Ghrelin is a stomach-derived hormone and the only known circulating peptide that stimulates appetite. Animal studies have conclusively shown that ghrelin increases dopaminergic neurotransmission and, thereby, enhances effort. However, similar evidence on the putative role of ghrelin in humans is still lacking. Here, the investigators propose to conduct a [11C]-raclopride PET/MR study after intravenous administration of ghrelin vs. saline in healthy individuals. First, during an intake visit, the investigators will assess fasting blood levels of hormones involved in appetitive behavior such as ghrelin, leptin, and insulin. In addition, the investigators will conduct a set of tasks that have been associated with dopamine function (i.e., effort and reinforcement learning). Second, the investigators will assess the effects of intravenous administration of ghrelin on dopamine signaling using a double-blind randomized cross-over design. To this end, participants will be infused with ghrelin (vs. saline) while we determine dopamine release (via PET imaging) and assess cerebral blood flow and functional connectivity at rest (via concurrent MR imaging). Furthermore, the investigators will conduct an instrumental motivation task (IMT) where participants have to exert physical effort to obtain rewards. Based on preclinical studies and indirect evidence from human studies, the investigators hypothesize that ghrelin will increase dopamine release in the striatum and that this will, in turn, lead to an increase in the willingness to work for rewards. Moreover, the investigators expect that ghrelin-induced dopamine release will be associated with an elevated tracking of reward utility in the mesolimbic circuit during the IMT, which is known to be associated with response vigor. Collectively, the proposed project would provide a unique resource to test an important link between the gut and the brain in the regulation of appetitive behavior. If ghrelin were to enhance effort expenditure for rewards via dopamine signaling in humans, then restoring sensitivity to ghrelin might be the more promising therapeutic approach compared to antagonizing the ghrelin receptor.

Inclusion Criteria:

• Healthy control participants: never fulfilled the criteria of any mood or anxiety disorder (except specific phobia)
• Patients with major depressive disorder: diagnosis according to DSM-5 within 12 months before enrollment and presence of at least mild symptoms at enrollment (BDI II >= 14)

Exclusion Criteria:

• lifetime history of a brain injury, schizophrenia, bipolar disorder, and a severe substance use disorder according to DSM-5
• obsessive-compulsive disorder, trauma- and stressor-related disorder, somatic symptom disorder, and eating disorder within a 12-month interval before the test day.
• Neuroimaging Study involving ghrelin infusion: contraindication for PET/MR (e.g., metal implants or prostheses, pregnancy, claustrophobia)

After the publication of the key results of the study, all anonymized imaging data will be made publicly available (e.g., at openfmri.org). Behavioral data will be shared after aggregation at the trial or participant level.

Data will become available after an embargo period of 12 months after completion of the study.

Until the data is publicly available, researchers may contact the lead PI to gain access.

Neither participants nor investigators will know whether the participant receives a ghrelin or saline infusion, which will be prepared by independent members of the university hospital.