REVIEW ISJ 5: 12-19, 2008 ISSN 1824-307X REVIEW Norepinephrine and octopamine: linking stress and immune function across phyla SA Adamo Department of Psychology, Dalhousie University, Canada Accepted February 8, 2008 Abstract In species from three widely divergent phyla (Arthropoda, Mollusca and Chordata) tyrosine derivatives (norepinephrine or octopamine) mediate a response to acute stress. Part of this response is a change in immune function that results in a decrease in resistance to pathogens. This decrease in disease resistance appears maladaptive. However, if the connections between norepinephrine/octopamine and immune function were maladaptive, they should have been selected against. None of the four commonly proposed adaptive explanations for acute stress-induced changes in immune function fit the available data for species from all three phyla. However, this result is probably due to the lack of information about acute stress-induced immunosuppression in invertebrates and a lack of ecologically valid studies in vertebrates. Understanding why immune function and disease resistance changes during acute stress will require greater comparative study. Key words: immunocompetence; immunosuppression; insect; mollusc; vertebrate; adaptive benefits Introduction When responding to danger, animals from across the animal kingdom alter their physiology in order to optimize it for the performance of flight-or- fight behaviours (Wingfield, 2003). This reaction is called the acute stress response. Species from at least three diverse phyla (Chordata, Mollusca and Arthropoda) coordinate their acute stress response using chemically similar derivatives of the amino acid tyrosine (Ottaviani and Franceschi, 1996). Vertebrates (Cooper et al., 2003) and molluscs (Lacoste et al. 2001a) release norepinephrine (NE) during acute stress, while insects release norepinephrine’s chemical cousin, octopamine (O) (Orchard et al., 1993). In both vertebrates (Charmandari et al., 2005) and invertebrates (Roeder, 2005) NE and OA mediate a range of stress related responses. Most of these responses prepare the animal for extreme physical exertion (Charmandari et al., 2005; Roeder, 2005). However, the acute stress response also has complex, but largely immunosuppressive effects in a wide range of animals (Adamo and Parsons, 2006). The acute stress response can influence immune function ___________________________________________________________________________ Corresponding author: Shelley A Adamo Department of Psychology Dalhousie University Halifax, NS, B3H 4J1, Canada E-mail: sadamo@dal.ca because immune cells in vertebrates (e.g. Webster et al., 2002; Madden, 2003), molluscs (Lacoste et al., 2001b) and insects (Gole et al., 1982; Orr et al., 1985) have receptors for NE or OA. The consistent connection between acute stress, NE (vertebrates and molluscs) or OA (insects) and immune function suggests that modulating immune function during acute stress serves an important adaptive function. In this paper, I use a comparative approach to examine four adaptive explanations for the existence of acute stress-induced immunosuppression. Two tyrosine derivatives: norepinephrine and octopamine OA and NE are both derived from the amino acid tyrosine, although via different pathways (Cooper et al., 2003). OA is synthesized from tyramine, while NE is synthesized from dopamine (Fig. 1). OA and NE are identical in structure, except for the number of hydroxyl groups on the benzene ring (Fig. 1). Molluscs use both NE and OA as a signaling molecule (e.g. NE: Sloley et al., 1990, Lacoste et al., 2001a, b; OA Vehovszky et al., 2005). Insects, on the other hand, use OA, but not NE as a signaling molecule (Roeder, 1999). Vertebrates make extensive use of NE and its metabolite, epinephrine as signaling molecules (Cooper et al., 12 mailto:sadamo@dal.ca Fig. 1. Biosynthetic pathways for norepinephrine and octopamine. Adapted from Cooper et al. (2003). 2003), but make little, if any, use of OA (Roeder, 1999; Pflüger and Stevenson, 2005; Farooqui, 2007). Most invertebrate OA receptors have substantial sequence homology with vertebrate adrenergic (e.g. NE) receptors (Evans and Maqueira, 2005; Roeder, 2005). Also, OA receptors in invertebrates have similar pharmacological profiles to vertebrate adrenergic receptors (e.g. Farooqui, 2007; Evans and Maqueira, 2005). Moreover, pharmcological studies of invertebrate OA receptors demonstrate that they show some affinity for NE (Evans and Maqueira, 2005). For example, in the aquatic snail Lymnaea stagnalis the cloned OA receptor has high affinity for OA, but also exhibits some affinity for NE (Gerhardt et al., 1997). Similarly, human alpha-adrenergic receptors (subtypes 2a, b, c) have high affinity for NE, but