Research Article

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.
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