Barry R. Komisaruk
> We've recently looked very hard to find oxytocin release
>in response to mating stimulation (15 intromissions including
>ejaculations)in the cycling proestrus rat, but were unable to
>see any increase in plasma oxytocin (Polston, Centorino &
>Erskine, 1998, Endocrinology 139:4849-4859). However, there
>were increased numbers of FOS-positive oxytocin cells in the
>PVN and SON after mating at 0600h. What do you think might
>account for the difference between your finding of increased
>plasma OT and our failure to find it?
Thank you for your comments. The pupil dilatation in the female in response to natural mating stimuli and to artificial vaginocervical stimulation is striking, robust, and extremely reliable. However, we can only speculate as to its possible function. Three speculations are as follows: 1) accommodation to close vision, of possible significance to visually adjusting to close physical contact, 2) which may be related to adjustment also to low light conditions during which mating normally occurs, and 3) possible social signal function, as in the work of Hess showing that in humans pupil dilatation occurs in an individual showing interest in what the individual is observing. Do you (or anyone) have any suggestions as to its possible function? Pupil dilatation has also been reported in women during sexual intercourse.
I am aware of the unusual pathway that visual input takes to the pineal, via the accessory optic tract, through the brainstem, down to the spinal cord, out to the superior cervical ganglion and up to the pineal. It is an interesting question as to whether the pupil dilatation during mating could have a neuroendocrine triggering function via this pathway.
Regarding your not finding oxytocin in plasma in female rats after multiple intromissions and ejaculation, but finding increased fos expression in SON and PVN oxytocinergic neurons: is it possible that oxytocin was in fact released, but over a time course that you missed
in the sampling timing that you selected? I need to see your recent paper. In our study with vaginocervical stimulation in urethane-treated rats, we saw an oxytocin peak in plasma after about 15 minutes from the start of the stimulation.