Stimulation-induced [14C]2-deoxyglucose labeling of synaptic activity in the central auditory system.

The relative contribution of active synapses and discharging neurons to [14C]2-DG labeling in film autoradiographs of the auditory system was studied in a series of three experiments, two in cat, one in chick. In the first, the lateral superior olive in cats was specially prepared so that its inhibitory afferents could be stimulated without concurrent stimulation of its excitatory afferents. The film autoradiographs showed clear 2-DG labeling in the vicinity of the activated inhibitory synapses. In the second experiment, the medial superior olive in cat was specially prepared so that it could be stimulated antidromically without concurrent orthodromic stimulation. The film autoradiographs showed little or no elevations in 2-DG labeling of the antidromically stimulated nucleus over its unstimulated contralateral control despite heavy labeling of nearby orthodromically stimulated nuclei. In the third experiment, the highly polarized nucleus laminaris of a chick was specially prepared so that one set of its excitatory afferents could be stimulated without concurrent stimulation of the other set. The film autoradiographs showed that the distribution of heavy 2-DG labeling matched the distribution of the activated synapses and not the distribution of discharging postsynaptic membrane. The outcomes of the three experiments taken together suggest that it is active synapses and not actively discharging neurons that dominate typical [14C]2-DG film autoradiographs, at least of the vertebrate central auditory system. It follows that [14C]2-DG labeling of central auditory system tissue is not necessarily evidence of local cell discharge but instead evidence of synaptic activity whether excitatory or inhibitory, and whether or not it is accompanied by significant levels of postsynaptic cell discharge.