Introduction

Critical questions in neurobiology revolve around the issue of specificity. How do neurons receive, process, and coordinate responses to specific information? How do developing neurons organize themselves into specific arrays, and how do growing neuronal processes find their way to and recognize their ultimate targets? At the sensory level, questions of specificity focus on issues of signal recognition. How does an animal perceive a specific signal against a background of apparent noise? These questions are put into sharp focus when we consider them in a specific sensory context, such as olfaction. For example, how do we discriminate the odor of a banana from that of a fragrant flower, a mountain forest, or a summer beach? This chapter presents the moth olfactory system as a model for identifying molecular genetic mechanisms encoding neuronal specificity.

Perception of an odor is a quality of the brain involving the coordination of many biochemical events. The process begins with odor molecules binding to selective receptor proteins in the membranes of first-order olfactory receptor neurons. This binding is then transduced into an electrical signal, and the brain is informed of this event through synaptic connections between first- and second-order neurons. These synapses are organized as distinct glomeruli, grapelike structures, in the olfactory lobe in insects and in the olfactory bulb in vertebrates. The first-order sensory neurons can be viewed as functionally identifiable based on the odorantsensitive phenotype they express; olfactory receptor neurons express different receptor proteins with selective odorant or ligand binding specificities.