The impact of a competitive and a non-competitive NMDA receptor antagonist on dopaminergic neurotransmission in the rat ventral tegmental area and substantia nigra

The study compares effects of the competitive and non-competitive NMDA receptor antagonists, CGP 40116 and MK-801 respectively, on the metabolism of dopamine and on the density of D-1 and D-2 dopaminergic receptors in the rat ventral tegmental area and substantia nigra. The effects of CGP 40116 were tested in a range of doses which either were devoid of or had locomotor- or stereotypy-stimulating effects. It was found that (1) CGP 40116 given in a dose of 5 mg/kg enhanced the locomotor activity of rats and evoked a stereotypy-like activity; doses of 1.25 and 2.5 mg/kg were devoid of such effects; (2) CGP 40116 (5 mg/kg) enhanced the concentrations of dopamine, DOPAC and HVA in the ventral tegmental area, whereas the lowest dose, 1.25 mg/kg was without effect; a dose of 2.5 mg/kg increased the concentration of dopamine only; the only effect of CGP 40116 (5 mg/kg) observed in substantia nigra, was an increase in dopamine concentration; its doses of 1.25 and 2.5 mg/kg were ineffective. (3) MK-801 (0.2 and 0.4 mg/kg) enhanced the concentrations of dopamine, DOPAC and HVA in both structures. A dose of 0.1 mg/kg increased the dopamine concentration only. The effects of MK-801 in substantia nigra were quantitatively weaker than those observed in ventral tegmental area. (4) Both CGP 40116 (5 mg/kg) and MK-801 (0.4 mg/kg) evolved alterations in the density of dopaminergic receptors. D-2 receptors, were up-regulated by MK-801 in ventral tegmental area and subregions of substantia nigra, i.e. pars compacta and pars reticulata, whereas CGP 40116 evoked similar effects in ventral tegmental area only. D-1 receptors in pars compacta and pars reticulata of substantia nigra were down-regulated after administration of either drug.It is concluded that competitive NMDA receptor antagonists in doses which evoke hyperlocomotion and stereotypy-like activity, may have a substantial impact on the dopaminergic neurotransmission in the rat ventral tegmental area and substantia nigra, similar to that described for MK-801, a non-competitive NMDA receptor antagonist. The obtained results may suggest that CGP 40116 and, possibly, other competitive NMDA antagonists may have dopaminomimetic properties, and that their clinical potentials may be limited by the risk of evoking dopamine-dependent psychotomimetic and abusing effects, similar to those described for MK-801.     

Specific neurokinin receptors mediate plasma extravasation in the rat knee joint.

1 Plasma extravasation in the rat knee joint was induced by intra-articular injection of neurokinins and specific neurokinin receptor agonists. 2 Pronounced plasma extravasation was produced by substance P (SP, 4-185 microM) and to a lesser extent by neurokinin-B (NKB, 83-413 microM), whereas neurokinin-A (NKA, 88-440 microM) and calcitonin gene-related peptide (CGRP, 26-130 microM) had no significant effect. 3 The specific neurokinin1 receptor agonist [Sar9, Met(O2)11]-substance P (NK1 agonist) in doses of 0.4-70 microM appeared to be more potent than SP in eliciting plasma extravasation. The neurokinin2 receptor agonist [Nle10]-neurokinin A4-10 (NK2 agonist) was not effective at 70 microM but produced a small and significant effect at 330 microM, whereas the neurokinin3 receptor agonist [MePhe7]-neurokinin B (NK3 agonist) was without effect at 40 microM or 400 microM. 4 Injections of SP or NKA into the synovial cavity of the rat knee were equally effective in producing marked plasma extravasation in remote sites such as the forelimb and hindlimb paws. 5 Co-administration experiments showed that the effects of SP were synergistic with NKA or the NK1 receptor agonist, but not with CGRP or the NK2 receptor agonist. 6 The rank order of potency was NK1 agonist greater than or equal to SP greater than NKB greater than NK2 agonist suggesting that NK1 receptors mediate plasma extravasation in the rat knee joint.     

