Dendrites Of Dopaminergic Neurons Research Articles

Melatonin attenuates MPTP-induced neurotoxicity via preventing CDK5-mediated autophagy and SNCA/α-synuclein aggregation

Autophagy is involved in the pathogenesis of neurodegenerative diseases including Parkinson disease (PD). However, little is known about the regulation of autophagy in neurodegenerative process. In this study, we characterized aberrant activation of autophagy induced by neurotoxin 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) and demonstrated that melatonin has a protective effect on neurotoxicity. We found an excessive activation of autophagy in monkey brain tissues and C6 cells, induced by MPTP, which is mediated by CDK5 (cyclin-dependent kinase 5). MPTP treatment significantly reduced total dendritic length and dendritic complexity of cultured primary cortical neurons and melatonin could reverse this effect. Decreased TH (tyrosine hydroxylase)-positive cells and dendrites of dopaminergic neurons in the substantia nigra pars compacta (SNc) were observed in MPTP-treated monkeys and mice. Along with decreased TH protein level, we observed an upregulation of CDK5 and enhanced autophagic activity in the striatum of mice with MPTP injection. These changes could be salvaged by melatonin treatment or knockdown of CDK5. Importantly, melatonin or knockdown of CDK5 reduced MPTP-induced SNCA/α-synuclein aggregation in mice, which is widely thought to trigger the pathogenesis of PD. Finally, melatonin or knockdown of CDK5 counteracted the PD phenotype in mice induced by MPTP. Our findings uncover a potent role of CDK5-mediated autophagy in the pathogenesis of PD, and suggest that control of autophagic pathways may provide an important clue for exploring potential target for novel therapeutics of PD.

Influence of r-type (Ca v2.3) and t-type (Ca v3.1–3.3) antagonists on nigral somatodendritic dopamine release measured by microdialysis

The release of dopamine from soma and dendrites of dopaminergic neurons in substantia nigra has been reported to be calcium-dependent, but it remains to be determined which calcium channels mediate this effect. We have used in vivo microdialysis in rat substantia nigra and striatum to investigate the effect of Ca v3.1–3.3 (T-type) and Ca v2.3 (R-type) calcium channel antagonists on somatodendritic and terminal dopamine release. Local reverse dialysis administration of 0.1–10 μM of the Ca v2.3 inhibitor SNX-482, or 100 μM of mibefradil, decreased the concentrations of dopamine and its metabolites in dialysate from substantia nigra, whereas 1 μM mibefradil or 40–80 μM nickel(II) induced an increase in nigral dialysate dopamine concentrations. Dopamine concentrations in striatal dialysates were decreased only by 10 μM of SNX-482 or 100 μM of mibefradil. Nickel(II) induced an increase in striatal dialysate dopamine concentration similar to that in substantia nigra. The results indicate a role for Ca v2.3 (R-type) voltage sensitive calcium channels in the calcium dependency of somatodendritic dopamine release, but argue against a calcium dependency mediated substantially by Ca v3.1–3.3 (T-type) channels.

Control of subcellular localization and function of inwardly rectifying K channels by PDZ-anchoring proteins

