Co-occurrence of gamma-aminobutyric acid, parvalbumin and the neurotensin-related neuropeptide LANT6 in pallidal, nigral and striatal neurons in pigeons and monkeys

Neurochemistry and circuit organization of the lateral spiriform nucleus of birds: A uniquely nonmammalian direct pathway component of the basal ganglia

We used diverse methods to characterize the role of avian lateral spiriform nucleus (SpL) in basal ganglia motor function. Connectivity analysis showed that SpL receives input from globus pallidus (GP), and the intrapeduncular nucleus (INP) located ventromedial to GP, whose neurons express numerous striatal markers. SpL-projecting GP neurons were large and aspiny, while SpL-projecting INP neurons were medium sized and spiny. Connectivity analysis further showed that SpL receives inputs from subthalamic nucleus (STN) and substantia nigra pars reticulata (SNr), and that the SNr also receives inputs from GP, INP, and STN. Neurochemical analysis showed that SpL neurons express ENK, GAD, and a variety of pallidal neuron markers, and receive GABAergic terminals, some of which also contain DARPP32, consistent with GP pallidal and INP striatal inputs. Connectivity and neurochemical analysis showed that the SpL input to tectum prominently ends on GABAA receptor-enriched tectobulbar neurons. Behavioral studies showed that lesions of SpL impair visuomotor behaviors involving tracking and pecking moving targets. Our results suggest that SpL modulates brainstem-projecting tectobulbar neurons in a manner comparable to the demonstrated influence of GP internus on motor thalamus and of SNr on tectobulbar neurons in mammals. Given published data in amphibians and reptiles, it seems likely the SpL circuit represents a major direct pathway-type circuit by which the basal ganglia exerts its motor influence in nonmammalian tetrapods. The present studies also show that avian striatum is divided into three spatially segregated territories with differing connectivity, a medial striato-nigral territory, a dorsolateral striato-GP territory, and the ventrolateral INP motor territory.

Targeting parvalbumin-expressing neurons in the substantia nigra pars reticulata restores motor function in parkinsonian mice

The activity of substantia nigra pars reticulata (SNr) neurons, the main output structure of basal ganglia, is altered in Parkinson's disease (PD). However, neither the underlying mechanisms nor the type of neurons responsible for PD-related motor dysfunctions have been elucidated yet. Here, we show that parvalbumin-expressing SNr neurons (SNr-PV+) occupy dorsolateral parts and possess specific electrophysiological properties compared with other SNr cells. We also report that only SNr-PV+ neurons' intrinsic excitability is reduced by downregulation of sodium leak channels in a PD mouse model. Interestingly, in anesthetized parkinsonian mice in vivo, SNr-PV+ neurons display a bursty pattern of activity dependent on glutamatergic tone. Finally, we demonstrate that chemogenetic inhibition of SNr-PV+ neurons is sufficient to alleviate motor impairments in parkinsonian mice. Overall, our findings establish cell-type-specific dysfunction in experimental parkinsonism in the SNr and provide a potential cellular therapeutic target to alleviate motor symptoms in PD.

Calcium-binding proteins typify the dopaminergic neuronal subtypes in the ventral tegmental area of zebra finch, Taeniopygia guttata

Calcium-binding proteins (CBPs) regulate neuronal function in midbrain dopamine (DA)-ergic neurons in mammals by buffering and sensing the intracellular Ca²⁺ , and vesicular release. In birds, the equivalent set of neurons are important in song learning, directed singing, courtship, and energy balance, yet the status of CBPs in these neurons is unknown. Herein, for the first time, we probe the nature of CBPs, namely, Calbindin-, Calretinin-, Parvalbumin-, and Secretagogin-expressing DA neurons in the ventral tegmental area (VTA) and substantia nigra (SN) in the midbrain of zebra finch, Taeniopygia guttata. qRT-PCR analysis of ventral midbrain tissue fragment revealed higher Calbindin- and Calretinin-mRNA levels compared to Parvalbumin and Secretagogin. Application of immunofluorescence showed CBP-immunoreactive (-i) neurons in VTA (anterior [VTAa], mid [VTAm], caudal [VTAc]), SN (compacta [SNc], and reticulata [SNr]). Compared to VTAa, higher Calbindin- and Parvalbumin-immunoreactivity (-ir), and lower Calretinin-ir were observed in VTAm and VTAc. Secretagogin-ir was highly localized to VTAa. In SN, Calbindin- and Calretinin-ir were higher in SNc, SNr was Parvalbumin enriched, and Secretagogin-ir was not detected. Weak, moderate, and intense tyrosine hydroxylase (TH)-i VTA neurons were demarcated as subtypes 1, 2, and 3, respectively. While subtype 1 TH-i neurons were neither Calbindin- nor Calretinin-i, ∼80 and ∼65% subtype 2 and ∼30 and ∼45% subtype 3 TH-i neurons co-expressed Calbindin and Calretinin, respectively. All TH-i neuronal subtypes co-expressed Parvalbumin with reciprocal relationship with TH-ir. We suggest that the CBPs may determine VTA DA neuronal heterogeneity and differentially regulate their activity in T. guttata.

Cellular and Synaptic Dysfunctions in Parkinson's Disease: Stepping out of the Striatum

The basal ganglia (BG) are a collection of interconnected subcortical nuclei that participate in a great variety of functions, ranging from motor programming and execution to procedural learning, cognition, and emotions. This network is also the region primarily affected by the degeneration of midbrain dopaminergic neurons localized in the substantia nigra pars compacta (SNc). This degeneration causes cellular and synaptic dysfunctions in the BG network, which are responsible for the appearance of the motor symptoms of Parkinson’s disease. Dopamine (DA) modulation and the consequences of its loss on the striatal microcircuit have been extensively studied, and because of the discrete nature of DA innervation of other BG nuclei, its action outside the striatum has been considered negligible. However, there is a growing body of evidence supporting functional extrastriatal DA modulation of both cellular excitability and synaptic transmission. In this review, the functional relevance of DA modulation outside the striatum in both normal and pathological conditions will be discussed.

Dorsal pallidal neurons directly link the nidopallium and midbrain in the zebra finch (Taeniopygia guttata)

The dorsal pallidum in birds is considered similar, if not homologous, to the globus pallidus (GP) of mammals. The dorsal pallidum projects to both thalamic and midbrain targets similar to the direct and indirect pathways arising from the internal and external segments of the GP. In the present study, retrograde and anterograde tracing studies revealed a previously undescribed projection of the avian dorsal pallidum. This arises from a specific dorsomedial component, which terminates in the intercollicular nucleus and partly surrounds the avian equivalent of the central nucleus of the inferior colliculus. The respiratory-vocal dorsomedial nucleus of the intercollicular complex, however, does not receive these projections. The somata of the pallidal neurons retrogradely labeled from injections in the intercollicular nucleus were large and generally multipolar and had extensive, sparsely branching central processes (presumptive dendrites) that together extended up to 2 mm dorsally into the intermediate and caudomedial nidopallium. The size and morphology of these neurons were similar to those of large pallidal neurons labeled by calretinin immunoreactivity, which could be co-localized to the same cells. Thus, rather than being directly involved in the control of movement, the large dorsomedial neurons of the caudal dorsal pallidum may be involved in sensory processing, in that they provide an unusual direct link between sensory (auditory/somatosensory) regions of the nidopallium and sensory regions of the intercollicular nucleus of the midbrain. J. Comp. Neurol. 525:1731-1742, 2017. © 2016 Wiley Periodicals, Inc.

