The peptide hormone oxytocin is synthesised by oxytocin neurons of the supraoptic nucleus (SON) and paraventricular nucleus and is well-known for its role in uterine contraction at parturition and milk-ejection during lactation. However, the mechanisms that cause oxytocin neurons to release pulses during parturition and lactation are poorly understood. We have recently found that central injection of kisspeptin causes a transient increase in oxytocin neuronal firing rate in late pregnant rats but not in virgin rats, suggesting that the emergence of an excitatory kisspeptin input to oxytocin neurons might contribute to pulsatile oxytocin secretion.
Here, we report significantly greater density of kisspeptin-positive fibres in the perinuclear zone (PNZ), immediately dorsal to the SON, in rats at day 21 of pregnancy (P21) compared to virgin rats (P < 0.001; Student’s t-test). Only 1 – 2% of kisspeptin-fibres were also positive for neurokinin B, suggesting that the fibres are unlikely to originate from KNDy (kisspeptin-neurokinin B-dynorphin) neurons of the arcuate nucleus. Furthermore, the number of kisspeptin-positive cell bodies in the arcuate nucleus was significantly lower in P21 rats than in virgin rats (P = 0.003; Student’s t-test) By contrast the number of kisspeptin-positive cell bodies in the rostral periventricular area of the third ventricle (RP3V) was significantly greater in P21 rats than in virgin rats (P < 0.001; Student’s t-test). These results suggest that kisspeptin projections from the RP3V to the PNZ of the SON increase during pregnancy and this might be involved in activation of oxytocin neurons during parturition and lactation.