Optical Recording of Neuronal Activity in the Insect Central Nervous System: Odorant Coding by the Antennal Lobes of Honeybees

Voltage‐sensitive dyes and activity‐dependent intrinsic optical signals were used to study the spatio‐temporal activity in the antennal lobes of honeybees. The intrinsic signals are somewhat slower than the dye signals but show a 10‐fold larger intensity change. These intrinsic signals consist of at least two components—one is wavelength‐independent and the other strongly wavelength‐dependent, with a maximum at ∼500 nm. Local inhibitory connections within the antennal lobes were examined by recording the activity elicited by an electrical stimulus to the antennal nerve of a slice preparation before and after applying picrotoxin to manipulate GABAergic inhibitory synapses. The inhibition starts with a delay of ∼10 ms after onset of the response and has at least two components. The spatial distribution of the inhibition is extremely inhomogeneous, with areas of small inhibition adjacent to areas of large inhibition. Thus inhibitory interactions in the antennal lobes are not evenly distributed among the glomerular organization. Stimulation of an in vivo preparation with an odour yields a spatially restricted activity. However, the spatial map appears highly dynamic in time because the size of the activated area is a function of the time during and after the stimulus.