTP-induce motor deficits and other behavioural abnormalities. The decrease in substance P mRNA in the striatum of MPTP-treated animals was reversed by L-DOPA treatment and reached levels above those found in normal animals. In contrast, the increase in enkephalin mRNA in marmosets treated with MPTP was not altered by L-DOPA treatment. In the nucleus accumbens the levels of peptide mRNA were not affected by L-DOPA treatment. Loss of nigral dopamine cells in a primate species causes opposing alterations in the expression of enkephalin and substance P mRNA in the caudate nucleus and putamen. No changes were observed in the nucleus accumbens, which reflects the resistance of the mesolimbic neurons to MPTP toxicity. While the decrease in substance P mRNA was reversed by L-DOPA treatment, the increase in enkephalin mRNA was not. This may partly indicate the greater effect of L-DOPA on the direct GABA pathway compared to the indirect output pathway from the striatum.

The expression of proenkephalin and prodynorphin genes and the induction of c-fos gene by dopaminergic drugs are not altered in the straitum of MPTP-treated mice

The expression of proenkephalin (PENK), prodynorphin (PDYN) and c-fos genes was studied in the striatum of C57B1/6 mice treated with 1-methyl-4-phenyl-1,2,3,6,-tetrahydropyridine (MPTP), which are used as a rodent model of Parkinson's disease (PD). Two weeks after systemic administration of MPTP (2 x 40 mg/kg, s.c. 18h apart), the lesion of the substantia nigra (SN) could be visualised by loss of the nigral tyrosine hydroxylase (TH) mRNA hybridization signal and by a 91% decrease in striatal dopamine levels. The levels of PENK and PDYN mRNAs were not significantly changed in the striatum of the lesioned mice, as compared to non-treated controls. The induction of the immediate early gene c-fos by the dopamine D2 receptor antagonist haloperidol was not altered, while the selective D1 receptor agonist SKF 38393 failed to induce c-fos in the striatum of MPTP-treated mice. These results are in contrast to the data concerning rats with the 6-hydroxydopamine (6-OHDA) lesion of the SN, which serve as another rodent model of PD. In the striata of 6-OHDA-lesioned rats, PENK gene is upregulated, PDYN gene is down-regulated and the induction of c-fos gene by D2 receptor antagonists is abolished, whereas selective D1 receptor agonists induce c-fos gene, which does not occur in non-lesioned rats. We presume that the lack of influence of the MPTP lesion in mice on the striatal gene expression was mainly caused by insufficient dopamine depletion in the striatum, which could not be increased in this model. The importance of the changes observed in 6-OHDA-lesioned rats has been discussed in the context of the mouse and primate MPTP models of PD.

Preproenkephalin and preprotachykinin messenger RNA expression in normal human basal ganglia and in Parkinson's disease

Striatal expression of preproenkephalin and preprotachykinin messenger RNA was studied in normal controls and in patients with Parkinson's disease using in situ hybridization histochemistry. In controls, preproenkephalin messenger RNA was expressed in a population of medium-sized neurons of mean cross-sectional area 165 microns 2, accounting for 66% of striatal medium-sized neurons, whereas preprotachykinin messenger RNA was expressed in a population of medium-sized neurons of mean cross-sectional area 204 microns 2 (23% larger than those expressing enkephalin, P < 0.05), accounting for 58% of medium-sized striatal neurons. Much lower levels of both preproenkephalin messenger RNA and preprotachykinin messenger RNA were expressed by large neurons in the globus pallidus and substantia nigra reticulata. In addition, preproenkephalin messenger RNA was expressed at low levels by neurons in the subthalamic nucleus. In Parkinson's disease cases, there was a statistically significant increase in preproenkephalin messenger RNA expression in the body of the caudate (109% increase, P < 0.05) and in the intermediolateral putamen (55% increase, P < 0.05) due to an increase in the level of gene expression per neuron rather than an increase in the number of neurons expressing preproenkephalin messenger RNA. Similar increases were observed in other putaminal subregions and in the putamen as a whole, but these did not reach statistical significance. No change in preprotachykinin messenger RNA expression was detected. These findings demonstrate selective up-regulation of a striatal neuropeptide system in Parkinson's disease compatible with increased activity of the "indirect" striatopallidal pathway, which is thought to play a crucial role in the pathophysiology of akinesia and rigidity in this condition.

