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Brazilian Journal of Medical and Biological Research (2000) 33: 1121-1131 HPA axis in stress 1121 ISSN 0100-879X

Role of the hypothalamic pituitary adrenal axis in the control of the response to stress and infection

S.M. McCann1, 1Pennington Biomedical Research Center (LSU), Baton Rouge, LA, USA J. Antunes-Rodrigues2, Departamentos de Fisiologia da 2Faculdade de Medicina e da C.R. Franci2, 3Faculdade de Odontologia de Ribeirão Preto, Universidade de São Paulo, J.A. Anselmo-Franci3, Ribeirão Preto, SP, Brasil S. Karanth1 and 4Centro de Estudios Farmacologicos y Botanicos, Consejo Nacional de Investigaciones V. Rettori4 Cientificas y Tecnicas (CEFYBO-CONICET), Buenos Aires, Argentina

Abstract

Correspondence The release of adrenocorticotropin (ACTH) from the corticotrophs is Key words S.M. McCann controlled principally by and corticotropin-releasing hor- · Corticotropin-releasing Pennington Biomedical Research mone (CRH). may augment the release of ACTH under hormone (CRH) Center (LSU) · certain conditions, whereas atrial natriuretic peptide acts as a cortico- Vasopressin Baton Rouge, LA 70808-4124 · Oxytocin tropin release-inhibiting factor to inhibit ACTH release by direct USA · Atrial natriuretic peptide Fax: +1-225-763-3030 action on the pituitary. Glucocorticoids act on their receptors within (ANP) E-mail: [email protected] the and anterior to suppress the release · ACTH of vasopressin and CRH and the release of ACTH in response to these · Cortisol Presented at the First neuropeptides. CRH neurons in the paraventricular nucleus also project · Norepinephrine International Meeting on Adrenal to the cerebral cortex and subcortical regions and to the locus ceruleus · Acetyl choline Disease: Basic and Clinical (LC) in the brain stem. Cortical influences via the limbic system and · Nitric oxide Aspects, Ribeirão Preto, SP, Brazil, August 31-September 2, 1999. possibly the LC augment CRH release during emotional stress, whereas peripheral input by pain and other sensory impulses to the LC causes Research supported by the National stimulation of the noradrenergic neurons located there that project a Institutes of Health (Nos. DK43900 their to the CRH neurons stimulating them by -adrenergic and MH51853) to S.M. McCann. receptors. A muscarinic cholinergic receptor is interposed between the Publication supported by FAPESP. a-receptors and nitric oxidergic interneurons which release nitric oxide that activates CRH release by activation of cyclic guanosine monophosphate, cyclooxygenase, lipoxygenase and epoxygenase. Va- Received December 20, 1999 sopressin release during stress may be similarly mediated. Vaso- Accepted March 10, 2000 pressin augments the release of CRH from the hypothalamus and also augments the action of CRH on the pituitary. CRH exerts a positive ultrashort loop feedback to stimulate its own release during stress, possibly by stimulating the LC noradrenergic neurons whose axons project to the paraventricular nucleus to augment the release of CRH.

Introduction itary hormones. In the mid-1940’s it was recognized that neural stimuli could evoke Dramatic progress has been made in pep- release of hormones from the anterior pitui- tide research over the last 45 years, and we tary. Examples included coitus-induced ovu- now know the specific hypothalamic pep- lation in birds, ferrets, cats, and other mam- tides that control the release of various pitu- mals, suckling-induced prolactin (PRL) re-

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lease, and stress-induced release of adrenal mediately transplanted under the ME, so that corticotropic hormone (ACTH). One of the it was vascularized by the portal vessels, most important discoveries in the field was pituitary function returned to normal. A cor- that of the hypophyseal portal system of ollary of this experiment was carried out by veins by Popa and Fielding in 1933. These Nikovitch-Winer and Everett who showed originate from capillaries in the median emi- that pituitary grafts under the kidney capsule nence (ME), drain blood down the hypophy- did not sustain normal pituitary function but seal stalk, and supply the sinusoids of the later regrafting under the ME returned func- anterior lobe of the pituitary gland. Although tion to normal (for reviews, see Refs. 1 and blood flow was initially thought to be up- 2). ward in these vessels, Houssay and his col- leagues observed downward flow in the liv- Corticotropin-releasing factors ing toad in 1935. This observation was con- firmed in the living rat by Green and Harris The next logical step was to make ex- in 1947. It is apparent then that the primary tracts of the ME and to evaluate their effects flow is downward from the ME to the pitui- on pituitary hormone secretion. Because of tary; however, there is evidence that upward the early development of sensitive assays for flow may indeed occur under certain condi- determining ACTH release, attention first tions. focused on the ability of such extracts to In the meantime, it became apparent that evoke ACTH release. Because of the ready there was little if any functional innervation availability of partially purified extracts of to the anterior lobe, prompting a number of the , which were used in workers to suggest that hypothalamic con- the preparation of commercial vasopressin trol over the pituitary might be mediated via (Pitressin) and oxytocin (Pitocin), these ex- neurohumoral agents released into the capil- tracts were tested in early experiments. To laries of the hypophyseal portal system that make extracts of the stalk ME required labo- would pass down the pituitary stalk and stimu- rious collection of the tissue at the slaughter- late the release of particular pituitary hor- house. The initial choice of posterior pitui- mones from the cells in the gland specialized tary extracts for testing was unfortunate but to produce each hormone. Indeed, deficits in led to the discovery of the ACTH-releasing pituitary hormone secretion followed lesions activity of vasopressin (3). Just after this, placed in various hypothalamic loci, and vasopressin was synthesized by Du Vigneaud conversely, electrical stimulation of these and colleagues (see Ref. 4), and the synthetic same regions was shown to evoke release of material was shown to be active in vivo to gonadotropins and ACTH. That the portal release ACTH in animals with ME lesions system was a crucial link between brain and that blocked the ubiquitous release of ACTH pituitary was suggested by the experiments from stress. It was reported that a substance of Harris and his collaborators who showed different from vasopressin could be isolated that following stalk section the portal vessels from posterior pituitary extracts that also frequently regenerated, and a return of nor- released ACTH; this substance was named mal pituitary function accompanied this re- corticotropin-releasing factor (CRF) (5,6). generation. On the other hand, a plate placed Early claims for a CRF other than vaso- between the cut ends of the stalk to block pressin in posterior pituitary extracts have such regeneration caused a permanent im- not been confirmed (7,8). Guillemin and pairment in pituitary function. In other ex- Schally even claimed to have partial struc- periments it was shown that if the pituitary tures of ß CRF and a I and II CRF. Again, was removed from its capsule and then im- these claims have not been confirmed (8). At