The control of subcellular localization is an essential mechanism for ion channels to play specific functional roles in various polarized cells, such as neurons and epithelial cells. We found that anchoring proteins that contain PDZ domains such as PSD-95/SAP90 and SAP97 play critical roles in the control of subcellular localization and function of some of inwardly rectifying K (Kir) channels. Kir4.1/KAB-2, cloned by our group, was shown to be predominantly expressed in glial cells in the brain and retina1.Takumi T. Ishii T. Horio Y. et al.A novel ATP-dependent inward rectifier potassium channel expressed predominantly in glial cells.J Biol Chem. 1995; 270: 16339-16346Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar, 2.Horio Y. Hibino H. Inanobe A. et al.Clustering and enhanced activity of an inwardly rectifying potassium channel, Kir4.1, by an anchoring protein, PSD-95/SAP 90 family.J Biol Chem. 1997; 272: 12885-12888Crossref PubMed Scopus (121) Google Scholar, 3.Ishii M. Horio Y. Tada Y. et al.Expression and clustered distribution of an inwardly rectifying potassium channel, KAB-2/Kir4.1, on mammalian retinal Müller cell membrane: their regulation by insulin and laminin signals.J Neurosci. 1997; 17: 7725-7735Crossref PubMed Google Scholar, 4.Nagelhus E.A. Horio Y. Inanobe A. et al.Subcellular coexpression of the inwardly rectifying K+ channel Kir4.1 and water aquaporin-4 in retinal Müller cells suggests coupling between K+ siphoning and water transport.Glia. 1999; 26: 47-54Crossref PubMed Scopus (388) Google Scholar. We found that (1) Kir4.1 is distributed in a clustered manner on the retinal Mueller cell membrane; (2) the clustering distribution is probably controlled by the interaction between Kir4.1 and SAP97, a PSD anchoring protein; (3) the C-terminal amino acids, SNV, in Kir4.1 may be critical for the protein-protein interaction; (4) laminin and insulin signals are essential for maintaining the clustered distribution of Kir4.1, which involves tyrosine kinase activity; (5) aquaporin 4 (AQP4), whose amino acids at the C-terminal end are LSSV, is co-localized with Kir4.1 on retinal Mueller cells, which seems to be controlled by associated proteins other than SAP97; and (6) SAP97 enhances the channel activity of Kir4.1 in a heterologous expression system. G protein-gated K (KG) channels generate slow inhibitory postsynaptic potentials in the brain5.Yamada M. Inanobe A. Kurachi Y. G protein regulation of potassium ion channels.Pharmacol Rev. 1998; 50: 723-757PubMed Google Scholar. Current opinion suggests that neuronal KG channels are heterotetramers of Kir3.1 and Kir3.2. Kir3.2 has at least four splicing variants named Kir3.2a-Kir3.2d. We found that the KG channels in substantia nigra (SN) are an assembly of Kir3.2a and Kir3.2c6.Inanobe A. Yoshimoto Y. Horio Y. et al.Characterization of G protein-gated K+ channels composed of Kir3.2 subunits in substantia nigra.J Neuroscience. 1999; 19: 1006-1017PubMed Google Scholar. Kir3.2c is 11 amino acids longer than Kir3.2a, which is the only divergent part between the two clones. The C-terminal amino acids of Kir3.2c are ESKV. The channels were localized specifically at the postsynaptic membrane on the dendrites of dopaminergic neurons. Kir3.2c, but not Kir3.2a, could bind a PDZ domain-containing protein, PSD-95. The heterologously expressed KG channels composed of Kir3.2a and Kir3.2c or Kir3.2a alone were activated by G-protein stimulation, but expression of Kir3.2c alone was not. However, the anchoring proteins confer the G-protein sensitivity to the homomeric Kir3.2c channel through their GK domain7.Hibino H. Inanobe A. Tanemoto M. et al.Anchoring proteins confer G protein sensitivity to an inwardly rectifying K+ channel through the GK domain.EMBO J. 2000; 19: 78-83Crossref PubMed Scopus (47) Google Scholar. Therefore, not only subcellular localization, but also the pore function of some Kir channels are critically controlled by PDZ domain-containing anchoring proteins in vivo.

Characterization of G-Protein-Gated K+Channels Composed of Kir3.2 Subunits in Dopaminergic Neurons of the Substantia Nigra

