The microbiota-gut-brain axis: neurobehavioral correlates, health and sociality

Recent data suggest that the human body is not such a neatly self-sufficient island after all. It is more like a super-complex ecosystem containing trillions of bacteria and other microorganisms that inhabit all our surfaces; skin, mouth, sexual organs, and specially intestines. It has recently become evident that such microbiota, specifically within the gut, can greatly influence many physiological parameters, including cognitive functions, such as learning, memory and decision making processes. Human microbiota is a diverse and dynamic ecosystem, which has evolved in a mutualistic relationship with its host. Ontogenetically, it is vertically inoculated from the mother during birth, established during the first year of life and during lifespan, horizontally transferred among relatives, mates or close community members. This micro-ecosystem serves the host by protecting it against pathogens, metabolizing complex lipids and polysaccharides that otherwise would be inaccessible nutrients, neutralizing drugs and carcinogens, modulating intestinal motility, and making visceral perception possible. It is now evident that the bidirectional signaling between the gastrointestinal tract and the brain, mainly through the vagus nerve, the so called “microbiota–gut–vagus–brain axis,” is vital for maintaining homeostasis and it may be also involved in the etiology of several metabolic and mental dysfunctions/disorders. Here we review evidence on the ability of the gut microbiota to communicate with the brain and thus modulate behavior, and also elaborate on the ethological and cultural strategies of human and non-human primates to select, transfer and eliminate microorganisms for selecting the commensal profile.

Expectations and associations that heal: Immunomodulatory placebo effects and its neurobiology

The use of placebo may have accompanied healing and medical practices since their origins (Plato; Charmides, 155-156). Recent experimental data indicate that we would be well advised to further consider placebo effects in future therapeutic strategies, with a better knowledge of their potency, psychological basis and underlying neurobiological mechanisms. Current research in the areas of pain, depression and Parkinson's disease has uncovered some of the potential neurobiological mechanisms of placebo effects. These data indicate that conscious expectation and unconscious behavioral conditioning processes appear to be the major neurobiological mechanisms capable of releasing endogenous neurotransmitters and/or neurohormones that mimic the expected or conditioned pharmacological effects. To date, research on placebo responses affecting immune-related diseases is scarce, but there are consistent indications that skin and mucosal inflammatory diseases, in particular, are strongly modulated by placebo treatments. However, the brain's capability to modulate peripheral immune reactivity has been impressively demonstrated by paradigms of behavioral conditioning in animal experiments and human studies. Thus, placebo effects can benefit end organ functioning and the overall health of the individual through positive expectations and behavioral conditioning processes.

The learned immune response: Pavlov and beyond

The ability to associate physiological changes with a specific flavor was most likely acquired during evolution as an adaptive strategy aimed at protecting the organism while preparing it for danger. The behaviorally conditioned or learned immune response is an exquisite example of the bidirectional communication between the central nervous system (CNS) and the peripheral immune system. How is it possible that specific immuno-modulating properties of a drug or substance (unconditioned stimulus) can be re-enlisted just by the mere re-exposure to a particular taste, odor or environment (conditioned stimulus)? To answer this key question, we review the neurobiological mechanism mediating this type of associative learning, as well as the pathways and mechanisms employed by the brain to harness the immune system during the execution of the conditioned immune response. Finally, we focus on the potential therapeutic relevance of such learned immune responses, and their re-conceptualization within the framework of "learned placebo effects".

Acute amygdaloid response to systemic inflammation

The amygdala, a group of nuclei located in the medial temporal lobe, is a key limbic structure involved in mood regulation, associative learning, and modulation of cognitive functions. Functional neuroanatomical studies suggest that this brain region plays also an important role in the central integration of afferent signals from the peripheral immune system. In the present study, intracerebral electroencephalography and microdialysis were employed to investigate the electrophysiological and neurochemical consequences of systemic immune activation in the amygdala of freely moving rats. Intraperitoneal administration of bacterial lipopolysaccharide (100 μg/kg) induced with a latency of about 2 h a significant increase in amygdaloid neuronal activity and a substantial rise in extracellular noradrenaline levels. Activated neurons in the amygdaloid complex, identified by c-Fos immunohistochemistry, were mainly located in the central nucleus and, to a lesser extent, in the basolateral nucleus of the amygdala. Gene expression analysis in micropunches of the amygdala revealed that endotoxin administration induced a strong time-dependent increase in IL-1β, IL-6, and TNF-α mRNA levels indicating that these cytokines are de novo synthesized in the amygdala in response to peripheral immune activation. The changes in amygdaloid activity were timely related to an increase in anxiety-like behavior and decreased locomotor activity and exploration in the open-field. Taken together, these data give novel insights into different features of the acute amygdaloid response during experimental inflammation and provides further evidence that the amygdala integrates immune-derived information to coordinate behavioral and autonomic responses.