Braz J Med Biol Res 33(10) 2000 HPA axis in stress 1123 this point in time, it is clear that there is such vasopressin antagonists (17). There are va- a factor in posterior lobe extracts; however, sopressin type II receptors similar to those in it is a minor contaminant and the major the vascular system on ACTH-releasing activity of such extracts is corticotrophs that act to increase intracellu- accounted for by vasopressin (8,9). lar calcium in these cells (12), inducing a It was only when attention was turned to release of ACTH. On the other hand, CRH extracts of the stalk ME that it became clear acts on CRH type I receptors on the cortico- that indeed there was a separate corticotro- troph to activate adenylyl cyclase, leading to pin-releasing hormone (CRH) different from an increase in cyclic adenosine monophos- vasopressin (9). This material was purified phate (cAMP) that induces ACTH release and separated from vasopressin (10), but its (12). isolation and determination of structure An early candidate for CRF was epineph- proved difficult. In fact, although CRH was rine; however, its direct CRF activity ap- the first releasing factor to be discovered, it peared to be ruled out by the ability of ME was one of the last whose structure was lesions to block the response to epinephrine. elucidated, probably because it was a much This early concept is now reemerging in larger polypeptide. This feat was finally ac- view of the ability of epinephrine to evoke complished by Vale and Rivier and their ACTH release both in vivo and in vitro by an colleagues in 1981 (11). CRH turned out to action on ß-receptors which have been dis- be a 41-amino acid peptide, completely dis- covered in the gland. Epinephrine is released tinct from the octapeptide vasopressin, and during stress and could activate ACTH se- with no disulfide group in the molecule. cretion either by the increased concentra- It is now clear that vasopressin and CRH tions in the circulation or by its release into share physiological roles as CRFs. The vaso- portal blood, which would provide much pressin neuronal system has cell bodies pri- higher concentrations of the catecholamine. marily in the (SON) but In fact, high concentrations of epinephrine also in the paraventricular nucleus (PVN) have recently been reported in portal blood. and axons that project primarily to the neural From all of this, it is apparent that there is no lobe; however, some of these axons, particu- single CRF. Instead there is a constellation larly those from the PVN, terminate in the of agents that can release ACTH and may external layer of the ME in juxtaposition to play physiological roles in evoking the stress- portal vessels. High concentrations of vaso- induced release of the peptide (12). pressin and CRH are found in the portal The perikarya of CRH and vasopressin vessels, and both peptides release ACTH in neurons are located predominately in parvo- concentrations similar to those found in por- cellular neurons of the SON and PVN. The tal blood (12). There is a defect, albeit small, preponderance of vasopressin neurons is in in ACTH secretion in animals with heredi- the SON and of CRH neurons in the PVN. tary diabetes insipidus that lack vasopressin The axons project to the ME where the pre- secretion (13). Vasopressin can potentiate ponderance of vasopressin axons continues the response to CRH at the pituitary level as into the neural lobe to release the peptide revealed by both in vivo (14) and in vitro there. The preponderance of CRF neurons experiments (15). The peptide is also active terminates in the external layer of the ME in the hypothalamus to potentiate the release and only a few project to the neural lobe. of CRH (16). Antisera directed against vaso- Early studies indicated that lesions in the pressin injected into the pro- ME that induce severe diabetes insipidus duce a partial block in ACTH secretion, and block ACTH secretion, presumably because a partial block has also been achieved with they block these axons of vasopressin and

