Neurophysiological and endocrine consequences of immune activity

A physiological profile approach to animal temperament: How to understand the functional significance of individual differences in behaviour

Animal behaviour research has experienced a renewed interest in consistent individual differences (i.e. animal personality or temperament). Recent ecological studies have identified environmental conditions that give rise to the development and evolution of temperaments and to fitness-related outcomes of temperament. Additional literature has also described relationships between temperaments and physiological regulation. However, one-to-one relationships between one behavioural trait and one physiological system do not account for co-selection of behavioural and physiological traits, nor the complex signalling among physiological systems. In the current paper, we review the literature on multiple physiological processes associated with temperament, propose temperament-specific physiological profiles, and focus on next steps to understand the functional significance, evolution and maintenance of temperaments. We propose that to understand causes and consequences of temperament we need to characterize integrative physiological profiles associated with different temperaments.

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.

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.

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.

Mechanisms mediating the effects of cytokines on neuroendocrine functions in the rat

Exposure to an antigen causes significant endocrine changes, some of which in turn affect immune functioning. Proteins produced by activated immune cells, cytokines, act as messengers between the immune and the endocrine systems, and convey to the brain the occurrence of immune activation. We have investigated the ability of interleukin 1 (IL-1) alpha and beta to alter endocrine functioning in the adult rat. Acute peripheral injection of IL-1 alpha or beta causes dose-dependent increases in plasma adrenocorticotropic hormone (ACTH) and corticosterone secretion. These changes are primarily dependent upon increased release of corticotropin-releasing factor (CRF) into the portal circulation, and recent studies have indicated that the paraventricular nucleus (PVN) of the hypothalamus is the main source of this CRF. This conclusion is based on our finding that intravenous injection of IL-1 increases CRF biosynthesis in the PVN, and that lesion of this hypothalamic area interferes with IL-1's stimulatory action on ACTH secretion. Indomethacin partially reverses the effect of IL-1, suggesting that increased prostaglandin synthesis plays some part in this activation. Administration of IL-1 beta into the brain, but not into the general circulation, interferes with secretion of luteinizing hormone (LH) and ovulation through mechanisms involving endogenous opiates. Because neither CRF antagonists, nor lesions of the PVN, alter the inhibitory effect of IL-1 on LH release, CRF perikarya in the PVN do not appear to be involved in this phenomenon. Central administration of IL-1 beta strongly increases c-Fos immunoreactivity in the PVN, mainly within CRF neurons. Infusion of IL-1 beta into the PVN does not induce measurable changes in release of gonadotropin-releasing hormone (GnRH), but infusion of IL-1 directly into the median preoptic area (MPOA), a region rich in GnRH perikarya, markedly decreases GnRH secretion in rats bearing a push-pull cannula in the median eminence. Furthermore, central administration of IL-1 beta during the critical phase of pro-oestrus (1600-1930) also inhibits the expression of c-fos in GnRH cell bodies in the MPOA. Thus, we suggest that IL-1 interferes with reproductive functioning through a direct action at the level of the MPOA. These results indicate that circulating cytokines can alter the activity of the hypothalamo-pituitary-adrenal axis by increasing CRF release, probably through both immediate stimulation of CRF terminals within the median eminence and stimulation of CRF synthesis in the PVN. In contrast, cytokine-induced changes in LH and GnRH secretion are mediated through pathways lying primarily beyond the blood-brain barrier.

Infection as a stressor: a cytokine-mediated activation of the hypothalamo-pituitary-adrenal axis?

