Cyclic adenosine monophosphate regulates vasoactive intestinal polypeptide and enkephalin biosynthesis in cultured bovine chromaffin cells

Dopamine D2 receptors bidirectionally regulate striatal enkephalin expression: Implications for cocaine reward

Low dopamine D2 receptor (D2R) availability in the striatum can predispose for cocaine abuse; though how low striatal D2Rs facilitate cocaine reward is unclear. Overexpression of D2Rs in striatal neurons or activation of D2Rs by acute cocaine suppresses striatal Penk mRNA. Conversely, low D2Rs in D2-striatal neurons increases striatal Penk mRNA and enkephalin peptide tone, an endogenous mu-opioid agonist. In brain slices, met-enkephalin and inhibition of enkephalin catabolism suppresses intra-striatal GABA transmission. Pairing cocaine with intra-accumbens met-enkephalin during place conditioning facilitates acquisition of preference, while mu-opioid receptor antagonist blocks preference in wild-type mice. We propose that heightened striatal enkephalin potentiates cocaine reward by suppressing intra-striatal GABA to enhance striatal output. Surprisingly, a mu-opioid receptor antagonist does not block cocaine preference in mice with low striatal D2Rs, implicating other opioid receptors. The bidirectional regulation of enkephalin by D2R activity and cocaine offers insights into mechanisms underlying the vulnerability for cocaine abuse.

Low striatal D2 receptor levels are associated with cocaine abuse. Dai et al. bidirectionally alter striatal D2 receptor levels to probe the downstream mechanisms underlying this abuse liability. They provide evidence that enhanced enkephalin tone resulting from low D2 receptors is associated with suppressed intra-striatal GABA and potentiated cocaine reward.

Regulated expression of atrial natriuretic peptide-like immunoreactivity in cultured bovine adrenomedullary chromaffin cells

We show that cultured bovine adrenal chromaffin cells can be stimulated to produce atrial natriuretic peptide-like immunoreactivity (ANP). ANP levels increased 5-fold in response to either forskolin or phorbol ester treatment, and 17-fold after depolarization by 40 mM potassium. These agents appear to act through distinct second messenger mediated pathways to cause increased accumulation of ANP. When forskolin and phorbol ester were added simultaneously, their effects were synergistic: ANP levels increased 30-fold, more than the product of the increases achieved by treatment with either drug alone. The ANP present in bovine chromaffin cell extracts appeared immunologically identical to human ANP(1-28). By gel filtration, the immunoreactive ANP extracted from chromaffin cells was in a high molecular weight form, although HPLC fractionation revealed that a portion of the total immunoreactivity could be eluted with the same retention time as synthetic ANP standards.

Regulation of neuronal pituitary adenylate cyclase-activating polypeptide expression during culture of guinea-pig cardiac ganglia

The trophic neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) increases in many different neuron types following injury; a response postulated to support cell survival and regeneration. In acutely isolated cardiac ganglia, approximately 1% of the cardiac neurons exhibited PACAP immunoreactivity whereas after 72 h in culture, approximately 25% of the neurons were PACAP immunoreactive. In contrast, there was no increase in vasoactive intestinal polypeptide (VIP)-immunoreactive (IR) cells. Using a combination of immunocytochemical and molecular techniques, we have quantified PACAP expression, during explant culture of guinea-pig cardiac ganglia. Using real time polymerase chain reaction, PACAP transcript levels increased progressively up to 48 h in culture with no further increase after 72 h. PACAP transcript levels were reduced by neurturin at 48 h in culture but not after 24 or 72 h in culture. In addition, neurturin partially suppressed the percentage of PACAP-IR neurons after 72 h in culture, an effect mediated by activation of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase signaling pathways. The addition of different known regulatory molecules, including ciliary neurotrophic factor (CNTF), interleukin-1 beta (Il-1beta), tumor necrosis factor-alpha (TNFalpha), fibroblast growth factor basic (bFGF), transforming growth factor-beta (TGF-beta) and nerve growth factor (NGF) did not increase the percentage of PACAP-IR neurons after 24 h in culture; a result indicating that the generation and secretion of these factors did not stimulate PACAP expression. The presence of 20 nM PACAP or 10 muM forskolin increased the percentage of PACAP-IR cardiac neurons in 24 h cultures, but not in 72 h cultures. Neither treatment enhanced the number of VIP-IR neurons. The addition of the PACAP selective receptor (PAC(1)) receptor antagonist, M65 (100 nM) suppressed the 20 nM PACAP-induced increase in percentage of PACAP-IR cells in 24 h cultures indicating the effect of PACAP was mediated through the PAC(1) receptor. However, 100 nM M65 had no effect on the percentage of PACAP-IR cells in either 24 or 48 h cultures not treated with exogenous PACAP, suggesting that endogenous release of PACAP likely did not contribute to the enhanced peptide expression. We postulate that the enhanced PACAP expression, which occurs in response to injury is facilitated in the explant cultured cardiac ganglia by the loss of a target-derived inhibitory factor, very likely neurturin. In intact tissues the presence of neurturin would normally suppress PACAP expression. Lastly, our results indicate that many common trophic factors do not enhance PACAP expression in the cultured cardiac neurons. However, the stimulatory role of an, as yet, unidentified factor cannot be excluded.

