3-(15-Hydroxypentadecyl)-2,4,4-trimethyl-2-cyclohexen-1-one and its effect on neuropeptide secretion

The aim of the present study was to describe the synthesis of a trimethyl cyclohexenonic long chain fatty alcohol (t-CFA), and analyze its biological activity. Specifically, 3-(15-hydroxypentadecyl)-2,4,4-trimethyl-2-cyclohexen-1-one, the t-CFA containing 15 carbon atoms on the side chain (t-CFA n = 15) stimulated arginine vasopressin secretion in nerve terminals of the neurohypophysis. This effect was inhibited by extracellular calcium depletion, which suggests that t-CFA n = 15 stimulates neuropeptide secretion through a calcium-dependent exocytosis mechanism.

Differential screening of mutated SOD1 transgenic mice reveals early up-regulation of a fast axonal transport component in spinal cord motor neurons

In the present study we analyze the molecular mechanisms underlying motor neuron degeneration in familial amyotrophic lateral sclerosis (FALS). For this, we used a transgenic mouse model expressing the Cu/Zn superoxide dismutase (SOD1) gene with a Gly(86) to Arg (G86R) mutation equivalent to that found in a subset of human FALS. Using an optimized suppression subtractive hybridization method, a cDNA specifically up-regulated during the asymptomatic phase in the lumbar spinal cord of G86R mice was identified by sequence analysis as the KIF3-associated protein (KAP3), a regulator of fast axonal transport. RT-PCR analysis revealed that KAP3 induction was an early event arising long before axonal degeneration. Immunohistochemical studies further revealed that KAP3 protein predominantly accumulates in large motor neurons of the ventral spinal cord. We further demonstrated that KAP3 up-regulation occurs independent of any change in the other components of the kinesin II complex. However, since the ubiquitous KIF1A motor is up-regulated, our results show an early and complex rearrangement of the fast axonal transport machinery in the course of FALS pathology.

Alteration of the Bcl-x/Bax ratio in a transgenic mouse model of amyotrophic lateral sclerosis: evidence for the implication of the p53 signaling pathway

Molecular mechanisms promoting neuronal death in amyotrophic lateral sclerosis (ALS) were investigated using transgenic mice that overexpressed the G86R mutated form of the Cu/Zn superoxide dismutase (SOD1) gene. We observed: (i) alteration of the Bcl-x/Bax ratio and (ii) activation of the transcription factor p53, as deduced from its location within neuron nuclei. We further demonstrated that ectopic expression of the G86R mutant SOD1 in PC12 cells enhanced both p53 expression and phosphorylation, leading to transcriptional stimulation of p53-responsive genes. These findings provide evidence that the p53 signaling pathway is activated in SOD1-linked familial ALS and may play a causative role in spinal cord neuron apoptosis by modulating the Bcl-x/Bax ratio.

A mouse model of familial amyotrophic lateral sclerosis expressing a mutant superoxide dismutase 1 shows evidence of disordered transport in the vasopressin hypothalamo-neurohypophysial axis

Amyotrophic lateral sclerosis (ALS) is a fatal, paralytic disorder that primarily affects motoneurons. By combining physiological and morphological approaches, we examined the effect of a murine superoxide dismutase 1 (SOD1) mutation (G86R), which induces neurological disorders resembling human familial ALS (FALS), on the arginine vasopressin (AVP) hypothalamo-neurohypophysial axis, an unmyelinated tract poor in neurofilaments. First, we observed that G86R mice progressively consumed more water than wild-type littermates. Furthermore, levels of plasma AVP and neurohypophysial AVP content were decreased in the SOD1 mutant mice, whereas the amount of hypothalamic AVP increased in an age-dependent manner. However, hypothalamic AVP mRNA levels were not significantly modified in these animals. At the ultrastructural level, we found that the neurohypophysis of G86R mice had a decreased number of neurosecretory axons. Conversely, the presence of large axon swellings was more pronounced in the SOD1 mutant mice. In addition, the size of neurosecretory granules was higher in G86R than in wild-type animals. All these findings strongly suggest that the FALS-associated SOD1 mutation injures the hypothalamo-neurohypophysial axis by provoking early, progressive disturbances in the axonal transport of neurosecretory products from neuronal perikarya to nerve terminals. This blockade could ultimately result in degeneration of the tract, as proposed for the myelinated, neurofilament-enriched motor axons affected by ALS.

Oxidative stress and a murine superoxide dismutase-1 mutation promoting amyotrophic lateral sclerosis alter neurosecretion in the hypothalamo-neurohypophyseal axis

In this study, we examined the effects of oxidative stress on a nitric oxide (NO)-regulated neuroendocrine function, the release of arginine vasopressin (AVP) by the hypothalamo-neurohypophyseal axis. Treatment of mouse-isolated hypothalami and neurointermediate lobes (NIL) with H2O2 increased AVP release. This effect was inhibited by copper-zinc superoxide dismutase-1 (SOD1) analogs. By measuring cGMP accumulation as an indicator of biologically active NO, we found that H2O2 treatment decreased cGMP formation in both hypothalami and NIL. We have previously shown that NO inhibits AVP release by a cGMP-independent mechanism. Given that H2O2 stimulated AVP release, while it reduced cGMP production, our findings strongly suggest that oxidative damage affects neurosecretion by reducing NO availability. To test whether such a mechanism may operate under pathological conditions with pronounced oxidative stress, we compared neurosecretion in wild-type and transgenic mice carrying a mutated form of SOD1 associated with human familial amyotrophic lateral sclerosis. Reminiscent of the data obtained from H2O2-treated tissues, hypothalami and NIL from SOD1 mutants displayed decreased cGMP accumulation and increased AVP release, compared with tissues from wild-type littermates. Since neuronal NO synthase expression was not modified, we conclude that the perturbed free radical metabolism associated with the SOD1 mutation is likely to trap NO, and thereby alter neurosecretion, a mechanism that can be exacerbated in specific physiopathological conditions.