Infections are associated with increased plasma concentrations of adrenocorticotropic hormone (ACTH) and corticosterone. Hypothalamo-pituitary-adrenal (HPA) responses have also been observed with immunological stimuli that are not infective. Although such responses have been suggested to be mediated by ACTH secreted by lymphocytes, adrenocortical activation by immunological stimuli requires a functional pituitary. The most likely mechanism by which immunological stimuli activate the HPA axis involves production of cytokines by lymphocytes. The prime candidate is interleukin 1 (IL-1), because IL-1 production follows activation of the immune system and IL-1 administration is a potent activator of the HPA axis. However, other cytokines, such as tumour necrosis factor, may also be involved. Most immunological stimuli and IL-1 also activate both peripheral and central noradrenergic neurons. IL-1-induced activation of the HPA axis in vivo depends upon secretion of corticotropin-releasing factor (CRF), an intact pituitary, and the ventral noradrenergic bundle which innervates the CRF-containing neurons in the paraventricular nucleus (PVN) of the hypothalamus. Besides elevating body temperature, administration of IL-1 elicits a number of behavioural responses in rats and mice, including anorexia, increased sleep time, decreased investigation of novel objects and other animals, increased defensive withdrawal and other behaviours characteristic of sickness. Some of these responses can be reversed by CRF-antagonists and mimicked by CRF administration. Thus, endogenous production of IL-1 can account for a range of physiological and behavioural responses characteristic of sickness. Nevertheless, definitive evidence that IL-1 mediates these responses in sick animals is lacking.

Cytokines as mediators of depression: what can we learn from animal studies?

It has recently been postulated that cytokines may cause depressive illness in man. This hypothesis is based on the following observations: 1. Treatment of patients with cytokines can produce symptoms of depression; 2. Activation of the immune system is observed in many depressed patients; 3. Depression occurs more frequently in those with medical disorders associated with immune dysfunction; 4. Activation of the immune system, and administration of endotoxin (LPS) or interleukin-1 (IL-1) to animals induces sickness behavior, which resembles depression, and chronic treatment with antidepressants has been shown to inhibit sickness behavior induced by LPS; 5. Several cytokines can activate the hypothalamo-pituitary-adrenocortical axis (HPAA), which is commonly activated in depressed patients; 6. Some cytokines activates cerebral noradrenergic systems, also commonly observed in depressed patients; 7. Some cytokines activate brain serotonergic systems, which have been implicated in major depressive illness and its treatment. The evidence for each of these tenets is reviewed and evaluated along with the effects of cytokines in classical animal tests of depression. Although certain sickness behaviors resemble the symptoms of depression, they are not identical and each has distinct features. Thus the value of sickness behavior as an animal model of major depressive disorder is limited, so that care should be taken in extrapolating results from the model to the human disorder. Nevertheless, the model may provide insight into the etiology and the mechanisms underlying some symptoms of major depressive disorder. It is concluded that immune activation and cytokines may be involved in depressive symptoms in some patients. However, cytokines do not appear to be essential mediators of depressive illness.

The peptide molecular links between the central nervous and the immune systems

The central nervous system (CNS) and the immune system were for many years considered as two autonomous systems. Now, the reciprocal connections between them are generally recognized and very well documented. The links are realized mainly by various immuno- and neuropeptides. In the review the influence of the following immunopeptides on CNS is presented: tuftsin, thymulin, thymopoietin and thymopentin, thymosins, and thymic humoral factor. On the other side, the activity in the immune system of such neuropeptides as substance P, neurotensin, some neurokinins, enkephalins, and endorphins is discussed.

A cytokine-based neuroimmunologic mechanism of cancer-related symptoms

While many of the multiple symptoms that cancer patients have are due to the disease, it is increasingly recognized that pain, fatigue, sleep disturbance, cognitive dysfunction and affective symptoms are treatment related, and may lead to treatment delays or premature treatment termination. This symptom burden, a subjective counterpart of tumor burden, causes significant distress. Progress in understanding the mechanisms that underlie these symptoms may lead to new therapies for symptom control. Recently, some of these symptoms have been related to the actions of certain cytokines that produce a constellation of symptoms and behavioral signs when given exogenously to both humans and animals. The cytokine-induced sickness behavior that occurs in animals after the administration of infectious or inflammatory agents or certain proinflammatory cytokines has much in common with the symptoms experienced by cancer patients. Accordingly, we propose that cancer-related symptom clusters share common cytokine-based neuroimmunologic mechanisms. In this review, we provide evidence from clinical and animal studies that correlate the altered cytokine profile with cancer-related symptoms. We also propose that the expression of coexisting symptoms is linked to the deregulated activity of nuclear factor-kappa B, the transcription factor responsible for the production of cytokines and mediators of the inflammatory responses due to cancer and/or cancer treatment. These concepts open exciting new avenues for translational research in the pathophysiology and treatment of cancer-related symptoms.

