Pass the salt: the central control of sodium intake

Salt Sensation and Regulation

Taste sensation and regulation are highly conserved in insects and mammals. Research conducted over recent decades has yielded major advances in our understanding of the molecular mechanisms underlying the taste sensors for a variety of taste sensations and the processes underlying regulation of ingestion depending on our internal state. Salt (NaCl) is an essential ingested nutrient. The regulation of internal sodium concentrations for physiological processes, including neuronal activity, fluid volume, acid–base balance, and muscle contraction, are extremely important issues in animal health. Both mammals and flies detect low and high NaCl concentrations as attractive and aversive tastants, respectively. These attractive or aversive behaviors can be modulated by the internal nutrient state. However, the differential encoding of the tastes underlying low and high salt concentrations in the brain remain unclear. In this review, we discuss the current view of taste sensation and modulation in the brain with an emphasis on recent advances in this field. This work presents new questions that include but are not limited to, “How do the fly’s neuronal circuits process this complex salt code?” and “Why do high concentrations of salt induce a negative valence only when the need for salt is low?” A better understanding of regulation of salt homeostasis could improve our understanding of why our brains enjoy salty food so much.

Regulation of Thirst and Vasopressin Release

Recent experiments using optogenetic tools facilitate the identification and functional analysis of thirst neurons and vasopressin-producing neurons. Four major advances provide a detailed anatomy and physiology of thirst, taste for water, and arginine-vasopressin (AVP) release: ( a) Thirst and AVP release are regulated by the classical homeostatic, interosensory plasma osmolality negative feedback as well as by novel, exterosensory, anticipatory signals. These anticipatory signals for thirst and vasopressin release concentrate on the same homeostatic neurons and circumventricular organs that monitor the composition of blood. ( b) Acid-sensing taste receptor cells (TRCs) expressing otopetrin 1 on type III presynaptic TRCs on the tongue, which were previously suggested as the sour taste sensors, also mediate taste responses to water. ( c) Dehydration is aversive, and median preoptic nucleus (MnPO) neuron activity is proportional to the intensity of this aversive state. ( d) MnPO^GLP1R neurons serve as a central detector that discriminates fluid ingestion from solid ingestion, which promotes acute satiation of thirst through the subfornical organ and other downstream targets.

Allostasis: A Brain-Centered, Predictive Mode of Physiological Regulation

Although the concept of allostasis was proposed some 30 years ago, doubts persist about its precise meaning and whether it is useful. Here we review the concept in the context of recent studies as a strategy to efficiently regulate physiology and behavior. The brain, sensing the internal and external milieu, and consulting its database, predicts what is likely to be needed; then, it computes the best response. The brain rewards a better-than-predicted result with a pulse of dopamine, thereby encouraging the organism to learn effective regulatory behaviors. The brain, by prioritizing behaviors and dynamically adjusting the flows of energy and nutrients, reduces costly errors and exploits more opportunities. Despite significant costs of computation, allostasis pays off and can now be recognized as a core principle of organismal design.

The Amphibious Mudskipper: A Unique Model Bridging the Gap of Central Actions of Osmoregulatory Hormones Between Terrestrial and Aquatic Vertebrates

Body fluid regulation, or osmoregulation, continues to be a major topic in comparative physiology, and teleost fishes have been the subject of intensive research. Great progress has been made in understanding the osmoregulatory mechanisms including drinking behavior in teleosts and mammals. Mudskipper gobies can bridge the gap from aquatic to terrestrial habitats by their amphibious behavior, but the studies are yet emerging. In this review, we introduce this unique teleost as a model to study osmoregulatory behaviors, particularly amphibious behaviors regulated by the central action of hormones. Regarding drinking behavior of mammals, a thirst sensation is aroused by angiotensin II (Ang II) through direct actions on the forebrain circumventricular structures, which predominantly motivates them to search for water and take it into the mouth for drinking. By contrast, aquatic teleosts can drink water that is constantly present in their mouth only by reflex swallowing, and Ang II induces swallowing by acting on the hindbrain circumventricular organ without inducing thirst. In mudskippers, however, through the loss of buccal water by swallowing, which appears to induce buccal drying on land, Ang II motivates these fishes to move to water for drinking. Thus, mudskippers revealed a unique thirst regulation by sensory detection in the buccal cavity. In addition, the neurohypophysial hormones, isotocin (IT) and vasotocin (VT), promote migration to water via IT receptors in mudskippers. VT is also dipsogenic and the neurons in the forebrain may mediate their thirst. VT regulates social behaviors as well as osmoregulation. The VT-induced migration appears to be a submissive response of subordinate mudskippers to escape from competitive and dehydrating land. Together with implications of VT in aggression, mudskippers may bridge the multiple functions of neurohypophysial hormones. Interestingly, cortisol, an important hormone for seawater adaptation and stress response in teleosts, also stimulates the migration toward water, mediated possibly via the mineralocorticoid receptor. The corticosteroid system that is responsive to external stressors can accelerate emergence of migration to alternative habitats. In this review, we suggest this unique teleost as an important model to deepen insights into the behavioral roles of these hormones in relation to osmoregulation.