ENDOTHELIN MECHANISMS IN THE CENTRAL-NERVOUS-SYSTEM - A TARGET FOR DRUG DEVELOPMENT

Endothelin (ET), a vasoconstrictor peptide, was isolated and characterized in 1988. Since then extensive studies have been done in different animal species to identify its receptors in central nervous system (CNS) and peripheral tissues. Three forms of ETs have been identified: ET-1, ET-2, and ET-3. Their distribution pattern and pharmacological functions have been defined. ET-1 and ET-2 are mainly distributed in the CNS, including spinal cord and the peripheral tissues, while ET-3 although present in other tissues is mainly concentrated in the pituitary. ET receptors have been found to be coupled to intracellular pathways involving stimulation of Ca++ fluxes, through phospholipase C and inhibition of adenyl cyclase. The literature concerning central actions of ET has been reviewed. The bulk of the evidence shows that it may be a regulator of regional cerebral blood flow, and a modulator of the release of posterior pituitary hormones (oxytocin and vasopressin). It may also regulate the release of prolactin, growth, thyroid stimulating, or luteinizing hormones. The regulatory role of ET on cardiovascular system can be exerted by central as well as peripheral actions. Besides direct regulation, ET modulates the release of endothelium derived relaxing factor/nitric oxide (EDRF/NO) and increases the sensitivity of peripheral alpha-adrenoceptors. Thus ET appears to establish a balance between vasodilatation induced by EDRF/NO release and vasoconstriction due to increased sensitivity of vascular alpha-adrenoceptors. The level of ET was found to be raised in several pathological conditions like subarachnoid hemorrhage, myocardial infarction, cardiogenic and septic shock, Raynaud's disease, and pulmonary hypertension. Its level was also raised in patients undergoing chronic hemodialysis. On the other hand, the level of ET was significantly lower in the cerebrospinal fluid (CSF) of patients of depression. The available data suggest that ET has a neuromodulatory role in the CNS and may be involved in the pathophysiological processes of several diseases. ET mechanisms in the CNS could provide potential targets for the development of newer drugs.