Nucleus tegmenti pedunculopontinus: Efferent connections with special reference to the basal ganglia,studied in the rat by anterograde and retrograde transport of horseradish peroxidase

The efferent connections of the brain stem nucleus tegmenti pedunculopontinus were studied in the rat using the techniques of anterograde and retrograde transport of the enzyme horseradish peroxidase, laying particular emphasis on that part of pedunculopontinus which receives direct descending projections from the basal ganglia and related nuclei. In a preliminary series of experiments horseradish peroxidase was injected into either the entopeduncular nucleus or the subthalamic nucleus and, following anterograde transport of enzyme, terminal labelling was identified in nucleus tegmenti pedunculopontinus, surrounding the brachium conjunctivum in the caudal mesencephalon.In a subsequent series of experiments, horseradish peroxidase was injected into that region of nucleus tegmenti pedunculopontinus which receives entopeduncular and subthalamic efferents and its efferent projections were studied by anterograde transport of the enzyme. The results indicate that nucleus tegmenti pedunculopontinus gives rise to widely distributed efferent projections which terminate rostrally in mesencephalic, diencephalic and telencephalic structures and caudally in the pontine tegmentum. In the mesencephalon, terminal labelling was found in the pars compacta of the ipsilateral substantia nigra and sometimes in the adjoining ventral tegmental area. Labelling was also found in the ipsilateral half of the periaqueductal grey. In the diencephalon terminal labelling occurred bilaterally in the subthalamic nucleus and ipsilaterally in the intralaminar nuclei of the thalamus. Further rostrally, terminal labelling was particularly evident in the ipsilateral pallidal complex, especially in the caudal two-thirds of the entopeduncular nucleus and the ventral half of the caudal third of the globus pallidus. Caudal to pedunculopontine injection sites dense labelling was observed in the reticular formation of the pontine tegmentum.In a final series of experiments, confirmation of apparent pedunculopontine efferent projections was sought using the retrograde transport of horseradish peroxidase. Enzyme was injected into sites possibly receiving pedunculopontine efferents and the peribrachial area of the brain stem was examined for retrograde cell labelling. In this way, pedunculopontine projections were confirmed to the globus pallidus, entopeduncular nucleus, subthalamic nucleus, substantia nigra, parafascicular nucleus and pontine reticular formation. Injections into the globus pallidus or subthalamic nucleus gave rise to retrograde cell labelling bilaterally in pedunculopontinus. In addition, retrograde transport studies alone demonstrated projections from pedunculopontinus to the cerebral cortex and to the spinal cord.It is concluded that the nucleus tegmenti pedunculopontinus has reciprocal relationships with parts of the basal ganglia and some functionally related nuclei (in particular, the pallidal complex, subthalamic nucleus and substantia nigra). These connections support the view that nucleus tegmenti pedunculopontinus is likely to be involved in the subcortical regulation and mediation of basal ganglia influences upon the lower motor system. This suggests a potential role for pedunculopontine afferent and efferent pathways in the pathophysiology of basal ganglia related disorders of movement.     

Origin,course and termination of dopaminergic substantia nigra neurons projecting to the amygdaloid complex in the cat

The organization of dopaminergic neurons projecting to the amygdala was examined using retrograde (horseradish peroxidase histochemistry) and anterograde ([³H]leucine autoradiography) transport methods and Falck-Hillarp histofluorescence techniques combined with microspectrofluorometry and radiofrequency lesions. Cell bodies located within the pars lateralis and pars compacta of the substantia nigra were found to project to the lateral and central amygdaloid nuclei, respectively. Both of these areas within the substantia nigra contained dopaminergic perikarya, while the central and lateral amygdaloid nuclei contained fluorescent varicosities with features indicative of dopaminergic neurons. Lesions restricted to the pars lateralis of the substantia nigra resulted in a loss of fluorescence in the lateral amygdaloid nucleus. Autoradiographic experiments revealed that the projections from the pars lateralis did not run with fibers originating from the pars compacta in the nigrostriatal tract but rather had their own course occupying a lateral position adjacent to the cerebral peduncle and joining the ventral amygdalo-fugal bundle.The data indicate that, in the cat, there are two separate dopaminergic projections to the amygdala arising from the substantia nigra.     

