Pituitary Cyclic AMP Research Articles

Lutzomyia longipalpisSalivary Peptide Maxadilan Alters Murine Dendritic Cell Expression of CD80/86, CCR7, and Cytokine Secretion and Reprograms Dendritic Cell-Mediated Cytokine Release from Cultures Containing Allogeneic T Cells

Leishmania protozoan parasites, the etiologic agent of leishmaniasis, are transmitted exclusively by phlebotomine sand flies of the genera Phlebotomus and Lutzomyia. In addition to parasites, the infectious bite inoculum contains arthropod salivary components. One well-characterized salivary component from Lutzomyia longipalpis is maxadilan (MAX), a vasodilator acting via the type I receptor for the pituitary cyclic AMP activating peptide. MAX has been shown to elicit immunomodulatory effects potentially dictating immune responses to Leishmania parasites. When exposed to MAX, both resting and LPS-stimulated dendritic cells (DCs) show reduced CD80 and CD86 expression on most DCs in vitro. However, CD86 expression is increased significantly on a subpopulation of DCs. Furthermore, MAX treatment promoted secretion of type 2 cytokines (IL-6 and IL-10) while reducing production of type 1 cytokines (IL-12p40, TNF-alpha, and IFN-gamma) by LPS-stimulated DCs. A similar trend was observed in cultures of MAX-treated DCs containing naive allogeneic CD4(+) T cells: type 2 cytokines (IL-6 and IL-13) increased while type 1 cytokines (TNF-alpha and IFN-gamma) decreased. Additionally, the proinflammatory cytokine IL-1beta was increased in cultures containing MAX-treated mature DCs. MAX treatment of LPS-stimulated DCs also prevented optimal surface expression of CCR7 in vitro. These MAX-dependent effects were evident in DCs from both Leishmania major-susceptible (BALB/c) and -resistant (C3H/HeN) murine strains. These data suggest that modification of DC phenotype and function by MAX likely affects crucial cellular components that determine the pathological response to infection with Leishmania.

Pituitary adenylate cyclase-activating polypeptide and vasoactive intestinal peptide-stimulated cyclic AMP synthesis in rat cerebral cortical slices: interaction with noradrenaline, adrenaline, and forskolin.

Pituitary adenylate cyclase-activating polypeptide (PACAP; 0.001-1 microM) and vasoactive intestinal peptide (VIP; 0.01-1 microM) produced a concentration-dependent stimulation of cyclic AMP (cAMP) formation in rat cerebral cortical slices prelabeled with [3H]adenine. The effects of PACAP38 and PACAP27 were similar, and more efficacious (at 0.1 and 1 microM) than those of VIP. Adrenaline and noradrenaline (each at 100 microM) also stimulated cAMP formation, with the latter compound being more effective. Combination of PACAP38, PACAP27 (each at 0.1 microM) and VIP (1 microM) with adrenaline or noradrenaline resulted in most cases in additive effects, with some supraadditive (PACAP27 plus adrenaline) or subadditive (PACAP38 or VIP plus noradrenaline) fluctuations. In contrast, combination of each of the three peptides with 3 microM forskolin resulted in synergistic effects. These results indicate that in rat cerebral cortex there is no synergism between PACAP or VIP with noradrenaline or adrenaline; however, based on the forskolin data, it seems likely that synergistic effects may take place with VIP or PACAP and other cAMP-stimulating neuroregulators.

Stimulation of the hypothalamo-pituitary-adrenal axis in the rat by the type 4 phosphodiesterase (PDE-4) inhibitor, denbufylline.

