Can conditioned histamine release occur under urethane anesthesia in guinea pigs?

Placebo and nocebo effects for itch and itch-related immune outcomes: A systematic review of animal and human studies

Placebo and nocebo effects can influence somatic symptoms such as pain. For itch and other dermatological symptoms these effects have been far less investigated. This review systematically integrates evidence from both animal (mainly rodents) and human trials on placebo and nocebo effects in itch, itch-related symptoms and conditions of the skin and mucous membranes, and related immune outcomes (e.g., histamine). Thirty-one animal studies, and fifty-five human studies (k = 21 healthy participants, k = 34 patients) were included. Overall, studies consistently show that placebo and nocebo effects can be induced by various methods (e.g., suggestions, conditioning and social cues), despite high heterogeneity across studies. Effects of suggestions were found consistently across subjective and behavioral parameters (e.g., itch and scratching in humans), whereas conditioning was likely to impact physiological parameters under certain conditions (e.g., conditioning of histamine levels in stressed rodents). Brain areas responsible for itch processing were associated with nocebo effects. Future research may investigate how variations in methods impact placebo and nocebo effects, and whether all symptoms and conditions can be influenced equally.

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.

Applications and limitations of behaviorally conditioned immunopharmacological responses

The importance of placebo responses for the treatment of various medical conditions has increasingly been recognized, whereas knowledge and systematic application in clinical settings are still sparse. One possible application for placebo responses in pharmacotherapy is given by learning paradigms, such as behaviorally conditioned immunosuppression, aiming at drug dose reduction while maintaining therapeutic efficacy of drug treatment. In an established learning paradigm of conditioned taste aversion/avoidance (CTA) in both, rats and humans, respectively, a novel-tasting drinking solution (conditioned stimulus, CS) is paired with an injection of the immunosuppressive drug cyclosporine A (CsA) as unconditioned stimulus (US). The conditioned response, evoked by re-presenting the CS alone at a later time, is reflected by avoidance behavior of consuming the solution (conditioned taste aversion; CTA) and a diminished interleukin (IL)-2 and interferon (IFN)-γ cytokine production as well as mRNA expression of rat splenic T cells or human peripheral T lymphocytes, closely mimicking the immunosuppressive effects of CsA. However, due to unreinforced CS-re-exposure conditioned responses progressively decreases over time (extinction), reflecting a considerable challenge for potential clinical applications of this learned immunosuppression. The present article discusses and critically reviews actual approaches, applications but also limitations of learning paradigms in immune pharmacotherapy.

Neuro-Bio-Behavioral Mechanisms of Placebo and Nocebo Responses: Implications for Clinical Trials and Clinical Practice

The placebo effect has often been considered a nuisance in basic and particularly clinical research. This view has gradually changed in recent years due to deeper insight into the neuro-bio-behavioral mechanisms steering both the placebo and nocebo responses, the evil twin of placebo. For the neuroscientist, placebo and nocebo responses have evolved as indispensable tools to understand brain mechanisms that link cognitive and emotional factors with symptom perception as well as peripheral physiologic systems and end organ functioning. For the clinical investigator, better understanding of the mechanisms driving placebo and nocebo responses allow the control of these responses and thereby help to more precisely define the efficacy of a specific pharmacological intervention. Finally, in the clinical context, the systematic exploitation of these mechanisms will help to maximize placebo responses and minimize nocebo responses for the patient's benefit. In this review, we summarize and critically examine the neuro-bio-behavioral mechanisms underlying placebo and nocebo responses that are currently known in terms of different diseases and physiologic systems. We subsequently elaborate on the consequences of this knowledge for pharmacological treatments of patients and the implications for pharmacological research, the training of healthcare professionals, and for the health care system and future research strategies on placebo and nocebo responses.

Learned placebo responses in neuroendocrine and immune functions

The phenomenon of learned placebo responses in neuroendocrine and immune functions is a fascinating example of communication between the brain and both the endocrine and peripheral immune systems. In this chapter, we will give a short overview of afferent and efferent communication pathways, as well as the central mechanisms, which steer the behavioral conditioned immune response. Subsequently, we will focus on data that provides evidence for learned immune responses in experimental animals and learned neuroendocrine and immune placebo responses in humans. Finally, we will take a critical look at these learning protocols, to determine whether or not they can be considered a viable additional treatment option to pharmacological regimens in clinical routine. This is fundamental, since there are still a number of issues, which need to be solved, such as the potential reproducibility, predictability, and extinction of the learned neuroendocrine and immune responses. Together, these findings not only provide an excellent basis to increase our understanding of human biology but may also have far reaching clinical implications. They pave the way for the ultimate aim of employing associative learning protocols as supportive treatment strategies in pharmacological regimens. As a result, medication levels may be reduced, as well as their unwanted side effects, providing a maximized therapeutic outcome to the benefit of the patient.