Topographic projections from the basal ganglia to the nucleus tegmenti pedunculopontinus pars compacta of the cat with special reference to pallidal projections

Summary Projections from the basal ganglia to the nucleus tegmenti pedunculopontinus pars compacta (TPC) were studied by using anterograde and retrograde tracing techniques with horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) in the cat. Following WGA-HRP injections into the medial TPC area, a substantial number of retrogradely labeled cells were seen in the entopeduncular nucleus (EP) and medial half of the substantia nigra pars reticulata (SNr), whereas following WGA-HRP injections into the lateral TPC area, labeled cells were marked in the caudal half of the globus pallidus (GP) and lateral half of the SNr. To confirm the retrograde tracing study, WGA-HRP was injected into the EP or the caudal GP, and anterograde labeling was observed in the TPC areas. Terminal labeling was located in the medail TPC area in the EP injection case, while terminal labeling was observed in the lateral TPC area in the caudal GP injection case. Projections from the striatum to the pallidal complex (the EP and the caudal GP) were also studied autoradiographically by injecting amino acids into various parts of the caudate nucleus and the putamen. Terminal labeling was distributed over the whole extent of the EP and the rostral GP following injections into the rostral striatum (the head of the caudate nucleus or the rostral part of the putamen), while terminal labeling was distributed over the caudal GP following injections into the caudal striatum (the body of the caudate nucleus or the caudal part of the putamen). From these findings, we conclude that there exists a medio-lateral topography in the projection from the basal ganglia to the TPC: The EP receives afferent projections from the rostral striatum and projects to the medial TPC area, whereas the caudal GP receives projections from the caudal striatum and sends fibers to the lateral TPC area.Abbreviations BC brachium conjunctivum - CD caudate nucleus - CP cerebral peduncle - DBC decussation of the brachium conjunctivum - EP entopeduncular nucleus - GP globus pallidus - IC internal capsule - ICo inferior colliculus - LH lateral habenular nucleus - ML medial lemniscus - PN pontine nuclei - PUT putamen - SCo superior colliculus - SI substantia innominata - SN substantia nigra - SNc substantia nigra pars compacta - SNr substantia nigra pars reticulata - STN subthalamic nucleus - TH thalamus - TPC nucleus tegmenti pedunculopontinus pars compacta     

Basal ganglia and other afferent projections to the peribrachial region in the rat: A study using retrograde and anterograde transport of horseradish peroxidase

The afferent projections to the peribrachial region in the rat were studied using retrograde and anterograde transport of horseradish peroxidase. Particular attention was paid to descending projections from the basal ganglia and related nuclei to the region of nucleus tegmenti pedunculopontinus. Following injection of peroxidase into nucleus tegmenti pedunculopontinus, few retrogradely-labelled neurons were found in the entopeduncular nucleus proper, but larger numbers were found with a wide distribution within the boundaries of the internal capsule and cerebral peduncle. Labelled cells were also consistently observed in the amygdala, the caudal globus pallidus, the subthalamus including zona incerta and subthalamic nucleus, the hypothalamus, the substantia nigra and the ventral tegmental area. Following iontophoretic injections of horseradish peroxidase into the entopeduncular nucleus, lateral hypothalamus, subthalamic nucleus or ventral tegmental area, terminal labelling was observed in and around the branchium conjunctivum in an area apparently corresponding to nucleus tegmenti pedunculopontinus in the rat.     

Projections of the pallidal complex: An autoradiographic study in the cat

The efferent projections of the external pallidal segment (‘globus pallidus’), and the internal pallidal segment (entopeduncular nucleus) were studied in separate experiments in the cat by the auto-radiographic tracing method. Injections of tritiated amino acids into the external pallidal segment resulted in labelling of axon systems distributed not only to the subthalamic nucleus but also in sparser density to the nucleus reticularis thalami, the substantia nigra, the caudate nucleus, the putamen, and as yet undefined areas of the cortex. Injections of tritiated amino acids into the internal pallidal segment resulted in labelling of axon systems distributed to the ventrolateral-ventroanterior complex of the thalamus, to the centrum medianum, the lateral habenular nucleus, and the mesencephalic nucleus tegmenti pedunculopontinus, pars compacta. Less prominent termination may also occur in the parafas-cicular nucleus, the nuclei of the fields of Forel, and in the mesencephalic tegmentum rostral to the nucleus tegmenti pedunculopontinus, pars compacta.The fact that this and previous studies show that the projections of the pallidal complex are more widespread than would be expected if it was only involved in motor functions, raises questions about the functional organization of the basal ganglia. These are discussed in the following paper.     