Dopamine autoreceptor mediation of the effects of apomorphine on serotonin neurons

The effects of direct apomorphine (APO) infusion to the dorsal raphe and the substantia nigra on serotonergic neurons were examined in male rats. The results showed that APO infusion to the dorsal raphe failed to produce a significant effect on serotonin (5-HT) neurons in the dorsal raphe or 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the corresponding projection site, the striatum. Conversely, direct APO infusion to the substantia nigra mimicked the effects of systemic APO, namely, elevated 5-HT fluorescence in the dorsal raphe and increased 5-HT and 5-HIAA concentrations in the striatum. Serotonin neurons in the median raphe and its projection site, the hippocampus, were unaffected. Furthermore, horseradish peroxidase injection to the dorsal raphe resulted in specific cell labelling in the substantia nigra and fiber labelling in the ventral tegmental area. Together with previous findings that the serotonergic actions of systemic APO were antagonized by haloperidol or intraventricular 6-hydroxydopamine pretreatment; and the selective dopamine (DA) autoreceptor agonist 3-3-hydroxyphenyl-N-n-propyl-piperidine mimicked the effects of APO on 5-HT neurons, these results suggest that the observed effects of APO on the mesostriatal serotonergic system are probably mediated through DA autoreceptors in the substantia nigra and possibly by a direct nigroraphe pathway.     

Electrophysiological evidence that spiperone is an antagonist of 5-HT1A receptors in the dorsal raphe nucleus

The neuroleptic spiperone, which binds to 5-HT1A, 5-HT2 and dopamine (DA) receptors, was studied for its effects on serotonin (5-HT) and DA neurons in dorsal raphe nucleus and substantia nigra pars compacta, respectively. We found that 1 mg/kg i.v. spiperone, but not LY53837 (a 5-HT2 antagonist), antagonized the inhibition induced by 5-HT1A agonists 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) and buspirone in the dorsal raphe nucleus. Lower spiperone doses blocked DA receptors in substantia nigra pars compacta, but did not affect 5-HT neurons. Doses of 8-OH-DPAT completely silencing dorsal raphe neurons were ineffective in substantia nigra pars compacta. However, buspirone antagonized DA receptors in substantia nigra pars compacta with doses similar to those depressing dorsal raphe neurons. It is concluded that spiperone is an antagonist of 5-HT1A receptors in the dorsal raphe nucleus.     

Neurokinin peptides and neurokinin receptors as potential therapeutic intervention targets of basal ganglia in the prevention and treatment of Parkinson's disease

Parkinson's disease (PD) is a serious motor disorder and it is the second most common brain degenerative disease in human. PD is known to be caused by degeneration of dopamine neurons in the substantia nigra but the cause of cell death is largely unknown. Mammalian neurokinins [NKs] are a group of neuropeptides that include substance P (SP; neurokinin-1, NK-1), substance K (SK; NK-2; neurokinin A), and neuromedin K (NK; NK-3; neurokinin B). Their biological effects as neurotransmitters, neuromodulators, or neurotrophic-like factors are mediated by three distinct neurokinin receptors, namely SP receptor (SPR: NK-1 receptor, NK-1R), SKR (NK-2R), and NKR (NK-3R). Several lines of evidence have indicated that neurokinins are implicated in the pathogenesis of PD. First, decreases of SP level and SP-immunoreactivity have been found in nigral and striatal tissues of animals with PD and postmortem PD patients. Second, NKs exert neuroprotective effects on neurons. In addition, NK receptors, namely NK-1 and NK-3 receptors, are abundantly localized in dopaminergic and cholinergic neurons of the basal ganglia, indicating that these neurons are under the physiological regulation of NKs. Moreover, modulation in motor activity occurred in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice, PD animal model, after systemic administration of NK receptor agonists. NKs and NK receptors, therefore, might be important molecules that are associated with functions and survival of neurons in the basal ganglia, in particular the dopamine neurons. Further studies should be devoted to elucidate the functional roles of NK systems in (a) the neuropathogenesis and neuroprotection during the course of PD, (b) the efficacy of NK receptor drugs towards PD, and (c) potential therapeutic intervention that targets at the prevention or treatment of PD.     