Morphine and naltrexone modulate D2 but not D1 receptor induced motor behavior in MPTP-lesioned monkeys

Interactions at the behavioral level between dopamine (DA) and opioid receptors in the mammalian brain have been amply demonstrated. Considering the pivotal role for DA receptors in the pharmacotherapy of Parkinson's disease (PD), these interactions might be clinically relevant. Therefore, in the present study the effects of the opioid antagonist naltrexone and agonist morphine on D1 and D2 receptor induced stimulation of motor behavior in the unilateral MPTP monkey model (n = 5) of PD were investigated. The results show that both naltrexone and morphine [0.1-1.0 mg/kg; intramuscular injection (IM)] inhibited D2 receptor stimulated contralateral rotational behavior and hand use induced by administration of quinpirole (LY 171555; 0.01 mg/kg, IM) in a dose-related way. However, no effects of these opioid drugs were observed on D1 receptor stimulated contralateral rotational behavior and hand use induced by administration of SKF 81297 (0.3 mg/kg, IM). Interestingly, the action of the alleged preferential mu-receptor antagonist naltrexone was mimicked by the selective delta-opioid antagonist naltrindole (0.5 mg/kg, IM). From this study it is concluded that in a non-human primate model of PD, alteration of opioid tonus leads to modulation of D2 receptor but not D1 receptor controlled motor behavior. The possible underlying mechanisms and clinical relevance of these findings are discussed.

Naltrexone attenuates the antiparkinsonian effects of picoTesla range magnetic fields

Extracranial treatment with magnetic fields (MF) in the picoTesla range has been shown an efficacious treatment modality in the management of Parkinsonism. The mechanisms by which such extremely weak MF improve Parkinsonian symptoms are unknown. As the pineal gland has been shown to function as a "magnetosensor" and since exposure to various intensities of MF disrupts melatonin secretion, it has been proposed that the beneficial effects of MF in Parkinsonism are partly mediated through the actions of pineal melatonin. Animal studies indicate that externally applied MF also influence the activity of the opioid peptides which have been implicated in a broad range of pathological conditions including Parkinsonism. To explore whether the beneficial effects of MF in Parkinsonism involve the mediation of the opioid systems and following informed consent, we administered the opiate receptor antagonist naltrexone (50 mg, P.O.) to a Parkinsonian patient after he showed improvement of symptoms with application of MF. Results of the trial showed that naltrexone partially reversed the antiparkinsonian effects of MF thus suggesting that opioid peptides are involved in mediating the clinical effects of these extremely weak MF in Parkinsonism. These results also suggest that intact opioid systems may be required for a full expression of the antiparkinsonian effect of picoTesla range MF.

Naltrexone, an opiate antagonist, fails to modify motor symptoms in patients with Parkinson's disease

One month of adjunct treatment with naltrexone (100 mg/day) was compared with placebo in a double-blind, randomized, cross-over design in two groups of patients with Parkinson's disease. The first group was composed of 10 patients with a moderate motor impairment insufficiently controlled by monotherapy with bromocriptine. The second group was composed of eight patients with L-dopa-induced peak-dose dyskinesia. Naltrexone as compared with placebo did not demonstrate any significant change in motor function in either group. These negative clinical results do not support a significant role of endogenous opioid systems in the pathophysiology of motor impairment in Parkinson's disease.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) effects on enkephalinergic neurons in various regions of mouse brain

In order to elucidate the effects of MPTP on enkephalinergic neurons, dopamine (DA), norepinephrine (NE), proenkephalin (PE) mRNA and met-enkephalin (ME) were measured in striatum, olfactory tubercle, and prefrontal cortex of C57/B16 mice 1 day-2 weeks following treatment with 96 mg/kg MPTP HCl (24 mg/kg i.p., twice/day for 2 days). DA and its metabolites were depleted 70% in striatum and 40% in olfactory tubercle within 1 day. In cortex, DA was unchanged, whereas homovanillic acid and NE were depleted 50 and 40% respectively by 3 days. ME increased in all three brain regions at different times whereas PE mRNA showed a different pattern in each region, with an increase in olfactory tubercle, a decrease in cortex, and in striatum, a decrease at 1 day followed by an increase at 3 days. Thus enkephalinergic neurons in each region respond differently to MPTP treatment. In striatum and olfactory tubercle. DA is depleted sufficiently to release its tonic inhibition on the enkephalinergic neurons, thereby leading to increased enkephalin synthesis. In cortex, the change in NE metabolism appears to cause a decrease of ME release and thereby a depression of PE synthesis. The possible relationship between these results and the changes observed in Parkinson's disease are discussed.