Neurochemical compartmentalization within the pigeon basal ganglia

The goals of this study were to use multiple informative markers to define and characterize the neurochemically distinct compartments of the pigeon basal ganglia, especially striatum and accumbens. To this end, we used antibodies against 12 different neuropeptides, calcium-binding proteins or neurotransmitter-related enzymes that are enriched in the basal ganglia. Our results clarify boundaries between previously described basal ganglia subdivisions in birds, and reveal considerable novel heterogeneity within these previously described subdivisions. Sixteen regions were identified that each displayed a unique neurochemical organization. Four compartments were identified within the dorsal striatal region. The neurochemical characteristics support previous comparisons to part of the central extended amygdala, somatomotor striatum, and associational striatum of mammals, respectively. The medialmost part of the medial striatum, however, has several unique features, including prominent pallidal-like woolly fibers and thus may be a region unique to birds. Four neurochemically distinct regions were identified within the pigeon ventral striatum: the accumbens, paratubercular striatum, ventrocaudal striatum, and the ventral area of the lateral part of the medial striatum that is located adjacent to these regions. The pigeon accumbens is neurochemically similar to the mammalian rostral accumbens. The pigeon paratubercular and ventrocaudal striatal regions are similar to the mammalian accumbens shell. The ventral portions of the medial and lateral parts of the medial striatum, which are located adjacent to accumbens shell-like areas, have neurochemical characteristics as well as previously reported limbic connections that are comparable to the accumbens core. Comparisons to neurochemically identified compartments in reptiles, mammals, and amphibians indicate that, although most of the basic compartments of the basal ganglia were highly conserved during tetrapod evolution, uniquely avian compartments may exist as well.

Evolutionary and developmental contributions for understanding the organization of the basal ganglia

Herein we take advantage of the evolutionary developmental biology approach in order to improve our understanding of both the functional organization and the evolution of the basal ganglia, with a particular focus on the globus pallidus. Therefore, we review data on the expression of developmental regulatory genes (that play key roles in patterning, regional specification and/or morphogenesis), gene function and fate mapping available in different vertebrate species, which are useful to (a) understand the embryonic origin and basic features of each neuron subtype of the basal ganglia (including neurotransmitter/neuropeptide expression and connectivity patterns); (b) identify the same (homologous) subpopulations in different species and the degree of variation or conservation throughout phylogeny, and (c) identify possible mechanisms that may explain the evolution of the basal ganglia. These data show that the globus pallidus of rodents contains two major subpopulations of GABAergic projection neurons: (1) neurons containing parvalbumin and neurotensin-related hexapetide (LANT6), with descending projections to the subthalamus and substantia nigra, which originate from progenitors expressing Nkx2.1, primarily located in the pallidal embryonic domain (medial ganglionic eminence), and (2) neurons containing preproenkephalin (and possibly calbindin), with ascending projections to the striatum, which appear to originate from progenitors expressing Islet1 in the striatal embryonic domain (lateral ganglionic eminence). Based on data on Nkx2.1, Islet1, LANT6 and proenkephalin, it appears that both cell types are also present in the globus pallidus/dorsal pallidum of chicken, frog and lungfish. In chicken, the globus pallidus also contains neurons expressing substance P (SP), perhaps originating in the striatal embryonic domain. In ray-finned and cartilaginous fishes, the pallidum contains at least the Nkx2.1 lineage cell population (likely representing the neurons containing LANT6). Based on the presence of neurons containing enkephalin or SP, it is possible that the pallidum of these animals also includes the Islet1 lineage cell subpopulation, and both neuron subtypes were likely present in the pallidum of the first jawed vertebrates. In contrast, lampreys (jawless fishes) appear to lack the pallidal embryonic domain and the Nkx2.1 lineage cell population that mainly characterize the pallidum in jawed vertebrates. In the absence of data in other jawless fishes, the ancestral condition in vertebrates remains to be elucidated. Perhaps, a major event in telencephalic evolution was the novel expression of Nkx2.1 in the subpallium, which has been related to Hedgehog expression and changes in the regulatory region of Nkx2.1.

The avian subpallium: new insights into structural and functional subdivisions occupying the lateral subpallial wall and their embryological origins

The subpallial region of the avian telencephalon contains neural systems whose functions are critical to the survival of individual vertebrates and their species. The subpallial neural structures can be grouped into five major functional systems, namely the dorsal somatomotor basal ganglia; ventral viscerolimbic basal ganglia; subpallial extended amygdala including the central and medial extended amygdala and bed nuclei of the stria terminalis; basal telencephalic cholinergic and non-cholinergic corticopetal systems; and septum. The paper provides an overview of the major developmental, neuroanatomical and functional characteristics of the first four of these neural systems, all of which belong to the lateral telencephalic wall. The review particularly focuses on new findings that have emerged since the identity, extent and terminology for the regions were considered by the Avian Brain Nomenclature Forum. New terminology is introduced as appropriate based on the new findings. The paper also addresses regional similarities and differences between birds and mammals, and notes areas where gaps in knowledge occur for birds.

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.

GABA distribution in lamprey is phylogenetically conserved

The localization of gamma-aminobutyric acid (GABA) has been well described in most classes of vertebrates but not in adult lampreys. The question if the GABA distribution is similar throughout the vertebrate subphylum is therefore still to be addressed. We here investigate two lamprey species, the sea lamprey, Petromyzon marinus, and the river lamprey, Lampetra fluviatilis, and compare the GABA pattern with that of other vertebrates. The present immunohistochemical study provides an anatomical basis for the general distribution and precise localization of GABAergic neurons in the adult lamprey forebrain and brainstem. GABA-immunoreactive cells were organized in a virtually identical manner in the two species. They were found throughout the brain, with the following regions being of particular interest: the granular cell layer of the olfactory bulb, the nucleus of the anterior commissure, the septum, the lateral and medial pallia, the striatum, the nucleus of the postoptic commissure, the thalamus, the hypothalamus, and pretectal areas, the optic tectum, the torus semicircularis, the mesencephalic tegmentum, restricted regions of the rhombencephalic tegmentum, the octavolateral area, and the dorsal column nucleus. The GABA distribution found in cyclostomes is very similar to that of other classes of vertebrates, including mammals. Since the lamprey diverged from the main vertebrate line around 450 million years ago, this implies that already at that time the basic vertebrate plan for the GABA innervation in different parts of the brain had been developed.