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CRH neurons reaching the ME from both the Similarly, angiotensin II neurons within PVN and SON (3). Lesions of the PVN did the hypothalamus that project to the ME may not block the stress response when studied 2 play a small augmenting role since the pep- to 3 weeks later, but were associated with tide is present in portal blood and has an adrenal atrophy (3); however, later studies intrinsic capacity to stimulate the cortico- showed that lesions of the PVN were effec- trophs (20). There are also receptors for this tive to block ACTH secretion when tested peptide on pituitary cells. Angiotensin II is only 5 days after lesions (18). These experi- an important hormone that is released in ments support the concept that both PVN states of reduction of the circulating blood and SON are involved in ACTH secretion by volume and that is associated with ACTH release of CRH and vasopressin, the former release (20). Atrial natriuretic peptide (ANP) into the hypophyseal portal vessels in the has opposite effects; namely it is released in ME and the latter also into the short portal states of volume expansion and causes di- vessels in the neural lobe. These supply 1/3 uresis and natriuresis (20,21). ANP also acts of the blood to the anterior lobe and prob- centrally to inhibit vasopressin secretion and ably play an important role in control of has now been clearly shown to have an anterior lobe function through release of inhibitory action to suppress CRH release hypothalamic factors into these vessels, in intrahypothalamically and inhibit the pitui- particular, vasopressin in the case of ACTH tary’s response to CRH, leading some to call secretion, a fact that has been largely ig- it a corticotropin release-inhibiting factor nored. (20,21). Some of the CRH neurons in the PVN Many of these actions of angiotensin II projecting to the ME also contain vasopressin, and ANP are physiologically significant as and the relative abundance of vasopressin is revealed by injection of antisera against these increased in the adrenalectomized animal. peptides into the third cerebral ventricle Therefore, vasopressin appears particularly (19,22). ANP is also synthesized in the ante- to be important in the increase in ACTH rior pituitary gland and there are receptors in secretion that occurs following adrenalec- the gland to mediate its action (22). tomy. Indeed, vasopressin appears to be more The principal ANP receptor is guanylyl important than CRH in control of ACTH cyclase. It causes conversion of guanosine secretion in sheep. The relative importance triphosphate into cyclic guanosine mono- of vasopressin and CRH appears to vary phosphate (cGMP) that induces a decrease depending on the stress imposed and the in intracellular free calcium (Ca2+) in the details of this relationship have yet to be corticotroph, thereby inhibiting ACTH se- fully determined. As pointed out above, the cretion (Antunes-Rodrigues J, Yu WH and two synergize to stimulate ACTH release McCann SM, unpublished results). Interest- (see Ref. 17 for a review). ingly, neural nitric oxide (NO) synthase Oxytocin, another peptide released from (nNOS) is also localized in pituitary cells. the magnocellular neurons in the PVN and When activated by transmitters it produces SON, is also released from axonal terminals an increase in intracellular free Ca2+ in the in juxtaposition to the portal vessels. It is pituitary cells containing the enzyme that present in high concentrations in portal blood generates NO by conversion of arginine in and also has the capacity to potentiate the the presence of oxygen and various cofac- response to CRH at the pituitary level. Since tors. This NO diffuses to the corticotroph oxytocin is released in stress as well as vaso- and activates guanylyl cyclase, leading to pressin and CRF, it may play a small role in increased cGMP formation that decreases the stress-induced release of ACTH (19). intracellular free Ca2+, thereby inhibiting

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ACTH secretion. Thus, agents that generate as occurs for example in emotional arousal, NO in the pituitary, such as cytokines would can activate CRH release followed by ACTH be expected to suppress rather than stimulate and cortical hormone secretion (27,28). The ACTH secretion. Indeed, experiments have mechanism of this pathway is not known; indicated that NO will suppress ACTH se- however, this may be by activation of locus cretion by direct action on the gland and it ceruleus (LC) neurons and other brain stem has been shown that increased NO will in- noradrenergic (NAergic) neurons that are hibit the stimulatory effect of vasopressin on known to project NAergic axons to the PVN. ACTH secretion (23). Indeed, lesions of the LC will temporarily completely block the release of ACTH in- Control of vasopressin CRH and duced by ether anesthesia in the rat (29). ACTH secretion by negative Indeed, the LC might be characterized as feedback of cortical steroids a head ganglion of the sympathoadrenal sys- tem, so that it is activated by stressful stimuli It was shown by Ingle in 1936 that there in the periphery and by afferent input by pain was a negative feedback action of adrenal and other sensory modalities (29). It also can cortical steroids to suppress adrenal cortical be stimulated by activation from the cerebral hypertrophy following unilateral adrenalec- cortex via the limbic system (30). The result tomy. This action is mediated by a direct is not only activation of CRH release by suppression of the response of the cortico- projections to the CRH neurons in the PVN, troph to vasopressin (24) and CRH (25). The but also activation of the peripheral sympa- mechanism of this has yet to be fully eluci- thoadrenomedullary system by producing a dated. Furthermore, there is also an action of massive discharge of sympathetic terminals glucocorticoids on the hypothalamus to sup- into the vascular blood system and also on press the release not only of vasopressin the producing what Cannon but also of CRH. Following adrenalec- called the flight or fight response. With the tomy the release of vasopressin and CRH realization that the hypothalamic pituitary is increased and vasopressin appears to be adrenal (HPA) axis also participated in this colocalized with CRH in more neurons response, interest in the activation of the than in the intact animal. The actions at peripheral sympathoadrenomedullary sys- both the pituitary and hypothalamic levels tem has waned; however, evidence that are mediated by glucocorticoid receptors has accrued in recent years indicates that which have been localized in the pituitary there is a direct sympathetic innervation of gland and also in the region of the CRH the and that adrenomedullary neurons in the PVN (26). rests are present in the adrenal cortex. Fur- thermore, catecholamines can stimulate re- Distribution and control of CRH lease of adrenocortical steroids from the neurons by intra- and extra- ACTH-primed adrenal cortex. Indeed, in hypothalamic neuronal systems chronic stress the continued activation of the sympathoadrenomedullary system sensitizes Not only do vasopressinergic and the cortex to the action of CRH and can even CRHergic neurons project to the ME and activate the cortex directly. Therefore, con- neural lobe of the pituitary gland, but also tinued activation of the sympathoadreno- they project to cortical and subcortical struc- medullary system can contribute to the in- tures and caudally to the brain stem (26). creased adrenal cortical steroid production Therefore, it is not surprising that input from in chronic stress, even though the elevated the cerebral cortex via limbic system areas, plasma cortical hormone levels will inhibit