Chronic MPTP treatment reduces substance P and met-enkephalin content in the basal ganglia of the marmoset

Common marmosets were treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 1.25-2.5 mg/kg s.c., twice a week) for 5-10 consecutive months. The initial doses of MPTP produced a severe parkinsonian syndrome but motor activity was partially recovered at the end of treatment. Fifteen days or 6 months after the last MPTP dose, monkeys were sacrificed. In addition to a strong decrease of dopamine in the striatum, there were significant reductions in substance P and Met-enkephalin content in the substantia nigra, caudate nucleus and putamen. In the globus pallidus, the reduction in peptide levels did not reach statistical significance as compared to controls. Neurotensin levels were also decreased in the caudate nucleus. The chronic administration of MPTP for 5-10 months induces changes in substance P and Met-enkephalin systems which resemble the degeneration found in brains from parkinsonian patients.

Cellular basis for interactions between catecholaminergic afferents and neurons containing leu-enkephalin-like immunoreactivity in rat caudate-putamen nuclei

Dopaminergic afferents to the dorsal striatum, caudate-putamen nuclei, are known to modulate the levels and synthesis of endogenous opiate peptides (Leu5 and Met5-enkephalins). We examined the dual immunocytochemical localization of antisera raised against Leu5-enkephalin and the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), to determine the cellular substrates for these and/or other functional interactions. The antisera were identified by combined immunogold-silver and immunoperoxidase labeling in single coronal sections through the caudate-putamen nuclei of adult rats. These animals were given intraventricular injections of colchicine, and the brains were fixed by acrolein perfusion prior to immunocytochemical labeling. By light microscopy, perikarya and processes containing enkephalin-like immunoreactivity (ELI) were seen in close proximity to varicose processes immunoreactive for TH. Electron microscopy further demonstrated that the ELI was localized to perikarya, dendrites, and axon terminals, whereas the TH was exclusively in axons and terminals. The dendrites containing ELI were postsynaptic to terminals that were either (1) without detectable immunoreactivity, or (2) immunoreactive for TH or enkephalin. Nonsynaptic portions of the dendrites containing ELI were covered with astrocytic processes or were in direct apposition to unlabeled dendrites. Terminals containing ELI were densely immunoreactive and were in direct contact with (1) unlabeled and occasionally enkephalin-labeled proximal dendrites, and (2) TH-labeled and unlabeled terminals. In comparison with the opiate terminals, most catecholaminergic terminals were lightly immunoreactive for TH and usually contacted more distal unlabeled dendrites or spines and, more rarely, dendrites containing ELI. In a few favorable planes of section, the terminals containing ELI and those containing TH (1) converged on common unlabeled dendrites, or (2) formed dual contacts on two different labeled or unlabeled targets. Junctions formed by terminals containing ELI and TH were sometimes characterized by symmetric synaptic densities. However, numerous other dendritic and all axonal appositions were without recognized membrane densities. The findings of the study provide anatomical substrates for multilevel interactions between catecholamines, mostly dopamine, and enkephalin in rat dorsal striatum. These include (1) monosynaptic input from dopaminergic terminals to neurons containing enkephalin, (2) presynaptic modulation of transmitter release through axonal appositions, and (3) dual regulation of common targets through convergent input. In addition, the findings suggest that both enkephalin and dopamine may have similar modulatory roles in synchronizing the activity of dual targets postsynaptic to individual axon terminals. Alterations in any one of these multiple types of interactions could account for noted motor or sensory symptoms in neurological disorders characterized by depletion of dopamine or endogenous opiate peptides, or both.

Decreased levels of [Met]enkephalin, neuropeptide Y, substance P, and vasoactive intestinal peptide in parkinsonian adrenal medulla

Adrenal medullary tissue was collected from parkinsonian patients at autopsy and at the time of autologous transplantation of the adrenal medulla to the caudate nucleus, and from nonparkinsonian patients at autopsy and during nephrectomy. Levels of the following neuropeptides were measured by radioimmunoassay in samples of the medullary tissue: neuropeptide Y (NPY), substance P (SP), [Met]enkephalin ([Met]ENK), vasoactive intestinal peptide (VIP), peptide YY, and bombesin-like immunoreactivity. Regression analysis was used to establish a relationship between patient age, time to organ harvest, and peptide levels in nonparkinsonian tissue. Levels of [Met]ENK, VIP, NPY, and SP were significantly lower in parkinsonian adrenal medullae than that predicted from the control group. These results suggest that the adrenal medulla of a parkinsonian patient is severely compromised, either by the disease process itself or by the antiparkinsonian medications used to treat the symptoms of the disease.