Oleoylethanolamide exerts partial and dose-dependent neuroprotection of substantia nigra dopamine neurons

Oleoylethanolamide (OEA), agonist of nuclear PPAR-α receptors and antagonist of vanilloid TRPV1 receptors, has been reported to show cytoprotective properties. In this study, OEA-induced neuroprotection has been tested in vitro and in vivo models of 6-OHDA-induced degeneration of substantia nigra dopamine neurons. First, PPAR-α receptors were confirmed to be located in the nigrostriatal circuit, these receptors being expressed by dopamine neurons of the substantia nigra, and intrinsic neurons and fibers bundles of the dorsal striatum. In the substantia nigra, their location was confined to the ventral tier. The in vitro study showed that 1 μM OEA exerted a significantly neuroprotective effect on cultured nigral dopamine neurons, effects following U-shaped dose-response curves. Regarding the in vivo study, rats were locally injected with OEA into the right striatum and vehicle into the left striatum 30 min before 6-OHDA-induced striatal lesion. In the short term, signals of heme oxygenase-1 (oxidation marker, 24 and 48 h post-lesion) and OX6 (reactive microglia marker, 96 h post-lesion) were found to be significantly less intense in the striatum pretreated with 5 μM OEA. In the long term (1 month), reduction in striatal TH and synaptophysin was less intense whether the right striatum was pretreated with 5 μM OEA, and nigral TH+ neuron death was significantly reduced after pretreatment with 1 and 5 μM OEA. In vivo effects also followed U-shaped dose-response curves. In conclusion, OEA shows U-shaped partial and dose-dependent neuroprotective properties both in vitro and in vivo models of substantia nigra dopamine neuron degeneration. The occurrence of U-shaped dose-response relationships normally suggests toxicity due to high drug concentration or that opposing intracellular pathways are activated by different OEA doses.     

Substance P neurokinin 1 receptor activation within the dorsal raphe nucleus controls serotonin release in the mouse frontal cortex

Preclinical studies suggest that substance P (SP) neurokinin 1 (NK1) receptor antagonists are efficient in the treatment of anxiety and depression. This therapeutic activity could be mediated via stimulation of serotonin (5-HT) neurons located in the dorsal raphe nucleus (DRN), which receive important SP-NK1 receptor immunoreactive innervations. The present study examined the effects of intraraphe injection of SP on extracellular 5-HT levels in the frontal cortex, ventral hippocampus, and DRN by using intracerebral microdialysis in conscious mice. Intraraphe SP injection dose dependently decreased cortical 5-HT release, whereas no effects were detected in the ventral hippocampus. Cortical effects were blocked by the selective NK1 receptor antagonist N-[[2-methoxy-5-[5-(trifluoromethyl)tetrazol-1-yl]phenyl]methyl]-2-phenylpiperidin-3-amine (GR205171) and completely dampened in mice lacking NK1 receptors. Furthermore, genetic (in knockout 5-HT1A(-/-) mice) or pharmacological inactivation of 5-HT1A autoreceptors blocked cortical responses to SP. Contrasting with its cortical effects, intraraphe SP injection increased 5-HT outflow in the DRN in wild-type mice; this effect was potentiated by a local perfusion of the selective 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY100635). Finally, SP-induced changes in frontal cortex and DRN dialysate 5-HT levels were blocked by the DRN perfusion of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate ionotropic receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). These data support the hypothesis that SP-induced over-activation of 5-HT1A autoreceptors within the DRN limits cortical 5-HT release. A better knowledge of the complex relationship between tachykininergic, serotonergic, and glutamatergic systems within the DRN might help better understand the pathophysiology and subsequent treatment of depression.     

