Prof. Dr.Mohamed Hamdy Abouel-Hassan

Opioids
iterature
Review of l
Endogenous Opioids are known to affect sexual function, but the mechanism
of action is far from clear. Injection of small amounts of morphine into the
MPOA facilitates sexual behavior in rats. Larger doses, however, inhibit
both penile erection and yawning induced by oxytocin or apomorphine. It is
suggested that endogenous Opioids may exert an inhibitory control on
central Oxytocinergic transmission (Argiolas, 1992).
Oxytocin
Oxytocin is a neural hormone secreted by the neurons into the circulation.
Besides the posterior pituitary gland, o
xytocin-secreting
neurons are also
found in the neurons projecting from the paraventricular nuclei to the brain
stem and spinal cord and, thus, oxytocin can also function as a
neurotransmitter. The blood level of oxytocin is increased during sexual
activity in humans and animals. It produces yawning and penile erection
when injected into the paraventricular area in rats. It has been suggested that
calcium is the second messenger mediating oxytocin-mediated penile
erection. Because neurons in the paraventricular area have been shown to
contain NOS, and NOS inhibitors prevent apomorphine- and oxytocin-
induced erection, it is suggested that oxytocin acts on neurons whose activity
is dependent on certain levels of NO (Vincent and Kimura, 1992; Melis and
Argiolas, 1993).
Prolactin
Increased levels of Prolactin suppress sexual function in men and
experimental animals. In rats, high levels of Prolactin decrease the genital
reflex and disturb copulatory behavior. It is suggested that the mechanism of
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