Ectopic sodium channel expression decreases excitability of Drosophila Kenyon cells

Abstract

Neurons stabilize their physiological properties in part by homeostatic compensation between different ion channel conductances. However, little is known about this process in the central brain in vivo. We studied this problem in Kenyon cells, the third-order olfactory neurons in the fruit fly Drosophila that store olfactory associative memories. We investigated whether Kenyon cells regulate their excitability homeostatically by testing how their activity is affected by ectopic expression of the bacterial voltage-gated sodium channel NaChBac, a manipulation previously reported to increase neuronal excitability in other systems. Surprisingly, NaChBac expression decreases Kenyon cell excitability. Whether expressed constitutively (throughout development) or only in the adult, NaChBac expression in Kenyon cells suppresses Kenyon cell spiking, reduces odour-evoked calcium influx, and prevents olfactory aversive conditioning. However, odour-evoked calcium influx in Kenyon cell axons and dendrites (but not somata) is normal after 4-day adult-only NaChBac expression (but not 2-day adult-only or constitutive expression), suggesting limited homeostatic regulation of calcium influx that is prevented by developmental NaChBac expression. NaChBac expression also decreases expression of endogenous voltage-gated sodium channels (Para) in the spike initiation zone, suggesting homeostatic regulation of sodium influx. Indeed, a compartmental model best fits the data when the exogenous NaChBac conductance is accompanied by a decrease in endogenous sodium conductance. These results suggest that manipulating neuronal activity with ion channels can have unexpected effects depending on compensatory plasticity. KEY POINTS: Neurons stabilize their physiological properties through compensation between key ion channels, but little is known about this process in the central brain in vivo. Here we test homeostatic compensation in vivo in Drosophila Kenyon cells, the neurons that store olfactory associative memories, by ectopically expressing NaChBac, a bacterial voltage-gated sodium channel commonly used to increase neuronal excitability. Surprisingly, NaChBac expression in Kenyon cells decreases their excitability: it reduces expression of endogenous sodium channels, prevents spiking, reduces odour-evoked calcium influx, and impairs learning. The electrophysiological phenotype is reproduced in a compartmental model. However, odour-evoked dendritic/axonal calcium influx (but not spiking or learning) returns to normal if NaChBac is expressed for 4 days only in adults (not during development), suggesting limited homeostatic regulation of voltage-gated calcium influx. These results show that manipulating ion channels can have unexpected effects depending on homeostatic compensation.

Keywords: Kenyon cell; NaChBac; drosophila; homeostatic plasticity; ion channel; mushroom body; sodium channel.