Neural substrates for behaviorally conditioned immunosuppression in the rat

We have previously demonstrated behaviorally conditioned immunosuppression using cyclosporin A as an unconditioned stimulus and saccharin as a conditioned stimulus. In the current study, we examined the central processing of this phenomenon generating excitotoxic lesions before and after acquisition to discriminate between learning and memory processes. Three different brain areas were analyzed: insular cortex (IC), amygdala (Am), and ventromedial nucleus of the hypothalamus (VMH). The results demonstrate that IC lesions performed before and after acquisition disrupted the behavioral component of the conditioned response (taste aversion). In contrast, Am and VMH lesions did not affect conditioned taste aversion. The behaviorally conditioned suppression of splenocyte proliferation and cytokine production (interleukin-2 and interferon-gamma) was differentially affected by the excitotoxic lesions, showing that the IC is essential to acquire and evoke this conditioned response of the immune system. In contrast, the Am seems to mediate the input of visceral information necessary at the acquisition time, whereas the VMH appears to participate within the output pathway to the immune system necessary to evoke the behavioral conditioned immune response. The present data reveal relevant neural mechanisms underlying the learning and memory processes of behaviorally conditioned immunosuppression.

Reciprocal Interactions Between Gut Microbiota and Host Social Behavior

Animals harbor an extensive, dynamic microbial ecosystem in their gut. Gut microbiota (GM) supposedly modulate various host functions including fecundity, metabolism, immunity, cognition and behavior. Starting by analyzing the concept of the holobiont as a unit of selection, we highlight recent findings suggesting an intimate link between GM and animal social behavior. We consider two reciprocal emerging themes: (i) that GM influence host social behavior; and (ii) that social behavior and social structure shape the composition of the GM across individuals. We propose that, throughout a long history of coevolution, GM may have become involved in the modulation of their host’s sociality to foster their own transmission, while in turn social organization may have fine-tuned the transmission of beneficial endosymbionts and prevented pathogen infection. We suggest that investigating these reciprocal interactions can advance our understanding of sociality, from healthy and impaired social cognition to the evolution of specific social behaviors and societal structure.

Hepatic-portal vein infusions of glucagon-like peptide-1 reduce meal size and increase c-Fos expression in the nucleus tractus solitarii, area postrema and central nucleus of the amygdala in rats

We recently reported that brief, remotely controlled intrameal hepatic-portal vein infusions of glucagon-like peptide-1 (GLP-1) reduced spontaneous meal size in rats. To investigate the neurobehavioural correlates of this effect, we equipped male Sprague-Dawley rats with hepatic-portal vein catheters and assessed (i) the effect on eating of remotely triggered infusions of GLP-1 (1 nmol/kg, 5 min) or vehicle during the first nocturnal meal after 3 h of food deprivation and (ii) the effect of identical infusions performed at dark onset on c-Fos expression in several brain areas involved in the control of eating. GLP-1 reduced (P < 0.05) the size of the first nocturnal meal and increased its satiety ratio. Also, GLP-1 increased (P < 0.05) the number of c-Fos-expressing cells in the nucleus tractus solitarii, the area postrema and the central nucleus of the amygdala, but not in the arcuate or paraventricular hypothalamic nuclei. These data suggest that the nucleus tractus solitarii, the area postrema and the central nucleus of the amygdala play a role in the eating-inhibitory actions of GLP-1 infused into the hepatic-portal vein; it remains to be established whether activation of these brain nuclei reflect satiation, aversion, or both.

