A distinctive feature in the topographic organization of the olfactory system in mammals is the dual function of the odorant receptor (OR): it detects odors in the nasal epithelium and plays an instructive role in the axonal convergence of olfactory sensory neurons (OSN) into the olfactory bulb (OB). The latter function is supported by genetic experiments and by the expression of the OR not only on the cilia, but also on the axon termini of the OSN. The signaling pathway coupled to the OR on the cilia is well known and is recognized to involve cAMP and Ca 2؉ , whereas, until now, nothing was known on the functional characteristics of the OR on the axon termini-growth cone. Here, by analyzing the spatiotemporal dynamics of cAMP and Ca 2؉ in living OSN in vitro and in situ, we found that the OR at the growth cone is capable of binding odors and is coupled to cAMP synthesis and Ca 2؉ influx through cyclic nucleotide gated (CNG) channels. Furthermore we found that selective odor activation of the OR on the growth cone is followed by nuclear translocation of protein kinase A catalytic subunit. These results define the functional properties of the OR on the growth cone and suggest a potential role of OR activation in axonal convergence and sensory map formation.olfactory sensory neurons ͉ real-time imaging ͉ second messengers I n sensory systems, neurons in the peripheral sheet send their axons in precise loci of the CNS to create an internal representation of the external world. The spatial segregation of afferent inputs from primary sensory neurons provides a topographic map that defines the quality and the location of the stimulus. In the olfactory system, each OSN expresses only one OR gene out of a repertoire of approximately 1000 (1). OSNs expressing different ORs are randomly dispersed in the nasal epithelium. However, spatial order is achieved in the olfactory bulb (OB), where OSNs expressing the same odorant receptor converge with exquisite precision to form glomeruli both on the lateral and the medial side of each OB. Each odor is thus encoded by a specific spatial pattern of activated glomeruli. The OR, a G-protein-coupled receptor, upon binding of odorant ligands at the cilia, activates a specific G protein, Golf, that stimulates adenylyl cyclase, AC, to synthesize cAMP. The cAMP then directly activates cyclic nucleotide gated (CNG) channels, leading to action potential generation (2, 3).A unique feature in the topographic organization of the OB is the ''dual role'' of the OR. Although it is well established that the OR is involved in the transduction of chemical signals (odors) at the cilia level, a number of evidence suggests that this receptor plays also an instructive role in glomerular convergence of OSN axons to the bulb (4-6). The latter property is supported by genetic observations demonstrating that alterations of OR sequence perturb the sensory map (7-9) and by the demonstration that the OR is expressed not only at the cilia but also on the OSN axon termini (10, 11). The OR is not the only determ...

ThemechanismofcGMPproductioninolfactorysensoryneurons(OSNs)ispoorlyunderstood,althoughthismessengertakespartinseveralkey processes such as adaptation, neuronal development, and long-term cellular responses to odorant stimulation. Many aspects of the regulation of cGMP in OSNs are still unknown or highly controversial, such as its subcellular heterogeneity, mechanism of coupling to odorant receptors and downstream targets. Here, we have investigated the dynamics and the intracellular distribution of cGMP in living rat OSNs in culture transfected with a genetically encoded sensor for cGMP. We demonstrate that OSNs treated with pharmacological stimuli able to activate membrane or soluble guanylyl cyclase (sGC) presented an increase in cGMP in the entire neuron, from cilia-dendrite to the axon terminus-growth cone. Upon odorant stimulation, a rise in cGMP was again found in the entire neuron, including the axon terminus, where it is locally synthesized. The odorant-dependent rise in cGMP is due to sGC activation by nitric oxide (NO) and requires an increase of cAMP. The link between cAMP and NO synthase appears to be the rise in cytosolic Ca 2ϩ concentration elicited by either plasma membrane Ca 2ϩ channel activation or Ca 2ϩ mobilization from stores via the guanine nucleotide exchange factor Epac. Finally, we show that a cGMP rise can elicit both in vitro and in vivo the phosphorylation of nuclear CREB, suggesting that this signaling pathway may be relevant for both local events (pathfinding, neurotransmitter release) and more distal processes involving gene expression regulation.

The nuclear receptor Chicken Ovalbumin Upstream Promoter-Transcription Factor II (COUP-TFII) is an important coordinator of glucose homeostasis through its function in different organs such as the endocrine pancreas, adipose tissue, skeletal muscle, and liver. Recently we have demonstrated that COUP-TFII expression in the hypothalamus is restricted to a subpopulation of neurons expressing the steroidogenic factor 1 transcription factor, known to play a crucial role in glucose homeostasis. To understand the functional significance of COUP-TFII expression in the steroidogenic factor 1 neurons, we generated hypothalamic ventromedial nucleus-specific COUP-TFII KO mice using the cyclization recombination/locus of XoverP1 technology. The heterozygous mutant mice display insulin hypersensitivity and a leaner phenotype associated with increased energy expenditure and similar food intake. These mutant mice also present a defective counterregulation to hypoglycemia with altered glucagon secretion. Moreover, the mutant mice are more likely to develop hypoglycemia-associated autonomic failure in response to recurrent hypoglycemic or glucopenic events. Therefore, COUP-TFII expression levels in the ventromedial nucleus are keys in the ability to resist the onset of hypoglycemia-associated autonomic failure.H ypothalamic neurons are able to detect changes in the concentration of circulating hormones and nutrients and to relay this information into adaptive signals toward peripheral organs aimed at maintaining glucose homeostasis and energy balance. Among these neurons, several studies have shown that the steroidogenic factor 1 (SF1) expressing ones, which are restricted to the hypothalamic ventromedial nucleus (VMN), are important modulators of metabolic functions through their ability to detect variations of extracellular concentrations of glucose, insulin, leptin, and orexin to coordinate insulin/glucose homeostasis (1-5). The SF1 transcription factor itself is a key actor of energy homeostasis ,as SF1 knockout (KO) mice are obese (6). SF1 KO mice display an abnormal organization of the VMN, as SF1 is necessary for the terminal differentiation of the VMN neurons (7,8). SF1 itself directs a genetic program involved in energy homeostasis and leptin action in the VMN (9). Outside these functions, SF1 neurons are also part of the neurocircuitry involved in the counterregulatory response (CRR) to hypoglycemia. To prevent hypoglycemia, organisms have developed several survival mechanisms of CRR such as decreased insulin secretion and increased glucagon secretion to quickly reestablish euglycemia (10, 11). The decreased insulin secretion is a direct effect of hypoglycemia on the pancreatic β cells, while the other CRRs are mainly directed by the central nervous system and in part by the SF1 neurons (12)(13)(14). Overall the SF1 neuronal population is at the interface between sensing of glucose concentrations and modulation of insulin-dependent metabolism and energy homeostasis.Recently we have observed that Chicken Ovalbumin Upstre...