they also show some affinity for OA (Gerhardt et al., 1997). The similarity between OA and NE receptors suggests that both had a common origin millions of years ago (Pflüger and Stevenson, 2005). OA and NE transporters also seem to have had a common origin (Caveney et al., 2006). Interestingly, molluscs appear to lack both an OA and an NE transporter, even though they contain both compounds. Some insects also lack an OA transporter (e.g. Drosophila) and must deactivate OA enzymatically (Caveney et al., 2006). The chemical similarity between OA and NE, the similarity in the enzymes involved in their synthesis, and the similarities in the sequences of their receptor and transporter molecules support the argument that OA and NE pathways arose from the same ancestral pathway (Caveney et al., 2006). Both OA and NE play a role in stress adaptation, suggesting that this is an ancient conserved function for these compounds (Gerhardt et al., 1997; Roeder, 1999). Norepinephrine, octopamine and acute stress Molluscs (bivalves) react to stressful stimuli by contracting the large muscles that hold the shell closed (Moore, 2006). This is the bivalve equivalent of flight-or-fight behaviour. It also results in an increase in NE in the hemolymph (Lacoste et al., 2001a). NE is released by chromaffin-like cells in the oyster heart (Lacoste et al., 2001a). In vertebrates, the sympathetic nervous system (SNS) is activated in response to flight-or-fight situations and releases NE into immune organs (Nance and Sanders, 2007). All primary and secondary immune organs receive noradrenergic innervation, as do all body surfaces that are potential sites of microbial invasion (e.g. skin, gut or oral mucosa) (Nance and Sanders, 2007). NE also increases in concentration in the plasma (Sachser, 1987; Matt et al., 1997). Therefore NE can reach the entire vertebrate immune system. In insects, OA is released as a neurohormone during flight-or-fight behaviours (Orchard et al., 1997; Pflüger and Stevenson, 2005; Roeder, 2005). OA is released into the periphery by dorsal unpaired medial cells (DUM cells) (Roeder, 2005). DUM cells are considered to be the insect equivalent of the vertebrate SNS based on their anatomy and pattern of innervation (Evans and Maqueira, 2005; Roeder, 2005). Therefore NE, or its chemical cousin OA, is released by a wide range of animals in response to acute stress. These compounds are widely disseminated allowing NE (Charmandari et al., 2005) and OA (Orchard et al, 1993; Roeder, 2005) 13 to affect the immune system as well as mediating other stress responses. Norepinephrine, octopamine and immune function Immune cells release NE and it appears to have a paracrine-like function in both molluscs (Ottaviani et al., 1993; Ottaviani and Franceschi, 1996; Lacoste et al., 2001b) and vertebrates (Flierl et al., 2007). Therefore, the role of NE as an immunoregulator may be very ancient (Ottaviani and Franceschi, 1996). In molluscs, acute stress transiently suppresses immune function and increases susceptibility to bacterial infection (Table 1). This increased susceptibility to disease is caused, at least in part, by the release of NE during acute stress. Molluscan hemocytes contain receptors for NE (Lacoste 2001b), and NE has negative effects on hemocyte function (Table 1). Injections of NE result in decreased bacterial clearance and increased mortality in oysters challenged with a bacterial pathogen (Lacoste et al., 2001c). Acute stress results in a transient decline in resistance to bacterial infection in insects as well (crickets, Adamo and Parsons, 2006). Some of this decrease in disease resistance may be mediated by OA. Injections of OA prior to a bacterial challenge results in increased mortality (Adamo and Parson, 2006). However, OA also has immunoenhancing effects (Brey, 1994; Table 2). It can even increase resistance to infection when the pathogen is co- incubated with OA (Baines et al., 1992; Baines and Downer, 1992). As in insects, acute stress in vertebrates results in a mix of immunosuppressive and immunoenhancing effects (Dhabhar, 2002; Ortega, 2003; Gleeson et al., 2004; Glaser and Kiecolt- Glaser, 2005; Nance and Sanders, 2007; Ortega et al., 2007). Despite this complex mix of positive and negative effects, acute stress increases susceptibility to pathogens (e.g. Cao and Lawrence, 2002). One bout of intense exercise in mice (e.g. Davis et al., 1997) or humans (Gleeson et al., 2004) leads to an increased risk of disease and/or mortality in response to a pathogen challenge. NE appears to be causally involved in the increase in disease susceptibility after acute stress (e.g. Kohut et al., 1998; Cao et al., 2003; Emeny et al., 2007). The complexity of the effects of NE on vertebrate immune function has