DOUBLE-LABELING OF GABA AND CYTOCHROME-OXIDASE IN THE MACAQUE VISUAL-CORTEX - QUANTITATIVE EM ANALYSIS

In the primate striate cortex, cytochrome oxidase (CO)-rich puffs diff er from CO-poor interpuffs in their metabolic levels and physiological properties. The neurochemical basis for their metabolic and physiolog ical differences is not well understood. The goal of the present study was to examine the relationship between the distribution of gamma ami nobutyric acid (GABA)/non-GABA synapses and CO levels in postsynaptic neuronal profiles and to determine whether or not a difference existed between puffs and interpuffs. By combining CO histochemistry and post embedding GABA immunocytochemistry on the same ultrathin sections, the simultaneous distribution of the two markers in individual neuronal p rofiles was quantitatively analyzed. In both puffs and interpuffs, GAB A-immunoreactve (GABA-IR) neurons were the only cell type that receive d both non-GABA-IR (presumed excitatory) and GABA-IR (presumed inhibit ory) axosomatic synapses, and they had three times as many mitochondri a darkly reactive for CO than non-GABA-IR neurons, which received only GABA-IR axosomatic synapses. GABA-IR neurons and terminals in puffs h ad a larger mean size, about twice as many darkly reactive mitochondri a, and a higher ratio of non-GABA-IR to GABA-IR axosomatic synapses th an those in interpuffs (2.3:1 vs. 1.6:1; P < 0.01). There were signifi cantly more synapses of both non-GABA-IR and GABA-IR types in the neur opil of puffs than of interpuffs; however, the ratio of non-GABA-IR to GABA-IR synapses was significantly higher in puffs (2.86:1) than in i nterpuffs (2.08:1; P < 0.01). Our results are consistent with the hypo thesis that the level of oxidative metabolism in postsynaptic neurons and neuronal processes is tightly governed by the strength and proport ion of excitatory over inhibitory synapses. Thus, the present results suggest that (1) GABA-IR neurons in the macaque striate cortex have a higher level of oxidative metabolism than non-GABA ones because their somata receive direct excitatory synapses and their terminals are more tonically active; (2) the higher proportion of presumed excitatory sy napses in puffs imposes a greater energy demand there than in interpuf fs; and (3) excitatory synaptic activity may be more prominent in puff s than in interpuffs because puffs receive a greater proportion of exc itatory synapses from multiple sources including the lateral geniculat e nucleus, which is not known to project to the interpuffs. (C) 1995 W iley-Liss, Inc.