The mammalian main olfactory epithelium (MOE) recognizes and transduces olfactory cues through a G protein-coupled, cAMPdependent signaling cascade. Additional chemosensory transduction mechanisms have been suggested but remain controversial. We show that a subset of MOE neurons expressing the orphan receptor guanylyl cyclase GC-D and the cyclic nucleotide-gated channel subunit CNGA3 employ an excitatory cGMP-dependent transduction mechanism for chemodetection. By combining gene targeting of Gucy2d, which encodes GC-D, with patch clamp recording and confocal Ca 2؉ imaging from single dendritic knobs in situ, we find that GC-D cells recognize the peptide hormones uroguanylin and guanylin as well as natural urine stimuli. These molecules stimulate an excitatory, cGMP-dependent signaling cascade that increases intracellular Ca 2؉ and action potential firing. Responses are eliminated in both Gucy2d-and Cnga3-null mice, demonstrating the essential role of GC-D and CNGA3 in the transduction of these molecules. The sensitive and selective detection of two important natriuretic peptides by the GC-D neurons suggests the possibility that these cells contribute to the maintenance of salt and water homeostasis or the detection of cues related to hunger, satiety, or thirst.O dor recognition by canonical olfactory sensory neurons (OSNs) of the main olfactory epithelium (MOE) begins when odor molecules bind to any one of many hundred G protein-coupled odorant receptors (1-4). Upon ligand binding, odorant receptors activate the G protein G␣ olf , which stimulates type III adenylyl cyclase to increase intracellular levels of cAMP (4-9). This second messenger then directly opens cAMPsensitive, cyclic nucleotide-gated (CNG) channels in the plasma membrane, resulting in the initial depolarization of the cell (3, 4). OSNs that use this cAMP-mediated transduction cascade respond to a wide variety of odors, including food odors, volatile pheromones, and peptides that bind major histocompatibility complex proteins (4, 10-12). A recently discovered second family of G protein-coupled receptors expressed in the MOE, the trace amine-associated receptors, may also function in the recognition of some odors or pheromones by coupling to the canonical cAMP pathway (13). However, there has been intense debate as to whether the cAMP cascade is the only excitatory sensory transduction mechanism in the MOE (11,14,15).A subpopulation of ciliated MOE neurons that express the orphan receptor guanylyl cyclase GC-D (16) are prime candidates to mediate cAMP-independent odor recognition. GC-D cells lack key components of the canonical OSN odor transduction cascade, including G␣ olf , type III adenylyl cyclase, the Ca 2ϩ /calmodulin-dependent phosphodiesterase PDE1C2, the cAMP-specific phosphodiesterase PDE4A, and the cAMPsensitive CNG channel subunits CNGA2 and CNGB1b (17,18). Instead, these neurons express a cGMP-specific CNG channel subunit, CNGA3, and a cGMP-stimulated phosphodiesterase, PDE2 (17,18). Although these cells were identified more than a de...

Summary Olfactory signals influence social interactions in a variety of species [1, 2]. In mammals, pheromones and other social cues can promote mating or aggression behaviors, can communicate information about social hierarchies, genetic identity and health status, and can contribute to associative learning [1–5]. However, the molecular, cellular and neural mechanisms underlying many olfactory-mediated social interactions remain poorly understood. Here, we report that a specialized olfactory subsystem that includes olfactory sensory neurons (OSNs) expressing the receptor guanylyl cyclase GC-D, the cyclic nucleotide-gated channel subunit CNGA3 and the carbonic anhydrase isoform CAII (GC-D+ OSNs) [6–11] is required for the acquisition of socially transmitted food preferences (STFPs) in mice. Using electrophysiological recordings from gene-targeted mice, we show that GC-D+ OSNs are highly sensitive to the volatile semiochemical carbon disulfide (CS2), a component of rodent breath and a known social signal mediating the acquisition of STFPs [12–14]. Responses to sub-micromolar concentrations of CS2 in the main olfactory epithelium or in identified GC-D+ OSNs are absent in mice lacking CNGA3 or CAII and drastically reduced in mice lacking GC-D. Mice in which GC-D+ OSN transduction mechanisms have been disrupted fail to acquire STFPs from either live or surrogate demonstrator mice and do not exhibit neuronal activation of the ventral subiculum of the hippocampus, a brain region implicated in STFP retrieval [15]. Our findings indicate that GC-D+ OSNs detect chemosignals that facilitate food-related social interactions.

