Figure 2. Concentration-dependent effects of episodic intrathecal 5-HT injections (3 × 6 μl) on integrated phrenic (A) and XII (B) nerve amplitude at 60 min post-injection.

At progressively higher 5-HT concentrations, pMF increased, reaching a peak at 1 mm. At concentrations higher than 1 mm, pMF decreased and was no longer significant at 100 mm, thus revealing a bell-shaped dose–response curve. Open bar in A indicates the average effect of a single 1 mm injection of 5-HT. There was no effect of intrathecal 5-HT on XII motor activity at any concentration (see also Fig. 1A). Values are mean ± 1 s.e.m.; n values for each treatment group are indicated in parentheses. *Significant difference from baseline; #significant difference from time control (TC) group (P < 0.05).