1. Preliminary studies in our laboratories showed that the synthetic xanthine analogue denbufylline, a selective type 4 phosphodiesterase (PDE-4) inhibitor, is a potent activator of the hypothalamo-pituitary-adrenal (HPA) axis when given orally to adult male rats. This paper describes the results of experiments in which well established in vivo and in vitro models were used to (a) examine further the effects of denbufylline on HPA function and (b) identify the site and mode of action of the drug within the axis. 2. In vivo, administration of denbufylline (0.1-2.5 mg kg-1, i.p.) produced a significant increase in the serum corticosterone concentration; maximal responses were attained at a dose of 1.0 mg kg-1 (P < 0.01 vs. vehicle control, Scheffe's test). However, when denbufylline was administered by intracerebroventricular injection (0.05-1 micrograms kg-1) it failed to influence significantly the serum corticosterone concentration (P > 0.05 vs. vehicle control, Scheffe's test). The adrenocortical responses to peripheral injections of denbufylline (1 mg kg-1, i.p.) were reduced in rats in which the secretion of endogenous corticotrophin releasing factors (CRFs) from the hypothalamus was blocked pharmacologically (P < 0.01 vs. controls, Scheffe's test). However, denbufylline (0.1 mg kg-1, i.p.) potentiated the significant (P < 0.01) increases in serum corticosterone concentration provoked in "CRF blocked rats' by hypothalamic extract (5 hypothalamic extracts kg-1, i.v.) although it failed to influence (P > 0.05) the relatively moderate increases in corticosterone secretion evoked by CRH-41 (2 mg kg-1, i.v.). 3. In vitro, denbufylline (0.01-1 mM) evoked small but significant (P < 0.05) increases in the release of ACTH from rat anterior pituitary segments; furthermore, at these and lower concentrations (0.01 microM-1 mM), it potentiated the adrenocorticotrophic responses to sub-maximal concentrations of hypothalamic extract (P < 0.01) and forskolin (0.1 mM, P < 0.01) but not those to CRH-41 (10 nM) or 8-bromo-cyclic AMP (1-100 microM). In addition, denbufyline (0.1 mM) increased the anterior pituitary cyclic AMP content (P < 0.05) and potentiated the rises in tissue content of the cyclic nucleotide induced by hypothalamic extract (0.1 hypothalamic equivalents ml-1, P < 0.01) and forskolin (0.1 mM, P < 0.01) but not by CRH-41 (10 nM, P < 0.05). By contrast, denbufylline (1 microM-1 mM) failed to influence the release of AVP from rat isolated hypothalami and stimulated the secretion of CRH-41 (P < 0.01) release only at the highest concentration tested (1 mM). 4. The results suggest that the stimulatory actions of denbufylline on the hypothalamo-pituitary-adrenocortical axis are exerted predominantly at the level of the anterior pituitary gland and that they may be attributed, at least in part, to inhibition of type 4 phosphodiesterase enzymes.

Functional stimulation of natural killer cells by corticotropin-releasing factor

The corticotropin-releasing factor (CRF) analog CRF 9-41 inhibits CRF, but not forskolin or dibutyryl cyclic AMP, stimulated release of ACTH from isolated pituitary cells. CRF 9-41 also blocks CRF-stimulated accumulation of cyclic AMP in a parallel dose dependent fashion. CRF 9-41 has no effect on basal ACTH release or cAMP levels. This substantiates that the analog acts as a direct CRF antagonist and that the site of this inhibition is most likely at the level of binding of CRF to its receptor on the corticotrope. Various substances, including most prominently glucocorticoids, inhibit release of ACTH from the pituitary (1,2). In an effort to develop another class of inhibitors, Rivier et al (3) recently synthesized analogs of corticotropin releasing factor (CRF). One among these, alpha-helical ovine CRF 9-41 blunts adrenalectomy and stress induced ACTH release in non-anesthetized rats (3,4). At micromolar concentrations, CRF 9-41, shifts rightward the dose response of isolated pituitary cells to ovine CRF (3). Thus, the authors suggested that CRF 9-41 acts as a competitive antagonist to CRF-induced ACTH secretion. CRF appears to act through stimulation of adenylate cyclase (5–7). To determine the potential site of action of CRF 9-41 in the activation sequence for adenylate cyclase, we studied its effects on pituitary cyclic AMP formation and ACTH secretion from dispersed anterior pituitary cells derived from normal adult rats, as well as, its interaction with cyclic nucleotide agonists.

