Co-localization of tyrosine hydroxylase and glutamate decarboxylase in a subpopulation of single nigrotectal projection neurons

Reexamination of Dopaminergic Amacrine Cells in the Rabbit Retina: Confocal Analysis with Double- and Triple-labeling Immunohistochemistry

Dopaminergic amacrine cells (DACs) are among the most well-characterized neurons in the mammalian retina, and their connections to AII amacrine cells have been described in detail. However, the stratification of DAC dendrites differs based on their location in the inner plexiform layer (IPL), raising the question of whether all AII lobules are modulated by dopamine release from DACs. The present study aimed to clarify the relationship between DACs and AII amacrine cells, and to further elucidate the role of dopamine at synapses with AII amacrine cell. In the rabbit retina, DAC dendrites were observed in strata 1, 3, and 5 of the IPL. In stratum 1, most DAC dendritic varicosities—the presumed sites of neurotransmitter release—made contact with the somata and lobular appendages of AII amacrine cells. However, most lobular appendages of AII amacrine cells localized within stratum 2 of the IPL exhibited little contact with DAC varicosities. In addition, double- or triple-labeling experiments revealed that DACs did not express the GABAergic neuronal markers anti-GABA, vesicular GABA transporter, or glutamic acid decarboxylase. These findings suggest that the lobular appendages of AII amacrine cells are involved in at least two different circuits. We speculate that the circuit associated with stratum 1 of the IPL is modulated by DACs, while that associated with stratum 2 is modulated by unknown amacrine cells expressing a different neuroactive substance. Our findings further indicate that DACs in the rabbit retina do not use GABA as a neurotransmitter, in contrast to those in other mammals.

Effect of methylphenidate on visual responses in the superior colliculus in the anaesthetised rat: Role of cortical activation

The mechanism of action of psychostimulant drugs in the treatment of Attention Deficit Hyperactivity Disorder is still largely unknown, although recent evidence suggests one possibility is that the drugs affect the superior colliculus (SC). We have previously demonstrated that systemically administered d-amphetamine attenuates/abolishes visual responses to wholefield light flashes in the superficial layers of the SC in anaesthetised rats, and the present study sought to extend this work to methylphenidate (MPH). Anaesthetised rats were administered MPH at a range of doses (or saline) and subjected to monocular wholefield light flashes at two intensities, juxta-threshold and super-threshold. In contrast to d-amphetamine, systemic MPH produced an enhancement of visual activity at both intensities. Methylphenidate was also found to produce activation of the cortical EEG in anaesthetised rats. Furthermore, cortical activation induced by electrical stimulation of the pons was found to enhance visual responses in superficial layers of the SC, and when MPH was paired with pontine-induced cortical activation, the response-enhancing effects of MPH were substantially attenuated. Taken together, the results suggest that the enhancement of visual responses in the superficial layers of the SC by MPH in the anaesthetised rat is an artefact of the drug's interaction with cortical arousal.

Extrasynaptic release of GABA and dopamine by retinal dopaminergic neurons

In the mouse retina, dopaminergic amacrine (DA) cells synthesize both dopamine and GABA. Both transmitters are released extrasynaptically and act on neighbouring and distant retinal neurons by volume transmission. In simultaneous recordings of dopamine and GABA release from isolated perikarya of DA cells, a proportion of the events of dopamine and GABA exocytosis were simultaneous, suggesting co-release. In addition, DA cells establish GABAergic synapses onto AII amacrine cells, the neurons that transfer rod bipolar signals to cone bipolars. GABAA but not dopamine receptors are clustered in the postsynaptic membrane. Therefore, dopamine, irrespective of its site of release-synaptic or extrasynaptic-exclusively acts by volume transmission. Dopamine is released upon illumination and sets the gain of retinal neurons for vision in bright light. The GABA released at DA cells' synapses probably prevents signals from the saturated rods from entering the cone pathway when the dark-adapted retina is exposed to bright illumination. The GABA released extrasynaptically by DA and other amacrine cells may set a 'GABAergic tone' in the inner plexiform layer and thus counteract the effects of a spillover of glutamate released at the bipolar cell synapses of adjacent OFF and ON strata, thus preserving segregation of signals between ON and OFF pathways.

Vesicular expression and release of ATP from dopaminergic neurons of the mouse retina and midbrain

Vesicular nucleotide transporter (VNUT) is required for active accumulation of adenosine tri-phosphate (ATP) into vesicles for purinergic neurotransmission, however, the cell types that express VNUT in the central nervous system remain unknown. This study characterized VNUT expression within the mammalian retina and brain and assessed a possible functional role in purinergic signaling. Two native isoforms of VNUT were detected in mouse retina and brain based on RNA transcript and protein analysis. Using immunohistochemistry, VNUT was found to co-localize with tyrosine hydroxylase (TH) positive, dopaminergic (DA) neurons of the substantia nigra and ventral tegmental area, however, VNUT expression in extranigral non-DA neurons was also observed. In the retina, VNUT labeling was found to co-localize solely with TH-positive DA-cells. In the outer retina, VNUT-positive interplexiform cell processes were in close contact with horizontal cells and cone photoreceptor terminals, which are known to express P2 purinergic-receptors. In order to assess function, dissociated retinal neurons were loaded with fluorescent ATP markers (Quinacrine or Mant-ATP) and the DA marker FFN102, co-labeled with a VNUT antibody and imaged in real time. Fluorescent ATP markers and FFN102 puncta were found to co-localize in VNUT positive neurons and upon stimulation with high potassium, ATP marker fluorescence at the cell membrane was reduced. This response was blocked in the presence of cadmium. These data suggest DA neurons co-release ATP via calcium dependent exocytosis and in the retina this may modulate the visual response by activating purine receptors on closely associated neurons.

Prenatal and acute cocaine exposure affects neural responses and habituation to visual stimuli

Psychostimulants have many effects on visual function, from adverse following acute and prenatal exposure to therapeutic on attention deficit. To determine the impact of prenatal and acute cocaine exposure on visual processing, we studied neuronal responses to visual stimuli in two brain regions of a transgenic larval zebrafish expressing the calcium indicator GCaMP-HS. We found that both red light (LF) and dark (DF) flashes elicited similar responses in the optic tectum neuropil (TOn), while the dorsal telencephalon (dTe) responded only to LF. Acute cocaine (0.5 μM) reduced neuronal responses to LF in both brain regions but did not affect responses to DF. Repeated stimulus presentation (RSP) led to habituation of dTe neurons to LF. Acute cocaine prevented habituation. TOn habituated to DF, but not LF, and DF habituation was not modified by cocaine. Remarkably, prenatal cocaine exposure (PCE) prevented the effects of acute cocaine on LF response amplitude and habituation later in development in both brain regions, but did not affect DF responses. We discovered that, in spite of similar neural responses to LF and DF in the TO (superior colliculus in mammals), responses to LF are more complex, involving dTe (homologous to the cerebral cortex), and are more vulnerable to cocaine. Our results demonstrate that acute cocaine exposure affects visual processing differentially by brain region, and that PCE modifies zebrafish visual processing in multiple structures in a stimulus-dependent manner. These findings are in accordance with the major role that the optic tectum and cerebral cortex play in sustaining visual attention, and support the hypothesis that modification of these areas by PCE may be responsible for visual deficits noted in humans. This model offers new methodological approaches for studying the adverse and therapeutic effects of psychostimulants on attention, and for the development of new pharmacological interventions.

