Abstract
BackgroundA reduction of dopamine release or D2 receptor blockade in the terminal fields of the mesolimbic system clearly reduces conditioned fear. Injections of haloperidol, a preferential D2 receptor antagonist, into the inferior colliculus (IC) enhance the processing of unconditioned aversive information. However, a clear characterization of the interplay of D2 receptors in the mediation of unconditioned and conditioned fear is still lacking.MethodsThe present study investigated the effects of intra-IC injections of the D2 receptor-selective antagonist sulpiride on behavior in the elevated plus maze (EPM), auditory-evoked potentials (AEPs) to loud sounds recorded from the IC, fear-potentiated startle (FPS), and conditioned freezing.ResultsIntra-IC injections of sulpiride caused clear proaversive effects in the EPM and enhanced AEPs induced by loud auditory stimuli. Intra-IC sulpiride administration did not affect FPS or conditioned freezing.ConclusionsDopamine D2-like receptors of the inferior colliculus play a role in the modulation of unconditioned aversive information but not in the fear-potentiated startle response.
Highlights
Several studies have suggested modulatory roles for c-aminobutyric acid (GABA), serotonin, opioid peptides, excitatory amino acids, and neuropeptides in the so-called encephalic aversion system (EAS), which includes the dorsal periaqueductal gray, deep layers of the superior colliculus, amygdala, medial hypothalamus, and inferior colliculus (IC) [1,2,3,4]
We showed that fear-evoking stimuli increase the magnitude of auditory-evoked potentials (AEPs) that were directly recorded from the ventral aspects of the IC in response to an aversive auditory stimulus (AAS) [7,8]
The analysis of variance (ANOVA) did not indicate a statistically significant effect of treatment on the frequency of open arm entries (F3,29 = 2.14, p.0.05) or percentage of time spent in the open arms (F3,29 = 0.78, p.0.05)
Summary
Several studies have suggested modulatory roles for c-aminobutyric acid (GABA), serotonin, opioid peptides, excitatory amino acids, and neuropeptides in the so-called encephalic aversion system (EAS), which includes the dorsal periaqueductal gray (dPAG), deep layers of the superior colliculus, amygdala, medial hypothalamus, and inferior colliculus (IC) [1,2,3,4]. Little is known about the role of dopamine (DA) in the mediation of aversive states in the EAS. The IC is a primary acoustic structure of the brainstem and the most caudal structure of the EAS that integrates sensory information of an aversive nature. A reduction of dopamine release or D2 receptor blockade in the terminal fields of the mesolimbic system clearly reduces conditioned fear. Injections of haloperidol, a preferential D2 receptor antagonist, into the inferior colliculus (IC) enhance the processing of unconditioned aversive information. A clear characterization of the interplay of D2 receptors in the mediation of unconditioned and conditioned fear is still lacking
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