Profile of monoamine and excitatory amino acid release in rat supraoptic nucleus over parturition.

The magnocellular oxytocin neurons of the hypothalamic supraoptic (SON) and paraventricular nuclei play an important role in the initiation and maintenance of parturition in the rat. As little is known about the neural inputs responsible for activating oxytocin neuron activity at this time, we used the technique of microdialysis to examine the profile of monoamine and excitatory amino acid neurotransmitter release within the SON before and during parturition. Microdialysis probes were implanted into the SON of anesthetized pregnant rats (n = 8) on the morning of the day preceding parturition (day 20), and 15-min dialysate samples were collected from freely moving animals over the following 2 days until 3 h after birth of the last pup. On the day of parturition (day 21), dialysate concentrations of norepinephrine were significantly increased (P < 0.05) in the hour leading up to the expulsion of the first pup and, compared with those on the previous day, remained at significantly (P < 0.05) elevated levels throughout the course of parturition. A significant (P < 0.01) increase in glutamate concentrations was also detected, although in this case, it was only elevated transiently in the 15-min period immediately before the onset of pup expulsion. Mean levels of dopamine were not different between days 20 and 21, but a significant increase in dopamine release was detected specifically during the second half of parturition. No significant changes in serotonin and aspartate concentrations were observed on days 20 and 21 or in relation to parturition. This study provides an analysis of neurotransmitter release in the SON over parturition and indicates that norepinephrine concentrations are elevated well in advance of the onset of pup expulsion, whereas a burst of glutamate release occurs immediately before the birth of the first pup. Such changes are likely to reflect activity in afferent inputs to the SON and may represent neurochemical events involved in the initiation and maintenance of parturition.