Continuous activation of pituitary adenylate cyclase-activating polypeptide receptors elicits antipodal effects on cyclic AMP and inositol phospholipid signaling pathways in CATH.a cells: role of protein synthesis and protein kinases

Continuous exposure of cells to agonists develops a process that determines the extent to which the cells eventually respond to further stimuli. Here we used CATH.a cells (a catecholaminergic neuron-like cell line), which express pituitary adenylate cyclase-activating polypeptide (PACAP) receptors linked to both adenylyl cyclase and phospholipase C-beta pathways, to investigate the influence of prolonged hormonal treatment on dual signaling and gene transcription. Prolonged incubation of cells with PACAP failed to down-regulate the density and affinity of membrane binding sites and caused opposite changes in messenger systems: PACAP-stimulated cyclic AMP accumulation was attenuated in a time- and dose-dependent fashion (t(1/2) = 6.7 h and IC50 = 0.1 nM), whereas phosphoinositide turnover was overstimulated. Both effects were insensitive to pertussis toxin, whereas the drop in cyclic AMP concentration was also unchanged in the presence of 3-isobutyl-1-methylxanthine, indicating that neither Gi-like proteins nor cyclic nucleotide phosphodiesterases play a critical role in these processes. Blockade of protein synthesis with cycloheximide, as well as inhibition by H89 of cyclic AMP-dependent protein kinase (but not by bisindolylmaleimide of protein kinase C) antagonized the influences exerted by PACAP on adenylyl cyclase activity and inositol phosphate formation. Transcription of the chimeric GAL4-CREB construct, transiently transfected into CATH.a cells, was stimulated by PACAP, and this effect was potentiated as a result of chronic PACAP treatment. The results of the present investigation provide new insight into the possible differential regulation and cross-talks of transduction signals of receptors linked to multiplex signaling. They demonstrate that prolonged exposure of CATH.a cells to PACAP results in the desensitization of the cyclic AMP pathway and superinduction of the inositol phosphate signal, through protein neosynthesis and cyclic AMP-dependent protein kinase activation. At the same time, they show that desensitization of cyclic AMP signaling not only fails to hamper, but actually amplifies PACAP-stimulated CREB-regulated transcription.

Evidence for the presence of receptors for pituitary adenylate cyclase-activating polypeptide in the neurohypophysis that are positively coupled to cyclic AMP formation and neurohypophyseal hormone secretion

This study examines the neural lobe of the pituitary gland for the presence of receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) and their possible involvement in the regulation of neurosecretion. The presence of PACAP receptors of type I was revealed in the neural lobe, as well as in anterior and intermediate lobes, by means of RT-PCR amplification using selective oligonucleotide pairs of primers. They appeared to be expressed in the tissues as a short form together with an isoform of heavier molecular weight. Activation of receptors in the presence of PACAP stimulated both formation of cyclic AMP (cAMP) and secretion of arginine vasopressin (AVP) in neural lobes, in a dose-related fashion, with half-maximum (EC50) values of 1.0 +/- 0.2 x 10(-9) M and 1.4 +/- 0.3 x 10(-8) M, respectively. Parallel with AVP, PACAP also stimulated oxytocin (OXT) output, with an EC50 value of 0.6 +/- 0.1 x 10(-8) M. In an attempt to localize receptors on cells (mainly astrocyte-like glials or pituicytes) and/or on nerve fibers of the gland, we used cultures of neural lobe cells and explants (in which nerve fibers undergo degeneration), as well as isolated nerve endings. In both cells and nerve terminals, PACAP enhanced accumulation of cAMP, while it triggered AVP secretion from the latter. The stimulatory effect of PACAP on both AVP and OXT release was mimicked by dbcAMP and blocked by H89, an inhibitor of cAMP-dependent protein kinase. We conclude that in the neural lobe, PACAP receptors are localized on both nerve terminals and pituicytes, which participate in the modulation of secretion of neurohypophyseal hormones in an interactive way and mainly through the cAMP signalling route.

Pituitary adenylate cyclase-activating polypeptide induces expression of corticosteroid-binding globulin in cultured fetal hepatocytes: synergy with tri-iodothyronine

The purpose of the present study was to determine whether functional receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) are expressed in cultured rat fetal hepatocytes and eventually play a role in regulating gene expression of corticosteroid-binding globulin (CBG). We found PACAP38 and PACAP27 to elevate cAMP levels in hepatocytes in a dose-dependent manner, with a plateau being achieved at 10 nM and EC50 values of about 0.5-1 nM. PACAP failed to alter the turnover of inositol phosphates, whereas PACAP and VIP stimulated cAMP accumulation in an equipotent manner, suggesting the presence in these cells of type II receptor isoforms. As revealed by measurements of both CBG mRNA levels and concentrations of binding sites, long-term treatment of fetal cells with 10 nM PACAP, although resulting in partial desensitization of peptide-induced cAMP accumulation, caused a significant 3-fold elevation in CBG synthesis. This stimulatory influence of PACAP was mimicked by the cell permeant N6,2'-O-dibutyryladenosine 3',5'-phosphate (dbcAMP). Treatment of hepatocytes with tri-iodothyronine (T3) enhanced CBG expression and, most interestingly, appeared to synergize with PACAP to elicit a 2-3-fold amplification of CBG synthesis. This study thus provides first evidence for the up-regulation by PACAP and cAMP of CBG expression in fetal hepatocytes and for T3's playing a synergistic role in enhancing PACAP-induced synthesis of the binder.

Multifactorial regulation of pituitary adenylate cyclase-activating polypeptide (PACAP)-induced production of cyclic AMP in ATT-20 corticotrophs: major involvement of Rolipram-sensitive and insensitive phosphodiesterases

Cyclic nucleotide phosphodiesterases (PDEs) appear to play a major role in the modulation of cellular accumulations of cAMP/cGMP and hence the magnitude of the cell response to a hormone signal. These enzymes are present in cells as multiple isoforms and lie under control of various protein kinases. Because PACAP, unlike corticotropin-releasing factor (CRF), may stimulate a dual signalling pathway in pituitary cells (activating both adenylyl cyclase and phospholipase C), we used AtT-20 corticotrophs and primary cultures of rat pituitary cells to study the effect and possible differential influence of these peptides on cAMP formation. Time-course analysis indicated that, both in the absence and the presence of Rolipram (a selective type IV PDE inhibitor), PACAP stimulated a rapid and short-lived accumulation of cAMP in tumor corticotrophs, while in the presence of the non-selective inhibitor IBMX, the peptide produced a sustained high plateau level of second messenger (10 times the level generated with Rolipram at 20 min). On the contrary, when exposed to CRF, cAMP production augmented in parallel, irrespective of whether Rolipram or IBMX were present. The differential effects of the PDE inhibitors were seen with PACAP concentrations ranging from 0.1 to 100 nM, and could also be demonstrated in primary cultures of pituitary cells. Co-incubation of AtT-20 cells with Rolipram along with inhibitors of type I (but not of type III) PDEs, enhanced cAMP formation elicited by PACAP to a level significantly higher than that induced by CRF.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of protein phosphatases by okadaic acid and calyculin-A differentially modulates hormonal- and forskolin-stimulated formation of cyclic AMP in AtT-20 corticotrophs: effect of pituitary adenylate activating polypeptide and corticotropin-releasing factor

