Fig. 5. Nociceptive responses to noxious chemical stimulation of cutaneous or visceral tissue. (A and B) Formalin-evoked hindpaw licking behavior. (A) Open columns represent total time for licking and/or biting in phase 1 (1–7 min after injection); filled columns, total licking and/or biting time in phase 2 (10–47 min after injection). +/+, n = 20; +/–, n = 27; –/–, n = 14. (B) The licking and/or biting time is plotted against the time after formalin injection. Circles, wild-type; squares, heterozygote; triangles, homozygous mutant. Responses in the early half of phase 2 (phase 2A, 10–27 min after injection) were almost completely suppressed in the homozygous mutant. ** represents a significant difference at P <0.01 between +/+ and –/–, and # represents a significant difference at P <0.05 between +/– and –/– at each time point. (C) Visceral nociceptive response (abdominal writhes) produced by intraperitoneal injection of 0.6% acetic acid (+/+, n = 8; +/–, n = 5; –/–, n = 6). (D) Effects of sensitization by a noxious visceral conditioning stimulus on the formalin-evoked somatic nociception. In wild-type mice, which had received a noxious visceral stimulus (0.6% acetic acid injection) 3 weeks before, the phase 2 response (filled column) was considerably reduced compared with the control (gray column). The phase 2 response in homozygous mutant mice after sensitization was significantly (P <0.05) facilitated compared with that of naive homozygous mutants. +/+, n = 6; +/–, n = 4; –/–, n = 6. Data of naive mice (columns with broken lines) are presented for comparison.