ACTH, prolactin, corticosterone and pituitary cyclic AMP responses to repeated stress

The present experiment was conducted to determine whether the plasma hormonal and pituitary cyclic AMP responses observed following a single exposure to an acute stressor would diminish following reexposures to the same stressor. Fifteen-min stress exposures (forced running) were separated by 45-min recovery periods. Separate groups of control and stressed animals were sacrificed before and after each of four 15-min stress periods and after each recovery period. The first exposure to 15 min of forced running raised plasma ACTH, corticosterone and pituitary cyclic AMP levels approximately 6-fold and more than tripled levels of plasma prolactin. Plasma ACTH and pituitary cyclic AMP responses to the second, third and fourth stress exposures were very similar to the responses to the first stress exposure, and levels of these substances returned to prestress levels during each 45-min recovery period. Plasma prolactin responses to the four stress sessions were somewhat variable but no significant trend among the responses was seen. Plasma prolactin levels also returned to prestress levels between stress exposures. Corticosterone levels were similar following each of the four stress sessions but levels remained elevated compared to prestress levels between stress exposures. These data suggest that pituitary responses to acute stress are rapid, that return to prestress levels is also rapid, with the exception of corticosterone, and that repeated responses of the same magnitude may be evoked when stressors are separated by short recovery periods.

Dexamethasone suppresses ACTH release without attenuating pituitary cyclic AMP response to stress in vivo

Dexamethasone, a synthetic glucocorticoid, has been shown to decrease basal and stress-elevated levels of the pituitary hormone ACTH. Glucocorticoids are known to bind to multiple sites within the brain and pituitary and it is not known which site(s) is most important in mediating the observed inhibition of ACTH release. At the level of the corticotroph, there is contradictory data from in vitro studies regarding whether dexamethasone acts proximal or distal to the formation of the cyclic AMP second messenger that has been shown to be involved in CRF-stimulated ACTH release. In the present report, we have examined the effects of dexamethasone pretreatment on stress-induced elevations in pituitary cyclic AMP and the release of ACTH in vivo . Acute stress (15 min of intermittent footshock) elevated levels of pituitary cyclic AMP and plasma ACTH consistent with previous studies. Dexamethasone administration (0.4mg/kg 24 hr prior to sacrifice plus 0.2mg/kg 2 hr prior to sacrifice) inhibited stress-induced elevations in plasma ACTH but did not affect pituitary cyclic AMP response to acute stress. These findings suggest that dexamethasone inhibits the release of ACTH via an action distal to the generation of cyclic AMP.

Psychological stress increases pituitary cyclic AMP

Exposure to physical stressors has been shown to produce increases in pituitary cyclic AMP in laboratory rats. In this experiment, the generality of these findings has been extended to include a psychological stressor, defined as returning the animals to a situation in which they had been exposed to footshock for four days. Rats in the psychological stress group exhibited increases in pituitary cyclic AMP and in plasma corticosterone and prolactin which were similar to those seen in animals that received the physical stressor on the test day. At present it is not known whether the effects are due to associative conditioning or to sensitization of the neuroendocrine system following repeated presentations of the physical stressor.

Regulation of pituitary cyclic AMP, plasma prolactin and POMC-derived peptide responses to stressful conditions.

We have been investigating the role of pituitary cyclic AMP in regulating the stress-induced release of pituitary hormones in vivo. This paper will review a series of experiments describing the effects of both physical and psychological Stressors on these responses and exploring possible mechanisms of regulation. Exposure to stressful conditions elevates plasma levels of corticosterone (CS) (1–3), and increases plasma levels of several hormones secreted from the anterior pituitary, including adrenocorticotrophic hormone (ACTH), beta-endorphin (β-EP), beta-lipotropin (β-LPH), and prolactin (PRL) (3–8). ACTH, β-LPH and β-EP are derived from a common precursor, a 31,000 dalton glycoprotein currently referred to as proopiomelanocortin (POMC) (9,10). β-LPH is a 91 amino acid peptide (11) which contains β-EP as the 61–91 segment (12). In the rat, the three POMC-derived peptides are co-localized in the same secretory granules in anterior pituitary corticotrophs (13,14), while the intermediate lobe contains and releases predominantly β-EP (15–18). Plasma levels of β-EP are high relative to β-LPH (8,19), but exposure to stressors markedly increases plasma levels of all three POMC-derived peptides (5,8,19).