The effect of phosphatase inhibitors okadaic acid and calyculin-A on cAMP formation and adrenocorticotropic hormone (ACTH) secretion in AtT-20 corticotrophs was investigated. Both okadaic acid and calyculin-A inhibited dose-dependently the accumulation of cAMP in cells stimulated with pituitary adenylate cyclase activating factor (PACAP) and corticotropin-relating hormone (CRF). While in the case of okadaic acid the half-maximum inhibiting concentration was similar for both peptides (IC50 = 4 x 10(-7) M), it appeared that calyculin-A was about one order of magnitude more efficient in inhibiting the effect of PACAP than that of CRF (IC50 = 3.8 x 10(-9) M vs 2.0 x 10(-8) M, respectively). Importantly, the inhibitors blocked the activation by cholera toxin (which acts on Gs-like proteins) of cAMP formation, but failed to alter the effect of forskolin (which bypasses the receptor-G protein complex and activates adenylyl cyclase directly). Treatment of cells with calyculin-A significantly dampened adenylyl cyclase activity in cell membrane fraction, though to a lesser extent than it blocked cAMP formation in the whole cell. Both okadaic acid and calyculin-A inhibited CRF- and PACAP-induced secretion of ACTH. Our data hint that in AtT-20 corticotrophs, inhibition of phosphatases by modulating the state of phosphorylation of the receptor-G proteins complexes for CRF and PACAP, regulates cAMP formation and ACTH secretion.

Evidence for an inhibitory effect of nitric oxides on neuropeptide secretion from isolated neural lobe of the rat pituitary gland

The present study aims at investigating the effect of pharmacological manipulation of nitric oxides (NOs) formation in the rat neurohypophysis on the secretion of vasopressin (AVP). We found that the NO synthase antagonist L-NAME and free-ferrous hemoglobin (an NO inactivator) produced a transient and significant enhancement of basal secretion of AVP from incubated glands. Conversely, the NO precursor L-arginine (but not its inactive counterpart D-arginine) antagonized the stimulatory influence of L-NAME on both AVP and oxytocin (OT) output. Elevation of NOs formation triggered by means of the NO donor SIN-1 likewise dampened spontaneous, as well as stimulated, AVP release. It is concluded that NOs molecules show up as potent regulators of neuropeptide secretion at the level of nerve terminals in the neurohypophysis.

Vasopressin, unlike phorbol ester, fails to synergistically interact with pituitary adenylate cyclase activating polypeptide (PACAP) in stimulating cyclic AMP formation and ACTH secretion in cultured anterior pituitary cells

In an attempt to determine if PACAP synergistically interacts with vasopressin (VP) and protein kinase C (PKC) to enhance cyclic AMP formation and adrenocorticotrophic hormone (ACTH) secretion, the effects of PACAP, either alone or together with VP and the phorbol ester phorbol 12-myristate 13-acetate (PMA) were examined in primary cultures of rat anterior pituitary cells. VP failed to potentiate the stimulatory effect of PACAP on cyclic AMP formation, while it dramatically enhanced the effect of corticotropin-releasing factor (CRF). However, activation of PKC upon exposure of cells to PMA amplified cyclic AMP production induced by both peptides, though in the case of PACAP, contrary to that of CRF, potentiation was markedly dependent on the blockade of phosphodiesterase (PDE) activity, for it was undetectable in the absence of the inhibitor Rolipram. Depletion of PKC by long-term treatment of pituitary cells with PMA abolished the synergistic influence of PMA. There was no significant effect of PACAP, either alone or together with PMA, on ACTH secretion, while PMA enhanced peptide secretion elicited by CRF. The data show that in anterior pituitary cells cyclic AMP accumulation induced by PACAP and CRF was differentially modulated by PKC and PDE activities and that the potentiation of PACAP-stimulated cyclic AMP accumulation by PMA was not reflected by parallel increment of ACTH secretion.

Pituitary adenylate cyclase polypeptide (PACAP) stimulates cyclic AMP formation in pituitary fibroblasts and 3T3 tumor fibroblasts: lack of enhancement by protein kinase C activation

A number of neuropeptides were shown to produce potent mitogenic effects on Swiss 3T3 fibroblasts by activating the phospholipase C pathway. Here we provide evidence for the activation by PACAP of the adenylate cyclase pathway in 3T3, as well as in non-tumoral pituitary fibroblasts, similarly to what was seen in pituitary endocrine cells. In these cells, PACAP triggered elevation of both intracellular and extracellular contents of cAMP and the effect was time- and dose-dependent, with half-maximal stimulations being induced with about 0.1 nM. Following activation of protein kinase C (PKC) by the phorbol ester phorbol 12-myristate 13-acetate (PMA), PACAP-induced cAMP production was amplified in pituitary endocrine cells, but was either unchanged or dampened in 3T3 and pituitary fibroblasts, respectively. Pretreatment of cells with pertussis toxin (PT) failed to change the effect of PMA on PACAP-stimulated adenylate cyclase activity, irrespective of the cell type being used. However, PT dramatically reduced the potentiation by PMA of cAMP production enhanced by forskolin in 3T3 cells. These results provide new evidence pointing to the presence in fibroblasts of receptors for PACAP, coupled to cAMP production, which may play a role in the modulation of the mitogenic signal. They also indicate that, compared with pituitary endocrine cells, PKC activation in fibroblasts differentially affected PACAP-induced cAMP formation and that these effects were unaltered upon inhibition by PT of Gi-like proteins.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates cyclic AMP formation as well as peptide output of cultured pituitary melanotrophs and AtT-20 corticotrophs

The present study was aimed at investigating whether PACAP stimulates accumulation of cAMP, as well as hormonal secretion of homogeneous populations of pituitary proopiomelanocortin (POMC) cells, namely melanotrophs and AtT-20 corticotrophs. PACAP was shown to enhance cAMP accumulation in a dose-dependent fashion in both cell types (with EC50 values of approx. 10(-10) M) and elicited additive increases of cAMP production with CRF in melanotrophs, but not in corticotrophs. PACAP also stimulated dose-dependently the secretion of alpha-MSH and ACTH, with EC50 concentrations of about 10(-9) M. In melanotrophs, bromocriptine significantly depressed PACAP-induced cAMP formation and blunted by more than 90% stimulated alpha-MSH release. This study shows that (1) pituitary POMC cells did respond to PACAP by enhancing cAMP accumulation and elevating hormone secretion as well; (2) the effect of PACAP was additive with CRF on cAMP production in melanotrophs, but not in corticotrophs, while there was no additivity on peptide output from both cell types; (3) activation of dopamine receptors in melanotrophs dampened both cAMP formation and peptide secretion. These findings are consistent with PACAP playing a possible hypophysiotropic role in the regulation of pituitary POMC cell activity.

