THE SPREAD OF EXCITATION IN NEOCORTICAL COLUMNS VISUALIZED WITH INFRARED-DARK-FIELD VIDEOMICROSCOPY

A combination of darkfield techniques and infrared videomicroscopy was used to measure the intrinsic optical signal (IOS) in slices of adult rat neocortex. The IOS, which reflects changes in light transmittance and scattering, provides a means of studying the spread of neuronal e xcitation and its modulation with high sensitivity and spatial resolut ion. The column-like IOS elicited by orthodromic stimulation is in acc ordance with models of neocortical circuitry. Blockade of synaptic tra nsmission by the glutamate receptor antagonists 6-cyano-7-nitroquinoxa line-2,3-dione (CNQX) and D-2-amino-5-phosphovaleric acid (D-APV) redu ced the IOS. The GABA(A) agonist muscimol and the neuroactive steroid 5 alpha-tetrahydrodeoxy-corticosterone (5 alpha-THDOC) decreased the s pread of excitation, whereas the GABA(A) antagonist bicuculline increa sed it. The present data suggest that the spatial spread of excitation in different neocortical layers is delimited by GABAergic inhibition mediated by the activation of GABA(A) receptors.