Pavlovian Conditioning of Immunological and Neuroendocrine Functions

The phenomenon of behaviorally conditioned immunological and neuroendocrine functions has been investigated for the past 100 yr. The observation that associative learning processes can modify peripheral immune functions was first reported and investigated by Ivan Petrovic Pavlov and his co-workers. Their work later fell into oblivion, also because so little was known about the immune system’s function and even less about the underlying mechanisms of how learning, a central nervous system activity, could affect peripheral immune responses. With the employment of a taste-avoidance paradigm in rats, this phenomenon was rediscovered 45 yr ago as one of the most fascinating examples of the reciprocal functional interaction between behavior, the brain, and peripheral immune functions, and it established psychoneuroimmunology as a new research field. Relying on growing knowledge about efferent and afferent communication pathways between the brain, neuroendocrine system, primary and secondary immune organs, and immunocompetent cells, experimental animal studies demonstrate that cellular and humoral immune and neuroendocrine functions can be modulated via associative learning protocols. These (from the classical perspective) learned immune responses are clinically relevant, since they affect the development and progression of immune-related diseases and, more importantly, are also inducible in humans. The increased knowledge about the neuropsychological machinery steering learning and memory processes together with recent insight into the mechanisms mediating placebo responses provide fascinating perspectives to exploit these learned immune and neuroendocrine responses as supportive therapies, the aim being to reduce the amount of medication required, diminishing unwanted drug side effects while maximizing the therapeutic effect for the patient’s benefit.

Vagal afferents mediate early satiation and prevent flavour avoidance learning in response to intraperitoneally infused exendin-4

Glucagon-like peptide-1 receptor (GLP-1R) agonists such as exendin-4 (Ex-4) affect eating and metabolism and are potential candidates for treating obesity and type II diabetes. In the present study, we tested whether vagal afferents mediate the eating-inhibitory and avoidance-inducing effects of Ex-4. Subdiaphragmatic vagal deafferentation (SDA) blunted the short-term (< 1 h) but not long-term eating-inhibitory effect of i.p.-infused Ex-4 (0.1 μg/kg) in rats. A dose of 1 μg/kg Ex-4 reduced 0.5, 1, 2 and 4 h cumulative food intake in SDA and sham-operated rats to a similar extent. Paradoxically, SDA but not sham rats developed a conditioned flavour avoidance (CFA) after i.p. Ex-4 (0.1 μg/kg). SDA completely blunted the induction of c-Fos expression by Ex-4 in the hypothalamic paraventricular nucleus. Ex-4, however, increased the number of c-Fos expressing cells, independent of intact vagal afferents, in the nucleus accumbens and in the central nucleus of the amygdala, the lateral external parabrachial nucleus, the caudal ventrolateral medulla and the dorsal vagal complex. These data suggest that intact vagal afferents are only necessary for the full expression of the early satiating effect of Ex-4 but not for later eating-inhibitory actions, when circulating Ex-4 might reach the brain via the circulation. Our data also dissociate the satiating and avoidance-inducing effects of the low Ex-4 dose tested under our conditions and suggest that vagal afferent signalling may protect against the development of CFA. Taken together, these findings reveal a complex role of vagal afferents in mediating the effects of GLP-1R activation on ingestive behaviour.

Palatable Hyper-Caloric Foods Impact on Neuronal Plasticity

Neural plasticity is an intrinsic and essential characteristic of the nervous system that allows animals “self-tuning” to adapt to their environment over their lifetime. Activity-dependent synaptic plasticity in the central nervous system is a form of neural plasticity that underlies learning and memory formation, as well as long-lasting, environmentally-induced maladaptive behaviors, such as drug addiction and overeating of palatable hyper-caloric (PHc) food. In western societies, the abundance of PHc foods has caused a dramatic increase in the incidence of overweight/obesity and related disorders. To this regard, it has been suggested that increased adiposity may be caused at least in part by behavioral changes in the affected individuals that are induced by the chronic consumption of PHc foods; some authors have even drawn attention to the similarity that exists between over-indulgent eating and drug addiction. Long-term misuse of certain dietary components has also been linked to chronic neuroimmune maladaptation that may predispose individuals to neurodegenerative conditions such as Alzheimer’s disease. In this review article, we discuss recent evidence that shows how consumption of PHc food can cause maladaptive neural plasticity that converts short-term ingestive drives into compulsive behaviors. We also discuss the neural mechanisms of how chronic consumption of PHc foods may alter brain function and lead to cognitive impairments, focusing on prenatal, childhood and adolescence as vulnerable neurodevelopmental stages to dietary environmental insults. Finally, we outline a societal agenda for harnessing permissive obesogenic environments.