Useful cell lines derived from the adrenal medulla

Five approaches for the preparation of adrenal chromaffin cell lines have been developed. Initially, continuous chromaffin lines were derived from spontaneous pheochromocytoma tumors of the medulla, either from murine or human sources, such as the rat PC12 cell line and the human KNA and KAT45 cell lines. Over the last few decades, more sophisticated molecular methods have allowed for induced tumorigenesis and targeted oncogenesis in vivo, where isolation of specific populations of mouse cell lines of endocrine origin have resulted in model cells to examine a variety of regulatory pathways in the chromaffin phenotype. As well, conditional immortalization with retroviral infection of chromaffin precursors has provided homogeneous and expandable chromaffin cells for transplant studies in animal models of pain. This same strategy of immortalization with conditionally expressed oncogenes has been expanded recently to create the first disimmortalizable chromaffin cells, with an excisable oncogenic cassette, as might be envisioned for the creation of human chromaffin cell lines. Eventually, as we increase our understanding of regulating the phenotypic fate of chromaffin cells in vitro, stem or progenitor adrenal medullary cell lines will be derived as an alternative source for expansion and clinical use.

Coincident elevation of cAMP and calcium influx by PACAP-27 synergistically regulates vasoactive intestinal polypeptide gene transcription through a novel PKA-independent signaling pathway

Pituitary adenylate cyclase-activating polypeptide (PACAP) causes calcium influx, intracellular calcium release, and elevation of cAMP in chromaffin cells. Calcium influx is required for PACAP-stimulated secretion of catecholamines and neuropeptides. The role of cAMP elevation in the action of PACAP at either sympathetic or adrenomedullary synapses, however, is unknown. Here, we show that PACAP-27-induced calcium influx through voltage-sensitive calcium channels (VSCCs), together with elevation of intracellular cAMP, was sufficient to stimulate vasoactive intestinal polypeptide (VIP) biosynthesis at least 40-fold. Combined treatment of chromaffin cells with 40 mm KCl, which elevates intracellular calcium, and 25 micrometer forskolin, which elevates intracellular cAMP, caused an increase in VIP peptide and mRNA much greater than that elicited by either agent alone, and comparable to the increase caused by 10-100 nm PACAP-27. Elevation of VIP mRNA by either KCl plus forskolin, or PACAP, (1) was independent of new protein synthesis, (2) was blocked by inhibition of calcium influx through voltage-sensitive calcium channels, (3) was calcineurin dependent, and (4) was dependent on MAP kinase activation but not activation of protein kinase A. The degree of activation of two different second-messenger pathways, calcium influx and cAMP elevation, appears to determine the magnitude of transcriptional activation of the VIP gene in chromaffin cells. Maximal stimulation of VIP biosynthesis by PACAP appears to require the coincident activation of both of these pathways.

