Figure 7. Morphine’s spinal level analgesia is reduced in mice that lack e37a.

Comparison of thermal pain thresholds in WT (n = 20), b*b (n = 20), and aa* (n = 14) mice before (a) and after (b, c, d) intrathecal morphine. a, Average, basal paw withdrawal latency times (Hargreaves’s test) in response to noxious thermal stimuli measured in WT, b*b, and aa* mice were not significantly different (2 way ANOVA or Student’s t test, P > 0.5). b, Comparison intrathecal morphine analgesia expressed as the integral of the maximum possible effect (MPE) for each group as follows: (PWL_morphine − PWL_baseline) ×100/(20 − PWL_baseline) to normalize for differences in baseline responsiveness among mice. Morphine is significantly less effective against thermal stimuli in b*b mice compared to WT mice (Student’s t test P = 0.004) but has similar efficacy in aa* compared to WT mice (Student’s t test, P = 0.77). c and d, Time course of morphine analgesia following intrathecal injection (3 μg) at time 0 in WT and b*b mice (c) and in WT and aa* mice (d) measured every 10 min for 60 min. The efficacy of morphine is reduced in b*b mice relative to WT. P values at 10, 20, 30, 40, 50, and 60 min were 0.613, 0.190, 0.043, 0.023, 0.061, 0.008, and 0.044, respectively comparing average responses in b*b and WT mice (Student’s two tailed t test; c). Values shown are averages ± s.e.m.