G-protein-gated K+ (KG) channels generate slow inhibitory postsynaptic potentials in the brain. Current opinion suggests that neuronal KG channels are heterotetramers of Kir3.1 and Kir3.2. In substantia nigra (SN), however, mRNA of Kir3.1 does not express, whereas that of Kir3.2 clearly does. Therefore, we have characterized the KG channels containing Kir3.2 subunits in SN using biochemical and immunological techniques. We found that they were composed of only Kir3.2 subunits and did not contain significant amounts of either Kir3.1 or Kir3.3. Furthermore, at least some of the KG channels in SN were assemblies of the splicing variants Kir3. 2a and Kir3.2c. The channels were localized specifically at the postsynaptic membrane on the dendrites of dopaminergic neurons. Kir3. 2c, but not Kir3.2a, could bind a PDZ domain-containing protein, PSD-95. The heterologously expressed KG channels composed of Kir3.2a plus Kir3.2c or Kir3.2a alone were activated by G-protein stimulation, but expression of Kir3.2c alone was not. This study reveals that the Kir3.2 splicing variants play distinct roles in the control of function and localization of some of the KG channels in dopaminergic neurons of SN.

An ultrastructural double-label immunohistochemical study of the enkephalinergic input to dopaminergic neurons of the substantia nigra in pigeons.

Electron microscopic immunohistochemical double-label studies were carried out in pigeons to characterize the ultrastructural organization and postsynaptic targets of enkephalinergic (ENK+) striatonigral projection. ENK+ terminals in the substantia nigra were labeled with antileucine-enkephalin antiserum by using peroxidase-antiperoxidase methods, and dopaminergic neurons were labeled with anti-tyrosine hydroxylase antiserum by using silver-intensified immunogold methods. ENK+ terminals on dopaminergic neurons were equal in abundance to ENK+ terminals on nondopaminergic neurons, although the former were typically somewhat smaller than the latter (mean size: 0.50 vs. 0.75 micron, respectively). ENK+ terminals were evenly distributed on the cell bodies and dendrites of dopaminergic neurons, and they were evenly distributed on dendrites but rare on perikarya of nondopaminergic neurons. Transection of the basal telencephalic output revealed that 75% of the nigral ENK+ terminals were of basal telencephalic origin. These telencephalic ENK+ terminals included over 80% of those smaller than 0.80 micron on dopaminergic neurons and smaller than 1.0 micron on nondopaminergic neurons, and none greater than this in size. Both telencephalic and the nontelencephalic ENK+ nigral terminals made predominantly symmetric synapses on nigral neurons. Although the basal telencephalic ENK+ terminals uniformly targeted dendrites and perikarya, nontelencephalic ENK+ terminals seemed to avoid perikarya. The results indicate that ENK+ striatonigral neurons in birds may directly influence both dopaminergic and nondopaminergic neurons of the substantia nigra. Based on similar data for substance P-containing striatonigral terminals, the roles of enkephalin and substance P in influencing nigral dopaminergic neurons may differ slightly, as they appear to target preferentially different portions of dopaminergic neurons. The overall results in pigeons are similar to those for ENK+ terminals in the ventral tegmental area in rats, suggesting that the synaptic organization of the ENK+ input to the tegmental dopaminergic cell fields is similar in mammals and birds.

Ultrastructure and synaptic targets of the raphe-nigral projection in the rat

A major input to the substantia nigra is from the 5-hydroxytryptamine-containing neurons in the dorsal raphe nucleus. In order to examine the morphology and distribution of this projection, rats were given injections of the anterograde tracers, Phaseolus vulgaris-leucoagglutinin or biocytin, in the dorsal raphe nucleus and the substantia nigra was examined at both the light and electron microscopic levels. In addition, sections of the substantia nigra were immunostained for 5-hydroxytryptamine and examined in both the light and electron microscopes. Since dopaminergic neurons of the substantia nigra are known to be responsive to stimulation of the raphe and to applied 5-hydroxytryptamine, sections that contained anterogradely labelled terminals were further processed to reveal tyrosine hydroxylase immunoreactivity to determine whether the raphe input makes direct synaptic contact with dopaminergic neurons. Light microscopic analysis revealed that all divisions of the substantia nigra received input from the dorsal raphe which, in agreement with previous observations, showed a topographical organization. In the electron microscope the anterograde markers were present in axon terminals that formed asymmetrical synaptic contact with dendritic shafts and spines. The synaptic boutons were often associated with subjunctional dense bodies. Terminals that displayed immunoreactivity for 5-hydroxytryptamine had a similar morphology, synaptic specialisations and postsynaptic targets to the anterogradely labelled terminals. In those sections that were stained for both anterogradely labelled terminals and tyrosine hydroxylase, the raphe-nigral terminals were seen to form asymmetrical synaptic contact with the dendrites of the dopaminergic neurons. It is concluded that dendrites of dopaminergic neurons in the substantia nigra pars reticulata represent at least one of the synaptic targets of the raphe-nigral projection and that these contacts provide an anatomical substrate for the effects of the dorsal raphe, and presumably 5-hydroxytryptamine, on dopaminergic systems in the substantia nigra.

