Role of hyperpolarization generated by the Na+ - K+ pump in the trans-synaptic induction of RNA synthesis in sympathetic neurons.

Abstract

1. The mechanism which enables activation of the nicotinic receptors to modify the synthesis of RNA was investigated in the incubated superior cervical ganglion of the rat. RNA labelling with [5-3H] uridine was used in order to screen the effects of varying the ionic environment and altering Na+ channels on the sequence of the three changes in ganglionic RNA synthesis induced by stimulation, i.e. a) an initial decrease, b) an increase during the late stages of stimulation, c) another increase taking place after the end of stimulation. These successive variations were obtained by either repetitive excitation of the preganglionic nerve, or application of ACh or carbachol to the ganglion. 2. The three changes in RNA synthesis mediated by ACh or carbachol were prevented when the KCl concentration of the medium was increased up to 37 mM or when NaCl of the medium was replaced with Tris or sucrose. This confirmed previous indications that the sequence of activity-induced changes is initiated by the transmembrane ionic fluxes mediated by nicotinic activation and not by depolarization per se. 3. Application of aconitine to resting ganglia for 1 h decreased the RNA synthesis to the same extent as a 1 h preganglionic or ACh stimulation. This effect was prevented by a concomitant application of tetrodotoxin (TTX) which also restored the ability of carbachol to modify RNA synthesis. This suggested that the initial decrease in RNA synthesis is caused by the increase in [Na+]i which seems to interfere directly with the transcription process. 4. The increase of RNA synthesis occurring during the late stages of synaptic activation was selectively inhibited by replacing Cl- of the medium with SO4-. On the other hand, the post-stimulation increase was selectively inhibited when the generation of after-hyperpolarization resulting from the electrogenic extrusion of Na+ was prevented by substituting LiCl for NaCl. This indicated that increases in RNA synthesis during and after the stimulation are triggered by different ionic events. 5. An induction of RNA synthesis was obtained, without previous activation of the nicotinic receptors, by incubating the ganglia at rest in conditions which entail the generation of an hyperpolarization resulting from the activation of the Na(+)-K- pump, i.e. low external KCl as well as application of TTX following an aconitine treatment. However, in these cases, the increase in RNA synthesis was delayed by about 2 h as compared to that observed after the end of nicotinic activation.(ABSTRACT TRUNCATED AT 400 WORDS)