The cocaine- and amphetamine-regulated transcript (CART) neuropeptide has been implicated in the neural regulation of energy homeostasis across vertebrate phyla. By using gene-specific in situ hybridization, we have mapped the distribution of the four CART mRNAs in the central nervous system of the adult zebrafish. The widespread neuronal expression pattern for CART 2 and 4 suggests a prominent role for the peptide in processing sensory information from diverse modalities including olfactory and visual inputs. In contrast, CART 1 and 3 have a much more restricted distribution, predominantly located in the nucleus of the medial longitudinal fasciculus (NMLF) and entopeduncular nucleus (EN), respectively. Enrichment of CART 2 and 4 in the preoptic and tuberal areas emphasizes the importance of CART in neuroendocrine functions. Starvation resulted in a significant decrease in CART-positive cells in the nucleus recessus lateralis (NRL) and nucleus lateralis tuberis (NLT) hypothalamic regions, suggesting a function in energy homeostasis for these neurons. Similarly, the EN emerges as a novel energy status-responsive region. Not only is there abundant and overlapping expression of CART 2, 3, and 4 in the EN, but also starvation induced a decrease in CART-expressing neurons in this region. The cellular resolution mapping of CART mRNA and the response of CART-expressing nuclei to starvation underscores the importance of CART neuropeptide in energy processing. Additionally, the regional and gene-specific responses to energy levels suggest a complex, interactive network whereby the four CART gene products may have nonredundant functions in energy homeostasis.

Modulation of sensory perception by homeostatic feedback is critical for survival. Gating of sensory inputs by internal states, such as energy availability, enables the animal to issue measured responses, like food-seeking and consumptive behaviors. Olfaction is an important sensory modality in food-seeking behavior providing predictive information on the availability, quality, and associated reward

BackgroundModulation of sensory perception by homeostatic feedback from physiological states is central to innate purposive behaviours. Olfaction is an important predictive modality for feeding-related behaviours and its modulation has been associated with hunger-satiety states. However, the mechanisms mapping internal states to chemosensory processing in order to modify behaviour are poorly understood.ResultsIn the zebrafish olfactory epithelium, a subset of olfactory sensory neurons (OSNs) and the terminal nerve projections express neuropeptide Y (NPY). We find that NPY signalling in the peripheral olfactory system of zebrafish is correlated with its nutritional state and is both necessary and sufficient for the olfactory perception of food related odorants. NPY activity dynamically modulates the microvillar OSN activation thresholds and acts cooperatively with amino acid signalling resulting in a switch-like increase in OSN sensitivity in starved animals. We suggest that cooperative activation of phospholipase C by convergent signalling from NPY and amino acid receptors is central to this heightened sensitivity.ConclusionsThis study provides ethologically relevant, physiological evidence for NPY signalling in peripheral modulation of OSN sensitivity to food-associated amino acid cues. We demonstrate sensory gating directly at the level of OSNs and identify a novel mechanistic framework for tuning olfactory sensitivity to prevailing energy states.

Dynamic re-configuration of circuit function subserves the flexibility of innate behaviours tuned to physiological states. Internal energy stores adaptively regulate feeding-associated behaviours by integrating opposing hunger and satiety signals at the level of neural circuits. Across vertebrate lineages, the neuropeptides CART and NPY have potent anorexic and orexic functions, respectively, and show energy state-dependent expression in interoceptive neurons. However, how the antagonistic activities of these peptides modulate circuit plasticity remain unclear. Using behavioural, neuroanatomical and activity analysis in adult zebrafish, along with pharmacological interventions, we show that CART and NPY activities converge on a population of neurons in the dorsomedial telencephalon (Dm). While CART facilitates glutamatergic neurotransmission at the Dm, NPY dampens the response to glutamate. In energy-rich states, CART enhances NMDA receptor (NMDAR) function by PKA/PKC mediated phosphorylation of the NR1 subunit of the NMDAR complex. Conversely, starvation triggers NPY-mediated reduction in phosphorylated NR1 via calcineurin activation and inhibition of cAMP production leading to reduced responsiveness to glutamate. Our data identify convergent integration of CART and NPY inputs by the Dm neurons to generate nutritional state-dependent circuit plasticity that is correlated with the behavioural switch induced by the opposing actions of satiety and hunger signals.

Dynamic re-configuration of circuit function subserves the flexibility of innate behaviours tuned to physiological states. Internal energy stores adaptively regulate feeding-associated behaviours by integrating opposing hunger and satiety signals at the level of neural circuits. Across vertebrate lineages, the neuropeptides CART and NPY have potent anorexic and orexic functions, respectively, and show energy state-dependent expression in interoceptive neurons. However, how the antagonistic activities of these peptides modulate circuit plasticity remains unclear. Using behavioural, neuroanatomical and activity analysis in adult zebrafish of both sexes, along with pharmacological interventions, we show that CART and NPY activities converge on a population of neurons in the dorsomedial telencephalon (Dm). While CART facilitates glutamatergic neurotransmission at the Dm, NPY dampens the response to glutamate. In energy-rich states, CART enhances NMDA receptor (NMDAR) function by PKA/PKC mediated phosphorylation of the NR1 subunit of the NMDAR complex. Conversely, starvation triggers NPY-mediated reduction in phosphorylated NR1 via calcineurin activation and inhibition of cAMP production leading to reduced responsiveness to glutamate. Our data identify convergent integration of CART and NPY inputs by the Dm neurons to generate nutritional state-dependent circuit plasticity that is correlated with the behavioural switch induced by the opposing actions of satiety and hunger signals.SIGNIFICANCE STATEMENT:Internal energy needs reconfigure neuronal circuits to adaptively regulate feeding behaviour. Energy state-dependent neuropeptide release can signal energy status to feeding-associated circuits and modulate circuit function. CART and NPY are major anorexic and orexic factors, respectively, but the intracellular signalling pathways utilised by these peptides to alter circuit function remain uncharacterised. We show that CART and NPY-expressing neurons from energy state interoceptive areas project to a novel telencephalic region, Dm, in adult zebrafish. CART increases the excitability of Dm neurons while NPY opposes CART activity. Antagonistic signalling by CART and NPY converge onto NMDA-receptor function to modulate glutamatergic neurotransmission. Thus opposing activities of anorexic CART and orexic NPY reconfigure circuit function to generate flexibility in feeding behaviour.