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the response of the pituitary to CRH and also inhibit GH and LH release and this probably diminish the release of CRH into the hypo- has a protective value by turning off repro- physeal portal vessels (31). ductive function and growth to mobilize all of the resources of the body to combat the Ultrashort loop positive feedback of imposed stress. CRH to stimulate its own secretion Role of the hypothalamic pituitary Microinjection of tiny amounts of CRH adrenal system in stress into the third ventricle that had no effect on plasma ACTH resulted in a rapid augmenta- Stress may be defined as any stimulus tion of the ACTH release in response to ether that distorts homeostasis in the organism. stress (32). Therefore, it appears that an Therefore, stress can be physical as in the ultrashort loop positive feedback of CRH case of excessive heat or cold, trauma, surgi- exists to augment its own secretion that op- cal operations or infection or it can be emo- erates only during stress. The site of action tional and psychological. Selye, who coined of CRH to augment its own release has not the term, even talked about the stress of life been determined. However, this may be me- itself. It is the stresses of life that contribute diated by the stimulatory effect of the LC on to degenerative disease and old age (38). CRH release since the intraventricularly in- Stress can affect every organ system in jected CRH would reach the LC and might the body, but the principal mediators of the stimulate it to send NAergic input into the response to stress are the sympathetic ner- CRH neurons, thereby stimulating additional vous system and the HPA axis. Various hy- CRH release. Further experiments need to pothalamic releasing and inhibiting hormones be performed to determine if this is the mech- are released from their terminals in the anism by which CRH augments its own re- ME into the capillaries of the hypophyseal lease. The positive feedback of CRH to aug- portal veins that transport them to the ante- ment its own release appears to be physi- rior lobe of the pituitary gland. There they ologically significant since antiserum against stimulate or inhibit the release of the various CRH injected into the third ventricle aug- pituitary hormones that act either directly on mented ACTH release during stress (33). body tissues or on their target glands, which in turn release hormones that act on various Intrahypothalamic action of CRH to tissues of the body. Examples of the latter inhibit growth hormone (GH) and are ACTH, follicle-stimulating hormone luteinizing hormone (LH) release (FSH), LH and -stimulating hormone (TSH), and of the former, are PRL and GH. Not only does CRH augment its own Physiological stress is presumably medi- release by an ultrashort loop positive feed- ated by anxiety recognized in the cerebral back, but it also can inhibit GH and LH cortex that stimulates limbic system struc- release following its intraventricular injec- tures and probably the LC and other brain tion (33-36). It is known that in the rat stress stem NAergic neurons that activate CRH inhibits GH and LH release (33,37) and this release by NAergic axons which terminate intrahypothalamic action of CRH may play in the vicinity of CRH neurons in the PVN. an important physiological role in inhibiting Vasopressin release is also stimulated; how- the release of these hormones, thereby in- ever, this pathway has not been elucidated. ducing the profile of release of these hor- In the case of physical stress, such as pain, mones which occurs in stress. In essence in cold, heat or trauma, there is afferent input the rat all stresses, if severe enough, will by sensory neurons to the brain stem again

