Effects of EDCs on Neurodevelopmental and Neuroendocrine Systems 215
LH secreted by the pituitary, which itself is stimulated by gonadotropin releas
ing hormone (GnRH) from the hypothalamus. On the other hand, circulating
testosterone has, together with estrogen, an inhibitory effect on both GnRH
and LH secretion. In the adult male, a balance between LH and testosterone
level is thus achieved through a finely regulated hormonal negative‐feedback
loop, which is part of the hypothalamo–pituitary–testis hormone axis. About
98–989 of circulating testosterone and estrogen is bound to serum proteins
called sex steroid binding protein (SHBG) testosterone and only 1–2% of circu
lating testosterone occurs free in the bloodstream. The target tissues of the
body, including the hypothalamus and pituitary, only sense the free‐unbound
sex steroids, and SHBG serum level is therefore an important regulator in the
GnRH–LH–testosterone hormonal feedback loop. In several studies, data
indicate that both SHBG and free‐unbound testosterone levels are declining
significantly in males. In addition, sperm count is also declining in males from
developed nations [129].
Change in Sex Ratio
Furthermore, the sex ratio of offspring is also changing in a significant manner.
Generally, there are 105 male births to 100 female births, leading to 51.5% rate
of male birth. However, recent data suggest a decline in the sex ratio in many
developed nations [129].
Effects of EDCs on Neurodevelopmental and Neuroendocrine Systems
and Neuroendocrine Systems
Parts of the brain function as an endocrine gland. In Chapter 4, we covered
certain aspects of the human brain’s endocrine function and role of oxytocin
and AVP and their respective receptors in ASD. Specialized kinds of neurons
release neurohormones, and oxytocin and AVP are neuropeptides that func
tion like hormones in human brains. These are evolutionarily conserved
neuropeptides and their origin can be tracked back to many early vertebrate
life forms.
The central neuroendocrine region is located at the base of the brain, the
hypothalamus, and is integrally involved in controlling how the body adapts to
the environment and in the regulation of several peripheral endocrine systems.
Different groups of hypothalamic neurons control reproduction, growth,
metabolism, lactation, stress responsiveness, uterine contractions at parturi
tion, energy balance, circadian rhythms, temperature regulation, and electro
lyte balance, among other functions. We will focus primarily on the
hypothalamic–pituitary–gonadal (HPG) and neuroendocrine axes controlling
reproduction and stress, respectively (Figure 7.16). Importantly, the steroid
