Intracellular polyamines enhance astrocytic coupling

Abstract

Spermine (SPM) and spermidine, endogenous polyamines with the ability to modulate various ion channels and receptors in the brain, exert neuroprotective, antidepressant, antioxidant, and other effects in vivo such as increasing longevity. These polyamines are preferably accumulated in astrocytes, and we hypothesized that SPM increases glial intercellular communication by interacting with glial gap junctions. The results obtained in situ, using Lucifer yellow propagation in the astrocytic syncitium of 21-25-day-old rat CA1 hippocampal slices, showed reduced coupling when astrocytes were dialyzed with standard intracellular solutions without SPM. However, there was a robust increase in the spreading of Lucifer yellow through gap junctions to neighboring astrocytes when the cells were patched with intracellular solutions containing 1 mM SPM, a physiological concentration in glia. Lucifer yellow propagation was inhibited by gap junction blockers. Our findings show that the glial syncitium propagates SPM through gap junctions and further indicate a new role of polyamines in the regulation of the astroglial network under both normal and pathological conditions.

Figure 1. Microscopic images of Lucifer yellow propagation in the astrocytic syncytium of adult rat

A. Two cells were coupled in the control experiment using 117 mM K+ gluconate-based intracellular solution containing no spermine, while in B. injection of the cell with the same intracellular solution enriched by 1 mM spermine revealed a robust increase in cell-to-cell coupling (18 cells stained total). C. The SPM-induced increase in coupling was abolished in slices pretreated with 200 µM carbenoxolone. Images were taken 10 min after loading the fluorescent dye through the patch pipette. Note the different scale bar in B.

Figure 2. Intracellular spermine increases Lucifer yellow coupling between astrocytes located in the stratum radiatum of CA1 hippocampus

A. Cell-to-cell coupling is not dependent on the membrane potential either in the control group (white-filled squares) or in the spermine-treated group (black-filled circles) B. Intracellular loading of fluorescent dye (2 mM Lucifer yellow) revealed significant difference between the number of cells coupled in the control group (2.7±0.2) and in the spermine-treated group (11±1.4). The asterisk indicates a significant difference between control and SPM-treated groups (p<0.01, N=15 in each group).