Naltrexone blocks the expression of the conditioned elevation of natural killer cell activity in BALB/c mice

Brain behavior and immunity: twenty years of T cells

During the period from 1987 to 2007, our understanding of nervous system-T lymphocyte bi-directional communication advanced exponentially. Progress in exploring these relationships was aided by the constant development of new, cutting-edge technologies, and by a steady growth in interest, and number, of scientists who recognized the need to conduct cross-disciplinary research. In this brief review of 20 years of Brain, Behavior, and Immunity (BBI), we highlight just a small number of the important studies published in the journal that collectively have provided the foundation for our current understanding of brain, behavior, and, specifically, T cells.

Psychoneuroendocrine immunology: perception of stress can alter body temperature and natural killer cell activity

Psychoimmunology has been credited with using the mind as a way to alter immunity. The problem with this concept is that many of the current psychoimmunology techniques in use are aimed at alleviating stress effects on the immune system rather than at direct augmentation of immunity by the brain. Studies in animals provide a model that permits us to approach the difficulties associated with gaining an understanding of the CNS-immune system connection. A particular advantage of using animals over humans is that psychological and social contributions play a less prominent role for animals than for human subjects, since the animals are all inbred and reared under identical controlled conditions. If the insightful information provided by animal studies is correct, then psychotherapy for the treatment of diseases might be made more effective if some aspect of this knowledge is included in the design of the treatment. We emphasize conditioning as a regimen and an acceptable way to train the brain to remember an output pathway to raise immunity. We propose that a specific drug or perception (mild stress, represented by rotation, total body heating or handling) could substitute and kindle the same output pathway without the need for conditioning. If this view is correct, then instead of using conditioning, it may be possible to use an antigen to activate desired immune cells, and substitute a drug or an external environmental sensory stimulus (perception) to energize the output pathway to these cells. Alternatively, monitoring alterations of body temperature in response to a drug or perception might allow us to follow how effectively the brain is performing in altering immunity. Studies with animals suggest that there are alternative ways to use the mind to raise natural or acquired immunity in man.

Involvement of catecholamines in recall of the conditioned NK cell response

The primary goal of the study was to identify the types of catecholamines and the associated receptors which might be involved in the recall of the conditioned NK cell response. Specific catecholamine receptor antagonists were selected to block the conditioned NK cell response at the recall step. The regional contents of dopamine (DA), norepinephrine (NE), and epinephrine were determined in the brain of the conditioned animals by using the high performance liquid chromatography with electrochemical detection (HPLC/ED). Results showed that pre-disruption of the central alpha1-, alpha2-, beta1-, beta2-, D1-, or D2-receptors at the conditioned recall stage, interrupted the conditioned enhancement in NK cell activity. The NE contents at the cerebellum, and DA contents at the striatum and hippocampus, were significantly higher in the brain of the conditioned animals when compared to that of the control animals. These information indicated the possible roles of the central noradrenergic and dopaminergic systems in regulating the recall of the conditioned NK cell response.

Psychoneuroendocrine immunology: site of recognition, learning and memory in the immune system and the brain

