Salt Appetite, and the Influence of Opioids

Association of Salty and Sweet Taste Recognition with Food Reward and Subjective Control of Eating Behavior

Sweet and salty tastes are highly palatable and drive food consumption and potentially uncontrolled eating, but it remains unresolved whether the ability to recognize sweet and salty affects food reward and uncontrolled eating. We investigate the association of sweet and salty taste recognition with liking and wanting and uncontrolled eating. Thirty-eight, mainly female (68%) participants of the Obese Taste Bud study, between 22 and 67 years old, with a median BMI of 25.74 kg/m2 (interquartile range: 9.78 kg/m2) completed a taste test, the Leeds Food Preference Questionnaire to assess food reward, the Power of Food Scale (PFS) and the Three-Factor Eating Questionnaire to assess different aspects of uncontrolled eating. Better salty taste recognition predicted greater implicit wanting for high-fat savory foods (β = 0.428, p = 0.008) and higher PFS total (β = 0.315; p = 0.004) and PFS present subscale scores (β = 0.494, p = 0.002). While neither sweet nor salty taste recognition differed between lean individuals and individuals with obesity, those with greater trait uncontrolled eating showed significantly better salty taste recognition (U = 249.0; p = 0.009). Sweet taste recognition did not associate with food reward or uncontrolled eating. Better salty but not sweet taste recognition associates with a greater motivation for, but not liking of, particularly savory high-fat foods and further relates to greater loss of control over eating. Salty taste perception, with taste recognition in particular, may comprise a target to modulate food reward and uncontrolled eating.

Ionic mechanisms involved in arginine vasopressin-mediated excitation of auditory cortical and thalamic neurons

The axons containing arginine vasopressin (AVP) from the hypothalamus innervate a variety of structures including the cerebral cortex, thalamus, hippocampus and amygdala. A plethora amount of evidence indicates that activation of the V1a subtype of the vasopressin receptors facilitates anxiety-like and fear responses. As an essential structure involved in fear and anxiety responses, the amygdala, especially the lateral nucleus of amygdala (LA), receives glutamatergic innervations from the auditory cortex and auditory thalamus where high density of V1a receptors have been detected. However, the roles and mechanisms of AVP in these two important areas have not been determined, which prevents the understanding of the mechanisms whereby V1a activation augments anxiety and fear responses. Here, we used coronal brain slices and studied the effects of AVP on neuronal activities of the auditory cortical and thalamic neurons. Our results indicate that activation of V1a receptors excited both auditory cortical and thalamic neurons. In the auditory cortical neurons, AVP increased neuronal excitability by depressing multiple subtypes of inwardly rectifying K+ (Kir) channels including the Kir2 subfamily, the ATP-sensitive K+ channels and the G protein-gated inwardly rectifying K+ (GIRK) channels, whereas activation of V1a receptors excited the auditory thalamic neurons by depressing the Kir2 subfamily of the Kir channels as well as activating the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and a persistent Na+ channel. Our results may help explain the roles of V1a receptors in facilitating fear and anxiety responses. Categories: Cell Physiology.

Leptin excites basolateral amygdala principal neurons and reduces food intake by LepRb-JAK2-PI3K-dependent depression of GIRK channels

Leptin is an adipocyte-derived hormone that modulates food intake, energy balance, neuroendocrine status, thermogenesis, and cognition. Whereas a high density of leptin receptors has been detected in the basolateral amygdala (BLA) neurons, the physiological functions of leptin in the BLA have not been determined yet. We found that application of leptin excited BLA principal neurons by activation of the long form leptin receptor, LepRb. The LepRb-elicited excitation of BLA neurons was mediated by depression of the G protein-activated inwardly rectifying potassium (GIRK) channels. Janus Kinase 2 (JAK2) and phosphoinositide 3-kinase (PI3K) were required for leptin-induced excitation of BLA neurons and depression of GIRK channels. Microinjection of leptin into the BLA reduced food intake via activation of LepRb, JAK2, and PI3K. Our results may provide a cellular and molecular mechanism to explain the physiological roles of leptin in vivo.

