The ascending projections of the dopaminergic neurons of the substantia nigra, zona reticulata: a combined retrograde tracer-immunohistochemical study

The impact of cell structure, metabolism and group behavior for the survival of bacteria under stress conditions

Microbes from diverse types of habitats are continuously exposed to external challenges, which may include acidic, alkaline, and toxic metabolites stress as well as nutrient deficiencies. To promote their own survival, bacteria have to rapidly adapt to external perturbations by inducing particular stress responses that typically involve genetic and/or cellular changes. In addition, pathogenic bacteria need to sense and withstand these environmental stresses within a host to establish and maintain infection. These responses can be, in principle, induced by changes in bacterial cell structure, metabolism and group behavior. Bacterial nucleic acids may serve as the core part of the stress response, and the cell envelope and ribosomes protect genetic structures from damage. Cellular metabolism and group behavior, such as quorum sensing system, can play a more important role in resisting stress than we have now found. Since bacteria survival can be only appreciated if we better understand the mechanisms behind bacterial stress response, here we review how morphological and physiological features may lead to bacterial resistance upon exposure to particular stress-inducing factors.

Inhibitory synaptic transmission from the substantia nigra pars reticulata to the ventral medial thalamus in mice

The cortico-basal ganglia-thalamic loop circuit is involved in variety of motor, association and limbic functions. The basal ganglia receive neural information from various areas of the cerebral cortex and transfer them back to the frontal and motor cortex via the ventral medial (VM), and the anterior-ventral lateral thalamic complex. The projection from the basal ganglia to the thalamus is GABAergic, and, therefore, the output from the basal ganglia cannot directly evoke excitation in the thalamic nuclei. The mechanism underlying the information transfer via the inhibitory projection remains unclear. To address this issue, we recorded electrophysiological properties of nigro-thalamic synapses from the VM neuron. We developed a nigro-thalamic slice preparation, in which the projection from the substantia nigra pars reticulata (SNr) to VM nucleus is stored, to enable the selective activation of the projection from the SNr. We characterized synaptic properties and membrane properties of the VM neuron, and developed a VM neuron model to simulate the impacts of SNr inputs on VM neuron activity. Neural simulation suggested that the inhibitory projection from SNr can control neural activity in two ways: a disinhibition from the spontaneous nigral inhibition and a β-band synchronization evoked by combination of excitation and inhibition of SNr activity.

The substantia nigra conveys target-dependent excitatory and inhibitory outputs from the basal ganglia to the thalamus

The basal ganglia (BG), which influence cortical activity via the thalamus, play a major role in motor activity, learning and memory, sensory processing, and many aspects of behavior. The substantia nigra (SN) consists of GABAergic neurons of the pars reticulata that inhibit thalamic neurons and provide the primary output of the BG, and dopaminergic neurons of the pars compacta that modulate thalamic excitability. Little is known about the functional properties of the SN→thalamus synapses, and anatomical characterization has been controversial. Here we use a combination of anatomical, electrophysiological, genetic, and optogenetic approaches to re-examine these synaptic connections in mice. We find that neurons in the SN inhibit neurons in the ventroposterolateral nucleus of the thalamus via GABAergic synapses, excite neurons in the thalamic nucleus reticularis, and both excite and inhibit neurons within the posterior nucleus group. Glutamatergic SN neurons express the vesicular glutamate receptor transporter vGluT2 and receive inhibitory synapses from striatal neurons, and many also express tyrosine hydroxylase, a marker of dopaminergic neurons. Thus, in addition to providing inhibitory outputs, which is consistent with the canonical circuit, the SN provides glutamatergic outputs that differentially target thalamic nuclei. This suggests that an increase in the activity of glutamatergic neurons in the SN allows the BG to directly excite neurons in specific thalamic nuclei. Elucidating an excitatory connection between the BG and the thalamus provides new insights into how the BG regulate thalamic activity, and has important implications for understanding BG function in health and disease.