Effects of NMDA and MK-801 injected into the substantia nigra pars reticulata on jaw movements evoked by dopamine D1-/D2 receptor stimulation in the ventrolateral striatum: studies in freely moving rats

The effects of NMDA and MK-801 injected into the substantia nigra pars reticulata on jaw movements evoked by dopamine D1/D2 receptor stimulation in the ventrolateral striatum were examined in freely moving rats, by using a magnet-sensing system combined with intracerebral drug microinjection technique. Bilateral injections of a mixture of SKF 82958 (5 microg) and quinpirole (10 microg), agonist at dopamine D1 and D2 receptors respectively, into the ventrolateral striatum elicited repetitive jaw movements. Bilateral injections of NMDA (0.01 and 0.05 microg/0.2 microl in each side) into the substantia nigra pars reticulata, which alone did not produce jaw movements, reduced the repetitive jaw movements evoked by the dopamine D1/D2 receptor agonist mixture in a dose-dependent manner. Injection of the non-competitive NMDA receptor antagonist, MK-801 (0.1 and 0.5 microg/0.2 microl in each side), into the substantia nigra pars reticulata, which alone did not produce jaw movements, prevented the dopaminergic jaw movements in a dose-dependent manner. Moreover, other behaviors such as grooming, rearing, yawning, vacuous chewing, and locomotor activity that occurred after injections of the dopamine receptor agonist mixture were not significantly altered by the bilateral injections of NMDA or MK-801 into the substantia nigra pars reticulata. Given our previous results showing that both agonist and antagonist of GABA(A) receptors injected into the substantia nigra pars reticulata inhibit the jaw movements elicited by dopamine D1/D2 receptor stimulation in the ventrolateral striatum, the present results suggest that there are complex functional interactions between NMDA and GABA(A) receptors within the substantia nigra pars reticulata that may be responsible for the common profiles in the effects of NMDA and GABA(A) receptor agents.     

Comparison of changes in locomotor activity with striatal homovanillic acid and 3,4-dihydroxyphenylacetic acid concentrations following the bilateral intranigral injection of dopamine agonist drugs in rats

Bilateral injection of apomorphine (2.5 micrograms) into the substantia nigra zona reticulata of rats reduced both locomotor activity and striatal HVA and DOPAC concentrations. Bilateral injection of dopamine (10 micrograms) did not affect locomotor activity whereas a higher dose of dopamine (50 micrograms) enhanced locomotor activity. Striatal HVA and DOPAC concentrations were unchanged following injection of dopamine. Bilateral injection of (+/-)-3PPP (0.1 or 2.5 micrograms) into the zona reticulata of the substantia nigra did not alter locomotor activity while a higher dose (10 micrograms) enhanced locomotion. Injection of (+/-)-3PPP (0.1-10 micrograms) into the zona reticulata was without effect on striatal HVA or DOPAC concentrations. The bilateral manipulation of nigral dopaminergic neurotransmission alters motor activity and nigrostriatal dopamine turnover in conscious rats. However, the changes in motor activity are not necessarily related to altered nigrostriatal activity, suggesting the involvement of dopamine receptors located at non-dopaminergic sites within the substantia nigra.     

Biochemical and immunohistological changes in the brain of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mouse.