Separating desire from prediction of outcome value

Individuals typically want what they expect to like, often based on memories of previous positive experiences. However, in some situations desire can decouple completely from memories and from learned predictions of outcome value. The potential for desire to separate from prediction arises from independent operating rules that control motivational incentive salience. Incentive salience, or 'wanting', is a type of mesolimbic desire that evolved for adaptive goals, but can also generate maladaptive addictions. Two proof-of-principle examples are presented here to show how motivational 'wanting' can soar above memory-based predictions of outcome value: (i) 'wanting what is remembered to be disgusting', and (ii) 'wanting what is predicted to hurt'. Consequently, even outcomes remembered and predicted to be negatively aversive can become positively 'wanted'. Similarly, in human addictions, people may experience powerful cue-triggered cravings for outcomes that are not predicted to be enjoyable.

Neuromedin B excites central lateral amygdala neurons and reduces cardiovascular output and fear-potentiated startle

Neuromedin B (NMB) and gastrin-releasing peptide (GRP) are the two mammalian analogs in the bombesin peptide family that exert a variety of actions including emotional processing, appetitive behaviors, cognition, and tumor growth. The bombesin-like peptides interact with three receptors: the NMB-preferring bombesin 1 (BB1) receptors, the GRP-preferring bombesin 2 (BB2) receptors and the orphan bombesin 3 (BB3) receptors. Whereas, injection of bombesin into the central amygdala reduces satiety and modulates blood pressure, the underlying cellular and molecular mechanisms have not been determined. As administration of bombesin induces the expression of Fos in the lateral nucleus of the central amygdala (CeL) which expresses BB1 receptors, we probed the effects of NMB on CeL neurons using in vitro and in vivo approaches. We showed that activation of the BB1 receptors increased action potential firing frequency recorded from CeL neurons via inhibition of the inwardly rectifying K+ (Kir) channels. Activities of phospholipase Cβ and protein kinase C were required, whereas intracellular Ca2+ release was unnecessary for BB1 receptor-elicited potentiation of neuronal excitability. Application of NMB directly into the CeA reduced blood pressure and heart rate and significantly reduced fear-potentiated startle. We may provide a cellular and molecular mechanism whereby bombesin-like peptides modulate anxiety and fear responses in the amygdala.

Central regulation of body fluid homeostasis

Extracellular fluids, including blood, lymphatic fluid, and cerebrospinal fluid, are collectively called body fluids. The Na⁺ concentration ([Na⁺]) in body fluids is maintained at 135-145 mM and is broadly conserved among terrestrial animals. Homeostatic osmoregulation by Na⁺ is vital for life because severe hyper- or hypotonicity elicits irreversible organ damage and lethal neurological trauma. To achieve "body fluid homeostasis" or "Na homeostasis", the brain continuously monitors [Na⁺] in body fluids and controls water/salt intake and water/salt excretion by the kidneys. These physiological functions are primarily regulated based on information on [Na⁺] and relevant circulating hormones, such as angiotensin II, aldosterone, and vasopressin. In this review, we discuss sensing mechanisms for [Na⁺] and hormones in the brain that control water/salt intake behaviors, together with the responsible sensors (receptors) and relevant neural pathways. We also describe mechanisms in the brain by which [Na⁺] increases in body fluids activate the sympathetic neural activity leading to hypertension.

Salt as a non-caloric behavioral modifier: A review of evidence from pre-clinical studies

Though excess salt intake is well-accepted as a dietary risk factor for cardiovascular diseases, relatively little has been explored about how it impacts behavior, despite the ubiquity of salt in modern diets. Given the challenges of manipulating salt intake in humans, non-human animals provide a more tractable means for evaluating behavioral sequelae of high salt. By describing what is known about the impact of elevated salt on behavior, this review highlights how underexplored salt's behavioral effects are. Increased salt consumption in adulthood does not affect spontaneous anxiety-related behaviors or locomotor activity, nor acquisition of maze or fear tasks, but does impede expression of spatial/navigational and fear memory. Nest building is reduced by heightened salt in adults, and stress responsivity is augmented. When excess salt exposure occurs during development, and/or to parents, offspring locomotion is increased, and both spatial memory expression and social investigation are attenuated. The largely consistent findings reviewed here indicate expanded study of salt's effects will likely uncover broader behavioral implications, particularly in the scarcely studied female sex.