Topographical organization of GABAergic neurons within the ventral tegmental area of the rat

The ventral tegmental area (VTA), the origin of dopaminergic cell bodies that comprise the mesocorticolimibic DA system, is widely implicated in drug and natural reward, cognition, and several psychiatric disorders. In addition to dopaminergic neurons, this region is populated by GABAergic neurons, which both regulate the firing of their dopaminergic counterparts and send projections throughout the brain. Although the dopaminergic neurons of the VTA have been extensively characterized neuroanatomically, much less is known about the GABAergic neurons in this region. Recent data suggest that the rostro-caudal topographic organization of these GABAergic neurons may correspond to their ability to regulate drug reward. In the present study, we used immunohistochemical techniques to examine the frequency and topography of GABAergic neurons throughout the rostro-caudal axis of the VTA and the extent to which they coexpress other proteins, including tyrosine hydroxylase (a marker of DA neurons), cholecystokinin, parvalbumin, calretinin, and calbindin d 28k.

Presynaptic modulation of GABA release in the basal ganglia

Presynaptic receptors provide plasticity to GABAergic synapses in the basal ganglia network, in which GABA neurons outnumber all other neurons. Presynaptic receptors, mostly of the metabotropic type, enhance or reduce the strength of synaptic inhibition and are activated by ligands being released from the GABA terminals themselves (autoreceptors) or by ligands coming from other sources (heteroreceptors), including the target neurons innervated by the GABA terminals. The latter mechanism, termed retrograde signaling, is given particular emphasis as far as it occurs in substantia nigra.

Cellular Localization of AMPA Type Glutamate Receptor Subunits in the Basal Ganglia of Pigeons (Columba livia)

Corticostriatal and thalamostriatal projections utilize glutamate as a neurotransmitter in mammals and birds. The influence on striatum is mediated, in part, by ionotropic AMPA-type glutamate receptors, which are heteromers composed of GluR1-4 subunits. Although the cellular localization of AMPA-type subunits has been well characterized in mammalian basal ganglia, their localization in avian basal ganglia has not. We thus carried out light microscopic single- and double-label and electron microscopic single-label immunohistochemical studies of GluR1-4 distribution and cellular localization in pigeon basal ganglia. Single-label studies showed that the striatal neuropil is rich in GluR1, GluR2, and GluR2/3 immunolabeling, suggesting the localization of GluR1, GluR2 and/or GluR3 to the dendrites and spines of striatal projection neurons. Double-label studies and perikaryal size distribution determined from single-label material indicated that about 25% of enkephalinergic and 25% of substance P-containing striatal projection neuron perikarya contained GluR1, whereas GluR2 was present in about 75% of enkephalinergic neurons and all substance-P -containing neurons. The perikaryal size distribution for GluR2 compared to GluR2/3 suggested that enkephalinergic neurons might more commonly contain GluR3 than do substance P neurons. Parvalbuminergic and calretininergic striatal interneurons were rich in GluR1 and GluR4, a few cholinergic striatal interneurons possessed GluR2, but somatostatinergic striatal interneurons were devoid of all subunits. The projection neurons of globus pallidus all possessed GluR1, GluR2, GluR2/3 and GluR4 immunolabeling. Ultrastructural analysis of striatum revealed that GluR1 was preferentially localized to dendritic spines, whereas GluR2/3 was found in spines, dendrites, and perikarya. GluR2/3-rich spines were generally larger than GluR1 spines and more frequently possessed perforated post-synaptic densities. These results show that the diverse basal ganglia neuron types each display different combinations of AMPA subunit localization that shape their responses to excitatory input. For striatal projection neurons and parvalbuminergic interneurons, the combinations resemble those for the corresponding cell types in mammals, and thus their AMPA responses to glutamate are likely to be similar.

The distribution and cellular localization of glutamic acid decarboxylase-65 (GAD65) mRNA in the forebrain and midbrain of domestic chick

The distribution and cellular localization of GAD65 mRNA in the forebrain and midbrain of domestic chick were examined by in situ hybridization histochemistry with (35)[S]-UTP labeled cRNA probes, using film and emulsion autoradiography. Film autoradiograms showed intense GAD65 labeling in many structures of the basal telencephalon, such as the medial and lateral striatum, the septum, the olfactory tubercle, the lateral bed nucleus of the stria terminalis, and the intrapeduncular nucleus, while the pallial telencephalon showed only a low level of labeling. Emulsion-coated sections revealed that GAD65 mRNA-containing neurons were at least six times more abundant in striatum than pallium, with only a uniformly scattered subpopulation labeled in pallium, and that the vast majority of the large scattered projection neurons of globus pallidus were heavily labeled for GAD65. Prominent labeling was also evident in the nucleus taeniae and subpallial amygdala, but not in the arcopallium in film autoradiograms. Within the diencephalon, the hypothalamus was more GAD65-rich than the thalamus. Additional subtelencephalic cell groups showing prominent labeling included the thalamic reticular nucleus and ventral lateral geniculate nucleus of the diencephalon, the nucleus pretectalis, subpretectalis and spiriformis lateralis of the pretectum, and the magnocellular isthmic nucleus of the optic lobe. Tectal layers 9-10 were also rich in GAD65. These results further clarify GABAergic circuits of the avian forebrain and midbrain, and show them to closely resemble those in mammals.

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 avian song system in comparative perspective

The song system of oscine birds has become a versatile model system that is used to study diverse problems in neurobiology. Because the song system is often studied with the intention of applying the results to mammalian systems, it is important to place song system brain nuclei in a broader context and to understand the relationships between these avian structures and regions of the mammalian brain. This task has been impeded by the distinctiveness of the song system and the vast apparent differences between the forebrains of birds and mammals. Fortunately, accumulating data on the development, histochemistry, and anatomical organization of avian and mammalian brains has begun to shed light on this issue. We now know that the forebrains of birds and mammals are more alike than they first appeared, even though many questions remain unanswered. Furthermore, the song system is not as singular as it seemed-it has much in common with other neural systems in birds and mammals. These data provide a firmer foundation for extrapolating knowledge of the song system to mammalian systems and suggest how the song system might have evolved.

An immunohistochemical and pathway tracing study of the striatopallidal organization of area X in the male zebra finch

Area X is a nucleus within songbird basal ganglia that is part of the anterior forebrain song learning circuit. It receives cortical song-related input and projects to the dorsolateral medial nucleus of thalamus (DLM). We carried out single- and double-labeled immunohistochemical and pathway tracing studies in male zebra finch to characterize the cellular organization and circuitry of area X. We found that 5.4% of area X neuronal perikarya are relatively large, possess aspiny dendrites, and are rich in the pallidal neuron/striatal interneuron marker Lys8-Asn9-neurotensin8-13 (LANT6). Many of these perikarya were found to project to the DLM, and their traits suggest that they are pallidal. Area X also contained several neuron types characteristic of the striatum, including interneurons co-containing LANT6 and the striatal interneuron marker parvalbumin (2% of area X neurons), interneurons containing parvalbumin but not LANT6 (4.8%), cholinergic interneurons (1.4%), and neurons containing the striatal spiny projection neuron marker dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein (DARPP-32) (30%). Area X was rich in substance P (SP)-containing terminals, and many ended on area X neurons projecting to the DLM with the woolly fiber morphology characteristic of striatopallidal terminals. Although SP+ perikarya were not detected in area X, prior studies suggest it is likely that SP-synthesizing neurons are present and the source of the SP+ input to area X neurons projecting to the DLM. Area X was poor in enkephalinergic fibers and perikarya. The present data support the premise that area X contains both striatal and pallidal neurons, with the striatal neurons likely to include SP+ neurons that project to the pallidal neurons.

