GABAERGIC NEURONS IN MAMMALIAN THALAMUS - A MARKER OF THALAMIC COMPLEXITY

The present study evaluated the occurrence, distribution, and number o f GABAergic neurons in the thalamus of different mammalian species (ba t, mouse, rat, guinea pig, rabbit, cat, monkey, humans), by means of l ight microscopical immunoenzymatic localization of GABA or of its bios ynthetic enzyme glutamic acid decarboxylase and by ultrastructural imm unogold detection of GABA. Our data demonstrated that: 1) GABAergic lo cal circuit neurons were detected in the thalamic visual domain in all the species analyzed, whereas in other thalamic nuclei their presence and number varied among species; 2) the number of GABAergic local cir cuit neurons progressively increased in the dorsal thalamus of species with more complex behavior; 3) the presence of local circuit neurons conferred a similar intrinsic organization to the dorsal thalamic nucl ei, characterized by complex synaptic arrangements; 4) in the reticula r thalamic nucleus, whose neurons were GABA-immunoreactive in all the examined species, the cellular density decreased from the bat to human s. These findings strongly suggest that thalamic GABAergic local circu it neurons are not directly related to the ability to perform specific sensorimotor tasks, but they are likely to reflect an increasing comp lexity of the local information processing that occurs at thalamic lev el. Copyright (C) 1997 Elsevier Science Inc.