Synaptic organization of the globus pallidus.

The synaptic organization of the globus pallidus is reviewed with respect to present knowledge about neurons, fibers, axon terminals, and their intrinsic synaptic relationships. Information derived from studies employing Nissl stains, Golgi impregnations, lesion degeneration techniques, immunohistochemistry, and anterograde axonal labeling in various species are presented along with ultrastructural data. Studies indicate that the globus pallidus contains a principal efferent neuron with smooth or spiny dendrites and simple or complex terminal dendritic arborizations. This cell type receives convergent inputs from intrinsic and extrinsic sources and uses gamma-aminobutyric acid as a transmitter. A smaller and separate population of pallidal projection neurons contains acetylcholine. Two other less frequent neuronal types, of small and medium size, have also been recognized. Three to six types of axonal boutons forming synaptic contacts with pallidal neurons have been recognized in various studies. Among these, three types (types I, II, and III) are the most prevalent. Studies indicate that the most frequent category (type I) originates from neostriatal neurons via radial fiber projections and contains immunoreactive GABA and enkephalins. The synaptic architecture of the globus pallidus is dominated by a mosaic-like arrangement of long dendrites that are ensheathed by longitudinally oriented axons making synapses en passant. Triadic synapses involving dendrites that are pre- and postsynaptic are encountered infrequently. Because both striatopallidal and pallidothalamic connections are inhibitory, pallidal target neurons in the thalamus may be "disinhibited" when the neostriatum is activated.