Sachiko Take

The Autonomic Nervous System as a Communication Channel between the Brain and the Immune System

Much evidence from various fields has revealed multiple channels of communication between the brain and the immune system. Among the routes of signal transmission, this review focuses on the roles and mechanisms of neural communication between the two systems. As for the centrifugal neural pathway by which the brain modulates immunity, there are various requirements for the noradrenergic sympathetic innervation of the primary and secondary lymphoid organs. In addition to the presence of beta- and alpha-adrenergic receptors on different types of immunocompetent cells, histological studies have demonstrated direct contact between tyrosine-hydroxylase-positive nerve terminals and lymphocytes in the spleen and thymus. The exposure of lymphocytes and macrophages to adrenergic agonists in vitro modulates their functions. A surgical or chemical sympathectomy is known to alter the immune responses in rodents. Recent data from the rat show that stress-induced immunosuppression is only slightly affected, if at all, by hypophysectomy or adrenalectomy, whereas it is largely dependent on sympathetic innervation. The splenic sympathetic nerve alters the firing rate by an ablation or stimulation of the hypothalamus, the administration of cytokines or neuropeptides, and an exposure to stress. Furthermore, such procedures provoke the increase in the release of noradrenaline in the rat spleen as assessed by in vivo microdialysis. The altered activities of the splenic sympathetic nerves mentioned above have been found to be causally related to the alteration in immunological responses including natural killer cytotoxicity. The splenic sympathetic nerve may thus constitute a communication channel that mediates central modulation of peripheral cellular immunity. Although the roles and mechanisms of parasympathetic control of lymphoid organs still remain obscure, recent data suggest that the thymic vagal efferent nerve may be involved in central modulation of immunity. Finally, electrophysiological studies have shown that hepatic vagal afferents may be one of the pathways through which blood-borne cytokines signal the brain.

In vivo measurement of hypothalamic serotonin release by intracerebral microdialysis: Significant enhancement by immobilization stress in rats

Intracerebral microdialysis was used to measure extracellular serotonin and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the hypothalamus of unanesthetized rats. Increase in the concentration of K+ in the perfusing Ringer solution (70 mM) produced a sharp increase in serotonin release, which was significantly attenuated by omitting Ca2+ from the perfusion medium. Intraperitoneal injection of 5-hydroxytryptophan, a precursor of serotonin, or local perfusion of pargyline, a monoamine oxidase inhibitor, elevated the hypothalamic serotonin. Releasers or uptake inhibitors of serotonin, such as fenfluramine, cocaine, mazindol, or imipramine, when added to the perfusion medium, significantly increased serotonin level, whereas 5-HIAA was unaffected by these substances. Immobilization-stress caused an immediate increase in both the extracellular serotonin and 5-HIAA in the hypothalamus, suggesting that the hypothalamic serotonergic system is activated during immobilization stress. The present study indicates that the brain microdialysis is useful for analysis of local changes in serotonin concentration which directly reflect neuronal transmission.

Roles of sympathetic nervous system in the suppression of cytotoxicity of splenic natural killer cells in the rat.

1. We previously demonstrated that a central injection of interferon‐alpha in rats induced a suppression of cytotoxicity of splenic natural killer cells which depended upon intact splenic sympathetic innervation, suggesting the important role of the splenic nerve in immunosuppression. To further study the mechanisms of this phenomenon, we investigated: (1) the effects of a central injection of recombinant human interferon‐alpha on the electrical activity of the splenic nerve, and (2) the responses of splenic natural killer cytotoxicity on the electrical stimulation of the splenic nerve in urethane with alpha‐chloralose anaesthetized rats. 2. An injection of recombinant human interferon‐alpha (1.5 x 10(3) and 6.0 x 10(3) units (u) per rat) into the third cerebral ventricle produced a sustained and long lasting (at least for more than 60 min) increase in the electrical activity of splenic sympathetic nerve filaments in a dose‐dependent manner. Following an intra‐third‐ventricular injection of recombinant human interferon‐alpha at a dose of 6.0 x 10(3) u, the efferent discharges were elevated 2‐6 times that of the pre‐injection level with a mean onset latency of 12 min (8‐16 min). No changes in the arterial blood pressure and body temperature were observed after injections of recombinant human interferon‐alpha. 3. The excitation of the nerve activity induced by intra‐ventricular recombinant human interferon‐alpha was reversibly suppressed by an intravenous injection of an opioid antagonist, naloxone (1 mg/kg in 0.1 ml saline), whereas the injection of naloxone alone did not affect either the baseline level of the nerve activity or the systemic blood pressure. 4. The cytotoxicity of natural killer cells in the spleen measured by a standard chromium release assay was reduced 20 min after the laparotomy alone in anaesthetized rats. The reduced natural killer activity then recovered significantly when the splenic nerve was cut immediately after the laparotomy. When the peripheral cut end of the splenic nerve was subsequently stimulated (0.5 mA, 0.5 ms, 20 Hz for 20 min), a further suppression of natural killer cytotoxicity was observed. 5. The reduction of natural killer cytotoxicity produced by the stimulation of the splenic nerve was completely blocked by an intravenous injection of nadolol (a peripherally acting beta‐adrenergic receptor antagonist), but not by that of prazosin (an alpha‐antagonist). 6. These results indicate that a central injection of recombinant human interferon‐alpha activates the splenic sympathetic nerve through brain opioid receptors and thereby suppresses the natural killer cytotoxicity by beta‐adrenergic mechanisms.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypothalamic modulation of splenic natural killer cell activity in rats.