Cholinergic input to dopaminergic neurons in the substantia nigra: A double immunocytochemical study

In order to determine whether the cholinergic fibres that innervate the substantia nigra make synaptic contact with dopaminergic neurons of the substantia nigra pars compacta, a double immunocytochemical study was carried out in the rat and ferret. Sections of perfusion-fixed mesencephalon were incubated first to reveal choline acetyltransferase immunoreactivity to label the cholinergic terminals and then tyrosine hydroxylase immunoreactivity to label the dopaminergic neurons. Each antigen was localized using peroxidase reactions but with different chromogens. At the light microscopic level, in confirmation of previous observations, choline acetyltransferaseimmunoreactive axons and axonal boutons were found throughout the substantia nigra. The highest density of these axons was found in the pars compacta where they were often seen in close apposition to tyrosine hydroxylase-immunoreactive cell bodies and dendrites. In the ferret where the choline acetyltransferase immunostaining was particularly strong, bundles of immunoreactive fibres were seen to run through the reticulata perpendicular to the pars compacta. These bundles were associated with tyrosine hydroxylase-immunoreactive dendrites that descended into the reticulata. The choline acetyltransferase-immunoreactive fibres made “climbing fibre”-type multiple contacts with the tyrosine hydroxylase positive dendrites. At the electron microscopic level the choline acetyltransferase-immunoreactive axons were seen to give rise to vesicle-filled boutons that formed asymmetrical synaptic specializations with nigral dendrites and perikarya. The synapses were often associated with sub-junctional dense bodies. On many occasions the postsynaptic structures contained the tyrosine hydroxylase immunoreaction product, thus identifying them as dopaminergic. It is concluded that at least one of the synaptic targets of cholinergic terminals in the substantia nigra are the dendrites and perikarya of dopaminergic neurons and that in the ferret at least, the dendrites of dopaminergic neurons that descend into the pars reticulata receive multiple synaptic inputs from individual cholinergic axons.

Ventral tegmental area: site through which dopamine D2-receptor agonists evoke behavioural and electrocortical sleep in rats.