Catecholamine, indoleamine and corticosteroid responses in mice bearing tumors

The neurochemical and endocrine responses to inoculation of mice with the murine lymphoma cell line AW5E was studied. This cell line was chosen because it is NK cell lysis resistant and thus does not induce a normal immune response. Immune activation has long been known to be a potent stimulator of the hypothalamo-pituitary-adrenocortical (HPA) axis as well as brain catecholamine and indoleamine metabolism, involving increases in the brain concentrations of catabolites of norepinephrine (NE) and serotonin (5-HT), as well as free tryptophan. Mice injected intravenously with AW5E tumor cells exhibited small increases in plasma corticosterone and hypothalamic NE and 5-HT catabolites one day after injection. There were no significant changes after 6 or 8 days, but a sustained increase in hypothalamic NE and 5-HT metabolism appeared 10 days after injection. There were similar, but more limited changes in the brain stem and prefrontal cortex. On the last day tested (day 14), plasma corticosterone was slightly elevated, as were hypothalamic dopamine, NE and 5-HT catabolites and tryptophan. These results indicate that inoculation with AW5E tumor cells increases brain catecholamine and serotonin metabolism, the hypothalamus being the most sensitive region. The most marked increases occurred in the few days preceding death, and thus may be associated with the pathology of the tumor growth.

Influence of the specific immune response on some consistent murine behaviors

The author's goal was to discover if the generation and maintenance of the specific immune response resulted in alterations of reliable behaviors (i.e., behaviors correlated over time). The behaviors (ambulation, rearing, and interaction with a conspecific) of CD1 male mice were measured in a small open field, and several days later, the mice were immunized with antigens (either splenocytes from C57BL/6 mice or a mixture of sheep erythrocytes and goat serum). The same behaviors were recorded again some hours, or some days, after immunization. Immunizations and behavioral measurements were repeated at various intervals. Blood levels of antibodies to the antigens were measured 6 days after immunization. The recorded behaviors were consistent (according to Kendall coefficient of concordance). The mice mounted antibody responses to the antigens, yet no behavioral changes were apparent during the response. On the contrary, a single injection of E. coli lipopolysaccharide decreased ambulation and rearing. It is proposed that in healthy mice kept in normal conditions, the specific immune response may be unrelated to reliable behavioral changes.

Regulation of the hypothalamic-pituitary-adrenal axis by cytokines: actions and mechanisms of action

Glucocorticoids are hormone products of the adrenal gland, which have long been recognized to have a profound impact on immunologic processes. The communication between immune and neuroendocrine systems is, however, bidirectional. The endocrine and immune systems share a common "chemical language," with both systems possessing ligands and receptors of "classical" hormones and immunoregulatory mediators. Studies in the early to mid 1980s demonstrated that monocyte-derived or recombinant interleukin-1 (IL-1) causes secretion of hormones of the hypothalamic-pituitary-adrenal (HPA) axis, establishing that immunoregulators, known as cytokines, play a pivotal role in this bidirectional communication between the immune and neuroendocrine systems. The subsequent 10-15 years have witnessed demonstrations that numerous members of several cytokine families increase the secretory activity of the HPA axis. Because this neuroendocrine action of cytokines is mediated primarily at the level of the central nervous system, studies investigating the mechanisms of HPA activation produced by cytokines take on a more broad significance, with findings relevant to the more fundamental question of how cytokines signal the brain. This article reviews published findings that have documented which cytokines have been shown to influence hormone secretion from the HPA axis, determined under what physiological/pathophysiological circumstances endogenous cytokines regulate HPA axis activity, established the possible sites of cytokine action on HPA axis hormone secretion, and identified the potential neuroanatomic and pharmacological mechanisms by which cytokines signal the neuroendocrine hypothalamus.

