Evidence for a long Leu-enkephalin striopallidal pathway in rat brain

Antagonism of kappa opioid receptors accelerates the development of L-DOPA-induced dyskinesia in a preclinical model of moderate dopamine depletion

Levels of the opioid peptide dynorphin, an endogenous ligand selective for kappa-opioid receptors (KORs), its mRNA and pro-peptide precursors are differentially dysregulated in Parkinson's disease (PD) and following the development of l-DOPA-induced dyskinesia (LID). It remains unclear whether these alterations contribute to the pathophysiological mechanisms underlying PD motor impairment and the subsequent development of LID, or whether they are part of compensatory mechanisms. We sought to investigate nor-BNI, a KOR antagonist, 1) in the dopamine (DA)-depleted PD state, 2) during the development phase of LID, and 3) via measuring of tonic levels of striatal DA. While nor-BNI (3 mg/kg; s.c.) did not lead to functional restoration in the DA-depleted state, it affected the dose-dependent development of abnormal voluntary movements (AIMs) in response to escalating doses of l-DOPA in a rat PD model with a moderate striatal 6-hydroxdopamine (6-OHDA) lesion. We tested five escalating doses of l-DOPA (6, 12, 24, 48, 72 mg/kg; i.p.), and nor-BNI significantly increased the development of AIMs at the 12 and 24 mg/kg l-DOPA doses. However, after reaching the 72 mg/kg l-DOPA, AIMs were not significantly different between control and nor-BNI groups. In summary, while blocking KORs significantly increased the rate of development of LID induced by chronic, escalating doses of l-DOPA in a moderate-lesioned rat PD model, it did not contribute further once the overall severity of LID was established. While we observed an increase of tonic DA levels in the moderately lesioned dorsolateral striatum, there was no tonic DA change following administration of nor-BNI.

Antagonism of kappa opioid receptors accelerates the development of L-DOPA-induced dyskinesia in a preclinical model of moderate dopamine depletion

Levels of the opioid peptide dynorphin, an endogenous ligand selective for kappa-opioid receptors (KORs), its mRNA and pro-peptide precursors are differentially dysregulated in Parkinson disease (PD) and following the development of L-DOPA-induced dyskinesia (LID). It remains unclear, whether these alterations contribute to the pathophysiological mechanisms underlying PD motor impairment and the subsequent development of LID, or whether they are part of compensatory mechanisms. We sought to investigate nor-BNI, a KOR antagonist, 1) in the dopamine (DA)-depleted PD state, 2) during the development phase of LID, and 3) with measuring tonic levels of striatal DA. Nor-BNI (3 mg/kg; s.c.) did not lead to functional restoration in the DA-depleted state, but a change in the dose-dependent development of abnormal voluntary movements (AIMs) in response to escalating doses of L-DOPA in a rat PD model with a moderate striatal 6-hydroxydopamine (6-OHDA) lesion. We tested five escalating doses of L-DOPA (6, 12, 24, 48, 72 mg/kg; i.p.), and nor-BNI significantly increased the development of AIMs at the 12 and 24 mg/kg L-DOPA doses. However, after dosing with 72 mg/kg L-DOPA, AIMs were not significantly different between control and nor-BNI groups. In summary, while blocking KORs significantly increased the rate of development of LID induced by chronic, escalating doses of L-DOPA in a moderate-lesioned rat PD model, it did not contribute further once the overall severity of LID was established. While we saw an increase of tonic DA levels in the moderately lesioned dorsolateral striatum, there was no tonic DA change following administration of nor-BNI.

The Delta-Specific Opioid Glycopeptide BBI-11008: CNS Penetration and Behavioral Analysis in a Preclinical Model of Levodopa-Induced Dyskinesia

In previous work we evaluated an opioid glycopeptide with mixed μ/δ-opioid receptor agonism that was a congener of leu-enkephalin, MMP-2200. The glycopeptide analogue showed penetration of the blood-brain barrier (BBB) after systemic administration to rats, as well as profound central effects in models of Parkinson's disease (PD) and levodopa (L-DOPA)-induced dyskinesia (LID). In the present study, we tested the glycopeptide BBI-11008 with selective δ-opioid receptor agonism, an analogue of deltorphin, a peptide secreted from the skin of frogs (genus Phyllomedusa). We tested BBI-11008 for BBB-penetration after intraperitoneal (i.p.) injection and evaluated effects in LID rats. BBI-11008 (10 mg/kg) demonstrated good CNS-penetrance as shown by microdialysis and mass spectrometric analysis, with peak concentration levels of 150 pM in the striatum. While BBI-11008 at both 10 and 20 mg/kg produced no effect on levodopa-induced limb, axial and oral (LAO) abnormal involuntary movements (AIMs), it reduced the levodopa-induced locomotor AIMs by 50% after systemic injection. The N-methyl-D-aspartate receptor antagonist MK-801 reduced levodopa-induced LAO AIMs, but worsened PD symptoms in this model. Co-administration of MMP-2200 had been shown prior to block the MK-801-induced pro-Parkinsonian activity. Interestingly, BBI-11008 was not able to block the pro-Parkinsonian effect of MK-801 in the LID model, further indicating that a balance of mu- and delta-opioid agonism is required for this modulation. In summary, this study illustrates another example of meaningful BBB-penetration of a glycopeptide analogue of a peptide to achieve a central behavioral effect, providing additional evidence for the glycosylation technique as a method to harness therapeutic potential of peptides.

Cocaine Self-administration Regulates Transcription of Opioid Peptide Precursors and Opioid Receptors in Rat Caudate Putamen and Prefrontal Cortex

The brain opioid system plays an important role in cocaine reward. Altered signaling in the opioid system by chronic cocaine exposure contributes to cocaine-seeking and taking behavior. The current study investigated concurrent changes in the gene expression of multiple components in rat brain opioid system following cocaine self-administration. Animals were limited to 40 infusions (1.5 mg/kg/infusion) within 6 h per day for five consecutive days. We then examined the mRNA levels of opioid receptors including mu (Oprm), delta (Oprd), and kappa (Oprk), and their endogenous opioid peptide precursors including proopiomelanocortin (Pomc), proenkephalin (Penk), prodynorphin (Pdyn) in the dorsal striatum (CPu) and the prefrontal cortex (PFC) 18 h after the last cocaine infusion. We found that cocaine self-administration significantly increased the mRNA levels of Oprm and Oprd in both the CPu and PFC, but had no effect on Oprk mRNA levels in either brain region. Moreover, cocaine had a greater influence on the mRNA levels of opioid peptide precursors in rat CPu than in the PFC. In the CPu, cocaine self-administration significantly increased the mRNA levels of Penk and Pdyn and abolished the mRNA levels of Pomc. In the PFC, cocaine self-administration only increased Pdyn mRNA levels without changing the mRNA levels of Pomc and Penk. These data suggest that cocaine self-administration influences the expression of multiple genes in the brain opioid system, and the concurrent changes in these targets may underlie cocaine-induced reward and habitual drug-seeking behavior.

The combination of the opioid glycopeptide MMP-2200 and a NMDA receptor antagonist reduced l-DOPA-induced dyskinesia and MMP-2200 by itself reduced dopamine receptor 2-like agonist-induced dyskinesia

Dopamine (DA)-replacement therapy utilizing l-DOPA is the gold standard symptomatic treatment for Parkinson's disease (PD). A critical complication of this therapy is the development of l-DOPA-induced dyskinesia (LID). The endogenous opioid peptides, including enkephalins and dynorphin, are co-transmitters of dopaminergic, GABAergic, and glutamatergic transmission in the direct and indirect striatal output pathways disrupted in PD, and alterations in expression levels of these peptides and their precursors have been implicated in LID genesis and expression. We have previously shown that the opioid glycopeptide drug MMP-2200 (a.k.a. Lactomorphin), a glycosylated derivative of Leu-enkephalin mediates potent behavioral effects in two rodent models of striatal DA depletion. In this study, the mixed mu-delta agonist MMP-2200 was investigated in standard preclinical rodent models of PD and of LID to evaluate its effects on abnormal involuntary movements (AIMs). MMP-2200 showed antiparkinsonian activity, while increasing l-DOPA-induced limb, axial, and oral (LAO) AIMs by ∼10%, and had no effect on dopamine receptor 1 (D1R)-induced LAO AIMs. In contrast, it markedly reduced dopamine receptor 2 (D2R)-like-induced LAO AIMs. The locomotor AIMs were reduced by MMP-2200 in all three conditions. The N-methyl-d-aspartate receptor (NMDAR) antagonist MK-801 has previously been shown to be anti-dyskinetic, but only at doses that induce parkinsonism. When MMP-2200 was co-administered with MK-801, MK-801-induced pro-parkinsonian activity was suppressed, while a robust anti-dyskinetic effect remained. In summary, the opioid glycopeptide MMP-2200 reduced AIMs induced by a D2R-like agonist, and MMP-2200 modified the effect of MK-801 to result in a potent reduction of l-DOPA-induced AIMs without induction of parkinsonism.

