Central administration of the neurotensin receptor antagonist SR48692 attenuates vacuous chewing movements in a rodent model of tardive dyskinesia

Tardive dyskinesia is a movement disorder that develops in 20-30% of patients treated with chronic neuroleptics. Whilst the pathogenesis of tardive dyskinesia remains unclear, altered expression of neuropeptides in the basal ganglia has been implicated in its emergence. The peptide neurotensin is expressed in both dopamine D1 receptor-bearing neurons of the direct striatonigral pathway and dopamine D2 receptor-bearing neurons of the indirect striatopallidal pathway. Increased levels of striatal neurotensin messenger RNA (mRNA) are reported following chronic neuroleptic therapy. Chronic treatment with the typical antipsychotic haloperidol elicits neurotensin immunoreactivity in a large number of striatopallidal and a modest number of striatonigral projection neurons, whilst treatment with the potent dopamine releaser, methamphetamine, induces intense neurotensin immunoreactivity in striatonigral projection neurons. In order to determine whether increased levels of striatal neurotensin mRNA in the direct striatonigral or the indirect striatopallidal pathway play a more influential role in the development of tardive dyskinesia, we explored the effects of a specific neurotensin antagonist in a rodent model (vacuous chewing movements [VCMs] induced by chronic neuroleptics). Three groups of animals received injections of fluphenazine decanoate (25 mg/kg) or its vehicle sesame oil every 3 weeks for at least 18 weeks. They were then surgically implanted with bilateral guide cannulae aimed at the striatum, the substantia nigra pars reticulata, or the globus pallidus respectively. After recovery, animals were infused with 2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-imethoxyphenyl)pyrazol-3-yl)carbonylamino]tricyclo(3.3.1.1.(3.7))decan-2-carboxylic acid (SR48692; 0.25, 0.50, and 1.0 nmol/microl), or its vehicle (10% dimethyl sulfoxide [DMSO] in saline) and observed for 60 min. Intra-striatal, intra-nigral or intra-pallidal infusion of SR48692 attenuated neuroleptic-induced VCMs. These findings lend further support to a role for neurotensin in the development of VCMs but do not clarify which pathway plays a more important role. Thus, treatments that reduce or prevent the effects of increased neurotensin expression and release may be useful in the management of tardive dyskinesia.     

Dopamine D(1A) receptor function in a rodent model of tardive dyskinesia

Tardive dyskinesia develops as a common complication of long-term neuroleptic use. The emergence of such dyskinesias may reflect a shift in the balance of dopamine D(1) and D(2) receptor-mediated activity, with a relative increase in activity in the D(1) receptor-regulated direct striatonigral pathway. In rats, chronic treatment with the antipsychotic fluphenazine triggers a syndrome of vacuous chewing movements, which are attenuated by dopamine D(1) receptor antagonists. A similar syndrome can be seen in drug-naive animals following acute administration of selective dopamine D(1) receptor agonists. However, not all dopamine D(1) receptor agonists elicit these mouth movements. Thus, some investigators have suggested the existence of novel subtypes of the dopamine D(1) receptor. In these studies, we sought to clarify the role of the dopamine D(1A) receptor in vacuous chewing movements induced both by the selective dopamine D(1) receptor agonist SKF 38393, as well as by chronic neuroleptic administration, using in vivo oligonucleotide antisense to dopamine D(1A) receptor messenger RNA. Intrastriatal antisense treatment significantly and selectively attenuated striatal dopamine D(1) receptor binding, accompanied by reductions in SKF 38393- and chronic fluphenazine-induced vacuous chewing movements.These findings suggest that the dopamine D(1A) receptor plays an important role in the expression of vacuous chewing movements in a rodent model of tardive dyskinesia and may contribute to the pathogenesis of the human disorder. This may have important implications for the treatment of tardive dyskinesia in humans.     

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.     

Dopamine-opiate interaction in the regulation of neostriatal and pallidal neuronal activity as assessed by opioid precursor peptides and glutamate decarboxylase messenger RNA expression.

