VISIONS AND PERSPECTIVES ISJ 6: 1-6, 2009 ISSN 1824-307X VISIONS AND PERSPECTIVES Around the word stress: its biological and evolutive implications E Ottaviani, D Malagoli Department of Animal Biology, University of Modena and Reggio Emilia, Modena, Italy Accepted January 7, 2009 Abstract Stress is a general adaptive reaction crucial for survival and basically positive that involves the neuroendocrine and the immune systems. In all bilaterian metazoans, the molecular mediators of the stress response, i.e., corticotrophin-releasing hormone, corticotrophin, catecholamines and glucocorticoids, have been preserved during evolution, even if the increased complexity of animals have corresponded to a more articulated stress response that, following the eco-immunology perspective, we speculate to be hierarchically organized along three levels. Kew Words: stressors; stress response; vertebrates; invertebrates; evolution Eustress and distress, not simply “stress” Among the general public, the word stress evokes a concept of negativity, which is maintained even among those that have, or should have, knowledge of biology. This situation becomes even more embarrassing when considering that the scientific concept of stress has had the good fortune to become very popular, but at the same time the misfortune to be insufficiently understood. Moreover, the use of the term stress in the field of advertising has certainly not clarified its meaning. The present paper aims to provide a correct interpretation of the concept of stress, and especially to emphasize the importance of its positivity, i.e., the role played by stress response in the survival of all animal species on the Earth and maintained during evolution. The phenomenon of stress was identified and conceptualized by Hans Selye who in 1936 published a paper, entitled: "A syndrome produced by different nocuous agents ". Before describing the mechanisms of this phenomenon, we should underline some semantic details. Stress is fundamentally characterized by two moments and aspects, i.e., the “stimulus” and the “response”. The word stress can indicate both, so creating a possible semantic ambiguity. Selye (1978) suggested the word “stressors” (stressogenic ___________________________________________________________________________ Corresponding author: Enzo Ottaviani Department of Animal Biology via Campi 213/D, 41100 Modena, Italy E-mail: enzo.ottaviani@unimore.it agent) to indicate the causal agent, while keeping the word “stress” and “stress response” (response to stress) to indicate the final outcome. Moreover, according to Selye (1978), the word “stress” has meaning only if related to specific biological situations. Regarding the mechanisms of the response to stress, in mammals different organs belonging to the nervous and endocrine systems, such as the hypothalamus, the pituitary and adrenal glands, are involved (Selye, 1978). The response triggers physiological processes that operate along two routes. The first is the nervous pathway involving the autonomic nervous system and the medullar portion of adrenal glands leading to the release of catecholamines (CA) (epinephrine and norepinephrine). These molecules provoke a very rapid response, inducing physiological changes, such as the degradation of glycogen to glucose and its increase in the blood, so improving the quality of the life. This situation is further improved with activation of the second track, the endocrine pathway, in which the cortex portion of adrenal glands is involved. Schematically, the different stimuli that cause stress induce the release of the corticotrophin-releasing hormone (CRH) by the hypothalamus. In turn, the CRH provokes corticotrophin (ACTH) release from the pituitary. This hormone enters the bloodstream and binds specific receptors for ACTH present on the cells of the cortical portion of the adrenal glands and results in the release of steroid hormones such as glucorticoids (GC). These hormones (cortisol in humans and corticosterone in mice) have different 1 Bilatera Deuterostomia Protostomia Mollusca Annelida Arthropoda Echinodermata Urochordata Cephalochordata Vertebrata CRH POMC-derived peptides GC CA ? ? ? ? ? ? ? ? Fig. 1 Presence of the molecules involved in the stress response in the most representative taxa of Bilatera. effects; in particular they are involved in regulating the biosynthesis and release of CA. Altogether, it emerges that the stress pathway involves molecules in the following order: CRH, ACTH and CA. This complex mechanism that improves the quality of life by means of the release of CA, hormones and steroids is called “eustress” that means beneficial, positive stress. However, stress response must be of short duration. Indeed, prolonged exposure to stressors leads to a sustained release of CA and cortisol associated with psychological, functional and pathological symptoms (including bleeding and ulcers) described by Selye (1978). This overrun of the stress response is better defined as “distress” that means negative stress. The relationship between neuroendocrine and immune systems Stressor and stress response, by one side, and antigens and immune response, on the other, have always been considered as two distinct phenomena, having been discovered and studied separately and, consequently, having become the topics of specific disciplines. However, this division is inconsistent with reality, and the distinction between stressor and antigen or stress and immune response, is to be considered only quantitative and semantic. This dualism was first overcome in experiments undertaken by Hugo Besedovsky and colleagues (1987). They showed that interleukin (IL)-1, a classic mediator of the immune system, is able to activate the hypothalamus-pituitary-adrenal axis. This observation indicates that stressors that induce an immune response (bacteria, viruses, etc.) must also be inserted in the list of the stressogenic agents, suggesting that there is a deep correlation between the immune system and response to stress. Edwin Blalock and Eric Smith demonstrated that cells from the immune system, such as lymphocytes and macrophages may play a central role in the induction of stress (Blalock and Smith, 1985; Blalock et al., 1985; Blalock, 1989). Indeed, lymphocytes and macrophages, well-known producers of cytokines, have also to be considered as neuroendocrine cells being able to synthesize a variety of hormones (i.e., classical molecules produced by the endocrine system) and neuropeptides (i.e., classical molecules produced by the nervous system). Furthermore, lymphocytes and macrophages may, in turn, respond to hormones and neuropeptides produced by cells from the neuroendocrine system (Blalock and Smith, 1985; Blalock et al., 1985; Blalock, 1989). In summary, various levels of integration between the immune and neuroendocrine systems can be traced: • classical products from the immune system, i.e., cytokines, can act on cells from the neuroendocrine system, modifying the latter's functions; • immune stimuli and hypothalamic releasing factors induce immune cells to synthesize neuropeptides which, in turn, may influence the activity of the neuroendocrine system; • classical hormones and neurotransmitters bind to specific receptors on immune cells and modulate their activity; • cytokines and cytokine-like peptides that are potentially able to modulate immune cell activity are produced by cells from the nervous system. 