Generation of Antigen-Specific Th2 Cells from Unprimed Mice In Vitro: Effects of Dexamethasone and Anti-IL-10 Antibody

We describe a system for the in vitro production of Ag-specific mouse CD4 cell lines from unprimed mice. Purified CD4+CD45RBhigh T cells were exposed to Ag-pulsed accessory cells in serum-free medium for 24 h; cultured in the absence of Ag and in the presence of serum, IL-2, dexamethasone, and Abs to IL-10 for an additional 4 days; and then re-exposed to the original sensitizing Ag. The presence of dexamethasone and Abs to IL-10 during the initial expansion stage appeared to be critical for the ability of the stimulated and expanded T cells to respond to restimulation with the same Ag. Repeated cycles of in vitro stimulation led to increased specificity for the sensitizing Ag (in the current case, pigeon cytochrome c), a decline in production of IL-2 and IFN-γ, and increased production of IL-4, IL-5, and IL-10. This culture protocol provides a test system for exploration of factors that regulate the conversion of naïve cells to memory cells and the development of specific immune responses to protein Ags. The data are consistent with models that implicate glucocorticoids as regulators of immune response specificity.

CRH and the immune system

Inflammatory cytokines released during immune system activation can stimulate the hypothalamic-pituitary-adrenal axis and cause increased secretion of corticotropin-releasing hormone (CRH), adrenocorticotropin and glucocorticoids. Identification of CRH peptide and mRNA, as well as its receptors in immune tissues, suggested a role for this peptide as a mediator of the neuroendocrine-immune interactions. Experimental evidence suggests that CRH may modulate the immune and inflammatory responses via two pathways: an antiinflammatory one operated by centrally released CRH, most likely through stimulation of glucocorticoid and catecholamine release, and one proinflammatory, through direct action of peripherally released CRH. This review highlights these concepts. In addition preliminary data on immune activation and inflammatory response in CRH-deficient mice created in our laboratory are discussed.

Effect of hypothalamic lesion or chemical axotomy on restitution of immunoreactivity in mice after cyclophosphamide administration

Splenocytes harvested from mice that underwent chemical axotomy (by 6-hydroxydopamine) or that had electric lesions in their anterior hypothalami demonstrated a significant decrease in their proliferative ability after concanavalin-A stimulation in vitro. In axotomized mice pre-treated with cyclophosphamide, faster restitution of the proliferative activity was observed on days 6-8 after the drug's administration. Splenocytes isolated from mice treated with 6-hydroxydopamine did not exhibit any suppressive activity, which is characteristic of the restitution period following administration of cyclophosphamide. These results indicate that the autonomic nervous system exert a direct effect on immunoreactivity and on processes which are responsible for restitution after cyclophosphamide-induced immunodisorders.

Immune activation: the role of pro-inflammatory cytokines in inflammation, illness responses and pathological pain states

It has recently become accepted that the activated immune system communicates to brain via release of pro-inflammatory cytokines. This review examines the possibility that pro-inflammatory cytokines (interleukins and/or tumor necrosis factor) mediate a variety of commonly studied hyperalgesic states. We will first briefly review basic immune responses and inflammation. We will then develop the concept of illness responses and provide evidence for their existence and for the dramatic changes in neural functioning that they cause. Lastly, we will examine the potential roles that both pro-inflammatory cytokines and the neural circuits that they activate may play in the hyperalgesic states produced by irritants, inflammatory agents, and nerve damage. The possibility is raised that apparently diverse hyperalgesic states may converge in the central nervous system and activate similar or identical neural circuitry.

Cytokine-to-brain communication: a review & analysis of alternative mechanisms

It is becoming well accepted that products of the immune system (cytokines) can signal the brain that infection has occurred. This cytokine-to-brain communication can result in marked alterations in brain function and behavior. This review examines alternative mechanisms that have been proposed to explain how such immune products can reach the brain via the blood to cause centrally-mediated "illness" responses. Finally, we describe a new view which argues that cytokines signal brain in quite a different manner, by stimulating afferent terminals of peripheral nerves at local sites of synthesis and release.