Arginine-vasopressin in anterior pituitary cells: in situ hybridization of mRNA and ultrastructural localization of immunoreactivity

The hypothalamic nonapeptide arginine-vasopressin (AVP) exerts several distinct receptor-mediated actions on pituitary cells. Although hypothalamic AVP reaches the anterior pituitary via well-defined pathways, there is now accumulating evidence that AVP may also be produced endogenously in anterior pituitary cells. Using in situ hybridization, we demonstrate here the presence of AVP mRNA in the anterior pituitary of the rat. The observed grain density over pituitary cells was, however, greater than 10-fold lower than the one observed over AVP producing neurons present in the supraoptic and paraventricular nuclei of the hypothalamus. Immunoelectron microscopic analysis using two different AVP-specific antibodies revealed that the distribution of AVP-like immunoreactivity (AVP-LI) in the anterior pituitary is cell-specific. AVP-LI is most abundant in corticotrophs, followed by lactotrophs, gonadotrophs and thyrotrophs. On the other hand, there is complete absence of AVP-LI from somatotrophs. Interestingly, all pituitary cells in which AVP-LI is detected also represent potential target sites for AVP action. A minor fraction of AVP-LI was found to be membrane-associated and may originate, at least in part, from extrapituitary sources. This fraction likely represents receptor-bound peptide. The bulk of AVP-LI, however, was present in the cellular cytoplasm, not associated with any specific ultracellular structure. Specifically in corticotrophs, AVP-LI was excluded from secretory granules. However, our finding of AVP mRNA in anterior pituitary cells indicates that intracellular AVP-LI includes endogenously produced peptide, suggesting a paracrine and/or autocrine action.

Inhibition of protein kinase C activity in cultured pituitary cells attenuates both cyclic AMP-independent and -dependent secretion of ACTH

The present study examines the effect of reduction of protein kinase C (PKC) activity, as induced by either phorbol ester (PMA) down-regulation or staurosporine inhibition, on the secretion of ACTH from cultured anterior pituitary (AP) cells. Short-term (3 h) exposure of cells to 5 nM PMA resulted in almost complete desensitization to both PMA and vasopressin (AVP), while there was only a minor incidence on the effect of corticotropin-releasing factor (CRF). In contrast, long-term (12-24 h) exposure of cells to PMA, as well as pretreatment with staurosporine, dramatically reduced the stimulatory influence of CRF. This was shown not to be due to a decline in ACTH cells' stores, nor to the toxicity of phorbol ester or to a negative autofeedback of ACTH. Pretreatment of corticotrophs with PMA failed to dampen the CRF-induced cyclic AMP formation, while it caused a decline in the effects of forskolin and 8-bromoadenosine cyclic AMP. Stimulated ACTH secretion subsequent to either veratridine- or high K(+)-induced cell depolarization was likewise decreased. We conclude that in corticotrophs the stimulatory action of not only AVP, but also of that of CRF on ACTH secretion strongly relies on PKC activity. In the case of CRF, however, this may not be a primary consequence of receptor occupation, as evidence suggests an indirect relationship which may involve PKC regulation of Ca2+ channels and/or the ion's intracellular messenger function.

Activation of protein kinase C differentially regulates corticotropin-releasing factor-stimulated peptide secretion and cyclic AMP formation of intermediate and anterior pituitary cells in culture

The present study was aimed at investigating the effect of protein kinase C (PKC) activation on CRF receptor function of proopiomelanocortin (POMC) cells in culture. Incubation of tissues with the phorbol ester PMA selectively potentiated corticotropin-releasing factor (CRF)-stimulated ACTH secretion and cyclic AMP formation of anterior pituitary (AP) cells, while, in sharp contrast, it failed to similarly affect intermediate pituitary (IP) cells and AtT-20 corticotrophs exposed to CRF. Unexpectedly, however, long-term treatment of cultures with PMA, which depletes cell stores of PKC, resulted in a similar dramatic attenuation of stimulated peptide release from both corticotrophs and melanotrophs, while being without significant effect on cyclic AMP production. Exposure of cells to PMA did not change either basal or CRF-enhanced levels of POMC mRNA. We conclude that activation of PKC fails to synergize with CRF-mediated signalling in IP and AtT-20 cells, although optimal CRF receptor expression requires the presence of a functional kinase C pathway, thus suggesting cross-talks between both messenger systems.

The vasopressin receptor system in the neonatal pituitary gland: evidence for reduced binding capacity and signal transmission

The present study was aimed at evaluating the capacity of anterior pituitary cells from neonatal rats to bind arginine vasopressin (AVP) and show AVP-receptor-mediated signal transmission. We found that in cultures of pituitary cells of 10-day-old pups, in contrast to cultures of cells of adults, AVP was unable to trigger sustained adrenocorticotropin (ACTH) secretion and, in addition, was also less potent in synergizing with the effect of corticotropin-releasing factor (CRF) on both ACTH output and cyclic AMP formation. Binding studies revealed the existence of a much lower number of AVP receptor sites in membranes of neonatal pituitary gland than in those of adult tissue (32.3 +/- 9.0 and 137.6 +/- 6.2 fmol/mg protein, respectively), although the binding of agonists and the apparent molecular weight (Mr about 120,000) of the receptors were similar. Activation by phorbol ester PMA of protein kinase C, a messenger involved in AVP action, resulted in a dose-related enhancement of ACTH secretion that was 2-3 times smaller for immature corticotrophs than for mature ones. Importantly, PMA treatment allowed AVP to significantly stimulate ACTH secretion from neonatal cells, while it failed to similarly affect AVP-evoked hormone output from adult tissue. Our results indicate that pituitary corticotrophs of rat pups fail to properly transduce AVP-receptor-mediated signalling and, thereby, suggest an explanation for the postnatal 'stress nonresponsive period'.

Indirect evidence that protein kinase C plays a critical role in signal transduction of both vasopressin and corticotropin-releasing factor on pituitary cells in culture

The possible role of protein kinase C (PKC) in the cyclic AMP-dependent mechanism of action of corticotropin-releasing factor (CRF) on proopiomelanocortin cells of anterior and intermediate pituitary glands was examined after pretreatment of cells in culture with the PKC inhibitor retinal or the phorbol ester PMA, which depletes cell stores of the kinase. We found that these drugs not only abolished ACTH response to PMA and vasopressin, which both activate PKC, but unexpectably also dampened by 80-90% the stimulatory effect of CRF. Cell treatment with retinal failed to prevent CRF-induced accumulation of cyclic AMP. Retinal and PMA pretreatments of intermediate pituitary cells likewise inhibited alpha-MSH secretion stimulated by CRF. These data provide evidence to suggest that the mechanism of action of CRF on pituitary cells involves both cyclic AMP and PKC messenger systems.

