Environmental stimuli are received
and translated by the brain.
Stimuli of reproductive importance
are routed to a portion of the
brain called the hypothalamus.
The hypothalamus produces
gonadotropin releasing hormone
(GnRH) and also gonadotropin release
inhibiting factors. Experimental
results suggest that
dopamine is a substance that inhibits
the release of gonadotropin.
Gonadotropin releasing hormone
(GnRH) is thought to stimulate the
pituitary, a small gland located beneath
the brain, to produce and release
gonadotropin hormones
(GtH). Studies of induced ovulation
of many fishes using injected
pituitary extract indicate that an increased
blood GtH is a prerequisite
for ovulation.
Gonadotropin hormones (GtH) act
on the ovaries and testes (gonads).
Steroids and prostaglandins appear
to be the local ovarian mediators
of GtH action causing release
of the eggs. Elevated blood levels
of GtH trigger two distinct ovarian
processes: 1) final maturation of
the egg, which appears to be
stimulated by steroids (e.g., progesterone)
that are produced by
the follicle, and 2) rupture of the
follicle (ovulation), which evidently
is stimulated by prostaglandins.
Steroids also appear to
induce spermiation in the male.