prevented a clear adaptive explanation for these changes (Sternberg, 2006). Madden (2003), Maestroni (2005), and Kin and Sanders (2006) suggest that these complex effects are a result of NE playing a role in maintaining immune function homeostasis. NE and OA may play a similar role in invertebrates. In molluscs and insects, NE or OA are present in the hemolymph of resting animals. Although this could be because it is difficult to take blood from animals without stressing them, it may also indicate that OA and NE are chronically present in the hemolymph. OA has a half-life of 15 min or less in insect hemolymph (Goosey and Candy, 1982), and, therefore it should not be detectable unless it is constantly being released. A background level of OA or NE in non-stressed animals would be consistent with the hypothesis that these compounds help maintain normal immune function in invertebrates. However, if OA or NE helps maintain immune homeostasis in a variety of animals, why do the levels of OA and NE increase dramatically during acute stress? In other words, how does pushing the ‘immune thermostat’ towards an extreme end benefit animals during acute stress? Adaptive function of NE and OA effects on immune function during acute stress In molluscs, insects and vertebrates, the acute stress response results in a brief period during which the animal’s ability to fight off infection is reduced (Fig. 2, however see below). NE or OA appear to mediate some of this immunosuppression (Fig. 2). This effect of NE or OA on the immune system appears to be maladaptive. Increasing susceptibility to disease seems likely to reduce survival and reproductive success, and such a response should be selected against. As Dhabhar (2002) has pointed out, during fighting or fleeing, animals run a real risk of injury and, therefore, exposure to pathogens. Although it might make good adaptive sense to delay copulation, digestion, and egg laying until the predator has passed, the immune response may not be dispensable during flight-or-fight behaviours because of the increased risk of injury (Dhabhar, 2002). Nevertheless, the fact that animals from three different phyla exhibit the same apparently maladaptive response suggests that, despite the costs, it provides some benefit. Below I review some of the suggestions as to why animals display acute stress-induced immunosuppression. These hypotheses are not mutually exclusive. In particular I explore whether there are any explanations that might fit the evidence from animals across all three phyla. Table 1 Effects of norepinephrine on molluscan immune functions Immune functions Effects References Susceptibility to bacterial infection ↑ Lacoste et al., 2001c Phagocytosis at physiological concentrations ↓ Lacoste et al., 2002a Production of reactive oxygen species induced by interleukin-1 ↓ Lacoste et al., 2001d Apoptosis ↑ Lacoste et al., 2002b 14 I focus on evidence obtained from whole animal studies. Immune values taken in vitro are often different when measured in vivo (Nance and Sanders, 2007). More importantly, as Kohut et al. (2005) comments, there is often a lack of association between declines in various immune functions and actual disease resistance (also see Adamo, 2004). From an evolutionary perspective, it is the change in disease resistance that is important. 1. The ‘energy crisis’ hypothesis. One common hypothesis for the existence of acute stress-induced immunosuppression is that it allows animals to channel more energy into flight-or-fight behaviour (e.g. see Råberg et al., 1998; Segerstrom, 2007). However, it is unclear whether immunosuppression would save energy. For example, some mechanisms of immunosuppression, such as apoptosis, require an increase in energy expenditure (Dhabhar, 2002). At present there is little direct evidence supporting the ‘energy-crisis’ hypothesis (Adamo and Parsons, 2006). 2. The ‘resource crunch’ hypothesis. Animals make a number of physiological changes in order to make flight-or-fight possible (Wingfield, 2003; Charmandari et al., 2005). The ‘resource crunch’ hypothesis suggests that some of these changes will result in resources being shifted away from the immune system in order to optimize the flight-or- fight response. This hypothesis differs from the ‘energy-crisis’ hypothesis because it is not energy per se that is limiting, but specific molecules that are required for both immunity and some other physiological function. The ‘resource crunch’ hypothesis explains, at least in part, acute stress-induced immunosuppression in insects. In crickets, conflicts between immune function and lipid transport can lead to acute stress-induced immunosuppression (Adamo et al., 2008). Crickets release OA during flight-or-fight behaviours (Adamo et