How the interaction between the brain and immune system takes place has not been clearly defined. Because multiple changes are occurring simultaneously in all organ systems (e.g., cardiovascular, gastrointestinal, reproductive, renal, respiratory, immune, CNS), how many single systems interacts with the brain becomes extraordinarily difficult to understand. The problem boils down to developing an approach that not only allows one to study the whole organism and define the mediators of the interacting systems, but also permit one to establish the connection and physiologic relevance of the responses that are being evaluated. Conditioning, a phenomenon made popular by the work of Pavlov (1906, 1927), may provide insight into the pathways of communication between the brain and possibly any organ system of the body. Conditioning allows one to separate the afferent from the efferent circuits. That is, signals from the immune system to the CNS (IS-->CNS) can be effectively separated from signals from the CNS to immune system (CNS-->IS). This permits one to study each pathway individually. Simple, single association trial models to condition fever, natural killer (NK) cell and cytotoxic lymphocyte (CTL) activities have been developed to evaluate the pathways. Single trial learning is not new. Pavlov has observed that "The electric buzzer set going before administration of food established a conditioned alimentary reflex after only a single combination," whereas the reverse order of presentation failed to condition the animal (Pavlov 1927 p. 27). Thus, conditioning can be used to train the brain to activate the immune system and other organ systems participating in the response. During the course of the conditioned response, presumably the CNS via the hypothalamus integrates in a cohesive orderly fashion all input and output signals and coordinates the responses made by the brain to the organ systems. The odor of camphor, the conditioned stimulus (CS) can be associated with the response produced by an unconditioned stimulus (US). The unconditioned stimuli used are poly I:C to raise fever and nonimmunospecific NK cell activity or alloantigens to raise immunospecific CTL activity. The unconditioned stimulus serves only as a means to activate the immune system and unbalance the homeostasis so that a transient but new bidirectional communication loop can be established between the immune system and the CNS (IS<-->CNS). The expression of the conditioned response (i.e., elevation of fever, NK cell, or CTL activity) induced with the CS (odor stimulus) is an outcome of neural activity (CNS-->IS). This infers that during conditioning, the signals generated by the CS and US imprints a neural pathway located within the central nervous system and leaves behind a CS/US memory of the association. The immune activity (NK cell or CTL activity) which is modulated indicate that the memory pathway was activated in the brain of the animal expressing the conditioned response. The immune cells that are modulated can be considered to be casual bystander cells. These cells however must be in the proper (ready) state of activation to receive salient signals from the brain. Along with changes in the indicator cell population, other complex physiological processes are altered by the brain via sympathetic and neuroendocrine pathways to raise the fever response. These observations suggest that the physiological changes which are being evaluated such as fever, NK cell or CTL activities or perhaps blood pressure, heart rate, fat metabolism, oxygen consumption serve only as indicators (readouts), and infer that the CNS has made a coordinated reply in response to the CS signal.

Conditioned immunomodulation: investigations of the role of endogenous activity at mu, kappa, and delta opioid receptor subtypes

The present investigations were designed to determine the role of activity at mu, kappa, and delta opioid receptor subtypes in conditioned immunomodulation by evaluating the effects of selective opioid receptor antagonists on conditioned stimulus-induced alterations in immune status. Lewis rats were exposed to an aversive conditioned stimulus that was developed through pairings with electric footshock. This aversive conditioned stimulus induces a reduction in splenic natural killer cell activity, splenocyte proliferation in response to mitogens, and diminished levels of interferon-gamma (IFN-gamma) production by splenocytes. Intracerebroventricular (i.c.v.) administration of the opioid antagonist naltrexone or the mu 1-selective antagonist naloxonazine blocked conditioned alterations of immune status, indicating that activity at mu-opioid receptors is involved in conditioned immunomodulation. Further support for the involvement of mu-opioid receptors within the central nervous system is provided by data showing that peripheral administration of naloxonazine, at doses shown to be effective when administered i.c.v., had no effect on conditioned alterations of immune status. Ventricular administration of the kappa receptor antagonist nor-binaltorphimine (nor-BNI) did not antagonize the immunomodulatory effects of the conditioned stimulus. Administration of the delta receptor antagonist naltrindole also did not antagonize the conditioned alterations of immune status. Collectively, the results of this study indicate that the alterations of immune status produced by an aversive conditioned stimulus require activity at mu-opioid receptors, possibly mu 1, within the central nervous system.

Activation of mu-opioid receptors are required for the conditioned enhancement of NK cell activity

The type of opioid receptors involved in the conditioned enhancement of natural killer (NK) cell activity is identified in the present study. In our previous observations, we have demonstrated that the conditioned enhancement of NK cell activity was dependent on beta-endorphin and methionine-enkephalin, but not dynorphin. Based on the interaction of opioids with their homologous receptors, we concluded that mu- and delta-opioid receptors might be involved. To further classify the type(s) of opioid receptors involved in eliciting the conditioned NK cell activity, three opioid receptor antagonists, cyprodime hydrobromide, ICI-174864, and nor-binaltorphimine dihydrochloride, were used to block the conditioned NK cell activity in BALB/c mice. Blocking was conducted by intracisternal injection of the drugs. The results showed that the activation of mu-opioid receptors was required in the conditioned enhancement of NK cell activity, but not the delta- or kappa-type of receptors.

