Thirst deficits in aged rats are reversed by dietary omega-3 fatty acid supplementation

Thirst: neuroendocrine regulation in mammals

Animals can sense their changing internal needs and then generate specific physiological and behavioural responses in order to restore homeostasis. Water-saline homeostasis derives from balances of water and sodium intake and output (drinking and diuresis, salt appetite and natriuresis), maintaining an appropriate composition and volume of extracellular fluid. Thirst is the sensation which drives to seek and consume water, regulated in the central nervous system by both neural and chemical signals. Water and electrolyte homeostasis depends on finely tuned physiological mechanisms, mainly susceptible to plasma Na+ concentration and osmotic pressure, but also to blood volume and arterial pressure. Increases of osmotic pressure as slight as 1-2% are enough to induce thirst ("homeostatic" or cellular), by activation of specialized osmoreceptors in the circumventricular organs, outside the blood-brain barrier. Presystemic anticipatory signals (by oropharyngeal or gastrointestinal receptors) inhibit thirst when fluids are ingested, or stimulate thirst associated with food intake. Hypovolemia, arterial hypotension, Angiotensin II stimulate thirst ("hypovolemic thirst", "extracellular dehydration"). Hypervolemia, hypertension, Atrial Natriuretic Peptide inhibit thirst. Circadian rhythms of thirst are also detectable, driven by suprachiasmatic nucleus in the hypothalamus. Such homeostasis and other fundamental physiological functions (cardiocircolatory, thermoregulation, food intake) are highly interdependent.

The imperative of arachidonic acid in early human development

This review is about the role of arachidonic acid (ArA) in foetal and early growth and development. In 1975 and '76, we reported the preferential incorporation of ArA into the developing brain of rat pups, its conservation as a principal component in the brains of 32 mammalian species and the high proportion delivered by the human placenta for foetal nutrition, compared to its parent linoleic acid (LA). ArA is quantitatively the principal acyl component of membrane lipids from foetal red cells, mononuclear cells, astrocytes, endothelium, and placenta. Functionally, we present evidence that ArA, but not DHA, relaxes the foetal mesenteric arteries. The placenta biomagnifies ArA, doubling the proportion of the maternal level in cord blood. The proportions of ArA and its allies (di-homo-gamma-linolenic acid (DGLA), adrenic acid and ω6 docosapentaenoic acid) are similar or higher than the total of ω3 fatty acids in human milk, maintaining the abundant supply to the developing infant. Despite the evidence of the importance of ArA, the European Food Standard Agency, in 2014 rejected the joint FAO and WHO recommendation on the inclusion of ArA in infant formula, although they recommended DHA. The almost universal dominance of ArA in the membrane phosphoglycerides during human organogenesis and prenatal growth suggests that the importance of ArA and its allies in reproductive biology needs to be re-evaluated urgently.

The gut-brain axis and sodium appetite: Can inflammation-related signaling influence the control of sodium intake?

Sodium is the main cation present in the extracellular fluid. Sodium and water content in the body are responsible for volume and osmotic homeostasis through mechanisms involving sodium and water excretion and intake. When body sodium content decreases below the homeostatic threshold, a condition termed sodium deficiency, highly motivated sodium seeking, and intake occurs. This is termed sodium appetite. Classically, sodium and water intakes are controlled by a number of neuroendocrine mechanisms that include signaling molecules from the renin-angiotensin-aldosterone system acting in the central nervous system (CNS). However, recent findings have shown that sodium and water intakes can also be influenced by inflammatory agents and mediators acting in the CNS. For instance, central infusion of IL-1β or TNF-α can directly affect sodium and water consumption in animal models. Some dietary conditions, such as high salt intake, have been shown to change the intestinal microbiome composition, stimulating the immune branch of the gut-brain axis through the production of inflammatory cytokines, such as IL-17, which can stimulate the brain immune system. In this review, we address the latest findings supporting the hypothesis that immune signaling in the brain could produce a reduction in thirst and sodium appetite and, therefore, contribute to sodium intake control.