al., 1995). For about an hour after flying or fighting, crickets become more susceptible to bacterial infection (Adamo and Parsons, 2006). OA, either directly and/or indirectly, induces the mobilization of lipid from the fat body in order to fuel flight-or-fight behaviours (Orchard et al., 1993). As lipid levels in the hemolymph increase, the protein apolipophorin III (apoLpIII) changes its confirmation, and combines with high density lipophorin (HDLp) to form low density lipophorin (LDLp) which has an increased lipid carrying capacity (see Weers and Ryan, 2006, for review). However, in the unlipidated form, apoLpIII acts as an immune surveillance molecule (Weers and Ryan, 2006). Once apoLpIII becomes part of LDLp, it appears to loose that ability, resulting in a decline in immune surveillance. The decline in immune surveillance probably explains the increase in disease susceptibility after flying and fighting (Adamo et al., 2008). Therefore, in crickets, intense activity leads to transient immunosuppression because apoLpIII is co-opted into lipid transport and becomes unavailable as an immune surveillance molecule. Therefore, crickets become immunosuppressed during flight-or-fight even if they have abundant energy stores (Adamo et al., 2008). The ability of OA to mobilize lipid explains why OA can produce immunosuppression when injected into crickets. Injecting OA results in the release of lipid (Woodring et al., 1989), which leads to a decline in the immune surveillance molecule apoLpIII as it combines with HDLp to form LDLp (Weers and Ryan, 2006). But why does OA also have immunoenhancing effects (Table 2)? I hypothesize that OA also works to maintain immune system function as some of the components of the immune system are being siphoned off into lipid transport. In other words, OA helps liberate lipid stores (needed to fuel flight-or- fight behaviour) while simultaneously reconfiguring the immune system to maintain maximal function under the new physiological conditions. I predict that without the effects of OA on immune function, disease resistance would decline even more precipitously during flying or fighting in crickets. This hypothesis explains why OA can have both immunosuppressive and immunoenhancing effects. Why do crickets not make enough apoLpIII to support both immune surveillance and increased lipid transport? First, it would be energetically expensive to do so. ApoLpIII is already a very abundant protein in the hemolymph of many adult insects (Weers and Ryan, 2006). To produce more of this protein would decrease the energy available for reproduction and other activities. Furthermore, as apoLpIII concentrations increase, apoLpIII may begin to bind to the animal’s own molecules, initiating an inappropriate immune response. Such autoimmunity could be costly (e.g. Sadd and Siva- Jothy, 2006). Therefore, shuttling apoLpIII between immune surveillance and lipid transport may be the most adaptive response, even though it results in transient immunosuppression during flying or fighting. Table 2 Effects of octopamine on insect immune functions Immune functions Effects References Susceptibility to bacterial infection ↑ Adamo and Parsons, 2006 Phagocytosis ↑ Baines et al., 1992 Nodule formation (insect immune response) ↑ Baines et al., 1992 Hemocyte locomotion ↑ Dielh-Jones et al., 1996 Hemocyte number at low (physiological) doses ↓ Dunphy and Downer, 1994 Hemocyte number at higher (pharmacological) doses ↑ Dunphy and Downer, 1994 15 Fig. 2. Schematic outline of the connections between NE, OA, acute stress and immune function in different phyla. DUM cells, dorsal unpaired median cells; NE, norepinephrine; OA, octopamine; SNS, sympathetic nervous system. See text for references It is unclear whether a similar scenario can explain acute-stress induced immunosuppression in molluscs. In molluscs, the known effects of NE are all negative (Table 1). However, there have been few studies on acute stress-induced immunosuppression in molluscs. More data are needed to assess whether molluscs suffer from a ‘resource crunch’ during acute stress. In vertebrates a number of molecules are shared between the immune system and other physiological systems. For example, lipid metabolism and immune function are also intertwined in vertebrates (e.g. van Elzen et al., 2005). Mammalian lipoproteins transport lipid (e.g. cholesterol), but they also participate in innate immunity (Khovidhunkit et al., 2004). Lipid carriers such as very low density lipoprotein (VLDL) bind to and neutralize viruses and other pathogen products (Khovidhunkit et al., 2004). During infection, VLDL levels increase (Khovidhunkit et al., 2004). However, after a single bout of intense exercise, the total concentration of VLDL particles in