Stereoselective inhibition of natural killer activity by the sigma ligand (+)-pentazocine

The effect of sigma (sigma) receptor ligands on natural killer (NK) activity was examined both in vivo and in vitro. Following injection of mice with sigma receptor ligands such as (+)-pentazocine, (-)-pentazocine, BD 1073, BD 1165 and BD 737, NK activity was measured in poly-I.C.-stimulated mouse splenocytes. (+)-Pentazocine reduced NK cell activity in a dose-dependent fashion, while the (-) enantiomer was inactive in this measure. For example, at a dose of 50 mg/kg, (+)-pentazocine suppressed NK activity (using effector to target cell ratios of 200:1, 100:1 and 50:1) by > 70%, 24 h after injection while (-)-pentazocine was inactive. The other sigma ligands examined either slightly enhanced or had no effect on NK activity. Nonetheless, parenteral administration of the sigma receptor ligand BD 1165 blocked (+)-pentazocine-induced suppression of NK activity, while the opiate receptor antagonist naltrexone was ineffective. Addition of sigma receptor ligands (10(-11)-10(-5) M) to splenocyte cultures for 24 h did not affect NK activity. These findings indicate that while sigma receptor ligands are capable of modulating NK activity, this effect is not the result of an action on splenocyte sigma receptors, but may be mediated via sigma receptors either in the central or peripheral nervous systems.

The use of conditioning to probe for CNS pathways that regulate fever and NK cell activity

Immune and central nervous system (CNS) interactions are complicated because afferent signals from the immune system to the CNS in response to antigens or infections may elicit an immediate efferent response to the immune system. This communication loop is required for the homeostatic regulation of the immune system. Conditioning can be used as a tool to take the communication loop apart. In conditioned animals, the conditioned stimulus can be employed later to trigger the site of the association memory located within CNS, and set off the efferent pathway. Conditioning therefore allows one to isolate and identify the potential circuits in the brain that becomes conditioned. We have conditioned a pathway in the brain which can be used to modulate core body temperature (Tc) and natural killer (NK) cell activity. The Tc and NK cell activity are used as readouts to detect the expression of the conditioned response which is taking place in the brain. Since various cytokines (IFN, IL-1 etc) that are produced by antigenic stimulation invariably raise fever, it appears that the immune system could signal the CNS with nonspecific cytokines that activate the hypothalamic-pituitary pathway to modulate core body temperature. These observations infer that the thermoregulatory pathway in the brain becomes conditioned and points to a common pathway of communication in which interferon-beta, prostaglandin E2, CRH and ACTH appear to play a role in modulating both Tc and NK cell activity.

Corticotropin-releasing hormone is involved in conditioned stimulus-induced reduction of natural killer cell activity but not in conditioned alterations in cytokine production or proliferation responses

Research from our laboratory has demonstrated that the presentation of an aversive conditioned stimulus produces pronounced suppression of several in vitro measures of immune status. The present study was designed to evaluate the role of central corticotropin-releasing hormone (CRH) in the mechanisms mediating these conditioned effects. The aversive conditioned stimulus was a distinct environment that had previously been associated with electric footshock. Lewis rats received intraventricular administration of either buffered saline or a dose of the CRH-selective receptor antagonist alpha-helical CRH(9-41) (0, 0.5, 5, or 50 micrograms) prior to exposure to the aversive conditioned stimulus or home cage control treatment. The aversive conditioned stimulus produced decreases in splenic natural killer cell activity, splenocyte responsiveness to the mitogens concanavalin A (ConA), phytohemagglutinin (PHA), lipopolysaccharide (LPS), and the combination of ionomycin and phorbol myristate acetate (PMA), blood leukocyte responsiveness to ConA and PHA, and the production of interleukin-2 and interferon-gamma by activated splenocytes. The conditioned stimulus also produced an increase in plasma levels of corticosterone. Pretreatment with alpha-helical CRH(9-14) completely blocked the conditioned stimulus-induced suppression of natural killer cell activity. The CRH antagonist had no attenuative effect on the conditioned suppression of splenocyte or blood leukocyte proliferation in response to mitogens, or the production of interleukin-2 or interferon-gamma by activated splenocytes. There was also no effect of alpha-helical CRH(9-14) on the conditioned stimulus-induced increase in plasma corticosterone. These findings suggest that conditioned stimulus-induced suppression of natural killer cell activity is mediated by a mechanism that involves activity at central CRH receptors, and that this conditioned modulation is independent of HPA activation. Furthermore, these results indicate that the mechanisms involved in conditioned stimulus-induced suppression of proliferative or cytokine production responses are distinct from those involved in the modulation of natural killer cell activity.

