Intrinsic optical signal measurements reveal characteristic features during different forms of spontaneous neuronal hyperactivity associated with ECSshrinkage in vitro

We induced three different forms of spontaneous synchronous hyperactivity i n adult rat hippocampal-entorhinal cortex slices in order to investigate ef fects on the intrinsic optical signal and associated changes in the extrace llular space (ECS) volume. Low-Mg2+ artificial cerebrospinal fluid (ACSF) a nd the addition of 4-aminopyridine induced synchronous hyperactivity result ing mainly from increased synaptic transmission, while low-Ca2+ ACSF induce d hyperactivity in the absence of evoked synaptic transmission. In the two models of enhanced synaptic transmission, spontaneous activity lead to an i mmediate increase of light transmission. In contrast, a decrease of light t ransmission took place during low-Ca2+-induced hyperactivity, All three for ms of synchronous neuronal hyperactivity were associated with a shrinkage o f the ECS volume, as revealed by the tetraethylammonium signal, measured wi th ion-sensitive microelectrodes, This indicates that the change in the int rinsic optical signal is not simply related to a shrinkage in ECS volume. W e conclude that different forms of spontaneous synchronous neuronal hyperac tivity are associated with characteristic optical signals and that the dire ction of the change in intrinsic optical signal does not reflect ECS shrink age alone.