Panel and Study Groups

Background: Preclinical studies have demonstrated the role of stress-induced dynorphin release in mediating a component of the anxiogenic and aversive effects of stress exposure.These results suggest that kappa opioid receptor (KOR) antagonists or low efficacy partial agonists may promote stress-resilience in humans and may potentially be useful in treating certain forms of mood disorders exacerbated by stressful experience.In rodent models, stress-induced stimulation of dynorphin release produces aversion, reduces open arm time, potentiates cocaine and nicotine conditioned place preference, inhibits social interaction and reinstates extinguished drug seeking.Each of these responses have been shown to be blocked by prior treatment with KOR antagonists or gene deletion of either KOR or prodynorphin.Two classes of KOR antagonists have been developed and clinical trials of both types have begun, but important differences in the molecular mechanisms of antagonism have been revealed.In addition, partial agonists that activate KOR without stimulating p38 MAPK may also effectively promote analgesia and stress-resilience.Understanding these ligand-directed signaling differences has important implications for the optimization of future therapeutics that manipulate the functioning of the KOR system to affect mood.Methods: Mouse genetic and behavioral data were generated using conditional gene deletion approaches and behavioral stress response assays.Results: Long-acting KOR antagonists including norBNI, JDTic, and GNTI fail to evoke Gbg signaling typical of KOR agonists, but do initiate a Protein Kinase C (PKC) cascade resulting in C-Jun N-terminal Kinase (JNK) activation.Inhibition of PKC by Go6976 blocked norBNI increase in phospho-JNK-ir, and inhibition of JNK by SP600125 or gene deletion of JNK1 isoform blocked the long duration of norBNI antagonism.The JNK phosphorylation site has been localized to the KOR signaling complex, and ongoing sitedirected mutagenesis are being used to determine the regulatory site within KOR.Short acting antagonists (e.g.naloxone and buprenorphine) failed to activate PKC/JNK regulation of KOR.Interestingly, KOR agonists (U50,488 and dynorphin A(1-17)) activate phospho-JNK-ir following ligand binding, but do so through a G-protein kinase 3/beta arrestin dependent mechanism that does not result in long-lasting receptor inactivation.These strong agonists also activate p38a MAPK mechanisms resulting in aversion.KOR partial agonists have distinctly different signaling properties.For example, the mixed MOR/KOR partial agonist, pentazocine was significantly more potent in activating p38-MAPK in hKOR than rKOR in transfected HEK293 cells.In contrast, pentazocine was equally potent in arrestin-independent activation of ERK1/2 in hKOR and rKOR.We confirmed that pentazocine was a partial agonist at both receptors for both signaling pathways.Although pentazocine is reported to produce dysphoria in humans, its lower efficacy at p38 activation of rKOR suggests that it may be unlikely to produce aversion in rodents.Consistent with this prediction, pentazocine (10 mg/kg i.p.) produced analgesia and a MOR-dependent place preference but did not produce KOR-dependent aversion.Conclusions: The signaling differences identified in this study have important implications in screening different KOR agonists and antagonists having distinctly different ligand directed signaling properties.