THE DIFFERENTIAL CONTRIBUTION OF CAPSAICIN-SENSITIVE AFFERENTS TO BEHAVIORAL AND CARDIOVASCULAR MEASURES OF BRIEF AND PERSISTENT NOCICEPTION AND TO FOS EXPRESSION IN THE FORMALIN TEST

Intraplantar injection of dilute formalin evokes brief (Phase 1) and p ersistent (Phase 2) increases in primary afferent activity, pain behav ior, and cardiovascular responses, and induces spinal cord Fos-like im munoreactivity (Fos-LI). Although previous studies demonstrated that t he destruction of small diameter primary afferents with neonatal capsa icin treatment decrease formalin-evoked nociception, these studies onl y evaluated behavioral responses, and did not distinguish between Phas e 1 and 2. To address these questions, we simultaneously evaluated for malin-evoked pain behavior (flinching of the afflicted paw), cardiovas cular responses (heart rate and mean arterial pressure), and lumbar sp inal cord Fos expression in control rats and in rats treated with caps aicin (100 mg/kg) one day postpartum. We found that neonatal capsaicin -treated rats, compared to controls, exhibited similar cardiovascular responses and slightly less flinching behavior during Phase 1. During Phase 2, however, capsaicin treated rats exhibited 59% less flinching and 45% smaller heart rate responses. Also, in capsaicin-treated rats, we counted 59% fewer Fos-labeled neurons in the spinal cord. These re sults indicate that capsaicin-sensitive afferents contribute to formal in-evoked behavioral and cardiovascular responses and to spinal cord n euronal responses. The differential effect of neonatal capsaicin on no ciception during Phase 1 and Phase 2 suggests that sensitization mecha nisms during Phase 1 do not contribute to the magnitude of nociceptive responses during Phase 2.