Opioid peptides in Parkinson's disease: effects of dopamine repletion

Neurotransmitters other than dopamine, including neuropeptides, could have important pathophysiologic and therapeutic roles in Parkinson's disease. Both Met-enkephalin, the main transmitter of the striatopallidal pathway, and dynorphin, one of the co-transmitters of the striatonigral pathway display complex anatomic and biochemical interactions with the basal ganglionic dopamine system. In this study, the cerebrospinal fluid content of a proenkephalin derivative, Met5 enkephalin-Arg6-Gly7-Leu8 (MERGL), was found in significantly low concentrations in parkinsonian patients following overnight withdrawal of all medications compared with control subjects, and failed to change after at least 16 h of steady-state, optimal doses of levodopa infusion intravenously. MERGL levels increased with advancing age among normal individuals but not among patients with Parkinson's disease. In contrast dynorphin A(1-8) levels were not different between the two study groups, did not change with levodopa therapy, and failed to correlate with age or any indices of disease progression. These observations, consistent with post-mortem studies on Parkinson brains and contrary to findings in animal models of Parkinsonism, suggest that abnormality of the enkephalin system in this disease is due to involvement of these striatal neurons in the primary pathologic process.

Alterations in opiate receptor binding in MPTP-induced hemiparkinsonian monkeys

Quantitative autoradiography was used to study [3H]naloxone binding in the striatum of normal monkeys and monkeys made hemiparkinsonian by the unilateral infusion of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The density of [3H]naloxone binding sites was significantly higher in the caudate and putamen on the MPTP-treated side of hemiparkinsonian monkeys, as compared with binding on the untreated side and in the striatum of normal monkeys. A more extensive patchy distribution of binding sites was evident throughout the striatum on the MPTP-treated side than seen in the striatum of the untreated side or in normal striatum.

Neuropeptide levels in the basal ganglia of aged common marmosets following prolonged treatment with MPTP

Aged common marmosets were treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 0.5-2.0 mg/kg/week i.p.) for 16 or 24 weeks, observed for a total of 30 weeks and then killed for measurement of biochemical parameters in basal ganglia. The MPTP treatment induced a marked depletion in dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid levels in the caudate nucleus and putamen. In contrast, the concentrations of five neuropeptides: [Met5]-enkephalin, [Leu5]-enkephalin, cholecystokinin, substance P and neurotensin as measured by a combined HPLC/RIA method, remained unaltered in all basal ganglia regions examined. Enkephalin precursor levels, as reflected by cryptic [Met5]-enkephalin content, were increased in the putamen, but not in the caudate nucleus, as a consequence of MPTP administration. Cryptic [Leu5]-enkephalin content remained unchanged in the striatum of MPTP treated marmosets. Overall, these results suggest an increase in striatal [Met5]-enkephalin release following chronic MPTP treatment of aged marmosets. However, the chronic treatment of aged marmosets with MPTP does not reproduce the neuropeptide alterations characteristic of Parkinson's disease.

Preproenkephalin mRNA and methionine-enkephalin increase in mouse striatum after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment

Dopaminergic neurons that project to the striatum from the substantia nigra are thought to modulate methionine-enkephalin (Met-Enk) metabolism in the striatum. We administered a dose of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) that produces a moderate depletion of dopamine in striatum, about 50%, without overt motor deficits, and found that Met-Enk-like immunoreactivity and preproenkephalin mRNA content increased in the tissue. Pretreatment with the monoamine oxidase B inhibitor deprenyl or the dopamine transport blocker nomifensine prevented these changes, suggesting that the changes were related to the partial loss of dopaminergic neurons rather than to MPTP. Moreover, administering GM1 ganglioside, which partially restores the MPTP-induced dopaminergic deficit, partially corrected the Met-Enk changes in the striatum as well. These findings are consistent with the hypothesis that dopaminergic input to the striatum, in part, modulates Met-Enk metabolism. Moreover, they show that moderate nigrostriatal lesions are sufficient to elevate Met-Enk and preproenkephalin mRNA contents and that restoration of dopaminergic function, as in our studies with GM1 ganglioside, restores the content of Met-Enk.