Errata/corrigenda

Large neurons in the nucleus tegmenti pedunculopontinus pars compacta (TPC) of the rat were shown to have choline acetyltransferase (ChAT) immunoreactivity by light and electron microscopic immunohistochemistry using a monoclonal antibody to ChAT. The ChAT-positive TPC neurons had triangular or multipolar cell bodies with diameters of 20–50 μm. The ultrastructural features of the ChAT-positive TPC neurons were similar to those of ChAT-positive neurons in the caudoputamen; each neuron had a distinct, pale nucleus with an indented nuclear envelope and large, cytoplasm-containing, well-developed endoplasmic reticulum and pale mitochondria.     

A microiontophoretic study on the nature of the putative synaptic neurotransmitter involved in the pedunculopontine-substantia nigra pars compacta excitatory pathway of the rat

Summary The nature of the synaptic transmitter involved in the excitatory fibers linking the nucleus tegmenti pedunculopontinus (PPN) to the pars compacta of the substantia nigra (SNPC) was investigated using microiontophoretic techniques in rats anesthetized with ketamine. Among the SNPC cells activated orthodromically by PPN electrical stimulation, only a few cells were weakly excited by iontophoretically administered acetylcholine (Ach) while most were not affected. Conversely all cells were promptly and powerfully excited by short pulses of glutamate (GLU). The administration of the GLU antagonists glutamic acid diethylester (GDEE) and D--aminoadipic acid (DAA) reversibly and simultaneously suppressed both the PPN-evoked orthodromic response and the GLU-induced excitation of SNPC cells without affecting their response to iontophoretic Ach. GDEE was more effective than DAA in counteracting the synaptically evoked excitation. On the other hand, atropine, while antagonizing the Ach response in those cells which were cholinoceptive, did not affect either the PPN-evoked or the GLU-induced excitation. Hence, despite the presence of cholinergic cells in the PPN region, Ach does not appear to be involved in the excitatory PPNSNPC pathway. The present findings suggest that the excitatory PPN fibers innervating the SNPC may utilize GLU or a closely related amino acid as a neurotransmitter.Supported by grants from the Ministero della Pubblica Istruzione     

Laminar organization of the substantia nigra pars reticulata in the cat

Using a semihorizontal section plane tangential to the ventral surface of the cerebral peduncle, the authors re-examined cyto-, myelo- and dendroarchitecture, acetylcholinesterase activity, afferent fibers, and efferent projection neurons of the substantia nigra pars reticulata. In the semihorizontal section plane, the substantia nigra pars reticulata was a disc-shaped nucleus and contained two to three myelinated fiber bundles running from anteromedial to posterolateral. Bands of high acetylcholinesterase activity existed parallel to the anteromedial-posterolateral direction. The Golgi silver impregnation study revealed that many nigral neurons extended their varicose dendrites anteromedially and posterolaterally. In cases with injections of wheat germ agglutinated horseradish peroxidase into the neostriatum or injections of tritiated leucine into the subthalamic nucleus, anterogradely labeled afferent fibers and axon terminals in the substantia nigra pars reticulata were organized into bands in the same anteromedial-posterolateral direction. In cases with injections of wheat germ agglutinated horseradish peroxidase into either the superior colliculus, the pedunculopontine tegmental nucleus or the ventromedial nucleus of the thalamus, retrogradely labeled neurons were also clustered along the anteromedial-posterolateral direction with their dendrites extending anteromedially and posterolaterally. The present findings strongly suggest that the substantia nigra pars reticulata has a laminar organization.     

Effect of high-frequency stimulation of the subthalamic nucleus on the neuronal activities of the substantia nigra pars reticulata and ventrolateral nucleus of the thalamus in the rat

Electrophysiological recordings were made in anaesthetized rats to investigate the mode of function of high-frequency stimulation of the subthalamic nucleus used as a therapeutic approach for Parkinson's disease. High-frequency electrical stimulation of the subthalamic nucleus (130 Hz) induced a net decrease in activity of all cells recorded around the site of stimulation in the subthalamic nucleus. It also caused an inhibition of the majority of neurons recorded in the substantia nigra pars reticulata in normal rats (94%) and in rats with 6-hydroxydopamine lesions of the substantia nigra pars compacta (90%) or with ibotenic acid lesions of the globus pallidus (79.5%). The majority of cells recorded in the ventrolateral nucleus of the thalamus responded with an increase in their activity (84%).These results show that high-frequency stimulation of the subthalamic nucleus induces a reduction of the excitatory glutamatergic output from the subthalamic nucleus which results in deactivation of substantia nigra pars reticulata neurons. The reduction in tonic inhibitory drive of nigral neurons induces a disinhibition of activity in the ventrolateral motor thalamic nucleus, which should result in activation of the motor cortical system.     