Dopamine in motor cortex is necessary for skill learning and synaptic plasticity

Preliminary evidence indicates that dopamine given by mouth facilitates the learning of motor skills and improves the recovery of movement after stroke. The mechanism of these phenomena is unknown. Here, we describe a mechanism by demonstrating in rat that dopaminergic terminals and receptors in primary motor cortex (M1) enable motor skill learning and enhance M1 synaptic plasticity. Elimination of dopaminergic terminals in M1 specifically impaired motor skill acquisition, which was restored upon DA substitution. Execution of a previously acquired skill was unaffected. Reversible blockade of M1 D1 and D2 receptors temporarily impaired skill acquisition but not execution, and reduced long-term potentiation (LTP) within M1, a form of synaptic plasticity critically involved in skill learning. These findings identify a behavioral and functional role of dopaminergic signaling in M1. DA in M1 optimizes the learning of a novel motor skill.

Dopaminergic projections to the medial preoptic area of postpartum rats

Dopamine receptor activity in the rodent medial preoptic area (mPOA) is crucial for the display of maternal behaviors, as well as numerous other physiological and behavioral functions. However, the origin of dopaminergic input to the mPOA has not been identified through neuroanatomical tracing. To accomplish this, the retrograde tracer Fluorogold was iontophoretically applied to the mPOA of postpartum laboratory rats, and dual-label immunocytochemistry for Fluorogold and tyrosine hydroxylase later performed to identify dopaminergic cells of the forebrain and midbrain projecting to the mPOA. Results indicate that the number of dopaminergic cells projecting to the mPOA is moderate ( approximately 90 cells to one hemisphere), and that these cells have an unexpectedly wide distribution. Even so, more than half of the dual-labeled cells were found in either what has been considered extensions of the A10 dopamine group (particularly the ventrocaudal posterior hypothalamus and adjacent medial supramammillary nucleus), or in the A10 group of the ventral tegmental area. The rostral hypothalamus and surrounding region also contained numerous dual-labeled cells, with the greatest number found within the mPOA itself (including in the anteroventral preoptic area and preoptic periventricular nucleus). Notably, dual-labeled cells were rare in the zona incerta (A13), a site previously suggested to provide dopaminergic input to the mPOA. This study is the first to use anatomical tracing to detail the dopaminergic projections to the mPOA in the laboratory rat, and indicates that much of this projection originates more caudally than previously suggested.

The pars reticulata of the substantia nigra: a window to basal ganglia output

Together with the internal segment of the globus pallidus (GP(i)), the pars reticulata of the substantia nigra (SNr) provides a main output nucleus of the basal ganglia (BG) where the final stage of information processing within this system takes place. In the last decade, progress on the anatomical organization and functional properties of BG output neurons have shed some light on the mechanisms of integration taking place in these nuclei and leading to normal and pathological BG outflow. In this review focused on the SNr, after describing how the anatomical arrangement of nigral cells and their afferents determines specific input-output registers, we examine how the basic electrophysiological properties of the cells and their interaction with synaptic inputs contribute to the spatio-temporal shaping of BG output. The reported data show that the intrinsic membrane properties of the neurons subserves a tonic discharge allowing BG to gate the transmission of information to motor and cognitive systems thereby contributing to appropriate selection of behavior.

Regional distribution of SK3 mRNA-containing neurons in the adult and adolescent rat ventral midbrain and their relationship to dopamine-containing cells

SK3 small conductance, calcium-activated potassium channels play an important role in regulating the activity of mesencephalic dopamine (DA) neurons. In the present series of experiments, in situ hybridization techniques were used to compare SK3 and tyrosine hydroxylase (TH) mRNA expression throughout the rostrocaudal extent of the ventral midbrain in juvenile and adult rats. SK3 mRNA was found exclusively in areas that also contained large numbers of DA neurons including the substantia nigra (SN), the ventral tegmental area, and related cell groups (VTA-A10). An anteroposterior and mediolateral gradient in SK3 mRNA hybridization was apparent in the VTA-A10 but not in the SN. Younger rats appeared to possess higher levels and less regional variation in TH and SK3 transcripts. These results are consistent with previous studies reporting differential expression of SK3 protein within the midbrain and suggest that variations in SK3 channel distribution could contribute to differences in dopamine-related functions in the rat.

