Area Postrema

The area postrema is a circumventricular brain region positioned on the dorsal surface of the medulla on the floor of the fourth ventricle.

The  blood–brain barrier and the cerebrospinal fluid–brain barrier are absent in this region and consequently many substances that do not pass across capillaries in other regions of the brain can do so in the area postrema.

The chemoreceptor trigger zone (CTZ), located in the lateral area postrema is sensitive to blood-borne emetogens. Nerves from the CTZ connect with the vomiting centre.

Activins

Activins are growth and differentiation factors belonging to the transforming growth factor-β superfamily. They are dimeric proteins, consisting of two inhibin-β subunits.

The structure of activins is highly conserved during vertebrate evolution. Activins signal through type I and type II receptor serine/threonine receptor kinases. Subsequently downstream signals such as Smad proteins are phosphorylated.

Activins are present in many tissues of the mammalian organism, where they function as autocrine and/or paracrine regulators of various physiological processes, including reproduction.

In the hypothalamus, activins are thought to stimulate the release of gonadotropin-releasing hormone.

In the pituitary, activins increase folliclestimulating hormone secretion and up-regulate gonadotropin- releasing hormon receptor expression. In the ovaries, activins regulate processes such as folliculogenesis, steroid hormone production and oocyte maturation.

During pregnancy, activin-A is also involved in the regulation of placental functions.