Dopaminergic dendrites in the pars reticulata of the rat substantia nigra and their striatal input. combined immunocytochemical localization of tyrosine hydroxylase and anterograde degeneration

Abstract An antiserum prepared in the rabbit against bovine adrenal gland tyrosine hydroxylase has been used to identify by the immunoperoxidase method dopaminergic neurons in the rat substantia nigra. The purpose of this identification was (i) to assess the storing compartments and the release sites in the dopamine-containing processes of the pars reticulata; (ii) to determine if these processes receive a direct input from the neostriatum. Immunoreactive neurons were present in the three divisions of the substantia nigra (pars compacta, pars lateralis and pars reticulata), but they were much more numerous in pars compacta. The caudal half and the most rostral end of pars reticulata contained single and small clusters of reactive neurons, which were absent from the remaining regions. Processes emerging from the positive neurons, exhibiting also immunoperoxidase reactivity, spread throughout the whole pars reticulata. The ultrastructural study was limited to the region of the pars reticulata free of reactive perikarya, in order to analyze the processes that originate from neurons located in the pars compacta. Five hundred and eighty well-preserved immunoreactive processes were analyzed. Almost all of them (578) displayed cytological features allowing their identification as dendrites. Two of them corresponded to thin unmyelinated, non-synaptic segments of axons, probably in their way to their terminal fields outside the substantia nigra. The large majority of the reactive dendrites (82%) were postsynaptic to one or several axon terminals and did not establish direct appositions with other dendritic elements. Only 4.35% of the labeled dendrites were directly apposed to other reactive or unreactive dendrites. Two of the labeled dendrites (0.35%) contained synaptic-like vesicles. In one of them, the vesicles were clustered against a restricted area of the plasma-membrane, forming an active zone. In two animals, kainic acid was used to destroy neurons located within the central region of the main body of the neostriatum. Their projections were traced to the ipsilateral substantia nigra, in which dopaminergic neurons were visualized by the immunoperoxidase method. The axons originating from the injured neurons in the striatum established direct synaptic contacts with the immunoreactive dendrites in pars reticulata. These findings indicate that (i) there is no dopaminergic recurrent collateral axonal plexus in pars reticulata; (ii) the dopamine-storing compartment in the dendritic processes is not vesicular; the cisterns of the smooth endoplasmic reticulum might be such a compartment; (iii) The differentiation of presynaptic dendrites which establish typical junctional synaptic complexes does not occur in the dopaminergic dendrites present in pars reticulata; (iv) The proportion of presynaptic release sites observed in dopaminergic dendrites (1 active zone out of the 578 analyzed dendrites) is too low to account for the dendritic release revealed by biochemical analysis ( Nieoullou , Cheramy & Glowinski , 1977 a ). Therefore, the modality of transmitter release from dopaminergic dendrites must be different from that supposed in the vesicular theory; (v) combined anterograde degeneration and immunocytochemistry has allowed us to demonstrate a direct striatal input to the dopamine-containing dendrites present within the pars reticulata.