Bilateral tectal projection of single nigrostriatal dopamine cells in the rat

The Dopaminergic Control of Movement-Evolutionary Considerations

Dopamine is likely the most studied modulatory neurotransmitter, in great part due to characteristic motor deficits in Parkinson's disease that arise after the degeneration of the dopaminergic neurons in the substantia nigra pars compacta (SNc). The SNc, together with the ventral tegmental area (VTA), play a key role modulating motor responses through the basal ganglia. In contrast to the large amount of existing literature addressing the mammalian dopaminergic system, comparatively little is known in other vertebrate groups. However, in the last several years, numerous studies have been carried out in basal vertebrates, allowing a better understanding of the evolution of the dopaminergic system, especially the SNc/VTA. We provide an overview of existing research in basal vertebrates, mainly focusing on lampreys, belonging to the oldest group of extant vertebrates. The lamprey dopaminergic system and its role in modulating motor responses have been characterized in significant detail, both anatomically and functionally, providing the basis for understanding the evolution of the SNc/VTA in vertebrates. When considered alongside results from other early vertebrates, data in lampreys show that the key role of the SNc/VTA dopaminergic neurons modulating motor responses through the basal ganglia was already well developed early in vertebrate evolution.

The Lamprey Forebrain - Evolutionary Implications

The forebrain plays a critical role in a broad range of neural processes encompassing sensory integration and initiation/selection of behaviour. The forebrain functions through an interaction between different cortical areas, the thalamus, the basal ganglia with the dopamine system, and the habenulae. The ambition here is to compare the mammalian forebrain with that of the lamprey representing the oldest now living group of vertebrates, by a review of earlier studies. We show that the lamprey dorsal pallium has a motor, a somatosensory, and a visual area with retinotopic representation. The lamprey pallium was previously thought to be largely olfactory. There is also a detailed similarity between the lamprey and mammals with regard to other forebrain structures like the basal ganglia in which the general organisation, connectivity, transmitters and their receptors, neuropeptides, and expression of ion channels are virtually identical. These initially unexpected results allow for the possibility that many aspects of the basic design of the vertebrate forebrain had evolved before the lamprey diverged from the evolutionary line leading to mammals. Based on a detailed comparison between the mammalian forebrain and that of the lamprey and with due consideration of data from other vertebrate groups, we propose a compelling account of a pan-vertebrate schema for basic forebrain structures, suggesting a common ancestry of over half a billion years of vertebrate evolution.

The tectum/superior colliculus as the vertebrate solution for spatial sensory integration and action

The superior colliculus, or tectum in the case of non-mammalian vertebrates, is a part of the brain that registers events in the surrounding space, often through vision and hearing, but also through electrosensation, infrared detection, and other sensory modalities in diverse vertebrate lineages. This information is used to form maps of the surrounding space and the positions of different salient stimuli in relation to the individual. The sensory maps are arranged in layers with visual input in the uppermost layer, other senses in deeper positions, and a spatially aligned motor map in the deepest layer. Here, we will review the organization and intrinsic function of the tectum/superior colliculus and the information that is processed within tectal circuits. We will also discuss tectal/superior colliculus outputs that are conveyed directly to downstream motor circuits or via the thalamus to cortical areas to control various aspects of behavior. The tectum/superior colliculus is evolutionarily conserved among all vertebrates, but tailored to the sensory specialties of each lineage, and its roles have shifted with the emergence of the cerebral cortex in mammals. We will illustrate both the conserved and divergent properties of the tectum/superior colliculus through vertebrate evolution by comparing tectal processing in lampreys belonging to the oldest group of extant vertebrates, larval zebrafish, rodents, and other vertebrates including primates.

The basal ganglia downstream control of brainstem motor centres--an evolutionarily conserved strategy

The basal ganglia plays a crucial role in decision-making and control of motion. The output of the basal ganglia consists of tonically active GABAergic neurons, a proportion of which project to different brainstem centres and another part projecting to thalamus and back to cortex. The focus here is on the former part, which keeps the different brainstem motor-centres tonically inhibited under resting conditions. These centres will be disinhibited when called into action. In the control of motion the direct pathway will promote movement and the indirect pathway inhibit competing movement patterns counteracting the motor-command issued. The basal ganglia detailed structure and function are conserved throughout the vertebrate evolution, including the afferent (e.g. habenulae) and efferent control of the dopamine system.

