Estudio de las proyecciones eferentes de las porciones reticulada y compacta de la sustancia negra en la rata medieante el trazado axonico unitario

Resumen

#TITULO: ESTUDIO DE LAS PROYECCIONES EFERENTES DE LAS PORCIONES RETICULADA Y COMPACTA DE LA SUSTANCIA NEGRA EN LA RATA MEDIANTE EL TRAZADO AXONICO UNITARIO #RESUMEN: The substantia nigra (SN) isa component of the basal ganglia that lies in the midbrain and comprises a GABAergic population of neurons Tocated in its pars reticulata (SNr) subdivisión and numerous dopaminergic neurons harbored in its pars compacta (SNc) aspect. The SNr contains small and medium-sized GABAergic neurons and acts as one of the most important output of the basal ganglia. The dopaminergic cells of the SNc are the source of dopamine to the striatum and their degeneration underlies the motor symptoms of parkinsonian patients. The subset of dopaminergic neurons most severely affected in Parkinson's disease is located within the most ventrolateral aspect of the SNr. The aim of our study was two-fold: (1) to analyze in detail the efferent projections of SNr GABAergic neurons and SNc dopaminergic neurons "located within the ventrolateral aspect of the SNr; (2) to find out whether populations of neurons defined on the basis of their axonal projections share similar dendritic morphological features. We used the single axon tracing procedure to reconstruct entirely 30 axons of SNr neurons and other 35 axons of SNc neurons. The somatodendritic domain of 43 and 14 neurons of the SNr and SNc, respectively, was also completely reconstructed and then analyzed statistically. Our study has demonstrated the existence of four types of SNr projections neurons based on their axonal arborization pattern. The main target structures of SNr neurons are the thalamus, the superior colliculus, the pedunculopontine tegmental nucleus, the deep mesencephalic nucleus and the periaqueductal gray matter. Many of these structures were targeted by single SNr neurons through axon collaterals. Due to their highly patterned set of axon collaterals, the SNr neurons can exert a multifaceted influence on a wide variety of diencephalic and midbrain structures. The morphological features of the somatodendritic domain of SNr projection neurons appear be related to the type of inputs they receive from individual functional district of the striatum rather than to the pattern of their axonal projections. Likewise the SNr neurons, the dopaminergic neurons located within the ventrolateral aspect of the SNr in rodents are endowed with a highly patterned set of axon collaterals. Our study has revealed three types of SNc projection neurons based on their axonal branching pattern. One of these neuronal types innervates exclusively the basal ganglia. The other two types of SNc projection neurons project to the thalamus, either exclusively or in addition to the basal ganglia. The thalamic nuclei innervated by the SNc neurons are mainly the ventral posterolateral, the ventromedial and the reticular. Some of these nuclei, like for instance the ventral posterolateral and the reticular, were not seen to be targeted by any of the 30 SNr axons reconstructed in this investigation and, therefore, are likely to be innervated exclusively by the SNc. Although the patterns of axonal branching of the SNc neurons that abound within the ventrolateral aspect of the SNr are clearly different from those shown by SNc neurons located more dorsally, the morpnology of the somatodendritic domain of all these SNc neurons are mostly si mi lar.