Circulating glucagon-like peptide-1 (GLP-1) inhibits eating in male rats by acting in the hindbrain and without inducing avoidance

To address the neural mediation of the eating-inhibitory effect of circulating glucagon-like peptide-1 (GLP-1), we investigated the effects of 1) intra-fourth ventricular infusion of the GLP-1 receptor antagonist exendin-9 or 2) area postrema lesion on the eating-inhibitory effect of intrameal hepatic portal vein (HPV) GLP-1 infusion in adult male rats. To evaluate the physiological relevance of the observed effect we examined 3) the influence of GLP-1 on flavor acceptance in a 2-bottle conditioned flavor avoidance test, and 4) measured active GLP-1 in the HPV and vena cava (VC) in relation to a meal and in the VC after HPV GLP-1 infusion. Intrameal HPV GLP-1 infusion (1 nmol/kg body weight-5 min) specifically reduced ongoing meal size by almost 40% (P < .05). Intra-fourth ventricular exendin-9 (10 μg/rat) itself did not affect eating, but attenuated (P < .05) the satiating effect of HPV GLP-1. Area postrema lesion also blocked (P < .05) the eating-inhibitory effect of HPV GLP-1. Pairing consumption of flavored saccharin solutions with HPV GLP-1 infusion did not alter flavor acceptance, indicating that HPV GLP-1 can inhibit eating without inducing malaise. A regular chow meal transiently increased (P < .05) HPV, but not VC, plasma active GLP-1 levels, whereas HPV GLP-1 infusion caused a transient supraphysiological increase (P < .01) in VC GLP-1 concentration 3 minutes after infusion onset. The results implicate hindbrain GLP-1 receptors and the area postrema in the eating-inhibitory effect of circulating GLP-1, but question the physiological relevance of the eating-inhibitory effect of iv infused GLP-1 under our conditions.

Peripheral glucagon-like peptide-1 (GLP-1) and satiation

Peripheral GLP-1 is produced by post-translational processing of pro-glucagon in enteroendocrine L-cells and is released in response to luminal nutrient (primarily carbohydrate and fat) stimulation. GLP-1 is well known for its potent insulinotropic and gluco-regulatory effects. GLP-1 receptors (GLP-1R) are expressed in the periphery and in several brain areas that are implicated in the control of eating. Both central and peripheral administration of GLP-1 have been shown to reduce food intake. Unresolved, however, is whether these effects reflect functions of endogenous GLP-1. Data collected in our laboratory indicate that in chow-fed rats: 1) Remotely controlled, intra-meal intravenous (IV) or intraperitoneal (IP) GLP-1 infusions selectively reduce meal size; 2) hindbrain GLP-1R activation is involved in the eating-inhibitory effect of IV infused GLP-1, whereas intact abdominal vagal afferents are necessary for the eating-inhibitory effect of IP, but not IV, infused GLP-1; 3) GLP-1 degradation in the liver prevents a systemic increase in endogenous GLP-1 during normal chow meals in rats; and 4) peripheral or hindbrain GLP-1R antagonism by exendin-9 does not affect spontaneous eating. Also, although our data indicate that peripheral GLP-1 can act in two different sites to inhibit eating, they argue against a role of systemic increases in endogenous GLP-1 in satiation in chow-fed rats. Therefore, further studies should examine whether a local paracrine action of GLP-1 in the intestine or and endocrine action in the hepatic-portal area is physiologically relevant for satiation.

Priming of metabolic dysfunctions by prenatal immune activation in mice: relevance to schizophrenia

Schizophrenia is associated with increased risk for multiple metabolic abnormalities, including altered glucose homeostasis, type-2 diabetes, obesity, and cardiovascular disease. Some of the metabolic alterations can already exist in psychosis-prone subjects prior to the onset of chronic schizophrenic disease and pharmacotherapy, indicating that they may have a developmental origin. In the present study, we tested the hypothesis that metabolic alterations pertinent to schizophrenic disease can be primed by an environmental risk factor associated with the disorder, namely prenatal exposure to immune challenge. We used a well-established mouse model of prenatal immune challenge induced by maternal gestational treatment with poly(I:C) (="polyriboinosinic-polyribocytidilic acid"), an analog of double-stranded RNA that stimulates a cytokine-associated viral-like acute phase response. Metabolic effects were studied using high-resolution computed tomography and fully automated indirect calorimetry system, along with an oral glucose tolerance test and plasma cytokine and corticosterone measurements. We found that prenatal immune activation caused altered glycemic regulation and abnormal ingestive behavior in periadolescence and led to an adult onset of excess visceral and subcutaneous fat deposition. These effects were accompanied by age-dependent changes in peripheral secretion of proinflammatory (interleukin [IL]-6 and tumor necrosis factor [TNF]-α) and T cell-related (IL-2 and interferon [IFN]-γ) cytokines and by increased release of the stress hormone corticosterone in periadolescence. Our findings show that schizophrenia-relevant metabolic and physiological abnormalities can be primed by prenatal viral-like immune activation, but at the same time, our study emphasizes that this environmental insult is unlikely to precipitate the full spectrum of metabolic and immunological changes pertinent to chronic schizophrenic disease.