the blood declines by 38 % in humans (Børsheim et al., 1999). Whether changes in mammalian lipoprotein concentrations during intense activity results in acute stress-induced immunosuppression remains unknown. Some studies in mammals indirectly support the ‘resource crunch’ hypothesis. For example, despite the immunosuppressive effects of NE, mice that were stressed by minor surgery, and then exposed to infectious agents, were more likely to die from infection if they received ß-adrenoreceptor blockers (Schmitz et al., 2007). In another study, mice given ß-adrenoreceptor blockers prior to intense exercise were more likely to die after a viral challenge compared with controls (Kohut et al., 2005). These studies suggest that the effects of NE on immune function result in increased disease resistance when they occur within the context of an acute stress response. These results are consistent with the hypothesis that NE works to reconfigure the immune system in order to maintain immune function during a ‘resource crunch’. However, other studies have found that blocking ß1-adrenergic receptors decreased acute stress-induced immunosuppression (Cao et al., 2003). Emeny et al. (2007) found that mice lacking adrenoreceptors on their immune cells cleared a bacterial infection (Listeria monocytogenes) more quickly after acute stress than mice with immune cells capable of responding to NE. However, the relationship between the speed with which an animal can clear bacteria from liver and spleen and its ability to survive an infection was not stated in these studies (Cao and Lawrence, 2002; Cao et al., 2003; Emeny et al., 2007). When determining the effects of various drugs on disease resistance, Keil et al. (2001) used an LD10 dose of L. monocytogenes and measured the effect on mortality, not on bacterial clearance. In Drosophila melanogaster, the ability to clear bacteria from the hemocoel does not correlate with the ability to survive an infection (Corby-Harris et al., 2007). 3. The ‘over excitation’ hypothesis. Acute stress-induced immunosuppression may be beneficial because it prevents the immune system from becoming too active and harming the animal. During intense exercise, tissues such as muscle suffer minor damage, increasing the risk of inflammation and an autoimmune reaction (Råberg et al., 1998). Therefore, the immune system shifts towards a less inflammatory state, (i.e. a shift from Th1 to Th2 responses, Elenkov and Chrousos, 2006). This shift leads to a decrease in inflammation, but also an increased susceptibility to bacterial and viral pathogens. The cost of the increased risk of infection is thought to be less than an autoimmune reaction or damage from an overactive immune response. However, this key assumption remains untested. The ‘over-excitation’ hypothesis does raise the question as to why the prevention of ‘over- excitation’ occurs to the point that animals are left susceptible to bacterial infections during acute stress. It would be more adaptive to prevent autoimmunity and over-inflammation while maintaining normal anti-microbial defenses, unless there is some physiological constraint that makes this impossible. The ‘over-excitation’ hypothesis does fit the available data on molluscs (Table 1), but is not supported by the insect data (Table 2). In insects, immune cell activity appears to be up-regulated during acute stress (Table 2). Animals run the risk of having an over-active immune system during both an acute stress response and during an immune challenge. In vertebrates, an immune challenge also activates the acute stress response (Elenkov and Chrousos, 2006). This indirectly supports the ‘over-excitation’ hypothesis. However, insects release OA during an 16 immune challenge too (Dunphy and Downer, 1994), although its source is uncertain (Adamo, 2005). Given that OA appears to increase immune cell activity (Table 2), these data do not support the ‘over-excitation’ hypothesis. Immunologists should be wary of putting too much confidence in this hypothesis without more supporting data. 4. The ‘shift in focus’ hypothesis. This hypothesis suggests that during flight-or-fight animals are not immunosuppressed per se, but that they have shifted the focus of their immune effort from protecting against systemic invaders, to protecting against opportunistic organisms that might gain entry during wounding (Dhabhar, 2002). Dhabhar (2002) has shown that acute stress can increase a delayed-type hypersensitivity in rodents and that catecholamines (e.g. NE and epinephrine) play a role in this enhancement. This change could result in increased protection from wound infection (Dhabhar, 2002). However, tests with real bacteria have mixed results. Restraint stress produced a decrease in wound healing