Research ReportAccumbal opioid receptors modulate cue competition in one-trial overshadowing

•Examined the effect of accumbal morphine infusions on overshadowing in one trial fear conditioning.•We examined competition for association between a context and auditory CS with shock.•We manipulated salience using trace vs. delay conditioning, auditory cue intensity.•Accumbal morphine infusions disrupted learning about the stimulus of lower salience.

The contribution of opioid receptors in the nucleus accumbens to contextual and auditory fear conditioning was examined. Impairment in contextual fear conditioning was found when training occurred under accumbal infusions of the opioid receptor agonist morphine in a dose-dependent and receptor specific fashion, only when shock onset coincided with auditory stimulus offset. Contextual fear conditioning was spared, however when the delivery of shock was not signalled by an auditory stimulus, the auditory stimulus was of low intensity (70 dB), or an interval (10 s or 30 s) was interpolated between auditory stimulus offset and shock onset. These results provide evidence that opioid receptors in the nucleus accumbens regulate competition between contextual and discrete auditory stimuli for association formation.