Prenatal alcohol exposure selectively suppresses cell-mediated but not humoral immune responsiveness

The present study examined effects of fetal alcohol exposure (FAE) on the ability of peripubertal male and female rats to mount a humoral immune response against T-cell-dependent as well as independent antigens. The appropriate pair-fed (PF) and control (C) cohort rats were included. Serum immunoglobulins (Ig) levels were determined following a primary or secondary immune response. In addition, plasma corticosterone levels were measured in conscious, freely moving FAE, PF and C rats following sensitization with the T-cell-dependent antigen sheep red blood cells (SRBC). The study demonstrates that, in response to primary or secondary immunization, serum Ig levels in FAE rats were not significantly different from those in the PF or C cohorts. On the other hand, a marked reduction in mitogen-induced T-cell proliferative response was observed in FAE male rats in the same age group. Plasma corticosterone concentrations were increased almost four-fold 7 days after the primary immunization with SRBC, but there were no significant differences among the FAE, PF or C groups. Taken together, evidence from in vivo and in vitro studies indicates that FAE is associated with a selective impairment of cell-mediated immune function, including mitogen-induced T-cell proliferation, graft versus host as well as contact sensitization responses, but not of humoral immune function.

Interleukin-1 induces sleep-like behavior and alters call structure in juvenile rhesus macaques

To date, there have been no investigations of the behavioral effects of interleukin-1 (IL-1) in nonhuman primates. In this study the locomotor behavior and vocalizations of juvenile rhesus monkeys were monitored for 45 minutes following intravenous injections of recombinant human IL-1 alpha. In addition, their reaction to a broadcasted recording of infant monkey distress calls was determined 20 minutes after the beginning of each test session. IL-1 induced sleep-like inactivity and significantly diminished the monkey's behavioral and vocal responses to the broadcasted calls. The coo calls uttered by the monkeys following IL-1 treatment also had a longer duration and lower fundamental frequency than calls during the control condition. As several studies have indicated that behavioral effects of IL-1 may be mediated by corticotropin-releasing hormone (CRH), a second group of rhesus monkeys was given injections of CRH. CRH did not alter behavior or call structure at the dose administered. These results extend previous research on the behavioral effects of IL-1 to include the nonhuman primate and provide the first evidence that cytokines can affect vocal communication in rhesus monkeys. © 1995 Wiley-Liss, Inc.

Alpha-interferon inhibits adrenocortical secretion via mu 1-opioid receptors in the rat

The effect of recombinant human alpha-interferon on plasma corticosterone concentrations was investigated in adult male rats. Intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) administration of alpha-interferon (10-10(4) U i.p., and 1-10(3) U i.c.v.) decreased basal plasma corticosterone concentrations. This effect was evident at both the peak and nadir in the circadian rhythm of hypothalamo-pituitary-adrenocortical secretory activity. The same inhibitory effect was obtained with intra-paraventricular nucleus administration of the cytokine. Furthermore, alpha-interferon attenuated the effects of stressors such as handling, 1 min of forced swimming, and sound stress in a novel environment. The effect of alpha-interferon (10(2) U i.c.v.) was blocked by prior injection of the opioid receptor antagonist, naloxone (1 mg/kg i.p.). Similarly, the effect of 10(3) U alpha-interferon administered i.p. was blocked by i.c.v. injection of naloxone (1 microgram/kg), or of the mu 1-specific receptor antagonist, naloxonazine (1 microgram). The selective delta-opioid receptor antagonist, naltrindole (1 microgram i.c.v.) and the kappa-opioid receptor antagonist, nor-binaltorphimine (1 microgram i.c.v.) both failed to prevent the inhibitory effect of alpha-interferon (10(3) U i.p.) on adrenocortical secretion. The results obtained provide further evidence for a neuromodulatory effect of alpha-interferon and that this effect is mediated by central opioid receptors of the mu 1-subtype, delta- and kappa-opioid receptors not being involved.