Striatal cholinergic interneurons Drive GABA release from dopamine terminals

Striatal cholinergic interneurons are implicated in motor control, associative plasticity, and reward-dependent learning. Synchronous activation of cholinergic interneurons triggers large inhibitory synaptic currents in dorsal striatal projection neurons, providing one potential substrate for control of striatal output, but the mechanism for these GABAergic currents is not fully understood. Using optogenetics and whole-cell recordings in brain slices, we find that a large component of these inhibitory responses derive from action-potential-independent disynaptic neurotransmission mediated by nicotinic receptors. Cholinergically driven IPSCs were not affected by ablation of striatal fast-spiking interneurons but were greatly reduced after acute treatment with vesicular monoamine transport inhibitors or selective destruction of dopamine terminals with 6-hydroxydopamine, indicating that GABA release originated from dopamine terminals. These results delineate a mechanism in which striatal cholinergic interneurons can co-opt dopamine terminals to drive GABA release and rapidly inhibit striatal output neurons.

Dopamine, affordance and active inference

The role of dopamine in behaviour and decision-making is often cast in terms of reinforcement learning and optimal decision theory. Here, we present an alternative view that frames the physiology of dopamine in terms of Bayes-optimal behaviour. In this account, dopamine controls the precision or salience of (external or internal) cues that engender action. In other words, dopamine balances bottom-up sensory information and top-down prior beliefs when making hierarchical inferences (predictions) about cues that have affordance. In this paper, we focus on the consequences of changing tonic levels of dopamine firing using simulations of cued sequential movements. Crucially, the predictions driving movements are based upon a hierarchical generative model that infers the context in which movements are made. This means that we can confuse agents by changing the context (order) in which cues are presented. These simulations provide a (Bayes-optimal) model of contextual uncertainty and set switching that can be quantified in terms of behavioural and electrophysiological responses. Furthermore, one can simulate dopaminergic lesions (by changing the precision of prediction errors) to produce pathological behaviours that are reminiscent of those seen in neurological disorders such as Parkinson's disease. We use these simulations to demonstrate how a single functional role for dopamine at the synaptic level can manifest in different ways at the behavioural level.

Extrasynaptic release of GABA by retinal dopaminergic neurons

GABA release by dopaminergic amacrine (DA) cells of the mouse retina was detected by measuring Cl- currents generated by isolated perikarya in response to their own neurotransmitter. The possibility that the Cl- currents were caused by GABA release from synaptic endings that had survived the dissociation of the retina was ruled out by examining confocal Z series of the surface of dissociated tyrosine hydroxylase-positive perikarya after staining with antibodies to pre and postsynaptic markers. GABA release was caused by exocytosis because 1) the current events were transient on the millisecond time scale and thus resembled miniature synaptic currents; 2) they were abolished by treatment with a blocker of the vesicular proton pump, bafilomycin A1; and 3) their frequency was controlled by the intracellular Ca2+ concentration. Because DA cell perikarya do not contain presynaptic active zones, release was by necessity extrasynaptic. A range of depolarizing stimuli caused GABA exocytosis, showing that extrasynaptic release of GABA is controlled by DA cell electrical activity. With all modalities of stimulation, including long-lasting square pulses, segments of pacemaker activity delivered by the action-potential-clamp method and high-frequency trains of ramps, discharge of GABAergic currents exhibited considerable variability in latency and duration, suggesting that coupling between Ca2+ influx and transmitter exocytosis is extremely loose in comparison with the synapse. Paracrine signaling based on extrasynaptic release of GABA by DA cells and other GABAergic amacrines may participate in controlling the excitability of the neuronal processes that interact synaptically in the inner plexiform layer.

Phenotype, compartmental organization and differential vulnerability of nigral dopaminergic neurons

The degeneration of nigral dopaminergic (DA-) neurons is the histopathologic hallmark of Parkinson's disease (PD), but not all nigral DA-cells show the same susceptibility to degeneration. This starts in DA-cells in the ventrolateral and caudal regions of the susbtantia nigra (SN) and progresses to DA-cells in the dorsomedial and rostral regions of the SN and the ventral tegmental area, where many neurons remain intact until the final stages of the disease. This fact indicates a relationship between the topographic distribution of midbrain DA-cells and their differential vulnerability, and the possibility that this differential vulnerability is associated with phenotypic differences between different subpopulations of nigral DA-cells. Studies carried out during the last two decades have contributed to establishing the existence of different compartments of nigral DA-cells according to their neurochemical profile, and a possible relationship between the expression of some factors and the relative vulnerability or resistance of DA-cell subpopulations to degeneration. These aspects are reviewed and discussed here.

Methamphetamine induces ectopic expression of tyrosine hydroxylase and increases noradrenaline levels within the cerebellar cortex

Methamphetamine produces locomotor activation and typical stereotyped motor patterns, which are commonly related with increased catecholamine activity within the basal ganglia, including the dorsal and ventral striatum. Since the cerebellum is critical for movement control, and for learning of motor patterns, we hypothesized that cerebellar catecholamines might be a target of methamphetamine. To test this experimental hypothesis we injected methamphetamine into C57 Black mice at the doses of 5 mg/kg two or three times, 2 h apart. This dosing regimen is known to be toxic for striatal dopamine terminals. However, we found that in the cerebellum, methamphetamine increased the expression of the primary transcript of the tyrosine hydroxylase (TH) gene, followed by an increased expression of the TH protein. Increased TH was localized within Purkinje cells, where methamphetamine increased the number of TH-immunogold particles, and produced a change in the distribution of the enzyme by increasing the cytoplasmic percentage. Increased TH expression was accompanied by a slight increase in noradrenaline content. This effect was highly site-specific for the cortex of posterior vermal lobules, while only slight effects were detectable in the hemispheres. The present data indicate that the cerebellum does represent a target of methamphetamine, which produces specific and fine alterations of the catecholamine system involving synthesis, amount, and compartmentalization of TH as well as increased noradrenaline levels. This may be relevant for motor alterations induced by methamphetamine. In line with this, inherited cerebellar movement disorders in various animal species including humans are associated with increased TH immunoreactivity within intrinsic neurons of the same lobules of the cerebellar cortex.