Galanin expression is decreased by cAMP-elevating agents in cultured sympathetic ganglia

Galanin expression is co-regulated in peripheral neurons with that of vasoactive intestinal peptide (VIP) under a variety of conditions. For example, the expression of both increase after explantation of adult rat superior cervical ganglia (SCG). Because VIP participates in a positive feedback loop regulating its own expression, we examined whether VIP also increases galanin expression. Galanin mRNA and peptide are nearly undetectable in the SCG in vivo, but increase dramatically after 24-48 h in organ culture. Addition of VIP or forskolin to the culture medium reduced galanin mRNA expression by 75% and 77%, respectively, and reduced galanin peptide expression by 76% and 82%, respectively, compared with ganglia cultured in control medium. In contrast, isoproterenol stimulation did not significantly alter levels of galanin mRNA or peptide, consistent with previous observations that isoproterenol exerts its effect on SCG non-neuronal cells, but not on neurons. The results indicate that galanin and VIP are differentially regulated in sympathetic neurons by cAMP- elevating agents.

Regulation of VIP production and secretion by murine lymphocytes

Vasoactive intestinal peptide (VIP) is a neuropeptide present in the lymphoid microenvironment with a multiplicity of actions. Two sources for VIP have been described in the immune system, the terminals present in central and peripheral lymphoid organs and the immune cells. Although VIP is synthesized by lymphocytes, there is no evidence demonstrating that VIP is released, and which stimuli are able to induce VIP production and secretion. In this study, we demonstrated for the first time, that agents that mediate important immune functions, such as proliferation and antigenic stimulation (Con A, LPS, and anti-TCR antibody), inflammation (LPS, TNFalpha, IL-6 and IL-1beta) or apoptosis (dexamethasone) induce the production and release of VIP to the lymphoid microenvironment. We conclude that VIP is produced and secreted by lymphocytes and propose that during an immune response, the timely release of VIP within the lymphoid organs and peritoneum should influence the differentiation and/or downregulation of the ongoing response.

Cis-regulatory elements controlling basal and inducible VIP gene transcription

The cis-acting elements of the VIP gene important for basal and stimulated transcription have been studied by transfection of VIP-reporter gene constructs into distinct human neuroblastoma cell lines in which VIP transcription is constitutively high, or can be induced to high levels by protein kinase stimulation. The 5.2 kb flanking sequence of the VIP gene conferring correct basal and inducible VIP gene expression onto a reporter gene in these cell lines was systematically deleted to define its minimal components. A 425-bp fragment (-4656 to -4231) fused to the proximal 1.55 kb of the VIP promoter-enhancer was absolutely required for cell-specific basal and inducible transcription. Four additional components of the VIP gene were required for full cell-specific expression driven by the 425 bp TSE (region A). Sequences from -1.55 to -1.37 (region B), -1.37 to -1.28 (region C), -1.28 to -.094 (region D), and the CRE-containing proximal 94 bp (region E) were deleted in various combinations to demonstrate the specific contributions of each region to correct basal and inducible VIP gene expression. Deletion of region B, or mutational inactivation of the CRE in region E, resulted in constructs with low transcriptional activity in VIP-expressing cell lines. Deletion of regions B and C together resulted in a gain of transcriptional activity, but without cell specificity. All five domains of the VIP gene were also required for cell-specific induction of VIP gene expression with phorbol ester. Gelshift analysis of putative regulatory sequences in regions A-D suggests that both ubiquitous and neuron-specific trans-acting proteins participate in VIP gene regulation.

PACAP activates calcium influx-dependent and -independent pathways to couple met-enkephalin secretion and biosynthesis in chromaffin cells