2 These observations suggest that the three systems (immune, endocrine and nervous) should be considered as anatomically distinct components of a single integrated immuno-neuro-endocrine system involved in the maintenance of the body homeostasis, justifying the conclusion that the response to stress is essential for survival. Accordingly, It should be underlined that this interplay between the immune and neuroendocrine systems is not restricted to mammals or other vertebrates, but can be retrieved also in invertebrates (Ottaviani and Franceschi, 1996), where the molecular cascade of stress response described in the previous paragraph has been observed in immune-competent cells. CRH and ACTH CRH has been isolated and characterized by hypothalamic extracts of sheep by Vale’s group (1981). Later searches showed the presence of CRH also in not nervous tissue (Seasholtz et al., 2002). A similar picture has been detected in cartilaginous and bony fish as well as in tetrapods, i.e., in all vertebrates (Fig. 1) (Sato and George, 1973; Petrusz et al., 1983; Waugh et al., 1985; Panzica et al., 1986; Roubos, 1997; Lovejoy and Balment, 1999; Summers, 2001; Engelsma et al., 2002; Seasholtz et al., 2002; Malagoli et al., 2004; Huising et al., 2005). CRH-like molecules were also found in the nervous system of different invertebrate taxa, such as molluscs (Sonetti et al., 1986), annelids (Rèmy et al., 1982) and insects (Verhaert et al., 1984; Malagoli et al., 2002), as well as in the immunocytes and hemolymph of molluscs (Ottaviani et al., 1990). Unfortunately, no data are at present available for echinoderms, urochordates and cephalochordates, that represent the most important invertebrate taxa sharing the deuterostomian lineage with vertebrates (Fig. 1). ACTH is a small peptide enclosed within the pro-opiomelanocortin (POMC) precursor that was initially found in the human pituitary gland (Phifer et al., 1974; Eberle, 1988). Subsequently, ACTH was also detected in mammalian extra-pituitary areas (Ottaviani et al., 1997). As noted above for CRH, ACTH-like molecules were also found in intra- and extra-pituitary areas in other vertebrate taxa, namely fish, amphibians, reptiles and birds (Fig. 1) (Ottaviani et al., 1997; Roubos, 1997; Engelsma et al., 2002). Also different invertebrate taxa (molluscs, annelids, insects, urochordates and cephalochordates) contain immunoreactive ACTH molecules (Ottaviani et al., 1997). No data are available for echinoderms (Fig. 1). GC, CA and cytokines In 1985, David Norris identified the source of GC, in particular, of cortisol and corticosterone, in the cells of the adrenal cortex of mammals. Non- mammalian vertebrates also produce GC (Fig. 1) (Summers, 2001; Engelsma et al., 2002; Wada, 2008), but the typical adrenal glands found in mammals are not present in these animals. Fish present a group of cells homologue to adrenocortical and chromaffin mammalian tissue, and these two tissues are joined in various ways in tetrapods. The presence of GC-like molecules has also been detected in invertebrates, even if few studies are reported in literature. Cortisol immunoreactive molecules were detected in immunocytes from molluscs using an immunocytochemical method (Ottaviani et al., 1998), and cortisol and corticosterone have been recorded in the insect Calliphora vicina by autoradiography (Bidmon and Stumpf, 1991). No further data are available for other invertebrate taxa. As far as the presence of CA is concerned, these molecules were detected in all vertebrates (Leboulenger et al., 1984; Korte et al., 1997; Reid et al., 1998; Summers, 2001; Tsigos and Chrousos, 2002). In invertebrates CA were found in molluscs (Ottaviani and Franceschi, 1996; Lacoste et al., 2001; Hooper et al., 2007; Adamo, 2008), annelids (Díaz-Miranda et al., 1982; Fleming, 1993), arthropods (Murdock, 1971; Klemm, 1983; Adamo, 2008), echinoderms (Huet and Franquinet, 1981), urochordates (Kimura et al., 2003) and cephalochordates (Moret et al., 2004). Finally, as for CA, cytokines have been observed in all vertebrate lineages (Cohen and Haynes, 1991; Myers et al., 1992; Abbas et al., 1994; Scapigliati et al., 2000; Engelsma et al., 2002; Kaiser et al., 2004) and in some invertebrate taxa. With regard the latter, either cytokines or cytokine- like molecules were found in molluscs (Ottaviani et al., 2004; De Zoysa et al., in press), annelids (Ottaviani et al., 2004), arthropods (Morisato and Anderson, 1994; Agaisse et al., 2003; Kauppila et al., 2003; Söderhäll et al., 2005; Ottaviani et al., 2004; Lemaitre and Hoffmann, 2007; Malagoli et al., 2007) and urochordates (Parrinello et al., 2008; Zhang et al., 2008). A refined orchestra with the same players All the actors that play a role in the stress response must have appeared quite early in animals, since they can be retrieved in different bilaterian lineages (Fig. 1). It should be underlined that the cascade of molecular events involved in the stress response is the same in all the bilaterians analyzed so far, i.e., CRH, ACTH and CA. However, since invertebrates lack the organs usually related to vertebrate stress-response, i.e., hypothalamus, pituitary and adrenal glands, it remains to be established how invertebrate stress response can occur in such a simplified scenario. Our experiments in molluscs let us to speculate that in less complex organisms the stress response involves the circulating and phagocytic immunocyte endowed with the same molecules that are released and act in the same order described above (Ottaviani et al., 1997). However, if the primitive organization of stress response was restricted to single cells, how it came that it has been split up in different organs in vertebrates? In experiments in the catfish Ameiurus nebulosus we have observed that fish exposed to lipopolysaccharide (LPS) for 15 and 120 min showed an increase in proCRH-like molecules in the brain after 15 min but not after 120 min, while the increase in proCRH levels in the peripheral organs 3 such as the liver and head kidney persisted for the entire treatment. These findings suggest that stress response is hierarchically- and time-regulated (Malagoli et al., 2004). More precisely, the first and simpler level is the “cell” level by which circulating immunocytes and some cells in various organs, have maintained the capability to resume the stress response. The “cell” level can be taken to represent the persistence of the “ancestral” version of stress response in complex organisms. The second level is the “organ” level, representing a local stress response in which cells distributed within a whole organ are involved. In this case, other organs may not be interested by the stress response that is therefore managed by single components. Finally, the third level is the “body” levels, involving different organs connected in a functional net, coordinating the entire system, as it is for the hypothalamus- pituitary-adrenal gland axis (Ottaviani et al., 1998). This level represent the most complex machinery in stress response, but not necessary its activity is overlapped to that of the other levels (Malagoli et al., 2004). It may be surmized that during evolution of vertebrates, while circulating cells maintain their capability of promoting an immune-neuroendocrine response to stressor (“cell level”), some cells were specialized to respond to stressor within organs, thus constituting the “organ” level. The organization of a “system” or “body” level could derive form the constitution of a functional net between organs that were progressively specialized for the intertwined relations between increasingly complex nervous and endocrine systems. This concept of “hierarchy” is in agreement with the fundamental tenet of ecological immunology, i.e., to minimize the cost of biological responses (Lochmiller and Deerenberg, 2000). In this respect, the “organ” level described above represents a paradigmatic example. Fish challenged with LPS increased their expression of proCRH-like molecules in the brain after 15 min but not after 120 min, while after 120 min the increase in proCRH levels persisted in the liver and head kidney. In terms of energy expenditure, we can speculate that it is more convenient for the organism to face the stressor at first also with the “body” level, then, if the stressor does not change its intensity, the stress response is mainly