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.

Somatosensory cross-modal plasticity in the superior colliculus of visually deafferented rats

The effects of neonatal visual deafferentation on the final adult pattern of cortico-collicular connections from the rat primary somatosensory cortex barrel field were studied by injecting an anterograde tracer (BDA) into different locations of the barrel cortex. Collicular afferents originating in the barrel cortex normally end in the intermediate collicular strata (SGI and SAI). However, neonatal visual deafferentation caused an invasion of abundant somatosensory cortical afferents into the lateral portions of the superficial collicular strata (SGS and SO). Moreover, anterograde-labelled fibers in the intermediate strata were more densely packed in visually deafferented animals. In order to study the activity of the altered somatosensory cortico-collicular connection, the effects of two different types of whisker stimuli on c-fos expression in the SC were analyzed (apomorphine treatment and enriched environment exploration). In stimulated control animals, c-fos expression was clearly evident in neurons of the intermediate layers 2 h after whisker stimulation. Similar stimulation in adult animals that underwent neonatal visual deafferentation triggered higher levels of c-fos expression in the superficial collicular layers that were invaded by cortico-collicular axonal branches. In exploration experiments, increased levels of c-fos expression were also detected in lateral parts of the intermediate layers of visually deafferented animals. These results suggest that the ascending fibers of somatosensory cortical origin can recruit deafferented superficial collicular neurons that enabling them to participate in extravisual behavioural responses mediated by collicular circuits.

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.

Monoaminergic markers in the optic tectum of the domestic chick

The avian optic tectum has become a reliable model system to study the basic mechanisms that underlie the computation of visual stimuli. Many aspects of its cytoarchitecture, chemoarchitecture, connectivity and development are thoroughly characterized. However, knowledge about its monoaminergic innervation is still incomplete. As a prerequisite to understand a possible functional role of the monoaminergic neurotransmitters, the serotonergic, noradrenergic, and dopaminergic innervation of the optic tectum as well as the distribution of serotonin 2A receptors, the dopamine- and cAMP-regulated phosphoprotein DARPP-32 and calbindin D-28K was studied in domestic chicks by immunohistochemical techniques. Serotonergic, noradrenergic, and tyrosine hydroxylase positive axons and axon terminals were present in all layers of the optic tectum. Generally, the highest densities of serotonergic, noradrenergic, and tyrosine hydroxylase positive fibers were found in the superficial tectal layers 1-8, whereas only moderate densities of serotonergic, noradrenergic, and tyrosine hydroxylase positive fibers became obvious in the deep tectal layers 9-15. Serotonergic fibers were particularly abundant in layers 4, 5a and 7 and serotonin 2A receptors in layer 13. Noradrenergic fibers were densest in layers 4 and 5a, whereas tyrosine hydroxylase positive fibers showed a slightly different distribution pattern with additional dense labeling in layer 7. As revealed by double-labeling immunohistochemistry, serotonergic fibers were closely related to the cell bodies of calbindin-positive horizontal cells in layer 5b and tyrosine hydroxylase positive fibers often contacted DARPP-32+ dendritic shafts in layers 9 and 10. These findings indicate that the catecholaminergic innervation of the optic tectum consists of a noradrenergic and a dopaminergic component and that the noradrenergic, serotonergic, and dopaminergic system may be potentially involved in the modulation of retinal input in the superficial layers of the optic tectum as well as in the modulation of tectal output via the deep tectal layers.

The mammalian superior colliculus: laminar structure and connections

The superior colliculus is a laminated midbrain structure that acts as one of the centers organizing gaze movements. This review will concentrate on sensory and motor inputs to the superior colliculus, on its internal circuitry, and on its connections with other brainstem gaze centers, as well as its extensive outputs to those structures with which it is reciprocally connected. This will be done in the context of its laminar arrangement. Specifically, the superficial layers receive direct retinal input, and are primarily visual sensory in nature. They project upon the visual thalamus and pretectum to influence visual perception. These visual layers also project upon the deeper layers, which are both multimodal, and premotor in nature. Thus, the deep layers receive input from both somatosensory and auditory sources, as well as from the basal ganglia and cerebellum. Sensory, association, and motor areas of cerebral cortex provide another major source of collicular input, particularly in more encephalized species. For example, visual sensory cortex terminates superficially, while the eye fields target the deeper layers. The deeper layers are themselves the source of a major projection by way of the predorsal bundle which contributes collicular target information to the brainstem structures containing gaze-related burst neurons, and the spinal cord and medullary reticular formation regions that produce head turning.