Opioidergic Modulation of Striatal Circuits, Implications in Parkinson's Disease and Levodopa Induced Dyskinesia

The functional organization of the dorsal striatum is complex, due to the diversity of neural inputs that converge in this structure and its subdivision into direct and indirect output pathways, striosomes and matrix compartments. Among the neurotransmitters that regulate the activity of striatal projection neurons (SPNs), opioid neuropeptides (enkephalin and dynorphin) play a neuromodulatory role in synaptic transmission and plasticity and affect striatal-based behaviors in both normal brain function and pathological states, including Parkinson's disease (PD). We review recent findings on the cell-type-specific effects of opioidergic neurotransmission in the dorsal striatum, focusing on the maladaptive synaptic neuroadaptations that occur in PD and levodopa-induced dyskinesia. Understanding the plethora of molecular and synaptic mechanisms underpinning the opioid-mediated modulation of striatal circuits is critical for the development of pharmacological treatments that can alleviate motor dysfunctions and hyperkinetic responses to dopaminergic stimulant drugs.

CNS penetration of the opioid glycopeptide MMP-2200: a microdialysis study

Endogenous opioid peptides enkephalin and dynorphin are major co-transmitters of striatofugal pathways of the basal ganglia. They are involved in the genesis of levodopa-induced dyskinesia and in the modulation of direct and indirect striatal output pathways that are disrupted in Parkinson's disease. One pharmacologic approach is to develop synthetic glycopeptides closely resembling endogenous peptides to restore their normal functions. Glycosylation promotes penetration of the blood-brain barrier. We investigated CNS penetration of the opioid glycopeptide MMP-2200, a mixed δ/μ-agonist based on leu-enkephalin, as measured by in vivo microdialysis and subsequent mass spectrometric analysis in awake, freely moving rats. The glycopeptide (10 mg/kg) reaches the dorsolateral striatum (DLS) rapidly after systemic (i.p.) administration and is stably detectable for the duration of the experiment (80 min). The detected level at the end of the experiment (around 250 pM) is about 10-fold higher than the level of the endogenous leu-enkephalin, measured simultaneously. This is one of the first studies to directly prove that glycosylation of an endogenous opioid peptide leads to excellent blood-brain barrier penetration after systemic injection, and explains robust behavioral effects seen in previous studies by measuring how much glycopeptide reaches the target structure, in this case the DLS.

Effects of the novel glycopeptide opioid agonist MMP-2200 in preclinical models of Parkinson's disease

In Parkinson's disease (PD), the consequence of dopaminergic denervation is an imbalance in the activity of the direct and indirect striatofugal pathways, which include potentially important changes in opioid peptide expression and/or activity. The systemic administration of a novel glycosylated opioid peptide MMP-2200 (a.k.a. lactomorphin) was shown to have potent effects in two standard models of PD: 1) amphetamine-induced rotations in the hemi-Parkinsonian 6-hydroxydopamine (6-OHDA)-treated rat and 2) locomotion in the reserpine-treated rat. MMP-2200, an opioid mu and delta receptor agonist, reduced amphetamine-induced rotations in severely-lesioned hemi-Parkinsonian rats; this effect was fully blocked by naloxone, an opioid receptor antagonist. The selective δ-opioid receptor antagonist naltrindole only partially blocked the effect of MMP-2200. MMP-2200 alone did not induce rotations. This effect was also observed in a mild progressive rat 6-OHDA-lesion model. In animals treated with reserpine, profound akinesia was induced that was reversed with apomorphine. There was a prominent overshoot in animals that received apomorphine compared to non-reserpine treated animals, reflecting the well described phenomenon of dopamine supersensitivity indicating that apomorphine not only reversed akinesia but also induced hyper-kinesia. The opioid peptide MMP-2200 blocked the apomorphine-induced hyper-kinesia. This effect of MMP-2200 was prevented by pre-administration of naloxone. MMP-2200 had no effect in preventing the reserpine-induced akinesia, nor did it affect locomotion in control animals. Taken together, the results from these two models are consistent with the glycopeptide opioid agonist MMP-2200 having a potent effect on movements related to dopaminergic hyper-stimulation following striatal dopamine depletion that are best explained by a reduction in the downstream effects of dopamine agonists in these models.

Microdialysis and mass spectrometric monitoring of dopamine and enkephalins in the globus pallidus reveal reciprocal interactions that regulate movement

Pallidal dopamine, GABA and the endogenous opioid peptides enkephalins have independently been shown to be important controllers of sensorimotor processes. Using in vivo microdialysis coupled to liquid chromatography-mass spectrometry and a behavioral assay, we explored the interaction between these three neurotransmitters in the rat globus pallidus. Amphetamine (3 mg/kg i.p.) evoked an increase in dopamine, GABA and methionine/leucine enkephalin. Local perfusion of the dopamine D(1) receptor antagonist SCH 23390 (100 μM) fully prevented amphetamine stimulated enkephalin and GABA release in the globus pallidus and greatly suppressed hyperlocomotion. In contrast, the dopamine D(2) receptor antagonist raclopride (100 μM) had only minimal effects suggesting a greater role for pallidal D(1) over D(2) receptors in the regulation of movement. Under basal conditions, opioid receptor blockade by naloxone perfusion (10 μM) in the globus pallidus stimulated GABA and inhibited dopamine release. Amphetamine-stimulated dopamine release and locomotor activation were attenuated by naloxone perfusion with no effect on GABA. These findings demonstrate a functional relationship between pallidal dopamine, GABA and enkephalin systems in the control of locomotor behavior under basal and stimulated conditions. Moreover, these findings demonstrate the usefulness of liquid chromatography-mass spectrometry as an analytical tool when coupled to in vivo microdialysis.

Molecular profiling of striatonigral and striatopallidal medium spiny neurons past, present, and future

Defining distinct molecular properties of the two striatal medium spiny neurons (MSNs) has been a challenging task for basal ganglia (BG) neuroscientists. Identifying differential molecular components in each MSN subtype is crucial for BG researchers to understand functional properties of these two neurons. The two MSN populations are morphologically identical except in their projections through the direct verses indirect BG pathways and they are heterogeneously dispersed throughout the dorsal striatum (dStr) and nucleus accumbens (NAc). These characteristics have made it difficult for researchers to distinguish and isolate these two neuronal populations thereby hindering progress toward a more comprehensive understanding of their differential molecular properties. Researchers began to investigate molecular differences in the striatonigral and striatopallidal neurons using in situ hybridization (ISH) techniques and single cell reverse transcription-polymerase chain reaction (scRT-PCR). Currently the field is utilizing more advanced techniques for large-scale gene expression studies including fluorescence activated cell sorting (FACS) of MSNs, from which RNA is purified, from fluorescent reporter transgenic mice or use of transgenic mice in which ribosomes from each MSN are tagged and can be immunoprecipitated followed by RNA isolation, a technique termed translating ribosomal affinity purification (TRAP). Additionally, the availability of fluorescent reporter mice for each MSN subtype is allowing, scientists to perform more accurate histology studies evaluating differential protein expression and signaling changes in each cell subtype. Finally, researchers are able to evaluate the role of specific genes in vivo by utilizing cell type-specific mouse models including Cre driver lines that can be crossed with conditional overexpression or knockout systems. This is a very exciting time in the BG field because researchers are well equipped with the most progressive tools to comprehensively evaluate molecular components in the two MSNs and their consequence on BG functional output in the normal, diseased, and developing brain.