Stimulation by atrial natriuretic factor of cyclic GMP production in cultured anterior and intermediate pituitary tissues: evidence for a major contribution of proliferating nonendocrine cells

Primary cultures of anterior and intermediate pituitary tissues were monitored immunocytochemically for the presence of endocrine and nonendocrine cells and simultaneously tested for their ability to produce cyclic GMP in response to atrial natriuretic factor (ANF). Cells cultured for 3 days and 6 days, in which nonendocrine (vimentin-positive) cells were found to rapidly overgrow the endocrine cells, showed a dramatic elevation in cyclic GMP production stimulated by ANF, with maximum stimulation 300-700% that seen in 1-day cultured cells. Also, ANF-induced accumulation of cyclic GMP in an enriched population of vimentin-positive cells appeared to closely match that triggered in a 3-day culture of anterior pituitary cells, emphasizing the major role played by nonendocrine cells and their ability to synthesize cyclic GMP. In contrast, in the homogeneous population of tumor corticotrophs AtT-20, there was a close relationship between cyclic GMP formation and cell density. It thus appears that contamination of primary cultures of anterior and intermediate pituitary tissues by proliferating nonendocrine cells (mainly fibroblasts), in which ANF-induced accumulation of cyclic GMP may be confused with that of the very secretory cells, leads to overestimation and masking of guanylate cyclase activity of endocrine cells.

Corticotropin-releasing factor-like immunoreactivity, arginine vasopressin-like immunoreactivity and ACTH-releasing bioactivity in hypothalamic tissue from fetal and neonatal sheep

Corticotropin-releasing factor-like immunoreactivity (CRF-LI), arginine vasopressin-like immunoreactivity (AVP-LI) and ACTH-releasing bioactivity of hypothalamic tissue from 63- to 143-day-old ovine fetuses and from 7- and 150-day-old lambs have been assessed. CRF-LI and AVP-LI contents and concentrations increased steadily between 63 and 138 days of gestation, decreased at day 143 then rose again in postpartum animals. In terms of concentration, the AVP-LI/CRF-LI ratio remained close to 5 between 63 and 123 days of intrauterine life, decreased steadily in late gestation down to 1.2 at day 143 increased again in lambs. The total hypothalamic ACTH-releasing bioactivity increased 26-fold between 63 and 138 days of gestation and remained constant until day 143. The highest values were observed in 150-day-old-lambs. These data support the view that AVP and oCRF1-41 might be important hypothalamic factors involved in the regulation of ACTH release by the pituitary gland of the sheep fetus. In addition, they suggest that AVP is more important in young fetuses and lambs than in prepartum animals.

Indirect relationship between vasopressin-induced secretion of ACTH and cyclic nucleotides in cultured anterior pituitary cells

The present study examines whether a correlation exists between cyclic nucleotides and the mechanism of action of arginine vasopressin (AVP) on adrenocorticotropin hormone (ACTH) secretion from pituitary corticotrophs. Incubation of cultured anterior pituitary cells with 3-isobutyl-1-methylxanthine (IBMX) or Rolipram elevated the basal intracellular content of both adenosine 3':5'-cyclic monophosphate (cAMP) and guanosine 3':5'-cyclic monophosphate (cGMP) or cAMP alone, respectively. Both IBMX and Rolipram enhanced the AVP-stimulated secretion of ACTH in cultured anterior pituitary cells, but not in AtT-20 corticotrophs which lack functional AVP receptors. Rolipram was less potent than IBMX in this regard, which suggests a possible involvement of cGMP. In contrast, both drugs showed similar potency to stimulate CRF-induced ACTH secretion. Incubation of pituitary cells with atrial natriuretic factor elevated tissue cGMP levels and increased the ACTH response to AVP. The results of this study show that, although AVP fails to directly affect the levels of cAMP and cGMP in anterior pituitary cells, the stimulatory effect of AVP on ACTH secretion was modulated by the cellular cAMP content.

Binding and effect of atrial natriuretic factor on cyclic GMP formation and alpha-MSH secretion of intermediate pituitary cells

The present study shows for the first time that in proopiomelanocortin cells of the rat intermediate pituitary gland ANF binds to two receptor forms, with apparent molecular weights of 150K and 70K. Scatchard plots revealed specific and high affinity non-interacting sites, with a KD value of about 3 nM and a density of 7,000 sites/cell. The presence of these binding sites was further confirmed by autoradiographic studies. Activation of these receptors led to an increase in cellular content of cGMP, with half-maximal effect being elicited with about 5 nM ANF, while cAMP formation was unaltered. Alpha-MSH secretion of intermediate pituitary cells was unaffected by ANF, whether the cells were incubated in the absence or presence of corticotropin-releasing factor or bromocryptine. These data thus indicate the presence of multiple ANF receptor sites in the intermediate pituitary which are coupled to cell production of cGMP, but independent of alpha-MSH secretion.

Characterization of high affinity receptor sites for atrial natriuretic factor in anterior pituitary gland: evidence for the existence of two receptor forms

The present study shows that in rat anterior pituitary tissue atrial natriuretic factor (ANF) binds to two distinct receptor forms, with apparent molecular weights of about 166K and 58K. Binding assays carried out with (125I)-ANF revealed specific and high affinity non-interacting binding sites, with Ko values of 1-1.7 nM and a density of 10-15,000 sites/cell. rANF fragments (5-25), (5-27) and (5-28) exhibited apparent equipotency in displacing tracer binding, while fragment (13-28) and various other peptides were ineffective. ANF (5-25) was about 100-times less potent than ANF (8-33) in stimulating half-maximum pituitary cGMP production. These data indicate the presence of multiple binding sites for ANF in the pituitary gland and suggest that only part of these sites may be coupled to activation of guanylate cyclase.

Evidence that AVP receptors in AtT-20 corticotrophs are not coupled to secretion of POMC-derived peptides

This study reports the presence in AtT-20 corticotrophs of high affinity-low capacity receptors for arginine-vasopressin (AVP), whose binding capacity was considerably enhanced by the divalent metal ion nickel. These binding sites, when analyzed in the presence of nickel, showed high affinity for AVP, vasotocin and oxytocin, but recognized to a lesser extent the V2-agonist 1-deamino-AVP, as well as V1-antagonists. Surprisingly, AVP failed to alter secretion of proopiomelanocortin (POMC)-derived peptides from the cells or corticotropin-releasing factor (CRF)-induced cAMP synthesis, as reported in normal corticotrophs. Exposure of cells to CRF elicited an increase in mRNAPOMC levels, while, in contrast, AVP was without significant effect. It thus appears that in AtT-20 tumor cells, the AVP receptors are not coupled to either the biochemical or biological cellular response.

