Abstract
14
Group Report: Olfactory
Microcircuits
R. W. F
RIEDRICH
, Rapporteur
S. F
IRESTEIN
,C.G.G
ALIZIA
,C.A.G
REER
,G.L
AURENT
,
P. - M . L
LEDO
,P.M
OMBAERTS
,andS.S
ACHSE
OVERVIEW
Most organisms rely on an olfactory system to detect and analyze chemical cues
in the environment in the context of essential behaviors. The basic layout of the
first processing centers in the olfactory nervous system is remarkably similar in
diverse phylogenetic classes, including insects and vertebrates. Chemicals are
detected by odorant receptor proteins expressed by olfactory sensory neurons
(OSNs), which send an axon to the first processing center in the brain, the olfac-
tory bulb in vertebrates and the antennal lobe in insects. OSNs terminate in ana-
tomically distinct input modules, the olfactory glomeruli. In all vertebrate and
invertebrate species investigated to date, each OSN expresses only one or a few
odorant receptors, and each glomerulus receives convergent input from only
one type of OSN. Glomeruli are, therefore, considered functional units integrat
-
ing sensory input from idiotypic afferents. Even simple odors stimulate multiple
odorant receptors and thus evoke odor-specific patterns of afferent activity
across the array of glomeruli. Within glomeruli, OSNs make excitatory synap
-
ses onto the output neurons, the mitral cells in vertebrates, and projection neu
-
rons in insects, as well as with local inhibitory interneurons. As a result of synap
-
tic interactions within this network, the output of a given projection neuron is
not simply determined by the sensory input to the glomeruli it innervates, but
also by the activity of inputs channeled through other glomeruli. In addition,
synaptic interactions temporally pattern olfactory bulb/antennal lobe output ac
-
tivity on at least two timescales. It is currently debated how odor information is
encoded in the olfactory bulb/antennal lobe, and how neuronal circuits process
odor information conveyed by sensory afferents. Moreover, the development
and plasticity of olfactory circuits are only beginning to be elucidated. These
issues are of particular interest because OSNs and interneurons in the olfactory
bulb undergo continuous turnover throughout life in vertebrates
