Figure 2. Pharmacological manipulation reveals different mechanisms underlying the slow and fast gamma rhythms. A, Proportional change in mean (n = 5 slices from 5 rats) gamma power for slow, layer 2/3 gamma (30–45 Hz, gray bars) and fast, layer 4 gamma (50–80 Hz, black bars), induced by 800 nm kainate. Both rhythms were equally sensitive to GABAA receptor blockade (gabazine, 1 μm). Layer 4, high gamma was significantly less sensitive to AMPA receptor blockade (SYM2206, 10 μm) and nonspecific gap junction conductance decrease [carbenoxolone (cbx), 0.2 mm[rsqb]. Layer 2/3, slow gamma was insensitive to NR2C/D-containing NMDAR blockade (PPDA, 10 μm), whereas this manipulation almost abolished layer 4, fast gamma. Asterisks indicate significantly different effects of each drug on the two gamma rhythms (p < 0.05, n = 5 slices from 5 rats). B, Example pooled power spectra (n = 5 slices from 5 rats) for layer 4 population responses to 800 nm kainate showing the differential effects of PPDA (left graph) and carbenoxolone (right graph).