Effects of kappa-opioid receptor ligands on intracranial self-stimulation in rats.
BACKGROUND
Elevations in cAMP response element binding protein (CREB) function within the mesolimbic system of rats reduce cocaine reward in place conditioning studies and increase immobility in the forced swim test. Each of these behavioral adaptations can be interpreted as a depressive-like effect (i.e., anhedonia, despair) that may reflect reduced activity of brain reward systems. Furthermore, each effect appears due to increases in CREB-mediated expression of dynorphin, since each is attenuated by intracranial injections of the kappa-opioid receptor antagonist norBNI.


OBJECTIVE
Intracranial self-stimulation (ICSS) studies were conducted in rats to determine whether administration of a kappa-agonist would have depressive-like effects on brain stimulation reward, and whether pretreatment with a kappa-antagonist would attenuate any such effects. Conditions that have depressive effects in people (e.g., drug withdrawal) increase the threshold amounts of stimulation required to sustain ICSS in rats.


METHODS
Sprague-Dawley rats with lateral hypothalamic stimulating electrodes were tested in a "curve-shift" variant of the ICSS procedure after systemic administration of the kappa-agonist U-69593 alone, the novel kappa-antagonist 5'-acetamidinoethylnaltrindole (ANTI) alone, or co-administration of both drugs.


RESULTS
U-69593 dose dependently increased ICSS thresholds, suggesting that activation of kappa-receptors reduced the rewarding impact of the brain stimulation. ANTI had no effects on its own, but it attenuated increases in ICSS thresholds caused by the agonist.


CONCLUSIONS
These data provide further evidence that stimulation of brain kappa-receptors may trigger certain depressive-like signs, and that kappa antagonists may have efficacy as antidepressants without having reward-related actions of their own.
Todtenkopf
Mark S
MS
Behavioral Genetics Laboratory, Department of Psychiatry, Harvard Medical School, McLean Hospital, 115 Mill Street, MRC 217, Belmont, MA 02478, USA.
Marcus
Jacqueline F
JF
Portoghese
Philip S
PS
Carlezon
William A
WA
D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects*; D044482:Benzeneacetamides / Q000008:administration & dosage / Q000494:pharmacology; D001921:Brain / Q000187:drug effects* / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004399:Dynorphins / Q000378:metabolism; D004558:Electric Stimulation; D004567:Electrodes, Implanted; D007274:Injections, Intraperitoneal; D008024:Ligands; D008297:Male; D009271:Naltrexone / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000494:pharmacology; D011759:Pyrrolidines / Q000008:administration & dosage / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D017473:Receptors, Opioid, kappa / Q000819:agonists* / Q000037:antagonists & inhibitors*; D012201:Reward; D012653:Self Stimulation / Q000187:drug effects*; D013997:Time Factors
C493273:5'-acetamidinoethylnaltrindole; D044482:Benzeneacetamides; D008024:Ligands; D011759:Pyrrolidines; D017473:Receptors, Opioid, kappa; D009271:Naltrexone; D004399:Dynorphins; C045444:U 69593