Effect of GABAergic inhibition on odorant concentration coding in mushroom body intrinsic neurons of the honeybee.

Kenyon cells, the intrinsic neurons of the insect mushroom body, have the intriguing property of responding in a sparse way to odorants. Sparse neuronal codes are often invariant to changes in stimulus intensity and duration, and sparse coding often depends on global inhibition. We tested if this is the case for honeybees' Kenyon cells, too, and used in vivo Ca²⁺ imaging to record their responses to different odorant concentrations. Kenyon cells responded not only to the onset of odorant stimuli (ON responses), but also to their termination (OFF responses). Both, ON and OFF responses increased with increasing odorant concentration. ON responses were phasic and invariant to the duration of odorant stimuli, while OFF responses increased with increasing odorant duration. Pharmacological blocking of GABA receptors in the brain revealed that ionotropic GABA(A) and metabotropic GABA(B) receptors attenuate Kenyon cells' ON responses without changing their OFF responses. Ionotropic GABA(A) receptors attenuated Kenyon cell ON responses more strongly than metabotropic GABA(B) receptors. However, the response dynamic, temporal resolution and paired-pulse depression did not depend on GABA(A) transmission. These data are discussed in the context of mechanisms leading to sparse coding in Kenyon cells.