Neuronal interactions in the substantia nigra pars reticulata through axon collaterals of the projection neurons. An electrophysiological and morphological study.

Substantia nigra pars reticulata (SNr) neurons, antidromically activated following stimulation of the dorsal thalamus and/or superior colliculus were intracellularly stained with HRP. Light microscopic analysis revealed that the labeled SNr neurons have axon collaterals arborizing within SNr. Axon collaterals of SNr neurons partially overlapped with the dendritic fields of their parent cells and also extended beyond the parent dendritic fields. The labeled axon terminals did not closely appose the parent cell processes, suggesting that the collaterals most likely terminate on neurons other than the parent cell. Electrical stimulation of either the thalamus or the superior colliculus induced monosynaptic and polysynaptic IPSPs in SNr cells. The polysynaptic IPSPs evoked from thalamic stimulation disappeared following hemitransection of the brain just rostral to the thalamus while the monosynaptic IPSPs remained the same. Since there are no known afferents from either thalamus or superior colliculus to SNr, we consider that these monosynaptic IPSPs are due to activation of the recurrent collaterals of SNr projection cells. The results of this study indicate that projection neurons of SNr also have an inhibitory role within the SNr.