Neostriatal GABAergic neurons projecting to the globus pallidus synthesize the opioid peptide enkephalin, while those innervating the substantia nigra pars reticulata and the entopeduncular nucleus synthesize dynorphin. The differential control exerted by dopamine on the activity of these two efferent projections concerns also the biosynthesis of these opioid peptides. Using in situ hybridization histochemistry, we investigated the role of opioid co-transmission in the regulation of neostriatal and pallidal activity. The expression of the messenger RNAs encoding glutamate decarboxylase-the biosynthetic enzyme of GABA-and the precursor peptides of enkephalin (preproenkephalin) and dynorphin (preprodynorphin) were measured in rats after a sustained blockade of opioid receptors by naloxone (s.c. implanted osmotic minipump, eight days, 3 mg/kg per h), and/or a subchronic blockade of D2 dopamine receptors by haloperidol (one week, 1.25 mg/kg s.c. twice a day). The density of mu opioid receptors in the neostriatum and globus pallidus was determined by autoradiography. Naloxone treatment resulted in a strong up-regulation of neostriatal and pallidal mu opioid receptors that was not affected by the concurrent administration of haloperidol. Haloperidol alone produced a moderate down-regulation of neostriatal and pallidal micro opioid receptors. Haloperidol strongly stimulated the expression of neostriatal preproenkephalin and preprodynorphin messenger RNAs. This effect was partially attenuated by naloxone, which alone produced moderate increases in preproenkephalin and preprodynorphin messenger RNA levels. In the neostriatum, naloxone did not affect either basal or haloperidol-stimulated glutamate decarboxylase messenger RNA expression. A strong reduction of glutamate decarboxylase messenger RNA expression was detected over pallidal neurons following either naloxone or haloperidol treatment, but concurrent administration of the two antagonists did not result in a further decrease. The amplitude of the variations of mu opioid receptor density and of preproenkephalin and preprodynorphin messenger RNA levels suggests that the regulation of neostriatal and pallidal micro opioid receptors is more susceptible to a direct opioid antagonism, while the biosynthesis of opioid peptides in the neostriatum is more dependent on the dopaminergic transmission. The down-regulation of mu opioid receptors following haloperidol represents probably an adaptive change to increased enkephalin biosynthesis and release. The haloperidol-induced increase in neostriatal preprodynorphin messenger RNA expression might result from an indirect, intermittent stimulation of neostriatal D1 receptors. The haloperidol-induced decrease of pallidal glutamate decarboxylase messenger RNA expression suggests, in keeping with the current functional model of the basal ganglia, that the activation of the striatopallidal projection produced by the interruption of neostriatal dopaminergic transmission reduces the GABAergic output of the globus pallidus. The reduction of pallidal glutamate decarboxylase messenger RNA expression following opioid receptor blockade indicates an indirect, excitatory influence of enkephalin upon globus pallidus neurons and, consequently, a functional antagonism between the two neuroactive substances (GABA and enkephalin) of the striatopallidal projection in the control of globus pallidus output. Through this antagonism enkephalin could partly attenuate the GABA-mediated effects of a dopaminergic denervation on pallidal neuronal activity.     

D1 and D2 dopamine receptors differentially regulate c-fos expression in striatonigral and striatopallidal neurons.

The expression of Fos, the product of the proto-oncogene c-fos, is thought to be a marker of neuronal activity. D1, but not D2, dopamine receptor agonists have previously been shown to increase Fos immunoreactivity in striatonigral neurons ipsilateral to a 6-hydroxydopamine lesion of the nigrostriatal pathway. In the present study, it was demonstrated that the D1 receptor agonist SKF 38393 rarely increased Fos in striatopallidal neurons of the 6-hydroxydopamine denervated striatum. Conversely, in the intact striatum, the D2 receptor antagonist haloperidol enhanced Fos expression predominantly in striatopallidal neurons labelled retrogradely from the globus pallidus or with an oligonucleotide probe complementary to mRNA encoding enkephalin. These results are consistent with studies suggesting that D1 receptors are located predominantly on striatonigral neurons and that D2 receptors reside principally on enkephalin-containing striatopallidal neurons. They also provide a neuroanatomical basis for neurochemical and neurophysiological observations indicating that dopamine facilitates the activity of striatonigral neurons but inhibits striatopallidal neurons. In another experiment the selective D2 receptor agonist quinpirole was found to increase Fos immunoreactivity in the globus pallidus ipsilateral to a 6-hydroxydopamine lesion. It is proposed that this may have been due to a D2 receptor-mediated inhibition of enkephalin and GABA release from striatopallidal terminals that in turn disinhibited the pallidal neurons. In a final series of experiments, brain microdialysis was used to determine the location of dopamine receptors regulating striatal Fos expression. Local application of the selective D1 receptor agonist CY 208-243 in the 6-hydroxydopamine-denervated striatum, or of haloperidol in the intact striatum via the dialysis probe increased Fos immunoreactivity in the immediate vicinity of the probe. Hence, the inductive effects of these systematically administered compounds on Fos expression in the striatum are mediated at least partly by local dopamine receptors in the striatum. Taken together, these results suggest that the differential regulation of striatonigral and striatopallidal activity by dopamine is mediated by the largely separate location of D1 and D2 receptors on these outputs.     