Electroencephalographic changes in experimental autoimmune myasthenia gravis

A number of reports have suggested that central disturbances of cholinergic function may occur in patients with myasthenia gravis. The present study was designed in order to examine cortical electroencephalographic (EEG) activity in Lewis rats with experimental autoimmune myasthenia gravis (EAMG). Experiments were performed on conscious rats with clinical EAMG and demonstrable antibodies against the acetylcholine receptor. The animals showed no gross changes in cortical EEG discharge in terms of cycles and durations of wake, desynchronized sleep, and synchronized sleep, as compared with control rats. However, abnormalities characterized by single spikes or waves, and by spike and wave complexes, were observed, most commonly during synchronization of the EEG. Use of computerized frequency analysis of the EEG records revealed the presence of three basic differences in EEG discharge in myasthenic animals: (1) additional high-amplitude, low frequency (< 4 Hz) activity was recorded, especially during synchronized sleep; (2) decreases in mid-range (4-7 Hz) activity were recorded, particularly during periods of wakefulness; and, (3) increases in high frequency (> 8 Hz) spike discharge were observed at all times, although this was most evident during periods of synchronized sleep. The data provide further evidence for alterations in central cholinergic function in myasthenia gravis.

Effect of chronic treatment with recombinant interleukin-2 on the central nervous system of adult and old mice

We have studied the effects of chronic treatment with recombinant interleukin-2 on the central nervous system of adult and old mice. Treatment with high doses of recombinant interleukin-2, on a schedule similar to that used in humans, was started at the age of 4 and 17 months, respectively, and ended 3 months later. At that time, all the mice were tested for acquisition of a passive-avoidance task and then sacrificed for histological examination. Three of the four groups (treated and control adults and control old mice) did not differ from one another in task performance or neuron density in frontal cortex, cerebellum, dentate gyrus or CA1-2, CA3, CA4 hippocampal areas. The old treated mice were unique in showing impairment of the mnesic functions and marked neuronal cell loss and degenerative changes limited to the hippocampal regions. Immunohistochemical studies did not show any significant amount of immunoglobulins in affected areas. Our results suggest that in old mice the impairment of the mnesic functions after recombinant interleukin-2 administration is due to hippocampal neuronal damage.

Immunoregulators in the nervous system

The nervous system, through the production of neuroregulators (neurotransmitters, neuromodulators and neuropeptides) can regulate specific immune system functions, while the immune system, through the production of immunoregulators (immunomodulators and immunopeptides) can regulate specific nervous system functions. This indicates a reciprocal communication between the nervous and immune systems. The presence of immunoregulators in the brain and cerebrospinal fluid is the result of local synthesis--by intrinsic and blood-derived macrophages, activated T-lymphocytes that cross the blood-brain barrier, endothelial cells of the cerebrovasculature, microglia, astrocytes, and neuronal components--and/or uptake from the peripheral blood through the blood-brain barrier (in specific cases) and circumventricular organs. Acute and chronic pathological processes (infection, inflammation, immunological reactions, malignancy, necrosis) stimulate the synthesis and release of immunoregulators in various cell systems. These immunoregulators have pivotal roles in the coordination of the host defense mechanisms and repair, and induce a series of immunological, endocrinological, metabolical and neurological responses. This review summarizes studies concerning immunoregulators--such as interleukins, tumor necrosis factor, interferons, transforming growth factors, thymic peptides, tuftsin, platelet activating factor, neuro-immunoregulators--in the nervous system. It also describes the monitoring of immunoregulators by the central nervous system (CNS) as part of the regulatory factors that induce neurological manifestations (e.g., fever, somnolence, appetite suppression, neuroendocrine alterations) frequently accompanying acute and chronic pathological processes.