Patterns of axonal branching of neurons of the substantia nigra pars reticulata and pars lateralis in the rat

Axons from neurons of the rat substantia nigra pars reticulata (SNr) and pars lateralis (SNl) were traced after injecting their cell body with biotinylated dextran amine. Thirty-two single axons were reconstructed from serial sagittal sections with a camera lucida, whereas four other SNr axons were reconstructed in the coronal plane to determine whether they innervate the contralateral hemisphere. Four distinct types of SNr projection neurons were identified based on their main axonal targets: type I neurons that project to the thalamus; type II neurons that target the thalamus, the superior colliculus (SC), and the pedunculopontine tegmental nucleus (PPTg); type III neurons that project to the periaqueductal gray matter and the thalamus; and type IV neurons that target the deep mesencephalic nucleus (DpMe) and the SC. The axons of the SNl showed the same branching patterns as SNr axons of types I, II, and IV. The coronal reconstructions demonstrated that SNr neurons innervate the thalamus, the SC, and the DpMe bilaterally. At the thalamic level, SNr and SNl axons targeted preferentially the ventral medial, ventral lateral, paracentral, parafascicular, and mediodorsal nuclei. Axons reaching the SC arborized selectively within the deep layers of this structure. Our results reveal that the SNr and SNl harbor several subtypes of projection neurons endowed with a highly patterned set of axon collaterals. This organization allows single neurons of these output structures of the basal ganglia to exert a multifaceted influence on a wide variety of diencephalic and midbrain structures.

The fine organization of nigro-collicular channels with additional observations of their relationships with acetylcholinesterase in the rat

The nigro-collicular pathway that links the basal ganglia to the sensorimotor layers of superior colliculus plays a crucial role in promoting orienting behaviors. This connection originating in the pars reticulata and lateralis of the substantia nigra has been shown in rat and cat to be topographically organized. In rat, a functional compartmentalization of the substantia nigra has also been shown reflecting that of the striatum. In light of this, we reinvestigated the topographical arrangement of the nigro-collicular pathway by examining the innervation of each nigral functional zone. We performed small injections of either biocytin or wheatgerm agglutinin conjugated with horseradish peroxidase restricted to identified somatic, visual and auditory nigral zones. Frontally cut sections showed that innervations provided by the three main nigral zones form a mosaic of complementary domains stratified from the stratum opticum to the ventral part of the intermediate collicular layers, with the somatic afferents sandwiched between the visual and the auditory ones. When reconstructed from semi-horizontal sections, nigral innervations organized in the form of a honeycomb-like array composed of 100 cylindrical modules covering three-quarters of the collicular surface. Such a modular architecture is reminiscent of the acetylcholinesterase lattice we previously described in rat intermediate collicular layers. In the enzyme lattice, the surroundings of the cylindrical modules are composed of a mosaic of dense and diffuse enzyme subdomains. Thus, we compared the distribution of the overall nigral projection and of its constituent channels with the acetylcholinesterase lattice. The procedure combined axonal labelling with histochemistry on single sections for acetylcholinesterase activity. The results demonstrate that the overall nigral projection overlaps the acetylcholinesterase lattice and its constituent channels converge with either the dense or the diffuse enzyme subdomains. The stereometric arrangement of the nigro-collicular pathway is suggestive of an architecture promoting the selection of collicular motor programs for different classes of orienting behavior.

