GABAergic neurons in the rabbit visual cortex: percentage, layer distribution and cortical projections

Abstract

GABAergic neurons were studied in the rabbit visual cortex to estimate their percentage and their proportion among projection neurons to area 17. To retrogradely label projection neurons furnishing cortico-cortical connections to area 17, WGA-HRP injections were given to the visual cortex of rabbits. To determine the general incidence of GABAergic neurons in the visual cortex and the proportion of GABAergic neurons to tracer-labelled cortico-cortical cells, GABA post-embedding immunocytochemistry was made on 1-μm-semithin sections. GABAergic neurons accounted for 15.2% in area 17 and 15.5% in area 18. Except for layer 1, the highest incidence per layer was 25% in layer 5 of area 17. Areas 17 and 18 were nevertheless similar to each other in the distribution of their GABAergic neurons among layers. GABAergic neurons of every layer were double-labelled as furnishing connections intrinsic to area 17. At layer 2/3 and at the border between layers 5 and 6, the intrinsic GABAergic neurons were double-labelled up to 2500 μm from the site of tracer injections. Notably, none of >6250 neurons yielding either callosal or inter-areal ipsilateral projections extrinsic to area 17 was GABAergic. Comparing these findings with those reported for other mammals, it seems that the incidence and distribution of GABAergic neurons in the visual cortex is similar in rabbits and rats. In contrast to rats but akin to higher mammals, no GABAergic neuron was found to furnish cortico-cortical connections to area 17 other than intrinsic connections.