Expression of the conditioned NK cell activity is beta-endorphin dependent

We are interested in identifying the pathways which are responsible for triggering the conditioned enhancement of natural killer (NK) cell activity. Earlier studies have suggested that central opioid(s) are involved in eliciting the expression of the conditioned NK cell activity. The purpose of this study was to identify the central opioid peptides that allow the central nervous system (CNS) to communicate with the immune system. Mediators that activate the efferent pathway of communication between the CNS and immune system was examined by injection of the mediator via the cisterna magna (CM). Conditioning was used as a tool to show that the bi-directional communication between the CNS and the immune system does take place. We found that beta-endorphin but not dynorphin could stimulate NK cell activity, when beta-endorphin or dynorphin was injected into the CM. In addition, when anti-beta-endorphin or anti-dynorphin antibody was injected into the conditioned animals via CM the conditioned response was blocked by anti-beta-endorphin but not by anti-dynorphin antibody. These observations suggest that beta-endorphin appears to be one of the signals that is induced in the brain at the CS recall step of the conditioned response to trigger the elevation of NK cell activity.

Role of arcuate nucleus of the hypothalamus in the acquisition of association memory between the CS and US

A single trial association protocol was used to demonstrate a conditioned increase in natural killer (NK) cell activity. The signals used were odor of camphor as the conditioned stimulus (CS) and polyinosinic-polycytidylic acid (poly I:C) as the unconditioned stimulus (US). This model has been used to dissect the underlying mechanisms of interaction between the central nervous system (CNS) and the immune system (IS) and vice versa. Here, we demonstrate the potential role played by the arcuate nucleus of the hypothalamus in the acquisition of association memory between the CS and the US. Chemical destruction of the arcuate nucleus with monosodium glutamate (MSG) was used for this purpose. Mice with arcuate nucleus lesion prior to the association protocol did not demonstrate a conditioned increase in NK cell activity. However, the lesion has no effect if produced prior to exposure to the CS at recall. These studies demonstrate the significant role played by the hypothalamus (arcuate nucleus) in a conditioned response.

A multiple modality approach combining the effect of conditioning with adoptive chemoimmunotherapy

It has been demonstrated that significant protection against YC8 lymphoma can be induced in mice preimmunized with normal DBA/2 spleen cells. The DBA/2 spleen cells used as an alloantigen share minor histocompatibility determinants with the YC8 tumor. Our observations showed that once tumor was present in vivo, the use of a potent tumor specific vaccine that can confer 100% protection to preimmunized animals, can help in increasing survival but can no longer produce high incidence of regression and cure. We have used this model to show that adoptive chemoimmunotherapy (ACIT) can be used to regress tumors in mice with large body burden of tumor and that combination of conditioning with ACIT appears to enhance the effectiveness of treatment. The nature of the immunity conferred by conditioned resistance might be due to cytotoxic T-lymphocytes.

Conditioning of the allogeneic cytotoxic lymphocyte response

Allogeneic cytotoxic T-lymphocyte (CTL) response can be obtained following immunization of BALB/c mice with C57BL/6 spleen cells. We investigated the possibility of behaviorally conditioning this response by associating the C57BL/6 spleen cell immunization [unconditioned stimulus (US)] with camphor odor [conditioned stimulus (CS)]. We reported the possible mechanisms involved in the conditioning of natural killer cell activity. Similar approaches were used to investigate the mechanisms that participate in the conditioned CTL activity. The first mechanism of investigation utilized opioid receptor blockers naltrexone and quaternary naltrexone. Naltrexone, which blocks both the central and peripheral opioid receptors, blocked the recall of the conditioned response, whereas quaternary naltrexone, which does not penetrate the blood-brain barrier, was unable to block the conditioned response, demonstrating that centrally located opioid receptors play a role in the recall of the conditioned response. The studies are of interest because they indicate that resistance or susceptibility to various diseases such as cancer, autoimmunity, and infectious diseases might be influenced by the regulatory network of the CNS.

Identification of specific pathways of communication between the CNS and NK cell system

The specific signals and pathways utilized by the natural killer (NK) cell system and the central nervous system (CNS) that results in the conditioned response (CR) is not clearly understood. Single trial conditioning of the NK cell activity provides us with a model to probe the mechanisms of communication between two major systems (Immune and CNS) which are involved in the health and disease of the individual. The studies show that the IFN-beta molecules possess the properties attributed to the unconditioned stimulus (US). IFN-beta can penetrate the CNS and evoke the elevation of NK cell activity in the spleen. This unconditioned response (UR) can be linked to a specific conditioned stimulus (CS). Specific odors such as camphor provide a neural pathway for the CS to associate with the US. Evidence is presented that in conditioning there are two locations where memory develops. The CS/US association is made centrally and its memory is stored at a central location, but the memory for the specificity of the odor is presumably stored in the olfactory bulbs. The CS recalls the CR by triggering the olfactory neural pathway which, in turn, signals the hypothalamic-pituitary axis to release mediators that modulate the activity of NK cells in the spleen. These results imply that through conditioning one has direct input into the regulatory hypothalamus that controls the internal environment of the organism and the health and disease of the individual. Consequently, it is not inconceivable that through this approach we might be able to alter the course of a disease process.