Impaired Fluid Intake, but Not Sodium Appetite, in Aged Rats Is Mediated by the Cyclooxygenase-Prostaglandin E2 Pathway

Aging results in decreased fluid intake following dehydration and other dipsogenic stimuli; similar reductions in sodium intake have also been observed with aging. Given that cyclooxygenase (COX)-derived prostanoids are elevated in aged rats in the midbrain and proinflammatory prostanoids are known to decrease fluid intake in dehydrated rats, the aim of this study was to determine if the reductions of fluid intake and sodium intake in aging are mediated by proinflammatory eicosanoid signaling. Therefore, we examined the effect of acute COX inhibition in adult (4 months-old) and aged (30 months-old) rats prior to ingestive behavior challenges. COX inhibition, using acetylsalicylic acid (ASA), increased fluid intake in aged, but not adult, rats in response to 24-h dehydration. ASA had no effect on salt intake following sodium depletion and ASA did not change basal fluid or sodium consumption in either age group. Hypothalamic COX-1 and -2, prostaglandin E synthase (PGES) and inducible nitric oxide synthase (iNOS) mRNA expression were all elevated in aged animals, leading to elevated PGE₂ levels. COX expression in the hypothalamus was reduced by ASA treatment in rats of both ages resulting in reduced PGE₂ levels in aged ASA treated animals. These data indicate that the reduced fluid intake that occurs in aging is due to increased COX-PGE₂-mediated inflammation. However, the reduced sodium intake in these animals appears to occur via an alternate mechanism.

High Fat Diet Dysregulates Hypothalamic-Pituitary Axis Gene Expression Levels which are Differentially Rescued by EPA and DHA Ethyl Esters

SCOPE

Dietary fat composition can modulate gene expression in peripheral tissues in obesity. Observations of the dysregulation of growth hormone (GH) in obesity indicate that these effects extend to the hypothalamic-pituitary (H-P) axis. The authors thus determine whether specific high fat (HF) diets influence the levels of Gh and other key gene transcripts in the H-P axis.

METHODS AND RESULTS

C57BL/6 mice are fed a lean control diet or a HF diet in the absence or presence of OA, EPA, or DHA ethyl esters. Comparative studies are conducted with menhaden fish oil. The HF diet lowered pituitary Gh mRNA and protein levels, and cell culture studies reveal that elevated insulin and glucose can reduce Gh transcripts. Supplementation of the HF diet with OA, EPA, DHA, or menhaden fish oil do not improve pituitary Gh levels. The HF diet also impaired the levels of additional genes in the pituitary and hypothalamus, which are selectively rescued with EPA or DHA ethyl esters. The effects of EPA and DHA are more robust relative to fish oil.

CONCLUSION

A HF diet can affect H-P axis transcription, which can be mitigated in some genes by EPA and DHA, but not fish oil in most cases.

Prostaglandins and n-3 polyunsaturated fatty acids in the regulation of the hypothalamic-pituitary axis

The hypothalamic-pituitary (H-P) axis integrates complex physiological and environmental signals and responds to these cues by modulating the synthesis and secretion of multiple pituitary hormones to regulate peripheral tissues. Prostaglandins are a component of this regulatory system, affecting multiple hormone synthesis and secretion pathways in the H-P axis. The implications of these actions are that physiological processes or disease states that alter prostaglandin levels in the hypothalamus or pituitary can impinge on H-P axis function. Considering the role of prostaglandins in mediating inflammation, the potential for neuroinflammation to affect H-P axis function in this manner may be significant. In addition, the mitigating effects of n-3 polyunsaturated fatty acids (n-3 PUFA) on the inflammation-associated synthesis of prostaglandins and their role as substrates for pro-resolving lipid mediators may also include effects in the H-P axis. One context in which neuroinflammation may play a role is in the etiology of diet-induced obesity, which also correlates with altered pituitary hormone levels. This review will survey evidence for the actions of prostaglandins and other lipid mediators in the H-P axis, and will address the potential for obesity-associated inflammation and n-3 PUFA to impinge on these mechanisms.