Different motivational effects induced by the endogenous mu-opioid receptor ligands endomorphin-1 and -2 in the mouse

The present study was designed to investigate the motivational effects of the newly discovered endogenous mu-opioid receptor ligands, endomorphin-1 and endomorphin-2, using the conditioned place preference paradigm in mice. The binding properties of these peptides were first examined using an opioid binding assay. In membranes obtained from the mouse whole brain, the binding of [3H][D-Ala2, NMePhe4, Gly(ol)5]enkephalin (DAMGO; mu), but not of [3H][D-Phe2, D-Phe5]enkephalin (DPDPE; delta) or [3H]U69593 (kappa) selectively and concentration-dependently competed with that of endomorphin-1 and endomorphin-2, indicating that both endomorphin-1 and endomorphin-2 are specific ligands for mu-opioid receptors in the brain. Endomorphin-1 (1-30 nmol/mouse) given i.c.v. produced a dose-related place preference. This effect was abolished by pre-treatment with the mu-opioid receptor antagonist beta-funaltrexamine but not the delta-opioid receptor antagonist naltrindole or the kappa-opioid receptor antagonist nor-binaltorphimine. In contrast, endomorphin-2 (5.6 nmol/mouse) produced place aversion. This aversive effect was inhibited by nor-binaltorphimine as well as beta-funaltrexamine, but not by naltrindole. The place aversion produced by endomorphin-2 was also attenuated by pre-treatment with antiserum against the endogenous kappa-opioid receptor ligand dynorphin A (1-17). These findings indicate that endomorphin-1 may produce its rewarding effect via mu-opioid receptors. On the other hand, the aversive effect induced by endomorphin-2 may be associated with the stimulation of endomorphin-1-insensitive mu-opioid receptors and the activation of dynorphinergic systems in the mouse brain.     

The distribution of dynorphinergic terminals in striatal target regions in comparison to the distribution of substance P-containing and enkephalinergic terminals in monkeys and humans.

Single- and double-label immunohistochemical techniques using several different highly specific antisera against dynorphin peptides were used to examine the distribution of dynorphinergic terminals in globus pallidus and substantia nigra in rhesus monkeys and humans in comparison to substance P-containing and enkephalinergic terminals in these same regions. Similar results were observed in monkey and human tissue. Dynorphinergic fibers were very abundant in the medial half of the internal pallidal segment, but scarce in the external pallidal segment and the lateral half of the internal pallidal segment. In substantia nigra, dynorphinergic fibers were present in both the pars compacta and reticulata. Labeling of adjacent sections for enkephalin or substance P showed that the dynorphinergic terminals overlapped those for substance P in the medial half of the internal pallidal segment, but showed only slight overlap with enkephalinergic terminals in the external pallidal segment. The substance P-containing fibers were moderately abundant along the borders of the external pallidal segment, and enkephalinergic fibers were moderately abundant in parts of the internal pallidal segment. Dynorphinergic and substance P-containing terminals overlapped extensively in the nigra, and both extensively overlapped enkephalinergic fibers in medial nigra. Immunofluorescence double-labeling studies revealed that dynorphin co-localized extensively with substance P in individual fibers and terminals in the medial half of the internal pallidal segment and in substantia nigra. Thus, as has been found in non-primates, dynorphin within the striatum and its projection systems appears to be extensively localized to substance P-containing striatopallidal and striatonigral projection neurons. Nonetheless, our results also raise the possibility that a population of substance P-containing neurons that projects to the internal pallidal segment and does not contain dynorphin is present in primate striatum. Our results also suggest the possible existence of populations of striatopallidal and striatonigral projection neurons in which substance P and enkephalin or dynorphin and enkephalin, or all three, are co-localized. Thus, striatal projection neurons in primates may not consist of merely two types, one containing substance P and dynorphin and the other enkephalin.     