Time-dependent alterations of peripheral immune parameters after nigrostriatal dopamine depletion in a rat model of Parkinson's disease

Dysfunction of the central dopaminergic system is associated with neurodegenerative disorders and mental illnesses such as Parkinson's disease and schizophrenia. Patients suffering from these diseases were reported to exhibit altered immune functions compared to healthy subjects and imbalance of the central dopaminergic system has been suggested as one causative factor for the immune disturbances. However, it is unclear whether the observed immune changes are primary or secondary to the disease. Here we demonstrate that central dopamine (DA) depletion in a rat model of Parkinson's disease induced transient changes in blood leukocyte distribution and cytokine production that were apparent until four weeks after bilateral intrastriatal administration of the neurotoxin 6-hydroxydopamine (6-OHDA). Eight weeks after treatment, no differences in blood immune parameters were anymore evident between neurotoxin-treated and control animals. Nevertheless, animals with a widespread damage of dopaminergic neurons in the nigrostriatal system showed an exacerbated pro-inflammatory response following in vivo challenge with bacterial lipopolysaccharide. Our data indicate that peripheral immune perturbations in the early phase after intrastriatal 6-OHDA administration might have been related to the neurodegenerative process itself whereas the increased sensitivity to the inflammatory stimulus seems to have resulted from an impaired dopaminergic control of prolactin (PRL) and corticosterone (CORT) secretion. The findings demonstrate that the brain dopaminergic system is involved in peripheral immune regulation and suggest that central dopaminergic hypoactivity bears the risk of excessive inflammation, e.g., during infection or tissue injury.

Diacylglycerol acyltransferase-1 inhibition enhances intestinal fatty acid oxidation and reduces energy intake in rats

Acyl CoA:diacylglycerol acyltransferase-1 (DGAT-1) catalyzes the final step in triacylglycerol (TAG) synthesis and is highly expressed in the small intestine. Because DGAT-1 knockout mice are resistant to diet-induced obesity, we investigated the acute effects of intragastric (IG) infusion of a small molecule diacylglycerol acyltransferase-1 inhibitor (DGAT-1i) on eating, circulating fat metabolites, indirect calorimetry, and hepatic and intestinal expression of key fat catabolism enzymes in male rats adapted to an 8 h feeding-16 h deprivation schedule. Also, the DGAT-1i effect on fatty acid oxidation (FAO) was investigated in enterocyte cell culture models. IG DGAT-1i infusions reduced energy intake compared with vehicle in high-fat diet (HFD)-fed rats, but scarcely in chow-fed rats. IG DGAT-1i also blunted the postprandial increase in serum TAG and increased β-hydroxybutyrate levels only in HFD-fed rats, in which it lowered the respiratory quotient and increased intestinal, but not hepatic, protein levels of Complex III of the mitochondrial respiratory chain and of mitochondrial hydroxymethylglutaryl-CoA synthase. Finally, the DGAT-1i enhanced FAO in CaCo2 (EC50 = 0.3494) and HuTu80 (EC50 = 0.00762) cells. Thus, pharmacological DGAT-1 inhibition leads to an increase in intestinal FAO and ketogenesis when dietary fat is available. This may contribute to the observed eating-inhibitory effect.

