Effect of Ca²⁺-free saline, GABA, histamine, and antagonists on the olfactory lobe response evoked by paired pulses. A, Ca²⁺-free saline eliminates postsynaptic components of the conditioning response and increases the amplitude of the test response. GABA (100 μm) nearly eliminates both evoked responses, and the effect of GABA is blocked in the presence of 100 μmpicrotoxin. Finally, the evoked response recovers to its initial pattern after washout in normal saline. B, A different preparation than in A. Again, Ca²⁺-free saline eliminates postsynaptic activity and increases the test response amplitude. Histamine (30 μm) nearly eliminates the evoked response, and this effect is blocked by 5 mm cimetidine, followed by recovery in normal saline. In A and B, a reduction in absolute response amplitude attributable to dye washout can be seen by comparing the top and bottom traces. The change in test response amplitude at these two time points was used to estimate the rate of dye washout, which was used as a correction factor when normalizing response amplitudes for computing the values shown in Figures 3 and 5. The undershoot in the top andbottom traces is attributable to the digital high-pass RC filter. Digital filtering was performed as described in Figure 1using low- and high-pass cutoffs of 5.7 and 60 Hz.