Sodium channel expression in optic nerve astrocytes chronically deprived of axonal contact

Abstract

Immunocytochemical and electrophysiological methods were used to examine the effect of retinal ablation on the expression of sodium channels within optic nerve astrocytes in situ and in vitro. Enucleation was performed at postnatal day 3 (P3), and electron microscopy of the enucleated optic nerves at P28-P40 revealed complete degeneration of retinal ganglion axons, resulting in optic nerves composed predominantly of astrocytes. In contrast to control (non-enucleated) optic nerve astrocytes, which exhibited distinct sodium channel immunoreactivity following immunostaining with antibody 7493, the astrocytes in enucleated optic nerves did not display sodium channel immunoreactivity in situ. Cultures obtained from enucleated optic nerves consisted principally (greater than 90%) of glial fibrillary acidic protein (GFAP)+/A2B5- ("type-1") astrocytes, as determined by indirect immunofluorescence; GFAP+/A2B5+ ("type-2") astrocytes were not present, nor were GFAP-/A2B5+ (O-2A) progenitor cells. Sodium channel immunoreactivity was not present in GFAP+/A2B5- astrocytes obtained from enucleated optic nerves; in contrast, GFAP+/A2B5- astrocytes from control optic nerves exhibited 7493 immunostaining for the first 4-6 days in culture. Sodium current expression, studied using whole-cell patch-clamp recording, was attenuated in cultured astrocytes derived from enucleated optic nerves. Whereas 39 of 50 type-1 astrocytes cultured from intact optic nerves showed measurable sodium currents at 1-7 days in vitro, sodium currents were present in only 6 of 38 astrocytes cultured from enucleated optic nerves. Mean sodium current densities in astrocytes from the enucleated optic nerves (0.66 +/- 0.3 pA/pF) were significantly smaller than in astrocytes from control optic nerves (7.15 +/- 1.1 pA/pF). The h infinity-curves of sodium currents were similar in A2B5- astrocytes from enucleated and control rat optic nerves. These results suggest that there is neuronal modulation of sodium channel expression in type-1 optic nerve astrocytes, and that, following chronic loss of axonal association in vivo, sodium channel expression is down-regulated in this population of optic nerve astrocytes.

Voltage‐dependent sodium and potassium currents in cultured trout astrocytes

“…For rat astrocytes the importance of an ongoing neuronal influence on astroglial sodium current expression indeed has been demonstrated experimentally: thus an upregulation of sodium channel expression was effected by co-culture of optic nerve astrocytes with neurons, while transection of the optic nerve resulted in a severe loss of astroglial sodium current expression (Barres et al, 1990b;Minturn et al, 1992). Corresponding data for fish astrocytes are still lacking, though in view of the remarkable capacity of fish CNS fiber tracts for regenerative neuritic outgrowth such an approach would be of particular interest.…”

Intrinsic optical signals in the rat optic nerve: Role for K+ uptake via NKCC1 and swelling of astrocytes

“…We examined IOSs in the enucleated optic nerve to investigate the contribution of astrocytes. Enucleation of the retina at postnatal day 1 results in an optic nerve in the adult that is principally composed of astrocytes and is free of axons, oligodendrocytes, and myelination (Privat et al, 1981;Minturn et al, 1992). Obviously, in this preparation axonal stimulation cannot be used.…”

Schwann cells modulate sodium channel expression in spinal sensory neurons in vitro

“…A strong modulatory effect on sodium channel expression in cultured spinal cord astrocytes is exerted by DRG neurons, both when co-cultured with these astrocytes and via a factor in DRG neuron-conditioned medium (Thio et al, 1993). Modulation of sodium channel expression in cultured optic nerve astrocytes by retinal ganglion cells has also been described (Barres et al, 1990;Minturn et al, 1992). These observations, together with results presented here, clearly demonstrate that neuronal influences can modulate ion channel expression in glial cells, and that conversely, Schwann cells and astrocytes can modulate the expression of ion channels in neurons.…”