, 1999). This biphasic effect prevented

the determination of the EC50 for ACEA. Still, a two-way anova revealed significant effects of the variables ‘ACEA concentration’ (F5 = 5.9, P = 0.0005) and ‘stimulus’ (F1 = 799, P < 0.0001), and a significant interaction between them (F5 = 9.1, P < 0.0001). The electrical pulses used here (20 V, 0.4 ms) to stimulate the dorsal root recruits both A and C fibers. It is possible to selectively stimulate C fibers in the dorsal root by immersing it in capsaicin (Lao et al., 2003), because A 5FU fibers lack the TRPV1 channels activated by capsaicin. As in our previous study (Lao et al., 2003), capsaicin applied to the root induced NK1R internalization in about half the NK1R neurons in the ipsilateral dorsal horn (Fig. 6A). Absence of NK1R internalization contralaterally confirms that capsaicin did not reach the slice. In these conditions, AM251 (1 μm) also inhibited the evoked NK1R internalization. Two-way anova of results in Fig. 6A revealed significant PF-562271 in vivo effects of the variables ‘AM251’ (F1 = 29, P < 0.0001) and ‘stimulus’ (i.e. ipsilateral vs. contralateral to capsaicin on the root, F1 = 82, P < 0.0001), and a significant interaction between them (F1 = 18.5, P = 0.0004). This result

indicates that AM251 inhibits substance P release from C fibers. Incubating spinal cord slices with capsaicin is a powerful stimulus for inducing substance P release and subsequent NK1R internalization (Marvizon et al., 2003a; Nazarian et al., 2007). We have shown, however, that this stimulus bypasses the physiological control mechanisms of substance P release (Lao et al., 2003). Thus, capsaicin

causes Ca2+ entry through TRPV1 channels located in primary afferent terminals, so that inactivation of voltage-gated Ca2+ channels by GABAB receptors (Strock & Diverse-Pierluissi, 2004; Raingo et al., 2007) becomes ineffective in inducing substance P release (Lao et al., 2003). Fig. 6B shows that this also applies to the facilitation of substance P release by CB1 receptors. Incubating spinal cord slices with 0.3 μm capsaicin MTMR9 induced a large amount of NK1R internalization in lamina I neurons, which was not inhibited by 1 μm AM251 (Student’s t-test, nondirectional, P = 0.92). Next, we determined whether facilitation of substance P release by CB1 receptors could also be observed in vivo. Substance P release and subsequent NK1R internalization can be induced by applying a noxious stimulus to the hind paw of a rat (Abbadie et al., 1997; Allen et al., 1997; Honore et al., 1999; Kondo et al., 2005; Chen & Marvizon, 2009). In this experiment we anaesthetized rats with isoflurane and then clamped their hind paw with a hemostat for 30 s. This evoked in the ipsilateral dorsal horn a large amount of NK1R internalization, which was maximal in the L5 spinal segment (Fig. 7) which receives abundant innervation from the paw through the sciatic nerve.