Morphology and synaptic organization of non-dopaminergic nigral projections to the medio dorsal thalamic nucleus of the rat, a study by anterograde transport of PHA-L.

BACKGROUND Mediodorsal (MD) thalamic nucleus, which is considered to take place between extra pyramidal and limbic feedback circuit, receives projective fibers from ventrolateral neurons of reticular part of substantia nigra (SNr). In order to better understand the influence and chemical reaction of these fibers upon MD nucleus, the morphology and synaptology of them were examined in the present study. METHODS Phaseolous vulgaris-leucoagglutin (PHA-L) was injected into substantia nigra pars reticulate. After 3-4 days, the sections of SNr injection site and MD nucleus were prepared. Then, we examined organization, morphology and, synaptology of PHA-L labeled SNr fibers that go to caudal and lateral part of MD thalamic nucleus. RESULTS At the electron microscopic level, the SNr terminals made synapses predominantly with the medium to small dendrites and far less frequently with soma and large dendrites. These terminals were packed with polymorphic synaptic vesicles and formed symmetrical synapses; furthermore, it has been already recognized that cortico straital fibers from sensory-motor cortex go to region of the SNr that give rise to the nigrothalamic fibers. CONCLUSION This data suggest that upon the synaptic organization, morphology and chemical nature of GABAergic, SNr fibers may have different inhibitory influence on MD neurons regulating the thalamic output from MD to cerebral cortex in the control of limbic and extra pyramidal feedback system.