QUANTITATIVE ASPECTS OF THE GABA CIRCUITRY IN THE PRIMARY VISUAL-CORTEX OF THE ADULT-RAT

The number and size of synaptic contacts made by GABA-immunoreactive a xonal boutons were estimated in each layer of the primary visual corte x (area Oc1M) of adult rats by using the disector method. Immunoreacti vity for GABA was detected with the postembedding immunogold technique on ultrathin sections. Targets of GABA synaptic contacts were also id entified to predict the sites of GABA influence in the rat visual cort ex. For the total cortical depth, 82 million out of an overall populat ion of 666 million synaptic contacts per mm(3) of tissue (or 1 in 8 co ntacts, 12%) were GABA. Layer TV averaged 62% more GABA contacts per u nit volume than did any other cortical layer. Consequently, these repr esented a larger proportion (1 in 6, 17%) of the overall population of layer TV synaptic contacts. This higher number of GABA contacts was n ot due to a greater density of GABA boutons, but to an increased numbe r of contacts made by each layer IV GABA bouton (mean of 1.4 contacts per bouton compared to 1.1 in other cortical layers). The total area o ccupied by the contacts on an average GABA bouton was similar in all l ayers; the higher number of contacts per GABA bouton in layer IV being compensated for by their smaller size. This observed constancy in the area of synaptic contacts suggests the presence of one or more regula tory mechanisms maintaining optimal numbers of the different macromole cules forming the synaptic contacts.