State‐dependent changes in astrocyte regulation of extrasynaptic NMDA receptor signalling in neurosecretory neurons

Abstract

Despite the long-established presence of glutamate NMDA receptors at extrasynaptic sites (eNMDARs), their functional roles remain poorly understood. Factors influencing the concentration and time course of glutamate in the extrasynaptic space, such as the topography of the neuronal–glial microenvironment, as well as glial glutamate transporters, are expected to affect eNMDAR-mediated signalling strength. In this study, we used in vitro and in vivo electrophysiological recordings to assess the properties, functional relevance and modulation of a persistent excitatory current mediated by activation of eNMDARs in hypothalamic supraoptic nucleus (SON) neurons. We found that ambient glutamate of a non-synaptic origin activates eNMDARs to mediate a persistent excitatory current (termed tonic I(NMDA)), which tonically stimulates neuronal activity. Pharmacological blockade of GLT1 astrocyte glutamate transporters, as well as the gliotoxin α-aminodadipic acid, enhanced tonic I(NMDA) and neuronal activity, supporting an astrocyte regulation of tonic I(NMDA) strength. Dehydration, a physiological challenge known to increase SON firing activity and to induce neuroglial remodelling, including reduced neuronal ensheathment by astrocyte processes, resulted in blunted GLT1 efficacy, enhanced tonic I(NMDA) strength, and increased neuronal activity. Taken together, our studies support the view that glial modulation of tonic I(NMDA) activation contributes to regulation of SON neuronal activity, contributing in turn to neuronal homeostatic responses during a physiological challenge.

ATP stimulates rat hypothalamic sympathetic neurons by enhancing AMPA receptor-mediated currents

“…We found that P2 receptor activation by ATP did not induce significant changes in either the frequency, magnitude, or time course of glutamate receptor-mediated sEPSCs, suggesting that ATP did not affect the degree of glutamate release (i.e., no change in PSC frequency) or the magnitude/kinetics of postsynaptic receptors (i.e., no change PSC amplitude/kinetics) in PVN-RVLM neurons. This is different from previous reports in supraoptic nucleus (SON) and PVN magnocellular neurosecretory neurons showing that ATP increased glutamate EPSC frequency and amplitude (Gordon et al 2005;Vavra et al 2011), suggesting that ATP actions in the hypothalamus may be cell type dependent.…”

“…Under basal conditions, however, we found the extrasynaptic glutamate current to be mediated mostly by NMDA (but not AMPA) receptors (Fleming et al 2011;Naskar and Stern 2014;Potapenko et al 2012Potapenko et al , 2013. It is possible, however, that activation of P2 receptors increases extrasynaptic AMPA receptor density and/or glutamate sensitivity, contributing under this condition to glutamate-mediated extrasynaptic actions.…”

“…Because astrocytes constitute a major source of ATP within the SON and PVN, and this process can be evoked by physiologically relevant signals, including norepinephrine and dendritically-released vasopressin (Gordon et al 2005;Haam et al 2014), it will be important to assess in future studies whether astrocytes are key participants in osmotically driven changes PVN neuronal excitability and sympathoexcitatory regulation.…”

Organization, control and function of extrasynaptic NMDA receptors

“…However, the origin of the so-called ambient glutamate that allows such tonic activation is still unclear, and little evidence for the source exists, aside from the observation that it is of non-synaptic origin [58,59]. Interestingly, the amplitude of NMDAR-mediated tonic current is strongly enhanced either by blocking glutamate reuptake through the glutamate transporter 1 (GLT1) [58,60], which is predominantly present on glial cells and mediates 95% of glutamate transport, or when slices are challenged with glial toxins [60]. Additionally, a recent study showed that the extent of NMDAR-mediated tonic current depends on glial coverage in the supraoptic nucleus of the hypothalamus [60].…”

“…Interestingly, the amplitude of NMDAR-mediated tonic current is strongly enhanced either by blocking glutamate reuptake through the glutamate transporter 1 (GLT1) [58,60], which is predominantly present on glial cells and mediates 95% of glutamate transport, or when slices are challenged with glial toxins [60]. Additionally, a recent study showed that the extent of NMDAR-mediated tonic current depends on glial coverage in the supraoptic nucleus of the hypothalamus [60]. Hence, the activity of NMDARs that mediate tonic current probably relies on the regulation of extrasynaptic glutamate tone by glia.…”