Paraventricular Lesions Completely Eliminate the Stress‐induced Rise in Pituitary Cyclic AMP and Attenuate the Increases in Plasma Levels of POMC‐derived Peptides in the Ratab

Paraventricular Lesions Completely Eliminate the Stress‐induced Rise in Pituitary Cyclic AMP and Attenuate the Increases in Plasma Levels of POMC‐derived Peptides in the Ratab

Atropine sulfate increases pituitary responses to stress

The effects of atropine sulfate pretreatment on pituitary indices of stress response were examined. Pituitary cyclic AMP and plasma prolactin increases following 15 min of acute stress were used as measures of stress response. Over a range of doses (0, 5, 10, 30 and 60 mg/kg), pretreatment with atropine sulfate increased the measured stress responses to footshock but had little or no effect on resting or non-stressed levels of the substances measured. The effects of atropine on response to immobilization were tested only at 5 mg/kg. At this dose, atropine sulfate, but not methylatropine nitrate, increased pituitary cyclic AMP response to immobilization stress demonstrating that the potentiation of the pituitary cyclic AMP stress response was not limited to footshock stress and suggesting that this effect of atropine was central rather than peripheral. Neither atropine nor methylatropine pretreatment at this dose potentiated prolactic response to immobilization stress.

Site of inhibitory action of CRF 9-41 on ACTH release from isolated rat pituitary cells

The corticotropin-releasing factor (CRF) analog CRF 9-41 inhibits CRF, but not forskolin or dibutyryl cyclic AMP, stimulated release of ACTH from isolated pituitary cells. CRF 9-41 also blocks CRF-stimulated accumulation of cyclic AMP in a parallel dose dependent fashion. CRF 9-41 has no effect on basal ACTH release or cAMP levels. This substantiates that the analog acts as a direct CRF antagonist and that the site of this inhibition is most likely at the level of binding of CRF to its receptor on the corticotrope. Various substances, including most prominently glucocorticoids, inhibit release of ACTH from the pituitary (1,2). In an effort to develop another class of inhibitors, Rivier et al (3) recently synthesized analogs of corticotropin releasing factor (CRF). One among these, alpha-helical ovine CRF 9-41 blunts adrenalectomy and stress induced ACTH release in non-anesthetized rats (3,4). At micromolar concentrations, CRF 9-41, shifts rightward the dose response of isolated pituitary cells to ovine CRF (3). Thus, the authors suggested that CRF 9-41 acts as a competitive antagonist to CRF-induced ACTH secretion. CRF appears to act through stimulation of adenylate cyclase (5–7). To determine the potential site of action of CRF 9-41 in the activation sequence for adenylate cyclase, we studied its effects on pituitary cyclic AMP formation and ACTH secretion from dispersed anterior pituitary cells derived from normal adult rats, as well as, its interaction with cyclic nucleotide agonists.

Pituitary cyclic AMP and plasma hormone responses to epinephrine administration in vivo

The present study was conducted to characterize the in vivo effects of epinephrine administration on levels of pituitary cyclic AMP and plasma hormones. Rats were injected with saline or epinephrine bitartrate (1 mg/kg IP) and sacrificed by decapitation 1, 5, 15, 30 or 60 min post- injection. Levels of pituitary cyclic AMP and plasma ACTH, β-endorphin, β-LPH, corticosterone and prolactin were determined by radioimmunoassays. The injection procedure itself was somewhat stressful as demonstrated by increased levels of plasma prolactin and ACTH 5 min following either saline or epinephrine injection. This “stress” response was rapid and short-lasting for the pituitary hormones. The response of the adrenal hormone, corticosterone, to saline injection was slower in onset and longer in duration. Pituitary cyclic AMP levels did not increase following saline injection. Epinephrine-injected animals displayed markedly elevated plasma levels of ACTH, β-endorphin and β-LPH at 15, 30 and 60 min as compared to control or saline-injected rats. In addition, levels of pituitary cyclic AMP were increased over 10 fold at these times. Levels of plasma prolactin, a stress-responsive hormone, were not significantly increased in epinephrine-injected animals as compared to saline-injected rats indicating that these later responses seem to be specific to epinephrine rather than to stress.

