The effects of photoperiod on the responses of the hamster vas deferens to nerve stimulation.

Hamster vas deferens responds to nerve stimulation with a biphasic contraction which is completely blocked by guanethidine and tetrodotoxin. The second component of the contraction is inhibited by phentolamine, whereas the initial component is slightly potentiated by phentolamine. When hamsters are subjected to short photoperiods or castration, the vas deferens becomes supersensitive to nerve stimulation and shows spontaneous mechanical activity. The supersensitivity is not due to increased postsynaptic adrenergic supersensitivity, decreased neuronal uptake of catecholamine, decreased presynaptic feedback by prostaglandin or adrenergic agonist. The supersensitivity is still demonstrable in the presence of 4-aminopyridine, suggesting that the release of transmitter is also not different in animals kept in short photoperiods. The activity of the Na+/K+ pump is increased in vasa deferentia from hamsters kept in short photoperiods and the postsynaptic response to the ATP receptor agonist, diadenosine pentaphosphate, is also enhanced. It is suggested that the supersensitivity induced in the hamster vas deferens by short photoperiod is caused by testosterone withdrawal and is due to increased responsiveness to ATP, which could be acting as a cotransmitter in this tissue.