07 April 2008
The hypothalamic nonapeptide arginine-vasopressin (AVP) exerts several distinct receptor-mediated actions on pituitary cells. Although hypothalamic AVP reaches the anterior pituitary via well-defined pathways, there is now accumulating evidence that AVP may also be produced endogenously in anterior pituitary cells. Using in situ hybridization, we demonstrate here the presence of AVP mRNA in the anterior pituitary of the rat. The observed grain density over pituitary cells was, however, >10-fold lower than the one observed over AVP producing neurons present in the supraoptic and paraventricular nuclei of the hypothalamus. Immunoelectron microscopic analysis using two different AVP-specific antibodies revealed that the distribution of AVP-like immunoreactivity (AVP-LI) in the anterior pituitary is cell-specific. AVP-LI is most abundant in corticotrophs, followed by lactotrophs, gonadotrophs and thyrotrophs. On the other hand, there is complete absence of AVP-LI from somatotrophs. Interestingly, all pituitary cells in which AVP-LI is detected also represent potential target sites for AVP action. A minor fraction of AVP-LI was found to be membrane-associated and may originate, at least in part, from extrapituitary sources. This fraction likely represents receptor-bound peptide. The bulk of AVP-LI, however, was present in the cellular cytoplasm, not associated with any specific ultracellular structure. Specifically in corticotrophs, AVP-LI was excluded from secretory granules. However, our finding of AVP mRNA in anterior pituitary cells indicates that intracellular AVP-LI includes endogenously produced peptide, suggesting a paracrine and/or autocrine action.