1. In freely moving rats the effects on behaviour and electrocortical (ECoG) spectrum power of some dopamine agonists, i.e. apomorphine and (+)-3PPP, given directly into different areas of the rat brain were studied. In particular, dopamine agonists were microinfused in the ventral tegmental area (VTA) and substantia nigra (SN) or into the caudate nucleus, n. accumbens and prefrontal cortex. The ECoG spectrum power effects were continuously analysed by means of a computerized Berg-Fourier analyser as total spectrum power and power in preselected frequency bands. 2. Apomorphine and (+)-3PPP (0.01, 0.1 and 1.0 nmol) given bilaterally into the VTA produced behavioural and ECoG sleep in a dose-dependent fashion. A statistically significant (P less than 0.01) increase in ECoG total spectrum power with a predominant increase in the lower frequency bands (0.25-3, 3-6 and 6-9 Hz) occurred. No behavioural and ECoG changes were evoked by the same doses of apomorphine bilaterally microinfused into the SN or into the caudate nucleus or by (+)-3PPP (1.0 nml) microinjected into the n. accumbens or applied onto the prefrontal cortex. 3. Behavioural and ECoG sleep was also induced in rats after systemic administration of apomorphine (263 nmol kg-1, i.p.). 4. The behavioural and ECoG spectrum power effects of apomorphine (1.0 nmol) bilaterally micro-infused into the VTA were prevented by a previous microinjection into the same site of (-)-sulpiride (9.8 nmol). Similarly, behavioural and ECoG effects evoked by (+)-3PPP (0.1 nmol) given bilaterally into the VTA, were completely antagonized by a previous injection into the same site of haloperidol (16 pmol given 10 min before). In contrast, pretreatment with SCH 23390 (50 pgkg-1, s.c.), a selective antagonist at dopamine Dl-receptors, was unable to antagonize the behavioural and ECoG spectrum power effects of ( +)-3PPP. 5. Soporific effects induced by systemic administration of apomorphine were antagonized by (-)- sulpiride (9.8 nmol) given bilaterally into the VTA 10min before, whereas, yohimbine (1.3 nmol), (an antagonist at alpha 2-adrenoceptors) bilaterally microinfused into the VTA, was ineffective in this respect. 6. The present experiments provide evidence suggesting that stimulation of dopamine D2-receptors located at the cell body level and/or the dendrites of dopaminergic neurones in the VTA may represent the mechanism through which apomorphine or (+)-3PPP exert their soporific effects in rats.

Facilitation of a dendritic calcium conductance by 5-hydroxytryptamine in the substantia nigra

Within the substantia nigra, the dendrites of dopaminergic neurons that project to the striatum appear to play an active and nonclassical role in the physiology of the neuron in that they release transmitter and protein, but little is known of the factors controlling release of substances from these dendrites. In this study, we show that 5-hydroxytryptamine, which is contained in afferent fibres to the substantia nigra, is present in terminals making direct synaptic contact with dopaminergic neurons and also that it has a site-dependent, receptor-mediated, facilitatory effect on a specific dendritic calcium-dependent potential in nigrostriatal neurons in vitro. The ionic and spatial features of this response, which is insensitive to blockade by three different K+-channel antagonists, could correspond to those underlying the dendritic release of dopamine.

An electrophysiological action of acetylcholinesterase independent of its catalytic site.

Acetylcholinesterase (AChE) is released from the cell bodies and/or dendrites of dopaminergic neurones in the substantia nigra. Extracellular AChE can modify both the electrical activity of dopaminergic nigral neurones and the associated motor behaviour of the animal. These effects seem to be unrelated to hydrolysis of acetylcholine, but the underlying cellular mechanisms of these actions of AChE are unknown. The possible non-cholinergic action of AChE on the membrane properties of dopaminergic neurones was thus investigated by intracellular recording from midbrain slices in vitro. Application of AChE resulted in a marked hyperpolarization of the membrane accompanied by a decrease in input resistance, sometimes preceded by a period of spontaneous firing. Butyrylcholinesterase (BuChE) was without effect. AChE pre-treated with an irreversible inhibitor (Soman) of its enzymic activity caused similar changes to those seen following administration of untreated AChE. It is concluded that AChE can modify the membrane properties of nigrostriatal neurones in a way that is independent of its ability to hydrolyse acetylcholine. This novel biological property of AChE provides a possible mechanism by which this neurosecretory protein could modulate the functioning of the neurones from which it is secreted and suggests that other 'non-cholinergic' actions of AChE might exist.

6-Hydroxydopamine-induced degeneration of nigral dopamine neurons: Differential effect on nigral and striatal D-1 dopamine receptors

Dopamine-sensitive adenylate cyclase and 3H-SCH 23390 binding parameters were measured in the rat substantia nigra and striatum 15 days after the injection of 6-hydroxydopamine into the medial forebrain bundle. The activity of nigral dopamine-sensitive adenylate cyclase and the binding of 3H-SCH 23390 to rat nigral D-1 dopamine receptors were markedly decreased after the lesion. On the contrary, 6-hydroxydopamine-induced degeneration of the nigrostriatal dopamine pathway enhanced both adenylate cyclase activity and the density of 3H-SCH 23390 binding sites in striatal membrane preparations. The changes in 3H-SCH 23390 binding found in both nigral and striatal membrane preparations were associated with changes in the total number of binding sites with no modifications in their apparent affinity. The results indicate that: a) within the substantia nigra a fraction (30%) of D-1 dopamine receptors coupled to the adenylate cyclase is located on cell bodies and/or dendrites of dopaminergic neurons; b) striatal D-1 dopamine receptors are tonically invervated by nigrostriatal afferent fibers.