Dopamine cell degeneration induced by intraventricular administration of 6-hydroxydopamine in the rat: similarities with cell loss in parkinson's disease

In an attempt to find a convenient rat model to study cell vulnerability in Parkinson's disease, we have investigated the cell-loss profile in different midbrain dopaminergic nuclei and subnuclei of rats injected with 6-hydroxydopamine (6-OHDA) in the third ventricle. Following administration of different doses (5-1000 microgram) of 6-OHDA, motor behavior was evaluated and tyrosine hydroxylase-immunostained neurons were counted in the A8 group and different subdivisions of A9 and A10 groups. Animals developed hypokinesia, repetitive chewing movements, and catalepsia. Signs of cell degeneration were evident from the first day after injection, reaching the definitive pattern at the end of the first week. There was a similar degeneration in both brain sides, the A9 group showing the highest degree of cell-loss, followed by A8 and A10 groups. In the A9 group, the degeneration mostly affected those subgroups located in its ventral, lateral, and posterior regions. In the A10 group the degeneration mainly affected the parabrachial pigmented nucleus, the paranigral nucleus and the ventral tegmental area. This topographic pattern of degeneration is very similar to that previously described in Parkinson's disease, suggesting that this model may be a useful tool in the study of the cell vulnerability mechanisms in this neurodegenerative disorder. In addition, our results also showed that small dopaminergic neurons are more resistant to degeneration than the large ones. In some DA subgroups, the cells that contained calbindin but not calretinin were less vulnerable to the neurotoxic effect of 6-OHDA.

Distribution of serotonin 5-HT(2A) receptors in afferents of the rat striatum

Treatment with conventional antipsychotic drugs (APDs) is accompanied by extrapyramidal side effects (EPS), which are thought to be due to striatal dopamine D(2) receptor blockade. In contrast, treatment with atypical APDs is marked by a low incidence or absence of EPS. The reduced motor side effect liability of atypical APDs has been attributed to a high serotonin 5-HT(2A) receptor affinity coupled with a relatively low D(2) affinity. Despite the high density of 5-HT(2A) binding sites in the striatum, there are few detectable 5-HT(2A) mRNA-expressing neurons in the striatum. This suggests that most striatal 5-HT(2A) receptors are heteroceptors located on afferent axons. A combined retrograde tracer-immunohistochemistry method was used to determine the sites of origin of striatal 5-HT(2A)-like immunoreactive axons. 5-HT(2A)-like immunoreactive neurons in both the cortex and globus pallidus were retrogradely labeled from the striatum; very few nigrostriatal or thalamostriatal neurons expressed 5-HT(2A)-like immunoreactivity. Within the striatum, parvalbumin-containing interneurons displayed 5-HT(2A) immunolabeling; these neurons are the targets of cortical and pallidal projections. Our data indicate that cortico- and pallido-striatal neurons are the major source of 5-HT(2A) receptor binding in the striatum, and suggest that cortico- and pallido-striatal neurons are strategically positioned to reduce the motor side effects that accompany striatal D(2) receptor blockade or are seen in parkinsonism.

Effects of methamphetamine-induced neurotoxicity on the development of neural circuitry: a hypothesis

Exposure of the developing brain to methamphetamine has well-studied biochemical and behavioral consequences. We review: (1) the effects of methamphetamine on mature serotonergic and dopaminergic pathways; (2) the mechanisms of methamphetamine neurotoxicity and (3) the role of serotonergic and dopaminergic signaling in sculpting developing neural circuitry. Consideration of these data suggest the types of neural circuit alterations that may result from exposure of the developing brain to methamphetamine and that may underlie functional defects.