Tyrosine hydroxylase (TH)- and aromatic-L-amino acid decarboxylase (AADC)-immunoreactive neurons of the common marmoset (Callithrix jacchus) brain: an immunohistochemical analysis

From the perspective of comparative morphology, the distribution of non-monoaminergic neurons in the common marmoset (Callithrix jacchus) was investigated using an immunohistochemical method with specific antibodies to tyrosine hydroxylase (TH) and aromatic-L-amino acid decarboxylase (AADC).TH-immunoreactive (IR) neurons (but not AADC-IR) neurons were observed in the olfactory tubercle, preoptic suprachiasmatic nucleus, periventricular hypothalamic nucleus, arcuate nucleus, paraventricular nucleus, periaqueductal gray matter, medial longitudinal fasciculus, substantia nigra, and nucleus solitaris. In contrast, AADC-IR (but not TH-IR), small, oval and spindle-shaped neurons were sparsely distributed in the following areas: the hypothalamus from the anterior nucleus to the lateral nucleus, the dorsomedial nucleus, the dorsomedial area of the medial mammillary nucleus and the arcuate nucleus; the midbrain, including the stria medullaris and substantia nigra; and the medulla oblongata, including the dorsal area of the nucleus solitaris and the medullary reticular nucleus. The distribution of AADC-IR neurons was not as extensive in the marmoset as it is in rats. However, these neurons were located in the marmoset, but not the rat substantia nigra. Furthermore, AADC-IR neurons that are present in the human striatum were absent in that of the marmoset. The present results indicate that the distribution of non-monoaminergic neurons in the brain of the common marmoset is unique and different from that in humans and rodents.

Topographic and functional neuroanatomical study of GABAergic disinhibitory striatum–nigral inputs and inhibitory nigrocollicular pathways: Neural hodology recruiting the substantia nigra, pars reticulata, for the modulation of the neural activity in the inferior colliculus involved with panic-like emotions

Considering the influence of the substantia nigra on mesencephalic neurons involved with fear-induced reactions organized in rostral aspects of the dorsal midbrain, the present work investigated the topographical and functional neuroanatomy of similar influence on caudal division of the corpora quadrigemina, addressing: (a) the neural hodology connecting the neostriatum, the substantia nigra, periaqueductal gray matter and inferior colliculus (IC) neural networks; (b) the influence of the inhibitory neostriatonigral-nigrocollicular GABAergic links on the control of the defensive behavior organized in the IC. The effects of the increase or decrease of activity of nigrocollicular inputs on defensive responses elicited by either electrical or chemical stimulation of the IC were also determined. Electrolytic or chemical lesions of the substantia nigra, pars reticulata (SNpr), decreased the freezing and escape behaviors thresholds elicited by electrical stimulation of the IC, and increased the behavioral responses evoked by the GABAA blockade in the same sites of the mesencephalic tectum (MT) electrically stimulated. These findings were corroborated by similar effects caused by microinjections of the GABAA-receptor agonist muscimol in the SNpr, followed by electrical and chemical stimulations of the IC. The GABAA blockade in the SNpr caused a significant increase in the defensive behavior thresholds elicited by electrical stimulation of the IC and a decrease in the mean incidence of panic-like responses induced by microinjections of bicuculline in the mesencephalic tectum (inferior colliculus). These findings suggest that the substantia nigra receives GABAergic inputs that modulate local and also inhibitory GABAergic outputs toward the IC. In fact, neurotracing experiments with fast blue and iontophoretic microinjections of biotinylated dextran amine either into the inferior colliculus or in the reticular division of the substantia nigra demonstrated a neural link between these structures, as well as between the neostriatum and SNpr.

Synaptic microcircuitry of tyrosine hydroxylase-containing neurons and terminals in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys

A population of tyrosine hydroxylase (TH)-containing neurons that is up-regulated after lesion of the nigrostriatal dopaminergic pathway has been described in the primate striatum. The goal of this study was to examine the morphology, synaptology, and chemical phenotype of these neurons and TH-immunoreactive (-ir) terminals in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated rhesus monkeys. TH-ir perikarya were small (10-12 microm), displayed nuclear invaginations, and received very few synaptic inputs. On the other hand, TH-containing dendrites were typically large in diameter (>1.0 microm) and received scarce synaptic innervation from putative excitatory and inhibitory terminals forming asymmetric and symmetric synapses, respectively. More than 70% of TH-positive intrastriatal cell bodies were found in the caudate nucleus and the precommissural putamen, considered as the associative functional territories of the primate striatum. Under 10% of these cells displayed calretinin immunoreactivity. TH-ir terminals rarely formed clear synaptic contacts, except for a few that established asymmetric axodendritic synapses. Almost two-thirds of TH-containing boutons displayed gamma-aminobutyric acid (GABA) immunoreactivity in the striatum of parkinsonian monkeys, whereas under 5% did so in the normal striatum. These findings provide strong support for the existence of a population of putative catecholaminergic interneurons in the associative territory of the striatum in parkinsonian monkeys. Their sparse synaptic innervation raises interesting issues regarding synaptic and nonsynaptic mechanisms involved in the regulation and integration of these neurons in the striatal microcircuitry. Finally, the coexpression of GABA in TH-positive terminals in the striatum of dopamine-depleted monkeys suggests dramatic neurochemical changes in the catecholaminergic modulation of striatal activity in Parkinson's disease.

Time-dependent changes in gene expression profiles of midbrain dopamine neurons following haloperidol administration

Antipsychotic drugs require a treatment regimen of several weeks before clinical efficacy is achieved in patient populations. While the biochemical mechanisms underlying the delayed temporal profile remain unclear, molecular adaptations in specific neuroanatomical loci are likely involved. Haloperidol-induced changes in gene expression in various brain regions have been observed; however, alterations in distinct neuronal populations have remained elusive. The present study examined changes in gene expression profiles of ventral tegmental area (VTA) and substantia nigra (SN) tyrosine hydroxylase immunopositive neurons following 1, 10 or 21 days of haloperidol administration (0.5 mg/kg/day). Macroarrays were used to study the expression of receptors, signaling proteins, transcription factors and pre- and post-synaptic proteins. Data were analyzed using conventional statistical procedures as well as self-organizing maps (SOM) to elucidate conserved patterns of expression changes. Results show statistically significant haloperidol-induced and time-dependent alterations in 17 genes in the VTA and 25 genes in the SN, including glutamate and GABA receptor subunits, signaling proteins and transcription factors. SOMs revealed distinct patterns of gene expression changes in response to haloperidol. Understanding how gene expression is altered over a clinically relevant time course of haloperidol administration may provide insight into the development of antipsychotic efficacy as well as the underlying pathology of schizophrenia.