Pituitary adenylate cyclase activating polypeptide-27 (PACAP-27) caused a dose-dependent increase in met-enkephalin secretion and increased production of met-enkephalin peptide and proenkephalin A (PEnk) mRNA in bovine chromaffin cells, at concentrations as low as 300 pM. PACAP-38 was less potent than PACAP-27, but had similar effects. Vasoactive intestinal polypeptide (VIP) (1-100 nM) was without appreciable effect on either enkephalin secretion or biosynthesis, implicating PACAP type I receptors in PACAP-stimulated enkephalin secretion and synthesis. PACAP type I receptors can activate adenylate cyclase and stimulate phospholipase C through heterotrimeric G protein interactions, leading to increased intracellular cyclic AMP (cAMP), inositol triphosphate (IP3)-mediated calcium mobilization, and calcium- and diacylglycerol (DAG)-mediated protein kinase C (PKC) activation. Enkephalin secretion evoked by 10-100 nM PACAP-27 was not inhibited by 1 microM (-)-202-791, an L-type specific dihydropyridine calcium channel blocker, but was inhibited 65-80% by the arylalkylamine calcium channel blocker D600. Forty mM potassium-evoked secretion was inhibited > 90% by both D600 and (-)-202-791, 25 microM forskolin-induced secretion was blocked < 50% by D600 and was unaffected by (-)-202-791, and 100 nM phorbol myristate acetate (PMA)-induced secretion was unaffected by either D600 or (-)-202-791. Enkephalin biosynthesis was increased by 10 nM PACAP-27, as measured by increased met-enkephalin pentapeptide content and PEnk A mRNA levels. PACAP-, forskolin-, and PMA-stimulated enkephalin synthesis were not blocked by D600 or (-)-202-791. Elevated potassium-induced enkephalin biosynthesis upregulation was completely blocked by either D600 or (-)-202-791 at the same concentrations. PACAP acting through type I PACAP receptors couples calcium influx-dependent enkephalin secretion and calcium influx-independent enkephalin biosynthesis in chromaffin cells. Restriction of the effects of enhanced calcium influx to stimulation of secretion, but not of biosynthesis, is unique to PACAP. By contrast, potassium-induced enkephalin biosynthesis upregulation is completely calcium influx dependent, specifically via calcium influx through L-type calcium channels. We propose that subpopulations of voltage-dependent calcium channels are differentially linked to intracellular signal transduction pathways that control neuropeptide gene expression and secretion in chromaffin cells.

Vasoactive intestinal peptide enhances its own expression in sympathetic neurons after injury

Neurons in the adult rat superior cervical sympathetic ganglion (SCG) dramatically increase their content of vasoactive intestinal peptide (VIP) and its mRNA after axotomy in vivo and after explantation. Because the VIP gene contains a functional cAMP response element, the effects of cAMP-elevating agents on VIP expression were examined. VIP, forskolin, or isoproterenol increased cAMP accumulation in explanted ganglia. Secretin, a peptide chemically related to VIP, or forskolin increased VIP levels above those seen in ganglia cultured in control medium, whereas treatment with VIP or secretin increased the level of peptide histidine isoleucine (PHI), a peptide coded for by the same mRNA that encodes VIP. VIP or forskolin also increased VIP-PHI mRNA. In contrast, isoproterenol did not alter levels of VIP, PHI, or VIP-PHI mRNA. Although VIP or forskolin increased cAMP levels in both dissociated neurons and in non-neuronal cells, isoproterenol significantly stimulated cAMP accumulation only in the latter. VIP6-28 was an effective antagonist of the actions of exogenous VIP on cAMP and VIP-PHI mRNA in neuron-enriched cultures. When adult SCG explants were cultured in defined medium, endogenous VIP immunoreactivity was released. When VIP6-28 was added to such cultures, it significantly inhibited the increase in VIP-PHI mRNA that normally occurs. These data indicate that VIP, or a closely related molecule, produced by adult neurons after injury can enhance the expression of VIP. Such a mechanism may prolong the period during which VIP is elevated after axonal damage. The possibility is also discussed that, because VIP is present in preganglionic neurons in normal animals, its release during periods of increased sympathetic nerve activity could alter VIP expression in the SCG.

Regulation of the biosynthesis of large dense-core vesicles in chromaffin cells and neurons

1. The proteins of large dense-core vesicles (LDV) in neuroendocrine tissues are well characterized. Secretory components comprise chromogranins and neuropeptides. Intrinsic membrane proteins include cytochrome b-561, transporters, SV2, synaptotagmin, and synaptobrevin. 2. The effects of stimulation and of second messengers on the biosynthesis of LDV have been studied in detail. 3. Regulation of biosynthesis is complex. The cell can adapt to prolonged stimulation either by producing vesicles of normal size filled with a higher quantum of secretory peptides or by forming larger vesicles. In addition, some components, e.g., enzymes, can be upregulated specifically.