Braz J Med Biol Res 33(10) 2000 HPA axis in stress 1127 activating the LC that then triggers CRH and tially toxic quantities of NO. Following iv probably vasopressin secretion. injection of an intermediate dose of LPS, there was an induction of IL (interleukin)-1a The role of NO in the stress response immunoreactive neurons in the preoptic-hy- pothalamic region (37). These cells were In the dozen years since the discovery of shown to be neurons by the fact that double NO in the body and the enzymes forming it, staining revealed the presence of neuron it has become evident that NO plays a pivotal specific enolase. The neurons were found in role in the function of every organ system of saline-injected control animals suggesting the body. In this brief review, we will first that they are normally present, but they in- examine the role of NO under physiological creased in number by a factor of 2 within 2 h conditions and then indicate how this role is following injection of LPS. They are located affected by stress. In this connection, most in a region which also contains the thermo- attention has focused on the role of NO in the sensitive neurons. They may be the neurons induction of the responses to infection. Little that are stimulated to induce fever following attention has been given so far to its role in injection of LPS. They have short axons that psychological and other types of stress. did not clearly project to the areas containing Nearly all types of stress elicit a stereo- the various hypothalamic releasing and in- typed pattern of pituitary hormone secretion hibiting hormones, but they could also be in humans which consists of a rapid increase involved in the stimulation or inhibition of in ACTH, PRL and GH secretion (in the rat their release, which occurs following infec- GH secretion is inhibited) and inhibition of tion. the secretion of LH, TSH and to a lesser This study led to further research which extent FSH. The pattern is caused primarily demonstrated that ip injection of a moderate by alterations in the secretion of hypotha- dose of LPS induced IL-1ß and iNOS mRNA lamic releasing and inhibiting hormones. in the brain, anterior pituitary and pineal glands. The results were very exciting since The role of the hypothalamic an induction of IL-1ß and iNOS mRNA oc- pituitary adrenal system in infection curred with the same time-course as found in the periphery following injection of LPS, The hypothalamic-pituitary response to namely, clear induction of IL-1ß followed infection can be mimicked by the injection by iNOS mRNA within 2 h reaching a peak of bacterial lipopolysaccharide (LPS) iv or in 4-6 h, followed by a decline to near basal ip. This induces an identical pattern of pitu- levels at the next measurement by 24 h after itary hormone secretion as that seen in infec- the injection. The induction of both mRNAs tion. There is a very rapid increase in plasma occurred in the meninges, the choroid plexus, ACTH and PRL within a few minutes fol- the circumventricular organs, such as the lowing iv injection of LPS. The response is subfornical organ and ME, in the ependymal dose-related and is accompanied by a rapid cells lining the , and very inhibition of LH and TSH but not FSH secre- suprisingly in parvocellular neurons of the tion. GH secretion is stimulated in humans PVN and (AN), areas of but suppressed in the rat (37). particular interest since they contain the hy- Recent work indicates that central ner- pothalamic releasing and inhibiting hormone vous system infection is a powerful inducer producing neurons and also other neuro- of cytokine production in glia and neurons of transmitters controlled by NO (39). the brain, which causes induction of induc- The greatest induction occurred in the ible NOS (iNOS) and production of poten- anterior lobe of the pituitary, where the iNOS

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mRNA was increased at 2 h by a factor of 45 Unfortunately, the effects of inhibitors of and in the pineal where the activity was NOS on these later stages in the response to increased by a factor of 7 at 6 h, whereas the LPS or infection have not yet been studied. increase in the PVN was 5-fold. At 6 h, the Interestingly enough, in studies on LHRH medial basal hypothalamus was found to release induced by NO, it has been shown have an increased content of NOS measured that increasing concentrations of NO provid- in vitro and the collected ed by release from sodium nitroprusside pro- (CSF) had increased concentrations of the duce a bell-shaped dose-response curve in NO metabolite, nitrate. These results indi- terms of LHRH release with values reaching cate that the increase in iNOS mRNA was a peak and then declining as the concentra- followed by de novo synthesis of iNOS that tion of NO increases (41). Therefore, the liberated NO into the tissue and also into the massive increase in NO produced by iNOS CSF. Presumably, LPS was bound to its several hours after injection of LPS might receptors in the circumventricular organs actually reduce the effects of NO on releas- and in the choroid plexus. These receptors, ing hormone discharge below the peaks as in macrophages, activated DNA-directed achieved earlier. IL-1ß mRNA synthesis which, in turn, caused In addition to inducing production of the synthesis of IL-1ß. IL-1ß then activated proinflammatory cytokines such as IL-1, IL- iNOS mRNA and synthesis (39). 2, IL-6, and TNFa, LPS also induces pro- How can neurons in the AN and PVN be duction of anti-inflammatory cytokines, such activated since they are inside the blood as IL-10 and IL-13 and IL-1 receptor antago- brain barrier? In the case of the AN, the nist in the brain, pituitary and neurons may have axons that project to the (42). In the periphery these inhibit the in- ME. These neurons may have LPS receptors flammatory response induced by the proin- on their cell surface which then induce IL-ß flammatory cytokines. Limited studies indi- mRNA and IL-1ß synthesis. IL-1ß then in- cate that these anti-inflammatory cytokines duces iNOS mRNA followed by NO synthe- antagonize the actions of the proinflamma- sis. Alternatively, LPS acting on its recep- tory cytokines in the brain as well as the tors may simultaneously induce IL-ß mRNA hypothalamic-pituitary response to infection and iNOS mRNA. (42). Active transport mechanisms for IL-1 and The initial response to LPS is mediated other cytokines (40), and perhaps LPS, are by the constitutive nNOS present in the brain. present in the choroid plexus. The cells of There is no participation of the NO synthe- the choroid plexus, on the basis of our re- sized by iNOS in this initial response. In- sults, must have LPS receptors on them. LPS deed, the initial response must be due to then stimulates IL-1ß and iNOS mRNA fol- action on receptors for LPS on the endings of lowed by synthesis of IL-1ß and iNOS in the vagal afferents and also in areas where the choroid plexus. LPS and IL-1ß are then trans- blood brain barrier is not present, such as the ported into the CSF. LPS is carried by CSF choroid plexus, ME, organum vasculosum flow to the third ventricle, where it either , , and other crosses the ependyma or acts on terminals of circumventricular organs. Input to the hypo- PVN neurons in the ependyma to induce IL- from LPS by vagal afferents occurs 1ß and iNOS mRNA. at least in part by activation of the LC that This massive delayed increased NO pro- sends NAergic axons to the hypothalamus to duction should further increase the effects of activate CRH release. LPS alters the release NO to maintain the pattern of hypothalamic of the various hypophysiotropic hormones hormone secretion already induced by LPS. increasing release of CRH and vasopressin,

