Polyamine Fluxes via Hemichannels in Glia: Neuronal Network Regulation

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The polyamines, spermine/spermidine (SP/SD), are released in whole brain from unknown sources during neuronal activity. SP/SD dramatically alter the neuronal network and are neuroprotective against NMDA-induced neurotoxicity and ischemia. Intriguingly, endogenous SP and SD are predominantly stored in astrocytes, not in neurons. Our preliminary data suggest that SP/SD uptake and release by astrocytes may occur through unapposed gap junctions (hemichannels). This finding leads us to the novel working hypothesis that SP/SD are synthesized in neurons and permeate (i) connexin hemichannels in glia to ultimately be stored within the glial syncitium. (ii) Neuronal excitation results in a transient fall of [Ca2+]0 and [H+]0 together with increased [K+]0 that facilitates: (iii) opening of hemichannels in glia and (iv) release of SP from glia to the neuronal environment where it can act on AMPA, kainate and NMDA receptors, (v) Increased extracellular SP relieves residual hemichannel block by divalent cations, thus SP works as a positive feedback signal on glial hemichannels. We propose that SP/SD are signaling molecules that underlie a novel regulatory mechanism of neurons by glia. Furthermore, our preliminary experiments suggest that SP itself may facilitate hemichannel unblock, therefore, neuronal excitation may trigger a cascade, resulting in fast hemichannel opening. Here we ask: (i) what is the mechanism of SP permeation through hemichannels in glia, (ii) how is release of SP regulated under physiological conditions and (iii) what are the functional consequences of SP flux through the astrocytic membrane in a cortical slice preparation? These questions will be addressed by examining the mechanism of SP transport through hemichannels, by examining the effect of SP on heterologously expressed Cx hemichannels and by simultaneous recording from interneurons, astrocytes and principal cells while determining the relationship between opening of hemichannels, SP 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 and to minimizing neuronal damage during K+-spreading depression, stroke and ischemia.