Gustavo Pacheco-López

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.

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.

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.

Neurobehavioural activation during peripheral immunosuppression.

Like other physiological responses, immune functions are the subject of behavioural conditioning. Conditioned immunosuppression can be induced by contingently pairing a novel taste with an injection of the immunosuppressant cyclosporine A (CsA) in an associative learning paradigm. This learned immunosuppression is centrally mediated by the insular cortex and the amygdala. However, the afferent mechanisms by which the brain detects CsA are not understood. In this study we analysed whether CsA is sensed via the chemosensitive vagus nerve or whether CsA directly acts on the brain. Our experiments revealed that a single peripheral administration of CsA increases neuronal activity in the insular cortex and the amygdala as evident from increased electric activity, c-Fos expression and amygdaloid noradrenaline release. However, this increased neuronal activity was not affected by prior vagal deafferentation but rather seems to partially be induced by direct action of CsA on cortico-amygdaloid structures and the chemosensitive brainstem regions area postrema and nucleus of the solitary tract. Together, these data indicate that CsA as an unconditioned stimulus may directly act on the brain by a still unknown transduction mechanism.

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.

A comparison of heart rate variability in women at the third trimester of pregnancy and during low-risk labour.

Heart rate variability (HRV) has been recognised as a non-invasive method for assessing cardiac autonomic regulation. Aiming to characterize HRV changes at labour in women, we studied 10 minute ECG recordings from young mothers (n=30) at the third trimester of pregnancy (P) or during augmentation of labour (L) (n=30). Data of the L group were collected when no-contractions (L-NC) or the contractile activity (L-C) was manifested. Accordingly, the inter-beat interval (IBI) time series were processed to estimate relevant parameters of HRV such as the mean IBI (IBI¯), the mean heart rate HR¯, the root mean square of successive differences (RMSSD) in IBIs, the natural logarithm of high-frequency component (LnHF), the short-term scaling parameters from detrended fluctuation and magnitude and sign analyses such as (α1, α1(MAG), α1(SIGN)), and the sample entropy (SampEn). We found statistical differences (p<0.05) for RMSSD among P and L-NC/L-C groups (25 ± 13 vs. 36 ± 14/34 ± 16 ms) and for LnHF between P and L-NC (5.37 ± 1.15 vs. 6.05 ± 0.86 ms(2)). Likewise, we identified statistical differences (p<0.05) for α1(SIGN) among P and L-NC/L-C groups (0.19 ± 0.20 vs. 0.32 ± 0.17/0.39 ± 0.13). By contrast, L-NC and L-C groups showed statistical differences (p<0.05) in α1(MAG) (0.67 ± 0.12 vs. 0.79 ± 0.12), and SampEn (1.62 ± 0.26 vs. 1.20 ± 0.44). These results suggest that during labour, despite preserving a concomitant non-linear influence, the maternal short-term cardiac autonomic regulation becomes weakly anticorrelated (as indicated by α1(SIGN)); furthermore, an increased vagally mediated activity is observed (as indicated by RMSSD and LnHF), which may reflect a cholinergic pathway activation owing to the use of oxytocin or the anti-inflammatory cholinergic response triggered during labour.

Exogenous oxytocin reduces signs of sickness behavior and modifies heart rate fluctuations of endotoxemic rats

Besides the well-known roles of oxytocin on birth, maternal bonding, and lactation, recent evidence shows that this hypothalamic hormone possesses cardioprotective, anti-inflammatory and parasympathetic neuromodulation properties. In this study, we explore the heart rate fluctuations (HRF) in an endotoxemic rodent model that was accompanied by the administration of exogenous oxytocin. The assessment of HRF has been widely used as an indirect measure of the cardiac autonomic function. In this context, adult male Dark Agouti rats were equipped with a telemetric transmitter to continuously and remotely measure the electrocardiogram, temperature, and locomotion. In a between-subjects experimental design, rats received the following peripheral treatment: saline solution as a vehicle (V); lipopolysaccharide (LPS); oxytocin (Ox); lipopolysaccharide + oxytocin (LPS+Ox). Linear and non-linear parameters of HRF were estimated starting 3h before to 24h after treatments. Our results showed that exogenous oxytocin does not modify by itself the HRF of oxytocin-treated rats in comparison to vehicle-treated rats. However, in animals undergoing endotoxemia it: a) provokes a less anticorrelated pattern in HRF, b) decreased mean heart rate, c) moderated the magnitude and duration of the LPS-induced hyperthermia, and d) increased locomotion, up to 6h after the LPS injection. The less anticorrelated pattern in the HRF and decreased mean heart rate may reflect a cardiac pacemaker coupling with cholinergic influences mediated by oxytocin during LPS-induced endotoxemia. Finally, the anti-lethargic and long-term temperature moderating effects of the administration of oxytocin during endotoxemia could be a consequence of the systemic anti-inflammatory properties of oxytocin.

