Facilitation of self-stimulation in rats by methadone.

The effects of morphine and its derivatives on self-stimulation behavior have been widely studied. In those experiments which have used multiple injections (over days) and multiple post-injection tests (within days), the typical findings includes a depression of responding after the initial injections followed by a facilitation of responding on subsequent days. There have been only a few reports which have tested the effects of methadone in this paradigm. Some investigators have observed only depression of self-stimulation while others have reported both the transient depression and the subsequent facilitation generally obtained with morphine. In the present experiment we administered either 5 mg/kg or 10 mg/kg methadone IP over a five day period and tested MFB-LH self-stimulation at 2, 4, 6, 8, 10 and 23 hours post-injection. Compared to saline controls, the 10 mg/kg dose produced the typical opiate-induced changes in self-stimulation, i.e., an initial depression which lasted for two hours on the first two days but was replaced by significant facilitation by hour 4 of day 3. This facilitation persisted for at least 10 hours on all 5 days of the experiment. Except for a transient (days 2-3) depression of self-stimulation, 5 mg/kg was without effects. The present experiment demonstrates that methadone does facilitate self-stimulation but that its ability to do so is highly dose-dependent.