Quantification of DeQi sensation by visual analog scales in healthy humans after immunostimulating acupuncture treatment

Acupuncture is the most popular component of traditional Chinese medicine in Western countries. However, the mechanisms of its effects remain unclear. The therapeutic effect of acupuncture appears when a sensation of DeQi is achieved. We previously reported that repeated, but not single acupuncture treatment affected leukocyte circulation and blood pressure in healthy young humans. The objective of this study was to quantify DeQi sensation by using visual analog scales (VASs) and, to test whether DeQi induction is an important factor for the therapeutic effects of acupuncture in the same cohort. After either acupuncture or sham-acupuncture (placebo) treatment, a questionnaire containing five individual VASs was given to subjects to evaluate their DeQi sensation, including numbness, pressure, heaviness, warmth, and radiating paraesthesia, respectively. A separate VAS to measure their levels of anxiety during the treatment was also included. Our results showed that acupuncture significantly induced higher VAS values for numbness, pressure, warmth, and radiating paraesthesia, but not for heaviness than the placebo across three treatment sessions. Additionally, acupuncture did not induce higher anxiety levels than the placebo. These data confirm that VAS is an objective and reliable way to quantify DeQi sensation and, indicate that DeQi is unique to verum acupuncture treatment. Furthermore, either acupuncture-induced therapeutic effects or DeQi sensation should not be attributed to the stress-mediated effects. In summary, the induction of DeQi in each treatment session is an important factor for the physiological outcomes of repeated acupuncture treatment, and VASs offer objective, an easy and reliable way to assess it.

Calcineurin inhibition in splenocytes induced by pavlovian conditioning

Pavlovian conditioning is one of the major neurobiological mechanisms of placebo effects, potentially influencing the course of specific diseases and the response to a pharmacological therapy, such as immunosuppression. In our study with behaviorally conditioned rats, a relevant taste (0.2% saccharin) preceded the application of the immunosuppressive drug cyclosporin A (CsA), a specific calcineurin (CaN) inhibitor. Our results demonstrate that through pavlovian conditioning the particular pharmacological properties of CsA can be transferred to a neutral taste, i.e., CaN activity was inhibited in splenocytes from conditioned rats after reexposure to the gustatory stimulus. Concomitant immune consequences were observed on ex vivo mitogenic challenge (anti-CD3). Particularly, Th1-cytokine, but not Th2-cytokine, production and cell proliferation were impeded. Appropriate pharmacological and behavioral controls certify that all these changes in T-lymphocyte reactivity are attributable to mere taste reexposure. Furthermore, the underlying sympathetic-lymphocyte interaction was revealed modeling the conditioned response in vitro. CaN activity in CD4(+) T lymphocytes is reduced by beta-adrenergic stimulation (terbutaline), with these effects antagonized by the beta-adrenoreceptor antagonist nadolol. In summary, CaN was identified as the intracellular target for inducing conditioned immunosuppression by CsA, contributing to our understanding of the intracellular mechanisms behind "learned placebo effects."

Amygdaloid signature of peripheral immune activation by bacterial lipopolysaccharide or staphylococcal enterotoxin B

Activated immune cells produce soluble mediators that not only coordinate local and systemic immune responses but also act on the brain to initiate behavioral, neuroendocrine and metabolic adaptations. Earlier studies have shown that the amygdala, a group of nuclei located in the medial temporal lobe, is engaged in the central processing of afferent signals from the peripheral immune system. Here, we compared amygdaloid responses to lipopolysaccharide (LPS) and staphylococcal enterotoxin B (SEB), two prototypic bacterial products that elicit distinct immune responses. Intraperitoneal administration of LPS (0.1 mg/kg) or SEB (1 mg/kg) in adult rats induced substantial increases in amygdaloid neuronal activity as measured by intracerebral electroencephalography and c-fos gene expression. Amygdaloid neuronal activation was accompanied by an increase in anxiety-related behavior in the elevated plus-maze test. However, only treatment with LPS, but not SEB, enhanced amygdaloid IL-1β and TNF-α mRNA expression. This supports the view of the immune system as a sensory organ that recognizes invading pathogens and rapidly relays this information to the brain, independent of the nature of the immune response induced. The observation that neuronal and behavioral responses to peripheral immune challenges are not necessarily accompanied by increased brain cytokine expression suggests that cytokines are not the only factors driving sickness-related responses in the CNS.

