Electron microscopic study of the substantia nigra and the strio-nigral projection in the rat

Demonstration of a pallido-nigral projection innervating dopaminergic neurons

Afferents to the substantia nigra from the neostriatum and globus pallidus were studied in the rat by means of the autoradiographic tracing technique. 3H-leucine was injected stereotaxically into either the globus pallidus or neostriatum. Twenty-four hours later the axoplasmic transport of labelled proteins to the substantia nigra was studied by light and electron microscopic autoradiography. In animals used for electron microscopy, degeneration of dopaminergic cells in the substantia nigra was induced by intraventricular injection of 6-hydroxydopamine 72 hours before sacrifice. After neostriatal injections, light microscopic analysis revealed heavy labelling of the globus pallidus and entopeduncular nucleus, but only background labelling of the subthalamic nucleus. There was preferential labelling of the zone reticulata of the substantia nigra, with significantly less labelling of the zone compacta. After pallidal injections, light microscopic analysis showed very light labelling of those parts of the caudate-putamen in the vicinity of the injection site. There was intense labelling of the subthalamic nucleus and heavy labelling of the entopeduncular nucleus. The zona compacta of the substantia nigra was also heavily labelled. There was considerably less labelling of the zona reticulata. The electron microscopic analyses showed that after neostriatal injections, autoradiographic grains in the substantia nigra were located preferentially over boutons which terminated on normal dendritic processes. After pallidal injections, however, grains were preferentially located over boutons synapsing with degenerating dendritic processes. The degeneration produced in these dopaminergic processes by 6-hydroxydopamine was invariably of the dark type. Except for the different association with degenerating vs. non-degenerating dendrites, the subcellular distribution of autoradiographic grains in the substantia nigra was the same after injection into either the globus pallidue or caudate-putamen. Approximately 80 percent of the grains were over axons or boutons which invariably made symmetrical synaptic contacts. These observations demonstrate the existence of a pallido-nigral projection which terminates preferentially on dopaminergic cells in the pars compacta of the substantia nigra. They also confirm previous studies indicating that the strionigral projection terminates mainly in the pars reticulata. These terminations appear to be principally to non-dopaminergic cells.

High affinity uptake of GABA in presumed GABA-ERGIC nerve endings in rat brain

After interuption of the striato-nigral pathway uptake of gamma-aminobutyric acid (GABA) in the substatia nigra decresaed with 7 days to a constant level 30-40% of the normal. Concomitantly glutamate decarbosylase (GAD) was reduced to 10%. Hence about two-thirds of the GABA uptake activity in substantia nigra are localised, to the alleged GABA-ergic nerve elements originating from corpus striatum. The lesion resistant part of the uptake is probably not localised in cell bodies or large processes, since it was the same in tissuse prisms as in whole homogenates and crude nerve ending fractions. It was also not influenced by aminoozyacetic acid, which would argue against a localisation in glia. Whereas GAD was recovered mainly in a "heavy" nerve ending fraction, a large proportion of the GABA uptake was situated in a "light" fraction. After hemisections, GABA uptake was reduced to a similar extent in both fractions. It is suggested that whereas GAD is concentrated in nerve terminals, a significant proportion of the GABA uptake may be localised in preterminal axon branches in the substantia nigra. GABA uptake in the dorsal part of the lateral vestibular nucleus was not reduced by interruption of the Purkinje axons from the cerebellar vermis whereas GAD was reduced 50%. This indicates that the reuptake mechanism is not concentrated in the Prukinje axon terminals. In the hippocampus neither GABA nor GAD were reduced by lesions of afferent nerve pathoways, in accordance with previous results showing that in this region GABA producing neurones are intrinsic. The order of ratiols of GABA uptake to particulate GAD activity in different regions was: hippocampus greater than cerebellar cortex greater than substantia nigra greater than dorsal part of lateral vestibular necleus approximately equal to nucleus interpositus. The ratio may relfect the degree of specific localisation of the GABA uptake mechanism to the GABA-ergic structures.