Vaginocervical mechanostimulation (VS) was shown previously to release oxytocin 
within the spinal cord and to induce pupillary dilatation. In the present study, 
(a) injection of oxytocin directly to the spinal cord (10 or 25 microg 
intrathecally [i.t.] in 5 microl saline) induced pupillary dilatation when 
observed 1 min after the end of the injection and (b) injection of an oxytocin 
receptor antagonist ([d(CH2)5-Tyr (Me)2-Orn8]-Vasotocin [OTA]; 25 microg i.t. in 
5 microl saline) significantly attenuated the pupillary dilatation response to 
VS, when VS was applied 3 min after the end of the injection. Since activation 
of autonomic sympathetic preganglionic neurons in the thoracic spinal cord 
produces pupillary dilatation, we propose that oxytocin is a central nervous 
system neurotransmitter that stimulates these neurons directly, or perhaps 
indirectly, and thus is a mediator of VS-produced pupillary dilatation.