Characterization and modulation of corticotropin-releasing factor in the neurointermediate pituitary gland

The present study attempts to determine whether part of the corticotropin-releasing factor (CRF)-like materials present in the 'posterior pituitary' is composed of authentic CRF and examines whether the concentration of that peptide may be modulated by circulating glucocorticoids. Analysis of crude extracts of neurointermediate lobes (NIL) of rat pituitaries by reverse-phase HPLC, coupled with a specific radioimmunoassay (RIA), revealed the presence of a major component eluting with the same retention time as rat CRF (rCRF) and, importantly, which was indistinguishable by RIA from the synthetic peptide. Also, two minor forms eluted earlier than rCRF upon HPLC; one of these forms matched the elution position of r[Met(O)21,38]CRF. All three species did show biological activity and stimulated ACTH release from pituitary cells. Essentially the same elution profile was generated by median eminence (ME) extracts. Immunoreactive CRF (CRFi) content of the NIL was about 3% of that of the ME and was found to undergo a significant increase as a result of long-term adrenalectomy while, in contrast, CRFi content of the ME was decreased. This effect of adrenalectomy was completely antagonized by dexamethasone treatment. This study thus provides strong evidence for the presence of authentic CRF within the NIL of the rat pituitary and also shows that tissue concentration of that peptide was modulated by glucocorticoids.

Vasopressin and oxytocin levels in human neonates. Relationships with the evolution of labour and beta-endorphins

In order to find out whether arterial and venous cord levels of vasopressin (VP) and oxytocin (OT) might be linked to one or more obstetric parameters and to beta-endorphin (BEP) secretion, 42 successively delivered neonates were studied. Arterial and venous cord blood levels of these peptides were not statistically different whenever the neonates were born vaginally with or without foetal distress, after induction of labour by oxytocic drugs, or by elective caesarean section. BEP levels in cord and maternal blood do not seem to be linked with AVP or OT. The results of the group of infants born after uncomplicated vaginal delivery analyzed with regard to obstetric parameters, led to the following conclusions: arterial cord VP correlated with venous cord VP, with arterial cord OT and with the duration of membrane rupture; arterial cord OT correlated with venous cord OT and with the time taken by the cervix to dilate from 5 to 10 cm, suggesting that the foetal pituitary gland is sensitive to the evolution of labour.

Central nervous system control of pituitary vasopressin receptors: evidence for involvement of multiple factors

The regulation of pituitary vasopressin (VP) receptor concentration was investigated in rats with antero-lateral cuts (ALC) placed around the hypothalamus, as well as in Brattleboro homozygotes (HO) that genetically suffer from a lack of AVP. Hypothalamic ALCs caused a reduction in (3H)-AVP binding, while counteracting the dramatic fall in binding that normally occurs after adrenalectomy. Surprisingly, in HO rats, long-term adrenalectomy did cause pituitary AVP receptor number to decrease to an extent similar to that seen in normal rats. However, the receptor disappeared twice as rapidly in heterozygote controls than in HO animals, with calculated half-lives of 1.1 and 2.0 days, respectively. In HO, chronic administration of VP reduced receptor concentration by about 80%, while the same dose of oxytocin (OT) produced only a 20-30% reduction. Whereas dexamethasone injections did reverse the depressing effect of adrenalectomy on pituitary AVP receptors, they failed to enhance binding in sham-operated controls, treated or not with VP; thereby suggesting a central site of action of the steroid. In contrast, in rats with hypothalamic ALCs (i.e. with the pituitary lacking central control), corticosterone implants did antagonize the reduction in receptor density caused by adrenalectomy. We conclude that the pituitary AVP receptor system lies mainly under control of the central nervous system, through a mechanism of action that not only seems to imply AVP and OT, but probably also some other hypothalamic factor(s). Glucocorticoids appear to exert a dual effect, acting indirectly through negative feedback control of neuropeptide release and, possibly, also directly on the pituitary to regulate binding sites.

Modulation by Leu-enkephalin of peptide release from perifused neurointermediate pituitary. II. Inhibition of calcium-mediated secretion of alpha-MSH and beta-endorphin

The present study examines the effect of opiates on alpha-MSH and beta-endorphin release from perifused neurointermediate rat pituitaries, as stimulated by various secretagogues for which Ca ions and/or cAMP serve as messengers. alpha-MSH release stimulated by high K+ concentrations (5-min pulses) and veratridine depolarization, which is closely dependent on Ca2+ fluxes, was abolished by both Leu-enkephalin and beta-endorphin. A dose-response relationship between inhibition of alpha-MSH secretion and the concentration of Leu-enkephalin, with ED50 approximately 10(-9) M, was observed. High K+-induced release of beta-endorphin was likewise blunted by Leu-enkephalin. The stimulatory effect of the Ca2+ ionophore A 23187 was inhibited in a similar way as was that of CRF, which requires both Ca2+ fluxes and cAMP formation. The antagonist naloxone not only reversed the action of opiates, but also enhanced spontaneous hormonal output. In contrast, the effects of l-isoproterenol and forskolin, for which cAMP serves as a primary messenger, were unaffected in the absence of extracellular Ca ions and, also, in the presence of Leu-enkephalin. We conclude that opioid peptides may exert a direct inhibitory influence on the release of both alpha-MSH and beta-endorphin and do so by interfering with the Ca2+ messenger system. In addition, these data also suggest the existence of an opiate-opiate negative feedback mechanism.

Vasopressin secretion as a possible target of the gamma-aminobutyric acid-mediated component of the corticosteroid feedback effect

The inhibition of gamma-aminobutyric acid (GABA) synthesis did not interfere with the suppressive effect of dexamethasone on the stress-induced rise of plasma corticosterone levels in vasopressin-deficient homozygous Brattleboro rats. In dexamethasone-treated heterozygous rats corticosterone and vasopressin secretion increased after stress provided GABA synthesis was inhibited. The results indicate that inhibition of corticotrophin secretion by corticosteroids may in part be mediated by enhancement of GABA synthesis and a consequent inhibition of vasopressin release.

Specific receptors for vasopressin in the pituitary gland: evidence for down-regulation and desensitization to adrenocorticotropin-releasing factors

Specificity of binding of 3H-labeled arginine vasopressin [( 3H]AVP), down-regulation of receptors, and desensitization were studied in anterior pituitary glands of both Wistar and Brattleboro rats. Studies using both crude membrane fractions and isolated cells of anterior pituitaries revealed the presence of a single population of binding sites with a Kd of approximately 1 nM. The receptor recognized the following peptides, with AVP = lysine vasopressin = vasotocin greater than oxytocin = 1-deamino-(8-D-AVP) greater than d-(CH2)5-Tyr-(Me)-Val4-AVP greater than 1-deaminopenicillamine-(Val4-D-Arg8)VP. Neither corticotropin-releasing factor (CRF) nor any of the neuropeptides tested, including AVP ring and tail fragments, competed for tracer binding. Increased extracellular vasopressin levels due to chronic injections or long term adrenalectomy decreased receptor density by 80%, while oxytocin was less effective than AVP. Comparing binding data in Brattleboro homozygotes and heterozygotes revealed that AVP levels within the physiological range could down-regulate pituitary receptors as well. This could not be caused by occupation of sites by endogeneous vasopressin, since injection of large doses of peptide decreased tracer binding by less than 10%. Loss of pituitary receptors reduced 1) enhancement by AVP of CRF-induced cAMP accumulation, 2) intrinsic CRF-like activity and 3) synergistic effect of AVP on ACTH secretion elicited by CRF. This study thus provides evidence for the presence of highly specific vasopressin receptors in the anterior pituitary, which may undergo homologous down-regulation and desensitization in terms of cAMP production and ACTH release.