Serotonin-1B receptor-mediated inhibition of [(3)H]GABA release from rat ventral tegmental area slices

In order to assess a role of 5-HT(1B) receptors for regulation of GABA transmission in the ventral tegmental area (VTA), VTA slices from the rat were incubated with [(3)H]GABA and beta-alanine, and superfused in the presence of nipecotic acid and aminooxyacetic acid. [(3)H]GABA release was induced by exposures to the medium containing 30 mM potassium for 2 min. The results showed that high potassium-evoked [(3)H]GABA release was sensitive to calcium withdrawal or blockade of sodium channels by tetrodotoxin, suggesting that tritium overflow induced by high potassium derived largely from neuronal stores. Administration of CP 93129 (0.15 and 0.45 microM), a 5-HT(1B) receptor agonist, or RU 24969 (0.15 and 0.45 microM), a 5-HT(1B/1A) receptor agonist, but not 8-OH-DPAT (0.45 microM), a 5-HT(1A) receptor agonist, inhibited high potassium-evoked [(3)H]GABA release in a concentration-related manner. The RU 24969-induced inhibition of [(3)H]GABA release was antagonized by either SB 216641, a 5-H(1B) receptor antagonist, or cyanopindolol, a 5-HT(1B/1A) receptor antagonist, but not by WAY 100635, a 5-HT(1A) receptor antagonist. Pre-treatment with SB 216641 also antagonized CP 93129-induced inhibition of [(3)H]GABA release. The results support the hypothesis that 5-HT(1B) receptors within the VTA can function as heteroreceptors to inhibit GABA release.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

Role of the superior colliculus in the motor effects of cannabinoids and dopamine

We studied the cellular distribution of CB1 cannabinoid receptors in the superior colliculus of the rat using an antibody raised against the N-terminal of the receptor. The effect of unilateral cannabinoid receptor stimulation in the intermediate layers of the superior colliculus on rotational behavior in rats was also explored. The antibody against CB1 receptors outlined the crossed descending system of the superior colliculus (predorsal bundle output system) as well as the collicular commisure. The potent cannabinoid agonist CP55,940 (5 microgram/0.25 microliter) induced strong contralateral turning when microinjected unilaterally into the lateral intermediate layers of the superior colliculus. The levels of turning obtained with the intracollicular administration of the cannabinoid were comparable to the highest levels obtained with dopamine agonists in the basal ganglia. The D(2) dopamine agonist quinpirole or the D(1) dopamine agonist SKF82958 reversed this contralateral rotation but failed to affect motor behavior on their own. A new motor pathway for cannabinoids is discussed.

A retrograde double-labeling technique for light microscopy. A combination of axonal transport of cholera toxin B-subunit and a gold-lectin conjugate

A light microscopical, non-fluorescent, retrograde double-labeling technique is described. Cholera toxin B-subunit (CTb) and a conjugate of wheatgerm agglutinin and bovine serum albumin coupled to 10 nm gold particles (gold-lectin) are both excellent retrograde tracers and, when visualized by means of immunohistochemistry and silver intensification, respectively, may be readily identified within the same cell. This light microscopical retrograde double-labeling technique is illustrated in rat with experiments designed to investigate the collateralisation (1) of vestibular neurons to the spinal cord and oculomotor complex, (2) of spinal neurons to the left and right lateral reticular nucleus, and (3) of inferior olivary neurons to the uvula of the cerebellum. Advantages over fluorescent double-labeling experiments are found in the fact that the diaminobenzidine reaction product as well as the silver/gold deposits do not fade and can be examined in counterstained sections. Moreover, the injection sites can be kept quite small and may be guided by electrophysiological recording through the injection pipette.

Direct projections from the entopeduncular nucleus to the lower brainstem in the rat

The present study reports the existence of projection fibers from the entopeduncular nucleus to the superior colliculus and lateral parts of the pontobulbar tegmental regions (so-called lateral tegmental field) in the rat, suggesting that the entopeduncular nucleus may control eye-head and orofacial movements via these projection fibers. The anterograde axonal tracing with Phaseolus vulgaris-leucoagglutinin has revealed that the entopedunculotectal fibers terminate, bilaterally, with an ipsilateral predominance, in the deep layers of the superior colliculus through its rostral one-third level and that the entopedunculotegmental fibers terminate, bilaterally, with an ipsilateral predominance, in the parabrachial area, reticular formation surrounding the trigeminal motor nucleus, and parvicellular, dorsal, and ventral reticular nuclei. The cells of origin of the entopedunculotectal and entopedunculotegmental projections have been identified by retrograde axonal tracing with Fluoro-Gold and cholera toxin B subunit. The entopedunculotectal or entopedunculotegmental fibers originate, respectively, from the dorsal or ventral part of the entopeduncular nucleus. Additionally, a series of fluorescent retrograde double-labeling experiments with Fast Blue and Diamidino Yellow have indicated that single entopeduncular nucleus neurons projecting to the superior colliculus or lateral tegmental field often send their axon collaterals to the lateral habenular nucleus. The entopedunculotectal fibers are assumed to control head movements, which may be provoked via the tectospinal fibers, and further to participate in eye movements as the nigrotectal fibers that have been known to arise from the substantia nigra pars reticulata to end in the deep layers of the superior colliculus primarily through its caudal two-thirds level. The entopedunculotegmental fibers are presumed to be involved in control of orofacial movements, because the sites of termination of the entopedunculotegmental fibers correspond well with the reported areas of distribution of premotor interneurons for the trigeminal motor, facial, and hypoglossal nuclei.