Ionic and signaling mechanisms involved in neurotensin-mediated excitation of central amygdala neurons

Neurotensin (NT) serves as a neuromodulator in the brain where it regulates a variety of physiological functions. Whereas the central amygdala (CeA) expresses NT peptide and NTS1 receptors and application of NT has been shown to excite CeA neurons, the underlying cellular and molecular mechanisms have not been determined. We found that activation of NTS1 receptors increased the neuronal excitability of the lateral nucleus (CeL) of CeA. Both phospholipase Cβ (PLCβ) and phosphatidylinositol 4,5-bisphosphate (PIP₂) depletion were required, whereas intracellular Ca²⁺ release and PKC were unnecessary for NT-elicited excitation of CeL neurons. NT increased the input resistance and time constants of CeL neurons, suggesting that NT excites CeL neurons by decreasing a membrane conductance. Depressions of the inwardly rectifying K⁺ (Kir) channels including both the Kir2 subfamily and the GIRK channels were required for NT-elicited excitation of CeL neurons. Activation of NTS1 receptors in the CeL led to GABAergic inhibition of medial nucleus of CeA neurons, suggesting that NT modulates the network activity in the amygdala. Our results may provide a cellular and molecular mechanism to explain the physiological functions of NT in vivo.

Involvement of TRPC5 channels, inwardly rectifying K+ channels, PLCβ and PIP2 in vasopressin-mediated excitation of medial central amygdala neurons

KEY POINTS

Activation of V_1a vasopressin receptors facilitates neuronal excitability in the medial nucleus of central amygdala (CeM) V_1a receptor activation excites about 80% CeM neurons by opening a cationic conductance and about 20% CeM neurons by suppressing an inwardly rectifying K⁺ (Kir) channel The cationic conductance activated by V_1a receptors is identified as TRPC5 channels PLCβ-mediated depletion of PIP₂ is involved in V_1a receptor-elicited excitation of CeM neurons Intracellular Ca²⁺ release and PKC are unnecessary for V_1a receptor-mediated excitation of CeM neurons ABSTRACT: Arginine vasopressin (AVP) serves as a hormone in the periphery to modulate water homeostasis and a neuromodulator in the brain to regulate a diverse range of functions including anxiety, social behaviour, cognitive activities and nociception. The amygdala is an essential brain region involved in modulating defensive and appetitive behaviours, pain and alcohol use disorders. Whereas activation of V_1a receptors in the medial nucleus of the central amygdala (CeM) increases neuronal excitability, the involved ionic and signalling mechanisms have not been determined. We found that activation of V_1a receptors in the CeM facilitated neuronal excitability predominantly by opening TRPC5 channels, although AVP excited about one fifth of the CeM neurons via suppressing an inwardly rectifying K⁺ (Kir) channel. G proteins and phospholipase Cβ (PLCβ) were required for AVP-elicited excitation of CeM neurons, whereas intracellular Ca²⁺ release and the activity of protein kinase C were unnecessary. Prevention of the depletion of phosphatidylinositol 4,5-bisphosphate (PIP₂ ) blocked AVP-induced excitation of CeM neurons, suggesting that PLCβ-mediated depletion of PIP₂ is involved in AVP-mediated excitation of CeM neurons. Our results may provide a cellular and molecular mechanism to explain the anxiogenic effects of AVP in the amygdala.

Oxytocin receptors excite lateral nucleus of central amygdala by phospholipase Cβ- and protein kinase C-dependent depression of inwardly rectifying K+ channels

KEY POINTS

Activation of oxytocin receptors (OXTRs) facilitates neuronal excitability in rat lateral nucleus of central amygdala (CeL). OXTR-induced excitation is mediated by inhibition of inwardly rectifying K⁺ (Kir) channels. Phospholipase Cβ is necessary for OXTR-mediated excitation of CeL neurons and depression of Kir channels. OXTR-elicited depression of Kir channels and excitation of CeL neurons require the function of Ca²⁺ -dependent protein kinase C.

ABSTRACT

Oxytocin (OXT) is a nonapeptide that exerts anxiolytic effects in the brain. The amygdala is an important structure involved in the modulation of fear and anxiety. A high density of OXT receptors (OXTRs) has been detected in the capsular (CeC) and lateral (CeL) nucleus of the central amygdala (CeA). Previous studies have demonstrated that activation of OXTRs induces remarkable increases in neuronal excitability in the CeL/C. However, the signalling and ionic mechanisms underlying OXTR-induced facilitation of neuronal excitability have not been determined. We found that activation of OXTRs in the CeL increased action potential firing frequency recorded from neurons in this region via inhibition of the inwardly rectifying K⁺ channels. The functions of phospholipase Cβ and protein kinase C were required for OXTR-induced augmentation of neuronal excitability. Our results provide a cellular and molecular mechanism whereby activation of OXTRs exerts anxiolytic effects.

