LAMINAR PATTERN OF SYNAPTIC ACTIVITY IN RAT PRIMARY VISUAL-CORTEX - COMPARISON OF IN-VIVO AND IN-VITRO STUDIES EMPLOYING THE CURRENT SOURCEDENSITY ANALYSIS

In the present study we employed current source density analysis to st udy the major excitatory/inhibitory pathways in rat primary visual cor tex in vivo and in vitro. A natural photic stimulus was used in vivo a nd served as a baseline for understanding the results obtained from in vivo and in vitro studies employing electrical stimulation of the whi te matter. The temporal pattern of synaptic activity in the cortex rev ealed an early excitation, characterized by sinks of short duration an d high amplitude, that was followed by inhibition, characterized by lo ng lasting, low amplitude active sources. The spatial pattern of this synaptic activity displayed early excitatory inputs to layer IV and lo wer layer III. Supragranular layers exhibited synaptic activity of lon ger latency at more superficial layers. The excitatory activity of the infragranular layers was delayed relative to that in layer IV. This s patial and temporal pattern of synaptic activity supports the model of sequential information processing in visual cortex. Based on the resu lts of electrical and photic stimulations in vivo we conclude that ele ctrical stimulation of white matter activate the thalamo-cortical inpu t which results in a similar laminar pattern of postsynaptic activity evoked by photic stimulation. Electrical stimulation revealed addition al antidromic and anti-orthodromic activity (collaterals of descending axons to white matter), resulting in the early fast components and th e additional activity in layer VI. The major differences between in vi vo and in vitro laminar pattern of synaptic activity (applying electri cal stimulation) were reduced synaptic activity in layer IV and increa sed synaptic activity in the infragranular layers in the in vitro prep aration. We concluded that the visual cortex slice preparation preserv es the major pathways and electrophysiological function of this area. The technical advantages of the cortical slice preparation will facili tate studies and provide additional insight into this complex cortical network.