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.

Heterocarrier-mediated reciprocal modulation of glutamate and glycine release in rat cerebral cortex and spinal cord synaptosomes

The effects of glutamic acid (Glu) and glycine (Gly) on each others release were studied using rat brain cortex and spinal cord synaptosomes. Previously taken up [3H]Gly and [3H]D-aspartic acid ([3H]D-Asp) was employed as markers for Gly and Glu/Asp release, respectively. Glu enhanced the release of [3H]Gly (EC50 = 8.4 microM) from cortical synaptosomes. The effect of Glu was not mimicked by the glutamate receptor agonists N-methyl-D-aspartic acid (NMDA), kainic or quisqualic acid. The Glu effect was abolished by the Glu/Asp uptake inhibitor D-threo-hydroxy-aspartic acid and it was Na+ sensitive. D-Asp also increased [3H]Gly release (EC50 = 9.9 microM) and the effect was blocked by the Glu/Asp uptake inhibitor. In contrast to its effect in the cortex, Glu failed to increase the release of [3H]Gly from spinal cord synaptosomes. Gly enhanced the outflow of [3H]D-Asp from rat cerebral cortex and spinal cord synaptosomes (EC50 = 75.0 and 99.5 microM, respectively). Gly was much more potent a releaser of [3H]D-Asp in the spinal cord than in the cortex. The Gly effects were insensitive to strychnine or to 7-Cl-kynurenic acid, antagonists at the two known Gly receptors, but they were strongly Na+ dependent. Our results are compatible with the idea that high-affinity uptake systems specific for Glu/Asp or Gly are colocalized on the same nerve terminal in rat spinal cord and cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Glutamic acid and gamma-aminobutyric acid modulate each other's release through heterocarriers sited on the axon terminals of rat brain

The effects of gamma-aminobutyric acid (GABA) on the spontaneous release of endogenous glutamic acid (Glu) or aspartic acid (Asp) and the effects of Glu on the release of endogenous GABA or [3H]GABA were studied in superfused rat cerebral cortex synaptosomes. GABA increased the outflow of Glu (EC50 17.2 microM) and Asp (EC50 18.4 microM). GABA was not antagonized by bicuculline or picrotoxin. Neither muscimol nor (-)-baclofen mimicked GABA. The effects of GABA were prevented by GABA uptake inhibitors and were Na+ dependent. Glu enhanced the release of [3H]GABA (EC50 11.5 microM) from cortical synaptosomes. Glu was not mimicked by the glutamate receptor agonists N-methyl-D-aspartic, kainic, or quisqualic acid. The Glu effect was decreased by the Glu uptake inhibitor D-threo-hydroxyaspartic acid (THA) and it was Na+ sensitive. Similarly to Glu, D-Asp increased [3H]GABA release (EC50 9.9 microM), an effect blocked by THA. Glu also increased the release of endogenous GABA from cortex synaptosomes. In this case the effect was in part blocked by the (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, whereas the 6-cyano-7-nitroquinoxaline-2,3-dione-insensitive portion of the effect was prevented by THA. GABA increased the [3H]D-Asp outflow (EC50 13.7 microM) from hippocampal synaptosomes in a muscimol-, (-)-baclofen-, bicuculline-, and picrotoxin-insensitive manner. The GABA effect was abolished by blocking GABA uptake and was Na+ dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Bilateral projections of single retinal ganglion cells to the lateral geniculate nuclei and superior colliculi in the albino rat

