An olfactory subsystem that detects carbon disulfide and mediates food-related social learning

Social learning and traditions in animals: evidence, definitions, and relationship to human culture

The number of publications concerned with social learning in nonhuman animals has expanded dramatically in recent decades. In this article, recent literature addressing three issues that have been of particular concern to those with both an interest in social learning and a background in experimental psychology are reviewed: (1) the definition as well as (2) empirical investigation of the numerous behavioral processes that support social learning in animals, and (3) the relationship of the 'traditions' seen in animals to the 'culture' that is so important in shaping the development of behavioral repertoires in humans. WIREs Cogn Sci 2012 doi: 10.1002/wcs.1196 For further resources related to this article, please visit the WIREs website.

The wiring of Grueneberg ganglion axons is dependent on neuropilin 1

The Grueneberg ganglion is a specialized olfactory sensor. In mice, its activation induces freezing behavior. The topographical map corresponding to the central projections of its sensory axons is poorly defined, as well as the guidance molecules involved in its establishment. We took a transgenic approach to label exclusively Grueneberg sensory neurons and their axonal projections. We observed that a stereotyped convergence map in a series of coalescent neuropil-rich structures is already present at birth. These structures are part of a peculiar and complex neuronal circuit, composed of a chain of glomeruli organized in a necklace pattern that entirely surrounds the trunk of the olfactory bulb. We found that the necklace chain is composed of two different sets of glomeruli: one exclusively innervated by Grueneberg ganglion neurons, the other by axonal inputs from the main olfactory neuroepithelium. Combining the transgenic Grueneberg reporter mouse with a conditional null genetic approach, we then show that the axonal wiring of Grueneberg neurons is dependent on neuropilin 1 expression. Neuropilin 1-deficient Grueneberg axonal projections lose their strict and characteristic avoidance of vomeronasal glomeruli, glomeruli that are innervated by secondary neurons expressing the repulsive guidance cue and main neuropilin 1 ligand Sema3a. Taken together, our observations represent a first step in the understanding of the circuitry and the coding strategy used by the Grueneberg system.

Electrophysiological characterization of Grueneberg ganglion olfactory neurons: spontaneous firing, sodium conductance, and hyperpolarization-activated currents

Mammals rely on their acute olfactory sense for their survival. The most anterior olfactory subsystem in the nose, the Grueneberg ganglion (GG), plays a role in detecting alarm pheromone, cold, and urinary compounds. GG neurons respond homogeneously to these stimuli with increases in intracellular [Ca(2+)] or transcription of immediate-early genes. In this electrophysiological study, we used patch-clamp techniques to characterize the membrane properties of GG neurons. Our results offer evidence of functional heterogeneity in the GG. GG neurons fire spontaneously and independently in several stable patterns, including phasic and repetitive single-spike modes of discharge. Whole cell recordings demonstrated two distinct voltage-gated fast-inactivating Na(+) currents with different steady-state voltage dependencies and different sensitivities to tetrodotoxin. Hodgkin-Huxley simulations showed that these Na(+) currents confer dual mechanisms of action potential generation and contribute to different firing patterns. Additionally, GG neurons exhibited hyperpolarization-activated inward currents that modulated spontaneous firing in vitro. Thus, in GG neurons, the heterogeneity of firing patterns is linked to the unusual repertoire of ionic currents. The membrane properties described here will aid the interpretation of chemosensory function in the GG.

A single gene target of an ETS-family transcription factor determines neuronal CO2-chemosensitivity

Many animals possess neurons specialized for the detection of carbon dioxide (CO₂), which acts as a cue to elicit behavioral responses and is also an internally generated product of respiration that regulates animal physiology. In many organisms how such neurons detect CO₂ is poorly understood. We report here a mechanism that endows C. elegans neurons with the ability to detect CO₂. The ETS-5 transcription factor is necessary for the specification of CO₂-sensing BAG neurons. Expression of a single ETS-5 target gene, gcy-9, which encodes a receptor-type guanylate cyclase, is sufficient to bypass a requirement for ets-5 in CO₂-detection and transforms neurons into CO₂-sensing neurons. Because ETS-5 and GCY-9 are members of gene families that are conserved between nematodes and vertebrates, a similar mechanism might act in the specification of CO₂-sensing neurons in other phyla.