Metabotropic glutamate mGluR5 receptor blockade opposes abnormal involuntary movements and the increases in glutamic acid decarboxylase mRNA levels induced by l-DOPA in striatal neurons of 6-hydroxydopamine-lesioned rats

The present study examined the effect of a subchronic systemic administration of the glutamate metabotropic mGluR5 receptor antagonist MPEP on l-DOPA-induced dyskinesias and striatal gene expression in adult rats with a unilateral 6-OHDA lesion of dopamine neurons. The daily systemic administration of l-DOPA for 2 weeks induced a gradual increase in limb dyskinesia and axial dystonia. The subchronic systemic co-administration of MPEP reduced the severity of limb dyskinesia and axial dystonia over the whole duration of l-DOPA treatment. Subchronic l-DOPA administration was paralleled by a significant increase in mRNA levels of the two isoforms of the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD67 and GAD65) and preprodynorphin (PPD). Single cell analysis on emulsion radioautographs indicated that l-DOPA-induced increases in GAD67 occurred predominantly in preproenkephalin-unlabeled striatonigral and, to a lesser extent, in preproenkephalin-labeled striatopallidal neurons. MPEP completely reversed the effects of l-DOPA on GAD67 and reduced the increases in GAD65 and PPD mRNA levels in striatonigral neurons. MPEP also reversed the small l-DOPA-induced increase in GAD67 mRNA levels in striatopallidal neurons. Altogether, the findings support the idea that the relative efficacy of mGluR5 receptor antagonists to oppose l-DOPA-induced abnormal involuntary movements involves an ability to oppose increases in GAD gene expression and GABA-mediated signaling in striatonigral and striatopallidal neurons. The results also confirm the potential usefulness of antagonists of mGluR5 receptors as adjuncts in the treatment of l-DOPA-induced dyskinesia in patients with Parkinson's disease.     

Acute administration of haloperidol induces apoptosis of neurones in the striatum and substantia nigra in the rat.

Chronic administration of typical neuroleptics is associated with tardive dyskinesia in some patients. This dyskinetic syndrome has been associated with loss of GABAergic markers in the basal ganglia but the cause of these GABAergic depletions remains uncertain. Haloperidol, a commonly prescribed typical neuroleptic, is known to be toxic in vitro, possibly as a consequence of its conversion to pyridinium-based metabolites and potentially by raising glutamate-mediated transmission. We report here that the in vivo, acute administration of a large dose of haloperidol resulted in a microglial response indicative of neuronal damage. This was accompanied by an increase in the number of apoptotic cells in the striatum (especially in the dorsomedial caudate putamen) and in the substantia nigra pars reticulata. These apoptotic cells were characterised by the stereotaxic injection of a retrograde neuroanatomical tracer into the projection targets of the striatum and substantia nigra pars reticulata prior to the systemic injection of haloperidol. This procedure confirmed that the dying cells were neurones and demonstrated that within the striatum the majority were striatopallidal neurones though relatively high levels of apoptotic striatoentopeduncular neurones were also seen.The possibility that chronic administration of haloperidol could induce cumulative neuronal loss in the substantia nigra pars reticulata and thereby induce the pathological changes which lead to tardive dyskinesia is discussed.     