Employing fluorescent retrograde double/triple labeling, we investigated bilateral projections of single retinal ganglion cells to the lateral geniculate nuclei (LGN) and superior colliculi (SC) in the albino rat. After separate injections of Fast Blue (FB) and Diamidino Yellow (DY), respectively, into the right and left LGN, a large number of retrogradely-labeled cells were distributed all over the retina contralateral to each injection. Ipsilaterally projecting ganglion cells, which were labeled with one tracer injected into the LGN, were found predominantly in the lower-temporal retinal region; approximately 56% (120-140 cells per retina) of them were further labeled with the other tracer injected into the contralateral LGN. The vast majority of these double-labeled cells were of large type (more than 20 microns in diameter). Similar findings were obtained after separate injections of FB and DY, respectively, into the right and left SC, or respectively, into the right SC and left LGN. After separate injections of FB, DY and rhodamine-B-isothiocyanate, respectively, into the bilateral LGN and unilateral SC, or respectively, into the unilateral LGN and bilateral SC, a number of cells triple-labeled with all tracers were localized in the lower-temporal retinal region; most of them were of large type. Thus, the bilateral projections from the lower-temporal retinal region representing binocular vision in the rat are indicated to be achieved not only by separate populations of ganglion cells each exclusively serving one side of the brain, but also by axon collaterals from single ganglion cells; the ganglion cells projecting bilaterally to the LGN or/and SC are primarily of large type corresponding probably to the Y cell in the cat retina.

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.

Topographical organization of the nigrotectal projection in rat: evidence for segregated channels

Recent evidence suggests that projections from the superior colliculus to the brainstem in rat are organized into a series of anatomically segregated output channels. To understand how collicular function may be modified by the basal ganglia it is important to know whether particular output modules of the superior colliculus can be selectively influenced by input from substantia nigra. The purpose of the present study was, therefore, to examine in more detail topography within the nigrotectal system in the rat. Small injections (10-50 nl) of a 1% solution of wheatgerm agglutinin conjugated with horseradish peroxidase were made at different locations within substantia nigra and surrounding structures. A discontinuous puff-like pattern of anterogradely transported label was found in medial and caudal parts of the ipsilateral intermediate layers of the superior colliculus. In contrast, the rostrolateral enlargement of the intermediate layers contained a greater density of more evenly distributed terminal label. Injection sites associated with this dense pattern of laterally located label were concentrated in lateral pars reticulata, while the puff-like pattern was produced by injections into ventromedial pars reticulata. Retrograde tracing experiments with the fluorescent dyes True Blue and Fast Blue revealed that injections involving the rostrolateral intermediate layers were consistently associated with a restricted column of labelled cells in the dorsolateral part of ipsilateral pars reticulata. Comparable injections into medial and caudal regions of the superior colliculus produced retrograde labelling in ventral and medial parts of the rostral two-thirds of pars reticulata. Both anterograde and retrograde tracing data indicated that contralateral nigrotectal projections arise from cells located in ventral and medial pars reticulata. The present results suggest that the main ipsilateral projection from substantia nigra pars reticulata to the superior colliculus comprises two main components characterized by regionally segregated populations of output cells and spatially separated zones of termination. Of particular interest is the apparent close alignment between terminal zones of the nigrotectal channels and previously defined populations of crossed descending output cells in the superior colliculus. Thus, the rostrolateral intermediate layers contain a concentration of terminals specifically from dorsolateral pars reticulata and output cells which project to the contralateral caudal medulla and spinal cord. Conversely, the medial and caudal intermediate layers receive terminals from ventral and medial pars reticulata and contain cells which project specifically to contralateral regions of the paramedian pontine and medullary reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

gamma-Aminobutyric acid and glycine modulate each other's release through heterocarriers sited on the releasing axon terminals of rat CNS

