Differential regulation of neuronal nicotinic ACh receptor subunit genes in cultured neonatal rat sympathetic neurons: Specific induction of α7 by membrane depolarization through a Ca2+/calmodulin-dependent kinase pathway

Abstract

We have examined the regulation of neuronal nicotinic ACh receptor (nAChR) genes and ACh-evoked currents by neonatal rat sympathetic neurons developing in culture. These neurons contain 5 nAChR transcripts: α3, α5, α7, β2, and β4. When developing in culture, the neurons express 4 of these transcripts, α3, α5, β2, and β4, at levels similar to those in neurons developing in vivo: α3 mRNA levels increase two-to threefold over the first week, whereas the levels for α5, β2, and β4 remain essentially constant. In contrast, α7 mRNA levels drop by 60-75% within the first 48 hr and remain low. We show that during the first week, the ACh-evoked current densities on these cultured neurons increase twofold and correlate well with the increase in α3 mRNA levels. Depolarizing the neurons with 40 mM KCl for 1-2 d upregulates the α7 gene; this specific change in α7 mRNA level correlates with an increase in α-bungarotoxin (α-BTX) binding on the surface of the neurons. Depolarization has little effect on the expression of the other four transcripts, or on the magnitude or kinetics of the ACh-evoked currents. Furthermore, activators or inhibitors of protein kinase A (PKA), protein kinase C (PKC), or tyrosine kinase do not affect nAChR transcript levels in these cultured neurons. The effect of membrane depolarization on α7 expression is a result of Ca influx through L-type Ca channels, and we show that α7 is upregulated through a Ca2+/calmodulin-dependent protein kinase (CaM kinase) pathway. The identification of CaM kinase as a link between activity and neurotransmitter receptor expression may indicate a novel mechanism that underlies some forms of synaptic plasticity.