Involvement of 5-HT1, 5-HT2, and 5-HT3 receptors in the mediation of the prolactin response to serotonin and 5-hydroxytryptophan.

Serotonin (5-HT) is involved in the neuroendocrine regulation of prolactin (PRL) secretion as a stimulator. Within the last decade several 5-HT receptor types have been identified, but their individual role in the mediation of the PRL response to 5-HT is only partly understood. We investigated in conscious male rats the effect of different 5-HT1, 5-HT2, and 5-HT3 receptor antagonists on the PRL response to 5-HT or to the 5-HT precursor 5-hydroxytrytophan (5-HTP) which was administered in combination with the 5-HT reuptake inhibitor fluoxetine. 5-HT (0.5-5.0 mg/kg BW i.v.) or 5-HTP (25-100 mg/kg i.p.) in combination with saline or fluoxetine (10 mg/kg i.p.) increased the plasma PRL concentration dose-dependently. Pretreatment with the 5-HT1+2 receptor antagonist methysergide (2.5 mg/kg i.p.) prevented the stimulatory effect of 5-HT or 5-HTP + fluoxetine. Pretreatment with the 5-HT2 receptor antagonists ketanserin or LY 53857 (2.5 mg/kg i.p.) inhibited the PRL response to 5-HT by approximately 80% and to 5-HTP + fluoxetine approximately 100%. A higher dose (10 mg/kg) of the 5-HT2 receptor antagonists possessed only 50% inhibitory effect. Pretreatment with the 5-HT3 receptor antagonists ICS 205-930 or GR 38032F (0.05-2.5 mg/kg i.p.) inhibited the PRL response induced by 5-HT or by 5-HTP + fluoxetine. The maximal inhibitory effect (approximately 80%) was obtained by a dose of 0.1 mg/kg of both compounds.(ABSTRACT TRUNCATED AT 250 WORDS)     

Serotonergic involvement in stress-induced vasopressin and oxytocin secretion

OBJECTIVE: To investigate the involvement of serotonin (5-hydroxytryptamine - 5-HT) receptors in mediation of stress-induced arginine vasopressin (AVP) and oxytocin (OT) secretion in male rats. DESIGN: Experiments on laboratory rats with control groups. METHODS: Different stress paradigms were applied after pretreatment with intracerebroventricular infusion of saline or different 5-HT antagonists. RESULTS: Restraint stress (5 min), hypotensive hemorrhage or dehydration for 24 h increased AVP secretion fivefold and OT secretion threefold. Swim stress for 3 min had no effect on AVP secretion, but increased OT secretion threefold. Ether vapor or hypoglycemia had no effect on AVP or OT secretion. The restraint stress-induced AVP response was inhibited by pretreatment with the 5-HT(2A+2C) antagonists ketanserin (KET) and LY-53857 (LY) and the 5-HT(3+4) antagonist ICS-205930 (ICS), whereas the 5-HT(1A) antagonist WAY-100635 (WAY) had no effect. The OT response to restraint stress was inhibited by WAY, KET and LY but not by ICS. KET and LY inhibited OT response to dehydration, and LY inhibited OT response to hemorrhage. Neither of the antagonists affected AVP responses to dehydration or hemorrhage, nor the swim stress-induced OT response. CONCLUSION: 5-HT(2A), 5-HT(2C) and possibly 5-HT(3) and 5-HT(4) receptors, but not 5-HT(1A) receptors, are involved in the restraint stress-induced AVP secretion. 5-HT does not seem to be involved in the dehydration- or hemorrhage-induced AVP response. The restraint stress-induced OT response seems to be mediated via 5-HT(1A), 5-HT(2A) and 5-HT(2C) receptors. The dehydration and hemorrhage-induced OT responses are at least mediated by the 5-HT(2A) and 5-HT(2C) receptors. The 5-HT(3) and 5-HT(4) receptors are not involved in stress-induced OT secretion.     

Effect of serotonin 5-HT1, 5-HT2, and 5-HT3 receptor antagonists on the prolactin response to restraint and ether stress.