Molecular mechanisms underlying the regulation of proenkephalin gene expression in cultured spinal cord cells

The regulation of proenkephalin (proENK) mRNA levels by cAMP and protein kinase C (PKC) pathways was studied in cultured rat spinal cord cells in the present study. Spinal cord cells were cultured from 14 day (E 14) embryos of Sprague-Dawley rats. After 7 days in vitro, the spinal cord cells were incubated with either forskolin (5 microM) or phorbol-13-myristate acetate (PMA; 2.5 microM) for 1, 3, 6, 9, 12 or 24 h and total RNA and proteins were isolated for Northern and Western blot analyses. The proENK mRNA level began to increase within an hour, then reached and remained at a peak 3-12 h after stimulation by both forskolin and PMA. The increased proENK mRNA level in forskolin-treated cells was slightly decreased 24 h after the stimulation, whereas the level of proENK mRNA returned to basal levels in PMA-treated cells. A Western blot assay revealed that the intracellular level of proENK protein was not changed by treatment with either forskolin or PMA. Pretreatment of cells with cycloheximide (a protein synthesis inhibitor; 10 microM) did not affect the forskolin- or PMA-induced increase of proENK mRNA. However, pretreatment with nimodipine (an L-type Ca2+ channel blocker; 2 microM), omega-conotoxin (an N-type Ca2+ channel blocker; 1 microM), calmidazolium (a calmodulin antagonist; 1 microM) or KN-62 (a Ca2+/calmodulin-dependent protein kinase II inhibitor; 5 microM) attenuated the forskolin- or PMA-induced increase of proENK mRNA levels. Dexamethasone (1 microM) did not affect the forskolin-induced increase of proENK mRNA levels. Our results suggest that the elevation of proENK mRNA levels in the spinal cord is regulated by both cAMP and PKC pathways. Calcium influx through both L- and N-type calcium channels, calmodulin and Ca2+/calmodulin-dependent protein kinase II appear to be involved in the increase of proENK mRNA levels induced by either forskolin or PMA. Furthermore, ongoing protein synthesis is not required for forskolin- or PMA-induced alterations in proENK mRNA.

Neuropeptide genes: targets of activity-dependent signal transduction

Neuroendocrine cells respond to hormones and synaptic input by increasing or decreasing their own electrical activity and secretory output, and by changes in the repertoire of expression of neuronal genes. Neuropeptide genes are among those whose transcription rates can be dramatically up-and downregulated when neuronal activity is altered. In the last decade or so, our understanding of neuropeptide gene regulation has evolved from the concept of calcium-dependent coupling of neuropeptide secretion and biosynthesis to the current perspective of neuropeptide genes as the targets of multiple intracellular signaling pathways, entrained by intrinsic electrical activity and by transsynaptic influences. This review describes our current understanding of neuropeptide gene regulation in the adrenal gland as well as in the peripheral and central nervous systems. Particular emphasis is placed on the molecular mechanisms that allow unique patterns of expression of neuropeptide genes within specific types of neuroendocrine cells that contribute to the remarkable anatomical specificity of neuropeptide gene expression.

Molecular cloning and characterization of the hamster preproenkephalin A cDNA

The cDNA for hamster preproenkephalin A (ENK) was cloned from an adrenal gland cDNA library constructed in the lambda ZapII vector. A nearly full-length cDNA was obtained and its 5' end region was completed using the technique of rapid amplification of cDNA ends (RACE). The coding and 3' untranslated regions of the hamster ENK cDNA share a high sequence identity with the rat, human, and bovine cDNAs, whereas the sequence identity is lower for the 5' untranslated region. Southern blot analysis of genomic DNA digests showed that a single copy of the ENK gene is present in the hamster haploid genome. Northern blot analysis of poly(A)+RNA from various hamster tissues indicated the following rank order for ENK messenger RNA abundance: adrenal glands > right atrium > brain > left atrium > right ventricle > ventricular septum > left ventricle, whereas primer extension analysis showed a single, identical transcriptional initiation site for the ENK mRNA in all these tissues. The sequence of the 5' untranslated region of the heart ENK cDNA was found to be identical to that from adrenal glands. This rules out the possibility that structural divergences in the 5' untranslated region of the heart ENK mRNA could decrease its translation efficiency and contribute to the very low level of enkephalin-containing peptides in the heart, compared to the adrenal glands.