GABAergic synapses made by a retinal dopaminergic neuron

In the retina, dopaminergic amacrine (interplexiform) cells establish multiple synapses on the perikarya of AII amacrines, the neurons that distribute rod signals to on- and off-cone bipolars. We used triple-label immunocytochemistry and confocal microscopy to identify the receptors contained within the postsynaptic active zone of these synapses in both mouse and rat retinas. We found that at the interface between the dendrites of the dopaminergic neurons and the AII amacrine cell perikarya clusters of postsynaptic gamma-aminobutyric acid type A (GABA(A)) receptors are situated in register with aggregates of presynaptic organelles immunoreactive for GABA, the GABA vesicular transporter, and the vesicular monoamine transporter-2. D1 and D23 dopamine receptors, on the other hand, do not form clusters on the surface of the perikarya of AII amacrine cells. We suggest that the synapses between retinal dopaminergic neurons and AII amacrine cells are GABAergic and that both GABA and dopamine are released by the presynaptic endings. GABA acts on the ionotropic receptors clustered at the postsynaptic active zone, whereas dopamine diffuses to more distant, slower-acting metabotropic receptors.

A novel technique for quantitative immunohistochemical imaging of various neurochemicals in a multiple-stained brain slice

Here we describe a novel technique for comparative analysis of the distributions of various neurochemicals visualized using multiple immunohistochemistry in the same brain slice. As an example, the distributions of tyrosine hydroxylase, substance P and glutamate decarboxylase in coronal slices of rat brains were compared. Each slice was divided into approximately 220,000-300,000 microareas at 20-microm intervals, and the immunohistochemical intensities of the three substances in each microarea were analyzed independently using a brain mapping analyzer; a microphotometry system previously developed in our laboratory (Sutoo et al., J. Neurosci. Methods, 1998; 85: 161-73). No significant differences between the distribution of each substance were observed in single- and triple-labeled slices. We believe that this method will facilitate the investigation of the functions of the central nervous system and the disorders thereof in various diseases.

Molecular and physiological diversity of nicotinic acetylcholine receptors in the midbrain dopaminergic nuclei

Nicotinic acetylcholine receptors (nAChRs) on dopaminergic (DA) and GABAergic (Gaba) projection neurons of the substantia nigra (SN) and ventral tegmental area (VTA) are characterized by single-cell RT-PCR and patch-clamp recordings in slices of rat and wild-type, beta2-/-, alpha4-/-, and alpha7-/- mice. The eight nAChR subunits expressed in these nuclei, alpha3-7 and beta2-4, contribute to four different types of nAChR-mediated currents. Most DA neurons in the SN and VTA express two nAChR subtypes. One is inhibited by dihydro-beta-erythroidine (2 microm), alpha-conotoxin MII (10 nm), and methyllycaconitine (1 nm) but does not contain the alpha7 subunit; it possesses a putative alpha4alpha6alpha5(beta2)(2) composition. The other subtype is inhibited by dihydro-beta-erythroidine (2 microm) and has a putative alpha4alpha5(beta2)(2) composition. Gaba neurons in the VTA exhibit a third subtype with a putative (alpha4)(2)(beta2)(3) composition, whereas Gaba neurons in the SN have either the putative (alpha4)(2)(beta2)(3) oligomer or the putative alpha4alpha6alpha5(beta2)(2) oligomer. The fourth subtype, a putative (alpha7)(5) homomer, is encountered in less than half of DA and Gaba neurons, in the SN as well as in the VTA. Neurons in the DA nuclei thus exhibit a diversity of nAChRs that might differentially modulate reinforcement and motor behavior.

Immunohistochemical studies of the localization of neurons containing the enzyme that synthesizes dopamine, GABA, or gamma-hydroxybutyrate in the rat substantia nigra and striatum

gamma-Hydroxybutyrate (GHB) is an endogenous metabolite of gamma-aminobutyric acid (GABA), which is synthesized in the neuronal compartment of the central nervous system. This substance possesses several properties that support its role as a neurotransmitter/neuromodulator in brain. In particular, it is synthesized by a specific pathway that transforms GABA into succinic semialdehyde via GABA-T activity; then succinic semialdehyde is converted into GHB by a specific succinic semialdehyde reductase (SSR). The last enzyme is considered as a marker for neurons that synthesize GHB. This compound binds in brain to receptors whose distribution, ontogenesis, kinetics, and pharmacology are specific. Endogenous GHB, but also GHB exogenously administered to rats, participate in the regulation of dopaminergic activity of the nigrostriatal pathway. To investigate the distribution of GHB neurons in this pathway and the anatomic relationships between dopaminergic and GHB neurons, immunocytochemical identification of dopamine, GABA, and GHB neurons was carried out in the substantia nigra and striatum of the rat. The following markers for these neurons were used: anti-tyrosine hydroxylase (TH) antibodies for dopamine neurons, anti-glutamate decarboxylase (GAD) antibodies for GABA neurons, and anti-succinic semialdehyde reductase (SSR) antibodies for GHB neurons. GABA neurons were studied because GAD and SSR co-exist frequently in the same neuron, and GABA alone also exerts its own regulatory effects on dopaminergic neurons. This study reveals the co-existence of GAD/SSR and GAD/SSR/TH in numerous neurons of the substantia nigra. However, some neurons appear to be only GAD or SSR positive. In the striatum, TH-positive terminals surround many GHB neurons. GAD innervation is abundant in close contact with unlabeled neurons in the caudate-putamen, whereas distinct SSR-positive punctuates are also present. The existence of SSR-reactive synapses and neurons was confirmed in the striatum at the electron microscopic level. On the basis of these results, a clear anatomo-functional relationship between GHB and dopamine networks cannot be defined; however, we propose the modulation by GHB of striatal intrinsic neurons that could then interfere with the presynaptic control of dopaminergic activity.

Nitric oxide synthase expression in the cerebral cortex of patients with epilepsy

PURPOSE

Nitric oxide (NO), a short-lived radical synthesized from L-arginine by activation of the enzyme nitric oxide synthase (NOS), has been implicated in the pathophysiology of epilepsy by some investigators. However, the current data about NO and NOS in epilepsy are controversial and are derived only from animal models of epilepsy. In this study we investigated possible changes in NOS expression in the cerebral cortex of patients with epilepsy.

