Antennal lobes projections

Hansson B. S., Almaas T. J. and Anton S. (1995) Chemical communication in heliothine moths. V. Antennal lobe projection patterns of pheromone-detecting olfactory receptor neurons in the male Heliothis virescens (Lepidoptera Noctuidae). J. Comp. Physiol. A 177, 535-543. 

B.S. Hansson, Antennal lobe projection patterns of pheromone-specific olfactory receptor neurons in moths, in Insect pheromone research new directions, R.T. Carde, A.K. Minks (eds.), Chapman Hall New York, (1997), pp. 164-183. 

Axons of antennal ORCs project through the antennal nerve to enter the brain at the level of the ipsilateral antennal lobe (AL) of the deutocerebrum (52). ORC axons project from the flagellum to targets in the AL, but axons from antennal mechanosensory neurons bypass the AL and project instead to an "antennal mechanosensory and motor center" in the deutocerebrum posteroventral (with respect to the body axis of the animal) to the AL (52, 58, 64). In moths and certain other insect groups, sex-pheromonal information is processed in a prominent male-specific neuropil structure in each AL called the macroglomerular complex (MGC) (16, 52, 64, 65). 

Fig-1 Schematic view of the overall olfactory processing in insects. Pheromones and other semiochemicals are detected by specialized sensilla on the antennae, where the chemical signal is transduced into nervous activity. The olfactory receptor neurons in the semiochemi-cal-detecting sensilla are connected directly to the antennal lobe. Here the semiochemical-derived electrical signals are processed and sent out (through projection neurons) to the protocerebrum. Olfactory information is then integrated with other stimulus modalities, a decision is made, and the motor system is told what to do... 

Christensen T. A. and Hildebrand J. G. (1987) Male-specific, sex pheromone-selective projection neurons in the antennal lobes of the modi Mundue a sexta. J. Comp. Physiol. [A] 160, 553-569. 

Sun X. J., Tolbert L. P. and Hildebrand J. G. (1997) Synaptic organization of the uniglomerular projection neurons of the antennal lobe of the moth Manduca sexta a laser scanning confocal and electron microscopic study. J. Comp. Neurol. 379, 2-20. Takken W., van Loon J. J. A. and Adam W. (2001) Inhibition of host-seeking response and olfactory responsiveness in Anopheles gambiae following blood feeding. J. Insect. Physiol. 47, 303-310. 

Gao Q., Yuan B. and Chess A. (2000) Convergent projections of Drosophila olfactory neurons to specific glomeruli in the antennal lobe. Nat. Neurosci. 3, 780-785. 

Bhalerao S., Sen A., Stocker R. F. and Rodrigues V. (2003) Olfactory neurons expressing identified receptor genes project to subsets of glomeruli within the antennal lobe of Drosophila melanogaster, J. Neurobiol. 54, 577-592. 

Tissot M., Gendre N., Hawken A., Stortkuhl K. F. and Stocker R. F. (1997) Larval chemosensory projections and invasion of adult afferents in the antennal lobe of Drosophila. J. Neurobiol. 32, 281-297. 

Figure 24.1 A surface reconstruction of a male antennal lobe of the moth Spodoptera littoralis. The brain was immunostained with synapsin antibody and optically sectioned using a confocal microscope. Stacks of images were integrated with the software Imaris 2,7 (Bitplane AG, Switzerland) on a Silicon Graphics workstation to obtain surface projections of the lobe. The macroglomerular complex (MGC) is located close to the entrance of the antennal nerve. M, medial D, dorsal (modified from Carlsson et at, 2002). B Synaptic organization of the major types of antennal lobe neurons. Sensory neurons (ORNs) make uniglomerular synapses both directly with projection neurons (PNs) and indirectly via local interneurons (LNs). In addition, local interneurons innervate several glomeruli and generally make inhibitory synapses. Cell bodies of PNs and LNs are located within the antennal lobe.

Figure 24.4 Gray-scaled signals of optical recordings of calcium activity from the antennal lobe of the moth Helicoverpa zea. A, B and C show thresholded (>50 percent of maximum) responses to the pheromone components Z11-16 Ald and Z9-16 Ald and a behavioral antagonist Z11-16 Ac, respectively. D A schematic figure of the organization of the macroglomerular complex in H. zea (from Vickers et al., 1998). The position of the glomeruli coincides with the foci of calcium responses, i.e. response to Z11-16 Ald takes place in the cumulus, Z9-16 Ald in the DM-P glomerulus and Z11-16 Ac in the DM-A glomerulus. These activity patterns corroborate the innervation patterns of functionally identified projection neurons (redrawn from Vickers et al., 1998).

The axon wiring in the insect antennal lobe is anatomically and physiologically similar to that in the mammalian olfactory bulb. Insect ORNs target and synapse with two neurons projection neurons and GABA-mediated local intemeurons. Projection neurons are second-order olfactory intemeurons that transmit the information of odorants into the mushroom body calyx and lateral horn, which is the center for olfactory-related memory formation (Heisenberg... 

Fig. 5 Synaptic convergence onto a single glomerulus of ORN axons expressing the same ORs. Insect ORNs generally express two ORs on the surface of dendrites, a single OR plus a receptor of the Or83b family. The axons of ORNs expressing the same receptor combination synapse with projection neurons in unique glomeruli in the antennal lobe...

As detailed already, the olfactory system conveys odour information initially to the antennal lobe and then further via the projection neurons to the lateral horn and the mushroom bodies. From both these centres, premotor commands are thought to originate. The gustatory system, in contrast, carries sugar information to the suboesophageal ganglion, from where premotor commands likely can be triggered... 

Gao Q, Yuan B, Chess A (2000) Convergent projections of Drosophila olfactory neurons to specific glomeruli in the antennal lobe. Nat Neurosci 3 780-785 Garcia CC, Blair HJ, Seager M, Coulthard A, Tennant S, Buddies M, Curtis A, Goodship JA (2004) Identification of a mutation in synapsin I, a synaptic vesicle protein, in a family with epilepsy. J Med Genet 41 183-186... 

Among all animals, insects have one of the simplest olfactory systems. In particular the first stage of this system, the antennal lobe (AL), has been shown to encode odours. Many experimental data, and in particular from the work of Laurent s group from Caltech , have shown that subsets of AL projection neurons (PNs) - whose activity is projected on higher structures -get synchronized in presence of an olfactory stimulus and that these subsets change in time in an odour-specific manner. Such a transient synchronization encoding scheme or spatio-temporal code has already been reproduced and studied by means of a biologically detailed model of the insect antennal lobe... 

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