Attractiveness of illness-associated odorant cues in female rats is modulated by ovarian hormones, but not associated with pro-inflammatory cytokine levels

Odorant cues released by rodents play a key role in mate preference/selection. The goal of the following series of studies was to determine the impact of acute illness, and the potential role of the inflammatory response, on the release of illness-associated odor cues from female rats. Adult female Sprague-Dawley rats were injected with lipopolysaccharide (LPS, 100 μg/kg) and their soiled bedding was used as a stimulus to naïve male odor recipients. While odored bedding from sick males elicited a robust avoidance response evidenced by decreased sniffing, avoidance and burying behavior, odored bedding from sick females elicited only a reduction in sniffing, indicating a reduction in odor attractiveness. Odor cues from ovariectomized, but not sham-operated females decreased sniffing behavior and increased avoidance in male odor recipients. Acute estradiol benzoate (EB, 20 μg/kg) replacement into ovariectomized females restored the investigatory response of male recipients toward odor cues, while LPS administration into ovariectomized oil or EB treated females had little impact on odor attractiveness. Measurement of cytokines in both brain (the paraventricular nucleus of the hypothalamus) and blood from female odor donors indicated increased expression of TNF-α, IL-1β, and IL-6 following LPS, which was not affected by EB treatment. These findings illustrate a critical sexual dimorphism by demonstrating that acute illness reduces the attractiveness of female odor, whereas odor cues from sick males are highly aversive. Moreover, the attractiveness of female odor appears to be associated with circulating ovarian hormone levels, but not central or peripheral inflammatory cytokines.

The many facets of facial interactions in mammals

Facial interactions are prominent behaviors in primates. Primate facial signaling, which includes the expression of emotions, mimicking of facial movements, and gaze interactions, is visually dominated. Correspondingly, in primate brains an elaborate network of face processing areas exists within visual cortex. But other mammals also communicate through facial interactions using additional sensory modalities. In rodents, multisensory facial interactions are involved in aggressive behaviors and social transmission of food preferences. The eusocial naked mole-rat, whose face is dominated by prominent incisors, uses facial aggression to enforce reproductive suppression. In burrow-living mammals like the naked mole-rat in particular, and in rodents in general, somatosensory face representations in cortex are enlarged. Diversity of sensory domains mediating facial communication might belie underlying common mechanisms. As a case in point, neurogenetics has revealed strongly heritable traits in face processing and identified gene defects that disrupt facial interactions both in humans and rodents.

Social interaction promotes nicotine self-administration with olfactogustatory cues in adolescent rats

Cigarette smoking is a social behavior. Smoking is also accompanied by distinctive gustatory and olfactory stimulation. However, none of these factors affecting nicotine intake are modeled in existing preclinical studies. We report a novel model of adolescent nicotine self-administration (SA) in rats where licking on drinking spouts was used as the operant behavior to activate the concurrent delivery of nicotine (i.v.) and an appetitive olfactogustatory (OG) cue, and social interaction was required for stable SA. The operant chamber was divided by a panel that separated the SA rat and another rat serving as the demonstrator, who had free access to the OG cue but did not receive nicotine. Orofacial contacts were permitted by the divider. Conditioned taste aversion prevented solo rats to self-administer nicotine. However, stable nicotine (15-30 μg/kg, free base) SA was established in the presence of demonstrator rats with free access to the OG cue. Omitting the olfactory component of the cue prevented the acquisition of nicotine SA. Mecamylamine, a nicotinic antagonist, reduced licking behavior. Familiar peers were more effective demonstrators in facilitating the acquisition of nicotine SA than were unfamiliar rats. No sex difference in nicotine intake was found. These data indicate that the contingent OG cue is associated with the aversive property of nicotine that prevents subsequent drug intake. Social information encoded in olfaction not only permits the establishment of stable nicotine SA but also enhances nicotine intake. These findings implicate adolescent social interactions in promoting smoking behavior by surmounting the aversive property of nicotine.

Gastrointestinal hormones: uroguanylin-a new gut-derived weapon against obesity?

A recent report has identified uroguanylin as an endocrine signal that exerts a physiological role in energy homeostasis, adding another factor to the gut–brain axis. From a clinical point of view, several observations highlight the uroguanylin–guanylyl cyclase C pathway as a potential therapeutic target for the development of antiobesity drugs.

Guanylyl cyclase structure, function and regulation

Nitric oxide, bicarbonate, natriuretic peptides (ANP, BNP and CNP), guanylins, uroguanylins and guanylyl cyclase activating proteins (GCAPs) activate a family of enzymes variously called guanyl, guanylyl or guanylate cyclases that catalyze the conversion of guanosine triphosphate to cyclic guanosine monophosphate (cGMP) and pyrophosphate. Intracellular cyclic GMP is a second messenger that modulates: platelet aggregation, neurotransmission, sexual arousal, gut peristalsis, blood pressure, long bone growth, intestinal fluid secretion, lipolysis, phototransduction, cardiac hypertrophy and oocyte maturation. This review briefly discusses the discovery of cGMP and guanylyl cyclases, then nitric oxide, nitric oxide synthase and soluble guanylyl cyclase are described in slightly greater detail. Finally, the structure, function, and regulation of the individual mammalian single membrane-spanning guanylyl cyclases GC-A, GC-B, GC-C, GC-D, GC-E, GC-F and GC-G are described in greatest detail as determined by biochemical, cell biological and gene-deletion studies.