METHODS

Qualitative and quantitative parameters of the immunolabeling pattern of the neuronal, endothelial, and inducible isoforms of NOS were analyzed in biopsy material obtained from patients with short and long seizure history and from patients without epilepsy.

RESULTS

The comparative study showed that in the cerebral cortex of patients with epilepsy, particularly in those with a long seizure history, the number and labeling intensity of NOS-positive neurons increased, and that a subpopulation of nonpyramidal GABAergic neurons (type II NOS neurons) was responsible for this phenomenon.

CONCLUSIONS

The fact that NOS upregulation is more evident in patients with a long seizure history suggests that this is a consequence of seizures, acting probably as an adaptative response to the sustained release of excitatory amino acids.

Neurocircuitries of the basal ganglia studied in organotypic cultures: focus on tyrosine hydroxylase, nitric oxide synthase and neuropeptide immunocytochemistry

The nigrostriatal and mesolimbic systems of the rat were reconstructed using an organotypic culture model, whereby neonatal brain tissue was grown in vitro for approximately one month. The nigrostriatal system comprised of tissue from the substantia nigra, the dorsal striatum and the frontoparietal cortex, while the mesolimbic system included the ventral tegmental area, ventral striatum (including the fundus striati, accumbens nucleus, olfactory tubercle, lateral septum, ventral pallidum and piriform cortex) and cingulate cortex. These regions were also cultured alone or in pairs. The cultures were monitored in vitro, and after one month fixed in a formalin-picric acid solution, and processed for immunohistochemistry using antibodies raised against tyrosine hydroxylase, nitric oxide synthase, preprocholecystokinin, glutamate decarboxylase, neuropeptide Y, dopamine- and cyclic AMP-regulated phosphoprotein-32 and glial fibrillary acidic protein. The tissue survived in single, double or triple cultures, although differences were found depending upon the source and combination of cultured region. Neurons had localization and shape as in vivo. Local networks were especially prominent in the mesencephalon, where both tyrosine hydroxylase-positive axons spread from the "substantia nigra" to the rest of the tissue, and where nitric oxide synthase-positive networks also surrounded tyrosine hydroxylase-positive neurons. Glutamate decarboxylase-positive nerve terminals formed dense networks around tyrosine hydroxylase-positive neurons. In the striatum, nitric oxide synthase and dopamine- and cyclic AMP-regulated phosphoprotein-32 neurons were surrounded by tyrosine hydroxylase-positive nerve terminals. The nigral and ventral tegmental area dopamine neurons projected to striatal and cortical structures, but the projection from the ventral tegmental area to the cingulate cortex was more prominent. With regard to co-existence, preprochole-cystokinin-like immunoreactivities was found in many tyrosine hydroxylase-positive neurons and neuropeptide Y- and nitric oxide synthase-like immunoreactivity co-existed in striatal and cortical tissues. In general terms, the chemical neuroanatomy in the cultures was similar to that described earlier in vivo. Nitric oxide synthase staining was particularly intense. Taken together, the organotypic model captures many of the morphological and neurochemical features seen in vivo, providing a valuable model for studying neurocircuitries of the brain in detail, where 'normal' and 'pathological' conditions can be simulated.

Immunocytochemical detection of two nuclear proteins within the same neuron using light microscopy

We developed a method of double immunocytochemistry (ICC) that can be used with conventional light microscopy for localizing two different nuclear proteins. The procedure involves two sequential rounds of ICC that both employ the avidin and biotin conjugated enzyme (ABC) amplification method, separated by an Avidin D and biotin blocking step to reduce non-specific avidin-biotin reactions. Round one of ICC employs the use of avidin and biotin conjugated alkaline phosphatase (ABC-AP) and the Vector Red (VR) substrate, which produces a red colorimetric reaction product. The second round of ICC makes use of avidin and biotin conjugated peroxidase (ABC-HRP) and the Vector(R) SG substrate, which produces a gray colorimetric reaction product. Neuronal nuclei that are double-labeled for both proteins appear red with a gray core. This protocol allows the simultaneous detection of two proteins within the same subcellular compartment of a single neuron, without the need for epifluorescence or scanning confocal laser microscopy.

Electrophysiological and morphological evidence for a GABAergic nigrostriatal pathway

The electrophysiological and neurochemical characteristics of the nondopaminergic nigrostriatal (NO-DA) cells and their functional response to the degeneration of dopaminergic nigrostriatal (DA) cells were studied. Three different criteria were used to identify NO-DA cells: (1) antidromic response to striatal stimulation with an electrophysiological behavior (firing rate, interspike interval variability, and conduction velocity) different from that of DA cells; (2) retrograde labeling after striatal injection of HRP but showing immunonegativity for DA cell markers (tyrosine hydroxylase, calretinin, calbindin-D28k, and cholecystokinin); and (3) resistance to neurotoxic effect of 6-hydroxydomine (6-OHDA). Our results showed that under normal conditions, 5-8% of nigrostriatal neurons are immunoreactive for GABA, glutamic acid decarboxylase, and parvalbumin, markers of GABAergic neurons, a percentage that reached 81-84% after 6-OHDA injection. Electrophysiologically, NO-DA cells showed a behavior similar to that found in other nigral GABAergic (nigrothalamic) cells. In addition, the 6-OHDA degeneration of DA cells induced a modification of their electrophysiological pattern similar to that found in GABAergic nigrothalamic neurons. Taken together, the present data indicate the existence of a small GABAergic nigrostriatal pathway and suggest their involvement in the pathophysiology of Parkinson's disease.

Incremental conductance levels of GABAA receptors in dopaminergic neurones of the rat substantia nigra pars compacta

1. Molecular and biophysical properties of GABAA receptors of dopaminergic (DA) neurones of the pars compacta of the rat substantia nigra were studied in slices and after acute dissociation. 2. Single-cell reverse transcriptase-multiplex polymerase chain reaction confirmed that DA neurones contained mRNAs encoding for the alpha3 subunit of the GABAA receptor, but further showed the presence of alpha4 subunit mRNAs. alpha2, beta1 and gamma1 subunit mRNAs were never detected. Overall, DA neurones present a pattern of expression of GABAA receptor subunit mRNAs containing mainly alpha3/4beta2/3gamma3. 3. Outside-out patches were excised from DA neurones and GABAA single-channel patch-clamp currents were recorded under low doses (1-5 microM) of GABA or isoguvacine, a selective GABAA agonist. Recordings presented several conductance levels which appeared to be integer multiples of an elementary conductance of 4-5 pS. This property was shared by GABAA receptors of cerebellar Purkinje neurones recorded in slices (however, with an elementary conductance of 3 pS). Only the 5-6 lowest levels were analysed. 4. A progressive change in the distribution of occupancy of these levels was observed when increasing the isoguvacine concentration (up to 10 microM) as well as when adding zolpidem (20-200 nM), a drug acting at the benzodiazepine binding site: both treatments enlarged the occupancy of the highest conductance levels, while decreasing that of the smallest ones. Conversely, Zn2+ (10 microM), a negative allosteric modulator of GABAA receptor channels, decreased the occupancy of the highest levels in favour of the lowest ones. 5. These properties of alpha3/4beta2/3gamma3-containing GABAA receptors would support the hypothesis of either single GABAA receptor channels with multiple open states or that of a synchronous recruitment of GABAA receptor channels that could involve their clustering in the membranes of DA neurones.