The role of prolactin in autoimmune demyelination: suppression of experimental allergic encephalomyelitis by bromocriptine

Several lines of evidence suggest that the anterior pituitary hormone prolactin has a stimulatory role on immune function and that pharmacological suppression of prolactin secretion with the dopamine-agonist bromocriptine suppresses both humoral and cellular immunity. Here, we describe the effects of prolactin-suppression on the course of experimental allergic encephalomyelitis in female Lewis rats. Initiation of continuous bromocriptine treatment before immunization reduced both the severity and incidence of clinical signs of acute experimental allergic encephalomyelitis. Experimental allergic encephalomyelitis-immunized rats experienced a threefold rise in basal prolactin levels on day 4 after immunization and maintained elevated prolactin levels on day 10, before the onset of neurological signs of experimental allergic encephalomyelitis. Bromocriptine treatment reduced prolactin levels to those of sham-immunized rats. In vivo bromocriptine pretreatment inhibited splenic lymphocyte proliferative responses in vitro to the immunizing antigen and to concanavalin A. Moreover, bromocriptine therapy was protective when initiated 1 week after the initial immunization and was also effective in suppression of late disease. These results indicate that (1) prolactin levels are elevated after immunization and before the onset of experimental allergic encephalomyelitis, (2) bromocriptine inhibits both prolactin secretion and the severity of acute experimental allergic encephalomyelitis, and (3) inhibition is also present when treatment is begun after sensitization, suggesting an effect of prolactin on the effector limb of the immune response during experimental allergic encephalomyelitis.

Absence of neural responses following suppression of the immune response by cyclophosphamide

Injection of sheep red blood cells (SRBC) as an antigenic stimulus, causes significant increases (up to 300%) in multiunit neural activity in the preoptic area/anterior hypothalamus of conscious rats. This increase occurs on the fifth or sixth day after immunization, at the time of first appearance of circulating antibodies at a serum titer of 1:32, increasing to 1:128 by day 10 following sensitization. Treatment with the immunosuppressive drug cyclophoshamide was able to prevent both antibody production and the expected increases in electrical activity in 5 of 6 rats; the one remaining animal showed a low level of circulating anti-SRBC antibodies on day 10 (1:32) and also, a small increase (36%) in neural activity at the expected time. These results provide further evidence that activation of the immune system is able to alter neuronal activity in an area of the brain important in the regulation of both neuroendocrine and neuroimmunomodulatory mechanisms, and that such activity is probably due to soluble secretory products released from components of the immune system.

Alpha-interferon modifies cortical EEG activity: dose-dependence and antagonism by naloxone

Activation of the immune system is believed to provide signals in the form of chemical messengers that are able to change neural activity in a variety of regions of the central nervous system. In studies designed to examine the effects of alpha-interferon (alpha-IFN) upon the central nervous system, recordings of cortical EEG were made following intracerebroventricular injection of various doses of the cytokine. Administration of 25 U of alpha-IFN increased the amount of wake and decreased the amount of desynchronized sleep in the first hour following injection; an increase in synchronization being seen in the third hour. alpha-IFN at 250 U increased the amount of synchronization and decreased the amount of desynchronized sleep in the EEG, principally in the second hour, with 2,500 U having similar but more potent effects, mostly in the first hour. The (mu) opiate receptor antagonist, naloxone, was found to decrease the amount of EEG synchronization and blocked the increases in synchronized sleep produced by 250 U alpha-IFN. The data suggest that alpha-interferon increases EEG synchronization in a dose-dependent and specific manner, probably via central mu-opiate receptors. The increased wake in the EEG following 25 U suggests, however, that another discrete effect of alpha-IFN may also exist.

Selective facilitation of putative corticotropin-releasing factor-secreting neurones by interleukin-1

The activity of single hypothalamic paraventricular nucleus (PVN) neurones was recorded in order to examine the mechanism by which the endogenous pyrogen interleukin-1 (IL-1) increases activity of the hypothalamo-hypophyseal-adrenocortical axis. IL-1 injected intravenously caused a rapid increase in the electrical activity of putative corticotropin-releasing factor (CRF)-secreting neurones located within the PVN. The activity of neighboring, electrophysiologically identified, vasopressin-secreting neurones was not altered by the stimulus, indicating a lack of involvement of this secretagogue of adrenocorticotropic hormone (ACTH) in this response to IL-1. These results support the concept of a rapid and specific activating effect of IL-1 upon hypothalamic CRF secretion as a part of a bidirectional communicating link between immune and central nervous systems.

