**5. Morphology and immunocytochemistry of neurons in the antennal lobe**

Three classes of interneurons are present in the antennal lobes [11, 82]: (1) local, amacrine interneurons (LNs), with arborizations limited to the antennal lobe; (2) projection neurons (PNs) that send axons to higher order brain centers; and (3) centrifugal neurons that send axons from higher order brain centers into the antennal lobe (**Figures 2** and **3**). Sensory neurons from the antenna send their axon into one glomerulus only [9, 62] where it forms synapses with LNs, presumably mediated by acetylcholine [83]. The somata of antennal lobe LNs and PNs form three groups (lateral, medial, and anterior) [82]. There are about 360 LNs in each antennal lobe of *M. sexta*. They can innervate many, and perhaps all, glomeruli and appear to be mostly GABAergic [84, 85]. Different neurophysiological categories of local interneurons have been observed with respect to patterns of postsynaptic activity [13]. Evidence for unidirectional synaptic interactions between local interneurons and projection neurons as well as for disinhibitory pathways between these two types of neurons was found [13]. About 860 PNs project axons out of the antennal lobe through various antenno-cerebral tracts to different parts of the protocerebrum, for example, the calyces of the mushroom body and the lateral horn of the protocerebrum [11]. The third group of neurons, centrifugal neurons, is small in number and consists of a variety of cell types with unique morphologies, some of which innervate all glomeruli of one or both antennal lobes [82, 86]. The antennal lobe possesses a single serotonin-immunoreactive neuron [86]. This neuron has its soma in one antennal lobe, which innervates all glomeruli in the contralateral antennal lobe where it forms and receives synapses and has arborizations in the ipsilateral and contralateral protocerebrum [87].

Acetylcholine and GABA are the most prominent neurotransmitters in the antennal lobe [83]. Evidence that acetylcholine may serve as a transmitter has been reported for antennal sensory neurons [88] and some classes of projection neurons [89]. Acetylcholine may be released by primary afferent axons synapsing onto AL neurons [88, 90–93]. GABA is prominent in local interneurons and is also present in a subset of PNs [84]. GABA has an important role in the synaptic inhibition of PNs [85]. An IPSP is evoked when the antenna is stimulated with an odor. The IPSP is mediated by a chloride conductance and is sensitive to reversible blockade by picrotoxin and bicuculline. GABA hyperpolarizes neurons and inhibits their spontaneous nerve impulse firing. Several antennal lobe neurons are immunoreactive for biogenic amines. These neurons have wide dendritic arborizations and are thought to have widespread effects. Possibly, these neurons mediate central modulation of synaptic activity or threshold levels within the antennal lobe [86].

*Neuronal Architecture and Functional Organization of Olfactory Glomeruli DOI: http://dx.doi.org/10.5772/intechopen.108728*

#### **Figure 2.**

*Diagrammatic representation of sexually isomorphic glomeruli (G) and sexually dimorphic glomeruli in (a) male and (b) female Manduca sexta. (c) Laser-scanning confocal micrograph of an antennal lobe projection neuron in the moth antennal lobe of M. sexta. Image of a projection neuron in the macroglomerular complex (MGC-PN) with arborizations confined to the cumulus. The inset illustrates the organization of the antennal lobe with the macroglomerular complex (MGC) and other glomeruli (G). latLFG – Lateral large female glomerulus, medLFG – Medial large female glomerulus, smallFG – Small female glomerulus, C - cumulus, T1 - toroid-1, T2 - toroid-2, G – glomerulus, MGC – Macroglomerular complex, la – Lateral, do - dorsal. Scale bar: 100 μm. Modified from [66].*

In LNs and PNs, several putative neuropeptides appear to be colocalized with classical transmitters [89].