Alterations of neuronal activity in the superior colliculus of rotating animals

We have investigated the relationship between alterations in neuronal activity in the superior colliculus and behavioral responses which occur following disruption of basal ganglia circuitry. These changes were analysed following unilateral suppression of the immediate early genes, c-fos and ngfi-a, in the striatum and/or the globus pallidus. Animals with unilateral suppression of immediate early gene expression in the striatum exhibited robust circling activity, following administration of D-amphetamine, that was directed towards the side of suppression. The intensity of rotation was inversely related to the length of the recovery period following antisense infusion and increased significantly when the globus pallidus was infused simultaneously with the striatum. The difference between ipsiversive (towards the antisense-infused hemisphere) and contraversive rotations was calculated and animals were grouped by number according to their ipsiversive bias: I, <50 turns; II, 50-500 turns; III, 500-1000 turns; IV, >1000 turns. Immunohistochemical localization of Fos was used as an indicator of neuronal activity in the superior colliculus. While group I animals showed diffuse Fos-like immunoreactivity throughout the intermediate layers of the superior colliculus, those animals in groups II-IV showed increasing suppression of Fos-like immunoreactivity in the stratum album intermediale and marked enhancement in the stratum griseum intermediale. Correlation and regression analysis revealed a significant positive relationship between the number of ipsiversive rotations and the number of Fos-positive nuclei in the stratum griseum intermediale of the ipsilateral superior colliculus. These data suggest that the degree of rotation elicited in an animal may depend on reciprocal suppression/stimulation of adjacent intermediate strata of the superior colliculus. This study provides the first demonstration, using Fos immunohistochemistry, of changes in tectal activity produced by alterations in basal ganglia function. These findings support previous electrophysiological studies in this region and suggest that the nigrotectal projection may be an important site of altered basal ganglia output.

Dopamine neurons make glutamatergic synapses in vitro

Interactions between dopamine and glutamate play prominent roles in memory, addiction, and schizophrenia. Several lines of evidence have suggested that the ventral midbrain dopamine neurons that give rise to the major CNS dopaminergic projections may also be glutamatergic. To examine this possibility, we double immunostained ventral midbrain sections from rat and monkey for the dopamine-synthetic enzyme tyrosine hydroxylase and for glutamate; we found that most dopamine neurons immunostained for glutamate, both in rat and monkey. We then used postnatal cell culture to examine individual dopamine neurons. Again, most dopamine neurons immunostained for glutamate; they were also immunoreactive for phosphate-activated glutaminase, the major source of neurotransmitter glutamate. Inhibition of glutaminase reduced glutamate staining. In single-cell microculture, dopamine neurons gave rise to varicosities immunoreactive for both tyrosine hydroxylase and glutamate and others immunoreactive mainly for glutamate, which were found near the cell body. At the ultrastructural level, dopamine neurons formed occasional dopaminergic varicosities with symmetric synaptic specializations, but they more commonly formed nondopaminergic varicosities with asymmetric synaptic specializations. Stimulation of individual dopamine neurons evoked a fast glutamatergic autaptic EPSC that showed presynaptic inhibition caused by concomitant dopamine release. Thus, dopamine neurons may exert rapid synaptic actions via their glutamatergic synapses and slower modulatory actions via their dopaminergic synapses. Together with evidence for glutamate cotransmission in serotonergic raphe neurons and noradrenergic locus coeruleus neurons, the present results suggest that glutamatergic cotransmission may be the rule for central monoaminergic neurons.

Sex and the substantia nigra: administration, teaching, patient care, and research

The immature brain is most susceptible to the development of seizures. The substantia nigra may play a crucial role in the control of seizures as a function of age. In the adult substantia nigra pars reticulata (SNR), there are two regions that mediate opposing effects on seizures after infusions of GABA(A) agents. One region is located in the anterior SNR, and localized muscimol infusions mediate anticonvulsant effects. These anticonvulsant effects use a circuitry that may involve the ventromedial thalamic nucleus, the deep layer of the superior colliculus, or both. The second region is in the posterior SNR, and muscimol infusions produce proconvulsant effects, perhaps mediated by the striatum, the globus pallidus, the deep layer of the superior colliculus, or all three. In developing male rats, only the proconvulsant region is present up to the age of 21 days. In ongoing studies, it has been shown that, in the male rat, the transition from the immature to mature SNR-mediated seizure control occurs between the ages of 25 and 30 days, just before adolescence. In male rats castrated on the day of birth, the ensuing depletion of testosterone accelerates the development of the anterior SNR with its anticonvulsant features. Castration does not alter the development of the proconvulsant region. In the developing female SNR, muscimol infusions produce only anticonvulsant effects. The data indicate that gonadal hormones may have an important role in the maturation of systems involved in the containment of seizures.

Morphometric characteristics of cryopreserved mesencephalic dopamine neurons in culture

Blocks of embryonic rat mesencephalon were freeze-stored for 1-2 years in liquid nitrogen at -196 degrees C with 7.5% dimethyl sulfoxide (DMSO) as cryoprotectant. After thawing, pooled mesencephalic tissues were mechanically dissociated. The cells, plated at two different densities (4.10[5] and 2.10[5]/cm2) were cultured in a serum-supplemented medium for at least 2 weeks before immunocytochemical staining with highly specific antidopamine (DA) antibodies. The cryopreserved DA-immunoreactive (IR) neurons were compared, by means of computerized morphometry, to the fresh ones plated at the same densities. A separate analysis of the dendritic and axonal morphometric parameters revealed that the cryopreserved DA-IR cells, whatever the experimental conditions, had significantly larger dendritic fields and, less significantly, larger axonal fields than their fresh counterparts. A principal component analysis, mainly based on the dendritic morphometric parameters, allowed to individualize only two populations (cryopreserved and fresh) among the four groups studied. These findings underline the role of dendrites as potential sites of release and/or re-uptake of dopamine and their possible implications in functionally effective cryopreserved nigral grafts.