Time of neuron origin and gradients of neurogenesis in midbrain dopaminergic neurons in the mouse

Previous [³H]thymidine studies in Nisslstained sections in rats established that the substantia nigra pars compacta and the ventral tegmental area originate sequentially according to an anterolateral to posteromedial neurogenetic gradient. We investigated whether that same pattern is found in mice in the dopaminergic neurons in each of these structures. Using tyrosine hydroxylase immunostaining combined with [³H]thymidine autoradiography, the time of origin of dopaminergic midbrain neurons in the retrorubral field, the substantia nigra pars compacta, the ventral tegmental area, and the interfascicular nucleus was determined in postnatal day 20 mice. The dams of the experimental animals were injected with [³H]thymidine on embryonic days (E) 11–E12, E12–E13, E13–E14, and E14–E15. The time of origin profiles for each group indicated significant differences between populations. The retrorubral field and the substantia nigra pars compacta arose nearly simultaneously and contained the highest proportion of neurons, 49 to 37%, generated on or before E11. Progressively fewer early-generated neurons were found in the ventral tegmental area (20%), and the interfascicular nucleus (8.5%). In addition, anterior dorsolateral neurons in the substantia nigra and ventral tegmental area were more likely to be generated early than the posterior ventromedial neurons. These findings indicate that mouse and rat brains have nearly identical developmental patterns in the midbrain, and neurogenetic gradients in dopaminergic neurons are similar to those found in Nissl studies in rats.     

Ascending projections of presumed dopamine-containing neurons in the ventral tegmentum of the rat as demonstrated by horseradish peroxidase

The projections of presumed dopamine-containing neurons in the zona compacta of the substantia nigra and the ventral tegmental area were examined by stereotaxic injections of horseradish peroxidase into diverse cortical and subcortical regions which are known to include dopamine-containing terminals. Neurons in the lateral half of the substantia nigra pars compacta were labelled after injections into the caudolateral aspect of the caudate-putamen, while neurons in the medial part of the substantia nigra pars compacta and lateral aspect of the ventral tegmental area projected to the anteromedial portion of the caudate putamen. Injections of horseradish peroxidase into the amygdala resulted in the appearance of reactive neurons in the anterior portion of the ventral tegmental area, but the more caudally located entorhinal cortex received projections from the posterior half of the ventral tegmental area. Injections of horseradish peroxidase into the frontal cortex, anterior to the genu, produced scattered labelled cells in the rostral half of the ventral tegmental area, whereas more posterior injections into the cingulate cortex resulted in the appearance of reactive cells which were confined to the medial one-quarter of the substantia nigra pars compacta. The near-midline structure, the lateral septum, was innervated by neurons with cell bodies primarily in the medial half of the ventral tegmental area. Injections of horseradish peroxidase into the nucleus accumbens, which contains very high levels of dopamine, resulted in the appearance of many labelled neurons throughout the ventral tegmental area and some reactive neurons in the medial part of the substantia nigra pars compacta. A few labelled cells were also occasionally observed in the contralateral ventral tegmental area after accumbens injections.These results suggest that although there is considerable overlap, and that the same subdivisions within the substantia nigra pars compacta and the ventral tegmental area appear to innervate diverse regions of the forebrain, there also exists a general topographical organization with respect to the projections of these neurons.Injections of horseradish peroxidase into some of the forebrain regions also resulted in the appearance of reactive cells in mesencephalic nuclei not known to contain dopaminergic perikarya. For example, labelled cells were observed in the supramamillary nucleus after injections into the frontal cortex, entorhinal cortex, accumbens and lateral septum. Injections into the amygdala produced reactive cells in the suprageniculate nucleus, the peripeduncular nucleus, and the magnocellular nucleus of the medial geniculate. These latter results are discussed with reference to the possibility that such pathways may mediate the responsiveness of cells in the amygdala to a wide range of sensory stimuli.     

