Hironobu Tokuno

A morphological evidence for monosynaptic projections from the nucleus tegmenti pedunculopontinus pars compacta (TPC) to nigrostriatal projection neurons.

Electrolytic lesion was placed in the nucleus tegmenti pedunculopontinus pars compacta (TPC) in cats, which were injected simultaneously with horseradish peroxidase (HRP) into the caudate nucleus (Cd). After a survival period of 4 days, the substantia nigra pars compacta (SNC) was examined electron microscopically. Degenerating axon terminals were found to make asymmetric synaptic contacts on dendrites of SNC neurons which were retrogradely labeled with HRP. Thus, TPC neurons were indicated to project monosynaptically to SNC neurons which sent their axons to the Cd.

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

SummaryProjections 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.

Monosynaptic nigral inputs to the pedunculopontine tegmental nucleus neurons which send their axons to the medial reticular formation in the medulla oblongata. An electron microscopic study in the cat

An electron microscopic study in the cat has suggested that neurons in the substantia nigra pars reticulata (SNr) project to the medial reticular formation of the medulla oblongata (MRF) via the pedunculopontine tegmental nucleus (PPN): PPN neurons, which were labeled with a retrograde tracer (WGA-HRP; horseradish peroxidase conjugated to wheat germ agglutinin) injected into the MRF, were found to be in synaptic contact with axon terminals which were degenerated with neurotoxic agents applied into the SNr.

Organization of the nigrotectospinal pathway in the cat: a light and electron microscopic study

The relationships between nigrotectal terminals and tectospinal neurons in the deep layers of the superior colliculus have been examined light and electron microscopically by using antero- and retrograde transport of tracer, and anterograde degeneration. For light microscopy, horseradish peroxidase (HRP) was injected into the cervical spinal cord, and HRP conjugated to wheat germ agglutinin (WGA-HRP) into the substantia nigra at the same time. It appeared that about 22% of the retrogradely labeled tectospinal neurons were totally or partially in the anterogradely labeled nigral terminal clusters. This coexistence was particularly manifest in the dorso-lateral part of the intermediate gray layer. For electron microscopy, nigral lesions and HRP injections into the cervical spinal cord were combined in the same animals. Degenerating nigrotectal terminals were found in both axosomatic and axodendritic symmetrical synaptic contacts with retrogradely labeled tectospinal neurons in the intermediate gray layer. Present experiments show tectospinal neurons receiving monosynaptic input from the substantia nigra do exist in the intermediate gray layer of the superior colliculus, especially in its dorsolateral portion.

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.

Direct retinal projections to the lateroposterior thalamic nucleus (LP) in the mole

Direct retinothalamic projections in the mole, an animal with a highly reduced visual system, were studied after an intraocular injection of wheat germ-agglutinated horseradish peroxidase (WGA-HRP). Anterogradely labeled axon terminals were found not only in the dorsal and ventral nuclei of the lateral geniculate body but also in the lateroposterior nucleus on the side contralateral to the intraocular WGA-HRP injection.

Bilateral projections from the medial superior olivary nucleus to the inferior colliculus in the mole (Mogera robusta)

A retrograde tracing study in the mole using wheat germ-agglutinated horseradish peroxidase (WGA-HRP) indicated that the medial superior olivary nucleus (MSO) projects to the inferior colliculus (IC) bilaterally. Considering the strict ipsilateral projections from the MSO to the IC in all other eutherian species ever reported, the bilateral projection in the mole is quite unique. This may reflect specializations of the peripheral auditory apparatus of the underground dwellers and/or primitiveness of the insectivorous brains.

Synaptic organization of the pedunculopontine tegmental nucleus of the cat.

The synaptic organization of the feline pedunculopontine tegmental nucleus (PPN) was studied electron microscopically. The bouton covering ratios were calculated in various sizes of PPN neurons, and the ratios of large neurons (56%) were found to be much higher than those of small neurons (16%). The PPN neuron dendrites usually showed some varicosities, and spines were observed on both somatic and dendritic profiles. Among a total of 1021 synapses sampled at random, axosomatic, axodendritic and axospinous synapses comprised 21.7, 61.2 and 14.1%, respectively. On the basis of the postsynaptic junction, these synapses were classified into the symmetric (66.3%) and the asymmetric (33.7%) types. The percentage of symmetric synapses was much higher on the soma (91.0%), and the large (69.4%) and medium-sized (63.2%) dendrite, while that of asymmetric synapses showed a higher value on the small dendrite (55.5%) and the dendritic spine (50.8%). Axoaxonic, dendrodendritic and dendroaxonic synapses, although not so frequent, were, in part, involved in the serial synapse or the synaptic triad. It is concluded that some PPN neurons are spiny, and that axosomatic, axodendritic and axospinous synapses are the main synaptic constituents and besides those synapses a more complex synaptic organization exists in this nucleus.

Monosynaptic striatal inputs to the nigrotegmental neurons: an electron microscopic study in the cat

The cat substantia nigra (SN) pars reticulata (SNR) has been observed electron microscopically after ibotenic acid injections into the caudate nucleus (Cd) and horseradish peroxidase injections into the nucleus tegmenti pedunculopontinus pars compacta (TPC). As a result, degenerating striatal terminals were found to make symmetric synaptic contacts on retrogradely labeled SNR neurons. The present findings, together with our previous study on the tegmentonigrostriatal projection (Tokuno et al., Neurosci. Lett. 85 (1988) 1-4), confirm that the Cd has indirect reciprocal connections with the TPC via the SN.

Monosynaptic projection from the pedunculopontine tegmental nuclear region to the reticulospinal neurons of the medulla oblongata. An electron microscope study in the cat.

The magnocellular reticular nucleus (MRN) of the cat was observed electron microscopically following the combined application of kainic acid into the pedunculopontine tegmental nuclear region (PPN region) and horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) into the spinal cord. Some degenerating axon terminals arising from the PPN region were found to make synaptic contacts with retrogradely HRP-labeled reticulospinal neurons in the MRN.