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurotoxicity in the rat: characterization and age-dependent effects

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a potent dopaminergic toxin that has been found to produce Parkinson's disease-like symptoms in humans and monkeys. The neurotoxic effects of MPTP appear to be reduced in rodents where multiple dosing procedures are required to demonstrate long-lasting neuronal deficits. In the present study, the neurotoxic effects of MPTP were further characterized in the rat. Following the repeated administration of MPTP, pronounced (60-80%) and dose-dependent depletions of striatal dopamine and serotonin concentrations were found in the rat brain. Time-course studies revealed that while striatal dopamine concentrations remained consistently reduced for at least 8 weeks following MPTP treatment, striatal serotonin depletions as well as MPTP-induced monoamine depletions in other brain regions were transient in nature. Pretreatment with the MAO-B inhibitor pargyline afforded a selective and complete protection of striatal dopamine levels without significantly affecting MPTP-induced striatal serotonin depletions. Similarly, treatment with ascorbic acid was found to selectively attenuate MPTP-induced dopamine depletions in rats. The neurotoxic effects of MPTP were also found to increase in the developing rat. No significant brain monoamine depletions were observed in neonatal rats following the repeated administration of MPTP. However, MPTP-induced neurotoxicity progressively increased in older rats. The present results indicate that when appropriate treatment procedures are used, a pronounced, selective, age-dependent, and long-lasting MPTP-induced reduction in striatal dopamine concentrations can be observed in the rat brain. The present results are discussed in reference to the putative mechanisms and species differences of MPTP-induced neurotoxicity.

[Neural and ethological relations in the evaluation of motor control changes. II. Experimental models]

As a tool for the study of normal and pathological motor manifestations, the experimental models of motor alterations are discussed. Here, we present particularly the neural and ethological characteristics of experimental models as hemiplegia, akinesia, parkinsonism and epilepsy.

Medial forebrain bundle stimulation or D-2 dopamine receptor activation increases preproenkephalin mRNA in rat striatum

Electrical stimulation of the medial forebrain bundle, in a manner that augmented the release of dopamine in the forebrain, rapidly increased the striatal content of preproenkephalin (but not preprotachykinin) mRNA. This effect was mimicked by administration of either the indirect (dopamine-releasing) agonist methamphetamine or by the D-2 dopamine receptor agonist quinpirole, but not by the D-1 agonist SKF 38393. These data suggest that D-2 receptors, which mediate a stimulatory effect on enkephalin gene expression, may be subsaturated under basal conditions and, therefore, responsive to increases in synaptic dopamine.

Levels of Met-enkephalin, Leu-enkephalin, substance P and cholecystokinin in the brain of the common marmoset following long term 1-methyl-4-phenyl-1,2,3,6,-tetrahydropyridine treatment

Common marmosets were treated daily with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 7-9 mg/kg i.p.) for 25 days, and then kept out of drug for three months before biochemical measurements in various brain areas. This treatment induced a dramatic fall (-80%) in dopamine, homovanillic acid and dihydroxyphenylacetic acid levels in the putamen and caudate nucleus, and a significant but less pronounced reduction (less than or equal to 50%) in the levels of these compounds in the nucleus accumbens. In contrast, the concentrations of four neuropeptides: met-enkephalin, leu-enkephalin, substance P, and cholecystokinin, remained unaltered in all brain areas examined in MPTP-treated marmosets. Therefore the neuropeptide alterations previously reported in Parkinson's disease are probably not secondary to the severe lesion of dopaminergic neurones, but constitute another intrinsic feature of the disease.

An immunohistochemical study of the acute and long-term effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in the marmoset

Administration of the drug 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine induces a parkinsonian syndrome in primates. Intraperitoneal injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in the common marmoset (Callithrix jacchus) produced symptoms of rigidity, akinesia and tremor which persisted for at least one month. However, after this time, considerable behavioural recovery occurred, although animals were still severely bradykinetic compared with controls. Marmosets were allowed to survive for 1, 3 1/2 or 7 months prior to histological and immunocytochemical analysis. Detection of catecholaminergic neurons using antibodies directed against the enzyme tyrosine hydroxylase revealed a profound (80%) loss of dopaminergic cells from the substantia nigra one month after initiation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment. This was accompanied by a severe gliosis. Fewer cells were lost from the adjacent ventral tegmental area (45%), but dopamine-containing cells in other brain areas were not obviously affected. At longer survival times the substantia nigra was less damaged, with a proliferation of glia in the pars compacta and a loss of approximately 20% of the dopaminergic perikarya. Using immunohistochemical techniques, the distribution of neuropeptides substance P, [Met]enkephalin and dynorphin 1-17-like immunoreactivity were examined and found to exhibit distinctive patterns in the marmoset substantia nigra. The integrity of these systems appeared intact at all times after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment. These results support the hypothesis that the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine produces a clinical syndrome, indistinguishable from Parkinson's disease, via a selective destruction only of neurons with perikarya in the substantia nigra pars compacta and the ventral tegmental area. The findings that the peptidergic input to these cells together with most non-nigral dopaminergic cell groups are not damaged, indicate that the selectivity of the lesion produced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine appears greater than that seen in idiopathic Parkinson's disease. The neurotoxic effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in the marmoset may not be permanent since both behavioural and biochemical recovery were observed after several months.