transferred to the periphery and to the “organ” level, thus limiting the involvement of the central nervous system to just the first phase of the stress (Malagoli et al., 2004; Ottaviani et al., 2008). Conclusions In their response to agents that are potentially able to alter their homeostasis and threaten their survival, living organisms exploit a complex and integrated mechanism involving the immune- neuroendocrine system and molecules that have been preserved during evolution, though differently located as a consequence of the increasing complexity of the organisms. Stress is a general, adaptive reaction that is crucial for survival and basically positive. Most of the negative effects reported in the literature derive from the general perception of stress and refer to extreme conditions of excessive stress. The most common stress of moderate magnitude must be considered physiological and, as Seyle (1978) reported, represents “the spice of life”. References Abbas AK, Lichtman AH, Pober JS. Cellular and molecular immunology, 2nd ed. Saunders WB Company, Philadelphia, USA. Adamo SA. Norepinephrine and octopamine: linking stress and immune function across phyla. Inv. Surv. J. 5: 12-19, 2008. Agaisse H, Petersen UM, Boutros M, Mathey-Prevot B, Perrimon N. Signaling role of hemocytes in Drosophila JAK/STAT-dependent response to septic injury. Dev. Cell 5: 441-450, 2003. Berkenbosch F, van Oers J, del Rey A, Tilders F, Besedovsky H. Corticotropin-releasing factor- producing neurons in the rat activated by interleukin-1. Science 238: 524-526, 1987. Bidmon HJ, Stumpf WE. Uptake, distribution and binding of vertebrate and invertebrate steroid hormones and time-dependence of ponasterone A binding in Calliphora vicina. Comparisons among cholesterol, corticosterone, cortisol, dexamethasone, 5 alpha-dihydrotestosterone, 1,25-dihydroxyvitamin D3, ecdysone, estradiol- 17 beta, ponasterone A, progesterone, and testosterone. Histochemistry 96: 419-434, 1991. Blalock JE, Harbour-McMenamin D, Smith EM. Peptide hormones shared by the neuroendocrine and immunologic systems. J Immunol. 135 (2 Suppl): 858s-861s, 1985. Blalock JE, Smith EM. The immune system: our mobile brain? Immunol. Today 6: 115-117, 1985. Blalock JE. A molecular basis for bidirectional communication between the immune and neuroendocrine systems. Physiol. Rev. 69: 1- 32, 1989. Cohen N, Haynes L. The phylogenic conservation of cytokines. In: Warr GW, Cohen N (eds), Phylogenesis of immune function, CRC Press, Boca Raton, pp 241-268, 1991. De Zoysa M, Jung S, Lee J. First molluscan TNF- alpha homologue of the TNF superfamily in disk abalone: molecular characterization and expression analysis. Fish Shellfish Immunol. [in press]. Díaz-Miranda L, de Motta GE, García-Arrarás JE. Monoamines and neuropeptides as transmitters in the sedentary polychaete Sabellastarte magnifica: actions on the longitudinal muscle of the body wall. J. Exp. Zool. 263: 54-67, 1982. Eberle AN. The melanotropins. Karger, Basel, 1088. Engelsma MY, Huising MO, van Muiswinkel WB, Flik G, Kwang J, Savelkoul HF, et al. Neuroendocrine-immune interactions in fish: a role for interleukin-1. Vet. Immunol Immunopathol. 87: 467-479, 2002. Fleming MW. Catecholamines during development of the parasitic nematode, Haemonchus contortus. Comp. Biochem. Physiol. 104C: 333- 334, 1993. Hooper C, Day R, Slocombe R, Handlinger J, Benkendorff K. Stress and immune responses in abalone: limitations in current knowledge and investigative methods based on other models. Fish Shellfish Immunol. 22: 363-379, 2007. 4 Huet M, Franquinet R. Histofluorescence study and biochemical assay of catecholamines (dopamine and noradrenaline) during the course of arm-tip regeneration in the starfish, Asterina gibbosa (Echinodermata, Asteroidea). Histochemistry 72: 149-154, 1981. Huising MO, Metz JR, De Mazon AF, Verburg-van Kemenade BM, Flik G. Regulation of the stress response in early vertebrates. Ann. NY Acad. Sci. 1040: 345-347, 2005. Kaiser P, Rothwell L, Avery S, Balu S. Evolution of the interleukins. Dev. Comp. Immunol. 