Gamma oscillations have been implicated in higher cognitive processes and might critically depend on proper mitochondrial function. Using electrophysiology, oxygen sensor microelectrode, and imaging techniques, we investigated the interactions of neuronal activity, interstitial pO 2 , and mitochondrial redox state [NAD(P)H and FAD (flavin adenine dinucleotide) fluorescence] in the CA3 subfield of organotypic hippocampal slice cultures. We find that gamma oscillations and spontaneous network activity decrease significantly at pO 2 levels that do not affect neuronal population responses as elicited by moderate electrical stimuli. Moreover, pO 2 and mitochondrial redox states are tightly coupled, and electrical stimuli reveal transient alterations of redox responses when pO 2 decreases within the normoxic range. Finally, evoked redox responses are distinct in somatic and synaptic neuronal compartments and show different sensitivity to changes in pO 2 . We conclude that the threshold of interstitial pO 2 for robust CA3 network activities and required mitochondrial function is clearly above the "critical" value, which causes spreading depression as a result of generalized energy failure. Our study highlights the importance of a functional understanding of mitochondria and their implications on activities of individual neurons and neuronal networks.

No abstract

Unit activity of 135 GCR-cells in rat's LGNd was recorded extracellularly using micropipettes filled with trypan blue for subsequent marking of recording sites. The latencies of responses to electrical stimulation of the optic chiasma (OX) were mapped, and in accordance with Fukuda (1973) each GCR-cells was classified as fast or slow. Fast cells predominate in the rostroventromedial subregion and slow cells in the caudal part. Both fast and slow subtypes were found in the remaining rostrodorsolateral zone where slow cells made up two thirds of the subsample. The results fit the suggestion that in the LGNd of rats a fast retino-geniculate pathway relays in the ventromedial region, whereas slow pathways exist in the caudal as well as in the dorsolateral zone.