Effects of MPTP poisoning on central somatostatin and substance P levels in the mouse

The effects of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) on the levels of the neuropeptides somatostatin and substance P, were examined in various brain regions of C57 mice. Two weeks after injections of MPTP (2 X 30, 2 X 40 and 2 X 50 mg/kg i.p.) a dose-dependent decrease in striatal catecholamine levels was observed. There was also a dose-dependent increase in nigral somatostatin immunoreactivity and no reduction in striatonigral substance P levels. These results are in contrast with the changes observed in peptide levels in post-mortem Parkinson's brains.

Somatostatin immunoreactivity is reduced in Parkinson's disease dementia with Alzheimer's changes

Somatostatin-like immunoreactivity (SLI) was measured in postmortem brain tissue from 15 control patients, 7 non-demented parkinsonian patients and 7 demented parkinsonian patients who had Alzheimer-type cortical pathology. The non-demented parkinsonian patients had normal concentrations of SLI in the cerebral cortex, hippocampus, amygdala, putamen, caudate or globus pallidus. Demented parkinsonian patients with Alzheimer-type cortical pathology had significantly reduced (approximately 40%) levels of SLI in both the frontal (Brodmann area 6) and temporal (Brodmann area 21) cortex. These findings suggest that parkinsonian dementia with Alzheimer-type pathology like Alzheimer's disease itself, is associated with reduced concentrations of cortical somatostatin.

Neuropeptides in neurological disease

Neuropeptides are widely distributed in the central nervous system, where they serve as neuroregulators. Recent interest has focused on their role in degenerative neurological diseases. We describe the normal anatomy of neuropeptides in both the cerebral cortex and basal ganglia as a framework for interpreting neuropeptide alterations in Alzheimer's disease (AD), Huntington's disease, and Parkinson's disease. Concentrations of cortical somatostatin are reduced in AD and in dementia associated with Parkinson's disease. Concentrations of neuropeptide Y and corticotropin-releasing factor are also reduced in AD cerebral cortex. The reduced cortical concentrations of somatostatin and neuropeptide Y in AD cerebral cortex may reflect a loss of neurons or terminals in which these two peptides are co-localized. In Huntington's disease, basal ganglia neurons in which somatostatin and neuropeptide Y are co-localized are selectively preserved. Cerebrospinal fluid concentrations of neuropeptides in AD reflect alterations in cortical concentrations. Improved understanding of neuropeptides in degenerative neurological illnesses will help define which neuronal populations are specifically vulnerable to the pathological processes, and this could lead to improved therapy.

Lack of change in basal ganglia neuropeptide content following subacute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment of the common marmoset

Treatment of common marmosets with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 1-4 mg/kg for up to 4 days) caused a profound parkinsonian state. Ten days from the start of MPTP treatment, all animals showed marked motor impairment, consisting of bradykinesia and akinesia, limb rigidity, postural abnormalities, loss of vocalisation and blink reflex, and, on occasions, postural tremor. Measurement of caudate-putamen monoamine content at this time showed a profound loss in 3,4-dihydroxyphenylethylamine, homovanillic acid, and 3,4-dihydroxyphenylacetic acid concentrations. Measurement of neuropeptide concentrations in the caudate-putamen, internal and external segments of the globus pallidus, nucleus accumbens, substantia nigra, frontal cortex, and hippocampus showed met-enkephalin, leu-enkephalin, and cholecystokinin (CCK-8) concentrations to be unaffected by MPTP treatment. There was a small decrease in the substance P content of frontal cortex, but otherwise the content of this neuropeptide was unaltered. Parkinsonism in the marmoset, induced by MPTP treatment 10 days earlier, does not alter neuropeptide concentrations in the manner observed in Parkinson's disease.