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The role of OT is less clear. In females, OT plays a major role in milk letdown for nourishing the suckling infant (see also Chapter 14). Obviously there is no corresponding role for OT in males. In addition, oxytocin action on the uterine myometrium may play an important role in inducing the uterine contractions leading to the normal termination of pregnancy. Consequently, it is important to ensure that OT is not released prematurely during pregnancy, and indeed there are safeguards to ensure that this does not occur. Many central functions have been attributed to OT, or it may serve as a precursor for neurotransmitters that signal specific functions. Among these central nervous system activities are maternal behavior, penile erection, lordosis, copulatory behavior, yawning, memory and learning, tolerance and dependence mechanisms, feeding, grooming, and certain aspects of cardiovascular regulation.

Some responses are elicited by both VP and OT. Thus, VP also stimulates uterine contractions in humans and can also stimulate milk ejection to a slight degree. However, the stimuli for the release of each hormone are different, ensuring, to a certain extent, that only one of the hormones is released in response to a specific need. Also, OT and VP are produced in different cells where the hormones are synthesized and may act in a paracrine fashion. Similar biological activities can be expected from homologies in the molecular structures of VP and OT, and the homologous regions may be involved in the responses generated in common. Some functions are affected oppositely by the two hormones. While VP, in pharmacological amounts, elevates blood pressure, OT slightly lowers it, and the same trend is evident in the constriction of coronary arteries by VP and their dilation by OT.

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