Anterior Pituitary Nerve Fibers In The Functional Studies

The clasical theory of hypothalamic control of the anterior pituitary was founded by G. Harris more than half a century ago, which maintains that the anterior pituitary is regulated via hypothalamic hormones secreted at the median eminence but not by direct innervation, albeit it is acknowledged that there is a small amount of autonomic vascular nerve fibres. This concept is recently challenged by the discovery of substantial amount of substance P-and/ or calcitonin gene-related peptide-immunoreactive nerve fibres in the anterior pituitary of rat, dog, monkey, and human. The nerve fibres were found to mainly gather around gland cells, rather than along vascular walls. The varicosities of the nerve fibres could be seen in close proximity to the gland cells. Electron microscopic studies in the dog and rat demonstrated synapses with the gland cells, strongly suggesting that the nerve fibres could regulate the activities of the gland cells. Moreover, changes in endocrine status by adrenalectomy or ovariectomy in the rat profoundly increased the density of the nerve fibres in the anterior pituitary as a result of active axonal sprouting. All these lines of evidence imply a direct neural regulation of the anterior pituitary and a hypothesis of neural-humoral dual regulation of the anterior pituitary has been postulated .However, the physiological significance of the innervation remains an enigma.We addressed the issue by stimulating the an anterior pituitary slices in vitro. The slices were superfused with Krebs-Ringer bicarbonate-BSAbuffer (KRBGA)in a superfusion chamber for 30min before electrical field stimulation. A square current of 30 mA, lOHz and 0.5ms was then applied for lOmin. The perfusate was collected every lOmin and measured for ACTH by radioimmunoassay. At the end of each experiment (typically 60 min), KRBGA was replaced with high potassium KRBGA to evoke peak ACTH output for assessment of the functional viability of the tissue during the experimental period. Statistic analysis with ANOVA test was done to examine differences between group values, with the limit of significance set atp<0.05. The main results are as fallows:1. After 30min of equilibration, the KRBGA perfusion was continued for another SOmin. The basal release of ACTH remained constant throughout the experimental period. No significant changes in ACTH basal concentrations were noticed. Similarly, perfusion with KRBGA containing tetrodotxin(TTX, 1 ja M) for the same duration failed to affect the ACTH basal secretion.2. Electrical field stimulation(EFS)of 30mA, lOHz and 0.5ms for lOmin significantly suppressed basal ACTH concentrations. The suppressive effect of EFS was completely abolished by TTX(1 ju M).3. The addition of [Args]-vasopressin(10pg/ml) increased ACTH secretion from the basal concentration of 149pg/ml. to 325pg/ml Super-imposition of electrical field stimulation of vasopressin induced -very marked suppression.