Exploring neurocircuitries of the basal ganglia by intracerebral administration of selective neurotoxins

The detailed anatomy of the monoamine pathways of the rat, first described by the students of Nils Ake Hillarp in Sweden, provided the basis for a neurocircuitry targeted pharmacology, leading to important therapeutic breakthroughs. Progress was achieved by the introduction of accurate lesion techniques based on selective neurotoxins. Systematic intracerebral injections of 6-hydroxydopamine let Urban Ungerstedt at the Karolinska Institutet, Stockholm, Sweden, to propose the first stereotaxic mapping of the monoamine pathways in the rat brain; and the 'Rotational Behaviour', as a classical model for screening drugs useful for alleviating Parkinson's disease and other neuropathologies. The direction of the rotational behaviour induced by drugs administrated to unilaterally 6-hydroxydopamine-lesioned rats reveals their mechanism of action at dopamine synapses, as demonstrated when rotational behaviour was combined with microdialysis. The model was useful for proposing a role for dopamine receptors in the gating of the flow of information integrated and/or modulated by the basal ganglia, through different efferent pathways; notably the striatopallidal system, via D(2) receptors, and the striatonigral system, via D(1) receptors. The role of other dopamine receptor subtypes on rotational behaviour has not yet been clarified.

Translation of nondopaminergic treatments for levodopa-induced dyskinesia from MPTP-lesioned nonhuman primates to phase IIa clinical studies: keys to success and roads to failure

Studies in MPTP-lesioned nonhuman primates have demonstrated the potential of nondopaminergic drugs in reducing the problems of levodopa-induced dyskinesia (LID). Here we review the process of translating findings from the monkey to man. Agents targeting glutamate, adensosine, noradrenaline, 5-hydroxytryptamine, cannabinoid, and opioid transmitter systems have been assessed for antidyskinetic potential in human studies. Eleven nondopaminergic drugs with antidyskinetic efficacy in the MPTP primate have been advanced to proof-of-concept phase IIa trials in PD patients (amantadine, istradefylline, idazoxan, fipamezole, sarizotan, quetiapine, clozapine, nabilone, rimonabant, naloxone, and naltrexone). For all six nondopaminergic transmitter systems reviewed, the MPTP-lesioned primate correctly predicted phase II efficacy of at least one drug. Of the 11 specific molecules tested in both monkeys and humans, 8 showed clear antidyskinetic properties in both human and monkey. In the instances where the primate studies did not, or did not consistently, predict the outcome of the human studies, the discrepancy may reflect limitations in the validity of the model or limitations in the design of either the clinical or the preclinical studies. We find that the major determinant of success in predicting efficacy is to ensure that primate studies are conducted in a statistically rigorous way and incorporate designs and outcome measures with clinical applicability. On the other hand, phase IIa trials should strive to replicate the preclinical study, especially in terms of protocol, drug dose equivalence, and outcome measure, so as to test the same hypothesis. Failure to meet these criteria carries the risk of false negative conclusions in phase IIa trials.

Striatal delta opioid receptor binding in experimental models of Parkinson's disease and dyskinesia

Enhanced delta opioid receptor transmission may represent an endogenous compensatory mechanism in parkinsonism to reduce the activity of the indirect striatopallidal pathway following dopamine depletion. Furthermore, increased delta opioid receptor transmission may be causative in the production of dyskinesia following repeated dopaminergic treatment in Parkinson's disease. The present study employed radioligand receptor autoradiography, using [3H]naltrindole, a ligand selective for the delta opioid receptor, to assess delta opioid receptor binding sites in forebrain regions of reserpine-treated rats, and in parkinsonian nondyskinetic, and dyskinetic MPTP-lesioned macaques. In reserpine-treated animals, specific delta opioid binding was increased in premotor cortex (+30%), sensorimotor striatum (+20%), and associative striatum (+17%) rostrally, but was not changed in caudal forebrain. In contrast, delta opioid receptor binding was not significantly altered at any region analyzed, in either nondyskinetic or dyskinetic, MPTP-lesioned macaques, compared to normal. These results suggest that transient changes in delta opioid receptor binding may occur in motor circuits following acute dopamine depletion. However, in the more chronic MPTP-lesioned macaque model, simple changes in delta opioid receptor number or affinity are unlikely to contribute to mechanisms for abnormal opioid transmission in Parkinson's disease and dyskinesia.

Differential involvement of enkephalins in analgesic tolerance, locomotor sensitization, and conditioned place preference induced by morphine

In this study, the authors investigated the role of enkephalins in morphine-induced conditioned place preference, locomotor sensitization, and analgesic tolerance. Both preproenkephalin wild type (ppENK [+/+]) and knockout (ppENK [-/-]) mice showed similar preference for the morphine-paired chamber over the vehicle-paired chamber, indicating morphine induced comparable conditioned place preference in ppENK (+/+) and ppENK (-/-) mice. Sensitization developed to the motor stimulatory action of morphine after its repeated administration, but the magnitude of this response was not altered in ppENK (-/-) mice. However, as shown previously, ppENK (-/-) mice displayed blunted morphine analgesic tolerance. Taken together, the results suggest that enkephalins may be important for the development of analgesic tolerance but not for conditioned place preference or behavioral sensitization induced by morphine.

Ultrastructural localization of Leu5-enkephalin immunoreactivity in mesocortical neurons and their input terminals in rat ventral tegmental area

Enkephalin (ENK) immunoreactivity is widely distributed in the ventral tegmental area (VTA), where endogenous ENK and dynorphin opioid peptides are known to have opposing actions in reward, stress, cognition, and fear-related behaviors. Many neurons in the VTA give rise to mesocortical projections terminating in the medial prefrontal cortex (mPFC), and these projections have been implicated to varying extents in all these functions. To determine whether there is a synaptic basis for ENK and/or dynorphin modulation of cortically projecting neurons within the VTA, we combined retrograde tract-tracing from the mPFC with dual immunocytochemical-labeling electron microscopy in the rat VTA. The retrograde tracer Fluorogold (FG) was microinjected into mPFC. At optimal survival periods, sections through the VTA were processed for immunolabeling of anti-FG and a Leu(5)-ENK antibody recognizing both ENK and dynorphin peptides. Over 26% of the retrogradely labeled neuronal somatodendritic profiles (n = 177) were contacted by ENK-immunoreactive axonal profiles including small axons and axon terminals. The axon terminals varied in their subcellular distribution of ENK immunoreactivity and also differed in forming either inhibitory-type (symmetric) or excitatory-type (asymmetric) synapses. Many of the axonal profiles also were apposed to FG-labeled somata or dendrites without forming recognizable synapses. Approximately one-third of the mesocortical neuronal perikarya also showed sparsely distributed somatodendritic ENK-immunoreactivity. Our results provide ultrastructural evidence that ENK and possibly dynorphin in the rat VTA have distributions consistent with involvement in diverse physiological actions affecting the output of mesocortical neurons, some of which also contain one or both peptides.

Non-subtype-selective opioid receptor antagonism in treatment of levodopa-induced motor complications in Parkinson's disease

Opioid peptide transmission is enhanced in the striatum of animal models and Parkinson's disease (PD) patients with levodopa-induced motor complications. Opioid receptor antagonists reduce levodopa-induced dyskinesia in primate models of PD; however, clinical trials to date have been inconclusive. A double-blind, placebo controlled, crossover design study in 14 patients with PD experiencing motor fluctuations was carried out, using the non-subtype-selective opioid receptor antagonist naloxone. Naloxone did not reduce levodopa-induced dyskinesia. The duration of action of levodopa was increased significantly by 17.5%. Non-subtype-selective opioid receptor antagonism may prove useful in the treatment of levodopa-related wearing-off in PD but not in dyskinesia.

Chemical organization of projection neurons in the rat accumbens nucleus and olfactory tubercle

Projection neurons in the ventral striatum, the accumbens nucleus and olfactory tubercle, were examined by combining the retrograde tracing method and immunocytochemistry with antibodies against C-terminals of the preprodynorphin (PPD), preproenkephalin (PPE), preprotachykinin A (PPTA) and preprotachykinin B (PPTB). When the retrograde tracer was injected into the ventral pallidum, about 60% and 40% of retrogradely labeled neurons in the accumbens nucleus were immunoreactive for PPD and PPE, respectively. In contrast, all accumbens nucleus neurons projecting to the ventral mesencephalic regions including the substantia nigra and ventral tegmental area were immunopositive for PPD but not for PPE. Although no olfactory tubercle neurons projected fibers to the mesencephalic regions, 60% and 40% of olfactory tubercle neurons projecting to the ventrolateral portion of the ventral pallidum were immunoreactive for PPD and PPE, respectively, as were the accumbens nucleus neurons. About 70% of accumbens nucleus and olfactory tubercle neurons projecting to the ventral pallidum and all accumbens nucleus neurons projecting to the ventral mesencephalic regions showed PPTA immunoreactivity. A small population (2-12%) of accumbens neurons projecting to the ventral pallidum and mesencephalic regions displayed immunoreactivity for PPTB. Compared with the dorsal striatopallidal projection neurons that were reported to mostly express PPE, it was characteristic of the ventral striatum that only the smaller population (about 40%) of ventral striatopallidal projection neurons expressed PPE. This suggests that the ventral striatopallidal projection system is less specialized than the dorsal striatopallidal system in terms of peptide production, or that the ventral pallidum should be compared with a combined region of the globus pallidus and entopeduncular nucleus in the dorsal system.