Modulation by leu-enkephalin of peptide release from perifused neurointermediate pituitary. I. Selective effect on potassium-, veratridine- and isoproterenol-stimulated secretion of vasopressin

This study examines the effect of leu-enkephalin on K+, veratridine and isoproterenol stimulation of vasopressin (AVP) release from perifused neurointermediate pituitaries of rats. As opposed to catecholamine-evoked release of peptide, the secretory response to high K+ and veratridine involves Ca2+ influx, as the effect of both factors was blocked by Ca-chelation and the channel blocker D 600. Leu-enkephalin was found to antagonize AVP secretion induced by K+ and veratridine depolarization, by acting through a naloxone-sensitive receptor system. In contrast, the opiate failed to significantly affect isoproterenol-stimulated release of AVP, which we show to be correlated to cAMP accumulation in pure neurohypophyseal tissue. These results support the view that opiates modulate AVP secretion triggered by depolarization of nerve terminals by regulating Ca2+ fluxes.

Paradoxical involvement of glucocorticoid receptors in the aldosterone-induced impairment of ACTH secretion from perifused pituitary glands

The present investigation was aimed at examining whether interaction of aldosterone with specific mineralocorticoid receptors at the level of the pituitary gland may account for the inhibitory effect of that steroid on ACTH secretion. By using pituitaries from neonatal rats, which we show to completely lack specific mineralocorticoid receptors but to contain a functional glucocorticoid receptor system, we demonstrated the persistence of aldosterone-induced inhibition of ACTH release from perifused glands. Conversely, when the glucocorticoid receptors sites were blocked in pituitaries from mature rats by means of a potent antiglucocorticoid (RU 38486), thus leaving unaltered mineralocorticoid binder, aldosterone no longer dampened hormonal output. We conclude that the latter steroid affected corticotropic activity by interacting not with its proper and specific receptor, but rather with the glucocorticoid binding sites.

Endocrine concomitants of sweating and sweat depression

The effect of humid heat (Ta = 43 degrees C, Pa = 32 Torr) on sweat rate, plasma renin activity and plasma levels of aldosterone and antidiuretic hormone (ADH) was studied in four male subjects before and after repeated heat exposures. Over-sweating and sweat drippage followed by hidromeiosis were observed in three subjects during initial heat exposure. With repeated humid heat exposures increased sweat rates were accompanied by a more intense sweat depression (hidromeiosis) in all four subjects. In our conditions, no changes in plasma levels of aldosterone and ADH or plasma renin activity were observed with hidromeiosis. Plasma renin activity was slightly depressed by repeated exposures, whereas plasma volumes were enhanced, with no significant changes in plasma Na or K. The results suggest that neither ADH nor the components of the renin-angiotensin aldosterone system are involved in the hidromeiotic phenomenon.

Characterization of specific receptors for vasopressin in the pituitary gland

The present paper reports new findings concerning interaction of [3H]-Arginine-vasopressin with putative receptors in rat anterior pituitary gland. It shows the presence of a single type of receptor sites, with a limited binding capacity and a dissociation constant of nearly 1nM. The parent neurohormone oxytocin revealed weak affinity as compared with vasopressin [Ki = 100nM and Ki = 1nM, respectively]. None of the various peptides tested and, especially corticotropin-releasing factor CRF, competed for binding. Receptor characteristics appeared to be unaffected by lack of circulating vasopressin in Brattleboro rats presenting complete deficiency in synthesis of that peptide.

Light- and electron-microscope differentiation of axons containing vasopressin and oxytocin in the neurohypophyseal lobe of the rat

In neural lobes of Sprague-Dawley rats fixed with glutaraldehyde but without osmication, toluidine blue staining of semi-thin sections revealed two clear axon types: type I, highly stained, and type II, slightly stained. After uranyl acetate and lead citrate impregnation of adjacent ultrathin sections, electron microscopy showed that type I axons contained electron-dense granules whereas type II axons mostly contained pale granules. When adjacent semi-thin sections were stained with toluidine blue or treated for immunocytochemistry, type I axons were found to react with vasopressin antisera, and type II axons with oxytocin antisera. In neural lobes of Brattleboro rats, only type II axons were observed and contained pale granules. Both in light and electron microscopy, glutaraldehyde fixation without osmication, therefore provided a simple and reliable approach to distinguish between neurohypophyseal axons containing vasopressin and oxytocin, respectively.

Modulation by transcortin-like binding sites of uptake and distribution of glucocorticoids by dispersed pituitary cells

The influence of transcortin-like (TL) binding sites on uptake and distribution of glucocorticoids by dispersed pituitary cells was investigated. TL material, which combines corticosterone (CORT), but not dexamethasone (DEX), was previously found to be present on cell membranes and in cytosol of hypophysis. Exposure of cells at 0 degrees brought about striking differences in steroid binding, as labeled CORT was taken up more rapidly and to a significantly greater extent than DEX. This resulted from a higher concentration of binding sites and not from a difference in binding affinity. At 25 and 37 degrees, while the same relationship was apparent during the early events of steroid interaction with the cell, binding of DEX increased gradually with incubation time and finally exceeded that of the natural steroid. Also, time-course studies on nuclear translocation showed a biphasic pattern, which closely paralleled that of whole-cell binding. Interestingly, treatment of rats with transcortin antiserum caused a decrement of pituitary TL sites, as well as of CORT translocation. We conclude that the TL binder, which is probably of plasma origin, may be actively involved in the process of uptake and cellular distribution of corticosteroids in the pituitary gland.

Glucocorticoid binding and control ACTH secretion

This report attempts to summarize the present state of knowledge concerning interaction of glucocorticoids with brain and pituitary receptors, in relation to induction of specific biological effects. It examines the properties of the receptors, emphasizing the fact that transcortin-like binding molecules are present on the cell membrane in the pituitary gland. Also, it considers the functional aspects of the mechanism of action of steroids, including control, occupancy and heterogeneity of binding sites, as well as correlation with regulation of ACTH release.

