Table 1

Evidence of spontaneous withdrawal after pellet removal in morphine-dependent rats

Day 1Day 2Day 4Day 8
Cumulative Withdrawal Score
 Dependent rats11.9  (2.4)** 12.2  (1.8)** 12.5  (2.4)** 9.9  (2.0)
 Nondependent rats3.3  (0.5)3.4  (0.8)4.0  (0.8)5.7  (1.0)
Change in Body Weight
 Dependent rats−10.4  (1.4)** −19.4  (2.7)** −1.7  (1.8)** 27.3  (1.8)**
 Nondependent rats3.9  (2.9)11.0  (2.4)28.9  (1.5)49.4  (3.0)

Spontaneous withdrawal scores (±S.E.M.s) and changes in body weight of dependent and nondependent rats measured 1, 2, 4, or 8 days after pellet removal. The withdrawal scores represent the cumulative number of somatic signs (summed across all response categories) recorded during the 30-min period preceding locomotor activity tests. The changes in body weight reflect the differences (in grams) between the weights recorded immediately before pellet removal and those measured on the test day. An ANOVA followed by Fisher's LSD tests revealed that dependent rats showed significantly more spontaneous withdrawal signs than nondependent control rats on days 1, 2, and 4 after pellet removal, but these signs subsided by the 8th day of withdrawal. Dependent rats also showed substantial weight loss after pellet removal, with weights being reduced below prewithdrawal baseline values on days 1, 2, and 4. The weights of these animals then increased to above baseline levels by the 8th day after pellet removal, although they still remained significantly lower than those of nondependent rats.

  • The double asterisks indicate where somatic signs or weight changes in dependent rats differed significantly from those of nondependent animals with a probability of P < .05. Note that the data for NPA and vehicle-treated animals were combined for these analyses because these subgroups were treated similarly up to the point of drug testing.