Role of Dopaminergic D2 Receptors in the Regulation of Glutamic Acid Decarboxylase Messenger RNA in the Striatum of the Rat

Levels of messenger RNA (mRNA) encoding glutamic acid decarboxylase (GAD) and preproenkephalin (PPE) were measured by Northern blot and in situ hybridization analyses in the striatum of the rat, after chronic injections of two neuroleptics, sulpiride and haloperidol. The Northern blot analysis showed that the chronic injection of sulpiride at high doses (80 mg/kg, twice a day, 14 days) increased striatal GAD and PPE mRNA levels by 120% and 78% respectively, when compared to vehicle-injected rats. Haloperidol injections at relatively low doses (1 mg/kg, once a day, 14 days) produced parallel increases in GAD (40%) and PPE (52%) mRNA levels. After in situ hybridization densitometric measurements were performed on autoradiograms from rats treated with sulpiride, haloperidol or vehicle. The distribution of GAD and PPE mRNA signals in control rats was homogeneous along the rostrocaudal extension of the striatum. A similar increase was found along this axis after sulpiride (20%) and haloperidol (30%) treatments. The cellular observation of hybridization signals showed that grain density for GAD mRNA was increased in a majority of striatal cells after both treatments. By contrast, the PPE mRNA hybridization signal only increased in a subpopulation of neurons. The effects of such treatments were also analysed by measuring GAD activity in the striatum and in its output structures, the globus pallidus and the substantia nigra. After the administration of sulpiride, GAD activity was not modified in the striatum but increased in the globus pallidus (by 17%). After haloperidol treatment, GAD activity was increased in the globus pallidus (20%) and the substantia nigra (17%). It is concluded that the interruption of dopaminergic transmission, more precisely the D2 receptor blockade, promotes in striatopallidal neurons an increase in GAD mRNA accompanied by an increase in GAD activity and PPE mRNA. A possible regulation of GAD mRNA and GAD activity in striatonigral neurons is also discussed.

Antiparkinsonian action of a delta opioid agonist in rodent and primate models of Parkinson's disease

The opioid peptides localized in striatal projection neurons are of great relevance to Parkinson's disease, not only as a consequence of their distribution, but also due to the pronounced changes in expression seen in Parkinson's disease. It has long been suspected that increased expression of enkephalin may represent one of the many mechanisms that compensate for dopamine (DA) depletion in Parkinson's disease. Here we demonstrate that a systemically delivered, selective delta opioid agonist (SNC80) has potent antiparkinsonian actions in both rat and primate models of Parkinson's disease. In rats treated with either the D2-preferring DA antagonist haloperidol (1 mg/kg) or the selective D1 antagonist SCH23390 (1 mg/kg), but not a combination of D1 and D2 antagonists, SNC80 (10 mg/kg) completely reversed the catalepsy induced by DA antagonists. In rats rendered immobile by treatment with reserpine, SNC80 dose-dependently reversed akinesia (EC(50) 7.49 mg/kg). These effects were dose-dependently inhibited (IC(50) 1.05 mg/kg) by a selective delta opioid antagonist (naltrindole) and by SCH23390 (1 mg/kg), but not by haloperidol (1 mg/kg). SNC80 also reversed parkinsonian symptoms in the MPTP-treated marmoset. At 10 mg/kg (ip), scores measuring bradykinesia and posture were significantly reduced and motor activity increased to levels comparable with pre-MPTP-treatment scores. Any treatment that serves to increase delta opioid receptor activation may be a useful therapeutic strategy for the treatment of Parkinson's disease, either in the early stages or as an adjunct to dopamine replacement therapy. Furthermore, enhanced enkephalin expression observed in Parkinson's disease may serve to potentiate dopamine acting preferentially at D1 receptors.

Mu- and delta-opioid receptor antagonists reduce levodopa-induced dyskinesia in the MPTP-lesioned primate model of Parkinson's disease

Long-term treatment of Parkinson's disease with levodopa is complicated by the emergence of involuntary movements, known as levodopa-induced dyskinesia. It has been hypothesized that increased opioid transmission in striatal output pathways may be responsible for the generation of dyskinesia. In this study, we have investigated the effect of blockade of opioid peptide transmission on levodopa-induced dyskinesia in a primate model of Parkinson's disease-the MPTP-lesioned marmoset. Coadministration of nonselective and mu- or delta-subtype-selective opioid receptor antagonists with levodopa resulted in a significant decrease in dyskinesia. There was no attenuation of the anti-parkinsonian actions of levodopa. These data suggest that specific mu- or delta-opioid receptor antagonists might be applicable clinically in the treatment of levodopa-induced dyskinesia in Parkinson's disease.

The activation of mu opioid receptors promotes a small modulation of calcium currents in rat pallidal neurons

Globus pallidus receives, from dorsal neostriatum, a dense enkephalinergic innervation whose role is still uncertain. We examined the possibility that the activation of mu, delta or k opioid receptors modulate high-voltage-activated calcium currents in isolated GP neurons. Neither dynorphin nor DPEPE inhibited calcium current, whilst DAMGO produced a small (-16%) but consistent response, selectively antagonized by CTOP. The mu-mediated modulation required the activation of G-proteins but was voltage-independent. The pre-incubation in omega-conotoxinVIA abolished the response, implying the involvement of N-type calcium channels. These findings suggest that enkephalin may exert a direct influence on GP excitability also through post-synaptic effects. In degenerative conditions as Parkinsonism, an excessive stimulation of mu binding sites might induce a pathological inhibition of calcium signals, thus contributing to modify the GP firing pattern and transmitter release.

Third group of neostriatofugal neurons: neurokinin B-producing neurons that send axons predominantly to the substantia innominata

Neostriatal neurons that produce neurokinin B were investigated immunocytochemically in the rat brain with an antibody against the C-terminal portion of the precursor prepropeptide of neurokinin B, preprotachykinin B (PPTB). PPTB-immunoreactive neurons were scattered throughout the neostriatum and constituted 5.1% of neostriatal neurons. They were immunopositive for projection neuron markers, such as precursor peptides of substance P, enkephalins, and dynorphins, but negative for intrinsic neuron markers, suggesting that PPTB was expressed in neostriatal projection neurons. However, PPTB-immunoreactive neurons were immunonegative for dopamine- and cyclic AMP-regulated phosphoprotein, which is known to be produced by striatopallidal and striatonigral neurons. Furthermore, almost no PPTB-immunoreactive axon terminals were observed in the substantia nigra or globus pallidus. The authors then made large kainic acid lesions in the neostriatum to reveal the target areas of PPTB-producing neurons and observed a decrease in PPTB-immunoreactive fibers in the sublenticular portion of the substantia innominata and, to much lesser extent, in the bed nucleus of the stria terminalis and central nucleus of the amygdala. After injection of wheat germ agglutinin into the substantia innominata, PPTB immunoreactivity was detected in many retrogradely labeled neostriatal neurons. In contrast, no PPTB immunoreactivity was observed in striatonigral or striatopallidal neurons after injection of retrograde tracers into the substantia nigra or globus pallidus. Thus, neurokinin B-producing neostriatal neurons were considered to send projection fibers predominantly to the substantia innominata. Furthermore, PPTB-immunoreactive axonal swellings were closely apposed to neurokinin B receptor-immunoreactive dendrites in the substantia innominata. Overall, the present results indicate that the rat brain possesses a chemically and hodologically unique neostriatofugal pathway in addition to the direct and indirect pathways.

Striatal enkephalin gene expression does not reflect parkinsonian signs

Loss of striatal dopamine has been associated with an increase in striatal enkephalin expression. However, the relationship between increased striatal enkephalin expression and the manifestation of parkinsonian motor deficits is not clear. Administration of MPTP to cats produces a severe parkinsonian condition from which the animals spontaneously recover. Using in situ hybridization histochemistry, preproenkephalin (PPE) mRNA expression was examined in the striatum of cats when normal, symptomatic for or spontaneously recovered from MPTP-induced parkinsonism. In all areas of the striatum, PPE mRNA levels were significantly elevated in animals exhibiting severe parkinsonian motor deficits and remained elevated even after recovery of gross motor functioning. These results show that striatal PPE gene expression and parkinsonian motor deficits are not directly correlated.

