Neurons in the antennal lobe are primarily olfactory receptor neurons (their axons), local interneurons whose neurites are entirely confined to the antennal lobe, and antennal lobe projection neurons, which represent the output of the antennal lobe. A small number of efferent processes of neurons from other brain areas are also present.

Olfactory receptor neurons

Olfactory receptor neurons projecting to the antennal lobe are originating from the antennae in most insects and also from the maxillary palp (a moth part) in some more derived groups such as Lepidoptera and Diptera. Their somata are located in proximity to the sensillar structures. Olfactory receptor neurons expressing the same odorant receptor project to the same glomeruli [Vosshall et al., 2000]．Generally, a large number of olfactory recpetor neurons converge onto a projection neuron and synapses are located in the outer “cap” of the glomerulus.

Projection neurons

Projection neurons have dendrites in the antennal lobe and axons projecting to the protocerebrum. There are projection neurons whose dendrites are confined to a single glomerulus (uniglomerular) and projection neurons whose dendrites innervate more than one glomerulus (multiglomerular). In general, there are three projection tracts to the protocerebrum called antenno-cerebral tracts (ACTs) [Galizia & Rössler, 2010]．In the tobacco hornworm moth, a detailed analysis of these tracts has been made [Homberg et al., 1988]. In the silkmoth, it has also been shown that there is a topographic relationship between glomeruli, ACTs and projection areas in the protocerebrum [Kanzaki et al., 2003]．In most insects, uniglomerular projection neurons are the predominant type. In the locust (which has an unusually large number of glomeruli), multiglomerular projection neurons are typical.

Local interneurons

The neurites of antennal lobe interneurons are entirely confined to the antennal lobe. These cells are considered to be the main element of information processing in the antennal lobe. In the silkmoth, the first extensive morphological survey of local interneurons was provided. In combination with immunohistochemistry for GABA (an inhibitory neurotransmitter), it could be shown that local interneurons innervating many glomeruli are inhibitory. The study also showed a diversity of types of local interneurons [Seki & Kanzaki, 2008]．In the honey bee, a social insect, the morphology of local interneurons appears to be more complex.

Efferent innervation

Efferent innervation of the antennal lobe is less well studied that the other neuronal constituents. This is presumably due to the fact that the number of cells efferently innervating the antennal lobe is small.

In the honey bee, some putatively octopaminergic ventral unpaired median neurons whose somata are located in the suboesophageal ganglion, innervate the antennal lobe. For one of them, it has been shown that its activation can replace the unconditioned stimulus in classical olfactory conditioning [Hammer , 1997]．

Efferent innervation by serotonergic neurons is known for a number of insect species [Dacks et al., 2006], including the silkmoth [Hill et al., 2002](serotonin immunoreactive antennal lobe feedback neuron in silkmoth)．

References

Dacks AM, Christensen TA, Hildebrand JG (2006) Phylogeny of a serotonin-immunoreactive neuron in the primary olfactory center of the insect brain. J Comp Neurol 498:727-746.

Galizia CG, Rössler W (2010) Parallel olfactory systems in insects: anatomy and function. Annu Rev Entomol 55:399-420.

Hammer M (1997) The neural basis of associative reward learning in honeybees. Trends Neurosci 20:245-252.
Hill ES, Iwano M, Gatellier L and Kanzaki R (2002) Morphology and physiology of the serotone-immunoreactive putative antennal lobe feedback neuron in the male silkmoth Bombyx mori. Chemical Senses 27:475-483.

Homberg U, Montague RA, Hildebrand JG (1988) antenna-cerebral pathways in the brain of the sphinx moth Manduca sexta. Cell Tissue Res 254:255-281.

Seki Y, Kanzaki R (2008) Comprehensive morphological identification and GABA immunocytochemistry of antennal lobe local interneurons in Bombyx mori. J Comp Neurol 506: 93-107.

Vosshall LB, Wong AM, Axel R (2000) An olfactory sensory map in the fly brain. Cell 102:147159.