1. The cytotoxic activity of splenic natural killer cells measured by a standard chromium release assay in urethane and alpha‐chloralose‐anaesthetized rats was significantly suppressed 20 min after bilateral ablation of the medial part of the preoptic hypothalamus (MPO). The suppression was completely blocked by prior splenic denervation. The splenic natural killer cell activity of MPO sham‐lesioned rats or thalamus‐lesioned rats, both having an intact splenic innervation, were not different from that of a non‐treated control group. 2. Electrical stimulation of the bilateral MPO (0.1 ms, 0.1‐0.3 mA, 5‐100 Hz) suppressed the efferent activity of the splenic nerve in all six rats examined. The reduction of the nerve activity was accompanied by a transient fall in blood pressure. An I.V. injection of phenylephrine (3 micrograms/0.3 ml) also evoked a suppression of the nerve activity, which was accompanied by transient hypertension, suggesting that the suppressive effect of the MPO stimulation was independent of changes in blood pressure. On the other hand, a bilateral lesion of the MPO resulted in a sustained increase in the electrical activity of the splenic sympathetic nerve filaments which lasted for more than 2 h. 3. Microinjection of monosodium‐L‐glutamate (0.1 and 0.01 M in 0.1 microliters saline) unilaterally into the MPO evoked a transient suppression of the efferent discharge rate of the splenic nerve activity within 1 min, which was also accompanied by a decrease in blood pressure. The injection of saline (0.1 microliter) into the MPO had no effect. The microinjection of recombinant human interferon‐alpha (200 and 2000 U in 0.1 microliter saline) into the MPO dose dependently increased the splenic nerve activity without any change in blood pressure. 4. In contrast, microinjection of interferon‐alpha into the paraventricular nucleus of the hypothalamus (PVN) had no effect on splenic nerve activity, although an injection of glutamate increased the nerve activity. 5. The present results, taken together with previous reports, suggest that the neuronal networks between the MPO and the splenic sympathetic nerve, which may be activated by centrally administered interferon‐alpha, are important in the suppression of the splenic cellular immunity.

Immune cytokines and regulation of body temperature, food intake and cellular immunity

Interleukin-1 (IL-1) and interferon alpha (IFN alpha), cytokines originally detected in immunological cells, now have been shown to produce nonimmunological host defense responses of central and peripheral origins. These cytokines are released from glial cells in the brain in pathological states. Local application of IL-1 beta and IFN alpha to thermosensitive neurons in the preoptic and anterior hypothalamus and glucose responsive neurons in the ventromedial hypothalamus in vivo and in vitro, altered the activity in appropriate ways to explain the cytokines-induced fever and anorexia, respectively. The responses to IL-1 beta, but not to IFN alpha, were blocked by sodium salicylate, suggesting the involvement of synthesis of prostaglandins. alpha MSH, an endogenous antipyretic and a possible antagonist of IL-1 beta at lymphocytes, specifically depressed the responses to IL-1 beta, but not those to IFN alpha. In contrast, the action of IFN alpha was reversibly blocked by naloxone, suggesting the opioid receptor mediation. Intracerebral injection of IFN alpha and beta-endorphin in the rat and mouse resulted in the suppression of cytotoxic activity of natural killer cells in the spleen by activation of brain opioid receptor, which was shown to be mediated predominantly by splenic sympathetic nerves. The results suggest a view that immune cytokines may provide afferent links for the regulatory circuits between the brain and the immune system.

