Morphine-like ligand for opiate receptors in human CSF

Human CSF was fractionated chromatographically and tested for affinity to opiate receptors in synaptic plasma membrane preparations from rat brain. A unique fraction of an apparent molecular weight of 1000 to 1200 dalton, probably identical with that previously found by us in brain extracts, showed affinity. Since the active factor behaved similar to morphinomimetic analgesics in its interaction with the receptors and not like the narcotic antagonist naltrexone, it is termed MLF (morphine-like factor). The level of MLF varied between different individuals and was apparently lower in patients with trigeminal neuralgia than in other patients.