Enkephalin regulates acute D2 dopamine receptor antagonist-induced immediate-early gene expression in striatal neurons

Projection neurons of the striatum release opioid peptides in addition to GABA. Our previous studies showed that the opioid peptide dynorphin regulates that subtype of projection neurons which sends axons to the substantia nigra/entopeduncular nucleus, as indicated by an inhibitory action of dynorphin/agonists on D1 dopamine receptor-mediated immediate-early gene induction in these neurons. The other subtype of striatal projection neurons projects to the globus pallidus and contains the opioid peptide enkephalin. Here, we investigated whether enkephalin regulates the function of striatopallidal neurons, by analysing opioid effects on immediate-early gene induction by D2 dopamine receptor blockade that occurs in these neurons. Thus, the effects of systemic and intrastriatal administration of various opioid receptor agonists and antagonists on immediate-early gene expression (c-fos, zif 268) induced by the D2 receptor antagonist eticlopride were examined with in situ hybridization histochemistry. Intrastriatal infusion of enkephalin (delta and mu), but not dynorphin (kappa), receptor agonists suppressed immediate-early gene induction by eticlopride in a dose-dependent manner. This suppression was blocked by the opioid receptor antagonist naloxone, confirming the involvement of opioid receptors. Repeated treatment with D2 receptor antagonists produces increased enkephalin expression and diminished immediate-early gene inducibility in striatopallidal neurons, as well as behavioral effects that are attenuated compared to those of acute treatment (e.g., reduced akinesia). Naloxone reversed such behavioral recovery (i.e. reinstated akinesia), but did not significantly affect suppressed immediate-early gene induction. Our results indicate that enkephalin acts, via mu and delta receptors in the striatum, to inhibit acute effects of D2 receptor blockade in striatopallidal neurons. Moreover, the present findings suggest that increased enkephalin expression after repeated D2 receptor antagonist treatment is an adaptive response that counteracts functional consequences of D2 receptor blockade, but is not involved in suppressed immediate-early gene induction. Together with our earlier findings of the role of dynorphin, these results indicate that opioid peptides in the striatum serve as negative feedback systems to regulate the striatal output pathways in which they are expressed.

Distribution of catecholaminergic afferent fibres in the rat globus pallidus and their relations with cholinergic neurons

The topographical distribution of catecholaminergic nerve fibres and their anatomical relationship to cholinergic elements in the rat globus pallidus were studied. Peroxidase-antiperoxidase and two-colour immunoperoxidase staining procedures were used to demonstrate tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), phenylethanolamine N-methyltransferase (PNMT) and choline acetyltransferase (ChAT) immunoreactivities, combined with acetylcholinesterase (AChE) pharmacohistochemistry. TH immunoreactive nerve fibres were seen to enter the globus pallidus from the medial forebrain bundle. The greatest density of such fibres was found in the ventral region of the globus pallidus, which was also characterized by the greatest density of ChAT immunoreactive neurons. TH immunoreactive nerve fibres showed varicose arborizations and sparse boutons, which were occasionally seen in close opposition to cholinergic structures. In all regions of the globus pallidus, there were also larger, smooth TH immunoreactive nerve fibres of passage to the caudate putamen. A smaller number of DBH immunoreactive nerve fibres and terminal arborizations were found in the substantia innominata, internal capsule and in the globus pallidus bordering these structures. A few PNMT immunoreactive nerve fibres in the substantia innominata and internal capsule did not enter the globus pallidus. Electron microscopy revealed TH immunoreactive synaptic profiles in the ventromedial area of the globus pallidus corresponding to the nucleus basalis magnocellularis of Meynert (nBM). These made mainly symmetrical and only a few asymmetrical synaptic contacts with dendrites containing AChE reaction product. The results indicate that cholinergic structures in the nBM are innervated by dopaminergic fibres and terminals, with only a very small input from noradrenergic fibres.

Specific reductions of striatal prodynorphin and D1 dopamine receptor messenger RNAs during cocaine abstinence

It is well established that the opioid neuropeptide and dopamine systems are altered following the use of cocaine. However very little information is available about their possible involvement during cocaine abstinence. In the present study, the mRNA expression of the dopamine receptors, D1 and D2, and the opioid peptides, prodynorphin and proenkephalin, were analyzed in the rat striatum using in situ hybridization histochemistry. Saline or cocaine (30 mg/kg, i.p.) were administered to rats once daily for 1 or 10 days. To examine cocaine abstinence, animals were treated for 10 days as described followed by a 10-day drug free period. Acute and intermittent cocaine administration elevated the prodynorphin mRNA expression in the dorsal striatum, consistent with previous reports, while the abstinent phase resulted in a significant reduction of prodynorphin mRNA levels in the ventrorostral striatum. The D1-receptor mRNA was decreased in the caudorostral striatum during cocaine withdrawal, a finding opposite to the increase observed following a single injection of the drug. Proenkephalin and the D2-receptor mRNAs were not altered during cocaine abstinence, though proenkephalin was elevated following acute but not repeated cocaine administration. These results show long-term suppression on prodynorphin and D1-receptor systems in specific striatal populations localized mainly in rostral areas during withdrawal from cocaine.

Quantitative comparison of pretreatment regimens used to sensitize in situ hybridization using oligonucleotide probes on paraffin-embedded brain tissue

Paraffin embedding of tissue is generally perceived to dramatically reduce RNA detectability. As a consequence, in situ hybridization on paraffin-embedded tissue is largely confined to detection of high-copy RNA species (e.g., viral RNA) and/or to detection using typically more sensitive cDNA probes or riboprobes. In this study, several procedures for in situ hybridization on paraffin-embedded rat tissue using oligonucleotide probes complementary to cellular transcripts were developed and quantitatively compared. Certain pretreatments showed marked increases in sensitivity compared to untreated sections. Furthermore, through quantitative assessment using image analysis, sensitivity of optimal pretreatments was equal to that of routinely used fresh-frozen, postfixed tissue sections. The development of such techniques permitting in situ hybridization to be carried out on paraffin-embedded tissue allows a comparison of protein and mRNA distribution to be made in adjacent sections and provides the potential for double labeling by in situ hybridization and immunohistochemistry which may not be possible on post-fixed frozen sections.

Collateral projections from striatonigral neurons to substance P receptor-expressing intrinsic neurons in the striatum of the rat

It is well known that striatonigral neurons produce substance P (SP); however, no SP receptor (SPR) has so far been found in the substantia nigra. On the other hand, a previous study in the rat striatum indicated that SPR was expressed only in cholinergic or somatostatinergic intrinsic neurons (Kaneko et al. [1993] Brain Res. 631:297-303). Thus, it was assumed that SP produced by striatonigral neurons might be released through their intrastriatal axon collaterals to act upon intrinsic neurons in the striatum. To confirm this assumption, the distribution of axon collaterals of striatonigral neurons was examined in the striatum of the rat. The experiments were performed on brain slices by combining retrograde labeling with tetramethylrhodamine-dextran amine, electrophysiological recording, intracellular staining with biocytin, and immunocytochemistry for SPR. The distribution of axons of cholinergic striatal neurons (a group of SP-negative intrinsic striatal neurons) was also examined. It was observed that 16% of varicosities of intrastriatal axon collaterals of striatonigral neurons, as well as 6% of axonal varicosities of cholinergic neurons, were in close apposition to dendrites and cell bodies of SPR-immunoreactive striatal neurons. Since SPR-immunoreactive striatal neurons constituted only 2.7% of the total population of striatal neurons (Kaneko et al. [1993] Brain Res. 631:297-303), it appeared that axonal varicosities of striatonigral neurons were preferentially apposed to SPR-immunoreactive striatal neurons and that the varicosities in close apposition to SPR-immunoreactive neurons were derived more frequently from striatonigral neurons than from cholinergic interneurons. Confocal laser scanning microscopy indicated that axonal varicosities in close apposition to SPR-immunoreactive cells showed synaptophysin immunoreactivity, a marker of synaptic vesicles. In intrastriatal axons of striatonigral neurons, it was further revealed from electron microscopy that axonal varicosities in close apposition to SPR-immunoreactive dendrites, at least a part of them, made synapses of the symmetric type. Striatonigral neurons might release SP preferentially around cholinergic or somatostatinergic intrinsic neurons to regulate them through SP-SPR interactions.

