Fig. 4. Intrathecal morphine at postnatal day (P) 3 does not increase the number of degenerating neurons in the spinal cord 24 h or 7 days postinjection. (A –D ) Representative transverse spinal cord sections stained with hematoxylin and eosin 24 h after morphine 0.3 mg/kg (A and B ) or morphine 10 mg/kg (C and D ). Arrows point to degenerating neurons that have a shrunken appearance and condensed nuclei in the high-power expanded images (B and D ). (E ) Histogram shows the number of degenerating neurons for each treatment group at 24 h and 7 days after intrathecal (IT) injection of saline or morphine in P3 rat pups. The number of cells for each animal was averaged from at least four lumbosacral transverse spinal cord sections. Bars = mean ± SEM for n = 4 animals per treatment group.