We investigated neurochemically and neuropathologically the utility of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice as a model of Parkinson's disease. The changes in dopamine D1 and D2 receptors and dopamine uptake sites were determined by quantitative autoradiography using [3H]SCH23390, [3H]raclopride and [3H]mazindol, respectively. Dopamine and 3,4-dihydroxyphenyl acetic acid (DOPAC) contents in the striatum were measured by high-performance liquid chromatography. The distribution of nigral neurons and reactive astrocytes was determined by immunohistochemical staining with antibody against tyrosine hydroxylase (TH) and glial fibrillary acidic protein (GFAP). The mice received four intraperitoneal injections of MPTP (10 mg/kg) at 1-h intervals and then the brains were analyzed at 3 and 7 days after the treatments. No significant change in dopamine D1 receptors was observed in the striatum and substantia nigra after acute treatment with MPTP. Dopamine D2 receptors were reduced significantly in the substantia nigra only 7 days after the MPTP treatment, whereas striatum showed no significant change in the binding throughout the experiments. In contrast, dopamine uptake sites were reduced markedly in the striatum and substantia nigra 3 and 7 days after the MPTP treatment. Dopamine and DOPAC content were also reduced in the striatum 3 and 7 days after the MPTP treatment. An immunohistochemical study indicated a loss of the number of TH-positive neurons in the substantia nigra 7 days after the MPTP treatment. In contrast, numerous GFAP-positive astrocytes were evident in the striatum 7 days after the MPTP treatment. These results provide valuable information for the pathogenesis of acute stage of Parkinson's disease.     

Definition of the in-vivo binding of [3H]spiperone in rat brain using substituted benzamide drugs

The ability of some substituted benzamide drugs to define in-vivo the binding of [3H]spiperone to brain dopamine receptors in rats was assessed using behaviourally effective doses in comparison with haloperidol. As judged using haloperidol, [3H]spiperone identified dopamine receptors in the substantia nigra, striatum, tuberculum olfactorium and hypothalamus, but not in frontal cortex or nucleus accumbens. The substituted benzamide compounds alizapride, metoclopramide, clebopride and YM 09151-2 prevented the accumulation of [3H]spiperone in the substantia nigra, striatum, tuberculum olfactorium and hypothalamus. However, YM 09151-2 also caused displacement of [3H]spiperone accumulation in the nucleus accumbens and frontal cortex. (+/- )-Sulpiride, (+/- )-sultopride, amisulpiride and prosulpride all prevented the accumulation of [3H]spiperone in the hypothalamus but were ineffective in one or more of the other regions containing dopamine receptors defined by [3H]spiperone. The isomers of sulpiride and sultopride stereoselectively defined the accumulation of [3H]spiperone in dopamine containing brain regions. The (-)-isomers of both drugs prevented the accumulation of [3H]spiperone in the substantia nigra, striatum, tuberculum olfactorium and hypothalamus. In contrast, (+)-sulpiride and (+)-sultopride were ineffective. Selected substituted benzamide drugs can be used to define the interaction of ligands with dopamine receptors in-vivo. These substances may be useful in PET studies in man. The isomers of some substituted benzamine drugs may be used to define dopamine receptors in-vivo by enantiomeric selectivity.     

Demonstration of a 'septide-sensitive' inflammatory response in rat skin.

1. Measurement of plasma protein extravasation induced by the natural tachykinins following intradermal administration in rat skin indicated equipotency between substance P (SP), neurokinin A (NKA) and neurokinin B (NKB). The selective NK1 receptor agonist, [Sar9]SP sulphone was 10-100 times more potent than SP. The synthetic hexapeptide, septide, [pGlu6, Pro9]SP-(6-11), which has been proposed to act on a distinct NK1 receptor subtype/binding site was equipotent with [Sar9]SP sulphone. 2. The selective NK2 receptor agonist [beta Ala8]NKA(4-10) (0.1-1 nmol) and the selective NK3 receptor agonist, senktide (0.1-1 nmol) were both ineffective in producing oedema. The selective NK2 receptor antagonist, SR 48, 968 (0.3 mumol kg-1) had no significant inhibitory effects upon oedema induced by approximately equiactive doses of SP (0.2 nmol), septide (0.002 nmol), [Sar9]SP sulphone (0.002 nmol), or NKB (0.3 nmol). These results together suggest that neither NK2 nor NK3 receptors are involved in oedema formation in rat skin. 3. The non-peptide tachykinin NK1 receptor antagonist, RP 67,580 (1-3 mumol kg-1), inhibited plasma protein extravasation induced by septide (0.002 nmol) to a greater extent than that to SP (0.2 nmol). RP 67,580 (1 mumol kg-1) produced a significant inhibition of approximately 66% of the response to septide (0.002 nmol) only. Increasing the dose of RP 67,580 3 fold resulted in inhibition of the response to SP (0.2 nmol) and [Sar9]SP sulphone (0.002 nmol) by approximately 66% and 64% respectively with the response to septide being inhibited by approximately 70%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of naltrexone on the accumulation of L-3, 4-dihydroxyphenylalanine and 5-hydroxy-L-tryptophan and on the firing rate induced by acute ethanol administration

