The circadian rhythm of oxytocin in primate cerebrospinal fluid: effects of destruction of the suprachiasmatic nuclei.

The effects of lesions of the suprachiasmatic nuclei (SCN) on the circadian rhythm of oxytocin concentrations in cerebrospinal fluid (CSF) were examined in the rhesus monkey. In the two sham-operated animals, the rhythm was normally entrained to the daily light-dark cycle and persisted in constant lighting conditions at both 4 and 8 months after surgery. Similarly, the oxytocin rhythm was clearly manifested in the two animals with complete SCN ablation 4 months after surgery. At 8 months after surgery, however, the daily CSF rhythm was disrupted in one of the lesioned animals, while it persisted in the other animal. The data show that the SCN are not required for the expression of the daily rhythm of CSF oxytocin, suggesting that a circadian system outside the SCN normally generates the oxytocin rhythm.