Pertussis toxin enhances proenkephalin synthesis in bovine chromaffin cells

The synthesis of the neuropeptide precursor proenkephalin was measured in bovine adrenal chromaffin cells following radiolabeling with [35S]methionine. Treatment of chromaffin cells with pertussis toxin (100 ng/ml) approximately doubled proenkephalin synthesis without altering total protein synthesis. Pertussis toxin pretreatment also increased proenkephalin synthesis in chromaffin cells exposed to vasoactive intestinal peptide (VIP) and 3-isobutyl-1-methylxanthine (IBMX). Combinations of IBMX plus nicotine, VIP, or histamine also synergistically enhanced proenkephalin synthesis, with no further elevation when the cells were also pretreated with pertussis toxin. The action of forskolin, a direct activator of adenylate cyclase, on proenkephalin synthesis was similarly potentiated by pertussis toxin or IBMX, presumably reflecting the abilities of both the toxin and this phosphodiesterase inhibitor to enhance the cyclic AMP response to forskolin. In contrast, increased synthesis of proenkephalin in response to phorbol esters was not affected by pertussis toxin treatment. These results suggest that pertussis toxin potentiates proenkephalin synthesis primarily through inactivation of guanine nucleotide-binding proteins that inhibit adenylate cyclase, although other signaling pathways may also be involved.

Glycoprotein III (clusterin, sulfated glycoprotein 2) in endocrine, nervous, and other tissues: immunochemical characterization, subcellular localization, and regulation of biosynthesis

Specific antisera were raised against the A and B chains of glycoprotein III. Immunoblotting revealed that in adrenal medulla both chains migrate very closely together in two-dimensional electrophoresis. Both chains with slightly differing molecular sizes are found in several endocrine tissues and in brain, kidney, liver, and serum. The mRNA has an analogous widespread distribution. In primary cultures of chromaffin cells the level of message becomes significantly increased by treatment with histamine or 12-O-tetradecanoylphorbol 13-acetate/forskolin. However, the increase is small when compared with that of secretogranin II. The subcellular localization of glycoprotein III in endocrine organs and in the posterior pituitary was investigated by subcellular fractionation and immunoelectron microscopy. Glycoprotein III was found to be confined to the large dense-core vesicles of these organs. For a discussion of the function of glycoprotein III, its localization in these organelles has to be taken into account.

Regulation of the chromaffin granule catecholamine transporter in cultured bovine adrenal medullary cells: stimulus-biosynthesis coupling

The transsynaptic induction of the monoamine transporter present on the membrane of chromaffin granules was studied in primary cultures of dissociated bovine adrenomedullary cells submitted to a chronic secretory stimulation. The amount of the vesicular monoamine transporter was assayed by binding of the specific ligand [3H]-dihydrotetrabenazine. After several days of incubation in the presence of high potassium, the concentration of [3H]-dihydrotetrabenazine binding sites was increased by a 1.5-2.5 factor. This increase was smaller in the presence of the cholinergic agonist carbachol. The long-term inductions of the vesicular monoamine transporter, of tyrosine hydroxylase, and of acetylcholinesterase were of similar magnitude. Under the same conditions, we found no variation in either the activities of other catecholamine biosynthetic enzymes (dopamine beta-hydroxylase and DOPA decarboxylase), or in metabolic enzymes such as lactate dehydrogenase and cytochrome c oxidase, and a decrease in the cellular content of chromogranin A and cytochrome b-561. The induction of the vesicular monoamine transporter was inhibited by the calcium channel antagonists, fluspirilene and nifedipine, and was increased by the agonist Bay K 8644. It was abolished by cycloheximide and actinomycin D. These results indicate that calcium entry into chromaffin cells increases the synthesis of the vesicular monoamine transporter, presumably by transcriptional activation. Elevation of intracellular cyclic AMP concentration or activation of protein kinase C also induced an increase in the expression of the vesicular monoamine transporter. Our results confirm that components of storage vesicle membranes are differentially regulated in response to secretory stimulation, as are several cytosolic or intravesicular soluble proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasoactive intestinal peptide: autocrine growth factor in neuroblastoma