Systemic administration of adenosine A(2A) receptor antagonist reverses increased GABA release in the globus pallidus of unilateral 6-hydroxydopamine-lesioned rats: a microdialysis study

The ability of adenosine A(2A) receptor antagonists to exhibit antiparkinsonian activity has recently been reported, but the mechanisms of action are still unknown. Since A(2A) receptors have been localized to GABAergic striatopallidal neurons, it is probable that these antagonists affect the activity of these neurons. In the present study, extracellular GABA basal levels were increased in the ipsilateral striatum and globus pallidus following a unilateral 6-hydroxydopamine lesion of the nigrostriatal pathway. The A(2A) receptor-selective antagonist KW-6002 (3mg/kg, p.o.) caused a marked and sustained decrease of extracellular GABA levels in the globus pallidus of the 6-hydroxydopamine-lesioned rats, whereas no changes in GABA levels were observed in the globus pallidus of the non-lesioned rats. Microinjection of the A(2A) receptor agonist CGS21680 (0.005-0.5 microg) into the striatum of non-lesioned animals increased GABA concentrations in the globus pallidus, which was abolished by the voltage-dependent Na(+) channel blocker tetrodotoxin (1 micromol/l) delivered locally to the globus pallidus via the dialysis membrane. Furthermore, intrapallidal infusion of CGS21680 (10 micromol/l) also increased GABA levels in the globus pallidus.These data indicate that GABA release from striatopallidal neurons is regulated through A(2A) receptors in both the striatum and globus pallidus. The reversal of the 6-hydroxydopamine-induced increase in pallidal GABA levels by KW-6002 suggests that the antiparkinsonian effects of A(2A) receptor antagonists occur on the striatopallidal neurons.     

Roles of opioid receptor subtypes on the antinociceptive effect of intrathecal sildenafil in the formalin test of rats

Recently, it has been known that the antinociception of sildenafil, a phosphodiesterase 5 inhibitor, is mediated through the opioid receptors. There are common three types of opioid receptors mu, delta, and kappa. We characterized the role of subtypes of opioid receptor for the antinociception of sildenafil at the spinal level. Intrathecal catheters were placed for drug delivery and formalin solution (5%, 50 microl) was injected for induction of nociception within male SD rats. The effect of mu opioid receptor antagonist (CTOP), delta opioid receptor antagonist (naltrindole), and kappa opioid receptor antagonist (GNTI) on the activity of sildenafil was examined. Intrathecal sildenafil decreased the flinching responses during phases 1 and 2 in the formalin test. Intrathecal CTOP and naltrindole reversed the antinociception of sildenafil during both phases in the formalin test. Intrathecal GNTI reversed the effect of sildenafil during phase 2, but not phase 1. These results suggest that sildenafil is effective to acute pain and the facilitated pain state at the spinal level. Both mu and delta opioid receptors are involved. However, it seems that kappa opioid receptors play in the effect of sildenafil.     

Anatomically distinct output channels of the caudate nucleus and orofacial dyskinesia: critical role of the subcommissural part of the globus pallidus in oral dyskinesia

The findings in this feline study indicate that the enkephalin-positive subcommissural part of the globus pallidus, which is known to contain GABA and cholinergic cells projecting to the cortex, is innervated by the anterodorsal region of the caudate nucleus, but not by the core. Like stimulation of a particular subclass of dopamine receptors in the anterodorsal region of the caudate nucleus, inhibition of the GABA receptors in the noted part of the globus pallidus resulted in orofacial dyskinesia, viz. tic-like contractions of the facial, eye and ear muscles, and tongue protrusions. This phenomenon was elicited by intrapallidal injections of the GABA antagonist picrotoxin in a dose-dependent manner and could be attenuated by the GABA agonist muscimol. Previous studies have already shown that neither stimulation of the dopamine receptors in the core of the caudate nucleus nor any manipulation with the first- and second-order output-stations of the latter brain region, viz. (a) those regions of the substantia nigra, pars reticulata which receive afferents from the caudate nucleus, and (b) those regions of the intermediate layers of the superior colliculus which receive afferents from the latter nigral region, ever resulted in orofacial dyskinesia. These findings support the hypothesis that the anatomically distinct input-output channels of the caudate nucleus are differentially involved in orofacial dyskinesia. The clinical impact of these findings is discussed in view of the L-3,4-dihydroxyphenylalanine-induced tardive dyskinesia in man. In addition, the relevance of the anatomical data is discussed in view of the co-occurrence of Parkinson's Disease and Dementia of Alzheimer-type in certain patients.     

