Abstract
The ghrelin system was previously proposed to mediate an independent branch of the stress response that curbs fear processing. Interestingly, the ghrelin system was also shown to control the activity of midbrain dopamine neurons. Given that dopamine neurons of the ventral tegmental area appear to have a critical role in fear processing, we aimed to investigate their contribution to the effects of ghrelin on fear processing. Our data show that systemic administration of the ghrelin receptor agonist MK0677, in a dose that induces food intake, has no significant effect on auditory fear processing and does not significantly affect dopamine release in the nucleus accumbens of male C57BL/6J mice. Local administration of the ghrelin receptor agonist MK0677 into the ventral tegmental area significantly increases food intake and it also significantly increased dopamine release in the nucleus accumbens, the medial prefrontal cortex and the amygdala. Nevertheless, it did not significantly affect auditory fear extinction. Our data indicate that pharmacological activation of midbrain dopamine neurons using a ghrelin receptor agonist does not affect auditory fear extinction. We also investigated the effect of non-pharmacological manipulation of the ghrelin system on auditory fear processing. However, we found that neither overnight food deprivation nor genetic ablation of the ghrelin receptor had a significant effect on auditory fear extinction. We conclude that the effects of manipulation of the ghrelin system on fear processing are subject to boundary conditions that remain poorly understood.