Pallidotectal projection to the inferior colliculus of the rat

Summary After injection of fluorescent tracer into the inferior colliculus (IC), retrogradely labeled cells were observed not only in the temporoauditory cortex (ACx) and the substantia nigra pars lateralis, but also in the globus pallidus (GP). These labeled GP cells were localized exclusively in the caudal portion of the GP, which has been known to project to the ACx. Employing a retrograde fluorescent double labeling technique, the GP-IC neurons were found to be distributed in a separate manner from the GP-ACx neurons within the caudal GP. The present study provides further anatomical evidence that the caudal GP has a functional role in auditory processing.Abbreviations ACx temporoauditory cortex - BC Brachium conjunctivum - CP cerebral peduncle - CPu caudate putamen - DY Diamidino Yellow - EP entopeduncular nucleus - FG Fluoro-Gold - GP globus pallidus - I internal capsule - IC inferior colliculus - OT optic tract - SC superior colliculus - SN1 substantia nigra pars lateralis - T thalamus - TB True Blue - TPC nucleus tegmenti pedunculopontinus pars compacta     

Morphological study of the tegmental pedunculopontine nucleus, substantia nigra and subthalamic nucleus, and their interconnections in rat organotypic culture

The morphological organization of the tegmental pedunculopontine nucleus, midbrain extrapyramidal area, substantia nigra and subthalamic nucleus and their interrelationships were studied in rat organotypic culture using immunohistochemistry and NADPH-diaphorase histochemistry. Three coronal sections, one containing the tegmental pedunculopontine nucleus/midbrain extrapyramidal area, another with the substantia nigra and the third with the subthalamic nucleus, were obtained from postnatal 1-2-day-old rats. These sections were co-cultured for 3-4 weeks using the roller-tube technique. In the tegmental pedunculopontine nucleus/midbrain extrapyramidal area, the distribution pattern of cholinergic neurons was similar to that found in the in vivo study. We could, therefore, identify the subdivisions of the tegmental pedunculopontine nucleus (i.e., pars compacta and pars dissipata) and the midbrain extrapyramidal area. As in the in vivo situation, glutamate immunoreactive neurons were also located in these areas. Approximately 10% of NADPH-diaphorase positive neurons in the tegmental pedunculopontine nucleus, were glutamate immunoreactive. In the substantia nigra, as in the in vivo, tyrosine hydroxylase immunoreactive (putative dopaminergic) neurons were identified predominantly in the substantia nigra pars compacta, and parvalbumin immunoreactive neurons (putative GABAergic) mainly in the substantia nigra pars reticulata. The subthalamic nucleus was ladened with glutamate immunoreactive neurons. NADPH-diaphorase stained axons originating from the tegmental pedunculopontine nucleus were traced into the substantia nigra and subthalamic nucleus. They were often in close apposition to tyrosine hydroxylase immunoreactive neurons in the substantia nigra. Parvalbumin immunoreactive fibers from the substantia nigra projected heavily to the midbrain extrapyramidal area, but only sparsely to the tegmental pedunculopontine nucleus and the subthalamic nucleus. These findings indicate that the tegmental pedunculopontine nucleus/midbrain extrapyramidal area, substantia nigra and subthalamic nucleus in the organotypic culture have retained a basic morphological organization and connectivity similar to those seen in the in vivo situation. Therefore, this preparation could be a useful model to conduct further studies to investigate functional circuits among the structures represented.     

A horseradish peroxidase study of afferent connections of the globus pallidus in Macaca mulatta

