Dopamine-ghrelin interactions as novel targets for treatment-resistant anxiety

Anxiety disorders are the most common cause of psychiatric illness and are characterized by excessive fear. Many patients do not respond sufficiently to available treatments such as exposure therapy. This stresses the need for novel therapeutic approaches including non- classical targets such as neuropeptide systems. In mice, diet-induced obesity induces ghrelin resistance and high anxiety whereas caloric restriction reverses ghrelin resistance and decreases anxiety. This suggests that ghrelin resistance, the inability of ghrelin to activate its receptors in the brain, is a common factor in eating disorders and anxiety disorders. Ghrelin receptors are highly expressed on nerve cells that produce the neurotransmitter dopamine. These dopamine neurons control exposure-induced extinction of fear memory. We will investigate whether treatment resistance in anxiety disorders is related to altered ghrelin action on dopamine neurons. Central to our innovative research proposal is the use of a psychopathological mouse model with high relevance to human anxiety disorders that will allow us to identify maladaptive changes within the fear extinction circuit underlying treatment resistance. We propose that targeting ghrelin actions on neurons, directly using pharmacological tools, or indirectly through dietary restriction, may open new avenues to counter excessive fear by facilitating fear extinction.

Exposure-based therapy for anxiety- and trauma-related disorders relies on fear extinction and, thus, has only limited effect in individuals with an impaired ability to form these fear-inhibitory memories. Accumulating preclinical and clinical studies suggest that enhancing dopaminergic (DA) signaling facilitates fear extinction in extinction-competent as well as extinction-impaired individuals. Since the hunger hormone ghrelin has been shown to stimulate endogenous DA release in the brain, we hypothesized that activation of the ghrelin-DA axis could be an effective tool to augment fear extinction. In addition, we investigated whether alterations in the ghrelin system could contribute to impairments in fear extinction. The 129S1/SvImJ (S1) mouse strain shows deficits in fear extinction following Pavlovian fear conditioning. Using an ELISA assay we could demonstrate that this mouse strain was able to release ghrelin upon an overnight fasting challenge, but - in comparison to extinction-competent C57BL/6J (BL6) mice showed alterations in the ghrelin/ghrelin receptor (GHSR) system, i.e. higher baseline acyl ghrelin levels as well as altered central GHSR mRNA expression, but no evidence of a central ghrelin resistance as administration of ghrelin agonist MK0677 similarly stimulated food intake in S1 and BL6 mice. Moreover, neither a GHSR knock-out nor disruption of GHSR function by high-fat diet in BL6 mice affected fear extinction, suggesting that impairment in ghrelin signaling is not a main factor in disrupting the ability to extinguish learned fear. Nevertheless, we found that short-term fasting before fear extinction in extinction-impaired S1 mice supported the formation of a long-lasting extinction memory, which was accompanied by higher neuronal activation in the infralimbic cortex, a main projection target of VTA DAergic neurons and a key area of the extinction neurocircuitry. Whether this effect is mediated via the ghrelin/dopamine axis has yet to be determined. As a first step we could show that pharmacological stimulation of the dopamine signaling locally in the infralimbic cortex is sufficient to rescue impaired fear extinction in an enduring manner in S1. First experiments with the ghrelin agonist MK0677 could however not mimic the fasting effect, and also intra-VTA application of MK0677 had no influence on fear extinction in extinction-competent BL6 mice, despite increasing DA release in VTA projection areas. Overall, in this project we were able to show that functional ghrelin/GHSR signaling is not a pre-requisite to extinguish fear. While the GHSR agonist MK0677, despite its potential to activate midbrain DAergic neurons, might not be a suitable tool to enhance fear extinction, we identified overnight fasting as a successful means to do so. The concept of fasting as an augmentation strategy for exposure-based therapy in treatment-resistant patients would be relatively easy to implement and could have the potential to improve therapy effectiveness and reduce fear relapse in the long- term.