Interspecific differences in the expression of the AMPA-type glutamate receptors and parvalbumin in the nucleus of Edinger-Westphal of chicks and pigeons

The distribution of AMPA-type glutamate receptor (GluR) subunits was studied in the Edinger-Westphal nucleus (EW) of chicks and pigeons. GluR1, GluR2, GluR3 and GluR4 subunits appeared to be present in EW neurons of both species, but interspecific differences were observed in the abundance of the different types of subunits found in EW neurons. Of particular note, GluR2 immunoreactivity was present in the vast majority (ca. 80%) of neurons of pigeon EW but was found in only a small fraction (ca. 15%) of chick EW neurons. Scarcity of the GluR2 subunit in chick EW was confirmed by in situ hybridization. Because of the tendency for parvalbumin to be localized to neurons that are selectively deficient in GluR2, we also studied the localization of parvalbumin, as well as other calcium-binding proteins, in EW of chick and pigeon. Parvalbumin was found in more than 50% of chick EW neurons but was not detected in pigeon EW neurons. Our results suggest that there are major glutamatergic inputs to EW neurons in both pigeons and chicks. Furthermore, there are likely to be more AMPA-type calcium-permeable glutamate receptors in EW neurons of chick than in pigeon, since it is known that the subtype containing the edited GluR2 subunit is not calcium permeable.

Amphetamine-induced Fos expression is evident in gamma-aminobutyric acid neurons in the globus pallidus and entopeduncular nucleus in rats treated with intrastriatal c-fos antisense oligodeoxynucleotides

Double immunostaining for Fos and gamma-aminobutyric acid (GABA) was used in a previously established animal model of striatal dysfunction to examine whether GABA-immunoreactive neurons in the globus pallidus (GP) and entopeduncular nucleus (EP) are activated to express Fos immunoreactivity by intraperitoneal injection of amphetamine. Striatal efferent activity was suppressed by intrastriatal infusions of antisense oligodeoxynucleotide targeted to the messenger RNA of the immediate early gene, c-fos. This suppression produced robust rotational behavior and expression of Fos in the ipsilateral GP and EP following amphetamine challenge. The expression of Fos in the ipsilateral GP and EP following amphetamine challenge is not observed in naïve or control antisense-treated animals. Quantitative analysis revealed that a majority of the amphetamine-activated (Fos-immunoreactive) neurons in the GP and EP express GABA. The present results suggest that inhibitory GABAergic projection neurons within these two nuclei are regulated by inhibitory striatal output and suggests that decreased inhibitory striatal output may contribute to the motor dysfunction observed in patients with Huntington's disease.

Functional circuitry of the avian basal ganglia: implications for basal ganglia organization in stem amniotes

Histochemical, pathway tracing, and neuropeptide/neurotransmitter localization studies in birds, reptiles and mammals during the 1970s and 80s clearly showed that the telencephalon in all amniotes consists of a prominent ventrally situated subpallial region termed the basal ganglia, and a large overlying region involved in higher order information processing termed the pallium or cortex. These studies also showed that the basal ganglia in all extant amniote groups possessed neurochemically and hodologically distinct striatal and pallidal territories. More recently, studies of the localization of genes controlling regional brain development have confirmed the homology of the basal ganglia among amniotes. In our ongoing studies, we have identified several aspects of the functional organization of the basal ganglia that birds also share with mammals. These include: (1) an extensive glutamatergic "cortico"-striatal input and distinctive, cell-type specific localization of glutamate receptor subtypes; (2) an extensive, presumptively glutamatergic intralaminar thalamic input to striatal neurons; (3) an extensive dopaminergic input from the midbrain targeting both substance P (SP) type and enkephalin (ENK) type striatal projection neurons, with SP-type striatal neurons seemingly richer in the D-1 type dopamine receptor; and (4) SP+ and ENK+ striatal outputs giving rise to functionally distinct so-called direct and indirect motor output pathways, with the direct pathway having a pallido-thalamo-motor cortex loop and the indirect pathway relaying back to the direct circuit via the subthalamic nucleus. These findings suggest that the major aspects of the cellular organization and functional circuitry of the basal ganglia in stem amniotes were already as observed in living amniotes, as therefore presumably was its key role in movement control. Because the organization of the basal ganglia of anamniotes is clearly less elaborate than in amniotes, and because the basal ganglia and cortex in amniotes are clearly extensively interconnected structures, it seems likely that stem amniotes were characterized by a major step forward in the grade of telencephalic organization of both the basal ganglia and the pallium.

The deep mesencephalic nucleus as an output center of basal ganglia: morphological and electrophysiological similarities with the substantia nigra

The deep mesencephalic nucleus (DMN) is a large midbrain reticular region between the superior colliculus, the substantia nigra compacta, the periaqueductal gray, and the medial geniculate body. Although some data suggest that it is involved in nociception and visceral control, its functions remain unclear. In the present study, by using morphological (combination of anterograde and retrograde tracers with immunocytochemistry and in situ hibrydization) and electrophysiological (firing activity and transynaptic response to striatal stimulation) methods, we show that a subpopulation of DMN cells shares many morphological and electrophysiological characteristics with those of the substantia nigra reticulata (SNR). These similarities include the following: 1) firing rate, firing pattern, and conduction velocity; 2) expression of GAD65, GAD67, and PV; 3) excitatory and inhibitory inputs from the striatum; and 4) projections to the ventral thalamus, superior colliculus, and pedunculopontine tegmental nucleus. Some differences were also found. In comparison with SN, DMN cells and striatal afferents are more sparsely distributed and they show conspicuous contralateral projections to the thalamus and superior colliculus. This suggests that, similarly to the SNR, the DMN acts as an output center of basal ganglia and probably facilitates the inter-hemispheric regulation of these centers.