Cytokine modulation of pituitary hormone secretion

A rapidly expanding body of evidence indicates that cytokines do indeed regulate pituitary hormone secretion. Recent studies with cytokines in vivo and in vitro support the idea that cytokines are the principal mediators of the neuroendocrine responses previously observed in infectious and inflammatory states. The dominant route of this modulation appears to be via the brain and hypothalamus, although a role for direct effects on the pituitary has not been excluded. These effects may be mediated by circulating cytokines, endogenously produced cytokines, or both. A number of receptor systems and second messengers may be involved, and a role for arachidonate metabolite pathways appears particularly likely. A final question: Of what use to the organism is the ability of immune activation to control pituitary hormone secretion? For some pituitary secretions there is a reasonable basis for speculation. Glucocorticoids serve to limit the severity of immune responses and recent studies argue that defects in this pathway permit the expression of autoimmune disease. Inhibition of thyroid function may limit the catabolic side effects of infectious illness. Stimulation of growth hormone could have the same effect, and growth hormone and prolactin may serve to enhance some immune responses.

Freund's complete adjuvant induces ornithine decarboxylase activity in the central nervous system of male rats and triggers the release of pituitary hormones

In male rats, inoculation of Freund's complete adjuvant (FCA, 0.5 mg/rat of Mycobacterium butyricum in paraffin oil) induced high levels of ornithine decarboxylase (ODC) in the hypothalamus and pituitary gland (285% and 245% of controls, respectively, within 12 h to 2 days). ODC activity also was altered in the cerebellum and left neocortex, but not in the right neocortex. This activity reflected a dynamic equilibrium which is influenced by ODC synthesis, degradation, activation, etc. The circadian rhythms of pituitary ODC activity and plasma prolactin level, 3-4 days after FCA, showed that enhancement of enzymatic activity during the dark phase correlated with a marked release of prolactin (Prl). During this early period after FCA, changes in plasma levels of other pituitary hormones were not significant or were less important. Pretreatment with bromocriptine microcapsules inhibited both basal and FCA-induced pituitary ODC activity, as well as Prl secretion. Further, significant increases in plasma luteinizing hormone and adrenocorticotropic hormone were noted from days 4 and 8, respectively, and onwards. Finally, a phase of reduced corticosterone secretion occurred during the latency period. This study shows that FCA influences central nervous system pathways and supports the idea that endogenous Prl is involved in some early events which lead to the development of adjuvant arthritis.

Catecholaminergic projections to tuberoinfundibular neurones of the paraventricular nucleus: I. Effects of stimulation of A1, A2, A6 and C2 cell groups

Extracellular electrical activity was recorded from 203 paraventricular nucleus (PVN) neurones antidromically identified as projecting to the median eminence. Spontaneous activity and the effects of stimulation of the A1, A2, A6 and C2 catecholaminergic cell groups upon the PVN neurones were examined. Cells were located at a mean height 2.29 +/- 0.03 mm above the base of the brain, corresponding with the corticotropin-releasing factor (CRF) rich component of the nucleus. The mean firing rate was 3.2 +/- 0.3 Hz and antidromic invasion latency was 9.9 +/- 0.3 msec. Seventy-six % of cells tested were activated by painful somatosensory stimuli. Electrical stimulation of the A1 or A2 region evoked excitatory responses from the majority of cells tested (76% and 85%, respectively), whilst stimulation of the A6 and C2 regions evoked more inhibitory responses (43% and 59%, respectively). Most responses (56%), whether excitatory or inhibitory, were not clearly defined in terms of latency, and were only observed following delivery of 5-10 single shocks at 0.5 Hz. Excitation recorded following A1 and A2 stimulation suggests a facilitatory role for noradrenaline in the regulation of PVN activity. Inhibitory responses following C2 stimulation indicate that adrenaline may serve to inhibit such activity, whilst the more mixed responses following A6 stimulation suggest that the projections of this region differ in some way from those of the A1 and A2 cells. Response reversals were observed, after delivery of higher frequency stimulation, for a substantial proportion (20%) of the cells tested.