Serotonin (5-HT) appears to be involved in the central control of the prolactin (PRL) response to suckling and estrogen. Furthermore, 5-HT may participate in the mediation of stress-induced PRL release. In order further to elucidate the role of 5-HT and the type of 5-HT receptor(s) involved in the PRL response to stress, we investigated the effect of blockade of 5-HT1, 5-HT2 or 5-HT3 receptors on the restraint or ether stress-induced release of PRL in male rats. Pretreatment with the 5-HT1 + 2 receptor antagonist methysergide (0.5 or 2.5 mg/kg i.p.) inhibited or prevented the PRL response to restraint or ether stress. Pretreatment with the 5-HT2 receptor antagonists ketanserin or LY 53857 (0.5 or 2.5 mg/kg i.p.) inhibited the response to restraint or ether stress approximately 30 or 60%, respectively. Higher doses of both 5-HT2 receptor antagonists (10 mg/kg i.p.) had a minor inhibitory effect (5-30% for ketanserin and 50% for LY 53857). Prior intraperitoneal administration of the 5-HT3 receptor antagonists ICS 205-930 or GR 38032F (0.05-2.5 mg/kg i.p.) inhibited the restraint stress-induced PRL release dose-dependently. Both compounds inhibited the PRL response to ether stress, but only the effect of GR was dose-related. The maximal inhibitory effect (70% inhibition of the PRL response to restraint or ether stress) was obtained for both compounds at a dose of 0.1 mg/kg. We conclude that serotonergic neurons are involved in the mediation of the stress-induced PRL release by activation of 5-HT1, 5-HT2 as well as 5-HT3 receptors.     

Mechanisms of serotonin receptor agonist-induced activation of the hypothalamic-pituitary-adrenal axis in the rat

A substantial body of experimental evidence indicates that serotonin (5-HT) and several synthetic 5-HT receptor agonists activate the hypothalamic-pituitary-adrenal (HPA) axis. To explore the mechanism(s) by which 5-HT or 5-HT agonists enhance the activity of the HPA axis in vitro, we examined the stimulatory effects of the 5-HT1a agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), the 5-HT1c/5-HT1b agonist m-chlorophenylpiperazine (m-CPP), and the 5-HT2/5-HT1c agonist 1-(2,5-dimethoxy-4-iodophenyl)2-amino-propane (DOI) on plasma ACTH and corticosterone secretion in the rat. To test whether 8-OH-DPAT, m-CPP, or DOI increase plasma ACTH levels by stimulating the release of endogenous CRH, catheterized conscious male Sprague-Dawley rats were pretreated with hyperimmune CRH rabbit serum (TS-6) or normal rabbit serum and subsequently challenged with a maximally stimulatory dose of the above 5-HT agonists. Pretreatment with TS-6 completely suppressed the ACTH response to m-CPP and significantly blunted the responses to 8-OH-DPAT or DOI. To examine whether the remaining ACTH response to 8-OH-DPAT or DOI was also mediated by a pituitary site of action, we administered each of these agents to pituitary stalk-transected or sham-operated rats. The ACTH responses to 8-OH-DPAT and DOI in stalk-transected rats were preserved, although significantly blunted, compared to those in sham-operated rats. This suggested that both of these 5-HT agonists may also act at the pituitary level to stimulate ACTH release in vivo. Although the ACTH responses to 8-OH-DPAT, m-CPP, and DOI were blunted after both TS-6 pretreatment and pituitary stalk transection, corticosterone responses were only slightly affected, suggesting that some of these compounds may cause corticosterone release in the rat through another mechanism. To evaluate this hypothesis, ACTH and corticosterone responses to 8-OH-DPAT, m-CPP, and DOI were examined in rats whose HPA axis had been suppressed by a single high dose injection of dexamethasone. The corticosterone responses to 8-OH-DPAT and DOI were blunted compared to those of saline-pretreated rats, but were inappropriately high compared to the ACTH responses observed in these rats. On the other hand, both ACTH and corticosterone responses to m-CPP were completely abolished by dexamethasone.(ABSTRACT TRUNCATED AT 400 WORDS)     

Estrogen reduces serotonin-1A receptor-mediated oxytocin release and Galpha(i/o/z) proteins in the hypothalamus of ovariectomized rats

