Animals suffering from metabolic disorders caused by defective proline dehydrogenase 1 (PRODH), the mitochondrial enzyme that initiates the proline catabolic pathway, show increased concentration of L-proline in brain that leads to enhanced synaptic activity in hippocampus (Paterlini et al., 2005) and to behavioral phenotypes associated with mental illness, such as altered performance in cognitive tasks, sensorimotor gating and locomotor activity (Kanwar et al., 1975; Hayward et al., 1993; Paterlini et al., 2005). Here, PRODH is linked to metabolic disease.