Inconsistencies in the Hypophagic Action of Intracerebroventricular Insulin in Mice

Insulin inhibits eating after its intracerebroventricular (ICV) administration in multiple species and under a variety of conditions. Nevertheless, the results across reports are inconsistent in that ICV insulin does not always reduce food intake. The reasons for this variability are largely unknown. Using mice as a model, we performed several crossover trials with insulin vs. vehicle when infused into the third cerebral ventricle (i3vt) to test the hypothesis that recent experience with the i3vt procedure contributes to the variability in the effect of ICV insulin on food intake. Using a cross-over design with two days between injections, we found that insulin (0.4 μU/mouse) significantly reduced food intake relative to vehicle in mice that received vehicle on the first and insulin on the second trial, whereas this effect was absent in mice that received insulin on the first and vehicle on the second trial. Higher doses (i3vt 4.0 and 40.0 μU/mouse) had no effect on food intake in this paradigm. When injections were spaced 7 days apart, insulin reduced food intake with no crossover effect. Mice that did not reduce food intake in response to higher doses of i3vt insulin did so in response to i3vt infusion of the melanocortin receptor agonist melanotan-II (MT-II), indicating that the function of the hypothalamic melanocortin system, which mediates the effect of insulin on eating, was not impaired by whatever interfered with the insulin effect, and that this interference occurred upstream of the melanocortin receptors. Overall, our findings suggest that associative effects based on previous experience with the experimental situation can compromise the eating inhibition elicited by i3vt administered insulin.

Oxytocin’s role on the cardiorespiratory activity of endotoxemic rats

BACKGROUNDnRecent findings concerning oxytocin indicate its anti-inflammatory, cardioprotective and parasympathetic modulating properties. In this study, we investigated the effects of systemically applied oxytocin on the cardiorespiratory activity in a rodent model of moderate endotoxemia.nnnMETHODSnTelemetrically recorded electrocardiogram (ECGs) from animals which received lipopolysaccharide (LPS); oxytocin (Ox); lipopolysaccharide+oxytocin (LPS+Ox), or vehicle (V) were analyzed using the ECG-derived respiration (EDR) technique to estimate the respiratory rate. The mean R-R interval and the spectral parameters of heart rate variability (HRV), such as the natural logarithm of the high frequency (lnHF) and low frequency (lnLF) components were also estimated up to 24h after treatment.nnnRESULTSnThe endotoxemic animals (LPS) showed an elevated respiratory rate as well as a reduced mean R-R interval, lnHF and lnLF components compared to controls (V) from +5 to +12h after the treatment. The administration of oxytocin significantly attenuated the hyperventilation produced by the LPS-induced endotoxemia (LPS+Ox) and restored the values of the mean R-R interval and such spectral parameters at different time points.nnnCONCLUSIONSnOur results support the existence of a link among the respiratory, cardiovascular, and immune systems in which oxytocin seems to act as a potential cardioprotective peptide by favoring cardiac cholinergic autonomic coupling. As a result, oxytocin diminished animals endotoxemic tachypnea and restored the cardiorespiratory interactions, which was indicated by the spectral components of HRV.

Neuroautonomic activity evidences parturition as a complex and integrated neuro–immune–endocrine process

Parturition in mammals demands a precise coordination of several neuro–immune–endocrine interactions including: a sterile inflammatory response that involves secretion of inflammation mediators like cytokines/chemokines; changes in the secretion of hormones such as progestogen, estrogens, cortisol, and oxytocin; as well as adjustments of the neuroautonomic function. Specifically, the so‐called cholinergic anti‐inflammatory pathway seems to play a key role in the homeostasis of the neuro–immune–endocrine axis by adjusting the vagus nerve activity during parturition. Here, we provide insights into the importance of the vagus during parturition from an autonomic, endocrine, and immune interplay perspective, and describe the potential role of heart rate variability analysis to explore these interactions noninvasively, economically, and accessibly.