Identification of the anterior nucleus of the ansa lenticularis in birds as the homolog of the mammalian subthalamic nucleus

In mammals, the subthalamic nucleus (STN) is a glutamatergic diencephalic cell group that develops in the caudal hypothalamus and migrates to a position above the cerebral peduncle. By its input from the external pallidal segment and projection to the internal pallidal segment, STN plays a critical role in basal ganglia functions. Although the basal ganglia in birds is well developed, possesses the same major neuron types as in mammals, and plays a role in movement control similar to that in mammals, it has been uncertain whether birds possess an STN. We report here evidence indicating that the so-called anterior nucleus of the ansa lenticularis (ALa) is the avian homolog of mammalian STN. First, the avian ALa too develops within the mammillary hypothalamic area and migrates to a position adjacent to the cerebral peduncle. Second, ALa specifically receives input from dorsal pallidal neurons that receive input from enkephalinergic striatal neurons, as is true of STN. Third, ALa projects back to avian dorsal pallidum, as also the case for STN in mammals. Fourth, the neurons of ALa contain glutamate, and the target neurons of ALa in dorsal pallidum possess AMPA-type glutamate receptor profiles resembling those of mammalian pallidal neurons. Fifth, unilateral lesions of ALa yield behavioral disturbances and movement asymmetries resembling those observed in mammals after STN lesions. These various findings indicate that ALa is the avian STN, and they suggest that the output circuitry of the basal ganglia for motor control is similar in birds and mammals.

Localization of dopamine D1A and D1B receptor mRNAs in the forebrain and midbrain of the domestic chick

The distribution and cellular localization of dopamine D1A and D1B receptor mRNAs in the forebrain and midbrain of the domestic chick were examined using in situ hybridization histochemistry with 35[S]-dATP labeled oligonucleotide probes, visualized with film and emulsion autoradiography. Labeling for D1A receptor mRNA was intense in the medial and lateral striatum, and moderately abundant in the pallial regions termed the archistriatum and the neostriatum, in the hypothalamic paraventricular nucleus region, and in the superficial gray layer of optic tectum of the midbrain. D1B receptor mRNA was abundant in the medial and lateral striatum, and in the pallial region termed the hyperstriatum ventrale, and moderately abundant in the intralaminar dorsal and posterior thalamus and in the superficial gray of the optic tectum. At the cellular level, about 75% of neurons in the medial striatum and 59% of neurons in the lateral striatum were labeled for D1A receptor mRNA, whereas about 39% of the neurons in the medial striatum and 21% in the lateral striatum were labeled for D1B receptor mRNA. Large striatal neurons were not labeled for D1A or D1B receptor mRNA. The data suggest that while both D1A and D1B receptors mediate dopaminergic responses in many neurons of the avian striatum, primarily D1A receptors mediate dopaminergic responses in the archistriatum and the neostriatum, while primarily D1B receptors mediate dopaminergic responses in the hyperstriatum ventrale and the thalamus.

Succinic dehydrogenase histochemistry reveals the location of the putative primary visual and auditory areas within the dorsal ventricular ridge of Sphenodon punctatus

In turtles, crocodilians, lizards and snakes, the dorsal ventricular ridge (DVR) is a nuclear cell mass that contains distinct visual and auditory thalamorecipient cell groups. In the tuatara (Sphenodon punctatus), the DVR is not organized into diverse cell groups but instead possesses a trilaminar cytoarchitecture resembling that characteristic of the telencephalic cortex in reptiles. To determine if visual and auditory fields might also be present in the DVR of Sphenodon punctatus, we used succinic dehydrogenase (SDH) histochemistry, which has been shown to delineate the visual and auditory fields of the DVR in turtles, crocodilians and lizards. We also used acetylcholinesterase (AChE) histochemistry to determine the boundary between the DVR and the basal ganglia in Sphenodon. We found an SDH-rich region in the neuropil ventral to the cell plate of the rostrolateral DVR and a slightly less intense SDH-rich zone in the neuropil deep to the cell plate of the ventromedial DVR. These SDH-rich zones appear to be located at the apical dendrites of the neurons of the adjacent cell plate. These SDH-rich zones were clearly located within the DVR and were distinct from the AChE-rich striatal part of the basal ganglia, which occupied the ventrolateral wall of the telencephalon. Based on findings in other reptiles, it seems likely that the SDH-rich zone in rostrolateral DVR represents the zone of termination of nucleus rotundus visual input to the DVR, whereas the zone in ventromedial DVR represents the zone of termination of nucleus reuniens auditory input. Because a trilaminar DVR such as that in Sphenodon might be the primitive DVR condition for reptiles, our results suggest that the cytoarchitecture of the DVR and the synaptic organization of its thalamic sensory input in the common ancestor of living reptiles might have been much like of the dorsal cortex.

Immunohistochemical localization of NMDA- and AMPA-type glutamate receptor subunits in the basal ganglia of red-eared turtles

Corticostriatal and thalamostriatal projection systems have been shown to utilize glutamate as a neurotransmitter in mammals and birds. Although corticostriatal and thalamostriatal projection systems have been demonstrated in turtles, it is uncertain whether they too use glutamate as their neurotransmitter. Immunohistochemical localization of glutamate and of NMDA- and AMPA-type ionotropic glutamate receptor subunits (NMDAR2A/B, GluR1, GluR2/3, and GluR4) were used to address this issue. Numerous medium-sized neurons that were rich in NMDAR2A/B and GluR2/3 were observed in the striatal part of the basal ganglia of red-eared turtles. Smaller numbers of medium-sized neurons and some large neurons rich in the GluR1 and GluR4 subunits were also observed in the striatum. The striatal neuropil was notably rich in GluR1, GluR2/3 and NMDAR2A/B subunits. The pallidal region was specifically rich in large neurons possessing GluR4 subunits. Consistent with the glutamate receptors on striatal and pallidal neurons, sources of input to the striatum and pallidum in turtle such as the dorsomedial and dorsolateral thalamic nuclei (which appear to correspond to intralaminar thalamic nuclei), telencephalic pallial cell groups, and the apparent subthalamic nucleus homologue were rich in glutamatergic neurons. The results show that the thalamostriatal, corticostriatal and subthalamo-pallidal projection systems of turtles are glutamatergic and that similar basal ganglia cell types in turtles and mammals have largely similar glutamate receptor characteristics.

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.

Structural and functional evolution of the basal ganglia in vertebrates

While a basal ganglia with striatal and pallidal subdivisions is 1 clearly present in many extant anamniote species, this basal ganglia is cell sparse and receives only a relatively modest tegmental dopaminergic input and little if any cortical input. The major basal ganglia influence on motor functions in anamniotes appears to be exerted via output circuits to the tectum. In contrast, in modern mammals, birds, and reptiles (i.e., modern amniotes), the striatal and pallidal parts of the basal ganglia are very neuron-rich, both consist of the same basic populations of neurons in all amniotes, and the striatum receives abundant tegmental dopaminergic and cortical input. The functional circuitry of the basal ganglia also seems very similar in all amniotes, since the major basal ganglia influences on motor functions appear to be exerted via output circuits to both cerebral cortex and tectum in sauropsids (i.e., birds and reptiles) and mammals. The basal ganglia, output circuits to the cortex, however, appear to be considerably more developed in mammals than in birds and reptiles. The basal ganglia, thus, appears to have undergone a major elaboration during the evolutionary transition from amphibians to reptiles. This elaboration may have enabled amniotes to learn and/or execute a more sophisticated repertoire of behaviors and movements, and this ability may have been an important element of the successful adaptation of amniotes to a fully terrestrial habitat. The mammalian lineage appears, however, to have diverged somewhat from the sauropsid lineage with respect to the emergence of the cerebral cortex as the major target of the basal ganglia circuitry devoted to executing the basal ganglia-mediated control of movement.

