Thalamic-evoked synaptic interactions in barrel cortex revealed by opticalimaging

We used optical imaging of voltage-sensitive dye signals to study the spati otemporal spread of activity in the mouse barrel cortex, evoked by stimulat ion of thalamocortical afferents in an in vitro slice preparation. Stimulat ion of the thalamus, at low current intensity, results in activity largely restricted to a single barrel, and to the border between layers Vb and VI. Low concentrations of the GABAA receptor antagonist bicuculline increase th e amplitude of the optical signals, without affecting their spatiotemporal propagation. Higher concentrations of bicuculline result in paroxysmal acti vity, which propagates via intracolumnar and intercolumnar excitatory pathw ays. Enhancing the activity of NMDA receptors, by removing Mg2+ from the ex tracellular solution, dramatically alters the spatiotemporal pattern of exc itation: activity spreads to supragranular and infragranular layers and adj acent barrel columns. This enhanced propagation is suppressed by the NMDA r eceptor antagonist AP5. A similar enhancement of activity propagation can b e produced by stimulating the thalamus with a short, high-frequency pulse t rain. Application of AP5 suppresses the frequency-dependent spread of activ ity. These findings indicate that the spatiotemporal spread of activity in the barrel cortex is altered by varying the temporal patterns of thalamic i nputs, via an NMDA receptor-mediated mechanism, and suggest that a similar process occurs during repetitive whisking activity.