Prolonged effects of polyriboinosinic:polyribocytidylic acid on spontaneous running wheel activity and brain interferon-α mRNA in rats: A model for immunologically induced fatigue

Following 2 weeks acclimation to the running wheel in the home cages, an i.p. injection of a synthetic double-stranded RNA, polyriboinosinic:polyribocytidylic acid (poly I:C, 3 mg/kg), was performed to produce the immunologically induced fatigue in rats. The daily amounts of spontaneous running wheel activity decreased to about 40-60% of the preinjection level until day 9 with normal circadian rhythm, then gradually returned to the baseline level by day 14. Rats given a heat exposure (36 degrees C for 1 h) for the consecutive 3 days showed an increase in activity except for the first day. In the open field test, the total moving distance and the number of rearing of the poly I:C-injected rats decreased on day 1, but they were not different from the saline-injected group on day 7, suggesting that the poly I:C-induced fatigue on day 7 was not due to the peripheral problems such as muscle/joint pain, but involved the CNS. Quantitative analysis of mRNA levels using a real-time capillary reverse transcriptase-polymerase chain reaction (RT-PCR) method revealed that interferon-alpha (IFN-alpha) mRNA contents in the cortex, hippocampus, hypothalamic medial preoptic, paraventricular, and ventromedial nuclei were higher in the poly I:C group than those in the saline and heat-exposed groups on day 7, although the amount of interleukin-1 beta mRNA showed no differences. Serum adrenocorticotropic hormone and catecholamine levels were not significantly different between groups. The present results indicate that the prolonged fatigue induced by poly I:C, which is evaluated by the spontaneous running wheel activity, can be used as an animal model for the immunologically induced fatigue associated with viral infection, and suggest that brain IFN-alpha may play a role in this model.

Enhanced expression of brain interferon‐α and serotonin transporter in immunologically induced fatigue in rats

Immunologically induced fatigue was induced in rats by intraperitoneal injection of a synthetic double‐stranded RNA, polyriboinosinic : polyribocytidylic acid (poly I:C). An injection of poly I:C (3 mg/kg) decreased the daily amounts of spontaneous running wheel activity to ≈ 60% of the preinjection level until day 8. Quantitative analysis of mRNA levels demonstrated that interferon‐α (IFN‐α) and p38 mitogen‐activated protein kinase mRNAs increased in the medial preoptic, paraventricular and ventromedial hypothalamic nuclei and in cortex on both days 1 and 8, while interleukin‐1β and an inhibitor of nuclear factor κB (IκB)‐β mRNAs increased on day 1, but recovered within a week. Serotonin transporter (5‐HTT) mRNA also increased on days 1 and 8 after poly I:C injection in the same brain regions where IFN‐α mRNA increased. The increased 5‐HTT had a functional significance, because in vivo brain microdialysis revealed that an i.p. injection of poly I:C induced a decrease in the extracellular concentration of 5‐HT in the prefrontal cortex; the decrease was blocked by local perfusion with a nonselective 5‐HT reuptake inhibitor, imipramine. Finally, the poly I:C‐induced fatigue was attenuated by a 5‐HT1A receptor agonist but not by 5‐HT2, 5‐HT3 or dopamine D3 agonists. These findings, taken together, suggest that disorders in brain IFN‐α and 5‐HTT expression may be involved in the neuronal mechanisms of the poly I:C‐induced fatigue.