28: 375- 394, 2004. Kauppila S, Maaty WS, Chen P, Tomar RS, Eby MT, Chapo J, et al. Eiger and its receptor, Wengen, comprise a TNF-like system in Drosophila. Oncogene 22: 4860-4867, 2003. Kimura Y, Yoshida M, Morisawa M. Interaction between noradrenaline or adrenaline and the beta 1-adrenergic receptor in the nervous system triggers early metamorphosis of larvae in the ascidian, Ciona savignyi. Dev. Biol. 258: 129-140, 2003. Klemm N. Monoamine-containing neurons and their projections in the brain (supraoesophageal ganglion) of cockroaches. An aldehyde fluorescence study. Cell Tissue Res. 229: 379- 402, 1983. Korte SM, Beuving G, Ruesink W, Blokhuis HJ. Plasma catecholamine and corticosterone levels during manual restraint in chicks from a high and low feather pecking line of laying hens. Physiol. Behav. 62: 437-441, 1997. Lacoste A, Malham SK, Cueff A, Poulet SA. Stress- induced catecholamine changes in the hemolymph of the oyster Crassostrea gigas. Gen. Comp. Endocrinol. 122: 181-188, 2001. Leboulenger F, Charnay Y, Dubois PM, Rossier J, Coy DH, Pelletier G, et al. The coexistence of neuropeptides and catecholamines in the adrenal gland. Research on paracrine effects on adrenal cortex cells. Ann. Endocrinol. (Paris) 45: 217-227, 1984. Lemaitre B, Hoffmann J. The host defense of Drosophila melanogaster. Annu. Rev. Immunol. 25: 697-743, 2007. Lochmiller RL, Deerenberg C. Trade-offs in evolutionary immunology: just what is the cost of immunity? OIKOS 88: 87-98, 2000. Lovejoy DA, Balment RJ. Evolution and physiology of the corticotropin-releasing factor (CRF) family of neuropeptides in vertebrates. Gen. Comp. Endocrinol. 115: 1-22, 1999. Malagoli D, Conklin D, Sacchi S, Mandrioli M, Ottaviani E. A putative helical cytokine functioning in innate immune signalling in Drosophila melanogaster. Biochim. Biophys. Acta 1770: 974-978, 2007. Malagoli D, Mandrioli M, Ottaviani E. Cloning and characterisation of a procorticotrophin-releasing hormone in the IZD-MB-0503 immunocyte line from the insect Mamestra brassicae. Peptides 23: 1829-1836, 2002. Malagoli D, Mandrioli M, Ottaviani E. ProCRH in the teleost Ameiurus nebulosus: gene cloning and role in LPS-induced stress response. Brain Behav. Immun. 18: 451-457, 2004. Moret F, Guilland JC, Coudouel S, Rochette L, Vernier P. Distribution of tyrosine hydroxylase, dopamine, and serotonin in the central nervous system of amphioxus (Branchiostoma lanceolatum): implications for the evolution of catecholamine systems in vertebrates. J. Comp. Neurol. 468: 135-150, 2004. Morisato D, Anderson KV. The spätzle gene encodes a component of the extracellular signaling pathway establishing the dorsal- ventral pattern of the Drosophila embryo. Cell 76: 677-688, 1994. Murdock LL. Catecholamines in arthropods: a review. Comp. Gen. Pharmacol. 2: 254-274, 1971. Myers TJ, Lillehoj HS, Fetterer RH. Partial purification and characterization of chicken interleukin-2. Vet. Immunol. Immunopathol. 34: 97-114, 1992. Norris DO. Vertebrate endocrinology, 2nd ed. Lea and Febiger, Philadelphia, USA, 1985. Ottaviani E, Franceschi C. The neuroimmunology of stress from invertebrates to man. Prog. Neurobiol. 48: 421-440, 1996. Ottaviani E, Franchini A, Franceschi C. Presence of immunoreactive molecules to CRH and cortisol in invertebrate haemocytes and lower and higher vertebrate thymus. Histochem. J. 30: 61-67, 1998. Ottaviani E, Franchini A, Franceschi C. Pro- opiomelanocortin-derived peptides, cytokines, and nitric oxide in immune responses and stress: an evolutionary approach. Int. Rev. Cytol. 170: 79-141, 1997. Ottaviani E, Malagoli D, Capri M, Franceschi C. Ecoimmunology: is there any room for the neuroendocrine system? Bioessays 30: 868- 974, 2008. Ottaviani E, Malagoli D, Franceschi C. Common evolutionary origin of the immune and neuroendocrine systems: from morphological and functional evidence to in silico approaches. Trends Immunol. 