Spatiotemporal properties of an evoked population activity in rat sensory cortical slices

We have examined the spatiotemporal properties of ensemble activity, an evo ked all-or-none polysynaptic activity in rat neocortical slices. Ensemble a ctivity occurred in cortical slices bathed in normal artificial cerebrospin al fluid (ACSF) and was evoked by a single electrical shock either to the w hite matter or directly to the cortical tissue. This activity was seen in s lices of somatosensory and auditory cortices; in other cortical areas we ha ve not been able to evoke it. The activity developed 10 to 250 ms poststimu lus and lasted 280 +/- 120 ms in local field potential (LFP) recordings. Vo ltage-sensitive dye imaging showed that this activity was an area of activa tion 0.8 +/- 0.4 mm wide that propagated slowly (11.4 +/- 6.2 mm/s, n = 60, 6 animals) in the horizontal direction. Due to this propagation, the actua l duration in the whole tissue may be longer (similar to 400 ms) than that recorded by a single LFP electrode. Ensemble activity produced a low-amplit ude optical signal (7-14% of the interictal-like spikes in the same tissue) , suggesting a moderate net depolarization of the population. These were ve ry different from hyperexcitable (epileptiform) events in the same tissue t hat had about 10 times the optical signal amplitude and propagated at 125 /- 24 mm/s (n = 21, 6 animals). On a global spatial scale (similar to0.8 mm wide in layers II-III) ensemble activity had a smooth waveform in voltage- sensitive dye signals (population transmembrane potential). On a local scal e, field potential recordings showed large fluctuations with complex oscill ations and substantial trial-to-trial variation. This suggests that oscilla tions in cortical circuits occurred only in small clusters of correlated ne urons. Ensemble activity was sensitive to the excitation-inhibition balance of the local network. Antagonists of N-methyl-D-aspartate, alpha -amino-3- hydroxy-5-methyl-4-isoxazolepropionic acid, and GABAa receptors, and muscar inic agonists and other modest manipulations such as increasing bath concen tration of Mg2+ to 2.5-4 mM (normally at 2 mM), or K+ to 5-7 mM (normally 3 mM), all significantly reduced the probability of evoking the activity. Th e metabotropic glutamate receptor agonist, aminocyclopentane-1,3-dicarboxyl ic acid, blocked the activity at a low concentration (10-15 muM), while the antagonist (R,S)-alpha -methyl-4-carboxyphenylglycine had no effect even a t high concentration (240 muM). Our data suggest that locally organized neu ronal clusters may play a role in the organization of oscillatory activitie s in the gamma band and may participate in cortical integration/amplificati on occurring on a scale of similar to1 mm x 300 ms.