Comparison of the effects of CRF and stress on levels of pituitary cyclic AMP and plasma ACTH in vivo

We have previously reported that acute stress increases levels of rat pituitary cyclic AMP in vivo. The present study was conducted to test the hypothesis that stress-induced increases in pituitary cyclic AMP in vivo were mediated via CRF. We compared the effects of various stressors with the effects of CRF or epinephrine administration on pituitary cyclic AMP and plasma ACTH responses in vivo. Stressors, epinephrine or CRF increased levels of pituitary cyclic AMP. Pituitary cyclic AMP response to either immobilization or CRF was much greater at light onset than at lights off in rats maintained on at 12 hr light: 12 hr dark lighting regimen. In rats with pituitary stalk transections, footshock did not increase levels of pituitary cyclic AMP, suggesting that some factor of central origin was required for this stress response. Exogenous CRF administration did increase levels of pituitary cyclic AMP in stalk-transected rats, while epinephrine increased levels in sham-operated but not in stalk-transected rats. Antisera to CRF markedly decreased pituitary cyclic AMP response to exogenous CRF administered 6 min following antisera and partially attenuated pituitary cyclic AMP response to forced running. Taken as a whole these data support a major role for CRF in the pituitary cyclic AMP response to stress.

Diurnal variation in neuroendocrine response to stress in rats: plasma ACTH, beta-endorphin, beta-LPH, corticosterone, prolactin and pituitary cyclic AMP responses.

The effects of restraint stress applied at different times of the day on levels of five stress-responsive plasma hormones (ACTH, beta-endorphin, beta-LPH, corticosterone and prolactin) and pituitary cyclic AMP levels were assessed. Different groups of rats were subjected to 15 min of restraint stress at 2-hour intervals over a 24-hour period. Rats were sacrificed immediately upon removal from their home cage (controls) or immediately following restraint (stressed). The time of day of stress exposure markedly affected the stress responses measured. Generally, responses to stress applied at the beginning of the dark cycle (18:00) were less than those seen following stress applied at the beginning of the light cycle (06:00). Stress at 06:00 increased levels of pituitary cyclic AMP 10-fold, while stress applied at 18:00 did not significantly increase pituitary cyclic AMP levels. In stressed rats, high correlations were seen among levels of hormones derived from the common precursor, proopiomelanocortin (ACTH, beta-endorphin, beta-LPH) and between these hormones and levels of pituitary cyclic AMP. These findings support the hypothesis that pituitary cyclic AMP is involved in the stress-induced release or synthesis of the pituitary hormones ACTH, beta-endorphin, and beta-LPH.

Effect of somatostatin and dopaminergic agents on bovine pituitary phosphodiesterase activity.

The effects of somatostatin and dopaminergic agents on pituitary cyclic nucleotide phosphodiesterases that had been partially purified through DEAE cellulose column chromatography were studied. Somatostatin, L-dopa, dopamine and CB-154(2-bromo-alpha-ergocriptine) competitively inhibited the pituitary cyclic AMP phosphodiesterase activity, especially that of the low Km enzyme. This inhibition was most potent in the case of somatostatin and CB-154.

Beta-Adrenergic Stimulation of Anterior Pituitary Cyclic Amp is Enhanced by Tumor Promoters

Beta-adrenergic stimulation of cellular cyclic AMP accumulation was characterized in normal anterior pituitary cells in vitro. In the presence of isobutylmethylxanthine, the order of potency of catecholamine agonists, as well as the antagonism by propranolol and not phentolamine, aided in classifying the receptor as beta-adrenergic. Furthermore, this agonist effect was rapidly desensitized. Tumor promoters, which directly activate protein kinase C, enhanced the cyclic AMP levels achieved with beta-adrenergic agonists (1.5-fold average at 10 min). This acute effect occurred over 10-1000 nM phorbol dibutyrate or phorbol myristate acetate. Finally, 3H-phorbol dibutyrate binding to unstimulated anterior pituitary cells was predominantly associated with the cytosol (79%) versus membrane (21%) fractions. Thus, an acute role for protein kinase C in promoting beta-adrenergic receptor activation of adenylate cyclase activity is suggested for anterior pituitary cells.

