Rationale-The opioid peptide β-endorphin (β-E) is synthesized by the pro-opiomelanocortin gene in response to environmental stressors and alcohol administration and is implicated in the behavioral sequelae associated with these stimuli.Objectives-We sought to determine the influence of β-E on the stress response by evaluating basal measures of anxiety as well as on EtOH-induced anxiolytic behavior using transgenic mice that differ with respect to β-E.Methods-Anxious behavior was evaluated for male and female heterozygous, wild type, and β-E knockout mice using the Light-Dark Box and Plus Maze assays. Subsequent tests evaluated behavior 20 min after administration of intraperitoneal saline or EtOH (0.5, 1.0 and 1.5 g/kg).Results-We observed an inverse relationship between β-E levels and the percentage of entries into open arms of the Plus Maze as well as the time spent in either the open arms or the light compartment of the Light-Dark box during basal conditions, suggesting that this peptide normally inhibits anxious behavior. However, mice lacking β-E demonstrated an exaggerated anxiolytic response to EtOH in these assays.Conclusions-These data suggest that β-E moderates the response to stressful stimuli and supports the hypothesis that this peptide influences the behavioral effects of EtOH.
KeywordsAddiction; Ethanol; Anxiety; Opioids; Mice; Transgenic; Self-Medication β-endorphin (β-E) is a 31 amino-acid peptide that is cleaved from the carboxyl terminus of the Proopiomelanocortin (POMC) gene. As a member of the large family of opioid peptides that are widely and differentially distributed throughout the nervous system, it has been implicated in a variety of behaviors including the regulation of pain and reward, as well as in modulating neurocircuitry involved in learning and memory, motivation and processes associated with stress, fear or anxiety (e.g. Bloom 1980). The response to stressors involves a complex cascade of endocrine, autonomic and behavioral changes that seem to be generally aimed at maintaining or restoring homeostasis. At least in part because of its complexity, neither a precise definition nor thorough knowledge of the neurobiological underpinnings of the stress response has been elucidated (Pacak and Palkovitis 2001). Nonetheless, nearly any operational understanding includes activation of the hypothalamic-pituitary-adrenal (HPA) axis. This reaction involves Correspondence to: Judith E. Grisel, judy.grisel@furman.edu.
NIH Public AccessAuthor Manuscript Psychopharmacology (Berl). Author manuscript; available in PMC 2010 February 9. The most well-studied effect of β-E is its ability to modulate pain, but an early report by Fratta et al. (1981) suggested that central administration of β-E produced opposite effects to that of ACTH administration ("reciprocal antagonism") on several behaviors in rats including cataplexy, rigidity and analgesia, and supported the notion that β-E is directly implicated in the homeostatic regulation of ACTH response. Indeed, β-E synthesis and release is prec...