Neuroblastoma is the most common solid tumor of children less than 5 years of age; yet the biology of this tumor is poorly understood. Neuroblastoma tumors are derived from neural crest precursors; they synthesize both adrenergic and peptidergic neurotransmitters. This study determined VIP receptor expression in primary neuroblastoma tumors prior to chemotherapy. The VIP receptor was expressed in 12 of 15 neuroblastoma tumors as determined by direct binding studies (KD = 1.3-12.4 nM) and VIP-mediated stimulation of adenylate cyclase. The VIP stimulation index for adenylate cyclase in the primary tumor was inversely correlated with the VIP content of the tumor, suggesting that VIP regulates its own receptor expression. Similar observations were made in vitro by comparison of two human neuroblastoma cell lines, IMR32 and SKNSH. Both cell lines were demonstrated to express specific, high affinity VIP receptors (KD = 4 nM and 2.5 nM for IMR32 and SKNSH, respectively). IMR32 cells contained very low levels of VIP (0.6 pg VIP/10(6) cells). Exogenous VIP stimulated adenylate cyclase 22-fold over basal activity and VIP inhibited proliferation of IMR32 cells by 49% in 6-day cultures. On the other hand, SKNSH cells synthesized high levels of VIP (6.3 pg/10(6) cells), metabolized VIP rapidly and demonstrated a low level of VIP-mediated stimulation of adenylate cyclase; their proliferation rate was minimally inhibited by exogenous VIP. These observations help validate the hypothesis that VIP serves as an autocrine growth factor in neuroblastoma.

Histamine-induced increases in cyclic AMP levels in bovine adrenal medullary cells

1. The effect of histamine on cellular cyclic AMP levels in cultured bovine adrenal medullary cells has been studied. 2. Histamine (0.3-30 microM) increased cyclic AMP levels transiently, with a maximal response after 5 min, a smaller response after 20 min, and no increase seen after 80 or 180 min. The EC50 at 5 min was approximately 2 microM. Histamine had no effect on cyclic AMP release from the cells over 5 min, but increased it after 90 min. 3. The cyclic AMP response to 5 microM histamine was reduced by 45% by 1 microM mepyramine and by almost 30% by 1 microM cimetidine, and was abolished by the combination of both antagonists. Cimetidine at 100 microM did not inhibit the response to histamine more than 1 microM cimetidine. The H3-receptor antagonist, thioperamide (1 microM), had no effect on the response to histamine. 4. The H1-receptor agonist, 2-thiazolyethylamine (5-100 microM) and the H2-receptor agonist, dimaprit (5-100 microM), each induced a cyclic AMP response, and gave more-than-additive responses when combined. The H3 agonist (R) alpha-methylhistamine (100 microM) had no effect either on its own or in combination with either the H1 or the H2 agonist. The response to 100 microM 2-thiazolylethylamine was unaffected by cimetidine (100 microM). 5. The cyclic AMP responses to 5 microM histamine, 100 microM thiazolylethylamine and 100 microM dimaprit were each weakly enhanced in the presence of 1 mM 3-isobutyl-1-methylxanthine. The response to dimaprit was enhanced more than 10 fold in the presence of 0.3 microM forskolin, while the responses to histamine and thiazolylethylamine were weakly enhanced.6. The cyclic AMP response to 5 microM histamine was partially reduced in the absence of extracellular Ca2 and the residual response was fully antagonized by 1 microM cimetidine and was unaffected by 1 microM mepyramine.In the absence of Ca2 , the cyclic AMP response to 100 microM thiazolylethylamine was abolished, while that to 100 microM dimaprit was unaffected.7. Reincubation of 5 microM histamine solutions with a second set of chromaffin cells, following prior incubation with another set of cells, induced a cyclic AMP response in the fresh cells. This response was reduced by a combination of mepyramine and cimetidine to the same degree as the response to fresh 5 microm histamine solutions.8. The results indicate that histamine increases cellular cyclic AMP levels in bovine chromaffin cells by three mechanisms: by acting on H1 receptors, by acting on H2 receptors, and by an interaction between H, and H2 receptors. The H1 response does not require concomitant activation of H2 receptors, is fully dependent on extracellular Ca2 +, does not depend on secreted chromaffin cell products, and is not due to reduced cyclic AMP degradation or export. The H2 cyclic AMP response is the first functional response reported for H2 receptors on chromaffin cells, is independent of Ca2 , is not due to reduced cyclic AMP export or degradation, and is likely to be mediated via a direct action through Gs. The role of these different mechanisms in the regulation of cyclic AMP-dependent processes in chromaffin cells by histamine is under investigation.

