title: Glial regulation of extrasynaptic NMDA receptor-mediated excitation of supraoptic nucleus neurones during dehydration.
abstract: Magnocellular neurosecretory cells (MNCs) of the supraoptic nucleus (SON) project to the posterior pituitary gland where they release the hormones, vasopressin and oxytocin into the circulation to maintain plasma osmolality. Hormone release is proportionate to SON MNC action potential (spike) firing rate. When activated by ambient extracellular glutamate, extrasynaptic NMDA receptors (eNMDARs) mediate a tonic (persistent) depolarisation to increase the probability of action potential firing. In the present study, in vivo single-unit electrophysiological recordings were made from urethane-anaesthetised female Sprague-Dawley rats to investigate the impact of tonic eNMDAR activation on MNC activity. Water deprivation (for up to 48 h) caused an increase in the firing rate of SON MNCs that was associated with a general increase in post-spike excitability. To determine whether eNMDAR activation contributes to the increased MNC excitability during water deprivation, memantine, which preferentially blocks eNMDARs, was administered locally into the SON by microdialysis. Memantine significantly decreased the firing rate of MNCs recorded from 48-h water-deprived rats but had no effect on MNCs recorded from euhydrated rats. In the presence of the glial glutamate transporter-1 (GLT-1) blocker, dihydrokainate, memantine also reduced the MNC firing rate in euhydrated rats. Taken together, these observations suggest that GLT-1 clears extracellular glutamate to prevent the activation of eNDMARs under basal conditions and that, during dehydration, eNMDAR activation contributes to the increased firing rate of MNCs.
authors.affiliation: Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand.
authors.forename: N
authors.initials: N
authors.lastname: Joe
authors.forename: V
authors.initials: V
authors.lastname: Scott
authors.forename: C H
authors.initials: CH
authors.lastname: Brown
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D003681:Dehydration / Q000503:physiopathology*; D018691:Excitatory Amino Acid Antagonists / Q000008:administration & dosage / Q000494:pharmacology; D027342:Excitatory Amino Acid Transporter 2 / Q000037:antagonists & inhibitors*; D005260:Female; D007608:Kainic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D008559:Memantine / Q000008:administration & dosage / Q000494:pharmacology; D008845:Microinjections; D055099:Neuroendocrine Cells / Q000502:physiology*; D009457:Neuroglia / Q000502:physiology*; D051381:Rats; D016194:Receptors, N-Methyl-D-Aspartate / Q000819:agonists / Q000037:antagonists & inhibitors / Q000502:physiology*; D013495:Supraoptic Nucleus / Q000166:cytology* / Q000187:drug effects / Q000502:physiology*; D014868:Water Deprivation / Q000502:physiology
chemical_list: D018691:Excitatory Amino Acid Antagonists; D027342:Excitatory Amino Acid Transporter 2; D016194:Receptors, N-Methyl-D-Aspartate; C493395:Slc1a2 protein, rat; C020889:dihydrokainic acid; D007608:Kainic Acid; D008559:Memantine
keywords: dehydration; oxytocin; vasopressin; water deprivation