Involvement of mu- and delta-opioid receptors in the antinociceptive effects induced by AMPA receptor antagonist in the spinal cord of rats

The present study was performed to explore the involvement of opioid receptors in the antinociception induced by a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonist in rats. The hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation was assessed by hot plate test and the Randall Selitto Test. Intrathecal injection of 20 nmol of 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo[f]quinoxaline-7-sulfonamide (NBQX) disodium, a competitive AMPA receptor antagonist, increased significantly the HWLs to both thermal and mechanical stimulation in rats. The increased HWLs induced by NBQX were dose-dependently attenuated by the opioid receptor antagonist naloxone, while naloxone itself had no marked influences on the HWL of rats. Furthermore, the increased HWLs induced by NBQX were inhibited by the mu-opioid antagonist beta-funaltrexamine (beta-FNA) or the delta-opioid antagonist naltrindole, but not by the kappa-opioid antagonist nor-binaltorphimine (nor-BNI). The results suggest that mu- and delta-opioid receptors, not kappa-opioid receptor, are involved in the antinociception induced by AMPA antagonist in the spinal cord of rats.     

Presynaptic auto- and allelo-receptor regulation of hypothalamic opioid peptide release

Recent studies have shown that inhibitory feedback mechanisms regulate the release of the endogenous opioid peptides beta-endorphin (acting predominantly at mu opioid receptors in the brain), dynorphin (a kappa opioid receptor ligand) and [Met]enkephalin (a delta opioid receptor ligand) from the rat hypothalamus. By using specific antagonists of the various opioid receptor types, it is shown that the release of these peptides from hypothalamic slices in vitro is not only controlled by homologous (auto)-receptors, but that cross-regulation between the three neuronal opioid receptor types also occurs; thus, the delta receptor antagonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu increases the release of all three peptides, the mu receptor antagonist D-tetrahydroisoquinoline-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 increases that of beta-endorphin and dynorphin, and the kappa receptor antagonist nor-binaltorphimine increases that of dynorphin; all these effects occur in the presence of tetrodotoxin, indicating a presynaptic site of action. We propose the term "allelo-receptors" to describe this particular form of neuronal regulation in which an endogenous ligand, acting via its own specific receptor, also regulates the release of related peptides which activate different classes of opioid receptors.     

Differential role of GABAA and GABAB receptors in two distinct output stations of the rat striatum: studies on the substantia nigra pars reticulata and the globus pallidus

The role of GABA_A and GABA_B receptors in the substantia nigra pars reticulata and the globus pallidus in turning behaviour of rats was studied. Unilateral injection of the GABA_A receptor agonist muscimol (25 and 50 ng) into the substantia nigra pars reticulata elicited contralateral pivoting, namely tight head-to-tail turning marked by abnormal hindlimb backward stepping. This effect was GABA_A receptor specific, since it was dose-dependent and prevented by co-administration of the GABA_A receptor antagonist bicuculline (100 and 200 ng) which alone did not elicit turning behaviour. Unilateral injection of the GABA_B receptor agonist baclofen (100 and 200 ng) into the substantia nigra pars reticulata also produced contralateral pivoting. This effect was GABA_B receptor specific, since it was dose-dependent and inhibited by the GABA_B receptor antagonist CGP 55845 (200 ng) which alone did not elicit turning behaviour. In contrast, unilateral injection of bicuculline (100 and 200 ng) into the globus pallidus produced contralateral circling, namely turning marked by normal stepping. This effect was GABA_A receptor specific, since it was dose-dependent and prevented by muscimol (50 ng), which alone did not elicit turning behaviour. Unilateral injection of baclofen (100 and 200 ng) into the globus pallidus dose-dependently produced ipsilateral pivoting; this effect was inhibited by CGP 55845 (200 ng), which alone did not elicit turning behaviour. The present study demonstrates that GABA_A and GABA_B receptors in the globus pallidus and the substantina nigra pars reticulata play differential roles in the production of turning behaviour. This study underlines the notion that the two types of turning, namely pivoting and circling, are valid tools to map out the information flow across the basal ganglia.     