Chemo- and thermosensory responsiveness of Grueneberg ganglion neurons relies on cyclic guanosine monophosphate signaling elements

Neurons of the Grueneberg ganglion (GG) in the anterior nasal region of mouse pups respond to cool temperatures and to a small set of odorants. While the thermosensory reactivity appears to be mediated by elements of a cyclic guanosine monophosphate (cGMP) cascade, the molecular mechanisms underlying the odor-induced responses are unclear. Since odor-responsive GG cells are endowed with elements of a cGMP pathway, specifically the transmembrane guanylyl cyclase subtype GC-G and the cyclic nucleotide-gated ion channel CNGA3, the possibility was explored whether these cGMP signaling elements may also be involved in chemosensory GG responses. Experiments with transgenic mice deficient for GC-G or CNGA3 revealed that GG responsiveness to given odorants was significantly diminished in these knockout animals. These findings suggest that a cGMP cascade may be important for both olfactory and thermosensory signaling in the GG. However, in contrast to the thermosensory reactivity, which did not decline over time, the chemosensory response underwent adaptation upon extended stimulation, suggesting that the two transduction processes only partially overlap.

A case study in behavioral analysis, synthesis and attention to detail: social learning of food preferences

Philip Teitelbaum's focus on detailed description of behavior, the interplay of analysis and synthesis in experimental investigations and the importance of converging lines of evidence in testing hypotheses has proven useful in fields distant from the physiological psychology that he studied throughout his career. Here we consider the social biasing of food choice in Norway rats as an instance of the application of Teitelbaum's principles of behavioral analysis and synthesis and the usefulness of convergent evidence as well as the contributions of detailed behavioral analysis of social influences on food choice to present understanding of both sensory processes and memory.

Differential effects of dopamine receptor D1-type and D2-type antagonists and phase of the estrous cycle on social learning of food preferences, feeding, and social interactions in mice

The neurobiological bases of social learning, by which an animal can 'exploit the expertise of others' and avoid the disadvantages of individual learning, are only partially understood. We examined the involvement of the dopaminergic system in social learning by administering a dopamine D1-type receptor antagonist, SCH23390 (0.01, 0.05, and 0.1 mg/kg), or a D2-type receptor antagonist, raclopride (0.1, 0.3, and 0.6 mg/kg), to adult female mice prior to socially learning a food preference. We found that while SCH23390 dose-dependently inhibited social learning without affecting feeding behavior or the ability of mice to discriminate between differently flavored diets, raclopride had the opposite effects, inhibiting feeding but leaving social learning unaffected. We showed that food odor, alone or in a social context, was insufficient to induce a food preference, proving the specifically social nature of this paradigm. The estrous cycle also affected social learning, with mice in proestrus expressing the socially acquired food preference longer than estrous and diestrous mice. This suggests gonadal hormone involvement, which is consistent with known estrogenic regulation of female social behavior and estrogen receptor involvement in social learning. Furthermore, a detailed ethological analysis of the social interactions during which social learning occurs showed raclopride- and estrous phase-induced changes in agonistic behavior, which were not directly related to effects on social learning. Overall, these results suggest a differential involvement of the D1-type and D2-type receptors in the regulation of social learning, feeding, and agonistic behaviors that are likely mediated by different underlying states.

Out of thin air: sensory detection of oxygen and carbon dioxide

Oxygen (O₂) and carbon dioxide (CO₂) levels vary in different environments and locally fluctuate during respiration and photosynthesis. Recent studies in diverse animals have identified sensory neurons that detect these external variations and direct a variety of behaviors. Detection allows animals to stay within a preferred environment as well as identify potential food or dangers. The complexity of sensation is reflected in the fact that neurons compartmentalize detection into increases, decreases, and short-range and long-range cues. Animals also adjust their responses to these prevalent signals in the context of other cues, allowing for flexible behaviors. In general, the molecular mechanisms for detection suggest that sensory neurons adopted ancient strategies for cellular detection and coupled them to brain activity and behavior. This review highlights the multiple strategies that animals use to extract information about their environment from variations in O₂ and CO₂.

Defying the stereotype: non-canonical roles of the Peptide hormones guanylin and uroguanylin

The peptide hormones uroguanylin and guanylin have been traditionally thought to be mediators of fluid-ion homeostasis in the vertebrate intestine. They serve as ligands for receptor guanylyl cyclase C (GC-C), and both receptor and ligands are expressed predominantly in the intestine. Ligand binding to GC-C results in increased cyclic GMP production in the cell which governs downstream signaling. In the last decade, a significant amount of research has unraveled novel functions for this class of peptide hormones, in addition to their action as intestinal secretagogues. An additional receptor for uroguanylin, receptor guanylyl cyclase D, has also been identified. Thus, unconventional roles of these peptides in regulating renal filtration, olfaction, reproduction, and cell proliferation have begun to be elucidated in detail. These varied effects suggest that these peptide hormones act in an autocrine, paracrine as well as endocrine manner to regulate diverse cellular processes.