Alterations in opioid receptor binding in Parkinson's disease patients with levodopa-induced dyskinesias

Levodopa-induced dyskinesias remain a major challenge in the therapeutic management of Parkinson's disease (PD). Their etiology is unknown although dysfunction of striatal opioid transmission has been implicated in experimental models of PD. To determine whether the opioid system is involved in human dyskinetic PD, we measured in vivo opioid receptor binding in PD patients with and without levodopa-induced dyskinesias, using positron emission tomography (PET) and the opioid receptor ligand [11C]diprenorphine. Striatal and thalamic/occipital uptake ratios were calculated using a region of interest (ROI) approach. In addition, we used statistical parametric mapping (SPM) and images reflecting the volume of distribution of [11C]diprenorphine to assess changes in cerebral receptor binding on a voxel-by-voxel basis. By using the ROI approach, we found significantly reduced striatal and thalamic opioid binding in dyskinetic, but not in nondyskinetic, PD patients. The SPM approach confirmed reduced availability in these areas and, in addition, showed decreased cingulate and increased prefrontal opioid receptor binding in the dyskinetic patients. Our findings confirm that altered opioid transmission is part of the pathophysiology of levodopa-induced dyskinesias in PD and support further investigation into the role of opioid agents in the management of these involuntary movements.

Presynaptic versus postsynaptic localization of mu and delta opioid receptors in dorsal and ventral striatopallidal pathways

Parallel studies have demonstrated that enkephalin release from nerve terminals in the pallidum (globus pallidus and ventral pallidum) can be modulated by locally applied opioid drugs. To investigate further the mechanisms underlying these opioid effects, the present study examined the presynaptic and postsynaptic localization of delta (DOR1) and mu (MOR1) opioid receptors in the dorsal and ventral striatopallidal enkephalinergic system using fluorescence immunohistochemistry combined with anterograde and retrograde neuronal tracing techniques. DOR1 immunostaining patterns revealed primarily a postsynaptic localization of the receptor in pallidal cell bodies adjacent to enkephalin- or synaptophysin-positive fiber terminals. MOR1 immunostaining in the pallidum revealed both a presynaptic localization, as evidenced by punctate staining that co-localized with enkephalin and synaptophysin, and a postsynaptic localization, as evidenced by cytoplasmic staining of cells that were adjacent to enkephalin and synaptophysin immunoreactivities. Injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) or the retrograde tracer Texas Red-conjugated dextran amine (TRD) into the dorsal and ventral striatum resulted in labeling of striatopallidal fibers and pallidostriatal cell bodies, respectively. DOR1 immunostaining in the pallidum co-localized only with TRD and not PHA-L, whereas pallidal MOR1 immunostaining co-localized with PHA-L and not TRD. These results suggest that pallidal enkephalin release may be modulated by mu opioid receptors located presynaptically on striatopallidal enkephalinergic neurons and by delta opioid receptors located postsynaptically on pallidostriatal feedback neurons.

Preprodynorphin-, preproenkephalin-, and preprotachykinin-expressing neurons in the rat neostriatum: an analysis by immunocytochemistry and retrograde tracing

Specific antibodies were produced against C-terminal portions of rat preprodynorphin (PPD), preproenkephalin (PPE), and preprotachykinin A (PPT). PPD, PPE, and PPT C-terminal immunoreactivity was observed in many cell bodies of medium-sized neurons in the rat neostriatum (caudate-putamen). Intense PPE immunoreactivity was found in neuropil of the globus pallidus, whereas intense to moderate PPD and PPT immunoreactivity was distributed in neuropil of the substantia nigra and the entopeduncular nucleus. A double-immunofluorescence analysis revealed that PPE-immunoreactive neostriatal neurons rarely showed immunoreactivity for PPD (<1%) or PPT (<2%). In contrast, more than 95% of PPD-immunoreactive neostriatal neurons showed PPT immunoreactivity, and vice versa. No PPD-, PPE-, or PPT-immunoreactive neostriatal neurons showed immunoreactivity for the markers of neostriatal intrinsic neurons, such as calretinin, choline acetyltransferase, parvalbumin, or somatostatin. When tetramethylrhodamine-dextran amine (TMR-DA) was injected into the substantia nigra, almost all neurons that were labeled retrogradely with TMR-DA showed immunoreactivity for PPD (98%) or PPT (99%), but very few of them exhibited PPE immunoreactivity (1%). After injection of TMR-DA into the globus pallidus, 86%, 17%, and 10% of the retrogradely labeled neurons showed immunoreactivity for PPE, PPD, and PPT, respectively. These results support the notion that the neostriatal projection neurons are divided into at least two groups: The projection neurons of one group contain enkephalins and send projection fibers almost exclusively to the globus pallidus, and the others contain tachykinins and dynorphins/Leu-enkephalin and send projection fibers mainly to the substantia nigra.

Dopaminergic regulation of AP-1 transcription factor DNA binding activity in rat striatum

Dopaminergic modulation of the DNA binding activity of AP-1, Sp1, CREB and AP-2 transcription factors was examined in rat striatal nuclear extracts by gel shift assay. AP-1 binding was selectively increased in the striatum following depletion of dopamine by 6-hydroxydopamine-induced lesion of the nigrostriatal pathway or after reserpine treatment. The D1 agonist SKF 38393 dose-dependently increased AP-1 binding; this effect was significantly increased in reserpine-treated rats and even more markedly enhanced in denervated striatum. The D2/D3 agonist quinpirole, administered alone, did not affect striatal activator protein-1 binding; in combination, quinpirole and SKF 38393 acted synergistically in normal and reserpine-treated rats but not in 6-hydroxydopamine-lesioned rats, suggesting that mechanisms underlying D1-D2/D3 interactions are altered after dopamine denervation. Most, but not all, of the changes in AP-1 binding activity observed in this study are consistent with changes in levels of Fos/Jun family proteins observed after similar treatments. These results support the hypothesis that D1 receptor stimulation activates striatonigral neurons and modulates expression of AP-1-related genes in these neurons, while D2 receptor stimulation mediates tonic inhibition of AP-1 expression and activity in the striatopallidal neurons. Moreover, the findings provide evidence that the loss of dopaminergic input to the striatum, as occurs in Parkinson's disease, induces long-lasting alterations in the regulation of striatal gene expression which may contribute to the disease's progress.

Potential of opioid antagonists in the treatment of levodopa-induced dyskinesias in Parkinson's disease

Current treatments for Parkinson's disease (PD) rely on dopamine-replacing strategies, and centre around dopamine precursors (e.g. levodopa) or directly acting dopamine agonists. With long-term therapy these agents lose much of their clinical utility due to the appearance of adverse effects such as dyskinesias and/or a wearing off of efficacy. Although dyskinesias in Huntington's disease, hemiballism and experimental animals are thought to be associated with reductions in amino acid transmission within the lateral and medial segments of the globus pallidus, the neural mechanisms underlying treatment-related dyskinesias in PD are poorly understood. Recent evidence suggests that, within these regions of the brain, the opioid peptides enkephalin and dynorphin, acting at delta and kappa opioid receptors, respectively, can reduce the release of amino acid transmitters. Furthermore, the synthesis of these peptides appears to be enhanced in neurons projecting to the pallidal complex in animal models of PD following repeated treatment with dopamine-replacing agents that also cause dyskinetic adverse effects (e.g. levodopa and apomorphine). In contrast, dopamine receptor agonists such as bromocriptine and lisuride do not cause dyskinetic adverse effects following long-term treatment, and do not elevate peptide synthesis when given de novo. These data, together with recent data on the behavioural effects of opioid antagonists in a rodent model of levodopa-induced dyskinesia in PD, suggest the possibility that antagonists of opioid receptors may prove useful as adjuncts to levodopa. By limiting the severity of dyskinetic adverse effects, these drugs may help extend the time for which the antiparkinsonian effects of such compounds can be usefully exploited.

Differential messenger RNA expression of prodynorphin and proenkephalin in the human brain

The opiate system is involved in a wide variety of neural functions including pain perception, neuroendocrine regulation, memory, drug reward, and tolerance. Such functions imply that endogenous opioid peptides should have anatomical interactions with limbic brain structures believed to be involved in the experience and expression of emotion. Using in situ hybridization histochemistry, the messenger RNA expression of the opioid precursors, prodynorphin and proenkephalin, was studied in whole hemisphere human brain tissue. Different components of the limbic system were found to be characterized by a high gene expression of either prodynorphin or proenkephalin messenger RNA. Brain regions traditionally included within the limbic system (e.g. amygdala, hippocampus, entorhinal cortex and cingulate cortex) as well as limbic-associated regions including the ventromedial prefrontal cortex and patch compartment of the neostriatum showed high prodynorphin messenger RNA expression. In contrast, high levels of proenkephalin messenger RNA were more widely expressed in the hypothalamus, periaqueductal gray, various mesencephalic nuclei, bed nucleus of the stria terminalis, and ventral pallidum; brain regions associated with endocrine-reticular-motor continuum of the limbic system. The marked anatomical dissociation between the expression of these two opioid peptide genes, seen clearly in whole hemisphere sections, indicates that distinct functions must be subserved by the prodynorphin and proenkephalin systems in the human brain.