Cytoarchitectural distribution of calcium binding proteins in midbrain dopaminergic regions of rats and humans

The present study compares the distribution of three calcium binding proteins, calbindin-D28k, calretinin, and parvalbumin, in the midbrain tegmentum of rats and humans. In order to compare the distributions of these proteins directly, the cytoarchitecture of this region was evaluated by using immunohistochemistry for tyrosine hydroxylase and substance P in serial sections in both transverse and horizontal planes. There was a high degree of homology in the cytoarchitecture of the three main dopaminergic regions identified. The A8 group was localised in the retrorubral fields, which extended rostrally into the midbrain reticular fields in the human. The A9 group corresponded to the substantia nigra, which was delimited by its dense substance P innervation. The heterogeneous A10 group, situated along the dorsal border as well as medial to the A9 group, comprised multiple nuclei. The distribution of calcium binding proteins was similar in both species, although a larger proportion of neurons contained these proteins in the rat. Calbindin-D28k was localised in neurons within A8 and A10 nuclei and within the caudomedial A9 region (and rostrolateral A9 in the rat only). Calretinin was localised in similar regions. In contrast, neurons containing parvalbumin were concentrated in the substantia nigra pars reticulata. The results suggest that few dopaminergic neurons receiving striatal input in the substantia nigra contain calcium binding proteins; rather, the nondopaminergic nigral neurons contain parvalbumin. Interestingly, dopaminergic neurons are more numerous in humans, whereas nondopaminergic neurons predominate in rats, which suggests that functional differences may exist between rats and humans.

Alterations in TRH receptors in temporal lobe of schizophrenics: a quantitative autoradiographic study

We utilized quantitative autoradiography to determine the distribution of receptors for thyrotropin-releasing hormone (TRH) throughout the human temporal lobe and to examine the distribution of these receptors in discrete subregions of the temporal lobe from patients diagnosed premortem with schizophrenia. When compared to non-neurologic controls, schizophrenic patients demonstrated an increase of 51% in the concentration of TRH receptors in the molecular layer of the dentate gyrus. Within nuclei of the schizophrenic amygdala, marked decreases were found in the central (44%), medial (38%), cortical (36%), accessory cortical (52%), lateral (54%), and medial basal (22%) nuclei. We also examined postmortem brain samples from patients with Huntington's disease, amyotrophic lateral sclerosis, and Alzheimer's disease for alterations in the distribution of TRH receptors. No significant differences from non-neuropsychiatric controls were noted within the hippocampus in any of these disease states; however, slight alterations were noted in the central and medial basal amygdala in Huntington's disease and in the cortical amygdala in Alzheimer's disease. These disease-specific findings suggest that TRH may play a role in the neurochemical dysfunction of schizophrenia.

Postnatal changes in the distribution and morphology of rat substantia nigra dopaminergic neurons

Significant changes in the neurophysiology and neuropharmacology of nigral dopaminergic neurons take place in the first postnatal month. In order to correlate these changes with the postnatal development of dopaminergic neuron morphology and substantia nigra cytoarchitecture, brains from Sprague-Dawley rat pups of age postnatal days 1, 7, 14, 21 and 28 and adult rats were sectioned and processed for tyrosine hydroxylase immunocytochemistry. At postnatal day 1, pars compacta and pars reticulata were not clearly delineated; tyrosine hydroxylase positive neurons and a dense plexus of fibers were scattered throughout the substantia nigra. By day 7 the density of tyrosine hydroxylase positive neurons decreased markedly in ventral substantia nigra, and a dopaminergic pars compacta and a non-dopaminergic pars reticulata could be more clearly distinguished. By day 14 the substantia nigra appeared essentially as it does in the adult. Cell counts during development revealed that the number of tyrosine hydroxylase positive neurons/section in both pars compacta and pars reticulata decreased significantly from postnatal day 1 to postnatal day 14, while those in pars lateralis did not change. Tyrosine hydroxylase-positive somatic size increased modestly but significantly from postnatal day 1 to day 14 as did the diameter of the proximal and distal dendrites. However, even at day 1, the morphology of tyrosine hydroxylase positive neurons appeared essentially the same as in adults. Dendritic arborizations were well developed. The dendrites were non-varicose and modestly branched, with some of the longer ventrally directed dendrites passing through pars reticulata into the crus cerebri.(ABSTRACT TRUNCATED AT 250 WORDS)

The cortico-nigral projection in the rat: an anterograde tracing study with biotinylated dextran amine