Repeated acupuncture treatment affects leukocyte circulation in healthy young male subjects: a randomized single-blind two-period crossover study

Acupuncture is the most popular component of traditional Chinese medicine in western countries, which has been widely used in the treatment of numerous medical conditions, e.g., pain, emesis or asthma. However, the effects of acupuncture on neuroendocrine and immune functions in humans remain unclear. Therefore, the present study was performed to analyse whether acupuncture treatment affects leukocyte circulation as well as plasma levels of cortisol and norepinephrine in humans. Ten healthy young male subjects were enrolled in a randomized single-blind two-period crossover study. Each period contained three sessions of either acupuncture or sham acupuncture (placebo) treatment. After randomisation, the group 1 (n=5) received acupuncture treatment at acu-points ST36, LI11, SP10, and GV14, while sham acupuncture was performed for group 2 (n=5). Two weeks later, each group received the alternative treatment. Blood samples were taken before needling, 10 min after, and 30 min after removing the needles in the first and the third session. In addition, blood pressure and heart rate were determined simultaneously. Although acupuncture treatment did not affect leukocyte circulation in peripheral blood after the first session, we observed a significant decrease in leukocyte and lymphocyte values after the third session. In contrast, cortisol and norepinephrine plasma levels remained unchanged by acupuncture. These data indicate that repeated acupuncture treatment can affect leukocyte circulation in healthy humans by still unknown mechanisms.

Brain-immune interactions and the neural basis of disease-avoidant ingestive behaviour

Neuro-immune interactions are widely manifested in animal physiology. Since immunity competes for energy with other physiological functions, it is subject to a circadian trade-off between other energy-demanding processes, such as neural activity, locomotion and thermoregulation. When immunity is challenged, this trade-off is tilted to an adaptive energy protecting and reallocation strategy that is identified as 'sickness behaviour'. We review diverse disease-avoidant behaviours in the context of ingestion, indicating that several adaptive advantages have been acquired by animals (including humans) during phylogenetic evolution and by ontogenetic experiences: (i) preventing waste of energy by reducing appetite and consequently foraging/hunting (illness anorexia), (ii) avoiding unnecessary danger by promoting safe environments (preventing disease encounter by olfactory cues and illness potentiation neophobia), (iii) help fighting against pathogenic threats (hyperthermia/somnolence), and (iv) by associative learning evading specific foods or environments signalling danger (conditioned taste avoidance/aversion) and/or at the same time preparing the body to counteract by anticipatory immune responses (conditioning immunomodulation). The neurobiology behind disease-avoidant ingestive behaviours is reviewed with special emphasis on the body energy balance (intake versus expenditure) and an evolutionary psychology perspective.

Central catecholamine depletion inhibits peripheral lymphocyte responsiveness in spleen and blood

Experimental and clinical evidence has demonstrated extensive communication between the CNS and the immune system. To analyse the role of central catecholamines in modulating peripheral immune functions, we injected the neurotoxin 6-hydroxydopamine (6-OHDA) i.c.v. in rats. This treatment significantly reduced brain catecholamine content 2, 4 and 7 days after injection, and in the periphery splenic catecholamine levels were reduced 4 days after treatment. Central catecholamine depletion induced an inhibition of splenic and blood lymphocyte proliferation and splenic cytokine production and expression (interleukin-2 and interferon-gamma) 7 days after injection. In addition, central treatment with 6-OHDA reduced the percentage of spleen and peripheral blood natural killer (CD161 +) cells, and T-cytotoxic (CD8 +) cells in peripheral blood. The reduction in splenocyte proliferation was not associated with a glucocorticoid alteration but was completely abolished by prior peripheral sympathectomy. These data demonstrate a crucial role of central and peripheral catecholamines in modulating immune function.

Taste-immunosuppression engram: Reinforcement and extinction

Several Pavlovian conditioning paradigms have documented the brain's abilities to sense immune-derived signals or immune status, associate them with concurrently relevant extereoceptive stimuli, and reinstate such immune responses on demand. Specifically, the naturalistic relation of food ingestion with its possible immune consequences facilitates taste-immune associations. Here we demonstrate that the saccharin taste can be associated with the immunosuppressive agent cyclosporine A, and that such taste-immune associative learning is subject to reinforcement. Furthermore, once consolidated, this saccharin-immunosuppression engram is resistant to extinction when avoidance behavior is assessed. More importantly, the more this engram is activated, either at association or extinction phases, the more pronounced is the conditioned immunosuppression.