The influence of opioid dependence on salt consumption and related psychological parameters in mice and humans

BACKGROUND

The consumption of dietary salt (NaCl) is controlled by neuronal pathways that are modulated by endogenous opioid signalling. The latter is disrupted by chronic use of exogenous opioid receptor agonists, such as morphine. Therefore, opioid dependence may influence salt consumption, which we investigated in two complimentary studies in humans and mice.

METHODS

Human study: three groups were recruited: i. Individuals who are currently opioid dependent and receiving opioid substitution treatment (OST); ii. Previously opioid dependent individuals, who are currently abstinent, and; iii. Healthy controls with no history of opioid dependence. Participants tasted solutions containing different salt concentrations and indicated levels of salt 'desire', salt 'liking', and perceptions of 'saltiness'. Mouse study: preference for 0.1 M versus 0.2 M NaCl and overall levels of salt consumption were recorded during and after chronic escalating morphine treatment.

RESULTS

Human study: Abstinent participants' 'desire' for and 'liking' of salt was shifted towards more highly concentrated salt solutions relative to control and OST individuals. Mouse study: Mice increased their total salt consumption during morphine treatment relative to vehicle controls, which persisted for 3 days after cessation of treatment. Preference for 'low' versus 'high' concentrations of salt were unchanged.

CONCLUSION

These findings suggest a possible common mechanistic cross-sensitization to salt that is present in both mice and humans and builds our understanding of how opioid dependence can influence dietary salt consumption. This research may help inform better strategies to improve the diet and overall wellbeing of the growing number of individuals who develop opioid dependence.

Phenotyping neurons activated in the mouse brain during restoration of salt debt

Salt overconsumption contributes to hypertension, which is a major risk factor for stroke, heart and kidney disease. Characterising neuronal pathways that may control salt consumption is therefore important for developing novel approaches for reducing salt overconsumption. Here, we identify neurons within the mouse central amygdala (CeA), lateral parabrachial nucleus (LPBN), intermediate nucleus of the solitary tract (iNTS), and caudal NTS (cNTS) that are activated and display Fos immunoreactivity in mice that have consumed salt in order to restore a salt debt, relative to salt replete and salt depleted controls. Double-label immunohistochemical studies revealed that salt restoring mice had significantly greater densities of activated enkephalin neurons within the CeA and iNTS, while statistically significant changes within the LPBN and cNTS were not observed. Furthermore, within the CeA, restoration of salt debt conferred a significant increase in the density of activated calretinin neurons, while there was no change relative to control groups in the density of activated neurons that co-expressed protein kinase C delta (PKC-δ). Taken together, these studies highlight the importance of opioid systems within the CeA and iNTS in neuronal processes associated with salt restoration, and may aid the development of future pharmacological and other strategies for reducing salt overconsumption.

Chemosensory modulation of neural circuits for sodium appetite

Sodium is the main cation in the extracellular fluid that regulates various physiological functions. Sodium-depletion in the body elevates the hedonic value of sodium taste, which drives animals toward sodium consumption ^1,2. Conversely, oral sodium detection rapidly promotes satiation of sodium appetite ^3,4, suggesting that chemosensory signals have a central role in sodium appetite and its satiety. Nevertheless, the neural basis of chemosensory-based appetite regulation remains poorly understood. Here, we dissect genetically-defined neural circuits in mice that control sodium intake by integrating sodium taste and internal depletion signals. We show that a subset of excitatory neurons in the pre-locus coeruleus (pre-LC) that express prodynorphin (PDYN) serve as a critical neural substrate for sodium intake behavior. Acute stimulation of this population triggered robust sodium ingestion even from rock salt by transmitting negative valence signals. Inhibition of the same neurons selectively reduced sodium consumption. We further demonstrate that peripheral chemosensory signals rapidly suppressed these sodium appetite neurons. Simultaneous in vivo optical recording and gastric infusion revealed that sensory detection of sodium, but not sodium ingestion per se, is required for the acute modulation of pre-LC PDYN neurons and satiety of sodium appetite. Moreover, retrograde virus tracing showed that sensory modulation is partly mediated by specific GABAergic neurons in the bed nucleus of the stria terminalis. This inhibitory neural population is activated upon sodium ingestion, and sends rapid inhibitory signals to sodium appetite neurons. Together, this study reveals a dynamic circuit diagram that integrates chemosensory signals and the internal need to maintain sodium balance.