Total number of neurons in the neostriatal, pallidal, subthalamic, and substantia nigral nuclei of the rat basal ganglia: a stereological study using the cavalieri and optical disector methods

The total number of neurons within six subdivisions of the rat basal ganglia was estimated using unbiased stereological counting methods and systematic random sampling techniques. Six young adult rats were perfuse-fixed, their right cerebral hemispheres were embedded in glycolmethacrylate, and a complete set of serial 40-mu m sections was cut through each hemisphere. After a random start, a systematic subset (e.g., every tenth) of these sections was used to estimate the total volume of each subdivision using Cavalieri's method. The same set of sampled sections was used to estimate the number of neurons in a known subvolume (i.e., the Nv) by the optical disector method. The product of the total volume and the Nv by these methods yields an unbiased estimate of the total number of neurons. It was found that the right basal ganglia consisted, on average, of 2.79 million neostriatal or caudate-putamen neurons (with a coefficient of variation of 0.07), 46,000 external globus pallidus neurons (0.11), 3,200 entopeduncular/internal globus pallidus neurons (0.10), 13,600 subthalamic neurons (0.10), 7,200 substantial nigra pars compacta neurons (0.15), and 26,300 substantia nigra pars reticulata neurons (0.07).

The human neostriatum shows compartmentalization of neuropeptide gene expression in dorsal and ventral regions: an in situ hybridization histochemical analysis

Expression of neuropeptide messenger RNAs in striatal neurons was studied in post mortem human brain tissue by the use of in situ hybridization histochemistry. Clusters of cells expressing high levels of prodynorphin messenger RNA, and less strikingly, preprotachykinin messenger RNA, were prominent in the caudate nucleus and were present but less pronounced in the putamen. Proenkephalin and prosomatostatin messenger RNA-containing cells were more homogeneously distributed throughout the striatum, though the latter were much sparser. The four neuropeptide messenger RNA patterns in the nucleus accumbens were rather homogeneous compared with the dorsal striatum. Of these, prodynorphin messenger RNA showed a higher level of expression per cell in the nucleus accumbens relative to the dorsal striatum. The relationship of neuropeptide-containing cell clusters to the striosomal organization was characterized by looking at the register of these markers with patterns of low acetylcholinesterase activity and dense mu opiate receptor binding. In the caudate and putamen, clusters of cells expressing high levels of dynorphin and preprotachykinin messenger RNAs were clearly in register with the striosomes. The accumbens was defined by high prodynorphin messenger RNA levels, both low and high levels of acetylcholinesterase staining, and very low to absent mu opiate receptor binding. The distribution of high-expressing prodynorphin messenger RNA-containing cells--to the patch compartment and throughout the entire ventral striatum/nucleus accumbens region--defines the limbic domain of the neostriatum and suggests particular relevance to human striatal organization and function, because the distribution of this opioid neuropeptide is considerably more compartmentalized in human than in non-human species.

The development of enkephalin and substance P neurons in the basal ganglia: insights into neostriatal compartments and the extended amygdala

To study the comparative development of the two major neuropeptide genes of the striatum, we used immunocytochemistry to detect immunoreactivity (ir) for substance P and synenkephalin (the N terminus of proenkephalin), and in situ hybridization to detect proenkephalin mRNA. Earliest detection of substance P-ir was in the anlage of the bed nucleus of the stria terminalis (BST, at E15) and in the rostral-lateral caudate-putamen (CPu), at E16. Substance P in the BST was immediately subjacent to the medial ganglionic eminence, while immunoreactivity in the CPu was associated with the lateral ganglionic eminence. Earliest detection of synenkephalin-ir or proenkephalin mRNA was in the caudal-lateral CPu and the adjacent central nucleus of the amygdala (Ce), at E16. Over the next several days, expression of each neuropeptide spread toward the region of first expression of the other neuropeptide. The first overlap of expression of the two neuropeptides was at E18, at the level of the septum. Despite correspondence of substance P-ir and proenkephalin mRNA in patches at P0, very little co-expression of the two neuropeptides was evident in individual neurons. We propose a model in which the CPu develops primarily from the lateral ganglionic eminence, and the extended amygdala develops primarily from the medial ganglionic eminence. Within each structure, two poles of neuropeptide gene expression are established initially: substance P-ir in the rostral CPu and in the rostral-medial pole of the extended amygdala (represented by the BST), and synenkephalin/proenkephalin in the caudal CPu and in the caudal-lateral pole of the extended amygdala (represented by the Ce). A stream of substance P-ir cells connects the two poles of the extended amygdala, in the sublenticular substantia innominata.

Alterations in mRNA levels of D2 receptors and neuropeptides in striatonigral and striatopallidal neurons of rats with neuroleptic-induced dyskinesias

Chronic neuroleptic treatment in rat produces vacuous chewing movements (VCMs), analogous to TD in humans. We hypothesized that these hyperkinetic movements were due to alterations in striatonigral and striatopallidal GABAergic spiny II neurons. Rats were treated for 36 weeks with haloperidol decanoate and withdrawn for 28 weeks. Striatonigral and striatopallidal neurons were assessed using in situ hybridization histochemistry for mRNA levels of D1 and D2 dopamine receptors, preproenkephalin (ENK), prodynorphin (DYN), protachykinin (substance P), and glutamate decarboxylase (GAD67) in the dorsolateral and ventromedial striatum as well as the nucleus accumbens. Rats that did not develop VCMs (-VCM) had increased D2 receptor and DYN mRNA, and reduced substance P mRNA in the dorsolateral striatum. Rats with persistent VCMs (+VCM) had increased D2 receptor, ENK, and DYN mRNA in both striatal regions, and increased ENK and DYN mRNA in the nucleus accumbens, compared with controls. Relative to -VCM rats, however, +VCM rats only had increased ENK mRNA in the nucleus accumbens. Considering the overall pattern of mRNA changes, the data suggest that alterations in both the D1-mediated striatonigral and the D2-mediated striatopallidal pathways play a role in the expression of the VCM syndrome. To the extent that gene expression parallels changes in neuronal activity, this implies that the VCM syndrome is associated with increased activity in both pathways.

Differences between alcohol-preferring (AA) and alcohol-avoiding (ANA) rats in the prodynorphin and proenkephalin systems

The motivation to drink alcohol and the eventual risk of becoming addicted are in part genetically determined. Because opioid peptides are considered central to motivated behaviors, we have analyzed opioid peptides in relevant areas of the brain of two outbred lines of rats: the alcohol-preferring [Alko Alcohol (AA)] line who voluntarily drink alcohol and the alcohol-avoiding [Alko Non-Alcohol (ANA)] line with negligible intake. (Met)enkephalinArg6Phe7 (MEAP) was measured as a marker of proenkephalin, and dynorphin A, dynorphin B, and (Leu)enkephalinArg6 as markers of the prodynorphin system. The major line differences and effects of alcohol intake were observed in mesolimbic brain areas. The mesolimbic dopamine pathway, which projects from the ventral tegmental area (VTA) to the nucleus accumbens, is central in the reward system. Basal levels of MEAP and dynorphin peptides were low in the nucleus accumbens of AA rats, whereas (Leu)enkephalinArg6 levels were lower in the VTA of these rats. Alcohol drinking caused MEAP levels in the accumbens to rise, but had no effect on prodynorphin peptides. Opioids also influence the nigrostriatal dopamine pathway. However, this study showed no significant differences for any peptide between rat lines, or effect of alcohol intake, in either substantia nigra or striatum, except for a decrease of nigral and striatal (Leu)enkephalinArg6 levels in alcohol-drinking AA rats. Large line differences were observed in the pituitary gland. AA rats had high basal levels of MEAP, which became even higher after voluntary alcohol consumption for 4 weeks, and low levels of dynorphin peptides, not affected by alcohol drinking.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential dopaminergic regulation of the neurotensin striatonigral and striatopallidal pathways in the rat

Recently the existence of a neurotensin striatonigral pathway strongly up-regulated by methamphetamine has been demonstrated. The aim of the present study was to investigate, using immunohistochemistry and radioimmunoassay, the modulation of this pathway by dopamine antagonists. Rats were injected either with methamphetamine alone or together with the dopamine D1 receptor antagonist, SCH 23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-be nzapine hydrochloride), or with the dopamine D2 receptor antagonist, sulpiride. Both techniques showed that this neurotensin striatonigral pathway is regulated by dopamine D1 receptors, since SCH 23390 totally prevented the methamphetamine-induced increase in neurotensin-like immunoreactivity, both in the striatum and in the substantia nigra pars reticulata. Conversely, sulpiride was unable to counteract the effect of methamphetamine in these two areas, suggesting that dopamine D2 receptors are not involved in the regulation of this neurotensin pathway. On the other hand, neurotensin-like immunoreactivity was markedly increased in striatal cell bodies and in the globus pallidus after treatment with sulpiride, indicating that this pathway is mainly regulated by dopamine D2 receptors.

