Laminar pattern of synaptic activity in rat primary visual cortex: comparison of in vivo and in vitro studies employing the current source density analysis.

In the present study we employed current source density analysis to study the major excitatory/inhibitory pathways in rat primary visual cortex in vivo and in vitro. A natural photic stimulus was used in vivo and served as a baseline for understanding the results obtained from in vivo and in vitro studies employing electrical stimulation of the white matter. The temporal pattern of synaptic activity in the cortex revealed an early excitation, characterized by sinks of short duration and high amplitude, that was followed by inhibition, characterized by long lasting, low amplitude active sources. The spatial pattern of this synaptic activity displayed early excitatory inputs to layer IV and lower layer III. Supragranular layers exhibited synaptic activity of longer latency at more superficial layers. The excitatory activity of the infragranular layers was delayed relative to that in layer IV. This spatial and temporal pattern of synaptic activity supports the model of sequential information processing in visual cortex. Based on the results of electrical and photic stimulations in vivo we conclude that electrical stimulation of white matter activate the thalamo-cortical input which results in a similar laminar pattern of postsynaptic activity evoked by photic stimulation. Electrical stimulation revealed additional antidromic and anti-orthodromic activity (collaterals of descending axons to white matter), resulting in the early fast components and the additional activity in layer VI. The major differences between in vivo and in vitro laminar pattern of synaptic activity (applying electrical stimulation) were reduced synaptic activity in layer IV and increased synaptic activity in the infragranular layers in the in vitro preparation. We concluded that the visual cortex slice preparation preserves the major pathways and electrophysiological function of this area. The technical advantages of the cortical slice preparation will facilitate studies and provide additional insight into this complex cortical network.