Regulation of expression of functional voltage-gated ion channels for inward currents was studied in Schwann cells in organotypic cultures of dorsal root ganglia from E19 mouse embryos maintained in serum-free medium. Of the Schwann cells that did not contact axons, 46.5% expressed T-type Ca2+ conductances (ICaT). Two days or more after excision of the ganglia, and consequent disappearance of neurites, ICaT were detectable in only 10.9% of the cells, and the marker 04 disappeared. On Schwann cells deprived of neurons, T- (but not L-) type Ca2+ conductances were re-induced by weakly hydrolysable analogues of cAMP, and by forskolin (an activator of adenylyl cyclase) after long-term treatment (4 days). With CPT cAMP (0.1-2 mM), 8Br cAMP, db cAMP or forskolin (0.01 or 0.1 mM), the proportion of cells with ICaT was not significantly different from the proportion in the cultures with neurons. These agents also induced expression in some cells of tetrodotoxin-resistant Na+ currents, which were rarely induced by neurons, but 04 was not re-induced by cAMP analogue treatments that re-induced ICaT. Inward currents (Ba2+ or Na+) were partly restored (P < 0.05) on Schwann cells cultured for 6-7 days beneath a filter bearing cultured neurons. In contrast, addition of neuron-conditioned medium was ineffective. The results suggest that neurons activate, via diffusible and degradable factors, a subset of Schwann cell cAMP pathways leading to expression of IcaT, and activate additional non-cAMP pathways that lead to expression of 04.

The development, plasticity and regeneration of the nervous system appear to require extensive interactions between the neurones and the glial cells. Interactions between the axons and Schwann cells of peripheral nerve are relatively amenable to study, and it has been shown that signals from the axons influence expression of genes in the Schwann cells. Notably, the axon determines whether or not the ensheathing Schwann cell synthesizes molecules necessary for the formation of myelin or specific to non-myelinating Schwann cells (see Jessen & Mirsky, 1991, and Discussion). Changes in the expression of proteins that form functional ion channels can be quantified by measuring membrane currents. We have shown previously that in organotypic cultures of mouse dorsal root ganglia, isolated Schwann cells (i.e. that do not contact axons) possess two types of K+ conductance, inactivating and sustained, responsible for currents that we designate IA and IK (see Fig. 1A). However, on Schwann cells in contact with axons, only IK is present (Despeyroux, Amedee & Coles, 1994). One interpretation is that the axonal contact selectively downregulates IA conductance. It has been found that conditions that increase the intracellular concentration of cAMP in Schwann cells can reproduce many of the effects of axons on Schwann cells. We now report the effect of cAMP analogues on IA. We show that in the absence of neurones a long-term elevation of cAMP in mouse Schwann cells partially mimics the effect of axonal contact in selectively downregulating IA and that this response involves protein kinase A. This appears to be the first demonstration of downregulation of the expression of a functional K+ conductance by cAMP.

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It is well established that neurons regulate the properties of both central and peripheral glial cells. Some of these neuro-glial interactions are modulated by the pattern of neuronal electrical activity. In the present work, we asked whether blocking the electrical activity of dorsal root ganglion (DRG) neurons in vitro by a chronic treatment with tetrodotoxin (TTX) would modulate the expression of the T-type Ca(2+) channel by mouse Schwann cells. When recorded in their culture medium, about one-half of the DRG neurons spontaneously fired action potentials (APs). Treatment for 4 days with 1 microM TTX abolished both spontaneous and evoked APs in DRG neurons and in parallel significantly reduced the percentage of Schwann cells expressing Ca(2+) channel currents. On the fraction of Schwann cells still expressing Ca(2+) channel currents, these currents had electrophysiological parameters (mean amplitude, mean inactivation time constant, steady-state inactivation curve) similar to those of control cultures. Co-treatment for 4 days with 1 microM TTX and 2 mM CPT-cAMP, a cAMP analogue that induces the expression de novo of Ca(2+) channel currents in Schwann cells deprived of neurons, maintained the percentage of Schwann cells expressing Ca(2+) channel currents, showing that TTX does not directly affect the expression of Ca(2+) channel currents by Schwann cell. We conclude that blocking spontaneous activity of DRG neurons in vitro downregulates Ca(2+) channel expression by Schwann cells. These results strongly suggest that DRG neurons upregulate Ca(2+) channel expression by Schwann cells via the release of a diffusible factor whose secretion is dependent on electrical activity.