Glial polyamine regulation of the neuronal-glial network

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Extracellular spermine (SPM) facilitates brain slices and is neuroprotective against NMDA-induced neurotoxicity. Intriguingly, we have found that the polyamine SPM in the hippocampus is stored predominantly in and is released from astrocytes (during electrical depolarization, ischemia, gliotoxin). This leads us to suggest that endogenous glial SPM is released from depolarized glia to neighboring neurons where it plays a crucial role in regulating neuronal activity. Our preliminary data show that SPM (i) selectively blocks fast glutamate receptors (Ca2+-permeable AMPA receptors) on interneurons and (ii) potentiates pyramidal cells from extracellular sites. Therefore, selective blockade of interneurons together with potentiation of pyramidal cells may be a major mechanism for SPM/glia-dependent regulation of the neuronal network. Our preliminary data lead to the novel hypothesis that the gUal-neuronal relationship is based in part by extracellular polyamine fluxes between these cells. Our working hypothesis is that SPM accumulated in glial cells is released via (i) unopposed hemi-gap channels (hemichannels) from depolarized gila to neurons and acts from (ii) outside the neuronal receptor-channels to modulate neuronal activity. Specifically, during neuronal excitation (and ischemia) a transient fall of [H+]o and [Ca2+]otogether with increased [K+]o will depolarize glia and facilitate both the release and the effect of endogenous SPM. At higher concentrations, SPM depresses all kinds of glutamate receptors, resulting in a decrease of neuronal Ca 2+ entry through AMPA and NMDA receptors which may protect neurons against Ca2+-damage. Loss of SPM in glia leads to relief of rectification of glial K+-inwardly rectifying (Kir) channels, this may additionally protect neurons by removing excess [K+]ofrom brain to blood vessels, the "sinks" to which astrocytes are attached by endfeet. Thus, spermine is one of the major links between glia and neurons and if efficiently accumulated in glia, may be a basis of neuroprotection. Here we ask: how is SPM released from glia and how does this SPM regulate the neuronal network in whole brain? These questions will be addressed by examining the mechanism of SPM transport through hemichannels, by examining the effect of SPM on heterologously expressed Kir6.1/SUR1 and AMPARs and by simultaneous recording from interneurons, astrocytes and pyramidal cells while determining the relationship between opening of hemichannels, SPM release and alterations in neuronal excitability. These studies will provide a novel mechanism for understanding the newly elucidated role of glial cells in the regulation of neuronal activity.