• Gail Pereira Do Carmo, Glenn W Stevenson, William A Carlezon, and S Stevens Negus.
• Alcohol and Drug Abuse Research Center, McLean Hospital, Harvard Medical School, Belmont, MA, USA.
• Pain. 2009 Jul 1;144(1-2):170-7.

AbstractPain stimulates some behaviors (e.g., withdrawal responses) but depresses many other behaviors (e.g., feeding). Pain-stimulated behaviors are widely used in preclinical research on pain and analgesia, but human and veterinary medicine often rely on measures of functional impairment and pain-depressed behavior to diagnose pain or assess analgesic efficacy. In view of the clinical utility of measures of pain-depressed behaviors, our laboratory has focused on the development of methods for preclinical assays of pain-depressed behavior in rodents. The present study compared the effects of a chemical noxious stimulus (IP lactic acid injections) and an opioid analgesic (morphine) administered alone or in combination on the stretching response (a pain-stimulated behavior) and intracranial self-stimulation (ICSS; a behavior that may be depressed by pain) in rats. In the ICSS procedure, rats implanted with electrodes in the lateral hypothalamus responded to electrical stimulation across a range of current frequencies to permit rapid determination of frequency-rate curves and evaluation of curve shifts following treatment. Lactic acid alone produced a concentration-dependent stimulation of stretching and depression of ICSS, expressed as rightward shifts in ICSS frequency-rate curves. Morphine had little effect alone, but it produced a dose-dependent blockade of both acid-stimulated stretching and acid-depressed ICSS. Both lactic acid and morphine were equipotent in the stretching and ICSS procedures. These results suggest that ICSS may be useful as a behavioral baseline for studies of pain-depressed behavior.

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