Fig. 2.

The effects of TEA, 4-AP, and CO₂ on the membrane potential, input resistance, and firing rate in LC neurons. A: LC neuron in the presence of synaptic blockade medium (SNB), carbenoxolone (CAR), tetrodotoxin (TTX), and Co²⁺, which shows no action potentials. TEA induced a small, and 4-AP induced a larger, reversible depolarization. The combination of TEA and 4-AP resulted in the largest reversible depolarization, which was the sum of the effects of each drug alone. B: summary of the effects of TEA and 4-AP on Vm. The depolarization induced by 4-AP alone and in combination with TEA was significantly larger than the depolarization induced by TEA alone. The average effects of TEA, 4-AP, and the two together on Vm are shown for 4 LC neurons (all in the presence of 5% CO₂). C: effect of 15% CO₂, 4-AP, and the combination of the two on the change of Rin. Rin was significantly increased by increasing CO₂ from 5% to 15% and was also significantly increased by the addition of 4-AP (in the presence of 5% CO₂). However, in the maintained presence of 4-AP, raising CO₂ from 5% to 15% resulted in no additional increase of Rin. Thus, the increase in Rin induced by increasing CO₂ from 5% to 15% in the presence of 4-AP was significantly smaller than when CO₂ was increased in the absence of 4-AP. The average effects for 8 LC neurons is shown. D: effect of 15% CO₂, 4-AP, and the combination of the two on the change of firing rate. The firing rate of LC neurons was significantly increased by changing CO₂ from 5% to 15% and was also significantly increased by the addition of 4-AP (in the presence of 5% CO₂). The addition of 4-AP alone caused a significantly greater increase in firing rate than increasing CO₂ to 15%. However, in the maintained presence of 4-AP, increasing CO₂ from 5% to 15% resulted in no additional increase in firing rate. Thus, the increase in firing rate induced by increasing CO₂ from 5% to 15% in the presence of 4-AP was significantly smaller than when CO₂ was increased in the absence of 4-AP. The average effects for 8 LC neurons is shown. E: effect of 15% CO₂, TEA, and the combination of the two on the change of Rin. Rin was significantly increased by increasing CO₂ from 5% to 15% and was also significantly increased by the addition of TEA (in the presence of 5% CO₂). In the maintained presence of TEA, increasing CO₂ from 5% to 15% resulted in a significant increase of Rin. The increase in Rin induced by increasing CO₂ from 5% to 15% in the presence of TEA was significantly smaller than when CO₂ was increased in the absence of TEA. The average effects for five LC neurons are shown. F: effect of 15% CO₂, TEA, and the combination of the two on the change of firing rate. The firing rate was significantly increased by increasing CO₂ from 5% to 15% and was also significantly increased by the addition of TEA (in the presence of 5% CO₂). In the maintained presence of TEA, with the firing rate adjusted back to its initial value by injecting negative current, increasing CO₂ from 5% to 15% resulted in a significant increase of firing rate. Both increasing CO₂ from 5% to 15% and the addition of TEA (in 5% CO₂) resulted in a significant and nearly identical increase in firing rate. Further, in the maintained presence of TEA, increasing CO₂ from 5% to 15% caused a nearly identical increase in firing rate to that caused by raising CO₂ in the absence of TEA. The average effects for five LC neurons is shown. In all figures, bars show means ± SE. Differences were significant at either *P < 0.05 or at **P < 0.01.