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GHRH and somatostatin, particularly in the sone (46) and also by blockers of the three rat, which overpowers the effect on the re- pathways of AA metabolism, such as clotri- lease of GHRH (41). LPS decreases release mazole that blocks epoxygenase, which in of the PRL-inhibiting hormone, dopamine, turn converts AA into epoxides; by in- LHRH, TRH, but not FSHRF (37). All of domethacin that inhibits COX and by 5’8’11- these pathways involve participation of nNOS eicisotrionoic acid that blocks LOX. Thus, as described above (43). CRH release is activated by the AA cascade CRH release from hypothalami incubated (44). Alpha melanotropic-stimulating hor- in vitro is controlled by muscarinic cholin- mone also inhibits CRH release (46). ergic receptors since it can be blocked by Cyclosporin inhibits CRH release as well atropine (44,45). The acetylcholine-produc- (47), probably by inhibiting calcineurin. ing interneurons in the hypothalamus re- Calcineurin dephosphorylates NOS render- lease acetylcholine that stimulates a musca- ing it inactive. rinic-type receptor which in turn stimulates Of the many proinflammatory cytokines, CRH release from the CRH neurons. nNOS it has been shown that IL-1a or ß, TNFa, IL- has been located in neurons in the PVN of 6 and IL-2 can stimulate ACTH release from the hypothalamus. Stimulated CRH release the anterior pituitary in vitro and in vivo can be blocked by NG-monomethyl-L-argi- (43). The principal action probably occurs, nine, a competitive inhibitor of all forms of at least acutely, on the release of CRH and NOS. Consequently, CRH release from the vasopressin from the hypothalamus but also neurons in the PVN is stimulated by cholin- there are clear effects at the pituitary level. ergic neurons that synapse on these NOergic There have been few studies on the mechan- neurons to activate NOS. NOS synthesizes ism of this direct pituitary action of cyto- NO that diffuses into the CRH neurons and kines, however, several cytokines such as activates CRH release by activating cyclo- IL-6 have been found to be produced in oxygenase I (COX I), leading to the genera- pituitaries and there is also nNOS present in tion of prostaglandin E2 from arachidonate the gland as indicated earlier. There are indi- (AA). Prostaglandin E2 activates CRH via cations that NO participates in inhibiting the activation of adenylyl cyclase and genera- response of ACTH to vasopressin (23). tion of cAMP. cAMP activates protein ki- Whether it plays a role in the stimulatory nase A that induces exocytosis of CRH se- action of the various proinflammatory cyto- cretory granules into the hypophyseal portal kines on ACTH secretion has not yet been vessels that then activates ACTH release studied. from the corticotrophs of the anterior pitui- In our studies LPS itself had no acute tary gland. NO activates not only COX but effect on ACTH release from hemianterior also lipoxygenase (LOX) that also plays a pituitaries in vitro (37). However, LPS in- role in the activation of CRH release (46). duces cytokine production in the pituitary. NO also activates guanylyl cyclase that con- Cytokine production would be increased verts guanosine triphosphate into cGMP. in a few hours and undoubtedly would modify cGMP is postulated to increase intracellular the responses of the pituitary to the contin- 2+ Ca required to activate phospholipase A2 ued altered secretion of releasing and inhib- that converts membrane phospholipids into iting hormones. AA, the substrate for COX and LOX, per- In addition to the proinflammatory cyto- mitting generation of prostaglandins and kines which we have discussed extensively, leucotrienes, respectively (44,45). it is now clear that there are a number of anti- Activation of CRH release can be blocked inflammatory cytokines, the first one to be by the synthetic glucocorticoid dexametha- discovered being the IL-1 receptor antago-

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nist, but IL-10 and IL-13 also play this role hypothalamic and pituitary levels to dimin- as indicated above. These are also induced in ish the response to the proinflammatory cy- the brain by LPS and may play roles at tokines.

