Odors elicit spatio-temporal patterns of activity in the brain. Spatial patterns arise from the specificity of the interaction between odorants and odorant receptors expressed in different olfactory receptor neurons (ORNs). But the origin of temporal patterns of activity and their role in odor coding remain unclear. We investigate how physiological aspects of ORN response and physical aspects of odor stimuli give rise to diverse responses in Drosophila ORNs. We show that odor stimuli have intrinsic dynamics that depend on odor type and strongly affect ORN response. Using linear-nonlinear modeling to remove the contribution of the stimulus dynamics from the ORN dynamics we study the physiological properties of the response to different odorants and concentrations. For several odorants and receptor types the ORN response dynamics normalized by the peak response are independent of stimulus intensity for a large portion of the neuron’s dynamic range. Adaptation to a background odor changes the gain and dynamic range of the response but does not affect normalized response dynamics. Stimulating ORNs with various odorants reveals significant odor-dependent delays in the ORN response functions. These differences however can be dominated by differences in stimulus dynamics. In one case the response of one ORN to two odorants is predicted solely from measurements of the odor signals. Within a large portion of their dynamic range ORNs can capture information about stimulus dynamics independently from intensity while introducing odor-dependent delays. How insects might use odor-specific stimulus dynamics and ORN dynamics in discrimination and navigation tasks remains an open question.

The ability of an animal to detect, discriminate, and respond to odors depends on the function of its olfactory receptor neurons (ORNs), which in turn depends ultimately on odorant receptors. To understand the diverse mechanisms used by an animal in olfactory coding and computation, it is essential to understand the functional diversity of its odor receptors. The larval olfactory system of Drosophila melanogaster contains 21 ORNs and a comparable number of odorant receptors whose properties have been examined in only a limited way. We systematically screened them with a panel of ∼500 odorants, yielding >10,000 receptor-odorant combinations. We identify for each of 19 receptors an odorant that excites it strongly. The responses elicited by each of these odorants are analyzed in detail. The odorants elicited little cross-activation of other receptors at the test concentration; thus, low concentrations of many of these odorants in nature may be signaled by a single ORN. The receptors differed dramatically in sensitivity to their cognate odorants. The responses showed diverse temporal dynamics, with some odorants eliciting supersustained responses. An intriguing question in the field concerns the roles of different ORNs and receptors in driving behavior. We found that the cognate odorants elicited behavioral responses that varied across a broad range. Some odorants elicited strong physiological responses but weak behavioral responses or weak physiological responses but strong behavioral responses.T he olfactory system of the Drosophila larva achieves remarkable function with minimal structure. It detects and responds to spatial and temporal gradients of odorants, transforming chemical information into navigation via an elegant repertoire of head sweeps, runs, and turns (1-3). Its sophisticated function is based on the activities of 21 olfactory receptor neurons (ORNs), which innervate the dorsal organ of the head and send axons to the antennal lobe of the brain (4). The activities of the ORNs are in turn based on the responses of odor receptors (Ors). Thus, to understand the molecular basis of larval olfactory navigation, it is necessary to understand the function of the receptors.ORNs together express 25 members of the Or family of odor receptors and the Orco coreceptor (5-8). In each ORN, an Or and Orco together form a ligand-gated ion channel (9-11). Most ORNs express a single Or, although one ORN coexpresses Or94a and Or94b and another ORN coexpresses Or33b and Or47a (7). The significance of this coexpression remains speculative, but the response profiles of some coexpressed adult Ors are additive (12).The responses of the larval Or repertoire to a limited odorant panel was previously examined in an in vivo expression system known as the empty neuron system (8, 13). With the use of this system, 21 of the larval Ors were found to be functional. However, studies of the larval Or repertoire have been limited not only in the number of odorants examined, but also in their consideration of receptor sensitivity, tempor...

Animal behavior is, on the one hand, controlled by neuronal circuits that integrate external sensory stimuli and induce appropriate motor responses. On the other hand, stimulus-evoked or internally generated behavior can be influenced by motivational conditions, e.g., the metabolic state. Motivational states are determined by physiological parameters whose homeostatic imbalances are signaled to and processed within the brain, often mediated by modulatory peptides. Here, we investigate the regulation of appetitive and feeding behavior in the fruit fly, Drosophila melanogaster. We report that four neurons in the fly brain that release SIFamide are integral elements of a complex neuropeptide network that regulates feeding. We show that SIFamidergic cells integrate feeding stimulating (orexigenic) and feeding suppressant (anorexigenic) signals to appropriately sensitize sensory circuits, promote appetitive behavior, and enhance food intake. Our study advances the cellular dissection of evolutionarily conserved signaling pathways that convert peripheral metabolic signals into feeding-related behavior.

Most natural odors are mixtures and often elicit percepts distinct from those elicited by their constituents. This emergence of a unique odor quality has long been attributed to central processing. Here we show that sophisticated integration of olfactory information begins in olfactory receptor neurons (ORNs) in Drosophila. Odor mixtures are encoded in the temporal dynamics as well as in the magnitudes of ORN responses. ORNs can respond to an inhibitory odorant with different durations depending on the level of background excitation. ORNs respond to mixtures with distinctive temporal dynamics that reflect the physicochemical properties of the constituent odorants. The insect repellent DEET (N,N-diethyl-m-toluamide), which attenuates odor responses of multiple ORNs, differs from an ORNspecific inhibitor in its effects on temporal dynamics. Our analysis reveals a means by which integration of information from odor mixtures begins in ORNs and provides insight into the contribution of inhibitory stimuli to sensory coding.A fascinating aspect of the sense of smell is that odor mixtures often have distinctive emergent qualities, and their individual constituents are difficult to identify (1). How the emergent quality of an odor mixture arises is unknown. It is believed to originate mainly from information processing in the central nervous system, as in the insect antennal lobe (2-4) and the vertebrate olfactory bulb (5, 6). However, olfactory receptor neurons (ORNs) also contribute to the integration of olfactory information. For example, some individual rat ORNs respond to a binary odor mixture with response magnitudes, i.e., firing frequencies, that cannot be predicted simply from the responses to its components (7,8). In addition, studies in moths (9-14) and beetles (15) have provided evidence that information about odorants in a mixture can be integrated in the periphery. However, little is known about how information is processed in ORNs. It is unclear, for example, whether individual ORNs are capable of integrating information via means other than simple alterations of their response magnitudes.Equally intriguing in sensory coding is the role of inhibitory stimuli. In taste, certain stimuli inhibit vertebrate sweet receptors (16,17). In olfaction, inhibitory odorants have been identified for olfactory receptors of vertebrates (18-21) and invertebrates (13,(22)(23)(24)(25)(26). When delivered as a monomolecular odor stimulus, inhibitors reduce the spontaneous activity of ORNs. However, the low spontaneous activities of most ORNs (13,22,23,27) limit the dynamic range of inhibition, and inhibitory odorants may have greater functional significance when paired with excitatory odorants. Indeed, an inhibitory odorant reduced the intensity of a concurrent excitatory odor stimulus in a human psychophysical study (28), and odorants that inhibited CO 2 -sensing neurons prevented CO 2 -mediated avoidance behaviors in Drosophila (24). However, it is not clear whether a binary mixture of an excitatory stimulus and an inhibito...