Tetsuji Moriizumi

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.

Olfactory disturbance induced by deafferentation of serotonergic fibers in the olfactory bulb

The serotonergic neurons of the brain stem project widely throughout the central nervous system, and the olfactory bulb is one of the major forebrain targets of the ascending serotonin pathway. According to physiological studies, neurons of the olfactory bulb were found to reduce their spontaneous discharge rates by electrophoretically applied serotonin. However, roles of the bulbar serotonin in the sense of smell remain unanswered. In the present study, using 5,7-dihydroxytryptamine, a specific neurotoxin for serotonin, we found that the conditioned rats who learned to avoid a repellent by olfaction lost ability of discrimination by deafferentation of the bulbar serotonergic fibers. Such olfactory dysfunction did not occur in the early stage (three days after injection of the toxin) when the serotonergic fibers disappeared in the bulb, but developed a few weeks later. Interestingly, histological examination revealed marked shrinkage of the bulbar glomerulus which is a major termination site of the bulbopetal serotonergic fibers, and also a synaptic site of olfactory receptor cells and bulbar output neurons. The results indicate that depletion of the serotonergic fibers in the olfactory bulb causes glomerular atrophy and olfactory disturbance in the rat.

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.

Synaptic organization of the cat entopeduncular nucleus with special reference to the relationship between the afferents to entopedunculothalamic projection neurons: an electron microscope study by a combined degeneration and horseradish peroxidase tracing technique.

The synaptic organization of the feline entopeduncular nucleus was studied electron microscopically. After horseradish peroxidase injections into the ventral anterior and ventral lateral nuclear complex of the thalamus, normal axon terminals synapsing with entopedunculothalamic projection neurons were classified into four types on the basis of the size and shape of synaptic vesicles in them, and types of the postsynaptic membrane differentiation. Type I and type II axon terminals were characterized by symmetrical synaptic contacts, and large ovoid or small ovoid synaptic vesicles, respectively. Type II axon terminals were further classified into two subtypes as to their sizes: one was small (IIa), the other large (IIb). Type III and type IV axon terminals were characterized by asymmetrical synaptic contacts, and large ovoid or small ovoid synaptic vesicles, respectively. To determine the origin of each type of terminal, electrolytic lesions of the caudate nucleus or the subthalamic nuclear region were combined with horseradish peroxidase injections into the thalamus or the subthalamic nuclear region. After electrolytic lesions of the caudate nucleus, degeneration was seen in type I axon terminals contacting entopedunculothalamic projection neurons. Following electrolysis or horseradish peroxidase injection into the subthalamic nuclear region, type IIa and type IV axon terminals showed degenerations or horseradish peroxidase labelling. Such terminals also synapsed with entopedunculothalamic projection neurons. It was demonstrated that these projection neurons relay the striatal or subthalamic inputs directly to the thalamus. After horseradish peroxidase injection into the thalamus, many labelled type II axon terminals were observed to synapse with entopedunculothalamic projection neurons. Type III axon terminals were left unchanged throughout these experiments. In addition, the entopeduncular neuron was observed to receive convergent inputs from both the caudate nucleus and probably the subthalamic nucleus. Axoaxonal synapses were also found to be involved in the synaptic triad. These results indicate that type I axon terminals originate from the caudate nucleus, part of type IIa and type IV axon terminals originate from the subthalamic nucleus or caudal to the subthalamic nuclear region, and part of type IIa and type IIb terminals come from intrinsic axon collaterals.

Transection of the olfactory nerves induces expression of nerve growth factor receptor in mouse olfactory epithelium

Expression of nerve growth factor receptor (NGFR) was found in the mouse olfactory epithelium after olfactory nerve transection, although no immunoreactivity to NGFR was detectable in the olfactory epithelium in the control animals. After axotomy, however, NGFR-immunoreactive cells transiently appeared in the supporting cells, receptor neurons and basal cells between postoperative day 7 and 35, and thereafter disappeared by postoperative day 49. The results indicate that nerve growth factor may contribute to the regeneration of olfactory receptor neurons after olfactory nerve transection.

Electron microscopic study of synaptogenesis and myelination of the olfactory centers in developing rats

Development of the central olfactory system was studied in the rat with an electron microscope at three main structures: the olfactory bulb, the lateral olfactory tract, and the primary olfactory cortex (the piriform cortex). As a parameter of development, the synaptic density was examined quantitatively in the bulbar glomerulus and layer Ia (termination of bulbofugal fibers) of the piriform cortex, which are the key stations of the olfactory pathway. The synaptic densities in the glomerulus and those in layer Ia were 5.7% and 4.6% on embryonic day 19, 15.8% and 12.5% on postnatal day (P) 0, and 57.3% and 37.2% on P10, as compared with the adult (100%). As another parameter of development, the density of myelinated axons in the lateral olfactory tract was examined quantitatively. The densities of myelinated axons in the tract were 0% on P5, 15.1% on P10, and 73.5% on P21 of the adult density. Maturation in the tract was still progressing, even at P21, in terms of bundle formation and the thickness of myelin sheaths. The results show that synaptogenesis in the bulbar glomerulus is followed by synaptogenesis in layer Ia of the piriform cortex, and that myelination in the lateral olfactory tract occurs over a prolonged period, even in the stages after P21.

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.

Postnatal development of the projection from the medial superior olive to the inferior colliculus in the rat.

Normal projection development from the medial superior olive (MSO) to the inferior colliculus (IC) was examined by injecting Fluoro-Gold (FG), a retrograde tracer, into the IC unilaterally at postnatal days 0 (P0), P3, P7 and maturity. The rats were killed 1 day after FG injection. At all ages, labeled neurons in the MSO appeared on the ipsilateral side only, as in adult controls. The total number of MSO neurons counted in Nissl-stained sections was constant throughout the postnatal periods. The labeled frequency index of MSO neurons was increased stepwise (from 35% to 90%) with increasing postnatal stages (from P0 to adulthood), suggesting differential growth of early- and late-developing axons.