Novel insights in dopamine receptor physiology
The dopaminergic system has a pivotal role in the central nervous system but also plays important roles in the periphery, mainly in the endocrine system. Dopamine exerts its functions via five different receptors, named D1-D5, belonging to the category of G protein coupled membrane receptors. Dopamine receptors are heterogeneously expressed in different cells, tissues and organs, where they stimulate or inhibit different functions, including neurotransmission and hormone synthesis and secretion. In particular, the dopamineric system has a pivotal role in the physiological regulation of the hypothalamus-pituitary-adrenal axis. Recent data have demonstrated the expression and function of dopamine receptors not only in endocrine organs but also in endocrine tumors, mainly those belonging to the hypothalamus-pituitary-adrenal axis, and also in the so-called 'neuroendocrine' tumors. These data confirm the important role of the dopaminergic system in this endocrine axis, as well as in the neuroendocrine system. This review summarizes the main structural and functional characteristics of dopamine receptors, emphasizing the most recent novelties, and focused on the physiological and pathological regulation of the hypothalamus-pituitary-adrenal axis by the dopaminergic system. In addition, the recent findings on the relationship between dopamine receptors and neuroendocrine tumors are summarized.