What does Activin do?

2009 Review: The biology of activin: recent advances in structure, regulation and function.

2004 Review: Regulation of gonadotropins by inhibin and activin.
1) Activin can stimulate FSH sythesis and release from the pituitary gland via the gonadotrope cells.
2) Activin up-regulates gonadotropin-releasing hormone receptor (GnRHR) gene expression, leading to alterations in the synthesis and release of both gonadotropins in response to GnRH.
3) Activin can stimulate GnRH release from GnRH neurons in the hypothalamus and thereby affect FSH and LH secretion
Inhibins and folastatin apparently have the reverse effect.

**2002 Review: **

In vitro study: Activin-A stimulates GnRH in male rat hypothalamus
- Activin-A stimulated GnRH release. Maximal effect reached at a concentration of 10 ng/ml which increased GnRH output by about 75%.
- Inhibin abolished the stimulatory effect of a maximally effective concentration of activin-A in a dose-dependent manner, whereas alone it had no effect on GnRH output.
- As previously shown, testosterone (1 nmol/l) and dihydrotestosterone (DHT, 0ยท1 nmol/l) suppressed basal GnRH release, but only
testosterone was able to inhibit the release of GnRH stimulated by activin-A.
- Use of an aromatase inhibitor did not diminish the suppressive effect of testosterone. Suggesting effect is mediated via testosterone and does not require aromatisation.

In vivo rat study: Activin-A stimulates FSH, LH and Testosterone in adult male rats
"When activin-A was administered s.c. every 8 h for seven consecutive treatments, similar results were obtained for gonadotropin release; in addition, we observed a significant (p < 0.01) up-regulation of hypothalamic GnRH mRNA levels. The administration of activin-A intracerebroventricularly (i.c.v., 6 micrograms/rat) according to the same chronic schedule also produced a significant (p < 0.01) increase in FSH levels, as well as a modest, but detectable, elevation in LH concentrations, and a large augmentation of T secretion"

Impaired male sexual behavior in activin receptor type II knockout mice.
"These behavioral defects in the adult mice are independent of gonadotropin-releasing hormone (GnRH) homeostasis or mating-induced changes in luteinizing hormone (LH) and testosterone levels."
"Olfactory acuity tests confirmed that Acvr2-/- mice have no defects in general odor or pheromone recognition."

Activin A was also present in seminal plasma in normal men but was undetectable following vasectomy, thus deriving from the testis

suggests that all activin A in the circulation is bound to follistatin in males.