Connections of the superior colliculus with the tegmentum and the cerebellum in the hedgehog tenrec

Different tracer substances were injected into the superior colliculus (CoS) in order to study its afferents and efferents with the meso-rhombencephalic tegmentum, the precerebellar nuclei and the cerebellum in the Madagascan hedgehog tenrec. The overall pattern of tectal connectivity in tenrec was similar to that in other mammals, as, e.g. the efferents to the contralateral paramedian reticular formation. Similarly the origin of the cerebello-tectal projection in mainly the lateral portions of the tenrec's cerebellar nuclear complex corresponded to the findings in species with little binocular overlap. In comparison to other mammals, however, the tenrec showed a consistent projection to the ipsilateral inferior olivary nucleus, in addition to the classical contralateral tecto-olivary projection. The tenrec's CoS also appeared to receive an unusually prominent monoaminergic input particularly from the substantia nigra, pars compacta. There was a reciprocal tecto-parabigeminal projection, a distinct nuclear aggregation of parabigeminal neurons, however, was difficult to identify. The dorsal lemniscal nucleus did not show perikaryal labeling in contrast to the paralemniscal region. Similar to the cat but unlike the rat there were a few neurons in the nucleus of the central acoustic tract. Unlike the cat, but similar to the rat there was a distinct, predominantly ipsilateral projection to the magnocellular reticular field known to project spinalward.

Cryopreserved GABAergic neurons in cultures of rat cerebral cortex and mesencephalon: a comparative morphometric study with anti-GABA antibodies

Blocks of embryonic rat cerebral cortex and mesencephalon were cryopreserved and stored for up to 1 year in liquid nitrogen at -196 degrees C with 7.5% dimethylsulfoxide (DMSO) as cryoprotectant. After thawing, these tissues were only mechanically dissociated and the cells were cultured for 2-7 weeks before immunocytochemical staining with anti-GABA (gamma-aminobutyric acid) antibodies. The freeze-stored GABA-immunoreactive (IR) mesencephalic neurons were compared, with computerized morphometry, to fresh mesencephalic cells and to their fresh and frozen cerebral cortical counterparts. A part of the cortical cells was treated with thienyl-phencyclidine (TCP) in order to assess the potential morphological effects of this neuroprotective agent upon these cortical neurons. Two types of GABA-IR neurons (small and large neuritic field cells) could be evidenced in both structures without any difference between fresh and frozen materials, but with significant quantitative morphological differences linked to their anatomical source. GABAergic phenotype is expressed similarly in fresh and frozen cultured neurons with intrinsically programmed morphological features and only minor influences of epigenetic factors. Small and large neuritic field GABA-IR neurons represent, respectively, local and long-range circuits of inhibition, strongly reminiscent of those described in vivo and which remain unchanged in culture even after freeze-storage.

Choline acetyltransferase, glutamate decarboxylase, tyrosine hydroxylase, calcitonin gene-related peptide and opioid peptides coexist in lateral efferent neurons of rat and guinea-pig

The lateral efferent (olivocochlear) innervation of the cochlea originates in the brainstem lateral superior olive. It is likely to use acetylcholine, gamma-aminobutyric acid, dopamine and various neuropeptides as neurotransmitters and/or neuromodulators. In order to determine the different coexistence patterns of these molecules in lateral efferent perikarya, we have used double and triple immunofluorescence co-localization techniques to colocalize choline acetyltransferase, glutamate decarboxylase, tyrosine hydroxylase, calcitonin gene-related peptide and enkephalins in single sections of the lateral superior olive. We also used a non-radioactive in situ hybridization technique onto serial sections of this nucleus to confirm the immunofluorescence co-localization data at the mRNA level. Whatever the pair or triplet of primary antibodies tested was, a high ratio of coexistence was observed in the immunofluorescence experiments. In triple co-localization experiments, 90-93% of the choline acetyltransferase-like immunoreactive neurons were also immunoreactive to the two other antigens investigated. The in situ hybridization co-localization data, based on the use of biotin-labelled oligoprobes, qualitatively confirmed these immunofluorescence data. In conclusion, it can be postulated that acetylcholine, gamma-aminobutyric acid, dopamine, calcitonin gene-related peptide, enkephalins and dynorphins (whose coexistence with choline acetyltransferase and enkephalins has been previously described immunocytochemically) coexist in lateral efferent neurons. Based on these results, it is tempting to propose the lateral efferent innervation as a useful model with which the functional implications of the coexistence of neurotransmitters/neuromodulators can be investigated in vivo.

Presence of mRNA for glutamic acid decarboxylase in both excitatory and inhibitory neurons

Neurons in very low density hippocampal cultures that are physiologically identified as either GABAergic inhibitory or glutamatergic excitatory all contain mRNA for the gamma-aminobutyric acid (GABA) synthetic enzyme, glutamic acid decarboxylase (GAD), as detected by single cell mRNA amplification and PCR. However, consistent with the physiology, immunocytochemistry revealed that only a subset of the neurons stain for either GAD protein or GABA. A similar fraction hybridize with RNA probes for GAD65 and GAD67. Hippocampal CA1 pyramidal neurons in slice preparations, which are traditionally thought to be excitatory, also contain mRNA for GAD65 and GAD67. Hippocampal neurons in culture did not contain mRNA for two other neurotransmitter synthesizing enzymes, tyrosine hydroxylase, and choline acetyl transferase. These data suggest that in some neurons, presumably the excitatory neurons, GAD mRNA is selectively regulated at the level of translation. We propose that neurotransmitter phenotype may be posttranscriptionally regulated and neurons may exhibit transient phenotypic plasticity in response to environmental influences.