Evolution of the basal ganglia in tetrapods: a new perspective based on recent studies in amphibians

It has been postulated frequently that the fundamental organization of the basal ganglia (BG) in vertebrates arose with the appearance of amniotes during evolution. An alternative hypothesis, however, is that such a condition was already present in early anamniotic tetrapods and, therefore, characterizes the acquisition of the tetrapod phenotype rather than the anamniotic-amniotic transition. Re-examination of the BG organization in tetrapods in the light of recent findings in amphibians strongly supports the notion that elementary BG structures were present in the brain of ancestral tetrapods and that they were organized according to a general plan shared today by all extant tetrapods.

Distribution of calbindin D-28k and parvalbumin neurons and fibers in the rat basal ganglia

This review deals with the distribution of immunoreactivity for calbindin D-28k (CB) and parvalbumin (PV) in the different nuclei of the rodent basal ganglia analyzed with the data available after the use of single and double antigen procedures applied to single sections. These findings reveal that CB and PV are distributed according to a highly heterogeneous pattern in the caudate putamen complex (CPu), globus pallidus (GP), entopeduncular nucleus (EP), subthalamic nucleus (STh) and substantia nigra (SN) of the rat. In each basal ganglia structure, the two calcium-binding proteins label different neuronal subsets. Therefore, the use of CB and PV immunohistochemistry may be considered as an excellent tool to define distinct chemoarchitectonic and functional domains within the complex organization of the basal ganglia. Double immunohistochemical methods are also useful to illustrate the relationships between the different chemical subdivisions of the CPu, GP, EP, STh and SN and the chemically characterized connections with each other and with other forebrain and brainstem structures. However, specific rules should be followed when combining single and double immunostaining procedures, and the results of such studies must be evaluated with caution. When they are used properly, these methods can reveal hitherto unknown principles of organization of the basal ganglia and thus shed new light on the anatomical and functional organization of this set of subcortical structures involved in the control of motor behavior.

The efferent projections of the dorsal and ventral pallidal parts of the pigeon basal ganglia, studied with biotinylated dextran amine

In the present study we have investigated the efferent projections of both the dorsal and the ventral pallidum of the pigeon basal ganglia, using the sensitive anterograde tracer biotinylated dextran amine [Veenman C. L. et al. (1992) J. Neurosci. Meth. 41, 239-254]. Injections of biotinylated dextran amine in the pigeon dorsal pallidum produced numerous fibers and terminals in specific nuclei of the thalamus, hypothalamus, pretectum and midbrain tegmentum. In the thalamus, labeled fibers and terminals were observed in the avian thalamic reticular nucleus, the proposed motor part of the avian ventral tier (ventrointermediate area), the avian parafascicular nucleus (nucleus dorsointermedius posterior), as well as in the avian nucleus subrotundus (which may be comparable to the posterior intralaminar nuclei of mammals). Labeled fibers and terminals were also observed in the avian subthalamic nucleus (anterior nucleus of the ansa lenticularis), in the pretectum (nucleus spiriformis lateralis) and in the avian substantia nigra pars reticulata. Injections of biotinylated dextran amine in the pigeon ventral pallidum produced fibers and terminals in specific centers of the telencephalon, hypothalamus, thalamus, epithalamus, and midbrain and isthmic tegmentum. Labeled fibers and terminals were also observed in the avian subthalamic nucleus and the inmediately adjacent lateral hypothalamus, the avian thalamic reticular nucleus, the avian medidorsal nucleusaand posterior intralaminar nuclei, and the lateral habenula. Finally, labeled fibers and terminals were found in the ventral tegmental area, the avian substantia nigra pars compacta and the midbrain/isthmic tegmentum, which includes the pedunculopontine tegmental nucleus. Our results indicate that both the dorsal and ventral pallida of birds have unique and specific projection patterns, which are very similar to those of their counterparts in mammals. Our study suggests that these avian basal ganglia regions may be related mainly to somatomotor and limbic functions, respectively.

Distribution of neurotensin-containing neurons in the central nervous system of the pigeon and the chicken

Neurotensin is widely located in neurons of the central and peripheral nervous systems among mammalian species. To obtain a comparative evaluation, we examined the distribution of neurotensin-containing cell bodies and fibers in the central nervous system of the pigeon and the chicken. The pattern of localization of neurotensin immunoreactivity was similar in the two species. Abundant accumulations of neurotensin-containing cell bodies were found in the dorsolateral corticoid area, the piriform cortex, the parahippocampal area, the medial part of the frontal neostriatum, the lateral part of the caudal neostriatum, nucleus accumbens, the bed nucleus of the stria terminalis, ventral paleostriatum, the preoptic area, the ventromedial hypothalamic nucleus, the inferior hypothalamic nucleus, the infundibular hypothalamic nucleus, and the mammillary nuclei. Extremely dense networks of neurotensin-containing fibers were found in the pallial commissure, the lateral septal nucleus, the preoptic area, the periventricular gray around the third ventricle, the dorsalis hypothalamic area, the hypothalamic nuclei, the parabrachial nucleus, the locus ceruleus, and the dorsal vagal complex. Major differences of immunoreactivity between the two species were as follows. 1) The chicken neurohypophysis contained an extremely large accumulation of immunoreactive fibers, but there were few in the median eminence. The reverse was found in the pigeon. 2) The optic tectum in the pigeon contained immunoreactive cells and fibers in layers 2 and 4, but no immunoreactivity was seen in the chicken optic tectum. 3) The cerebellar cortex in the pigeon contained a small number of immunoreactive fibers, whereas that in the chicken did not. 4) The pigeon spinal cord contained immunoreactive neurons in the subependymal layer, but the chicken spinal cord did not. Our observations suggest the presence of a very wide network of neurotensin-containing neurons in the avian brain and spinal cord, which is also the case in mammals.

Colocalization of somatostatin, neuropeptide Y, neuronal nitric oxide synthase and NADPH-diaphorase in striatal interneurons in rats

The neuropeptides somatostatin (SS), neuropeptide Y (NPY), the enzyme neuronal nitric oxide synthase (nNOS) and enzymatic activity for NADPH diaphorase (NADPHd) are extensively colocalized in striatal interneurons, which has led to the widespread tendency to operationally treat all four substances as being completely colocalized within a single class of striatal interneurons. We have explored the validity of this assumption in rat striatum using multiple-labeling methods. Conventional epi-illumination fluorescence microscopy was used to examine tissue triple labeled for SS, NPY and nNOS, or double-labeled for SS and nNOS or for SS and NPY. In tissue double-labeled for SS and nNOs, confocal laser scanning microscopy (CLSM) images of SS and nNOS labeling were compared to subsequent NADPHd labeling. We found that SS, NPY and nNOS co-occurred extensively, but a moderately abundant population of neurons containing SS and nNOS but not NPY was also observed, as were small populations of SS only and nNOS only neurons. About 80% of SS+ neurons contained NPY, and no NPY neurons were devoid of SS or nNOS. All neurons containing nNOS in rat striatum were found to contain NADPHd. Combining our various quantitative observations, we found that of those striatal neurons containing any combination of SS, NPY, nNOS and NADPHd in rats, about 73% contained all four, 16% contained SS, nNOS and NADPHd, 5% contained SS only, and 6% contained only nNOS and NADPHd. These results indicate that while there is a large population of striatal neurons in which SS, NPY, nNOS and NADPHd are colocalized in rats, there may be smaller populations of striatal neurons devoid of NPY in which SS or nNOS/NADPHd are found individually or together.

