SYNAPTIC CONNECTIONS BETWEEN PARS COMPACTA AND PARS RETICULATA NEURONS - ELECTROPHYSIOLOGICAL EVIDENCE FOR FUNCTIONAL MODULES WITHIN THE SUBSTANTIA-NIGRA

Intracellular recordings were performed in vitro from both pars compac ta and pars reticulata neurones of the substantia nigra, and their pos tsynaptic responses to electrical stimulation within the nucleus were analysed. Intracellular staining by biocytin was used to reveal the mo rphology and location of the recorded neurone and its position and dis tance to the stimulating electrode. Inhibitory postsynaptic potentials in pars compacta neurones were evoked exclusively from a specific reg ion of stimulation within the pars reticulata; this field could be map ped out as a cone-shaped region surrounding the apical dendrite of the cell and perpendicular to the plane of the pars compacta. Furthermore , hemitransection, prior to the experiments which eliminated the most likely source of extrinsic inhibition (the GABAergic striatonigral pat hway) affected neither the generation of inhibitory postsynaptic poten tials nor its topographic pattern during pars reticulata stimulation. In contrast to the response of dopaminergic pars compacta neurones, pa rs reticulata neurones responded to stimulation over wide areas of sub stantia nigra, without any clear site-specific selectivity. It is conc luded that within the substantia nigra, dopaminergic neurones are arra nged in functional modules such that most inhibition is derived from a highly local circuit with the collaterals of adjacent pars reticulata cells. This module is intrinsic to the substantia nigra and may repre sent the basic functional unit of the nucleus.