Orcadian rhythm of oxytocin in the cerebrospinal fluid of rhesus and cynomolgus monkeys: Effects of castration and adrenalectomy and presence of a caudal-rostral gradient

Oxctocin and vasopressin (AVP) were previously reported to have a diurnal rhythm in cerebrospinal fluid (CSF) collected from the cervical cistern of chaired, intact male rhesus monkeys. In the present study, we continuously sampled CSF from temporary indwelling catheters placed in the spinal subarachnoid space of unanesthetized monkeys maintained on tether and swivel systems. CSF was collected From intact and castrate female rhesus monkeys and intact female and castrate, adrenalectomized male cynomolgus monkeys to determine if oxytocin and AVP rhythms are expressed in spinal subarachnoid CSF, if the magnitude of the CSF rhythm displays a rostral-caudal gradient, and if the rhythm is present in adrenalectomized and castrate monkeys, or is specific to the sex or species or macaque. Monkeys, maintained on a 12-hour light/dark cycle with lights on from 06.00 to 18.00 h, had I9-gauge epidural catheters introduced at the L₄-L₅ intervertebral space and advanced cephalad in the subarachnoid space. The proximal end of the catheter was connected to a peristaltic pump for continuous removal of CSF (0.5 ml/h) and hourly CSF samples were radioimmunoassayed for oxytocin and AVP. For rostra I-caudal studies, the distal lip of the catheter was repositioned every few days to collect CSF from 3 levels of the spinal subarachnoid space: C_5-6, T_5-6. T₁₂-L₁. Each animal had a diurnal CSF oxytocin rhythm with peak and trough oxytocin levels during early light and dark periods, respectively. The magnitude of the oxytocin rhythm differed among animals, but was consistent in an animal from day to day. Compared to oxytocin, AVP had a more subtle rhythm in CSF and was not present in all animals studied. The magnitude of the CSF oxytocin, but not AVP, rhythm was greatest in the caudal subarachnoid space (lumbar > thoracic>cervical). Oxytocin also displayed a rhythm in the CSF of adrenalectomized and castrate animals that was similar in pattern and magnitude to that in intact animals indicating that secretory products from the adrenal glands do not play a critical role in modulating the CSF oxytocin rhythm. The pronounced CSF oxytocin rhythm in the lumbar spinal subarachnoid space, an area of the cord known to contain high oxytocin concentrations, suggests local spinal cord secretion or regulation of the CSF oxytocin rhythm.