Summary The high tonic discharge rates of globus pallidus neurons in awake monkeys suggest that these neurons may receive some potent excitatory input. Because most current electrophysiological evidence suggests that the major described pallidal afferent systems from the neostriatum are primarily inhibitory, we used retrograde transport of horseradish peroxidase (HRP) to identify possible additional sources of pallidal afferent fibers. The appropriate location was determined before HRP injection by mapping the characteristic high frequency discharge of single pallidal units in awake animals. In animals with injections confined to the internal pallidal segment, retrograde label was seen in neurons of the pedunculopontine nucleus, dorsal raphe nucleus, substantia nigra, caudate, putamen, subthalamic nucleus, parafascicular nucleus, zona incerta, medial and lateral subthalamic tegmentum, parabrachial nuclei, and locus coeruleus. An injection involving the external pallidal segment and the putamen as well resulted in additional labeling of cells in centromedian nucleus, pulvinar, and the ventromedial thalamus.Abbreviations AC anterior commissure - CG central grey - CM centromedian nucleus - CN caudate nucleus - DM dorsomedial nucleus - DR dorsal raphe nucleus - DSCP decussation of superior cerebellar peduncle - GPe globus pallidus, external segment - GPi globus pallidus, internal segment - LC locus coeruleus - LL lateral lemniscus - MG medial geniculate nucleus - ML medial lemniscus - NVI abducens nucleus - OT optic tract - Pbl lateral parabrachial nucleus - Pbm medial parabrachial nucleus - Pf parafascicular nucleus - PPN pedunculopontine nucleus - PuO oral pulvinar nucleus - RN red nucleus - SCP superior cerebellar peduncle - SI substantia innominata - SNc substantia nigra, pars compacta - SNr substantia nigra, pars reticulata - STN subthalamic nucleus - TMT mamillothalamic tract - VA ventral anterior nucleus - VLc ventral lateral nucleus, pars caudalis - VLm ventral lateral nucleus, pars medialis - VLo ventral lateral nucleus, pars oralis - VPI ventral posterior inferior nucleus - VPM ventral posterior medial nucleus - VPLc ventral posterior lateral nucleus, pars caudalis - ZI zona incerta     

Electron microscopic identification of reticulo-subthalamic axon terminals in the cat

Synaptic boutons emanating from axons of nucleus tegmenti pedunculopontinus origin were identified by electron microscopy in the neuropil of the subthalamic nucleus. Such boutons measure 1.5-3 microns, contain round synaptic vesicles and make asymmetrical axodendritic and axosomatic synaptic contacts with large subthalamic neurons. Very few contacts with vesicle-containing dendrites, and no contacts with the perikarya of the small neurons were observed. The present findings, in keeping with the relevant light microscopic and electrophysiologic data, furnish evidence for a substantial bilateral tegmenti pedunculopontinofugal projection that excites monosynaptically the relay subthalamic neurons.     

Possible intermixing of neurons from the subthalamic nucleus and substantia nigra pars compacta in the guinea-pig

A population of cells in the anterior substantia nigra pars compacta (SNPc) of the guinea-pig have been reported previously that differ from classical dopaminergic neurons in terms of their active and passive membrane properties. To investigate this population further, anterior nigral neurons (n=17) were compared with neurons in the adjacent subthalamic nucleus (STN; n=26). The anterior nigral neurons were found to be indistinguishable from STN neurons in their action potential characteristics, firing rate, resting membrane potential and input resistance. A low-threshold calcium conductance and anomalous rectification could be demonstated in cells from both groups. Furthermore, the gross morphological characteristics of anterior nigral neurons and STN neurons were very similar, as assessed following the intracellular injection of biocytin. A further similarity was seen in the response of the two cell groups to cyanide (200 M) and apomorphine (500 M). Cyanide hyperpolarised the membrane potential of all STN neurons and the majority (77.8%) of anterior nigral neurons, in both cases producing a concomitant reduction in firing rate. These changes were accompanied by an increase in membrane conductance for potassium ions. Apomorphine depolarised the membrane potential of all STN neurons and anterior nigral neurons, in most cases increasing the input resistance (83.3% of STN neurons and 100% of anterior nigral neurons). In both groups of cells, when firing rate was affected, an increase was usually seen. Given the physiological, morphological and pharmacological similarities of STN and anterior nigral neurons, the most parsimonious interpretation is that the anterior nigral neurons belong to the STN. However, the anterior nigral neurons were found in slices that, when resectioned, contained tyrosine hydroxylase (TH)-immunoreactive cell bodies in every section, in a location corresponding to the SNPc. The implication is that in the guinea pig the SNPc and STN (usually considered to be anatomically distinct nuclei) intermix at this level for several hundred microns. This close association of the STN and the compacta was further demonstrated by the presence of TH-positive varicose and non-varicose neuronal processes within the STN.     