The projection from the cortex to the substantia nigra (SN) in the rat was studied using the biotinylated dextran amine (BDA) anterograde tracing method. Injections of BDA into the prefrontal cortex consistently yielded labeling of the thin nerve fibers with small boutons in the SN pars compacta (SNc) and the pars reticulata (SNr). The cortico-nigral projection had a loose medio-lateral and rostro-caudal arrangement. Injections of BDA into the precentral lateral, the parietal, the temporal, and the occipital cortices resulted in sparse or no labeling of boutons in the SN. The density of boutons in the most heavily labeled region of the SN was 0.23/10(3) micron3. For comparison, similar measurement was performed in the cortico-striatal projection. The density of the boutons in the the neostriatum was 4.9/10(3) micron3. This study indicates that the entire prefrontal cortex projects to the SNc and SNr and that the terminal density of this projection is much less than the cortico-striatal projection.

Haloperidol activates tyrosine hydroxylase gene-expression in the rat substantia nigra, pars reticulata

The cellular distribution of tyrosine hydroxylase (TH) and TH mRNA in the rat substantia nigra (SN) was investigated using immunohistochemistry (IMHC) and non-radioactive in situ hybridization histochemistry (ISH), respectively. Number and density of both TH immunoreactive and TH cRNA labeled cells were increased in the pars reticulata of the substantia nigra (SNr) 8 h after single administration of a dopamine antagonist haloperidol. At the same time number and density of TH positive cells remained unchanged in a ventro-medial, dorso-medial or lateral part of the pars compacta (SNc) and in the pars lateralis (SNl) of the substantia nigra. A D2 receptor-specific agonist, quinpirole, was without effect on either ISH or IMHC in any of these areas, including the SNr. These results reveal the existence of a population of TH-negative neurons in the SNr, in which TH gene-expression can be activated through a dopamine receptor-mediated mechanism, leading to detectable levels of both TH and TH mRNA. Furthermore they suggest that TH gene-expression in these neurons normally is inhibited by dopamine released from somata and dendrites in the SNr.

Chronic nicotine treatment counteracts nigral cell loss induced by a partial mesodiencephalic hemitransection: an analysis of the total number and mean volume of neurons and glia in substantia nigra of the male rat

This study combines immunocytochemical and stereological methods for the first time to obtain unbiased estimates of the number of cells in the entire substantia nigra and their respective mean volume. Nicotine, delivered by subcutaneously implanted osmotic pumps (0.125 mg/kg/h, 14 days) to male Sprague-Dawley rats with a partial unilateral mesodiencephalic lesion, caused a significant counteraction of the lesion-induced reduction in total number of nigral tyrosine hydroxylase-like immunoreactive neurons counterstained with Cresyl Violet compared with saline treated control animals. The number of Nissl stained neurons without tyrosine hydroxylase-like immunoreactivity was not affected by the lesion nor by nicotine. The numbers of non-neuronal glial fibrillary acidic protein-like immunoreactive cells counterstained with Cresyl Violet and smaller cells seen after Cresyl Violet staining alone, possibly representing microglia, were increased by the lesion but not affected by nicotine. No nicotine-induced effects were found on the number of nigral cells located contralateral to the lesion. The lesion-induced reduction in the mean volume of the nigral cells showing tyrosine hydroxylase-like immunoreactivity, as determined with the stereological rotator method, was not affected by nicotine. These findings suggest that continuous nicotine infusion exerts protective effects on lesioned nigroneostriatal dopamine systems and that these protective effects are selective for the nigral dopamine neurons not affecting other populations of neurons or non-neuronal cells. This neuroprotective effect might lead to new therapeutic strategies in clinical neurodegenerative disorders such as Parkinson's Disease.

GABAergic synaptic interactions in the substantia nigra

The substantia nigra receives a strong GABAergic input from the ipsilateral striatum and globus pallidus. Nigral GABAergic synaptic interactions have been described in the pars compacta (SNC) and pars reticulata (SNR) but not in the pars lateralis (SNL). The SNR and particularly the SNL are the nodal points of the GABAergic nigrotectal pathway. The present study analyzes the synaptic connections of GABAergic and dopaminergic neurons in each of the divisions of the substantia nigra by employing a double-labeling immunocytochemical technique at the light and electron microscope levels. Glutamic acid decarboxylase (GAD)-containing terminals make symmetrical synaptic contacts with dopaminergic neurons in the SNC and SNR. Neurons that contain GAD also receive a GABAergic input in the SNR and SNL. The proportion of GAD-GAD contacts appears to be highest in the SNL where virtually all GAD-positive terminals are found to be in synaptic contact with or apposed to GAD positive profiles. This study demonstrates a strong GABAergic input onto nigral dopaminergic neurons and GABAergic neurons in the SNR and SNL. This GABAergic influence which is ostensibly striatal or pallidal in origin is particularly prominent in relation to the SNL-mediated nigro-collicular pathway.