The ability of gamma-aminobutyric acid (GABA) and glycine (Gly) to modulate each other's release was studied in synaptosomes from rat spinal cord, cerebellum, cerebral cortex, or hippocampus, prelabeled with [3H]GABA or [3H]Gly and exposed in superfusion to Gly or to GABA, respectively. GABA increased the spontaneous outflow of [3H]Gly (EC50, 20.8 microM) from spinal cord synaptosomes. Neither muscimol nor (-)-baclofen, up to 300 microM, mimicked the effect of GABA, which was not antagonized by either bicuculline or picrotoxin. However, the effect of GABA was counteracted by the GABA uptake inhibitors nipecotic acid and N-(4,4-diphenyl-3-butenyl)nipecotic acid. Moreover, the GABA-induced [3H]Gly release was Na+ dependent and disappeared when the medium contained 23 mM Na+. The effect of GABA was Ca2+ independent and tetrodotoxin insensitive. Conversely, Gly enhanced the outflow of [3H]GABA from rat spinal cord synaptosomes (EC50, 100.9 microM). This effect was insensitive to both strychnine and 7-chlorokynurenic acid, antagonists at Gly receptors, but it was strongly Na+ dependent. Also, the Gly-evoked [3H]GABA release was Ca2+ independent and tetrodotoxin insensitive. GABA increased the outflow of [3H]Gly (EC50, 11.1 microM) from cerebellar synaptosomes; the effect was not mimicked by either muscimol or (-)-baclofen nor was it prevented by bicuculline or picrotoxin. The GABA effect was, however, blocked by GABA uptake inhibitors and was Na+ dependent. Gly increased [3H]GABA release from cerebellar synaptosomes (EC50, 110.7 microM) in a strychnine- and 7-chlorokynurenic acid-insensitive manner. This effect was Na+ dependent. The effects of GABA on [3H]Gly release seen in spinal cord and cerebellum could be reproduced also with cerebrocortical synaptosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Separate neuronal populations of the rat substantia nigra pars lateralis with distinct projection sites and transmitter phenotypes

The topographic organization of the nigral cells sending axons to the striatum, amygdala and inferior colliculus was studied in the rat substantia nigra pars lateralis by using retrograde fluorescent tracers. Nigral perikarya projecting to the inferior colliculus were located dorsolaterally within the substantia nigra pars lateralis, whereas nigral perikarya projecting to the striatum or to the amygdala were mostly situated ventromedially within the substantia nigra pars lateralis. The transmitter substances of the nigrotectal cells were examined by combining a retrograde tracing method with immunohistochemistry for tyrosine hydroxylase or glutamate decarboxylase. Nigral neurons projecting to the inferior colliculus lacked tyrosine hydroxylase immunoreactivity, but exhibited immunoreactivity for glutamate decarboxylase. The substantia nigra pars lateralis is made up of different neuronal populations: one projecting to the inferior colliculus and another directed to the striatum and amygdala. The pars lateralis pathway to the inferior colliculus utilized GABA as a neurotransmitter, whereas the previously characterized nigral cells projecting to the striatum and superior colliculus use GABA and dopamine as neurotransmitters.

Cellular basis for interactions between catecholaminergic afferents and neurons containing leu-enkephalin-like immunoreactivity in rat caudate-putamen nuclei

Dopaminergic afferents to the dorsal striatum, caudate-putamen nuclei, are known to modulate the levels and synthesis of endogenous opiate peptides (Leu5 and Met5-enkephalins). We examined the dual immunocytochemical localization of antisera raised against Leu5-enkephalin and the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), to determine the cellular substrates for these and/or other functional interactions. The antisera were identified by combined immunogold-silver and immunoperoxidase labeling in single coronal sections through the caudate-putamen nuclei of adult rats. These animals were given intraventricular injections of colchicine, and the brains were fixed by acrolein perfusion prior to immunocytochemical labeling. By light microscopy, perikarya and processes containing enkephalin-like immunoreactivity (ELI) were seen in close proximity to varicose processes immunoreactive for TH. Electron microscopy further demonstrated that the ELI was localized to perikarya, dendrites, and axon terminals, whereas the TH was exclusively in axons and terminals. The dendrites containing ELI were postsynaptic to terminals that were either (1) without detectable immunoreactivity, or (2) immunoreactive for TH or enkephalin. Nonsynaptic portions of the dendrites containing ELI were covered with astrocytic processes or were in direct apposition to unlabeled dendrites. Terminals containing ELI were densely immunoreactive and were in direct contact with (1) unlabeled and occasionally enkephalin-labeled proximal dendrites, and (2) TH-labeled and unlabeled terminals. In comparison with the opiate terminals, most catecholaminergic terminals were lightly immunoreactive for TH and usually contacted more distal unlabeled dendrites or spines and, more rarely, dendrites containing ELI. In a few favorable planes of section, the terminals containing ELI and those containing TH (1) converged on common unlabeled dendrites, or (2) formed dual contacts on two different labeled or unlabeled targets. Junctions formed by terminals containing ELI and TH were sometimes characterized by symmetric synaptic densities. However, numerous other dendritic and all axonal appositions were without recognized membrane densities. The findings of the study provide anatomical substrates for multilevel interactions between catecholamines, mostly dopamine, and enkephalin in rat dorsal striatum. These include (1) monosynaptic input from dopaminergic terminals to neurons containing enkephalin, (2) presynaptic modulation of transmitter release through axonal appositions, and (3) dual regulation of common targets through convergent input. In addition, the findings suggest that both enkephalin and dopamine may have similar modulatory roles in synchronizing the activity of dual targets postsynaptic to individual axon terminals. Alterations in any one of these multiple types of interactions could account for noted motor or sensory symptoms in neurological disorders characterized by depletion of dopamine or endogenous opiate peptides, or both.

