Cerebral insulin resistance plays an important role in the development of obesity and diabetes mellitus type 2. The aim of this project is to examine the effect of human nasal insulin on the dopaminergic system. Therefore, characteristics of cerebral dopamine receptors before and after administration of nasal insulin vs. placebo shall be analyzed in a randomized way. Moreover, the investigators plan to examine the insulin action on cortical and subcortical activation in humans and the interaction of dopamine metabolism with [11C]-Raclopride-PET/MRI. By performing fMRI measurements, insulin sensitivity of the central nervous system can be investigated simultaneously. Recruiting is planed as a two-step process.First 12 normal-weight (BMI 20-25 kg/m²) men should be examined. If first results show a insulin-dependent effect on the availability of dopamine receptors in the human brain, recruitment of 12 overweight men will get started.

Inclusion Criteria:

• Understand and voluntarily sign an informed consent document prior to any study related assessments/procedures.
• Intake of any medication
• HbA1C <6%
• BMI 20-25 kg/m²
• Clinical routine blood parameters within the normal ranges
• All participants must agree to get informed of unexpected detected, clinical relevant findings.

Exclusion Criteria:

• Acute diseases such as infections (e.g.)
• Any relevant cardiovascular disease
• Any surgery within the last three months
• Any neurologic or psychiatric disease
• Known allergies
• Hb < 13 g/dl
• Presence of any contraindication for the conduct of an MRI investigation, such as cardiac pacemakers, ferromagnetic haemostatic clips in the central nervous system, metallic splinters in the eye, ferromagnetic or electronically operated active devices like automatic cardioverter defibrillators, cochlear implants, insulin pumps and nerve stimulators, prosthetic heart valves etc.
• Claustrophobia