Comparative aspects of the basal ganglia-tectal pathways in reptiles

To determine how the basal ganglia in reptiles may influence visuomotor behavior, the connections from the basal ganglia to the tectum of the midbrain were studied in several species of reptiles. Immunohistochemical studies by means of antibodies against Leu-enkephalin (LENK) as well as experimental hodological studies with anterograde (PHA-L) and retrograde (HRP, Fluorogold, Cholera toxin) tracers were carried out. The results indicate that within the class of Reptilia, two different patterns occur: one in which information from the basal ganglia is relayed to the tectum via the substantia nigra as well as via a pretectal, enkephalinergic cell group, and another one in which only the ventral route, via the substantia nigra, is present. The former pattern is found in turtles, crocodiles, and the lacertid lizards Podarcis and Gallotia, and the latter pattern in the gekkonid lizards Gekko and Eublepharis, in Varanus, and in the snakes Python and Thamnophis. The presence or absence of the pretectal relay center is reflected in the laminar distribution of LENK immunoreactivity in the tectum. The apparent lack of a pretectal relay in nocturnal gekkonids and in snakes underlines the hypothesis (Reiner et al., '84: T.I.N.S. 7:320-325) that a de-emphasis of visual-basal ganglia mechanisms has occurred during the evolution of ancestral reptiles to modern mammals.

Organization of striatopallidal, striatonigral, and nigrostriatal projections in the macaque

The topographic organization of neostriatal connections was investigated by axonal transport of horseradish peroxidase, tritiated amino acids, or mixtures of both injected into the neostriatum of macaque monkeys. Striatal projections to pallidum and substantia nigra and the origin of projections to striatum from cerebral cortex and substantia nigra were examined. All striatal injections gave rise to projections to external and internal pallidum and to substantia nigra. Injections in caudate nucleus and in putamen both gave rise to substantial projections to pallidum and to substantia nigra, and the ratio of pallidal and nigral projections was generally similar. The striatopallidal projection showed prominent arborizations at right angles to the striatofugal pathway traversing the pallidum, forming in this manner terminal fields consisting of multiple bands or discs within a broad segment of the pallidum. Thus separate but neighboring regions of striatum appeared to have overlapping pallidal projection territories. In broad terms, rostral striatum projects to rostral pallidum, caudal striatum to caudal pallidum, and dorsal and ventral striatum, respectively, to dorsal and ventral pallidum. Inner (medial) and outer (lateral) putamen showed only subtle differences in pallidal projection patterns. The striatonigral projection from each injected area of striatum formed a longitudinal band extending over the entire length of the substantia nigra, with scattered, dense terminal fields occupying portions of pars compacta as well as pars reticularis. Rostral striatum projected to medial nigra and caudal striatum to lateral nigra. Terminal fields from ventral striatum were located somewhat more dorsally in the substantia nigra than those from dorsal striatum. Neighboring but separate regions of striatum appeared to have overlapping nigral projection territories, especially in caudal nigra. The nigrostriatal neurons projecting to an injected area of striatum generally were located in the same longitudinal band of the substantia nigra as the corresponding striatonigral projection. Labeled pars compacta neurons were often surrounded by a dense, labeled striatonigral terminal field, suggesting the existence of a striato-nigrostriatal loop. The rostromedial pars compacta contained labeled neuronal cell bodies in most cases, suggesting a widely divergent projection to striatum from this cell group. A slight tendency for preferential cell labeling rostrally in nigra with rostral striatal injection and caudally in nigra with caudal injections was noted. The preferred relationship of lateral nigra with caudal striatum and medial nigra with rostral striatum has implications for clinical expression of Parkinson's disease, which may vary with differential involvement of different nigral cell groups along the medial to lateral axis.(ABSTRACT TRUNCATED AT 400 WORDS)

