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The gustatory system in Drosophila is crucial for
detecting food, selecting sites to lay eggs and recognizing mates. Taste
neurons are distributed on many parts of the fly’s body surface and
they recognize familiar taste stimuli: sugars, salts, acids, alcohols and
noxious chemicals. We have recently characterized a large family of ~60
candidate gustatory receptor genes (GRs). These receptors provide
essential molecular markers that we are using to examine taste
recognition both in the periphery and in the CNS.
Ligand and Behavioral Specificity of Different GRs
To understand the function of different gustatory
neurons, we are determining the ligands that different taste neurons
recognize and the behaviors that they mediate. We are identifying ligands
by a combination of genetic cell ablations and receptor misexpression
studies, coupled with behavioral paradigms and calcium imaging
experiments to assay taste responses. For example, gustatory neurons
containing the same receptor gene can be ablated by genetically
expressing a toxin and taste defects can be tested by simple behavioral
assays, like food choice discrimination measured by food-coloring uptake.
We are also expressing calcium-sensitive fluorescent proteins in taste
neurons–this allows us to monitor taste responses in the entire
population of gustatory neurons in vivo with single cell resolution. To
determine the behaviors that different neurons mediate even in the
absence of identifying ligands, inducible activators will be expressed in
gustatory neurons, so that each neuron can be stimulated one by one to
examine the fly’s behavioral response. These studies will allow us
to identify a sensory neuron by the stimulus that it recognizes and the
response that it generates and will provide a starting point for
dissecting taste circuits.
Sensory maps in the fly brain
In other sensory systems, information from the
periphery is mapped in the brain to provide a representation of the
external world in the internal wiring. For example, there are tonotopic
maps of auditory projections and somatosensory maps of touch projections.
The gustatory system of the fly is interesting both because neurons
express unique complements of receptors and because neurons are
distributed in an orderly array along the body surface. We are using
molecular genetic approaches to examine whether gustatory projections are
segregated according to the receptor that is expressed in the periphery
and whether there is a topographic map of gustatory information.
Chemosensory bristles contain one mechanosensory neuron as well as taste
neurons, and we are examining how these two different sensory modalities
are represented in the brain. Our motivation is to understand the internal
representation of gustatory information in the first relay.
Information
processing in the fly brain
The subesophageal ganglion of the fly brain contains
both axons of gustatory neurons and dendrites of motor neurons involved
in taste behaviors. This suggests that the fly may have simple and
localized taste circuits, with few connections between sensory stimulus
and motor response. In addition, projection neurons may relay gustatory
information to higher brain centers, perhaps for more complex associations.
We are interested in mapping the functional and anatomical components of
taste circuits using a variety of approaches. Genetic approaches to label
subsets of neurons in the brain, behavioral screens for taste mutants,
and calcium imaging of taste responses in the brain will help elucidate
these circuits. These studies will provide insight into the integration
of gustatory cues and the difference between sweet versus bitter, and
will set the stage to examine how taste circuits are modified by learning
and other sensory stimuli. We plan to study increasingly complex problems
of neural integration by examining how different stimuli impinge upon
taste circuits.
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