Light and electron microscopic immunohistochemical study of dopaminergic terminals in the striatal portion of the pigeon basal ganglia using antisera against tyrosine hydroxylase and dopamine

A dopaminergic projection from the midbrain to the striatal portion of the basal ganglia is present in reptiles, birds, and mammals. Although the ultrastructure of these fibers and terminals within the striatum has been studied extensively in mammals, little information is available on the ultrastructure of this projection in nonmammals. In the present study, we used immunohistochemical labeling with antibodies against tyrosine hydroxylase (TH) or dopamine (DA) to study the dopaminergic input to the striatal portion of the basal ganglia in pigeons (i.e., lobus parolfactorius and paleostriatum augmentatum). At the light microscopic level, the anti-TH and anti-DA revealed a similar abundance and distribution of numerous labeled fine fibers and varicosities within the striatum. In contrast, the use of an antidopamine beta-hydroxylase antiserum (which labels only adrenergic and noradrenergic terminals) labeled very few striatal fibers, which were restricted to visceral striatum. These results demonstrate that anti-TH mainly labels dopaminergic terminals in the striatum. At the electron microscopic level, the anti-TH and anti-DA antisera labeled numerous axon terminals within the striatum (15-20% of all striatal terminals). These terminals tended to be small (with an average length of 0.6 microns) and flattened, and their vesicles tended to be small (35-60 nm in diameter) and pleomorphic. About 50% of the terminals were observed to make synaptic contacts in the planes of section examined, and nearly all of these synaptic contacts were symmetric. Both TH+ and DA+ terminals typically contacted dendritic shafts or the necks of dendritic spines, but a few contacted perikarya. No clear differences were observed between TH+ and DA+ terminals within medial striatum (whose neurons project to the nigra in birds) or between TH+ and DA+ terminals within lateral striatum (whose neurons project to the pallidum in birds). In addition, no differences were observed between medial and lateral striata in either TH+ or DA+ terminals. Thus, there is no evident difference in pigeons between striatonigral and striatopallidal neurons in their dopaminergic innervation. Our results also indicate that the abundance, ultrastructural characteristics, and postsynaptic targets of the midbrain dopaminergic input to the pigeon striatum are highly similar to those in mammals. This anatomical similarity is consistent with the pharmacologically demonstrable similarity in the role of the dopaminergic input to the striatum in birds and mammals.

Calretinin is largely localized to a unique population of striatal interneurons in rats

Previous studies have reported the presence of the calcium binding protein calretinin in neurons in the striatal part of the basal ganglia in rats and primates. In the present study, immunofluorescence double-labeling techniques and immunofluorescence combined with retrograde labeling were used in rats to determine whether calretinin is found in any of the known types of striatal neurons. The results showed that a small fraction of the calretinin-containing neurons (< 10%) contain parvalbumin, but none of the calretinin-containing striatal neurons contained markers for the other two major types of striatal interneurons (i.e., choline acetyltransferase-containing cholinergic neurons and somatostatin-containing neurons). Additionally, calretinin was not found in projection neurons, using either calbindin or DARPP32 as immunofluorescent markers of striatal projections neurons in general, or using retrograde labeling to specifically identify either striatonigral or striatopallidal neurons. Thus, calretinin appears to be largely found in a unique population of striatal interneurons in rats. This population appears to be about one third the abundance of any of the previously identified populations of striatal interneurons.

Ultrastructural morphology of synapses formed by corticostriatal terminals in the avian striatum

We studied the ultrastructural morphology of corticostriatal projections from two different avian 'neocortical' regions, namely, the hyperstriatum accessorium (HA) and the pallium externum (PE). Biotinylated dextran amine (BDA) was used to label the corticostriatal projection from either HA or PE to the striatum. The corticostriatal axons from both the PE and HA possessed numerous beaded varicosities with the striatum. These varicosities were filled with numerous round vesicles characterizing them as terminals. These terminals formed asymmetric synapses with spine heads and with dendrites of striatal neurons. The axospinous synapses outnumbered the axodendritic synapses by more than two to one. The diameters of labeled axons were typically 250-500 nm. The labeled terminals were typically 400-750 nm in diameter. No obvious differences between the ultrastructural morphology of the HA and the PE corticostriatal projections were observed. These data show that corticostriatal terminals and their synaptic contacts in birds are similar to those described in mammals.

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

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

Thalamostriatal projection neurons in birds utilize LANT6 and neurotensin: a light and electron microscopic double-labeling study

Based on its location, connectivity and neurotransmitter content, the dorsal thalamic zone in birds appears to be homologous to the intralaminar, midline, and mediodorsal nuclear complex in the thalamus of mammals. We investigated the neuroactive substances used by thalamostriatal projection neurons of the dorsal thalamic zone in the pigeon. Single-labeling experiments showed that many neurons in the dorsal thalamic zone are immunoreactive for neurotensin and the neurotensin-related hexapeptide, (Lys8,Asn9)NT(8-13) (LANT6). Double-labeling experiments, using the retrograde fluorescent tracer, FluoroGold, combined with fluorescence immunocytochemistry for either LANT6 or neurotensin, showed that neurotensin- and LANT6-containing neurons in the dorsal thalamic zone project to the striatum of the basal ganglia. Immunofluorescence double-labeling experiments showed that neurotensin and LANT6 are often (possibly always) co-expressed in neurons in the dorsal thalamic zone. Electron microscopic immunohistochemical double-labeling showed that LANT6 terminals in the striatum make asymmetric contacts with heads of spines labeled for substance P and heads of spines not labeled for substance P, suggesting that these terminals synapse with both substance P-containing and non-substance P-containing medium spiny striatal projection neurons. These findings indicate that LANT6 and neurotensin may be utilized as neurotransmitters in thalamostriatal projections in birds and raise the possibility that this may also be the case in other amniotes.

