M. Takada

Single thalamic dopaminergic neurons project to both the neocortex and spinal cord

Cells in the rat subparafascicular thalamic nucleus (Spf) belonging to the diencephalic A11 cell group, were immunohistochemically stained with antibodies against tyrosine hydroxylase (TH) and dopamine itself. Employing a combination of retrograde fluorescent double-labeling and TH immunofluorescence techniques, we revealed the existence of dopaminergic Spf cells, giving rise to collateral projections to the neocortex and spinal cord.

Co-localization of tyrosine hydroxylase and glutamate decar☐ylase in a subpopulation of single nigrotectal projection neurons

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

Astroglial ablation prevents MPTP-induced nigrostriatal neuronal death

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a potent neurotoxin which destroys nigrostriatal dopamine neurons, resulting in irreversible idiopathic parkinsonism. MPTP displays dopaminergic neurotoxicity to humans, monkeys, cats and rodents. The oxidative conversion of MPTP to 1-methyl-4-phenylpyridine (MPP+) is responsible for the generation of its neurotoxicity. This metabolism is mediated by the action of monoamine oxidase B, which in the substantia nigra pars compacta (SNc) is localized specifically in astroglia. Employing various combinations of intra-SNc injections of MPTP and the astroglia-specific toxin, L-alpha-aminoadipic acid (L-alpha-AA), we examined the effects of selective astroglial ablation on MPTP-induced nigrostriatal neuronal death in the rat. Varying nigrostriatal cell loss was assessed primarily by the aid of fluorescent retrograde axonal tracing. Treatment with MPTP alone caused tremendous nigrostriatal cell loss, while intra-SNc co-injections of MPTP and L-alpha-AA produced protection against MPTP neurotoxicity in a dose-dependent fashion. Similar effects of L-alpha-AA occurred in the SNc pretreated with the gliotoxin just prior to or 1 day before MPTP administration. However, this preventive action by L-alpha-AA was considerably reduced 3 days after its intra-SNc injection. Interestingly, 7 days following L-alpha-AA pretreatment, nigrostriatal cell loss was even enhanced rather than attenuated by MPTP administered into the SNc. Thus, our data provide clear morphological evidence for the critical importance of the presence of astroglia in the onset of MPTP neurotoxicity.

On the origin of the dopaminergic innervation of the paraventricular thalamic nucleus

The origin of the dopaminergic innervation of the paraventricular thalamic nucleus was examined in the rat. Employing a combination of fluorescent retrograde tracing and immunofluorescence histochemistry for tyrosine hydroxylase, we found that this innervation predominantly takes origin from the mesencephalic A8 and A10 catecholamine cell groups.

Tyrosine hydroxylase immunoreactivity in cerebellar Purkinje cells of the rat

Employing tyrosine hydroxylase (TH) immunohistochemistry, we have revealed that TH immunoreactivity occurs in cerebellar Purkinje cells in the rat. These TH-immunoreactive Purkinje cells were distributed predominantly in the crus I & II ansiform lobules and the paraflocculus, and to a lesser extent in the I & X vermal lobules. Since Purkinje cells in such cerebellar regions displayed no immunoreactivity to dopamine-beta-hydroxylase, the TH-immunoreactive Purkinje cells identified in the present study might contain dopamine or L-DOPA.

Bilateral tectal projection of single nigrostriatal dopamine cells in the rat

Employing fluorescent retrograde double/triple labeling techniques, we found that a substantial population of substantia nigra pars reticulata cells send divergent axon collaterals to both the ipsilateral striatum and bilateral superior colliculi in the rat. These multi-collateralized neurons were localized predominantly in the ventrolateral portion of the substantia nigra pars reticulata at its rostral level. Furthermore, tyrosine hydroxylase immunofluorescence histochemistry combined with fluorescent retrograde tracing techniques showed that the vast majority (more than 85%) of such specifically branched cells are dopaminergic. This novel nigral cell population seems to be in a strategic position to evoke dopamine-mediated motor impairments (i.e. abnormal saccadic eye movements in Parkinsonism) and/or behavioral syndromes (i.e. compulsive turning behavior) through the GABA-containing nigrotectal pathway.

Collateral projections from the substantia nigra to the cingulate cortex and striatum in the rat

After injecting a retrograde tracer into the posterior cingulate cortex, labeled neurons were found only in the substantia nigra pars compacta, leaving neurons in the ventral tegmental area totally unlabeled. The existence of collateral nigral projections to the cingulate cortex and striatum was clearly established using the fluorescent retrograde double-labeling technique. This constitutes a neuroanatomical substrate for psychotic symptoms expressed by a subpopulation of parkinsonian patients.

Two separate neuronal populations of the rat subthalamic nucleus project to the basal ganglia and pedunculopontine tegmental region

Employing a retrograde fluorescent double labeling technique, we compared the localization of subthalamic nucleus (STN) cells projecting to the striatum and nucleus tegmenti pedunculopontinus pars compacta (TPC), with that of STN cells sending axon collaterals to both the globus pallidus and substantia nigra in the rat. The STN-striatal projections were mostly the third branches of massive STN-pallidal and STN-nigral collateral projection neurons, whereas only very rarely did the STN-TPC projections contribute to these collateral projections. The TPC projecting STN cells, giving rise to an independent output of the nucleus, were located mainly in its thin lateral strip region. The STN may integrate the somatic motor information from various cortical/subcortical brain areas (including the motor cortex, striatum, globus pallidus, thalamus and TPC), and disperse it predominantly to the pallidal complex, substantia nigra and striatum by way of axon collaterals, and to a lesser degree to the TPC through separate fibers. Thus, the STN might be in a strategic position to exert a prominent control over the basal ganglia-related somatic motor functions.

Collateral projection from the substantia nigra to the striatum and superior colliculus in the rat

Our retrograde fluorescent double labeling study demonstrated the existence of divergent collateral projections from the substantia nigra to the striatum and superior colliculus in the rat. These bifurcating projection neurons were located predominantly in the ventrolateral portions of the substantia nigra pars reticulata at its rostral level, where they formed a narrow band along the boundary between the substantia nigra and cerebral peduncle. Such specific projection cells were also seen in the substantia nigra pars lateralis. However, nigral neurons did not give off axonal branches to the striatum and ventromedial thalamic nucleus. The new nigral cell population proposed here might constitute a neuroanatomical substrate for abnormal saccadic eye movements clinically manifested by many parkinsonian patients.

MPTP neurotoxicity to cerebellar Purkinje cells in mice

The pyridine derivative 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is recognized as a crucial neurotoxin which destroys nigrostriatal dopamine cells, thereby inducing neurological signs relevant to idiopathic Parkinsons disease. In the present study, we have revealed MPTP neurotoxicity to cerebellar Purkinje cells in mice. Systemic MPTP injections to mice resulted in a substantial loss of Purkinje cells in a dose-dependent fashion. The MPTP-induced Purkinje cell loss occurred markedly in the crus I and II ansiform lobules and the paraflocculus. Such a neurotoxic effect was largely prevented by the monoamine oxidase B inhibitors pargyline and deprenyl, and the dopamine uptake inhibitors mazindol and benztropine.