In order to characterize the effects of naltrexone, a mu-opioid receptor antagonist, on acute ethanol-induced functional modification of dopaminergic neurons in the nigrastriatal and mesolimbic dopamine systems, the accumulation of L-3, 4-dihydroxyphenylalanine (L-DOPA) in the cerebral cortex, dorsal striatum and nucleus accumbens and of 5-hydroxy-L-tryptophan (5-HTP) in the hippocampus was measured in normal rats using the mu-hydroxybenzylhydrazine dihydrochloride (NSD-1015) enzymatic inhibition method. In addition, the firing rates of dopaminergic neurons were recorded in the substantia nigra and ventral tegmental area. Naltrexone resulted in a decrease in the dopaminergic neuronal firing rates activated by ethanol and eventually in a reduction of the dopamine synthesis induced by ethanol in the dorsal striatum and nucleus accumbens, but not in the cerebral cortex. Mesolimbic dopamine neurons were slightly more sensitive to ethanol and naltrexone than were nigrostriatal dopamine neurons. The widespread inhibitory action of naltrexone also decreased the ethanol-induced stimulation of hippocampal serotonin synthesis.     

Influence of paraquat on dopaminergic transporter in the rat brain

Selective toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a parkinsonism inducing compound, is well known to be related to an uptake of its active metabolite MPP+ into dopaminergic neurons by dopamine transporter (DAT). The aim of the present study was to examine whether paraquat, a commonly used herbicide, which is an 1-methyl-4-phenyl-pyridinium ion (MPP+) analogue, affects DAT in vivo in rats. Paraquat administered at a dose of 10 mg/kg ip decreased the binding of [3H]GBR 12,935 to DAT measured by quantitative autoradiography in the dorsal and ventral caudate-putamen, but not in the substantia nigra pars compacta. Moreover, this compound increased the level of 3-methoxytyramine (3-MT) and 3-MT/dopamine ratio in the anterior and posterior caudate-putamen measured by HPLC with electrochemical detection. No other alterations in the levels of dopamine and its metabolites were found in the caudate-putamen and substantia nigra. The present study seems to suggest that systemic paraquat administration affects striatal DAT and dopamine metabolism in the nigrostriatal neurons in rats which may be crucial for its neurotoxic effects on dopaminergic neurons.     

Protective effect of 1-methyl-1,2,3,4-tetrahydroisoquinoline against dopaminergic neurodegeneration in the extrapyramidal structures produced by intracerebral injection of rotenone

