Osmotic regulation of neuronal activity: a new role for taurine and glial cells in a hypothalamic neuroendocrine structure

Maintenance of osmotic pressure is a primary regulatory process essential f or normal cell function. The osmolarity of extracellular fluids is regulate d by modifying the intake and excretion of salts and water. A major compone nt of this regulatory process is the neuroendocrine hypothalamo-neurohypoph ysial system, which consists of neurons located in the paraventricular and supraoptic nuclei. These neurons synthesize the neurohormones vasopressin a nd oxytocin and release them in the blood circulation. We here review the m echanisms responsible for the osmoregulation of the activity of these neuro ns. Notably, the osmosensitivity of the supraoptic nucleus is described inc luding the recent data that suggests an Important participation of taurine in the transmission of the osmotic information. Taurine is an amino acid ma inly known for its involvement in cell volume regulation, as it is one of t he major inorganic osmolytes used by cells to compensate for changes in ext racellular osmolarity. In the supraoptic nucleus, taurine is highly concent rated in astrocytes, and released in an osmodependent manner through volume -sensitive anion channels. Via its agonist action on neuronal glycine recep tors, taurine is likely to contribute to the inhibition of neuronal activit y induced by hypotonic stimuli. This inhibitory influence would complement the intrinsic osmosensitivity of supraoptic neurons, mediated by excitatory mechanoreceptors activated under hypertonic conditions. These observations extend the role of taurine from the regulation of cell volume to that of t he whole body fluid balance. They also point to a new role of supraoptic gl ial cells as active components in a neuroendocrine regulatory loop. (C) 200 0 Elsevier Science Ltd. All rights reserved.