and a decrease in the ability mice to clear bacteria introduced into a wound (Rojas et al., 2002). However, the duration of the stress (mice were restrained for 12 h at a time for 8 days) is more typical of a chronic than an acute stress. Campisi et al. (2002) found that after a series of tail shocks given over 2 h, acutely stressed rats recovered more quickly than unstressed rats from a subcutaneous injection of a relatively benign bacterium (there was no mortality) (Campisi et al., 2002). However, this experiment does not convincingly show that acutely stressed rats are less likely to develop infected wounds. The ‘shift in focus’ hypothesis does not appear to apply to insects. Although OA enhances hemolymph clotting in some arthropods (e.g. Battelle and Kravitz, 1978), in insects, flight-or-fight behaviour results in an increase in infection after wounding (Adamo and Parsons, 2006). There is no evidence available from the molluscs. Conclusions The involvement of NE (or OA) in mediating stress-induced changes in immune function may be ancient (Ottaviani and Franceschi, 1996). Regardless of whether these connections have been conserved over millions of years or have evolved independently in multiple lineages, their existence in animals from different phyla suggests that there is strong selection pressure for a change in immune function in response to acute stress. None of the four suggested adaptive functions reviewed here explains acute stress-induced immunosuppression in all species. In part this is due to a lack of information about acute stress-induced immunosuppression in invertebrates. All of the information presented here rests on a handful of studies. But it is also because the key experiments are missing in vertebrate studies. For example, despite work on the ‘shift in focus’ hypothesis for more than a decade, whether acute stress actually decreases susceptibility to opportunistic wound infections under real world conditions remains unknown. The lack of a real world test of the ‘shift in focus’ hypothesis highlights a general lack of ecological context in these studies. For example, acute stress is typically produced using highly artificial stimuli, such as restraint stress or tail shock. These stressors have an unknown connection to an ecologically relevant stressor (e.g. a predator). Nor is the duration or intensity of the artificial stressors correlated with data about what the animal would experience in the field. Studies using a more ecological perspective will be critical if we are to understand the adaptive significance of the changes in immune response that occur during acute stress. References Adamo SA. How should behavioural ecologists interpret measurments of immunity? Anim. Behav. 68: 1443-1449, 2004. Adamo SA. Parasitic suppression of feeding in the tobacco hornworm, Manduca sexta: parallels with feeding depression after an immune challenge. Arch. Insect Biochem. Physiol. 60: 185-197, 2005. Adamo SA, Linn CE, Hoy RR. The role of neurohormonal octopamine during 'fight or flight' behaviour in the field cricket Gryllus bimaculatus. J. Exp. Biol. 198: 1691-1700, 1995. Adamo SA, Parsons NM. The emergency life-history stage and immunity in the cricket, Gryllus texensis. Anim. Behav. 72: 235-244, 2006. Adamo SA, Roberts JL, Easy RH, Ross NW. Competition between immune function and lipid transport for the protein apolipophorin III leads to stress-induced immunosuppression in crickets. J. Exp. Biol. 211: 531-538, 2008. Baines D, DeSantis T, Downer R. Octopamine and 5-hydroxytryptamine enhance the phagocytic and nodule formation activities of cockroach (Periplaneta americana) haemocytes. J. Insect Physiol. 38: 905-914, 1992. Baines D, Downer RGH. 5-Hydroxytryptamine- sensitive adenylate cyclase affects phagocytosis in cockroach hemocytes. Arch. Insect Biochem. Physiol. 21: 303-316, 1992. Battelle BA, Kravitz EA. Targets of octopamine action in the lobster: cyclic nucleotide changes and physiological effects in hemolymph, heart and exoskeletal muscle. J. Pharmacol. Exp. Ther. 205: 438-448, 1978. Børsheim E, Knardahl S, Høstmark AT. Short-term effects of exercise on plasma very low density lipoproteins (VLDL) and fatty acids. Med. Sci. Sports Exer. 31: 522-530, 1999. Brey PT. The impact of stress on insect immunity. Bull. Inst. Pasteur 92: 101-118, 1994. Campisi J, Leem TH, Fleshner M. Acute stress decreases inflammation at the site of infection: a role for nitric oxide. Physiol. Behav. 77: 291- 299, 2002. Cao L, Hudson CA, Lawrence DA. Acute cold/restraint stress inhibits host resistance to Listeria monocytogenes via ß1-adrenergic receptors. Brain Behav. Immun. 17: 121-133, 2003. 17 Cao L, Lawrence DA. Suppression of host resistance to Listeria monocytogenes by acute cold/restraint stress: lack of direct IL-6 involvement. J. Neuroimmunol. 