The aim of this paper was to investigate whether rotenone, a pesticide causing experimental parkinsonism, causes direct damage to dopaminergic structure when injected intracerebrally and whether this action may be prevented by peripheral administration of 1-methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ), an endogenous compound with anti-dopaminergic activity. Male Wistar rats were injected unilaterally into the median forebrain bundle with 2 microg rotenone, and received 1MeTIQ, 50 mg/kg i.p. 1 h before and then daily for 21 d. To compare the effect of intracerebral and peripheral treatment, rotenone was also given once or for 7 d in a dose of 10 mg/kg s.c. Dopamine, serotonin and their metabolites were assessed by HPLC in the substantia nigra and striatum. While a single subcutaneous rotenone dose did not produce any change in striatal dopamine metabolism, the multiple treatments resulted in changes suggesting a shift in the metabolism towards oxidative desamination and reduction of O-methylation. In contrast to systemic injections, intracerebral-administered rotenone produced a decrease in dopamine and its metabolites content in the striatum (dopamine decrease by 70%) and substantia nigra (dopamine decrease by 35%), without affecting the serotonin system. As those changes were observed 21 d after the injection of rotenone, they suggest a durable neurotoxic effect. The treatment with 1MeTIQ strongly reduced the fall of striatal dopamine concentration. The data suggest that rotenone given peripherally affects metabolic processes in dopaminergic neurons, and this seems to result from its neurotoxic action, which may be observed after an intracerebral injection. 1MeTIQ is able to counteract the damaging action of rotenone and seems to be a potential neuroprotective agent.     

Detailed distribution of Neurokinin 3 receptors in the rat, guinea pig and gerbil brain: a comparative autoradiographic study

The neurokinin 3 (NK3) receptor is predominantly expressed in the central nervous system (CNS). Species differences in neurokinin 3 (NK3) receptor pharmacology have led to the preferential use of guinea pigs and gerbils in the characterization of non-peptide NK3 antagonists. Little is known about the central localization of NK3 receptors in the CNS of these species. To study this, [(3)H]senktide and [(3)H]SR 142801 were used in autoradiography experiments to visualize the NK3 receptors in the guinea pig and gerbil brain and compared to with the distribution of [(3)H]senktide binding sites in the rat brain. In the three species, the NK3 receptor was similarly distributed within the cerebral cortex, the zona incerta, the medial habenula, the amygdaloid complex, the superior colliculus and the interpeduncular nucleus. Outside of these structures, our study has revealed that each species displayed a specific distribution pattern of central NK3 receptors. The rat was the only species where NK3 receptors could be visualized in the striatum, the supraoptic nucleus and the paraventricular nucleus of the hypothalamus. The guinea pig differed mainly from the two other species by the absence of detectable binding sites in the substantia nigra pars compacta and the ventral tegmental area. A specific localization of NK3 receptors in the anterodorsal and anteroventral thalamic nuclei characterized the gerbil. This last species is also unique by in the higher level of NK3 receptors in the dorsal and median raphe nuclei. All these differences suggest that the NK3 receptor mediates different functions in different species.     

Baseline and apomorphine-induced extracellular levels of nigral substance P are increased in an animal model of Parkinson's disease.

Parkinson's disease arises from extensive premature loss of dopamine (DA)-utilizing midbrain neurons whose axons form the nigrostriatal pathway and densely innervate the striatum (caudate-putamen). The clinical symptoms of Parkinson's disease are improved by treatment with DA receptor agonists, including apomorphine. DA agonists modulate basal ganglia outflow at multiple sites and may affect synaptic activity of multiple basal ganglia neurotransmitters, including substance-P peptide (SP). With microdialysis in the substantia nigra reticulata (s.n.r.), a major striatal projection area, we monitored the extracellular level of SP-like immunoreactivity (SP-LI) in awake rats with neurotoxin-induced destruction of nigral DA neurons, before and after treatment with apomorphine (1 mg/kg i.p.). Loss of DA neurons produced a 60% increase in baseline extracellular SP-LI levels. Apomorphine treatment increased baseline extracellular SP-LI levels up to 60% in control rats and 100% in rats with loss of nigral DA neurons. Striatonigral SP endings increase SP release in response to loss of nigral target DA neurons, and DA agonist acting on supersensitive DA receptors, perhaps on SP terminals in the nigra, further increases synaptic SP concentrations.     

