Orexins and neuregulins as exemplars of peptide physiology in the brain: experimental challenges and implications for...

Neuropeptides and neurotrophic factors exert overlapping roles on synaptic maturation and plasticity. Although the relative intracellular signals are mostly elucidated, difficulties are met in discerning the dynamics of their extracellular action in vivo, which is largely determined by their peptide nature. We discuss these common mechanisms in two exemplary systems, which are otherwise biologically divergent. The hypothalamic orexin system has a low molecular complexity (two peptides, two receptors), but an intricate connection pattern covering subcortical and cortical structures. Orexins sustain motivated behavior during active waking and are implicated in narcolepsy with cataplexy, as well as psychiatric and metabolic disorders. Neuregulins control neuronal and synaptic maturation after proteolytic release in the extracellular space, or functioning as transmembrane cell-adhesion proteins. They are implicated in neurodevelopmental pathologies and display a wide molecular diversity, with multiple genes and tens of cell- and compartment-specific isoforms. Understanding how peptides regulate synaptic circuits is hampered by uncertainty about essential quantitative aspects: rate of release and diffusion; distribution and desensitization of receptor subtypes; extracellular matrix binding; kinetics of proteolytic degradation. Ongoing studies investigate the peptide-dependent interplay of different CNS populations, by applying optogenetics and ever more specific pharmacological agents to conditional animal models. These approaches need to be complemented by assessing peptide dynamics with novel amperometry or live imaging to monitor peptide release and spatiotemporal distribution, with photoactivatable drugs for spatiotemporal control of stimulus onset, with mass spectrometry to test the long-term cerebrospinal peptide concentration. Furthermore, experimental settings are needed that better mimic the situation in vivo.