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In the antennal lobe, information concerning temperature and humidity are
also processed and neurons with respones to mechanical stimuli are found.
In the present context, we mostly restrict this account to the predominant
role of odour information processing.
Odorant identification
Information
processing in the antennal lobe is crucial for identifying odours.
In bees, it has been shown that GABAergic inhbition in the antennal
lobe is necessary for the ability to discriminate odorants [Stopfer
et al., 1997]．Many
candidate
mechanisms and models have been proposed for the function of
the antennal lobe in olfactory information processing.
Sensitivity adjustment
Responses
to identical stimuli can vary depending on factors such as
circadian rhythms and stimulation history. See also 1.4.
Multimodal integration
Factors such as air
flow speed and temperature are parameters that can influence odour
respones [Zeiner & Tichy, 1998, 2000]．Changes
in responsiveness may occur at different levels, in olfactory
receptor neurons as well as within the antennal lobe, which
contains hygro- and thermosensitive receptor projections in
specific glomeruli [Nishino et al., 2003]. There are also
connections with the other part of the deutocerebrum, called
antennal mechanosensory and motor center (AMMC）or
dorsal lobe, which is also innervated by some local interneurons
(strictly speaking, local interneurons are thus deutocerebral local
interneurons).
References
Nishino
H, Yamashita S, Yamazaki Y, Nishikawa M, Yokohari F, Mizunami M
(2003) Projection neurons originating from thermo- and hygrosensory
glomeruli in the antennal lobe of the cockroach. J Comp Neurol 455:
40-55
Stopfer M, Bhagavan S, Smith BH, Laurent G (1997)
Impaired odour discrimination on desynchronization of
odour-encoding neural assemblies. Nature 390:70-74.
Zeiner
R, Tichy H (1998) Combined effects of olfactory and mechanical
inputs in antennal lobe neurons of the cockroach. J Comp Physiol A
182:467-473.
Zeiner R, Tichy H (2000) Integration of
temperature and olfactory information in cockroach antennal lobe
glomeruli. J Comp Physiol A 186:717-727.
Pheromone processing in the antennal lobe
In
numerous insect species, sex pheromones have been identified and in
some, conspicuous specialised glomeruli of large size are found
that exclusively process pheromone information. In moths, the unit
of these large glomeruli receiving projections of
pheromone-sensitive olfactory receptor neurons specific to males is
called the macroglomerular complex (MGC). In the silkmoth, it is
composed of three compartments called toroid, cumulus, and
horseshoe. Using recombinant silkmoths, we have confirmed recently
that olfactory receptor neurons expressing the bombykol receptor
(BmOR1) project to the cumulus while those expressing the bombykal
receptor (BmOR3) project to the toroid (Sakurai & Kanzaki in
preparation).
There are four types of antennal lobe
projection neurons from the MGC, cumulus projection neurons, toroid
projection neurons, cumulus and toroid projection neurons, and
horseshoe projection neurons. The responses specificities of
projection neurons whose dendrites innervate the cumulus and toroid
are as predicted from the specificities of the respective olfactory
receptor neurons. Horseshoe projection neurons respond to bombykal.
It appears that the number of compartments in the
MGC is related to the number of pheromone components.