The delta opioid receptor agonist SNC80 produces both antinociceptive and antidepressant effects in rodents. This profile suggests that SNC80 may also reverse prodepressant effects of
pain. Accordingly, this study compared SNC80 effects in complementary assays of
pain-
stimulated and
pain-depressed
behavior in rats. Intraperitoneal injection of dilute acid served as an acute noxious visceral stimulus in rats to stimulate abdominal stretching (a
pain-
stimulated behavior) or depress intracranial self-stimulation of the medial forebrain bundle (ICSS; a
pain-depressed
behavior). When administered once per week to minimize acute tolerance, SNC80 (1-10 mg/kg IP) decreased acid-
stimulated stretching but had little effect on acid-induced depression of ICSS. More frequent SNC80 administration produced tolerance to SNC80 effects on acid-
stimulated stretching, but unmasked antinociception in the assay of acid-depressed ICSS. SNC80 did not facilitate ICSS in the absence of
pain, and effects of SNC80 were not duplicated by ARM390, a reputed delta agonist congener of SNC80 that does not internalize delta receptors. These findings support continued consideration of delta agonists as candidate analgesics to treat prodepressant effects of
pain and illustrate the potential for diametrically opposite effects of drug treatments on preclinical measures of
pain-
stimulated and
pain-depressed
behavior.
Perspective
The delta opioid agonist SNC80 blocked pain-related depression of intracranial self-stimulation in rats, suggesting that delta agonists may be useful to treat prodepressant effects of pain. Repeated SNC80 produced tolerance to SNC80 antinociception in a conventional assay of pain-stimulated behavior but unmasked SNC80 antinociception in an assay of pain-depressed behavior.