The present study examined the effect of estradiol on hypothalamic serotonin-1A (5-HT(1A)) receptor signaling in female rats. We first examined the time-course effects of a single injection of the 5-HT(1A) receptor agonist (+/-)8-OH-DPAT (5, 15 or 30 min prior to decapitation), and dose response of (+)8-OH-DPAT (50, 100, 200 or 500 microg/kg, s.c.) on plasma hormones in ovariectomized rats that received a daily injection of beta-estradiol 3-benzoate (10 microg/day, s.c.) or vehicle (sesame oil) for 2 days. In vehicle- and estrogen-treated rats, the peak response of hormones occurred at 15 min after injection and the time-course of oxytocin and adrenocorticotropic hormone (ACTH) responses to an injection of 8-OH-DPAT were comparable. However, only the oxytocin response was reduced by estrogen treatment. A second experiment compared the ACTH and oxytocin responses with doses of 50 or 200 microg/kg, s.c. of (+)8-OH-DPAT vs. (+/-)8-OH-DPAT in ovariectomized rats that were treated with oil or beta-estradiol 3-benzoate (10 microg/day, s.c.) for 2 days. (+)8-OH-DPAT and (+/-)8-OH-DPAT produced a similar magnitude of increase in plasma levels of ACTH and oxytocin. Treatment with beta-estradiol 3-benzoate produced a significant and comparable reduction in the oxytocin response to the highest dose (200 microg/kg, s.c.) of both (+)8-OH-DPAT and (+/-)8-OH-DPAT but did not alter the ACTH response to either (+)8-OH-DPAT or (+/-)8-OH-DPAT. In the dose-response experiment, a dose of 50 microg/kg of (+)8-OH-DPAT produced a maximal increase in plasma levels of ACTH, while the maximal oxytocin response was achieved with a dose of 200 microg/kg, s.c. Treatment with beta-estradiol 3-benzoate reduced the maximal oxytocin response to (+)8-OH-DPAT (by 29%) but did not alter the ACTH response to any doses of (+)8-OH-DPAT. To examine potential mechanisms mediating the effects of estrogen on 5-HT(1A) receptor signaling, we measured the levels of Galpha(i), Galpha(o) and Galpha(z) proteins, which couple 5-HT(1A) receptors to their effector enzymes, in two subregions of the hypothalamus. The levels of Galpha(z) protein were reduced in the mediobasal hypothalamus (containing the ventromedial and arcuate nuclei), which mainly expresses estrogen receptor-alpha, but not in the paraventricular hypothalamus, which mainly expresses estrogen receptor-beta. Estradiol reduced the levels of Galpha(i2) and Galpha(i3 )proteins in both hypothalamic regions but did not affect Galpha(i1) levels in either area. Combined, the data suggest that racemic and stereoselective 8-OH-DPAT have similar neuroendocrine effects and that both estrogen receptor-alpha and estrogen receptor-beta mediate the reduction in levels of Galpha(i2,3) proteins.     

Mechanisms of 5-hydroxy-L-tryptophan-induced adrenocorticotropin release: a major role for central noradrenergic drive

The serotonin (5-hydroxytryptamine, 5-HT) precursor 5-hydroxy-L-tryptophan (5-HTP) and 5-HT antagonists, respectively, are able to stimulate and block pituitary adrenocorticotropin (ACTH) release. However, our previous data do not support a role of central serotoninergic systems in the neural control of ACTH release. We thus examined the hypothesis that 5-HTP given either alone or with uptake-blocking drugs such as fluoxetine caused stimulation of ACTH through activation of central noradrenergic neuronal activity (NNA). The hypothesis was tested in normal adult male rats by correlating medial basal hypothalamic NNA and serotoninergic neuronal activity (SNA) with serum ACTH following administration of 5-HTP (20 and 100 mg/kg, i.p.) in the presence or absence of fluoxetine (10 mg/kg, i.p.) or cyproheptadine (10 mg/kg, i.p.). The alpha 2-adrenergic agonist clonidine (150 micrograms/kg, i.p.) was used to inhibit central NNA and to examine the role of alpha 2-adrenoceptors in the actions of serotoninergic drugs. Computerized mass spectrometry was employed to specifically and precisely assay hypothalamic norepinephrine (NE), 3,4-dihydroxyphenylethyleneglycol (DHPG), 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) to obtain indices of hypothalamic NNA (DHPG/NE) and SNA (5-HIAA/5-HT). The administration of fluoxetine/5-HTP stimulated (p less than 0.01) both hypothalamic NNA and ACTH release.(ABSTRACT TRUNCATED AT 250 WORDS)     

Serotonin 5-HT1A receptors modulate depression-related symptoms following mild traumatic brain injury in male adult mice

Traumatic brain injury is a complex phenomenon leading to neurological diseases and persistent disability that currently affects millions of people worldwide. Increasing evidence shows that a wide range of patients with mild traumatic brain injury (mTBI) suffer from depression during the initial stages of injury and the post-acute stages of recovery. However, the underlying mechanisms involved in depression following mTBI are still not fully understood. The aim of this study was to determine whether serotonin 5-hydroxytryptamine-1A (5-HT1A) receptor is involved in the regulation of depression-related behaviors following mild traumatic brain injury in mice. Mice with or without mTBI received intracerebroventricular injections of 5-HT1A receptor agonist (8-OH-DPAT) or antagonist (WAY-100635) for 5 days, then animals were subjected to behavioral tests. Four behavioral tests including novelty-suppressed feeding test, forced swim test, sucrose preference test and tail suspension test were used to evaluate depression-related symptoms in animals. Our results indicated that mTBI induction increased depression-like symptoms through altering serotonin 5-HT1A receptor activity in the brain. Activation of 5-HT1A receptor by a subthreshold dose of 8-OH-DPAT led to a significant decrease in depression-like behaviors, whereas blockade of 5-HT1A receptor by a subthreshold dose of WAY-100635 resulted in a considerable increase in depression-like phenotypes in mTBI-induced mice. The major strength of the present study is that depression-related symptoms were assessed in four behavioral tests. The present study supports the idea that disturbances in the function of serotonergic system in the brain following mTBI can play an important role in the regulation of depression-related behaviors.

