Fig. 6.

Chronic morphine application in postnatal week 2 accelerates the maturation of RVM descending control of nociception. (A) Histogram showing the change in spinal nociceptive reflex excitability produced by electrical stimulation of the RVM (500 μsec, 5–200 μA, 10 Hz) in lightly anaesthetised P21 rats that had undergone chronic administration of morphine (0.175 mg/kg) via s.c. implanted osmotic mini-pumps for 7 days between P7 and P14. In these animals, an adult pattern of descending control has developed prematurely. (B) In control saline-treated animals, facilitation was observed only at the age that is the case in normal animals of this age [18]. Bars indicate standard error of the mean. Asterisks indicate significantly different from baseline in Bonferroni post-test. ^∗P < 0.05, ^∗∗P < 0.01.