Cellular Physiology of the Honey Bee Brain

Abstract

Membrane-bound ion channels determine the electrical activity of ­excitable cells. In this respect honey bee neurons within the olfactory pathways are among the physiologically best studied insect cells. Several ionic currents were characterized from identified central neurons in vitro, in particular mushroom body Kenyon cells (KCs) and antennal lobe (AL) neurons. They express voltage-sensitive Na+ and Ca2+ currents that depolarize the neurons upon activation. Outward K+ currents are rather diverse. At least four types exist: a delayed rectifier, a rapidly inactivating A-type, a slowly inactivating and a Ca2+-dependent K+ current. This diversity of K+ channels determines the threshold and shapes of single spikes and spike trains. Based on sequence analyses the honey bee genome contains genes coding for nine nicotinic acetylcholine receptor α-subunits, three GABA receptor subunits, one glutamate-chloride channel, three NMDA receptor subtypes, and two histamine-chloride channels. Acetylcholine-, GABA-, and glutamate-induced currents have been physiologically characterized. The ionotropic nicotinic cholinergic receptor is one of the major excitatory receptors of the olfactory pathway. It is involved during olfactory learning and therefore a good candidate for inducing learning-dependent synaptic plasticity (see Chap. 3.3). GABA-induced Cl− currents provide the major inhibitory system. In addition, glutamate-sensitive Cl− channels provide a parallel inhibitory network within the honey bee ALs. KCs express functional cation-selective AMPA-like receptors, whereas no physiological data exist on functioning NMDA-like receptors. Integrating the cell physiological data into a working model to explain experience-dependent neuronal plasticity is challenging, because the interactions of the various currents and signaling cascades and their contribution to experience-dependent plasticity remain to be analysed.