133: 132-143, 2002. Caveney S, Cladman W, Verellen L, Donly C. Ancestry of neuronal monoamine transporters in the Metazoa. J. Exp. Biol. 209: 4858-4868, 2006. Charmandari E, Tsigos C, Chrousos G. Endocrinology of the stress response. Ann. Rev. Physiol. 67: 259-284, 2005. Cooper JR, Bloom FE, Roth RR. The Biochemical Basis of Neuropharmacology. 8th edition ed. Oxford University Press, New York, 2003. Corby-Harris V, Habel KE, Ali FG, Promislow DEL. Alternative measures of response to Pseudomonas aeruginosa infection in Drosophila melanogaster. J. Evol. Biol. 20: 526- 533, 2007. Davis JM, Kohut ML, Colbert LH, Jackson DA, Ghaffer A, Mayer EP. Exercise, alveolar macrophage function and susceptibility to respiratory function. J. App. Physiol. 83: 1461- 1466, 1997. Dhabhar F. Stress-induced augmentation of immune function-The role of stress hormones, leuckocyte trafficking, and cytokines. Brain Behav. Immun. 16: 785-798, 2002. Diehl-Jones WL, Mandato CA, Whent G, Downer RGH. Monoaminergic regulation of hemocyte activity. J. Insect Physiol. 42: 13-19, 1996. Dunphy GB, Downer RGH. Octopamine, a modulator of the haemocytic nodulation response of non-immune Galleria mellonella. J. Insect Physiol. 40: 267-272, 1994. Elenkov IJ, Chrousos GP. Stress system - Organization, physiology and immunoregulation. Neuroimmunodulation 13: 257-267, 2006. Emeny RT, Gao D, Lawrence DA. ß1-Adrenergic receptors on immune cells impair innate defences against Listeria. J. Immunol. 178: 4876-4884, 2007. Evans PD, Maqueira B. Insect octopamine receptors: a new classification scheme based on studies of cloned Drosophila G-protein coupled receptors. Invert. Neurosci. 5: 111-118, 2005. Farooqui T. Octopamine-mediated neuromodulation of insect senses. Neurochem. Res. 32: 1511- 1529, 2007. Flierl MA, Rittirsch D, Nadeau BA, Chen AJ, Sarma JV, Zetoune FS, et al. Phagocyte-derived catecholamines enhance acute inflammatory injury. Nature 449: 721-726, 2007. Gerhardt CC, Bakker RA, Piek GJ, Planta RJ, Vreugdenhil E, Leysen JE, et al. Molecular cloning and pharmacological characterization of a molluscan octopamine receptor. Mol. Pharmacol. 51: 293-300, 1997. Glaser R, Kiecolt-Glaser JK. Stress-induced immune dysfunction: implications for health. Nature Rev. Immunol. 5: 243-251, 2005. Gleeson M, Nieman DC, Pedersen BK. Exercise, nutrition and immune function. J. Sports Sci. 22: 115-125, 2004. Gole JWD, Downer RGH, Sohi SS. Octopamine- sensitive adenylate cyclase in haemocytes of the forest tent caterpillar, Malacosoma disstria Hubner (Lepidoptera: Lasiocampidae). Can. J. Zool. 60: 825-829, 1982. Goosey MW, Candy DJ. The release and removal of octopamine by tissues of the locust Schistocerca americana gregaria. Insect Biochem. 12: 681-685, 1982. Keil D, Luebke RW, Pruett SB. Quantifying the relationship between multiple immunological parameters and host resistance: probing the limits of reductionism. J. Immunol. 167: 4543- 4552, 2001. Khovidhunkit W, Kim MS, Memon RA, Shigenaga JK, Moser AH, Feingold KR, et al. Effects of infection and inflammation on lipid and lipoprotein metabolism: mechanisms and consequences to the host. J. Lipid Res. 45: 1169-1196, 2004. Kin NW, Sanders VM. It takes nerve to tell T and B cells what to do. J. Leukoc. Biol. 79: 1093- 1104, 2006. Kohut ML, Davis JM, Jackson DA, Colbert LH, Strasner A, Essig DA, et al. The role of stress hormones in exercise-induced suppression of alveolar macrophage antiviral function. J. Neuroimmunol. 81: 193-200, 1998. Kohut ML, Martin AE, Senchina DS, Lee W. Glucorticoids produced during exercise may be necessary for optimal virus-induced IL-2 and cell proliferation whereas both catecholamines and glucorticoids may be required for adequate immune defense to viral infection. Brain Behav. Immun. 19: 423-435, 2005. Lacoste A, Cueff A, Poulet SA. P35-sensitive caspases, MAP kinases and Rho modulate beta-adrenergic induction of apoptosis in mollusc immune cells. J. Cell Sci. 115: 761- 768, 2002b. Lacoste A, Jalabert F, Malham S, Cueff A, Poulet S. Stress and stress-induced neuroendocrine changes increase the susceptibility of juvenile oysters (Crassostrea gigas) to Vibrio splendidus. Appl. Environ. Microbiol. 67: 2304- 2309, 2001c. Lacoste A, Malham SK, Cueff A, Jalabert F, Gélébart F, Poulet SA. Evidence for a form of adrenergic response to stress in the mollusc Crassostrea gigas. J. Exp. Biol. 204: 1247- 1255, 2001a. Lacoste A, Malham SK, Cueff A, Poulet SA. Noradrenaline modulates hemocyte reactive oxygen species production via beta-adrenergic receptors in the oyster Crassostrea giga. Dev. Comp. Immunol. 