Figure 1. Enhancement of the capsaicin-induced Ca²⁺ increase by nerve growth factor (NGF).

A, trace illustrating a typical experimental procedure. Ca²⁺ increases shown in a single neurone from a coverslip containing (typically) 10–30 neurones. Neurones loaded with the Ca²⁺-sensitive fluorophore fluo-4 as detailed in Methods. The Ca²⁺ increase observed on application of KCl (25 mm, 15 s first exposure) distinguished neuronal from non-neuronal cells. Subsequent applications of capsaicin (500 nm, 15 s) caused activation of TRPV1, which was potentiated by exposure to NGF (100 ng ml⁻¹, 2 min). NGF did not itself cause an increase in [Ca²⁺]_i in any experiment. The ratio b/a of the Ca²⁺ increases before and after exposure to NGF was used as an index of enhancement. B, NGF causes a long-lasting enhancement. Neurones (n_cell = 32) were separated into two groups: those in which an enhancement was (▪) or was not (•) observed in the first exposure following NGF addition (lower bar). Bars give ± s.e.m. Significance levels (two-tailed t test) are: **P < 10⁻²; ***P < 10⁻³. (NB in this experiment a continuous NGF exposure followed the third capsaicin application while in all others a 2 min exposure was given between the fifth and sixth capsaicin applications). C, collected ratio values obtained from experiments as in A. Open bars give ratios obtained without exposure to NGF (n_cell = 112, n_exp = 8). The distribution was well fitted by a Gaussian function with mean of 0.76, s.d. of 0.134 and upper 95 % two-tailed confidence limit of 1.023 (arrow). Filled bars give ratios following 2 min exposure to NGF (100 ng ml⁻¹; n_cell = 152, n_exp = 26). Following NGF exposure 38.74 ± 5.61 % of ratios exceeded the 95 % confidence limit, and the mean of these ratio values was 1.81 ± 0.14.