The 'marginal division': a new subdivision in the neostriatum of the rat

Using a combination of anterograde and retrograde (Phaseolus vulgaris leucoagglutinin; PHA-L and wheat germ agglutinin conjugated horseradish peroxidase; WGA-HRP) tract-tracing methods and histochemical techniques, a new subdivision of the neostriatum, the marginal division, has been found in the rat brain. The marginal division is approximately 120 microns wide and is located at the caudal extent of the neostriatum and surrounds the rostral edge of the globus pallidus. The neuronal somata of the marginal division are mostly fusiform in shape, with their long axes running parallel to the border between the striatum and the globus pallidus. Histochemically, the marginal division is lighter in AChE staining, is more densely filled with Met-enkephalin-immunoreactive terminals, and has fewer choline acetyltransferase (ChAT)-immunoreactive neurons than does the rest of the neostriatum. Injections of PHA-L or WGA-HRP demonstrated that the projections of the marginal division differ from those of the main body of the striatum. The striatopallidal projection from the marginal division terminates in the caudal-most part of the globus pallidus which is rich in cholinergic neurons. In contrast, the projection from the main region of the neostriatum terminates in two bands in the globus pallidus, both of which are rostral to the area of termination of the fibres from the marginal division. The striatonigral fibres from the marginal division terminate in the caudal part of the substantia nigra pars reticulata whereas the rest of neostriatum projects to a more rostral region. Based on its cellular morphology, immunohistochemistry and projection pattern, we conclude that the marginal division of the striatum is a distinct subdivision of the neostriatum.     

The pattern of neurodegeneration in Huntington's disease: a comparative study of cannabinoid, dopamine, adenosine and GABA(A) receptor alterations in the human basal ganglia in Huntington's disease

In order to investigate the sequence and pattern of neurodegeneration in Huntington's disease, the distribution and density of cannabinoid CB(1), dopamine D(1) and D(2), adenosine A(2a) and GABA(A) receptor changes were studied in the basal ganglia in early (grade 0), intermediate (grades 1, 2) and advanced (grade 3) neuropathological grades of Huntington's disease. The results showed a sequential pattern of receptor changes in the basal ganglia with increasing neuropathological grades of Huntington's disease. First, the very early stages of the disease (grade 0) were characterized by a major loss of cannabinoid CB(1), dopamine D(2) and adenosine A(2a) receptor binding in the caudate nucleus, putamen and globus pallidus externus and an increase in GABA(A) receptor binding in the globus pallidus externus. Second, intermediate neuropathological grades (grades 1, 2) showed a further marked decrease of CB(1) receptor binding in the caudate nucleus and putamen; this was associated with a loss of D(1) receptors in the caudate nucleus and putamen and a loss of both CB(1) and D(1) receptors in the substantia nigra. Finally, advanced grades of Huntington's disease showed an almost total loss of CB(1) receptors and the further depletion of D(1) receptors in the caudate nucleus, putamen and globus pallidus internus, and an increase in GABA(A) receptor binding in the globus pallidus internus.These findings suggest that there is a sequential but overlapping pattern of neurodegeneration of GABAergic striatal efferent projection neurons in increasing neuropathological grades of Huntington's disease. First, GABA/enkephalin striatopallidal neurons projecting to the globus pallidus externus are affected in the very early grades of the disease. Second, GABA/substance P striatonigral neurons projecting to the substantia nigra are involved at intermediate neuropathological grades. Finally, GABA/substance P striatopallidal neurons projecting to the globus pallidus internus are affected in the late grades of the disease. In addition, the finding that cannabinoid receptors are dramatically reduced in all regions of the basal ganglia in advance of other receptor changes in Huntington's disease suggests a possible role for cannabinoids in the progression of neurodegeneration in Huntington's disease.     