Morphine increases the activity of midbrain dopamine neurons in vitro

Morphine produced a dose-dependent increase in the activity of dopamine-containing neurons in the substantia nigra and ventral tegmental area recorded from mouse brain slices in vitro. The response was not changed in a low calcium/high magnesium incubation medium, indicating that the observed effects are the result of the direct action of morphine on dopamine neurons. Furthermore, a specific opiate antagonist, naloxone, reversed the excitatory effects of morphine in both brain regions, while naloxone alone had no significant effect on the activity of dopaminergic neurons. Morphine was more potent on ventral tegmental area than substantia nigra dopamine neurons. beta-Endorphin excited ventral tegmental area neurons but not substantia nigra cells, while [Leu5]enkephalin activated cells in both nuclei. These latter responses were blocked by naloxone. These data suggest that midbrain dopamine neurons contain receptors for opiates, and that ventral tegmental area neurons are more sensitive to the action of opiates than substantia nigra neurons.     

Substance P, neurokinins A and B, and synthetic tachykinin peptides protect mesencephalic dopaminergic neurons in culture via an activity-dependent mechanism

We evaluated the neuroprotective potential of tachykinin peptides using a model system in which mesencephalic dopaminergic (DA) neurons die spontaneously and selectively as they mature. The three native tachykinins, substance P (SP), neurokinin (NK) A, and NKB afforded substantial protection against DA cell demise. The selective NK1 receptor antagonist [D-Pro9,[spiro-gamma-lactam] Leu10,Trp11]substance P (GR71251) was sufficient in itself to suppress the effect of SP, whereas a cotreatment with GR71251 and the NK3 receptor antagonist (R)-N-[alpha-(methoxycarbonyl)benzyl]-2-phenylquinoline-4-carboxamide (SB218795) was required to prevent the effects of both NKA and NKB. Consistent with these results, D-Ala-[L-Pro9,Me-Leu8]substance P(7-11) (GR73632), a selective agonist of NK1 receptors and [pro7]-NKB, a selective agonist of NK3 receptors, conferred protection to DA neurons, whereas (Lys3, Gly8-R-gamma-lactam-Leu9)neurokinin A(3-10) (GR64349), which activates specifically NK2 receptors, did not. DA neurons rescued by tachykinins accumulated [3H]DA efficiently, which suggests that they were also totally functional. Neuroprotection by tachykinins was highly selective for DA neurons, rapidly reversed upon treatment withdrawal, and reproduced by but independent of glial cell line-derived neurotrophic factor. Survival promotion by tachykinins was abolished by blocking voltage-gated Na+ channels with tetrodotoxin or N-type voltage-gated Ca2+ channels with omega-conotoxin-MVIIA, which indicates that an increase in neuronal excitability was crucially involved in this effect. Together, these data further support the notion that the survival of mesencephalic DA neurons during development depends largely on excitatory inputs, which may be provided in part by tachykinins.     

Changes in muscle tone are regulated by D1 and D2 dopamine receptors in the ventral striatum and D1 receptors in the substantia nigra.

Muscle rigidity associated with antipsychotic drug treatment is believed to result from reduced striatal dopamine neurotransmission. In the current study the regulatory roles of dopamine D1 and D2 receptor subfamilies in the dorsal (DSTR) and ventral striatum (VSTR) and substantia nigra (SN) were investigated on muscle tone, assessed as increases in tonic electromyographic (EMG) activity. Rats were injected with the irreversible D1/D2 antagonist N-ethoxycarbonyl-2-ethoxy, -1,2-dihydroquinoline (EEDQ), the reversible D1 antagonist SCH23390, or D2 antagonist sulpiride. Increased EMG activity was observed following injection of EEDQ and SCH23390 into the SN or VSTR, and sulpiride into the VSTR. Mapping, using quantitative autoradiographic analysis of dopamine receptor occupancy after striatal injections, showed D1 and D2 receptors in discrete ventral sites were associated with EMG increases. Overall the results support roles for dopamine D1 and D2 receptors in the ventral striatum, and D1 receptors in the substantia nigra, in the regulation of muscle tone.