Anticonvulsant role of nigrotectal projection in the maximal electroshock model of epilepsy—II. Pathways from substantia nigra pars lateralis and adjacent peripeduncular area to the dorsal midbrain

Lesion evidence suggests that the superior colliculus is essential for mediating the anticonvulsant properties of nigral suppression in the electroshock model of epilepsy. However, our companion paper [Redgrave et al. (1991) Neuroscience 46, 379-390] established that the region of dorsal midbrain where bicuculline was most effective in suppressing tonic hindlimb extension did not correspond well with the known distribution of nigrotectal terminals. The purpose of the present anatomical study was, therefore, to investigate in more detail ventral midbrain connections to the dorsal midbrain anticonvulsant zone in rat. Small injections (10-20 nl) of a 1% solution of wheatgerm agglutinin conjugated with horseradish peroxidase were made specifically into the region of dorsal midbrain where bicuculline was maximally effective. Numerous retrogradely labelled cells were found in substantia nigra pars lateralis and adjacent peripeduncular area but not in substantia nigra pars reticulata. Retrogradely labelled cells were also located in ventral zona incerta. When wheatgerm agglutinin-horseradish peroxidase injections were made into lateral substantia nigra, a region of anterogradely transported reaction product characteristic of nerve terminals was observed in the caudolateral deep layers and underlying reticular tissue; this area corresponded well to the dorsal midbrain anticonvulsant zone. These data suggest that, in the electroshock model of epilepsy, direct connections between substantia nigra pars lateralis and adjacent peripeduncular area and the dorsal midbrain anticonvulsant zone could be critical for mediating the anticonvulsant properties previously attributed to substantia nigra pars reticulata. During the course of this study, anterograde projections from substantia nigra pars lateralis and adjacent peripeduncular area to both superficial and intermediate layers of the ipsilateral superior colliculus were noted. Additional experiments using retrograde transport of the fluorescent tracer Fast Blue confirmed these projections.

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

The neurotransmitter phenotype(s) of nigral neurons innervating the superior colliculus (SC) in the rat was examined using a combination of immunohistochemical techniques and fluorescent retrograde tracing. After double-immunofluorescent histochemistry for tyrosine hydroxylase (TH) and glutamate decarboxylase (GAD), single cells in the rostral ventrolateral portion of the substantia nigra pars reticulata (SNr) and to a lesser extent the substantia nigra pars lateralis (SN1) displayed immunoreactivity to both antigens. Furthermore, following True blue (TB) injections into the SC and incubation for both TH and GAD immunoreactivity, a considerable number of cells in the SNr retrogradely labeled with TB (approximately 10%) were also immunopositive for both synthetic enzymes. The present study provides evidence for the coexistence of TH and GAD and thus, the coexistence of dopamine and GABA in a subpopulation of single nigrotectal projection cells.

Pyramidal cells in rat temporoauditory cortex project to both striatum and inferior colliculus

A retrograde fluorescent double-labeling technique was employed to examine whether the cortico-striate fibers are collaterals of the corticofugal fibers directed to the inferior colliculus in the rat. Following injections of two different fluorescent tracers into the striatum and the inferior colliculus, double-labeled cells were found in layer V pyramidal cells of the temporal cortex. These double-labeled cells were located mostly in the area corresponding to the rat primary auditory cortex, and constituted 6.4% of the pyramidal cells projecting to the inferior colliculus. This study has revealed the existence of a common innervation of the basal ganglia and the auditory system by the pyramidal cell of the auditory cortex.