GABA axons in synaptic contact with dopamine neurons in the substantia nigra: double immunocytochemistry with biotin-peroxidase and protein A-colloidal gold

Two different antigens in the same ultrathin section of brain tissue can be revealed by ‘double immunocytochemistry’ in which one antigens is detected by horseradish peroxidase and the other by silver intensification of colloidal gold (SIG) adsorbed to Protein A. By means of this procedure it has been possible to show that GABAergic axon terminals (containing glutamate decarboxylase) are in synaptic contact with the cell bodies and dendrites of dopaminergic neurons (containing tyrosine hydroxylase) in the substantia nigra of the rat. Thus, several of the physiological and pharmacological effects of GABA and GABAergic drugs in this part of the brain are likely to be mediated by a direct action via postsynaptic GABAergic receptors located on dopaminergic nigrostriatal neurons.

Effects of unilateral electrical stimulation of various thalamic nuclei on the release of dopamine from dendrites and nerve terminals of neurons of the two nigrostriatal dopaminergic pathways

The role of several motor and intralaminar thalamic nuclei in the regulation of dopamine release from terminals and dendrites of the nigrostriatal dopaminergic neurons was investigated in halothane-anaesthetized cats. For this purpose, the effects of the unilateral electrical stimulation of various thalamic nuclei on the release of newly synthesized [ 3H]dopamine were simultaneously determined in both substantiae nigrae and caudate nuclei using the push-pull cannula method. The electrical stimulation of the motor nuclei was the only one to induce asymmetric changes in the four structures since [ 3H]dopamine release was enhanced in the ipsilateral caudate nucleus and reduced in the contralateral structure while opposite responses were observed in the corresponcling substantiae nigrae. A reduction of [ 3H]dopamine release occurred in the four structures or only in the contralateral substantia nigra and caudate nucleus following the stimulation of the parafascicularis nucleus and the adjacent. posterior part of the nucleus centrum medianum or of the nucleus centralis lateralis and the adjacent paralaminar part of the nucleus medialis dorsalis, respectively. The stimulation of the anterior part of the nucleus centrum medianum, which in contrast to other thalamic nuclei examined, receives few nigral inputs, selectively enhanced [ 3H]dopamine release in the contralateral substantia nigra. No significant changes in [ 3H]dopamine release were seen either in the substantiae nigrae or in the caudate nuclei following the stimulation of midline thalamic nuclei. These results indicate that the motor and intralaminar thalamic nuclei exert multiple and selective influences on the release of dopamine from terminals and/or dendrites of the dopaminergic neurons. They also further support a role of thalamic nuclei in the transfer of information from one substantia nigra to the contralateral dopaminergic neurons. The possible involvement of connections between paired thalamic nuclei was underlined by the observations of evoked potentials in contralateral homologous nuclei following unilateral stimulation of motor, or some intralaminar, nuclei. The present report provides new insights on the mechanisms contributing to the reciprocal and/or bilateral regulations of nigrostriatal dopaminergic pathways.

