Fig. 3.

The 5-HT₃R is required for the second phase of pain behavior and of spinal cord neuronal firing in the formalin test. a, Formalin-induced paw licking during phases 1 (0–10 min) and 2 (11–60 min) in both wild-type and 5-HT₃R-A null mutant mice did not differ, but the magnitude of second-phase pain behavior was significantly reduced in mutant animals (**p < 0.01; t test, comparing the two groups; n = 7–8 per group).b, Response of a lamina V neuron in the spinal cord to the injection of formalin. In wild-type mice, formalin produced a characteristic biphasic increase in neuronal activity, which resembles the time course of the behavioral response. In the 5-HT₃R mutant mice, the first-phase firing was comparable with wild-type mice, but there was only a modest second phase response. c, Summary of formalin-evoked neuronal activity; significantly fewer total spikes were recorded during the second phase in the mutant mice compared with wild-type mice (*p < 0.01; Mann–Whitney U test; n = 8).