Table 1.
The effects of ultralow doses of opioid receptor antagonists on the induction of tolerance to intrathecal morphine
| Chronic treatment (5 days) |
ED50 (mean±s.e.m.) (mg) |
|
|---|---|---|
| Tail flick | Paw pressure | |
| Saline | 5.8±0.5 | 5.1±1.4 |
| Morphine (15 μg) | 31.6±1.8a | 39.0±2.0a |
| Morphine/CTOP (0.01 ng) | 3.2±0.3 | 7.7±1.4 |
| Morphine/CTAP (0.001 ng) | 3.1±0.3 | 5.7±0.9 |
| Morphine/Naltrindole (0.01 ng) | 3.5±1.4 | 3.5±1.4 |
| CTAP (0.001 ng) | 4.6±0.5 | 7.7±0.5 |
| Naltrindole (0.01 ng) | 3.6±0.6 | 10.0±0.7 |
Following the end of the 5-day chronic treatment period, cumulative dose–response curves to acute morphine were generated on day 6, and ED50 values for morphine were derived from these curves.
a
Significant differences from saline group (P<0.001).