Honeybee Kenyon cells are regulated by a tonic GABA receptor conductance

The higher cognitive functions of insects are dependent on their mushroom bodies (MBs), which are particularly large in social insects such as honeybees. MB Kenyon cells (KCs) receive multisensory input and are involved in associative learning and memory. In addition to receiving sensory input via excitatory nicotinic synapses, KCs receive inhibitory GABAergic input from MB feedback neurons. Cultured honeybee KCs exhibit ionotropic GABA receptor currents but the properties of GABA-mediated inhibition in intact MBs are currently unknown. Here, using whole-cell recordings from KCs in acutely-isolated honeybee brain, we show that KCs exhibit a tonic current that is inhibited by picrotoxin but not by bicuculline. Bath application of GABA (5 µM) and taurine (1 mM) activate a tonic current in KCs, but L-glutamate (0.1-0.5 mM) has no effect. The tonic current is strongly potentiated by the allosteric GABAA receptor modulator pentobarbital, and is reduced by inhibition of Ca(2+) channels with Cd(2+) or nifedipine. Noise analysis of the GABA-evoked current gives a single-channel conductance value for the underlying receptors of 27 ± 3 pS, similar to that of RDL receptors. The amount of injected current required to evoke action potential firing in KCs is significantly lower in the presence of picrotoxin. KCs recorded in an intact honeybee head preparation similarly exhibit a tonic GABA receptor conductance that reduces neuronal excitability, a property that is likely to contribute to the sparse coding of sensory information in insect MBs.