References

1. McCann SM (1988). Saga of the discovery Proceedings of the Society for Experimen- Peptides, 18: 971-976. of the hypothalamic releasing and inhibit- tal Biology and Medicine, 121: 8-12. 21. Gutkowska J, Antunes-Rodrigues J & ing hormones. In: McCann SM (Editor), 11. Rivier J, Spiess J & Vale W (1983). Char- McCann SM (1997). Atrial natriuretic pep- People and Ideas in . Ameri- acterization of rat hypothalamic cortico- tide in brain and pituitary gland. Physi- can Physiological Society, Bethesda, MD, tropin-releasing factor. Proceedings of the ological Reviews, 77: 465-515. 41-62. National Academy of Sciences, USA, 80: 22. Franci CR, Anselmo-Franci JA & McCann 2. McCann SM (1999). Hypothalamic releas- 4851-4855. SM (1989). Opposite effects of central ing and inhibiting hormones. In: Adelman 12. McCann SM & Ojeda SR (1996). The an- immunoneutralization of angiotensin II or G & Smith BH (Editors), Encyclopedia of terior pituitary and hypothalamus. In: Grif- atrial natriuretic peptide on luteinizing hor- Neuroscience. Elsevier, New York, 927- fin JE & Ojeda SR (Editors), Textbook of mone release in ovariectomized rats. Neu- 930. Endocrine Physiology. 3rd edn. Oxford roendocrinology, 51: 683-687. 3. McCann SM & Brobeck JR (1954). Evi- University Press, New York, NY, 101-133. 23. Turnbull A & Rivier C (1996). Corticotro- dence for a role of the supraopticohypo- 13. McCann SM, Antunes-Rodrigues J, Nallar pin-releasing factor, vasopressin and pros- physeal system in regulation of adreno- R & Valtin H (1966). Pituitary adrenal func- taglandins mediate, and nitric oxide re- corticotrophin secretion. Proceedings of tion in the absence of vasopressin. Endo- strains, the hypothalamic-pituitary-adrenal the Society for Experimental Biology and crinology, 79: 1058-1064. response to acute local inflammation in Medicine, 87: 318-324. 14. Yates FE, Russell SM, Dallman MF, the rat. Endocrinology, 137: 455-463. 4. McCann SM & Fruit A (1957). Effect of Hedge GA, McCann SM & Dhariwal APS 24. McCann SM, Fruit A & Fulford BD (1958). synthetic vasopressin on release of (1971). Potentiation by vasopressin of cor- Studies on the loci of action of cortical adrenocorticotrophin in rats with hypotha- ticotropin release induced by corticotro- hormones in inhibiting the release of lamic lesions. Proceedings of the Society pin-releasing factor. Endocrinology, 88: 3- adrenocorticotrophin. Endocrinology, 63: for Experimental Biology and Medicine, 15. 29-42. 96: 556-567. 15. Gillies GE, Linton EA & Lowry PJ (1982). 25. Chowers I & McCann SM (1963). The ef- 5. Saffran M & Schally AV (1955). Release of Corticotropin releasing activity of the new fects on ACTH and gonadotrophin secre- corticotrophin by anterior pituitary tissue CRF is potentiated several times by vaso- tion of implants of gonadal steroids in the in vitro. Canadian Journal of Biochemistry pressin. Nature, 299: 355-357. hypothalamo-hypophysial region. Israel and Physiology, 33: 408-415. 16. Hedge GE, Yates MB, Marcus R & Yates Medical Journal, 22: 420-432. 6. Guillemin R & Hearn WR (1955). ACTH FE (1966). Site of action of vasopressin in 26. Reichlin S (1992). Neuroendocrinology. In: release by in vitro pituitary: effect of causing corticotropin release. Endocrinol- Foster DE & Wilson JD (Editors), Text- Pitressin and purified arginine vaso- ogy, 79: 328-340. book of Endocrinology. WB Saunders, pressin. Proceedings of the Society for 17. Ono N, Bedran de Castro JC, Khorram O Philadelphia, 135-220. Experimental Biology and Medicine, 89: & McCann SM (1985). Role of arginine 27. Goldstein LE, Rasmusson AM, Bunney 365-367. vasopressin in control of ACTH and LH BS & Roth RH (1996). Role of the amyg- 7. McCann SM (1957). The ACTH-releasing release during stress. Life Sciences, 36: dala in the coordination of behavioral, neu- activity of extracts of the posterior lobe of 1779-1786. roendocrine, and prefrontal cortical mono- the pituitary in vivo. Endocrinology, 60: 18. Tilders FJH, Schipper J, Lowry PJ & Ver- amine responses to psychological stress 664-676. mes I (1982). Effect of hypothalamus le- in the rat. Journal of Neuroscience, 16: 8. McCann SM & Porter JC (1969). Hypotha- sions on the presence of CRF-immunore- 4787-4798. lamic pituitary stimulating and inhibiting active nerve terminals in the median emi- 28. Arnsten AF, Mathew R, Ubriani R, Taylor hormones. Physiological Reviews, 49: nence and on the pituitary-adrenal re- JR & Li BM (1999). Alpha-1 noradrenergic 240-283. sponse to stress. Regulatory Peptides, 5: receptor stimulation impairs prefrontal 9. McCann SM & Haberland P (1959). Rela- 77-84. cortical cognitive function. Biological Psy- tive abundance of vasopressin and corti- 19. Gillies G & Lowry PJ (1979). Corticotro- chiatry, 45: 26-31. cotrophin-releasing factor in neurohypo- phin releasing factor may be modulated 29. Anselmo-Franci JA, Rocha-Barros VM, physeal extracts. Proceedings of the Soci- by vasopressin. Nature, 278: 463-464. Franci CR & McCann SM (1999). Locus ety for Experimental Biology and Medi- 20. Franci CR, Anselmo-Franci JA & McCann ceruleus lesions block pulsatile LH release cine, 102: 319-325. SM (1997). Angiotensinergic neurons in ovariectomized rats. Brain Research, 10. Dhariwal APS, Antunes-Rodrigues J, physiologically inhibit prolactin, growth 833: 86-92. Reeser F, Chowers I & McCann SM hormone, and thyroid-stimulating hor- 30. Foote SL (1999). . In: (1966). Purification of hypothalamic corti- mone, but not adrenocorticotropic hor- Adelman G & Smith BH (Editors), Ency- cotrophin-releasing factor of ovine origin. mone, release in ovariectomized rats. clopedia of Neuroscience. Elsevier, New

