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Figure 11.1 The hypothalamic
gonadal (HPG) axis. A scheme is presented of
the HPG axis, as master neurohormonal network for the control of reproduction. Hypo-
thalamic GnRH neurons receive transsynaptic inputs (of inhibitory and excitatory
nature) and glial-derived signals, which cooperate to modulate the pulsatile release
of GnRH. This neuropeptide drives the secretion of gonadotropins, LH, and FSH, from
the anterior pituitary, which in turn stimulate the maturation and function of the
gonads; note that both the ovary and testis are presented in the figure. These major
hormonal elements are connected via feedforward and feedback regulatory loops, in
which gonadal steroids play a prominent role: testicular testosterone (T) inhibits
GnRH/gonadotropin secretion (negative feedback), while ovarian steroids, mainly estra-
diol (E2) and progesterone (P), can carry out both negative and positive feedback
actions depending on the stage of the ovarian cycle. Other peripheral regulators of
the HPG axis mentioned in this review, such as the metabolic hormone, leptin, produced
by the white adipose tissue (WAT), are also presented in the scheme. Note that for the
sake of simplicity, central inhibitory and excitatory inputs to GnRH neurons are not
itemized; only Kiss1 neurons are highlighted. Adapted from references Pinilla, Aguilar,
Dieguez, Millar, and Tena-Sempere (2012) and Roa, Aguilar, Dieguez, Pinilla, and Tena-
Sempere (2008) , with modifications.
pituitary
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