-The glucose-dependent secretion of the insulinotropic hormone glucagon-like peptide-1 (GLP-1) is a critical step in the regulation of glucose homeostasis. Two molecular mechanisms have separately been suggested as the primary mediator of intestinal glucose-stimulated GLP-1 secretion (GSGS): one is a metabotropic mechanism requiring the sweet taste receptor type 2 (T1R2) ϩ type 3 (T1R3) while the second is a metabolic mechanism requiring ATP-sensitive K ϩ (KATP) channels. By quantifying sugar-stimulated hormone secretion in receptor knockout mice and in rats receiving Roux-en-Y gastric bypass (RYGB), we found that both of these mechanisms contribute to GSGS; however, the mechanisms exhibit different selectivity, regulation, and localization. T1R3Ϫ/Ϫ mice showed impaired glucose and insulin homeostasis during an oral glucose challenge as well as slowed insulin granule exocytosis from isolated pancreatic islets. Glucose, fructose, and sucralose evoked GLP-1 secretion from T1R3 ϩ/ϩ , but not T1R3 Ϫ/Ϫ , ileum explants; this secretion was not mimicked by the K ATP channel blocker glibenclamide. T1R2 Ϫ/Ϫ mice showed normal glycemic control and partial small intestine GSGS, suggesting that T1R3 can mediate GSGS without T1R2. Robust GSGS that was K ATP channeldependent and glucose-specific emerged in the large intestine of T1R3 Ϫ/Ϫ mice and RYGB rats in association with elevated fecal carbohydrate throughout the distal gut. Our results demonstrate that the small and large intestines utilize distinct mechanisms for GSGS and suggest novel large intestine targets that could mimic the improved glycemic control seen after RYGB.glucagon-like peptide-1; insulin; T1R3; glucose-stimulated potassium ion channel; enteroendocrine l cells THE BODY TIGHTLY REGULATES blood glucose levels, and disruption of the homeostatic mechanisms that underlie normal glycemic control can have significant deleterious effects. For example, the prolonged hyperglycemia associated with type 2 diabetes mellitus (T2DM) increases the risk of cardiovascular disease, neuropathy, retinopathy, kidney disease, and death (66). Hormonal signals arising in the gastrointestinal tract are key components of the homeostatic mechanisms controlling blood glucose levels after a meal. Ingestion of carbohydrate and other nutrients promotes the secretion of insulinotropic hormones such as glucagon-like peptide-1 (GLP-1) from the gut, resulting in a surge of insulin production before blood glucose levels rise (11,32). This early response contributes to increased glucose disposal during absorption and helps to prevent hyperglycemia. GLP-1 mimetics and inhibitors of GLP-1 degradation help increase insulin biosynthesis and secretion from pancreatic ␤-cells and are valuable additions to previous treatment regimens for T2DM patients (11,32).Despite the importance of intestinal glucose sensing and glucose-stimulated gut hormone secretion, the mechanisms underlying these processes have remained elusive. The distinct glucose-sensing mechanisms found in the pancreas and in the gustat...

Experimental rats had their right olfactory bulb removed on postnatal day 2 (P2) and their left olfactory bulb removed on P90. Control rats had one or both olfactory bulbs removed on P90. Before and after their adult-stage surgery, rats were trained using olfactometry and operant conditioning to detect and discriminate odors. Anterograde transport of horseradish peroxidase applied to the olfactory epithelium revealed numerous axons of olfactory sensory neurons in the right hemisphere of 27 experimental rats. These axons terminated in glomerular-like clusters within the frontal neocortex (n ϭ 5) or anterior olfactory nucleus with some axons extending into the subventricular epithelium (n ϭ 22). Seventeen of the experimental rats were able to detect a variety of odors and to discriminate between odors. Performance accuracy was related to the location and density of these anomalous inputs; experimental rats with inputs confined to frontal neocortex and those lacking any inputs to the forebrain were anosmic, as were adult-operated bilaterally bulbectomized rats. Our results provide strong support for the contention that, in the absence of the olfactory bulbs, olfactory connections to novel forebrain sites can support both odor detection and odor discrimination.

The mammalian nose employs several olfactory subsystems to recognize and transduce diverse chemosensory stimuli. These subsystems differ in their anatomical position within the nasal cavity, their targets in the olfactory forebrain, and the transduction mechanisms they employ. Here we report that they can also differ in the strategies they use for stimulus coding. Necklace glomeruli are the sole main olfactory bulb (MOB) targets of an olfactory sensory neuron (OSN) subpopulation distinguished by its expression of the receptor guanylyl cyclase GC-D and the phosphodiesterase PDE2, and by its chemosensitivity to the natriuretic peptides uroguanylin and guanylin and the gas CO2. In stark contrast to the homogeneous sensory innervation of canonical MOB glomeruli from OSNs expressing the same odorant receptor (OR), we find that each necklace glomerulus of the mouse receives heterogeneous innervation from at least two distinct sensory neuron populations: one expressing GC-D and PDE2, the other expressing olfactory marker protein. In the main olfactory system it is thought that odor identity is encoded by a combinatorial strategy and represented in the MOB by a pattern of glomerular activation. This combinatorial coding scheme requires functionally homogeneous sensory inputs to individual glomeruli by OSNs expressing the same OR and displaying uniform stimulus selectivity; thus, activity in each glomerulus reflects the stimulation of a single OSN type. The heterogeneous sensory innervation of individual necklace glomeruli by multiple, functionally distinct, OSN subtypes precludes a similar combinatorial coding strategy in this olfactory subsystem.