     

Role of serotonin in nocturnal and diurnal surges of prolactin in the pregnant rat

Two surges of PRL designated as nocturnal (N) and diurnal (D) are observed during early pregnancy. The present study was undertaken to determine the involvement of serotonin (5-HT) in regulating these PRL surges. On day 7 of pregnancy intracarotid catheters were implanted, and blood samples were obtained at 0200 and 0400 h on day 8 and between midnight and 0600 h on day 9 to monitor the N surge. Rats were killed by decapitation, brains were quickly removed, frozen on dry ice, and stored at -70 C until later determination of indole and catecholamines. Pretreatment 24 h earlier with para-chlorophenylalanine (PCPA; 250 mg/kg BW), an inhibitor of 5-HT biosynthesis, did not affect the N PRL surge, although the hypothalamic concentrations of 5-HT and its metabolite 5-hydroxyindole acetic acid were greatly reduced. On the other hand, administration at 2400 h of ketanserin (KET; 10 mg/kg BW, ip) or LY-53857 (5 mg/kg BW, ip), two selective 5-HT2 antagonists, significantly (P less than 0.025 for KET and P less than 0.01 for LY-53857) reduced the N PRL surge. Neither KET nor LY-53857 altered the hypothalamic content of biogenic amines compared to that in saline-treated controls. Intravenous treatment with LY-53857 (1 mg/kg) at 0200 h after the onset of the N surge induced no change in plasma PRL levels compared to those in controls. To test whether 5-HT plays a role in the D surge, rats were decapitated at 1800 h after drug injection. Pretreatment with the same dose of PCPA (24 h), KET (2 h), or LY-53857 (2 h) that was given to rats in the N surge study significantly (P less than 0.01) reduced plasma PRL levels. As observed in the N study, PCPA greatly diminished the hypothalamic content of 5-HT and 5-hydroxyindole acetic acid, and also reduced dopamine, but to a much lesser extent than 5-HT. It is concluded that 5-HT contributes significantly to the generation of the D surge. Its role during the N surge remains uncertain due to the contradictory effects of the synthesis inhibitor PCPA and the receptor antagonists KET and LY-53857.     

An endogenous serotonergic rhythm acting on 5-HT(2A) receptors may be involved in the diurnal changes in tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in female rats

The central serotonergic system has long been known to have a stimulatory role on the secretion of prolactin (PRL). The integrity of serotonergic neurotransmission is essential for the expression of the estrogen-induced afternoon PRL surge. Whether its effect on PRL involves change in the activity of tuberoinfundibular dopaminergic (TIDA) neurons has not been ascertained. In adult ovariectomized rats treated with estrogen, depletion of central serotonin (5-HT) by 5,7-dihydroxytryptamine (5,7-DHT, 200 microg/rat, i.c.v.) effectively prevented the afternoon fall in TIDA neuronal activity (using the levels of 3,4-dihydroxyphenylalanine and 3, 4-dihydroxyphenylacetic acid (DOPAC), and the ratio of DOPAC/dopamine in the median eminence as indices), and blunted the afternoon PRL surge. A single injection of a 5-HT(2A) receptor antagonist, ketanserin (5 mg/kg, i.p. at 12.00 h), also had the same effects on the diurnal changes in TIDA neuronal activity and PRL secretion as the treatment with 5,7-DHT did. Intracerebroventricular injection of a 5-HT(2) receptor agonist 2, 5-dimethoxy-4-iodoamphetamine (DOI) in the morning inhibited the TIDA neuronal activity and stimulated PRL secretion in a dose-dependent manner; while injection of a 5-HT(1) agonist, 8-hydroxy-dipropylaminotetralin, was without effect. Injection of DOI in 5,7-DHT-pretreated rats at 14.30 h also lowered the TIDA neuronal activity and reinstated the PRL surge. In all, endogenous 5-HT, acting through the 5-HT(2A) receptor, appears to exhibit an inhibitory effect on TIDA neuronal activity during the afternoon, which is essential for the PRL surge.     

