Effects of F8Famide analogs on intestinal transit in mice.

In an attempt to establish the role of F8Famide in opioid activity modulation, we examined the effects of intracerebroventricular administration of the F8Famide analogs (1DME)Y8Fa and (3D)Y8Fa on intestinal transit in mice. (1DME)Y8Fa (0.88 to 22 nmol) inhibited intestinal transit as did F8Famide and morphine. An IP injection of naloxone (2 mg/kg) decreased the morphine effect but had no effect on the response to (1DME)Y8Fa. In contrast, a subthreshold dose of morphine (0.22 nmol) inhibited the response to (1DME)Y8Fa, (3D)Y8Fa delayed intestinal transit only at large dose (22 nmol) but decreased (1DME)Y8Fa and morphine effects at lower ineffective doses. Our findings demonstrate that although F8Famide and morphine could induce the same pharmacological effect, F8Famide receptor activity was modulated by a low-level stimulation of opioid receptors. Furthermore, (3D)Y8Fa should be a useful probe to elucidate neuronal mechanisms controlled by opioids.