[Vasopressin and "CRF" in the regulation of ACTH secretion by the anterior and intermediate lobes of the pituitary gland (author's transl)]

The aim of this review was to summarize the present state of knowledge concerning the mode of action of vasopressin (VP) and the putative corticotropin releasing factor (CRF) on ACTH secretion from the anterior and intermediate lobes of the pituitary gland. In vitro data show that although both CRF and VP enhanced release of anterior pituitary ACTH, the pattern of hormonal release, based on kinetical and dose-dependent studies, appeared to be different. Also, the effect of VP most probably was mediated by specific putative receptor sites. In contrast, VP was found not to alter ACTH secretion from the intermediate lobe; that secretion seems to be regulated by CRF-like material and neurotransmitters. The importance of VP as a corticotropin agent is discussed.

Evidence for a direct inhibitory effect of morphine on the secretion of posterior pituitary hormones

The effect of morphine and naloxone on vasopressin and oxytocin release from incubated neurointermediate lobe and pars nervosa of rat pituitaries were investigated. It was shown that morphine and endorphins blocked hormonal output and that this inhibitory action was reversed by naloxone. It is concluded that opiates exert a direct inhibitory influence on vasopressin and oxytocin secretions from the neurohypophysis.

[Specificity of the effect of vasopressin at the anterior pituitary level (author's transl)]

The present investigation was conducted in order to get more insight into the mechanism of action of vasopressin (VP) on ACTH secretion and characterize VP interaction with putative receptor sites at the level of the anterior pituitary gland. The experimental procedure consisted of increasing ACTH release from incubated pituitary fragments as induced by VP in the presence or absence of oxytocin (OT) and various releasing factors. Our results show that, whereas OT was able to depress VP-induced release of ACTH (Fig. 2), TRF (Fig. 3), LH-RH (Fig. 4) and crude "CRF" extract (Fig. 1) it did not exhibit any significant inhibitory effect. This provides indirect evidence for the presence of specific pituitary sites for VP and related peptides such as OT. The latter, however, was devoided of corticotrophic releasing properties.

In vitro regulation of ACTH release from neurointermediate lobe of rat hypophysis. III. Effect of potassium (K+), calcium (Ca++) and dibutyryl cyclic 3',5'-adenosine monophosphate (dbc-AMP)

Alterations in the ionic composition of the medium and different secretagogues have been used in order to study the mechanism of release of ACTH from superfused neurointermediate lobes (NIL) of rat hypophysis. We showed that: (a) a tenfold increase of K+ in the medium caused a reversible and repeatable stimulation of the ACTH release; (b) removal of Ca++ reversibly abolished the stimulating effect of high K+; (c) removal of Ca++ had no effect on the stimulating effect of a hypothalamic extract (HE); (d) in the latter case, a reversible significant weakening was obtained by the addition of EDTA in the Ca++-free medium; (e) dbc-AMP caused a reversible and repeatable stimulation of the ACTH release; (f) comparable results were obtained for anterior lobes (AL) superfused in the same conditions. From these data we can conclude that Ca++ is necessary for the stimulation of the hormonal release and that factors such as K+ or dbc-AMP can mimic the in vitro stimulating effect of a HE. Similarities, which appear in this study, between the modulation of the ACTH release from NIL and AL, led us to support the idea that, in vivo, a release of ACTH from the NIL, may occur in physiological conditions.

Involvement of vasopressin in corticotropin-releasing effect of hypothalamic median eminence extract

Incubation of anterior pituitary (AP) fragments of rats was used to determine the specific role played by vasopressin (VP) in the overall effect of crude hypothalamic median eminence (HME) extract on ACTH release. Using the property of an AVP antiserum (AS) to completely abolish the CRF-like effect of hypothalamic VP without apparently affecting the effect of CRF, we show that under specific incubation conditions, the effect of the two secretagogues are additive at the pituitary level. ACTH secretion of pituitaries was enhanced when incubation was carried out in the presence of VP together with a maximum effective dose of VP-free HME extract (from Brattleboro rats). These observations favor the hypothesis that VP and CRF have different receptor sites in the anterior pituitary.

Effects of adrenalectomy and hypercorticism on the ACTH content of the anterior and posterior pituitary in rats with inherited diabetes insipidus (Brattleboro strain)

The effects of adrenalectomy (Adx) and hypercorticism on the ACTH content in the anterior (AH) and the neurointermediate lobe (NIL) of the pituitary in Long Evans (+/+), heterozygous (+/DI) and homozygous (DI/DI) Brattleboro rats were determined using dispersed adrenal cells bioassay. Adx decreased the NIL-ACTH content in +/DI and DI/DI rats and left it unchanged in the +/+ rats. Adx increased the AH-ACTH content in the three groups. Hypercorticism had a delayed decreasing effect both in the AH and in the NIL in all rats, with one exception for the NIL in DI/DI rats. Conversely to what appeared in Wistar rats, in Long Evans and Brattleboro rats the corticosterone administered in drinking water was unable to reduce the increase in AH-ACTH activity. These data suggest that Brattleboro, and, to a lesser extent, Long Evans rats from which the former are derived present some particularities in the regulation of their corticotropic function at the AH and the NIL level. We also observed that NaCl (0.9%) added to drinking water and hypercorticism are two factors able to increase diabetes insipidus in homozygous rats without modifying the water intake in Long Evans and heterozygous rats.

Relationship between ACTH secretion and corticoid binding to specific receptors in perifused adenohypophyses

This study reports on the kinetics of glucocorticoid-induced inhibition of ACTH release, together with steroid binding to specific pituitary receptors. It was shown that corticosterone (CORT) inhibited ACTH output provoked by either corticotropin-releasing factor (CRF) extracts or dbcAMP, in a manner which was both dose- and time-dependent. A close correlation appeared to exist between the degree of ACTH blockade and the percentage of filled steroid-binding sites. However, exposure of hypophyses to CORT for a critical period of time was a prerequisite for such a relationship to develop. Furthermore, it was found that dexamethasone (DEX) was more potent than CORT in inhibiting ACTH secretion and, in addition, bound to a greater extent to nuclei of pituitary cells. These data suggest the existence of a close correlation between occupancy of pituitary glucocorticoid receptors and modulation of ACTH secretion.

In vitro regulation of ACTH release from neurointermediate lobe of rat hypophysis. II. Effect of neurotransmitters

The effect of various neurotransmitters on adrenocorticitropic hormone (ACTH) release from superfused neurointermediate lobes of rat hypophysis (NIL) was investigated. Acetylcholine, serotonin, and histamine were shown to have no significant effect on the spontaneous release of ACTH. In contrast, dopamine (DA) and noradrenaline (NA) stimulated NIL ACTH release in a dose-dependent manner. The stimulating effect of DA was mimicked by apomorphine and inhibited by haloperidol, whereas that of NA was abolished by phentolamine. In a comparative study, none of these neurotransmitters is shown to have any significant stimulating effect on ACTH release from superfused anterior lobes. It is concluded that the ACTH release from NIL might be controlled, at least in part, by catecholaminergic innervation.