Neuroimmunomodulatory Actions of Hypothalamic Interferon-α

Recent studies have revealed that the brain produces interferon-α (IFN-α) in response to noninflammatory as well as inflammatory stress and that it might have a role in normal physiology. When administered intracerebrally, IFN-α causes diverse effects including fever, anorexia, analgesia and changes in the central neuronal activities. These responses are inhibited by the opioid receptor antagonist naloxone. This is consistent with the reports suggesting that recombinant human (rh) IFN-α binds to opioid receptors in rodent brain membrane. We revealed that rhIFN-α altered the activity of thermosensitive neurons in the medial preoptic area (MPO) and glucose-responsive neurons in the ventromedial hypothalamus in an opioid-receptor-dependent way. As a stress which produces opioid-dependent analgesia is known to suppress the cytotoxicity of splenic natural killer cells, we investigated whether the administration of β-endorphin and rhIFN-α may induce a similar immunosuppression. We found that central, but not peripheral, injection of both compounds inhibited natural killer (NK) cytotoxicity. Further studies revealed that rhIFN-α decreased the activity of MPO neurons via opioid receptors and the altered activity of MPO neurons in turn resulted in the activation of corticotropin-releasing factor neurons, thereby suppressing NK cytotoxicity predominantly through activation of the splenic sympathetic nerve and β-receptor mechanisms in splenocytes. Thus, IFN-α may alter the brain activity to exert a feedback effect on the immune system. Further detailed whole-cell clamping analyses on neuronal mechanisms in rat brain tissue slices showed that the inhibitory effect of rhIFN-α on N-methyl-D-aspartate-induced membrane current responses of MPO neurons was mediated not only by opioid receptors but also by the local production of reactive oxygen intermediates, nitric oxide and prostanoids, possibly due to neuron-glial cell interaction.

Plasmalogens Rescue Neuronal Cell Death through an Activation of AKT and ERK Survival Signaling

Neuronal cells are susceptible to many stresses, which will cause the apoptosis and neurodegenerative diseases. The precise molecular mechanism behind the neuronal protection against these apoptotic stimuli is necessary for drug discovery. In the present study, we have found that plasmalogens (Pls), which are glycerophospholipids containing vinyl ether linkage at sn-1 position, can protect the neuronal cell death upon serum deprivation. Interestingly, caspse-9, but not caspase-8 and caspase-12, was cleaved upon the serum starvation in Neuro-2A cells. Pls treatments effectively reduced the activation of caspase-9. Furthermore, cellular signaling experiments showed that Pls enhanced phosphorylation of the phosphoinositide 3-kinase (PI3K)-dependent serine/threonine-specific protein kinase AKT and extracellular-signal-regulated kinases ERK1/2. PI3K/AKT inhibitor LY294002 and MAPK/ERK kinase (MEK) inhibitor U0126 treatments study clearly indicated that Pls-mediated cell survival was dependent on the activation of these kinases. In addition, Pls also inhibited primary mouse hippocampal neuronal cell death induced by nutrient deprivation, which was associated with the inhibition of caspase-9 and caspase-3 cleavages. It was reported that Pls content decreased in the brain of the Alzheimer’s patients, which indicated that the reduction of Pls content could endanger neurons. The present findings, taken together, suggest that Pls have an anti-apoptotic action in the brain. Further studies on precise mechanisms of Pls-mediated protection against cell death may lead us to establish a novel therapeutic approach to cure neurodegenerative disorders.

Brain cytokines and the 5-HT system during poly I:C-induced fatigue

Abstract:  Fatigue is evoked not only by peripheral factors, such as muscle fatigue, but also by the central nervous system (CNS). For example, it is generally known that the feeling of fatigue is greatly influenced by psychological aspects, such as motivation. However, little is known about the central mechanisms of fatigue. The clinical symptoms of chronic fatigue syndrome (CFS) are shown to include disorders in neuroendocrine, autonomic, and immune systems. On the other hand, it has been demonstrated that cytokines produced in the brain play significant roles in neural–immune interactions through their various central actions, including hypothalamo‐pituitary and sympathetic activation, as well as immunosuppression. In this article, using the immunologically induced fatigue model, which was achieved by intraperitoneal (i.p.) injection of synthetic double‐stranded RNAs, polyriboinosinic: polyribocytidylic acid (poly I:C) in rats, we show an involvement of brain interferon‐α (IFN‐α) and serotonin (5‐HT) transporter (5‐HTT) in the central mechanisms of fatigue. In the poly I:C‐induced fatigue rats, expression of IFN‐α and 5‐HTT increased, while extracellular concentration of 5‐HT in the medial prefrontal cortex decreased, probably on account of the enhanced expression of 5‐HTT. Since the poly I:C‐induced reduction of the running wheel activity was attenuated by a 5‐HT1A receptor agonist, but not by 5‐HT2, 5‐HT3, or dopamine D3 receptor agonists, it is suggested that the decrease in 5‐HT actions on 5‐HT1A receptors may at least partly contribute to the poly I:C‐induced fatigue.