Electrical activity in rat cortico-limbic structures after single or repeated administration of lipopolysaccharide or staphylococcal enterotoxin B

Immune-to-brain communication is essential for an individual to aptly respond to challenging internal and external environments. However, the specificity by which the central nervous system detects or 'senses' peripheral immune challenges is still poorly understood. In contrast to post-mortem c-Fos mapping, we recorded neural activity in vivo in two specific cortico-limbic regions relevant for processing visceral inputs and associating it with other sensory signalling, the amygdala (Am) and the insular cortex (IC). Adult rats were implanted with deep-brain monopolar electrodes and electrical activity was monitored unilaterally before and after administration of two different immunogens, the T-cell-independent antigen lipopolysaccharide (LPS) or the T-cell-dependent antigen staphylococcal enterotoxin B (SEB). In addition, the neural activity of the same individuals was analysed after single as well as repeated antigen administration, the latter inducing attenuation of the immune response. Body temperature and circulating cytokine levels confirmed the biological activity of the antigens and the success of immunization and desensitization protocols. More importantly, the present data demonstrate that neural activity of the Am and IC is not only specific for the type of immune challenge (LPS versus SEB) but seems to be also sensitive to the different immune state (naive versus desensitization). This indicates that the forebrain expresses specific patterns of electrical activity related to the type of peripheral immune activation as well as to the intensity of the stimulation, substantiating associative learning paradigms employing antigens as unconditioned stimuli. Overall, our data support the view of an intensive immune-to-brain communication, which may have evolved to achieve the complex energetic balance necessary for mounting effective immunity and improved individual adaptability by cognitive functions.

Conditioning Immune and Endocrine Parameters in Humans: A Systematic Review

BACKGROUND

Conditioned pharmacological effects may provide relevant clinical opportunities to improve treatment for patients with a variety of conditions. The aim of this systematic review was to create an overview of studies in this field of research and to investigate whether specific characteristics of the study design make for successful conditioning.

METHODS

The protocol of this review was registered in Prospero (PROSPERO 2015: CRD42015024148). A systematic literature search was conducted in the databases PubMed, Embase, and PsychInfo. Studies were included if they were placebo-controlled trials in humans in which the effects of a pharmacological agent on immune or endocrine outcomes (e.g., interleukin-2 and cortisol) were conditioned, using a specific conditioned stimulus. The risk of bias of each study was assessed using the Cochrane risk-of-bias tool.

RESULTS

The final selection included 16 studies. Overall, those studies indicate that conditioning of immunosuppression, conditioning of allergic responses, and conditioning of insulin and glycemic responses is possible. Regarding immunostimulants, antiallergic effects, and cortisol conditioning, the preliminary results are promising, but additional studies are needed.

CONCLUSIONS

This systematic review shows classical conditioning of immune and endocrine responses for various pharmaceutical substances. The studies reviewed here indicate that the number of acquisition and evocation sessions, and characteristics of the unconditioned and conditioned stimuli, are important determinants of the effectiveness of pharmacological conditioning on immune and endocrine parameters. In the future, conditioned pharmacological effects may be used clinically as adjunct therapy in various patient populations.

Spatial Memory and Gut Microbiota Alterations Are Already Present in Early Adulthood in a Pre-clinical Transgenic Model of Alzheimer's Disease

The irreversible and progressive neurodegenerative Alzheimer's disease (AD) is characterized by cognitive decline, extracellular β-amyloid peptide accumulation, and tau neurofibrillary tangles in the cortex and hippocampus. The triple-transgenic (3xTg) mouse model of AD presents memory impairment in several behavioral paradigms and histopathological alterations from 6 to 16 months old. Additionally, it seems that dysbiotic gut microbiota is present in both mouse models and patients of AD at the cognitive symptomatic stage. The present study aimed to assess spatial learning, memory retention, and gut microbiota alterations in an early adult stage of the 3xTg-AD mice as well as to explore its sexual dimorphism. We evaluated motor activity, novel-object localization training, and retention test as well as collected fecal samples to characterize relative abundance, alpha- and beta-diversity, and linear discriminant analysis (LDA) effect size (LEfSe) analysis in gut microbiota in both female and male 3xTg-AD mice, and controls [non-transgenic mice (NoTg)], at 3 and 5 months old. We found spatial memory deficits in female and male 3xTg-AD but no alteration neither during training nor in motor activity. Importantly, already at 3 months old, we observed decreased relative abundances of Actinobacteria and TM7 in 3xTg-AD compared to NoTg mice, while the beta diversity of gut microbiota was different in female and male 3xTg-AD mice in comparison to NoTg. Our results suggest that gut microbiota modifications in 3xTg-AD mice anticipate and thus could be causally related to cognitive decline already at the early adult age of AD. We propose that microbiota alterations may be used as an early and non-invasive diagnostic biomarker of AD.