We begin with a brief description of the neurons in the insect antennal lobe and then draw comparisons between different insect species as well as to vertebrates. We then review physiological properties. Discussion about the significance of morphological and physiological findings for olfactory coding is discussed thereafter.
Morphological Elements ofthe Insect Olfactory System
Odors are recognized by primary olfactory sensory neurons (OSNs), which are located on the insect antenna and express odorant receptor genes that encode the odorant receptors (ORs). These are seven transmembrane domain receptor proteins that interact with odor molecules and transduce odorant binding to cellular excitation. The organization of the insect olfactory system is shown in Figure 13.1, using the honeybee as an example. The OSNs send their axons to the olfactory neuropil, the insect antennal lobe (Figure 13.1a, b), which consists of discrete neuropil structures called olfactory glomeruli. Each OSN expresses a single OR gene, and all OSNs expressing the same OR converge onto a common glomerulus. A glomerulus collects OSNs of only one type. Thus, the glomerulus acts as a "collecting basket" of OSNs with similar odor response profiles. This correspondence has been shown for Drosophila melanogaster (Vosshall 2001) and is also assumed for other insect species, but has not yet been experimentally proven. However, a few cases of 1:2 and of 2:1 innervation ratios in D. melanogaster have been described as well. A glomerulus receives not only the input from OSNs, but contains a highly ordered synaptic organization including synaptic microcircuits among OSNs, local interneurons, and projection neurons. The cell bodies of local interneurons and projection neurons are located outside of the glomeruli, in the periphery of the antennal lobe. Local inter-neurons branch exclusively within the antennal lobe and, in honeybees, can be further divided into two classes (Figure 13.1c): homogeneous local inter-neurons globally innervate many glomeruli, whereas heterogeneous local inter-neurons innervate densely a single glomerulus and diffusely a few others (Fonta et al. 1993). Projection neurons represent the antennal lobe output neurons and relay the olfactory information to higher processing centers, such as the lateral protocerebrum and the mushroom bodies. Projection neurons are either uni-glomerular neurons, and thus collect synaptic input in just one glomerulus, or multiglomerular neurons (Figure 13.1d). Uniglomerular projection neuron axons innervate both the lateral protocerebrum and the mushroom bodies, where they diverge onto many Kenyon cells, which in turn are read out by mushroom body extrinsic neurons. Multiglomerular projection neurons send their axons only to the lateral protocerebrum and around the a-lobe, bypassing the mushroom bodies. However, projection neurons in honeybees also have small branches at their exit points of the antennal lobe, which may form connections between cells outside their innervated glomeruli (Müller et al. 2002). The computational function of these lateral pathways is not yet understood.
Distinct classes of neurotransmitters mediate communication between different neuron types. OSNs release acetylcholine to excite local interneurons or projection neurons (Homberg and Müller 1999). Local interneurons are mainly inhibitory and use GABA and/or histamine to inhibit projection neurons, other
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