Discharge rate of substantia nigra pars reticulata neurons is reduced in non-parkinsonian monkeys with apomorphine-induced orofacial dyskinesia

Involuntary movements (dyskinesia) are a common symptom of dopamine-replacement therapy in parkinsonian patients, neuroleptic drug treatment of mental patients, and tic disorders. Levodopa-induced dyskinesia has been shown to be associated with substantial reduction of firing rate in the internal part of the globus pallidus. This study characterizes the changes that occur in the activity of the substantia nigra pars reticulata (SNr) of non-parkinsonian (normal) monkeys with apomorphine (APO)-induced orofacial dyskinesia. We conducted extracellular recordings of SNr neurons of two monkeys before and after induction of orofacial dyskinesia by systemic administration of APO. Involuntary orofacial movements appeared a few minutes after the injections and lasted 20-40 min. Almost all recorded neurons changed their firing rate after APO injection (96%), and most declined (70%). The mean amplitude of decreases was also larger than that of increases (40 vs. 21% of the control rate). Changes in firing pattern were not significant on average. Pairs of SNr neurons were uncorrelated before APO injection, similar to the normal pallidum. However, unlike the increased correlations in the pallidum that accompany parkinsonism, orofacilal dyskinesia in non-parkinsonian monkeys was not associated with changes in correlation between SNr neurons. We conclude that normal monkeys treated with APO can model orofacial dyskinesia and tic disorders that are a consequence of dopaminergic over-activity. These symptoms appear to be more related to reduced firing rate of SNr neurons and thus to disinhibition of their targets, than to changes in pattern and synchronization.     

Convergence of synaptic inputs from the striatum and the globus pallidus onto identified nigrocollicular cells in the rat: a double anterograde labelling study.

Two major sources of afferent synaptic inputs to projection neurons in the rat substantia nigra reticulata are the striatum and the globus pallidus. In order to understand better the functional relationships between these two afferents in the control of the activity of nigrofugal neurons, experiments have been performed to test the possibility that single nigrofugal cells receive convergent synaptic inputs from the striatum and the globus pallidus. To address this question we have used two different approaches. First, we have developed a double anterograde labelling technique suitable for both light and electron microscopy and combined this procedure with the retrograde transport of lectin-conjugated horseradish peroxidase in order to retrogradely label the nigrocollicular cells. Second, we have combined the anterograde transport of Phaseolus vulgaris-leucoagglutinin from the globus pallidus and immunocytochemistry for DARPP-32 as a marker for the striatal terminals, with the retrograde transport of lectin-conjugated horseradish peroxidase from the superior colliculus. In the double anterograde labelling experiment, biocytin was injected in the striatum, Phaseolus vulgaris-leucoagglutinin in the globus pallidus and lectin-conjugated horseradish peroxidase in the superior colliculus. Following these injections, rich plexuses of biocytin- and Phaseolus vulgaris-leucoagglutinin-labelled terminals were found in the ventral two-thirds of the substantia nigra. The biocytin-positive terminals (striatonigral) were generally small and formed rich plexuses without any apparent neuronal association whereas the Phaseolus vulgaris-leucoagglutinin-labelled terminals (pallidonigral) were much larger and formed baskets around the perikarya of retrogradely and non retrogradely labelled cells in the substantia nigra reticulata. In areas of the substantia nigra reticulata where the fields of biocytin- and Phaseolus vulgaris-leucoagglutinin-labelled terminals overlapped, the perikarya and the proximal dendrites of retrogradely and non retrogradely labelled cells were found to be apposed by numerous Phaseolus vulgaris-leucoagglutinin-immunoreactive pallidonigral terminals and a few biocytin-labelled striatonigral terminals. In the sections prepared for electron microscopy, the biocytin was localized using 3,3'-diaminobenzidine tetrahydrochloride whereas Phaseolus vulgaris-leucoagglutinin was localized using benzidine dihydrochloride. It was thus possible to distinguish the biocytin- from the Phaseolus vulgaris-leucoagglutinin-labelled terminals in the electron microscope by the texture of the reaction product associated with them.4+ Examination of 231 biocytin-labelled (striatonigral) terminals and 105 Phaseolus vulgaris-leucoagglutinin-immunoreactive (pallidronigral) terminals revealed that the striatonigral terminals were generally small, contained few mitochondria and formed symmetric synapses predominantly with the distal dendrites (77%) and far less frequently with the perikarya (3%) of substantia nigra reticulata cells.(ABSTRACT TRUNCATED AT 400 WORDS)