Braz J Med Biol Res 33(10) 2000 HPA axis in stress 1131

York, 1062-1063. Endocrinology, 114: 2409-2411. ological implications. Proceedings of the 31. Bornstein SR & Chrousos GP (1999). Clini- 37. Rettori V, Dees WL, Hiney JK, Lyson K & National Academy of Sciences, USA, 94: cal review: Adrenocorticotropin (ACTH)- McCann SM (1994). An interleukin-1-al- 227-232. and non-ACTH-mediated regulation of the pha-like neuronal system in the preoptic- 43. McCann SM, Karanth S, Kamat A, Dees adrenal cortex: neural and immune inputs. hypothalamic region and its induction by WL, Lyson K, Gimeno M & Rettori V Journal of Clinical Endocrinology and Me- bacterial lipopolysaccharide in concentra- (1994). Induction by cytokines of the pat- tabolism, 84: 1729-1736. tions which alter pituitary hormone re- tern of pituitary hormone secretion in in- 32. Ono N, Bedran de Castro J & McCann SM lease. Neuroimmunomodulation, 1: 251- fection. Neuroimmunomodulation, 1: 2- (1985). Ultrashort-loop positive feedback 258. 13. of corticotropin-releasing factor (CRF) to 38. McCann SM (2000). Nitric oxide. In: Fink 44. Karanth S, Lyson K & McCann SM (1993). enhance ACTH release in stress. Proceed- G (Editor), Encyclopedia of Stress. Vol. 3. Role of nitric oxide in interleukin 2-in- ings of the National Academy of Sciences, Academic Press, San Diego, CA, 53-61. duced corticotropin-releasing factor re- USA, 82: 3528-3531. 39. Wong M-L, Rettori V, Al-Shekhlee A, lease from incubated hypothalami. Pro- 33. Ono N, Samson WK, McDonald JK, Bongiorno PB, Canteros G, McCann SM, ceedings of the National Academy of Sci- Lumpkin MD, Bedran de Castro JC & Gold PW & Licinio J (1996). Inducible ni- ences, USA, 90: 3383-3387. McCann SM (1985). The effects of intra- tric oxide synthase gene expression in 45. Karanth S, Lyson K, Aguila MC & McCann venous and intraventricular injection of an- the brain during systemic inflammation. SM (1995). Effects of luteinizing-hor- tisera directed against corticotropin re- Nature Medicine, 2: 581-584. mone-releasing hormone, a-melanocyte- leasing factor (CRF) on the secretion of 40. Banks WA, Kastin AJ, Huang W, Jaspan stimulating hormone, naloxone, dexa- anterior pituitary hormones. Proceedings JB & Maness LM (1996). Leptin enters methasone and indomethacin on interleu- of the National Academy of Sciences, the brain by a saturable system independ- kin-2-induced corticotropin-releasing fac- USA, 82: 7787-7790. ent of insulin. Peptides, 17: 305-311. tor release. Neuroimmunomodulation, 2: 34. Ono N, Lumpkin MD, Samson WK, 41. Canteros G, Rettori V, Genaro A, Suburo 166-173. McDonald JK & McCann SM (1984). Intra- A, Gimeno M & McCann SM (1996). Nitric 46. Lyson K & McCann SM (1992). Involve- hypothalamic action of corticotrophin-re- oxide synthase content of hypothalamic ment of arachidonic acid cascade path- leasing factor (CRF) to inhibit growth hor- explants: Increased by norepinephrine ways in interleukin-6-stimulated cortico- mone and LH release in the rat. Life Sci- and inactivated by NO and cGMP. Pro- tropin-releasing factor release in vitro. ences, 35: 1117-1123. ceedings of the National Academy of Sci- Neuroendocrinology, 55: 708-713. 35. Rivier C & Vale W (1984). Influence of ences, USA, 93: 4246-4250. 47. Karanth S, Lyson K & McCann SM (1994). corticotropin-releasing factor on reproduc- 42. Wong M-L, Bongiorno PB, Rettori V, Cyclosporin A inhibits interleukin-2-in- tive functions in the rat. Endocrinology, McCann SM & Licinio J (1997). Interleu- duced release of corticotropin releasing 114: 914-921. kin (IL) 1ß, IL-1 receptor antagonist, IL-10, hormone. Neuroimmunomodulation, 1: 36. Rivier C & Vale W (1984). Corticotropin- and IL-13 gene expression in the central 82-85. releasing factor (CRF) acts centrally to in- nervous system and anterior pituitary dur- hibit growth hormone secretion in the rat. ing systemic inflammation: Pathophysi-

Braz J Med Biol Res 33(10) 2000