Behavioral Consequences of Delta-Opioid Receptor Activation in the Periaqueductal Gray of Morphine Tolerant Rats
Figure 1
Tolerance to
antinociception develops with repeated injections of morphine. The hot-plate
latency following morphine administration on trial 7 was significantly less
than on trial 1 for the morphine-treated groups (F(1, 39) = 31.38, P < .01). Antinociception was still evident with the administration of large doses
of morphine (10 and 20 mg/kg), but not as pronounced as on trial 1. Large
sample sizes were used for most groups (N = 19, 5, 19, and 17 for rats
pretreated with saline and morphine at 5, 10, and 20 mg/kg, resp.) because
subsets of these rats were subsequently tested using either the formalin test
or repeated hot-plate tests.