Diurnal variation in prolactin, adrenocorticotropin and corticosterone release induced by opiate agonists in intact and adrenalectomized rats

Diurnal variations of the effectivity of beta-endorphin (beta-End), dynorphin (DYN), Met-enkephalin (Met-Enk), D-Met2-Pro5-enkephalinamide (D-Met-Pro-Enk) and morphine to induce prolactin (PRL) and adrenocorticotropin (ACTH)/corticosterone (CS) release in intact and adrenalectomized rats have been examined. The response to morphine (10 mg/kg s.c.), Met-Enk (200 micrograms/rat i.c.v.) and D-Met-Pro-Enk (0.5 microgram/rat i.c.v.) did not change with different times of the day, while that to beta-End (0.5 microgram/rat i.c.v.), DYN (1 microgram/rat i.c.v.) and U50-488H, a selective kappa agonist (10 mg/kg s.c.), showed a circadian rhythm in stimulating PRL release, with a higher increase in the afternoon (16.00-17.00 h) than in the morning (08.00-09.00 h). In adrenalectomized rats the loss of this circadian rhythm was shown. The CS release evoked by morphine, D-Met-Pro-Enk, Met-Enk and DYN was demonstrable only in the morning when the basal CS level was significantly lower than in the afternoon. The afternoon release of ACTH by morphine was higher than in the morning in adrenalectomized rats. beta-End and U50-488H were equally active in the morning and in the afternoon in increasing CS secretion. The present results suggest that the diurnal rhythm in the response of CS and PRL release to opioids is in relation with the glucocorticoid secretion.     

Effect of selective blockade of catecholaminergic alpha and beta receptors on histamine-induced release of corticotropin and prolactin.

We investigated the role of adrenergic receptors in histamine (HA)-induced release of corticotropin (ACTH) and prolactin (PRL) in conscious male rats. Specific alpha- or beta-receptor antagonists were administered intracerebroventricularly in doses of 1 mmol at time -20 min, and HA (270 nmol), the H1 receptor agonist 2-thiazolylethylamine (2-TEA; 2,180 nmol) or the H2 receptor agonist 4-methylHA (4-MeHA; 790 nmol) were administered intracerebroventricularly at -15 min. The animals were decapitated at 0 min, and plasma was analyzed for ACTH and PRL. Administration of HA and the histaminergic agonists stimulated ACTH secretion equally, while only HA and the H2 receptor agonist stimulated PRL secretion. Pretreatment with the adrenergic receptor antagonists had no effect on the ACTH response to the histaminergic compounds. In contrast, the PRL response to HA or 4-MeHA was inhibited or prevented by the alpha-receptor antagonists phenoxybenzamine and phentolamine, the alpha1-receptor antagonist prazocin, the beta-receptor antagonist propranolol and the beta1-receptor antagonist atenolol, whereas the alpha2-receptor antagonist yohimbine or the beta2-receptor antagonist ICI-118-551 had no effect. The study indicates that histaminergic neurons interact with the catecholaminergic neuronal system in regulation of PRL secretion, and that this interaction is dependent upon activation of alpha1- and beta1-receptors. In contrast, histaminergic neurons stimulate ACTH secretion independently of adrenergic receptor activation.     

Mast Cell Degranulation Induces Delayed Rectal Allodynia in Rats: Role of Histamine and 5-HT

Visceral hypersensitivity is a common feature offunctional bowel disorders, where an increased number ofmast cells have often been described. Thus, weinvestigated the effect of an experimental mast cell degranulation induced by BrX-537A on somatic(tail heating) and visceral (rectal distension)sensitivity in rats and the involvement of histamineand/or serotonin on this last response. After BrX-537Aadministration, the latency of tail withdrawal reflex wasshortened within the 2- to 8-hr period. Moreover,BrX-537A reduced the distension volume threshold from0.8 ml to 0.4 ml inducing allodynia, from 6 to 12 hrafter its administration. This effect was suppressedby doxantrazole (mast cell stabilizing agent) and WAY100635 (5-HT_1A receptor antagonist), andreproduced by 5-HTP (5-HT precursor) and 8-OH-DPAT(5-HT_1A receptor agonist). However, neither granisetron(5-HT3 receptor antagonist) nor H1, H2, or H3 histaminereceptor antagonists modified the BrX-537A-inducedallodynia. Consequently, mast cell degranulation initiates a delayed somatic and visceralallodynia, with the participation of serotonin, through5-HT_1A receptor activation, on the visceralresponse.     

Prolactin release after 5-hydroxytryptophan treatment requires an intact neurointermediate pituitary lobe