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.

Behavioural conditioning as the mediator of placebo responses in the immune system

Current placebo research postulates that conditioning processes are one of the major mechanisms of the placebo response. Behaviourally conditioned changes in peripheral immune functions have been demonstrated in experimental animals, healthy subjects and patients. The physiological mechanisms responsible for this 'learned immune response' are not yet fully understood, but some relevant afferent and efferent pathways in the communication between the brain and the peripheral immune system have been identified. In addition, possible benefits and applicability in clinical settings have been demonstrated where behaviourally conditioned immunosuppression attenuated the exacerbation of autoimmune diseases, prolonged allograft survival and affected allergic responses. Here, we summarize data describing the mechanisms and the potential clinical benefit of behaviourally conditioned immune functions, with particular focus on learned placebo effects on allergic reactions.

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".

Diazepam attenuates conditioned histamine release in guinea pigs

To clarify the possibility of pharmacological mediation on classical conditioning-associated asthmatic response, the effect of diazepam on an odor-induced conditioned histamine release was investigated in ovalbumin (OA)-sensitized guinea pigs, i.e. a model of bronchial asthma. The animals received conditioning sessions in which an antigen (OA) as the unconditioned stimulus and an odor (dimethylsulfide) as the conditioned stimulus (CS) were simultaneously inhaled. After the animals were intraperitoneally injected with saline or diazepam (2.5 or 5 mg/kg), they underwent exposure to the CS and blood collecting. This procedure was repeated three times in order that the animals would have each kind of injection. The animals injected with saline showed significantly higher levels of plasma histamine following the exposure to the CS as a conditioning effect compared with the baselines (P<0.05), whereas the group injected with diazepam (5 mg/kg) did not indicate such elevations. The suppressing effect of diazepam on the conditioned histamine release was also confirmed by a multiple regression analysis (5 mg/kg) and an analysis of covariance (2.5 and 5 mg/kg), even after adjustments for several factors regarding immunological sensitization and conditionability. The present study suggests that diazepam attenuates a conditioned histamine release.

Fasting stress exacerbates classical conditioned histamine release in guinea pigs

To clarify the contribution of stress to classical conditioning-associated asthmatic responses, the effect of fasting stress on conditioned histamine release was investigated in a guinea pig model of asthma. The animals were randomly divided into 2 groups for Experiment 1 and 2, and received a conditioning procedure in which ovalbumin (OA) as an unconditioned stimulus (US) and dimethylsulfide (DMS, sulfur smelling) as a conditioned stimulus (CS) were simultaneously inhaled after fasting for 16 h. Then, one group was given food as a reward for respiratory distress, and the other group was denied it for more than 3 h, while being placed in front of the feeding group. After this procedure was repeated 5 times, the plasma histamine levels in response to the CS were measured in half of each group in Experiment 1, and the respiratory resistance (Rrs) was assessed similarly in the other half of each group in Experiment 2. The same experiments were again performed after exchanging assignments of feeding group or fasting group in both experiments. The control groups in both experiments received the CS and the US 10 times separately in a random order under 16 h fasting conditions and were provided food after the exposures. After these pseudo-conditioning presentations, the plasma histamine levels or the Rrs in response to the CS were measured. In Experiment 1, the plasma histamine levels in the fasting stress group after the first conditioning sessions were significantly higher than those of the other groups. This difference was not observed when the groups were exchanged. In Experiment 2, the fasting stress group showed higher values in the Rrs compared to the other groups, irrespective of the first or second conditionings; however, they were not significant. The present study indicates that fasting stress after the conditioning procedures exacerbates the following conditioned histamine release, although the stress effect on bronchoconstriction was not confirmed.

Effect of isolation on classical conditioned histamine release in guinea pigs

To investigate the effect of abrupt or non-abrupt isolation stress on the classical conditioned histamine release, socially isolated or paired guinea pigs underwent conditioning procedures in which ovalbumin (OA) as an unconditioned stimulus (US) and dimethylsulfide (sulfur smelling) as a conditioned stimulus (CS) were simultaneously inhaled, and the plasma histamine levels after the exposure to the CS in a state of isolation or pairing were measured. The plasma histamine levels significantly increased from baseline in response to the CS (P < 0.05), except in the animals which were abruptly isolated during the exposure to the CS. The guinea pigs which were isolated during either the conditioning procedures or the exposure to the CS displayed significantly lower levels of plasma histamine than did the paired animals during both periods (P < 0.01, respectively). The plasma histamine levels in the guinea pigs which were isolated or paired during both periods were significantly higher than those of the control animals which had received the CS and the US separately (P < 0.05 and P < 0.01). A change of social relations, particularly isolation during the presentation of the CS, may have a suppressive effect on immediate asthmatic responses due to the conditioning mechanism.