Endothelin-Mediated Calcium Responses in Supraoptic Nucleus Astrocytes Influence Magnocellular Neurosecretory Firing Activity

In addition to their peripheral vasoactive effects, accumulating evidence supports an important role for endothelins (ETs) in the regulation of the hypothalamic magnocellular neurosecretory system, which produces and releases the neurohormones vasopressin (VP) and oxytocin (OT). Still, the precise cellular substrates, loci and mechanisms underlying the actions of ETs on the magnocellular system are poorly understood. In the present study, we combined patch-clamp electrophysiology, confocal Ca ²⁺ imaging and immunohistochemistry to study the actions of ETs on supraoptic nucleus (SON) magnocellular neurosecretory neurones and astrocytes. Our studies show that ET-1 evoked rises in [Ca ²⁺] _i levels in SON astrocytes (but not neurones), an effect largely mediated by the activation of ET _B receptors and mobilisation of thapsigargin-sensitive Ca ²⁺ stores. The presence of ET _B receptors in SON astrocytes was also verified immunohistochemically. ET _B receptor activation either increased (75%) or decreased (25%) SON firing activity, both in VP and putative OT neurones, and these effects were prevented when slices were preincubated in glutamate receptor blockers or nitric oxide synthase blockers, respectively. Moreover, ET _B-mediated effects in SON neurones were also prevented by a gliotoxin compound, and when changes in [Ca ²⁺] _i were prevented with bath-applied BAPTA-AM or thapsigargin. Conversely, intracellular Ca ²⁺ chelation in the recorded SON neurones failed to block ET _B-mediated effects. In summary, our results indicate that ET _B receptor activation in SON astrocytes induces the mobilisation of [Ca ²⁺] _i, likely resulting in the activation of glutamate and nitric oxide signalling pathways, evoking in turn excitatory and inhibitory SON neuronal responses, respectively. Taken together, our study supports an important role for astrocytes in mediating the actions of ETs on the magnocellular neurosecretory system.