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: alpha 3, alpha 5, alpha 7, beta 2, and beta 4. When developing in culture, the neurons express 4 of these transcripts, alpha 3, alpha 5, beta 2, and beta 4, at levels similar to those in neurons developing in vivo: alpha 3 mRNA levels increase two- to threefold over the first week, whereas the levels for alpha 5, beta 2, and beta 4 remain essentially constant. In contrast, alpha 7 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 alpha 3 mRNA levels. Depolarizing the neurons with 40 mM KCl for 1–2 d upregulates the alpha 7 gene; this specific change in alpha 7 mRNA level correlates with an increase in alpha-bungarotoxin (alpha-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 alpha 7 expression is a result of Ca2+ influx through L-type Ca2+ channels, and we show that alpha 7 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.