28: 497-502, 2007. Ottaviani E, Malagoli D, Franchini A. Invertebrate humoral factors: cytokines as mediators of cell survival. Prog. Mol. Subcell. Biol. 34: 1-25, 2004. Ottaviani E, Petraglia F, Montagnani G, Cossarizza A, Monti D, Franceschi C. Presence of ACTH and beta-endorphin immunoreactive molecules in the freshwater snail Planorbarius corneus (L.) (Gastropoda, Pulmonata) and their possible role in phagocytosis. Regul. Pept. 27: 1-9, 1990. Panzica GC, Viglietti-Panzica C, Fasolo A, Vandesande F. CRF-like immunoreactive system in the quail brain. J. Hirnforsch. 27: 539- 547, 1986. Parrinello N, Vizzini A, Arizza V, Salerno G, Parrinello D, Cammarata M, et al. Enhanced expression of a cloned and sequenced Ciona intestinalis TNFalpha-like (CiTNFalpha) gene during the LPS-induced inflammatory response. Cell Tissue Res. 334: 305-317, 2008. Petrusz P, Merchenthaler I, Maderdrut JL, Vigh S, Schally AV. Corticotropin-releasing factor (CRF)-like immunoreactivity in the vertebrate endocrine pancreas. Proc. Natl. Acad. Sci. USA 80: 1721-1725, 1983. 5 Phifer RF, Orth DN, Spicer SS. Specific demonstration of the human hypophyseal adrenocortico-melanotropic (ACTH-MSH) cell. J. Clin. Endocrinol. Metab. 39: 684-692, 1974. Sonetti D, Vacirca F, Fasolo A. Localization of Sustance P (SP)-. Neuropeptide Y (NPY)- and Corticotropin-Releasing Factor (CRF)-like immunoreactive cells in the CNS of the freshwater Planorbis corneus. Neurosci. Lett. Suppl. 26: 322, 1986. Reid SG, Bernier NJ, Perry SF. The adrenergic stress response in fish: control of catecholamine storage and release. Comp. Biochem. Physiol. 120C: 1-27, 1998. Summers CH. Mechanisms for quick and variable responses. Brain. Behav. Evol. 57: 283-292, 2001. Rémy C, Tramu G, Dubois MP. Immunohistological demonstration of a CRF-like material in the central nervous system of the annelid Dendrobaena. Cell Tissue Res. 227: 569-575, 1982. Tsigos C, Chrousos GP. Hypothalamic-pituitary- adrenal axis, neuroendocrine factors and stress. J. Psychosom. Res. 53: 865-871, 2002. Roubos EW. Background adaptation by Xenopus laevis: a model for studying neuronal information processing in the pituitary pars intermedia. Comp. Biochem. Physiol. 118A: 533-550, 1997. Vale W, Spiess J, Rivier C, Rivier J. Characterization of a 41-residue ovine hypothalamic peptide that stimulates secretion of corticotropin and beta-endorphin. Science 213: 1394-1397, 1981. Sato M, George JC. Diurnal rhythm of corticotrophin-releasing factor activity in the pigeon hypothalamus. Canad. J. Physiol. Pharmacol. 51: 743-747, 1973. Verhaert P, Marivoet S, Vandesande F, De Loof A. Localization of CRF immunoreactivity in the central nervous system of three vertebrate and one insect species. Cell Tissue Res. 238: 49- 53, 1984. Scapigliati G, Bird S, Secombes CJ. Invertebrate and fish cytokines. Eur. Cytokine Netw. 11: 354-61, 2000. Wada H. Glucocorticoids: mediators of vertebrate ontogenetic transitions. Gen. Comp. Endocrinol. 156: 441-453, 2008. Seasholtz AF, Valverde RA, Denver RJ. Corticotropin-releasing hormone-binding protein: biochemistry and function from fishes to mammals. J. Endocrinol. 175: 89-97, 2002. Waugh D, Anderson G, Armour KJ, Balment RJ, Hazon N, Conlon JM. A peptide from the caudal neurosecretory system of the dogfish Scyliorhinus canicula that is structurally related to urotensin I. Gen. Comp. Endocrinol. 99: 333- 339, 1995. Selye H. A syndrome produced by diverse nocuous agents. Nature 138: 32, 1936. Selye H. The stress of life. McGraw-Hill Book Co., New York, USA, 1978. Zhang X, Luan W, Jin S, Xiang J. A novel tumor necrosis factor ligand superfamily member (CsTL) from Ciona savignyi: molecular identification and expression analysis. Dev. Comp. Immunol. 32: 1362-1373, 2008. Söderhäll I, Kim YA, Jiravanichpaisal P, Lee SY, Söderhäll K. An ancient role for a prokineticin domain in invertebrate hematopoiesis. J. Immunol. 174: 6153-6160, 2005. 6