The direction of the conditioned natural killer cell response can be re-directed with indomethacin and/or handling

We have used the pairing of camphor odor conditioned stimulus (CS) and injection of poly I:C unconditioned stimulus (US) in a short 3 day single trial conditioning paradigm. Conditioning was done by exposing mice to the CS/US combination on day 0 and reexposing the conditioned animals to the CS on day 2. This results in a conditioned augmentation of the natural killer (NK) cell response. Indomethacin treatment and/or handling stress induced by simply measuring rectal temperature was found to dramatically alter the direction of the conditioned NK cell response. Conditioning of indomethacin treated mice produced a conditioned suppression of the NK cell response mimicking a conditioned tolerance response. If handling stress was superimposed on day 2 the conditioned suppression response was replaced by a conditioned augmentation of the NK cell response. Even with one trial conditioning, drugs and handling stress can serve as additional cues to alter the direction of the conditioned response. The studies also show that the conditioning of the fever response is independent of conditioning of the NK cell response.

Indomethacin and sodium carbonate effects on conditioned fever and NK cell activity

The augmentation of natural killer (NK) cell activity and elevation of body temperature (fever) can both be conditioned using camphor odor as the conditioned stimulus (CS) and poly I:C as the unconditioned stimulus (US). While both responses can be conditioned in parallel fashion as shown previously, our results indicate the conditioned learning of these responses may not follow along a common path. We found that injection of a 1% solution of sodium carbonate was able to consistently block the CS/US learning of the NK cell response but did not block conditioning of the fever response. In contrary fashion, mice treated with indomethacin (which inhibits prostaglandin-induced fever) dissolved in the sodium carbonate solution did not learn in consistent fashion the fever response. However, indomethacin-treated animals were able to recall the NK cell response. These results support the view that although the same mediator, IFN-beta, is responsible for the conditioned learning of the NK cell and fever responses both the learning and recall of the responses are initiated along separate pathways.

The central effect of methionine-enkephalin on NK cell activity

The central effect of opioid peptide on natural killer (NK) cell activity in BALB/c mice was investigated. Injection of methionine-enkephalin (Met-Enk), 0.02 microgram/mouse or 1 microgram/kg, directly into the cisterna magna (CM) of the brain, resulted in a significant enhancement of NK cell activity. This enhancement was blocked by opiate antagonists, naltrexone and quaternary naltrexone. The same dose of Met-Enk had no effect on NK cell activity when given to the mouse intraperitoneally or intravenously. Moreover, des-tyrosine-methionine-enkephalin injected into the CM at 1 microgram/kg, had no effect on NK cell activity. The results indicate that activation of an opioid-mediated pathway in the central nervous system is capable of activating the pathways that stimulate the NK cell response in the periphery.

In vivo enhancement of NK cell activity with met-enkephalin and glycyl-glutamine: their possible role in the conditioned response

These studies investigated the effect of met-enkephalin, glycyl-glutamine, and naltrexone on NK cell activity in vivo and in vitro. It was found that both met-enkephalin (which shares the amino-terminal end of beta-endorphin) and glycyl-glutamine (which reflects the carboxyl-terminal end of beta-endorphin) can enhance the NK cell activity of mice prestimulated with a low dose (1 microgram/mouse) of poly I:C. Naltrexone had no effect. In vivo prestimulation of the mice with 1 microgram poly I:C was necessary as mice which were not pretreated with poly I:C did not show enhanced NK cell activity when treated with either met-enkephalin or glycyl-glutamine. In vitro studies however indicate that the drugs when cultured together with the NK cells from mice preactivated with poly I:C did not have a direct stimulatory effect on the NK cells. These studies imply that while beta-endorphin released from the pituitary could be involved in enhancement of activated NK cells in vivo other indirect peripheral pathways might be involved. The results suggest beta-endorphin probably reacts with other accessory type cells which in turn release the mediators which are required for the stimulation of NK cells in vivo.