Autoradiographic and electrophysiological evidence for the existence of neurotensin receptors on cultured astrocytes

By means of autoradiography we have studied the cellular localization of binding sites for [3H]neurotensin and its nonpeptide receptor antagonist [3H]SR-48692 in explant cultures of rat neocortex, striatum, brain stem and spinal cord. Binding sites for the peptide and its antagonist were observed on a great number of astrocytes in all CNS regions studied. Simultaneous staining of the cultures with a monoclonal antibody against glial fibrillary acidic protein has shown that the labelled cells in the outgrowth zone of the cultures were glial fibrillary acidic protein-positive and could therefore be identified as astrocytes. In addition to astrocytes, many neurons and outgrowing nerve fibres were labelled by the radioligands. Binding of [3H]neurotensin and [3H]SR-48692 (10(-8)M) to neurons and glial cells was markedly reduced or inhibited by the unlabelled compounds at high concentration (10(-6)M), suggesting "specific" binding of the radioligands. Electrophysiological studies have shown that addition of neurotensin to the bathing solution caused a hyperpolarization of the majority of astrocytes tested. There was a dose-response relationship between the magnitude of the hyperpolarization and the concentration of the peptide (10(-10)-10(-7)M); 10(-10)M being the threshold concentration. The specificity of the action of neurotensin was confirmed by the selective nonpeptide neurotensin receptor antagonist SR-48692 which reversibly blocked or markedly reduced the hyperpolarization by the peptide on all astrocytes tested. Our electrophysiological findings together with our autoradiographic data provide strong evidence for the presence of specific and functional neurotensin receptors on astrocytes.

Organization of the avian "corticostriatal" projection system: a retrograde and anterograde pathway tracing study in pigeons

Birds have well-developed basal ganglia within the telencephalon, including a striatum consisting of the medially located lobus parolfactorius (LPO) and the laterally located paleostriatum augmentatum (PA). Relatively little is known, however, about the extent and organization of the telencephalic "cortical" input to the avian basal ganglia (i.e., the avian "corticostriatal" projection system). Using retrograde and anterograde neuroanatomical pathway tracers to address this issue, we found that a large continuous expanse of the outer pallium projects to the striatum of the basal ganglia in pigeons. This expanse includes the Wulst and archistriatum as well as the entire outer rind of the pallium intervening between Wulst and archistriatum, termed by us the pallium externum (PE). In addition, the caudolateral neostriatum (NCL), pyriform cortex, and hippocampal complex also give rise to striatal projections in pigeon. A restricted number of these pallial regions (such as the "limbic" NCL, pyriform cortex, and ventral/caudal parts of the archistriatum) project to such ventral striatal structures as the olfactory tubercle (TO), nucleus accumbens (Ac), and bed nucleus of the stria terminalis (BNST). Such "limbic" pallial areas also project to medialmost LPO and lateralmost PA, while the hyperstriatum accessorium portion of the Wulst, the PE, and the dorsal parts of the archistriatum were found to project primarily to the remainder of LPO (the lateral two-thirds) and PA (the medial four-fifths). The available evidence indicates that the diverse pallial regions projecting to the striatum in birds, as in mammals, are parts of higher order sensory or motor systems. The extensive corticostriatal system in both birds and mammals appears to include two types of pallial neurons: 1) those that project to both striatum and brainstem (i.e., those in the Wulst and the archistriatum) and 2) those that project to striatum but not to brainstem (i.e., those in the PE). The lack of extensive corticostriatal projections from either type of neuron in anamniotes suggests that the anamniote-amniote evolutionary transition was marked by the emergence of the corticostriatal projection system as a prominent source of sensory and motor information for the striatum, possibly facilitating the role of the basal ganglia in movement control.

Brainstem motoneuron pools that are selectively resistant in amyotrophic lateral sclerosis are preferentially enriched in parvalbumin: evidence from monkey brainstem for a calcium-mediated mechanism in sporadic ALS

Some brainstem motoneuron groups appear more resistant to the process of neurodegeneration in ALS (for example, oculomotor, trochlear, and abducens nuclei) than others (for example, trigeminal, facial, ambiguus, and hypoglossal nuclei). The possibility that the differential presence of the calcium-chelating protein parvalbumin might underlie this difference in vulnerability was examined immunohistochemically as a way to determine whether a calcium-mediated mechanism might be involved in ALS. In normal monkey brainstem, we found that the abundance of parvalbumin-containing neurons in the oculomotor, trochlear, and abducens nuclei was approximately 90% of the abundance of choline acetyltransferase (CHAT)-containing motoneurons. In contrast, the abundance of parvalbumin-containing neurons in the other brainstem motor nuclei innervating skeletal muscle (trigeminal, facial, ambiguus, and hypoglossal) was only about 30-60% of the abundance of CHAT-containing motoneurons. Since some of these motoneuron pools contain nonmotoneuron internuclear neurons that might be parvalbumin-containing, we also carried out double-label studies to specifically determine the percentage of cholinergic motoneurons that contained parvalbumin in each of these motoneuron pools. We found that 85-100% of the oculomotor, trochlear, and abducens motoneurons were parvalbumin-containing. In contrast, only 20-30% of the trigeminal, facial, ambiguus, and hypoglossal motoneurons were parvalbumin-containing. These results raise the possibility that motoneuron death in sporadic ALS is related to some defect that promotes cytosolic calcium accumulation in motoneurons. This excess calcium entry may promote cell death via an excitotoxic pathway. Motoneurons rich in parvalbumin may resist the deleterious effects of this putative calcium gating defect because they are better able to sequester the excess calcium.

Distributions of GABAA, GABAB, and benzodiazepine receptors in the forebrain and midbrain of pigeons

Autoradiographic and immunohistochemical methods were used to study the distributions of GABAA, GABAB and benzodiazepine (BDZ) receptors in the pigeon fore- and midbrain. GABAA, GABAB and BDZ binding sites were found to be abundant although heterogeneously distributed in the telencephalon. The primary sensory areas of the pallium of the avian telencephalon (Wulst and dorsal ventricular ridge) tended to be low in all three binding sites, while the surrounding second order belt regions of the pallium were typically high in all three. Finally, the outermost rind of the pallium (termed the pallium externum by us), which surrounds the belt regions and projects to the striatum of the basal ganglia, was intermediate in all three GABAergic receptors types. Although both GABAA and benzodiazepine receptors were abundant within the basal ganglia, GABAA binding sites were densest in the striatum and BDZ binding sites were densest in the pallidum. Among the brainstem regions receiving GABAergic basal ganglia input, the anterior and posterior nuclei of the ansa lenticularis showed very low levels of all three receptors, while the lateral spiriform nucleus and the ventral tegmental area/substantia nigra complex contained moderate abundance of the three binding sites. The dorsalmost part of the dorsal thalamus (containing nonspecific nuclei) was rich in all three binding sites, while the more ventral part of the dorsal thalamus (containing specific sensory nuclei), the ventral thalamus and the hypothalamus were poor in all three binding sites. The pretectum was also generally poor in all three, although some nuclei displayed higher levels of one or more binding sites. The optic tectum, inferior colliculus, and central gray were rich in all three sites, while among the isthmic nuclei, the parvicellular isthmic nucleus was conspicuously rich in BDZ sites. The results show a strong correlation of the regional abundance of GABA binding sites with previously described distributions of GABAergic fibers and terminals in the avian forebrain and midbrain. The regional distribution of these binding sites is also remarkably similar to that in mammals, indicating a conservative evolution of forebrain and midbrain GABA systems among amniotes.