Architecture of the mammalian pituitary cholinergic system with observations on a putative blood acetylcholine sensor.

Acetylcholine (ACh) plays an important role in pituitary gland function. Little is known, however, about the source and trajectory of pituitary ACh, the location of pituitary cholinergic receptors, and the pathways along which the release of pituitary ACh is controlled. Therefore choline acetyltransferase (CHAT) immunoreactive profiles have been investigated in the rat median eminence and pituitary. Furthermore, both muscarinic- (mAChRp-L) and nicotinic receptor proteinlike (nAChRp-L) immunoreactivity have been examined in the rat, rabbit, and cat pituitary. The results have demonstrated that the rat pituitary ChAT network is composed of neurons in the hypothalamic arcuate nucleus and a great number of terminals in the median eminence. In the pituitary, ChAT immunolabeled profiles were virtually absent. This suggests that much of the ACh acting on pituitary cells is released as a humoral factor from the median eminence. All the examined animals expressed mAChRp-L immunostained endocrine cells in the intermediate lobe. Apart from this, marked species differences in AChRp-L immunolabeled profiles have been found. In addition, strong mAChRp-L immunoreactive rod to cone-shaped bodies were detected associated with blood vessels of the anterior and intermediate lobes in the rat and rabbit, but not in the cat. The immunolabeling was present in particles on the body plasma membrane. These characteristics suggest that the function of these structures might be to sense pituitary blood ACh levels. Consequently the name blood acetylcholine reading bodies (BARBs) was adopted to indicate these structures. It is proposed that the BARBs may play a role in the feedback control of ACh release from the median eminence.