The role of glutamine and glutamic acid in the pituitary gland involvement in thyroid-stimulating hormone release

Glutamate is a known excitatory neurotransmitter in the central nervous system, with an important role in brain function. The Glutamine/glutamate signaling system, which exists between the synaptic cells and glia cells, converts glutamine into/from glutamate in the nervous system. Large numbers of glutamate binding sites are localized in the pituitary gland. In addition, mRNA expression of many subtypes of glutamate receptors were found in peripheral tissues such as the adrenal gland, pancreas, and testis, suggesting that glutamate can act as an extracellular signal in these tissues. In this chapter, we describe the possible roles of glutamine and glutamate in an endocrine gland-the pituitary. Administration of glutamate stimulates the secretion of pituitary hormones such as prolactin, growth hormone, and cortisol. The pars tuberalis (PT), a part of the anterior pituitary gland, showed high expression of glutaminase and the amino acid transporter A2, known as the glutamine transporter, indicating increased glutamine/glutamate signaling in this structure. Furthermore, PT cells express the ionotropic glutamate receptor KA2, and administration of glutamate stimulates the secretion of thyroid stimulating hormone in the PT. We propose a new concept, wherein glutamate that is locally converted from glutamine acts as an extracellular signal in an autocrine and/or paracrine manner in the pituitary.