The present study was designed to evaluate the role of the neurointermediate pituitary lobe (NIL) in the 5-hydroxytryptophan (5-HTP)-induced increase in plasma PRL levels. The neurochemical mechanisms involved in this neuroendocrine regulation were also analyzed by examining the concentrations of serotonin (5-HT), norepinephrine, and dopamine as well as their primary metabolites in the median eminence (ME), NIL, and anterior pituitary (AP) after different treatments. Removal of the NIL (NIL-X) did not significantly affect basal plasma PRL concentrations or amine metabolism in the ME or AP. 5-HTP administration resulted in a 5-fold increase in plasma PRL in sham-operated (SHAM) animals, but NIL-X completely abolished this PRL response. 5-HTP injection elicited large increases in 5-HT and 5-hydroxyindole-3-acetic acid concentrations in ME, NIL, and AP in SHAM animals, and similar changes within the ME and AP in NIL-X animals, but did not significantly affect norepinephrine or dopamine metabolism in the ME, NIL, or AP of NIL-X or SHAM animals. Moreover, tissue concentrations of 5-HTP after 5-HTP injection increased similarly in SHAM and NIL-X animals. Inhibition of peripheral decarboxylase activity with MK486 prevented the 5-HTP-induced increase in PRL in SHAM animals and the increase in 5-HT metabolism in both SHAM and NIL-X groups. These data indicate that an intact NIL is required for the 5-HTP-induced increase in plasma PRL, but does not seem to be essential for the regulation of basal PRL release. They also demonstrate that the 5-HTP-induced activation of 5-HT metabolism in both ME and AP is not sufficient, by itself, to enhance PRL release.     

Histamine- and stress-induced secretion of ACTH and beta-endorphin: involvement of corticotropin-releasing hormone and vasopressin.

Histamine (HA) stimulates the release of the pro-opiomelanocortin (POMC)-derived peptides ACTH, beta-endorphin (beta-END), beta-lipotropin and alpha-melanocyte-stimulating hormone, and HA is involved in the mediation of the stress-induced release of these peptides. The effect of HA is indirect and may involve the hypothalamic regulating factors, corticotropin-releasing hormone (CRH) and/or arginine-vasopressin (AVP). We studied the effect of immunoneutralization with specific antisera against CRH or AVP on the response of ACTH and beta-END to HA, restraint stress, CRH, AVP or a posterior pituitary extract in male rats. Intracerebroventricular infusion of HA (34-540 nmol) increased the plasma levels of ACTH and beta-END immunoreactivity (beta-ENDir) in a dose-dependent manner. Pretreatment with antiserum to CRH or AVP prevented the ACTH response to 270 nmol HA and inhibited the beta-ENDir response by 30-60%. One to five minutes of restraint stress caused an increase in the plasma levels of ACTH and beta-ENDir. The increase was dependent on the duration of stress exposure. Pretreatment with CRH antiserum abolished the ACTH response to 5 min of restraint stress and inhibited the beta-ENDir response by 60%. Immunoneutralization with AVP antiserum had only half the inhibitory effect of that seen with CRH antiserum. CRH (100 pmol i.v.) increased the plasma levels of ACTH and beta-ENDir. This effect was abolished by pretreatment with CRH antiserum, whereas pretreatment with AVP antiserum prevented the CRH-induced ACTH release and inhibited the beta-ENDir response by 50%. AVP (24-800 pmol i.v.) stimulated ACTH and beta-ENDir in a dose-dependent manner. CRH and AVP antisera each prevented the effect of AVP (800 pmol) on ACTH secretion, whereas the beta-ENDir response to AVP was only inhibited by about 60% by the antisera. An extract of the posterior pituitary gland administered in a dose corresponding to 0.15 or 0.5 pituitary equivalents had no effect on ACTH secretion, while 1.0 pituitary equivalent increased the ACTH concentration in plasma. This effect was abolished by AVP antiserum. The posterior pituitary extract caused a dose-dependent rise in plasma beta-ENDir which might be due to an unavoidable contamination of the posterior pituitary extract by a small amount of beta-END from the intermediate lobe. Consistent with this view, AVP antiserum had no effect on the rise in the plasma concentration of beta-ENDir following administration of the posterior pituitary extract.(ABSTRACT TRUNCATED AT 400 WORDS)     

Serotonergic stimulation of prolactin release in the young turkey (Meleagris gallopavo)

Serum prolactin (PRL) levels were determined by homologous radioimmunoassay in 6- to 10-week-old domestic white turkeys treated by intraperitoneal injection of agents that alter serotonergic activity. Quipazine (0.1–10.0 mg/kg), a serotonin (5-hydroxytryptamine; 5-HT) agonist, induced a dose-dependent rise in serum PRL level 1 hr after injection. The 5-HT precursor, 5-hydroxytryptophan (5-HTP), at doses of 80 and 150 mg/kg produced over a twofold elevation in PRL level 1 hr after administration, though the 50 mg/kg dose failed to produce any change. Administration of fluoxetine (10 mg/kg), a 5-HT reuptake blocker, induced an elevation in PRL level persisting 3 hr. When fluoxetine injection preceded administration of a weakly stimulatory dose of 5-HTP, a prolonged elevation in PRL level was observed. Methysergide (MES), a 5-HT antagonist, administered in a 10 mg/kg dose produced no change in PRL level, while the 25 mg/kg dose initially produced a spike in PRL level which subsequently dropped slightly below the control level. Prior injection of 20 mg/kg MES completely blocked the serum PRL rises induced by quipazine and 5-HTP. These results suggest that serotonergic mechanisms are involved in the regulation of pituitary PRL release beyond basal levels in young domestic turkeys.     

