Abstract
Magnocellular neurosecretory cells (MNCs) of the supraoptic nucleus (SON) project their axons to the posterior pituitary gland where they release the hormones, vasopressin and oxytocin, into the periphery in proportion to their action potential firing rate. These hormones control plasma osmolality by regulating water and sodium levels; vasopressin inhibits diuresis and oxytocin promotes natriuresis. In the SON, glial cells have a fundamental role in regulating the activity of MNCs and one mechanism is by taking up glutamate via the glial glutamate transporter-1 (GLT-1). Extrasynaptic N-methyl-D-aspartate receptors (eNMDARs) are ionotropic glutamatergic receptors that, when activated by the binding of glutamate and glycine, mediate tonic (persistent) forms of excitatory current. The strength of the eNMDAR-mediated tonic current is influenced by glial glutamate uptake.
In vivo electrophysiology experiments were conducted on urethane-anaesthetised female Sprague-Dawley rats to investigate the presence of tonically-activated eNMDARs in the SON and its potential regulation by glial glutamate transporters. Experiments were conducted in rats that were euhydrated (control; free access to water) or dehydrated (48 hr water deprivation), a condition that is known to increase plasma osmolality, to elevate ambient levels of glutamate and to increase MNC firing rate. Memantine, an eNMDAR blocker, had no effect on the firing rate of SON MNCs in control conditions but decreased the firing rate of SON MNCs recorded from dehydrated rats. This suggests that, in basal conditions, SON MNC eNMDARs are not activated (or present) but are activated during dehydration. Memantine did, however, inhibit SON MNC firing rate recorded from euhydrated rats when firing rate had been increased by the GLT-1 blocker, dihydrokainic acid (DHK). This suggests that, in control conditions, eNMDARs are not active due to glial glutamate uptake. To determine whether dehydration-induced activation of SON eNMDARs was due to a change in SON GLT-1 expression, Western blot experiments were conducted. Western blots showed no change in SON GLT-1 expression; hence, it may be other mechanisms, such as glial retraction (a phenomenon that occurs in response to increases in plasma osmolality), that contribute to enhanced eNMDARs activation in dehydration.
This study shows that glial glutamate uptake in the SON prevents the activation of MNC eNMDARs in basal conditions. eNMDARs are active in dehydration, which may contribute to the increased firing rate of SON MNCs to increase vasopressin and oxytocin hormone secretion and thus return plasma osmolality back to physiological range.