Subsensitive pituitary cyclic AMP response to stress following adrenalectomy is not caused by loss of adrenal epinephrine

We have previously reported that various stressors acutely elevated levels of pituitary cyclic AMP in vivo and that this stress response is not seen in animals tested 7 or 30 days post-adrenalectomy. In this report we present data that demonstrate that the loss of the pituitary cyclic AMP stress response following adrenalectomy is not the result of the loss of stress-induced adrenal epinephrine release. These data show that (1) although administration of epinephrine to intact rats does elevate levels of pituitary cyclic AMP, administration of epinephrine to adrenalectomized animals does not elevate pituitary cyclic AMP levels in vivo ; (2) splanchnic denervation prevents stress-induced adrenal epinephrine release but does not abolish stress-induced increases in pituitary cyclic AMP ; and (3) the time course of the developing subsensitive pituitary cyclic AMP response to stress following adrenalectomy is much slower (2 to 3 days) than the loss of circulating epinephrine.

Tumor promoters enhance basal and growth hormone releasing factor stimulated cyclic AMP levels in anterior pituitary cells

Tumor promoters, such as phorbol esters and teleocidin, amplified the ability of growth hormone releasing factor to increase pituitary cyclic AMP levels. This effect of tumor promoters was concentration-dependent, could be observed in 5 minutes, and was over by 4 hours. Inactive tumor promoters (i.e., 4-alpha-didecanoate) had no effect on this sytem, whereas a synthetic diacylglycerol (i.e., 1-oleoly-2-acetyl glycerol), mimicked the action of tumor promoters. Due to the known stimulation of protein kinase C by both tumor promoters and diacylglycerols, we suggest that this calcium and phospholipid dependent protein kinase C can enhance the ability of the growth hormone releasing factor receptor to activate the cyclic AMP generating system.

Habituation to repeated stress is stressor specific

Rats were exposed to 15 min of restraint or footshock or forced running in an activity wheel once a day for 10 days. Control groups were handled only. On the 11th day, rats from each stressor group and controls were exposed to 15 min of one stressor in a crossed design such that all combinations of one chronic stressor and one acute stressor exposure on the 11th day and plasma corticosterone and prolactin and pituitary cyclic AMP levels were determined. There were no measured differences in these stress indices among groups of rats sacrificed immediately upon removal from their home cage on day 11 regardless of previous history on days 1 through 10. Plasma corticosterone and plasma prolactin and pituitary cyclic AMP levels were elevated in all rats exposed to any of the three stressors immediately prior to sacrifice as compared to all rats not exposed to stress immediately before sacrifice. However, plasma prolactin and pituitary cyclic AMP responses to each of the 3 stressors were attenuated in rats which had previous exposure to that specific stressor as compared to rats which had previous experience with a different or no stressor. We conclude that habituation results from behavioral experience with a particular stressor rather than biochemical adaptation resulting from repeated challenge to hormonal and neurochemical systems responsive to stress.

Adrenalectomy abolishes the stress-induced increase in pituitary cyclic AMP

We have previously demonstrated that various stressors increase pituitary cyclic AMP in vivo in the rat. In the course of studying the mechanisms mediating this response, we examined the effect of bilateral adrenalectomy on footshock-induced increases in pituitary cyclic AMP. In unoperated rats, intermittent footshock markedly increased pituitary levels of cyclic AMP and plasma levels of corticosterone and prolactin. Adrenalectomy completely abolished the stress-induced increase in pituitary cyclic AMP. The marked increase in plasma prolactin following footschock was not affected by adrenalectomy. Our results indicate that adrenal factors are involved in the stress-induced increase in pituitary cyclic AMP.