Endocrine activities of cortistatin-14 and its interaction with GHRH and ghrelin in humans

Cortistatin (CST)-14, a neuropeptide with high homology with somatostatin (SS)-14, binds all sst subtypes but, unlike SS, also ghrelin's receptor. In six normal adults, we studied the effects of CST-14 or SS-14 administration (2.0 micro g/kg/h iv) on: 1) GH and insulin secretion; 2) the GH response to GHRH (1.0 microg/kg i.v.); and 3) the GH, prolactin (PRL), ACTH, cortisol, insulin, and glucose responses to ghrelin (1.0 microg/kg i.v.). CST-14 inhibited GH and insulin secretion (P < 0.01) to the same extent of SS-14. The GH response to GHRH was similarly inhibited (P < 0.01) by either CST-14 or SS-14. Ghrelin released more GH than GHRH (P < 0.01); these responses were similarly inhibited (P < 0.05) by either CST-14 or SS-14, that made ghrelin-induced GH rise similar to that after GHRH alone. Neither CST-14 nor SS-14 modified PRL, ACTH, or cortisol responses to ghrelin. The inhibitory effect of CST-14 and SS-14 on insulin was unaffected by ghrelin that, in turn, reduced insulin secretion per se (P < 0.01). Ghrelin increased glucose levels (P < 0.05); CST-14 and SS-14 did not modify this effect. Thus, CST-14 inhibits both basal and stimulated GH secretion in humans to the same extent of SS-14. The GH-releasing activity of ghrelin seems partially resistant to CST-14 as well as SS-14. CST-14 and SS-14 do not affect PRL and ACTH secretion but, like ghrelin, inhibit insulin secretion; the ghrelin-induced inhibition is not additive with that of CST-14 or SS-14, suggesting a common mechanism of action on beta cell secretion.     

Impact of two or three daily subcutaneous injections of hexarelin, a synthetic growth hormone (GH) secretagogue, on 24-h GH, prolactin, adrenocorticotropin and cortisol secretion in humans

OBJECTIVE: To extend the insights on the action of GH secretagogues (GHS) on pituitary function, we studied the impact of intermittent daily s.c. administration of a peptidyl GHS, hexarelin (HEX), on 24-h GH, PRL, ACTH and cortisol release in healthy volunteers. DESIGN: We investigated the impact of two or three times daily s.c. administration of a short-acting peptidyl GHS, the hexapeptide HEX (1.5 microg/kg) on 24-h GH, PRL, ACTH and cortisol secretion (sampling every 20 min) in six normal young men. To monitor possible down-regulation, the effect of 1 microg/kg i.v. HEX at the end of each 24-h sampling period was studied. METHODS: Multi-parameter deconvolution analysis was used to quantitate pulsatile GH, PRL, ACTH and cortisol secretion and estimate the corresponding hormone half-lives. Complementary to deconvolution analysis, approximate entropy was used as a scale- and model-independent statistic to quantify the serial orderliness or pattern regularity of hormone measurements. RESULTS: Mean and integrated (24-h) serum GH concentrations were increased from baseline values to the same extent by two and three HEX injections. Both HEX schedules equally increased GH secretory burst mass (but not burst frequency), mean daily GH production rate, GH half-life and irregularity of GH release patterns. No change occurred in the secretion of IGF-I, PRL, ACTH and cortisol. Intravenous HEX at the end of each spontaneous 24-h profile induced a significant rise in GH, PRL, ACTH and cortisol. Prior HEX administration blunted the GH response, abolished that of ACTH and cortisol and did not modify the PRL increase. CONCLUSIONS: The study showed that two or three daily s.c. injections of HEX augmented 24-h GH secretion equally, amplifying selectively GH secretory pulse mass without altering lactotroph and corticotroph secretion. IGF-I levels were not modified by these 1-day HEX treatment schedules.     

Effect of sumatriptan on external pancreatic secretion and its interaction with endogenous norepinephrine in the rat.

