Physiological control of emotion-related behaviors by endogenous enkephalins involves essentially the delta opioid receptors.
The endogenous pentapeptide enkephalins bind to the mu and delta opioid receptors, with a slightly higher affinity for the latter. It remains a controversy regarding the respective physiological role of mu and delta opioid receptors in the control of emotion and motivation. One of the difficulties to investigate this problem is the low tonic extracellular release of enkephalins in various brain structures. To overcome this problem the synaptic levels of these pentapeptides were enhanced by inhibition of enzymes involved in their catabolism with the selective inhibitor H3N-CH(CH2-CH2-S-CH3)-CH2-S-S-CH2-CH(CH2phi)-CONH-CH(CH2phi)-COOCH2phi (RB101). This compound was shown to increase the extracellular levels and lifetime of endogenous enkephalins. Similar responses were obtained in wild-type and mu opioid receptor knockout mice following RB 101 administration in behavioral tests measuring locomotor activity, anxiety (elevated O-maze), and motivation (forced swim test and conditioned suppression of motility). In contrast, RB 101 led to antinociceptive responses only in wild-type animals using hot plate and tail immersion tests. These results clearly demonstrate the critical role of delta opioid receptors activated by the endogenous opioid peptides, in the physiological control of emotion- and motivation-related behaviors. In contrast, antinociceptive modulation, at least with respect to thermal nociceptive stimuli, involves enkephalin-activated mu opioid receptors. These findings could open new perspectives in the treatment of mood disorders using either inhibitors of enkephalin catabolism or delta opioid agonists.
Nieto
M Mas
MM
Neuropsychopharmacologie des Addictions, CNRS UMR 7157, Université René Descartes, Neuropsychopharmacologie des Addictions, 75270 Paris Cedex 06, France.
Guen
S L E
SL
Kieffer
B L
BL
Roques
B P
BP
Noble
F
F
D000704:Analysis of Variance; D000818:Animals; D001007:Anxiety / Q000503:physiopathology; D001522:Behavior, Animal; D004220:Disulfides / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004644:Emotions / Q000502:physiology*; D004745:Enkephalins / Q000502:physiology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005106:Exploratory Behavior / Q000187:drug effects; D046352:Freezing Reaction, Cataleptic / Q000187:drug effects / Q000502:physiology; D018782:Maze Learning / Q000187:drug effects / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009043:Motor Activity / Q000187:drug effects / Q000502:physiology*; D009270:Naloxone / Q000494:pharmacology; D009271:Naltrexone / Q000031:analogs & derivatives / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D010147:Pain Measurement / Q000379:methods; D010649:Phenylalanine / Q000031:analogs & derivatives / Q000494:pharmacology; D011930:Reaction Time / Q000187:drug effects / Q000502:physiology; D017465:Receptors, Opioid, delta / Q000502:physiology*; D017450:Receptors, Opioid, mu / Q000172:deficiency; D013550:Swimming; D013997:Time Factors
D004220:Disulfides; D004745:Enkephalins; D004791:Enzyme Inhibitors; D009292:Narcotic Antagonists; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; C074024:RB 101; D009270:Naloxone; D010649:Phenylalanine; D009271:Naltrexone; C055382:naltrindole