Lyckenvik, Tim2025-09-182025-09-182025-09-18978-91-8115-402-3 (PDF)ISBN 978-91-629-401-6 (PRINT)https://hdl.handle.net/2077/88260Cerebrospinal fluid (CSF) offers a rare vantage point into the hidden chemistry of the brain. Its composition arises from a finely tuned balance of production, circulation, and interstitial exchange, regulated by ion homeostasis, transporter activity, and physiological states such as sleep. This thesis explores CSF not only as a source of biomarkers but as a dynamic reporter of brain physiology. Paper I establishes reference values for CSF ion concentrations in healthy adults and shows that they differ markedly from serum, reflecting a CNS-specific ionic environment that stabilises neuronal excitability. These levels appear tightly regulated, largely unaffected by age, sex, or blood–brain barrier integrity, but modestly influenced by hydration. Paper II tests how deviations from this physiological profile affect human neuronal networks in vitro. BrainPhys™, despite its intent to mimic CSF, induces epileptiform activity likely due to elevated K+; raising K+ alone reproduces much of this effect, suggesting that serum-like media may drive hyperexcitation. Paper III examines sleep-dependent changes in CSF composition. Molecules associated with synaptic activity decline after sleep, whereas structural cell components remain unchanged. This pattern supports the idea of selective rather than bulk clearance during sleep and adds nuance to the glymphatic hypothesis. Paper IV investigates CSF in patients with opioid use disorder, showing elevated inflammatory markers and signs of ion dilution. By normalising to Aβ40 as a reference for dilution, group differences became clearer, highlighting the importance of accounting for fluid dynamics in CSF studies. Together, these studies illustrate how CSF composition varies across physiological and pathological contexts. Rather than providing definitive answers, these findings open new perspectives on how ion homeostasis, excitability, sleep, and inflammation intersect, and highlight the need for careful interpretation when linking CSF chemistry to brain function.engCerebrospinal fluidIon homeostasisSleepBiomarkerstext