25: 285-289, 2001b. Lacoste A, Malham SK, Cueff A, Poulet SA. Noradrenaline reduces the stimulatory effect of interleukin-1 alpha on reactive oxygen species production by oyster immunocytes. Invert. Biol. 120: 358-364, 2001d. Lacoste A, Malham SK, Gelebart F, Cueff A, Poulet SA. Stress-induced changes in the oyster Crassostrea gigas. Dev. Comp. Immunol. 26: 1- 9, 2002a. 18 Madden KS. Catecholamines, sympathetic innervation. and immunity. Brain Behav. Immun. 17: S5-S10, 2003. Maestroni GJ. Adrenergic modulation of dendritic cells function: relevance for the immune homeostasis. Curr. Neurovasc. Res. 2: 169- 173, 2005. Matt KS, Moore MC, Knapp R, Moore IT. Sympathetic mediation of stress and aggressive competition: plasma catecholamines in free-living male tree lizards. Physiol. Behav. 61: 639-647, 1997. Moore J. An Introduction to the Invertebrates. 2nd edition. Cambridge University Press, Cambridge, 2006. Munford RS. Detoxifying endotoxin: time, place and person. J. Endotoxin Res. 11: 69-84, 2005. Nance DM, Sanders VM. Autonomic innervation and regulation of the immune system (1987-2007). Brain Behav. Immun. 21: 736-745, 2007. Orchard I, Ramirez JM, Lange AB. A multifunctional role for octopamine in locust flight. Ann. Rev. Entomol. 38: 227-249, 1993. Orr GL, Gole JWD, Downer RGH. Characterization of an octopamine-sensitive adenylate cyclase in hemocyte membrane fragments of the American cockroach Periplaneta americana L. Insect Biochem. 15: 695-701, 1985. Ortega E. Neuroendocrine mediators in the modulation of phagocytosis by exercise: physiological implications. Exerc. Immunol. Rev. 9: 70-93, 2003. Ortega E, Giraldo E, Hinchado MD, Martin L, Garcia LL, De la Fuente M. Neuroimmunodulation during exercise: role of catecholamines as 'stress mediator' and/or 'danger signal' for the innate immune response. Neuroimmunodulation 14: 206-212, 2007. Ottaviani E, Caselgrandi E, Franchini A, Franceschi C. CRF provokes the release of norepinephrine by hemocytes of Viviparus ater (Gastropoda, Prosobranchia): further evidence in favour of the evolutionary hypothesis of the 'mobile immune brain'. Biochem. Biophys. Res. Comm. 193: 446-452, 1993. Ottaviani E, Franceschi C. The neuroimmunology of stress from invertebrates to man. Prog. Neurobiol. 48: 421-440, 1996. Pflüger HJ, Stevenson PA. Evolutionary aspects of octopaminergic systems with emphasis on arthropods. Arthropod Struct. Dev. 34: 379-396, 2005. Råberg L, Grahn M, Hasselquist D, Svensson E. On the adaptive significance of stress-induced immunosuppression. Proc. R. Soc. Lond. B 265: 1637-1641, 1998. Roeder T. Octopamine in invertebrates. Prog. Neurobiol. 59: 1-31, 1999. Roeder T. Tyramine and octopamine: ruling behavior and metabolism. Ann. Rev. Entomol. 50: 447-477, 2005. Rojas IG, Padgett DA, Sheridan JF, Marucha PT. Stress-induced susceptibility to bacterial infection during cutaneous wound healing. Brain Behav. Immun. 16: 74-84, 2002. Sachser N. Short-term responses of plasma norepinephrine, epinephrine, glucorticoid and testosterone titres to social and non-social stressors in male guinea pigs of different social status. Physiol. Behav. 39: 11-20, 1987. Sadd BM, Siva-Jothy MT. Self-harm caused by an insect's innate immunity. Proc. R. Soc. Lond. B 273: 2571-2574, 2006. Schmitz D, Wilsenack K, Lendemanns S, Schedlowski M, Oberbeck R. ß-Adrenergic blockade during systemic inflammation: impact on cellular immune functions and survival in a murine model of sepsis. Resuscitation 72: 286- 294, 2007. Segerstrom SC. Stress, Energy and Immunity: an ecological view. Curr. Direc. Psychol. Sci. 16: 326-330, 2007. Sloley BD, Juorio AV, Durdan DA. High- performance liquid-chromatographic analysis of monoamines and some of their gamma- glutamyl conjugates produced by the brain and other tissues of Helix aspersa (Gastropoda). Cell. Mol. Neurobiol. 10: 175-192, 1990. Sternberg EM. Neural regulation of innate immunity: a coordinated nonspecific response to pathogens. Nature Rev. Immunol. 6: 318-328, 2006. van den Elzen P, Garg S, Leon L, Brigl M, Leadbetter EA, Gumperz JE, et al. Apolipoprotein-mediated pathways of lipid antigen presentation. Nature 437: 906-910, 2005. Vehovszky A, Szabo H, Elliot CJH. Octopamine increases the excitability of neurons in the snail feeding system by modulation of inward sodium current but not outward potassium current. BMC Neurosci. 6: 70, 2005. Webster JI, Tonelli L, Sternberg EM. Neuroendocrine regulation of immunity. Ann. Rev. Immunol. 20: 125-163, 2002. Weers PMM, Ryan RO. Apolipophorin III: Role model apolipoprotein. Insect Biochem. Mol. Biol. 36: 231-240, 2006. Wingfield JC. Control of behavioural strategies for capricious environments. Anim. Behav. 66: 807-816, 2003. Woodring J, McBride LA, Fields P. The role of octopamine in handling and exercise-induced hyperglycaemia and hyperlipaemia in Acheta domesticus. J. Insect Physiol. 35: 613-617, 1989. 19