Organotypic slice cultures of dopaminergic neurons of substantia nigra

Morphological methods were used to study the plasticity of target-deprived mammalian dopaminergic (DA) neurons. Slices of substantia nigra (SN) were taken from the midbrain of rats aged one to twelve days, and cultured for one to two weeks. Localization of tyrosine hydroxylase (TH) was used to examine the distribution and shapes of DA neurons. Histochemical staining for acetylcholinesterase (AChE) was carried out to estimate both survival and biosynthesis of SN neurons. We found that some DA neurons can survive in vitro without their usual target neurons. This was demonstrated by injecting rhodamine-conjugated microspheres (RD) into the caudate putamen, a SN target area, at 6 to 8 days prior to culturing. RD-labeled cells survived in SN cultures and some of them were doubly labeled with AChE. TH neurons had different shapes and their axon terminals formed close contacts with adjacent nondopaminergic neurons. These findings suggest that a subset of DA neurons may switch targets, but the majority of them require target interactions with the caudate putamen for survival in vitro.

Trypan blue in vivo stains nigral dopaminergic neurons killed by 6-hydroxydopamine

Dopaminergic neurons in the substantia nigra killed by 6-hydroxydopamine were stained in vivo by intracerebral injections of trypan blue. Such staining appeared specific for dead neurons, although a proportion of these retained the ability to stain with Nissl dyes for at least 2 days. Neurons retained trypan blue in vivo for periods of up to 9 days. Trypan blue staining of some neurons outside the substantia nigra demonstrated the use of this dye in determining the degree of non-specific toxicity of 6-hydroxydopamine. Twenty-four hours after infusion of trypan blue almost no background staining was present and individually stained neurons were clearly visible. Thus the use of trypan blue may have a general application as a sensitive method for estimating discrete areas of toxin-induced neuronal death, and for estimating the degree of specificity of a toxin.

Nigral dopamine autoreceptors are exclusively of the D2 type: quantitative autoradiography of [125I]iodosulpride and [125I]SCH 23982 in adjacent brain sections

The effect of unilateral 6-hydroxydopamine lesions of the medial forebrain bundle on the specific binding of [125I]iodosulpride and [125I]SCH 23982 in the rat substantia nigra was determined by quantitative autoradiography of adjacent sections. The specific binding of [125I]iodosulpride was reduced by 40-70% on the lesioned side in the substantia nigra pars compacta, reticulata, lateralis and in the ventral tegmental area. In contrast, the specific [125I]SCH 23982 binding was unchanged in all subdivisions of the substantia nigra. The results indicate that dopamine autoreceptors are present in the substantia nigra and in the ventral tegmental area and that they are exclusively of the D2 type.

Dopaminergic nigrotectal projection in the rat

After injecting a fluorescent tracer (Fluoro-gold) into the rat superior colliculus, retrogradely labeled neurons in the rostral, ventrolateral portions of the substantia nigra pars reticulata were also immunohistochemically labeled with tyrosine hydroxylase antisera. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) pretreatment of the medial forebrain bundle resulted in the disappearance of the nigral neurons double-labeled with the two markers. The existence of a dopaminergic nigrotectal projection susceptible to MPTP might provide a morphological substrate for abnormal saccadic eye movements in parkinsonism.

Collateral projection from the substantia nigra to the striatum and superior colliculus in the rat

Our retrograde fluorescent double labeling study demonstrated the existence of divergent collateral projections from the substantia nigra to the striatum and superior colliculus in the rat. These bifurcating projection neurons were located predominantly in the ventrolateral portions of the substantia nigra pars reticulata at its rostral level, where they formed a narrow band along the boundary between the substantia nigra and cerebral peduncle. Such specific projection cells were also seen in the substantia nigra pars lateralis. However, nigral neurons did not give off axonal branches to the striatum and ventromedial thalamic nucleus. The new nigral cell population proposed here might constitute a neuroanatomical substrate for abnormal saccadic eye movements clinically manifested by many parkinsonian patients.