Neurochemical compartmentalization of the globus pallidus in the rat: an immunocytochemical study of calcium-binding proteins

The globus pallidus external segment forms a major target center of the mammalian striatum which is characterized by neurochemically distinct compartments. The present study was undertaken to determine if a corresponding compartmentalization exists within the globus pallidus external segment in the rat. Immunocytochemical examination of the calcium-binding proteins parvalbumin and calbindin D28kDa, which are present in neurons of the striatal matrix compartment, was employed. The results indicate three neurochemically distinct compartments within the globus pallidus external segment: 1) an area in the medial aspect of the entire length of the globus pallidus that contains dense immunoreactivity for calbindin D28kDa; 2) a narrow rim at the striatopallidal junction in the rostral two-thirds of the globus palidus that contains calbindin D28kDa immunoreactivity designated as the "border zone" of the globus pallidus; and 3) an area between these two zones showing very poor immunoreactivity for calbindin D28kDa but containing parvalbumin immunoreactive neurons. The calbindin D28kDa immunoreactive border zone corresponds to the area of the globus pallidus where striatal inputs converge extensively, whereas the rest of the nucleus is involved in segregated, topographically organized pathways. Parvalbumin-containing neurons are involved in the propagation of striatal output related to striosomal and sensorimotor aspects of basal ganglia function. The present results also indicate that calbindin D28kDa immunoreactivity is completely absent from striosomal neurons and is therefore a useful marker for striatal compartments.

Basal ganglia: functional anatomy and physiology. Part 1

Advances in knowledge about basal ganglia structure and connectivity from 1925 to date are reviewed. Current concepts about neuronal populations, transmitters, and input and output of each of the basal ganglia nuclei are presented. The portrayal by Wilson, in 1925, of the striatum as a simple homogeneous structure has been replaced by the recognition, based on staining characteristics, connectivity, and function, that the neostriatum is compartmentalized into striosomes, matrisomes, and matrix compartments. Electrophysiologic studies have further shown the existence, in the neostriatum, of neuronal clusters that represent basic functional units much like the functional columns described much earlier for the cerebral cortex. Whereas the neostriatum is considered the major receiving area of the basal ganglia, the globus pallidus and substantia nigra pars reticulata constitute the major output nuclei. Combined neuroanatomic and neurophysiologic studies have revealed precise somatotopic organization throughout the basal ganglia system such that the leg, arm, and face areas of the cerebral cortex related to respective topographic areas within the striatum, pallidum, substantia nigra, and subthalamus. The previous concept of an inhibitory role for dopamine on striatal neurons has been modified. It is now acknowledged that dopamine exerts an inhibitory effect on striatal neurons that project to the external pallidum and a facilitatory effect on striatal neurons that project to the internal pallidum and substantia nigra pars reticulata. The previous concept of serial connectivity of the neostriatum (funnel concept) has been replaced by the concept of parallel connectivity. Within the internal connectivity of the basal ganglia, there is a fast system in which the neurotransmitter is gamma-aminobutyric acid (GABA) and a slow system modulated by neuropeptides. The slow system is believed to give identity to an otherwise homogenous GABAergic system.

A selective lesion of striatonigral neurons decreases presynaptic binding of [3H]hemicholinium-3 to striatal interneurons

We have used the suicide transport agent, volkensin, to produce selective lesions of striatal efferent neurons projecting to the substantia nigra in the rat. In order to evaluate potential trans-synaptic effects, we examined cholinergic interneurons intrinsic to the striatum following destruction of striatonigral projection neurons by nigral injection of volkensin. There was no change in the number of large interneurons identified either by Nissl stain or by immunocytochemistry for choline acetyltransferase, indicating that volkensin was not directly toxic to this group of neurons. However, [3H]hemicholinium-3 binding to the choline re-uptake site on the presynaptic cholinergic terminals decreased. No change in [3H]hemicholinium-3 binding was seen after destruction of dopaminergic afferents with 6-hydroxydopamine. Striatonigral afferents to the cholinergic interneurons contain substance P which has been shown to stimulate acetylcholine release. The decrease in [3H]hemicholinium-3 binding may reflect loss of this afferent input. However, striatonigral neurons are an efferent target of the cholinergic interneuron as well, and a presynaptic effect due to loss of target neurons also may contribute.

Human brain methionine- and leucine-enkephalins and their receptors during ageing

Brain Met- and Leu-enk levels were investigated with radioimmunoassay and their binding to enkephalin receptors was studied with [3H]Met- and [3H]Leu-enkephalin in 56 human subjects (4-93 yr). Of the brain areas investigated, the head of the caudate nucleus and pallidum showed an age-associated decline for both Met- and Leu-enk content. In the substantia nigra and in putamen, no significant age-effect was seen. Binding of the enkephalins, which was studied in the head of the caudate nucleus and hippocampus, did not show significant age dependency. In conclusion, ageing affects human brain enkephalin levels but not their receptor binding, at least in brain areas investigated in the present study.

Alterations in delta opioid receptor levels in discrete areas of the neocortex and in the globus pallidus of the aging guinea pig: a quantitative autoradiographic study

The effect of aging on delta opioid receptors was examined in the brains of guinea pigs aged 1, 6, 24 and 36 months. Quantitative autoradiography was used to monitor the concentrations of delta receptors in various anatomical regions at five rostro-caudal levels. delta opioid receptor populations were found to be remarkably stable throughout the life span of this species. We have, however, discovered anatomical areas which offer striking exceptions. In the globus pallidus, progressive age-related losses of delta receptors reached 50% in the senescent animal. In contrast, laminae I, II of the lateral agranular frontal cortex and laminae I, II and III, IV of the primary somatosensory cortex demonstrated age-related increases in the concentrations of delta receptors ranging from 30 to 45%. These changes are discussed with the view to their being functionally related components of motor circuitry involving pyramidal and extrapyramidal elements.

Molecular alterations in the neostriatum of human cocaine addicts

Molecular changes in the neostriatum of human subjects who died with a history of cocaine abuse were revealed in discrete cell populations by means of the techniques of in situ hybridization histochemistry and in vitro receptor binding and autoradiography. Cocaine subjects had a history of repeated cocaine use and had cocaine and/or cocaine metabolites on board at the time of death. These subjects were compared to control subjects that had both a negative history and toxicology of cocaine use. Selective alterations in mRNA levels of striatal neuropeptides were detected in cocaine subjects compared to control subjects, especially for the opioid peptides. Marked reductions in the levels of enkephalin mRNA and mu opiate receptor binding were found in the caudate and putamen, concomitant with elevations in levels of dynorphin mRNA and kappa opiate receptor binding in the putamen and caudate, respectively. Dopamine uptake site binding was reduced in the caudate and putamen of cocaine subjects. The greater magnitude of changes in the dorsolateral striatum (caudate and putamen) as opposed to the ventromedial striatum (nucleus accumbens) suggests that cocaine abuse preferentially alters the biosynthetic activity of striatal systems associated with sensorimotor functioning. Additionally, an imbalance in the activity of the two major striatal output pathways in cocaine users is implicated because peptide mRNA levels were reduced in enkephalinergic striatopallidal neurons and increased in dynorphinergic striatonigral neurons. Another imbalance, that of reductions of transmitter mRNA and receptor expression associated with euphoria (enkephalin and mu opiate receptors), together with elevations in mRNAs of transmitter systems associated with dysphoria (dynorphin and kappa opiate receptors), suggests a model of dysphoria and craving in the human cocaine addict brain.