We reported previously that blocking norepinephrine reuptake by nisoxetine could modulate external pancreatic secretion in the rat. We report in this study the interaction of serotonin (5-HT) with endogenous catecholamines by using sumatriptan, an agonist of 5-HT1 receptors, in combination with nisoxetine. Urethane-anesthetized male Wistar rats were fitted with an acute pancreatic fistula. Nisoxetine (0.3 mg/kg, i.v.) and sumatriptan (0.1-1 mg/kg, s.c.) were administered alone or in combination. Pancreatic secretion was measured under stimulation by 2-deoxy-D-glucose (2DG; 75 mg/kg, i.v.), by vagal electrical stimulation (4 V, 2 ms, 10 Hz), or by acetylcholine (60-1,800 microg/kg.h). (i) 2DG: Nisoxetine alone inhibited 2DG-induced pancreatic secretion (p < 0.01). Sumatriptan alone also produced a dose-related inhibition of 2DG-induced pancreatic secretion (p < 0.01). When sumatriptan and nisoxetine were combined, protein response to 2DG remained inhibited, whereas water and electrolyte secretion was restored. (ii) Vagal stimulation: Nisoxetine did not modify water and electrolyte output in response to vagal electrical stimulation (VES), whereas it inhibited protein response by 75%. Sumatriptan alone strongly inhibited pancreatic response to VES (p < 0.01). When nisoxetine and sumatriptan were combined, the protein response to VES remained inhibited, whereas water and electrolyte response to VES was restored. (iii) Acetylcholine: Nisoxetine and sumatriptan alone or combined did not modify pancreatic response to acetylcholine. These results indicate that noradrenergic and serotonergic agents can indirectly affect pancreatic secretion through a modulation of the vagal cholinergic pathway. Nisoxetine and sumatriptan interact negatively on hydroelectrolytic pancreatic secretion, whereas they inhibit the secretion of enzymes both alone and in combination.     

Ovariectomy-induced increases in hypothalamic serotonin-1A receptor function in rats are prevented by estradiol

The present study investigated the effects of long-term estradiol withdrawal (ovariectomy) on hypothalamic serotonin-1A (5-HT(1A)) receptor signaling. Changes in neuroendocrine responses to the 5-HT(1A) agonist 8-OH-DPAT and levels of G(z) protein in the hypothalamus were used to examine 5-HT(1A) receptor signaling. Five days following ovariectomy, rats received daily injections of either 2 microg of beta-estradiol 3-benzoate or vehicle (subcutaneously) for 2, 4 or 14 days. Twenty-four hours after the last injection, and 15 min prior to sacrifice, rats were injected with (+/-)8-OH-DPAT (50 micro;g/kg, s.c.) or saline. Estradiol treatment did not alter basal corticotropin (ACTH) or oxytocin levels. Injection of (+/-)8-OH-DPAT produced significant increases in plasma ACTH and oxytocin levels. In the vehicle-treated rats, hormone responses to 8-OH-DPAT were enhanced in rats that received injections for 14 days compared with rats that received injections for either 2 or 4 days. Estradiol treatment for 4 or 14 days blunted this enhanced ACTH response to 8-OH-DPAT, whereas the oxytocin response to 8-OH-DPAT was only blunted after 14 daily injections of beta-estradiol 3-benzoate. The treatment with beta-estradiol 3-benzoate (2 microg/rat) did not reduce membrane-associated G(z) protein levels in the paraventricular nucleus of the hypothalamus. Hence, the inhibitory influence of a low dose of beta-estradiol 3-benzoate on 5-HT(1A) receptor signaling in the hypothalamus is not accompanied by a change in the levels of G(z) protein in the paraventricular hypothalamic nucleus. Results from the present study indicate a supersensitivity of 5-HT(1A) receptors after withdrawal of estradiol and suggest that estradiol suppresses 5-HT(1A) receptor signaling.     

Suppression of adrenocorticotrophic hormone secretion by simultaneous antagonism of vasopressin 1b and CRH-1 receptors on three different stress models

BACKGROUND/AIMS: Corticotrophin-releasing hormone (CRH), adrenocorticotrophic hormone (ACTH) and corticosterone are secreted during stress. These mediators may be involved in anxiety, depression and post-traumatic stress disorder, therefore antagonists have been developed to treat such conditions. METHODS: The non-peptide CRH receptor type 1 antagonist CP154,526 and the vasopressin receptor type 1b antagonist SSR149415 were used to suppress the secretion of ACTH induced by ether exposure, forced swimming and restraint in adult male Wistar rats. Doses ranged from 3 to 60 mg/kg s.c. (controls with vehicle) alone or in combination, in varying time schedules to assess the duration and effectiveness of treatments. RESULTS: Stressors increased plasma ACTH by 2.5- to 5-fold in control rats. SSR149415 at doses of 30 mg/kg was more effective at suppressing ACTH secretion after ether exposure and restraint but was ineffective against forced swimming. CP154,526 mildly affected ACTH rise after restraint at doses of 30 mg/kg. The combination of both antagonists at doses of 30 mg/kg effectively blocked the rise in plasma ACTH in all three stresses. The drug effects lasted less than 6 h. CONCLUSION: We demonstrated for the first time that simultaneous blockade of both vasopressin 1b and CRH-R1 receptors effectively abolish the ACTH response to physical and psychological stress modalities.