Anatomy and physiology of substantia nigra and retrorubral neurons studied by extra- and intracellular recording and by horseradish peroxidase labeling

The question of origin of the excitatory and inhibitory responses that occur in neostriatal neurons following electrical stimulation of the substantia nigra is complicated by the possible spread of stimulus currents to numerous unspecifiable systems of neuronal elements. The present work begins to address this problem through the study of conduction properties of specific nigral and perinigral neurons in the cat. Neurons of pars compacta of substantia nigra and of the retrorubral area were found to have similar latencies for antidromic activation, whether from caudate nucleus stimuli (6.8–8 ms) or medial forebrain bundle stimulation (2.4–6.4 ms).The soma-dendritic features of both pars compacta and retrorubral neurons (revealed by intracellular injection of horseradish peroxidase) resembled the sparsely-branched, medium-sized substantia nigra neurons known from Golgi studies to have long dendrites with scattered and mainly distally-located spine-like appendages. Two types of pars compacta neurons were found; one with an ascending axon lacking collateral branches, and another with a descending axon that issued collaterals which terminated in the compacta, in pars reticulata, and possibly in retrorubral areas. Despite failure to detect as ascending axonal trajectory for this latter neuron, both types of pars compacta cells responded antidromically to stimulation of the caudate nucleus or medial forebrain bundle.The conduction time for impulse propagation in axons of pars compacta or retrorubral neurons suggests that either may mediate at least some of the excitatory responses that are known to occur in neostriatal neurons following stimulation of the substantia nigra in the cat. However, these conduction times are not compatible with the production of other excitatory responses which are commonly observed in the cat striatum at latencies shorter than 6 to 7 ms following stimulation of the substantia nigra.     

Projections of the paleostriatum upon the midbrain tegmentum in the pigeon

Descending projections of neurons in the avian paleostriatal complex upon cell groups of the midbrain tegmentum were investigated in the pigeon using horseradish peroxidase histochemistry and autoradiography. The purpose of this study was to determine if pathways comparable to the mammalian striato-tegmental system (descending paths from neurons in the caudate nucleus, putamen and nucleus accumbens) exist in the avian brain. Although previous reports (Karten &Dubbeldam, 1973) have described pathways in the avian brain comparable to the mammalian pallido-tegmental system, no counterpart of the mammalian striato-tegmental system has been identified in birds.Our results indicate that a striato-tegmental system does exist in the pigeon. Neurons within a small-celled field containing a rich plexus of catecholamine-positive axons and terminals were found to project upon the midbrain tegmentum. This small-celled field includes the paleostriatum augmentatum, lobus parolfactorius and nucleus accumbens. Cells in all three components of this small-celled field project upon the pars compacta and the pars dorsomedialis of the nucleus tegmenti pedunculo pontinus of the midbrain. The pars compacta of this nucleus contains many catecholamine-positive cells and has been compared to the substantia nigra pars compacta of mammals on the basis of histochemistry and anatomical connections (Brauth,Ferguson &Kitt, 1978). The pars dorsomedialis of this nucleus has been shown to contain a plexus of substance P-positive axons and terminals (Reiner,Brauth,Kitt &Karten, 1980;Reiner,Karten &Korte, 1980). It is argued that the pars dorsomedialis of the pedunculo pontine nucleus of the pigeon is comparable to the mammalian substantia nigra pars reticulata. Other projection targets of the lobus parolfactorius and nucleus accumbens in the pigeon include the area ventralis of Tsai, the lateral habenular nucleus and the nucleus subceruleus dorsalis. The locus ceruleus also receives a projection from the nucleus accumbens. The results are compared to previously reported findings in mammals and reptiles.     

Systematic differences in time of dopaminergic neuron origin between normal mice and homozygous weaver mutants

Immunocytochemical labeling for tyrosine hydroxylase and [³H]thymidine autoradiography were combined in wild-type mice and in mice homozygous for the weaver mutant gene (wv) to see whether the neurogenetic patterns of midbrain dopaminergic neurons was normal in the mutants and whether the degeneration of dopaminergic neurons was linked to their time of origin. Dams of wild-type and homozygous weaver mice were injected with [³H]thymidine on embryonic days (E) 11–E12, E12–E13, E13–E14, and E14-E15 to label neurons in the retrorubral field, the substantia nigra pars compacta, the ventral tegmental area, and the interfascicular nucleus as they were being generated. The quantitatively determined time of origin profiles indicated that wv/wv mice have the same time span of neurogenesis as +/+ mice (E10 to E14), but have significant deficits in the proportion of late-generated neurons in each dopaminergic population. In the retrorubral field and substantia nigra, weaver homozygotes had substantial losses of dopaminergic neurons and had a greater deficit in the proportion of neurons generated late while, in the ventral tegmental area and interfascicular nucleus, there were slight losses of dopaminergic neurons and only slight deficits in the proportion of late-generated neurons. These findings lead to the conclusion that the weaver gene is specifically targeting dopaminergic neurons that are generated late, mainly on E13 and E14.