Salt Appetite, and the Influence of Opioids
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OVERVIEW
Salt Appetite, and the Influence of Opioids Craig M. Smith1,2 · Andrew J. Lawrence2,3
Received: 9 May 2017 / Revised: 15 June 2017 / Accepted: 19 June 2017 © Springer Science+Business Media, LLC 2017
Abstract Due to the biological importance of sodium and its relative scarcity within many natural environments, ‘salt appetite’ has evolved whereby dietary salt is highly sought after and palatable when tasted. In addition to peripheral responses, salt depletion is detected within the brain via circumventricular organs and 11β-hydroxysteroid dehydrogenase type 2 (HSD2) neurons to increase salt appetite. Salt appetite is comprised of two main components. One component is the incentive salience or motivation for salt (i.e. how much salt is ‘wanted’). Incentive salience is dynamic and largely depends on internal homeostatic conditions in combination with the detection of relevant cues. It involves the mesolimbic system and structures such as the central amygdala, and opioid signalling within these regions can increase salt intake in rodents. A second key feature is the hedonic palatability of salt (i.e. how much it is ‘liked’) when it is tasted. After detection on the tongue, gustatory information passes through the brainstem nucleus of the solitary tract and thalamus, before being consciously detected within the gustatory cerebral cortex. The positive or negative hedonic value of this stimulus is also dynamic, and is encoded by a network including the nucleus accumbens, ventral pallidum, and lateral parabrachial nucleus. Opioid signalling within these areas can alter salt intake, * Andrew J. Lawrence [email protected] 1
Faculty of Health, The School of Medicine, Deakin University, Geelong, VIC 3220, Australia
2
The Florey Institute of Neuroscience and Mental Health, The University of Melbourne, Melbourne, VIC 3052, Australia
3
The Florey Department of Neuroscience and Mental Health, The University of Melbourne, Melbourne, VIC 3052, Australia
and ‘liking’. The overconsumption of dietary salt likely contributes to hypertension and associated diseases, and hence further characterising the role played by opioid signalling has important implications for human health. Keywords Salt appetite · Opioids · Incentive salience · Palatability · Reward Abbreviations AT1a Angiotensin II receptor type 1a ACE Angiotensin-converting enzyme AgRP Agouti-related peptide CCK Cholecystokinin CeA Central amygdala CGRP Calcitonin gene-related peptide CN Cranial nerve CRF Corticotrophin releasing factor CVO Circumventricular organ DBH Dopamine β-hydroxylase DREADDs Designer receptors exclusively activated by designer drugs HSD2 11β-Hydroxysteroid dehydrogenase type 2 LPBN Lateral parabrachial nucleus MOR Mu-opioid receptor NAc Nucleus accumbens NTS Nucleus of the solitary tract PBN Parabrachial nucleus VP Ventral pallidum
Salt Appetite: An Instinctive Drive Elemental sodium is essential for a broad range of biochemical, cellular and physiological functions. Na+ is mostly obtained as die
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