Figure 3.

Average time course of the changes in spontaneous XII nerve activity after microinjections of prazosin and methysergide (A), prazosin only (B), and methysergide only (C) into the XII nucleus. (A) Combined antagonists were injected at three sites in the XII nucleus between time 0 and the point numbered 3; points 1–3 show the average activity levels measured 1 min after each of the three injections. XII nerve activity reached a minimum of 30.8 ± 2.7% of the control activity level about 60 min after the injections. A recovery to 65.3 ± 11% of the preantagonist level occurred over the next 2 h. (B) Prazosin only was injected at three sites in the XII nucleus (same format as in A). XII nerve activity declined to a minimum level corresponding to 19.1 ± 3.6% of the preantagonist level and then gradually recovered to 48.4 ± 7.7% over the next 2 h. (C) Methysergide only was injected at three sites in the XII nucleus (same format as in A). After these injections, XII nerve activity declined to 65.1 ± 6.6% of the control level during the first 20 min. It then transiently increased over the next 40 min to 89.2 ± 5.7% of the control level, suggesting that the drug diffused into a region where its action resulted in enhancement of XII nerve activity. Three hours after methysergide administration, XII nerve activity was still significantly depressed to 73.7 ± 7.4% of the control level. * Points at which XII nerve activity was significantly depressed (p < 0.05 relative to the level of activity at the beginning of these experiments); each curve is based on data from six animals. Pontine carbachol injections were made at greater than 30-min intervals during these experiments, both before and after antagonist injections, and each elicited characteristic responses, such as those illustrated in Figures 2 and 4, which lasted 3 to 6 min. The short horizontal bars mark the periods when carbachol responses described in text were elicited in individual animals.