Partial lesions of the dopaminergic nigrostriatal system in rat brain: biochemical characterization

Various doses of 6-hydroxydopamine injected into the rat substantia nigra produced partial, dose-dependent lesions of the dopaminergic nigrostriatal tract. The resulting reduction in striatal dopamine concentrations and tyrosine hydroxylase activities tended to be proportional, allowing these measurements to serve as indices for lesion severity in any particular animal. Lesions destroying two-thirds or more of the nigrostriatal neurons accelerated dopamine's synthesis in, and release from, surviving neurons, as indicated by increased striatal levels of the dopamine metabolites dihydroxyphenylacetic acid and homovanillic acid. Formation of these metabolites was also enhanced in dendrites of dopaminergic neurons in the substantia nigra. Supersensitivity of striatal postsynaptic receptors, as judged by induction of rotational behavior after apomorphine or l-DOPA administration, occurred when 90% or more of the nigrostriatal neurons had been destroyed. In contrast, rotational behavior could be induced by amphetamine in animals with only 50% of these neurons destroyed.

Effects of α-chloralose on the activity of the nigrostriatal dopaminergic system in the cat

The effects of α-chloralose anaesthesia were studied in the cat on the release of 3H-dopamine ( 3H-DA) from the caudate nucleus and the substantia nigra using the push-pull cannula method and continuous labelling of the structures with L-3,5- 3H-tyrosine. Results were compared to those obtained under the same experimental conditions in both halothane-anaesthetized animals and ‘encéphale isolé’ preparations. The α-chloralase-anaesthetized preparation showed the lowest level of 3H-DA release, both from dopaminergic nerve terminals in the caudate nucleus and from dendrites of dopaminergic neurons in the substantia nigra. Furthermore, the time required to reach a steady-state release of 3H-DA was longer in α-chloralose-anaesthetized cats than in animals under halothane. This indicated that the activity of the dopaminergic neurons is depressed under α-chloralose anaesthesia. In the second part of the study, we measured the effects of unilateral somatic stimulation on the release of 3H-DA from the caudate nuclei and substantiae nigrae on both sides of the brain. The results show that under α-chloralose anaesthesia, the electrical stimulation of the paw of the right forelimb does not significantly affect the release of 3H-DA from any of the above four structures. This contrasts with the changes of 3H-DA release seen in the same experimental conditions under halothane anaesthesia.

Self-inhibition by dopaminergic neurones: Disruption by (±)-α-methyl- p-tyrosine pretreatment or anterior diencephalic lesions

( + )-Amphetamine sulphate infused directly into the substantia nigra, pars compacta, at low rates and in low volumes and concentrations, produces a marked and prolonged inhibition of neuronal activity. This effect was significantly reduced or abolished following (∓)-α-methyl- p-tyrosine pretreatment or following electrolytic lesions in the diencephalon anterior to the substantia nigra. These results confirm previous reports of similar effects of such treatments on the depression of neuronal firing produced by systemically administered amphetamine. Further, our results suggest that (±)-α-methyl- p-tyrosine and anterior diencephalic lesions are affecting an indirect dopaminergic inhibitory mechanism intrinsic to the substantia nigra. Such evidence may also be regarded as consistent with the view that amphetamine produces an inhibition of dopaminergic neuronal activity by affecting release of dopamine from dendrites of dopaminergic neurones i.e. by affecting self-inhibition by dopaminergic neurones.

Release of dopamine from dendrites in rat substantia nigra.

THERE is increasing evidence that neurotransmitter synthesis, storage and release are not confined to axon terminals. Much of this evidence has come from studies of the dopaminergic neurones of the substantia nigra (see Fig. 1). The terminal dendritic processes of dopamine (DA) neurones in substantia nigra are varicose and contain clusters of small vesicles1,2; histochemical studies have shown that the dendrites contain DA and are able to take it up3,4. There is also evidence that the components necessary for the synthesis and storage of DA are transported somatofugally in dendrites of substantia nigra3,5. Finally, the local application of DA agonists to the substantia nigra produces an inhibition of firing of neurones in the pars compacta that is blocked by DA antagonists6,7. On the basis of these observations, it has been proposed that a local dendritic release of DA may modify the excitability of dopaminergic neurones in substantia nigra7. There has, however, been no direct demonstration of transmitter release from these or other dendrites. In this study we present evidence for the release of DA from dendrites of dopaminergic neurones in rat substantia nigra.