Tanycyte Regulation of Hypophysiotropic TRH Neurons

Tanycytes lining the third ventricle are emerging as an integral component of the hypothalamic regulatory system. Their functions, including the regulation of the hypothalamic-pituitary-thyroid (HPT) axis, extend far beyond their role as barrier cells. This chapter summarizes the various mechanisms by which tanycytes contribute to the control of the HPT axis. These cells modulate thyroid hormone negative feedback regulation of hypophysiotropic thyrotropin-releasing hormone (TRH) neurons by regulating local T3 availability. In addition, tanycytes control the availability of TRH transport to the anterior pituitary by at least three different mechanisms. These include degradation of TRH immediately following its release from hypophysiotropic TRH axon terminals, repositioning of tanycyte end feet processes between the TRH axons and the fenestrated capillaries of the portal system, and inhibiting the activity of TRH terminals via glutamate signaling and the endocannabinoid system.