Phenotypical characterization of neurons expressing the dopamine D3 receptor in the rat brain

We have established the cellular distribution of the dopamine D3 receptor using tritiated 7-hydroxy-N-N-di-n-propyl-2-aminotetralin and a complementary RNA probe to visualize autoradiographically the protein in binding studies and the gene transcripts by in situ hybridization, respectively. Studies with these two markers confirm the restricted expression of the D3 receptor in few brain areas, i.e. mainly the ventral striatal complex, the substantia nigra-ventral tegmental area and the cerebellum. In nucleus accumbens, the D3 receptor was mainly expressed in medium-sized neurons of the rostral pole and ventromedial shell subdivisions, but not of the core or septal pole, i.e. accumbal subdivisions expressing the D2 receptor. In the ventromedial shell, about 60% of the D3 receptor-expressing neurons were neurotensin neurons, presumably projecting to the ventral pallidum. In the islands of Calleja, both D3 receptor binding and messenger RNA were abundant in the entire population of granule cells. These cells are known to make sparse contacts with dopaminergic axons and also to express the D1 receptor. In the mesencephalon, low levels of D3 messenger RNA were detected in few dopamine neurons of substantia nigra pars lateralis and ventral tegmental area. In addition, some D3 receptor binding but not messenger RNA was detected in medial substantia nigra and lateral ventral tegmental area, where the receptor is presumably located presynaptically on afferents. In the archicerebellum, Purkinje cell perikarya in lobules 9 and 10 expressed the D3 receptor messenger RNA, whereas binding sites were found in the molecular layer, where corresponding dendrites but no known dopaminergic projection from mesencephalon are found. The occurrence of D3 receptor gene expression in some brain areas receiving low dopamine innervation supports the hypothesis that this receptor may mediate non-synaptic actions of dopamine.

Maturation and segregation of brain networks that modify seizures

The mature brain is less susceptible to seizures than the immature brain. We demonstrate that in the mature substantia nigra (SN) there are two topographically discrete GABAA-sensitive regions which differ in the amount of mRNA expression of the GABAA receptor alpha 1 subunit. These two regions mediate separate anticonvulsant and proconvulsant effects and use divergent projection networks. By contrast, in the immature SN there is no special topography of mRNA expression of the alpha 1 subunit and only the proconvulsant network is present. The decreased seizure susceptibility of the mature brain may be related to postnatal segregation of GABAA-sensitive networks.

Increase of tyrosine hydroxylase and its mRNA in the rat substantia nigra pars reticulata by diazepam and picrotoxin

An involvement of GABAA receptors in the regulation of tyrosine hydroxylase (TH) gene expression in the substantia nigra pars reticulata (SNr) was investigated using immunohistochemistry (IMHC) and nonradioactive in situ hybridization histochemistry (ISH). The number of TH-positive cells was increased for both ISH and IMHC 8 h after a single administration of benzodiazepine diazepam, which facilitates GABAA-receptor-mediated transmission and reduces dopamine release in the substantia nigra (SN). Such increase in TH staining was suppressed when a dopamine D2 receptor agonist quinpirole was administered 10 min after diazepam. Co-administration of diazepam with a dopamine antagonist haloperidol did not further elevate, but rather, reduced haloperidol-induced increases in TH labeling. These results suggest that haloperidol and diazepam regulate TH gene expression in the SNr commonly by depressing dopaminergic transmission, and that diazepam activates TH expression in a group of SNr neurons which express this gene after haloperidol treatment. Moreover, a GABAA receptor antagonist, picrotoxin, activated TH gene expression in the SNr, and diazepam antagonized picrotoxin effects. Since picrotoxin increases neuronal activity, additional mechanisms will operate on TH gene expression. In conclusion, GABAergic substances will activate TH gene expression in SNr neurons (1) through decreasing spontaneous somato-dendritic dopamine release in the substantia nigra and/or (2) by increasing the activity of these neurons.

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.

Conceptual history of the nigrostriatal dopamine system

The history of the nigrostriatal dopamine system may provide a prime example of the two faces of scientific development. First, a given concept is replaced by another simply as a result of methodologies being improved, and second, successive technical improvements make seemingly settled controversies even more complicated and disputable. The nigrostriatal pathway, which had been unrecognizable with Nauta's silver impregnation method, became apparent by use of the more sensitive silver impregnation method of Fink-Heimer. The sensitivity of the latter method, however, was still insufficient to reveal the whole extent of another ascending dopamine system, the mesocortical dopamine system, until its existence was established through the application of glyoxylic acid fluorescent histochemistry. Electron microscopic analysis of nigrostriatal dopamine synapses in properly fixed tissue was initiated by the demonstration of dark type terminal degeneration, which was induced by either electrolytic lesions or chemical destruction with a specific toxin (6-hydroxydopamine) of the substantia nigra and medial forebrain bundle. The degenerating terminal boutons, thus produced, invariably formed postsynaptic membrane specializations of asymmetric type. However, the asymmetric nature of the synaptic morphology, although later confirmed by the combined study of chemical lesions and autoradiographic anterograde tracing, was seriously challenged with the introduction of electron microscopic immunohistochemistry. The latter method has consistently revealed that symmetric en passant synapses or axonal varicosities with no synaptic membrane specializations are the only tissue compartments immunoreactive to antibodies against dopamine and its synthetic enzyme tyrosine hydroxylase. In view of the fact that more than 95% of the nigrostriatal projection neurons are dopaminergic, it is difficult to satisfactorily interpret all the available and seemingly paradoxical fine structural data. In this context, a novel concept has emerged in the process of eliminating all the possible alternative interpretations. The concept is that single nigrostriatal neurons form two chemically distinct types of synapses, one dopaminergic symmetric en passant bouton and another non-dopaminergic (still chemically unclassified) asymmetric terminal bouton. If the concept is a valid one, it contradicts Dale's long standing principle, as defined by Eccles: at all the axonal branches of a neuron there is liberation of the same transmitter substance or substances. Furthermore, a certain population of substantia nigra pars reticulata neurons has recently been recognized to be immunoreactive to both dopamine synthetic tyrosine hydroxylase and GABA synthetic glutamate decarboxylase. These single neurons send projections to both the striatum and superior colliculus by way of axon collaterals.(ABSTRACT TRUNCATED AT 400 WORDS)

Tyrosine hydroxylase immunoreactivity in cerebellar Purkinje cells of the rat

Employing tyrosine hydroxylase (TH) immunohistochemistry, we have revealed that TH immunoreactivity occurs in cerebellar Purkinje cells in the rat. These TH-immunoreactive Purkinje cells were distributed predominantly in the crus I & II ansiform lobules and the paraflocculus, and to a lesser extent in the I & X vermal lobules. Since Purkinje cells in such cerebellar regions displayed no immunoreactivity to dopamine-beta-hydroxylase, the TH-immunoreactive Purkinje cells identified in the present study might contain dopamine or L-DOPA.

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.

The lateroposterior thalamic nucleus and substantia nigra pars lateralis: origin of dual innervation over the visual system and basal ganglia

Employing the fluorescent retrograde double labeling technique, the present study demonstrated that in the rat single neurons in the lateroposterior thalamic nucleus (LP) project to both the visual cortex and striatum, and that single neurons in the substantia nigra pars lateralis project to both the LP and striatum, or to both the superior colliculus and striatum. These results provide morphological evidence for the functional correlation between the visual system and basal ganglia.