Regulation of cyclic AMP levels by calcium in bovine adrenal medullary cells

Both nicotine and histamine have been reported to increase cyclic AMP levels in chromaffin cells by Ca(2+)-dependent mechanisms. The present study investigated whether Ca2+ was an adequate and sufficient signal for increasing cyclic AMP in cultured bovine adrenal medullary cells. Depolarization with 50 mM K+ caused a two- to three-fold increase in cellular cyclic AMP levels over 5 min, with no change in extracellular cyclic AMP. This response was abolished by omission of extracellular Ca2+ and by 100 microM methoxyverapamil, and was unaffected by 1 microM tetrodotoxin and by 1 mM isobutylmethylxanthine. Veratridine (40 microM) also increased cellular cyclic AMP levels by two- to fourfold. This response was abolished by either methoxyverapamil or tetrodotoxin. The Ca2+ ionophore A23187 (10-50 microM) had little or no effect on cellular cyclic AMP levels. When the concentration of K+ used to depolarize the cells was reduced to 12-15 mM, the catecholamine release was similar to that induced by 50 microM A23187, and the cyclic AMP response was almost abolished. The results suggest that Ca2+ entry into chromaffin cells is a sufficient stimulus for increasing cellular cyclic AMP production. The possible involvement of a Ca2+/calmodulin-dependent isozyme of adenylate cyclase is discussed.

Comparative incorporation of proenkephalin-derived peptides, chromogranin A, and dopamine beta-hydroxylase into chromaffin vesicles

The incorporation of enkephalin-containing peptides (ECPs) derived from proenkephalin into chromaffin vesicles was examined in primary cultures of adrenal medullary chromaffin cells. Cells were pulse-labeled with [35S]methionine and chased for periods up to 24 h. Chromaffin vesicles in cell homogenates were then fractionated by density gradient centrifugation and the presence of [35S]Met-enkephalin sequences in gradient fractions determined. 35S-ECPs were incorporated into particles suggestive of immature vesicles within 1-2 h after radiolabeling. Vesicle maturation, measured by co-equilibration of 35S-ECPs and total ECPs in the gradients, was complete within 9-12 h and was unaffected by treatments that increase proenkephalin synthesis. Incorporation of [35S]chromogranin A into chromaffin vesicles followed a similar time course, but 35S-labeled dopamine beta-hydroxylase was much more slowly incorporated, possibly reflecting differences in incorporation of membrane and soluble components. In summary, the data demonstrate that ECPs are rapidly sequestered in immature chromaffin vesicles, a process unaltered by changing rates of proenkephalin synthesis.

Regulation of proenkephalin synthesis in adrenal medullary chromaffin cells

The synthesis of proenkephalin was assessed in primary cultures of bovine adrenal medullary chromaffin cells by incubation of the cells with [35S]methionine, digestion of proenkephalin-derived peptides with trypsin and carboxy-peptidase B, and quantitation of radioactivity incorporated into Met-enkephalin following reversed-phase HPLC. Nicotine, histamine, and vasoactive intestinal peptide each enhanced the rate of proenkephalin synthesis approximately 10-fold when examined between 16 and 32 h after the drug or hormone addition. Inclusion of nifedipine (1 microM) partially blocked the stimulatory effect of nicotine, but not that of vasoactive intestinal peptide or histamine, or proenkephalin synthesis. Theophylline, tetrabenazine, and angiotensin II also increased the rate of proenkephalin synthesis (three- to eight-fold). These increases in the apparent rate of proenkephalin synthesis were not attributable to altered [35S]methionine specific radioactivity or rates of turnover and did not reflect similar increases in total protein synthesis. The half-life for turnover of Met-enkephalin sequences was 3-4 days in the cultured chromaffin cell. These studies directly show that proenkephalin synthesis is the primary regulatory step in control of chromaffin cell opioid peptide content.

Vaginocervical stimulation of ferrets induces release of luteinizing hormone-releasing hormone

Abstract Vaginocervical stimulation of ovariectomized estradiol-primed ferrets (which are reflex ovulators) with a glass rod in the presence of a neck-gripping male induced an increase in plasma luteinizing hormone (LH) from undetectable levels (</=0.50 ng/ml) before stimulation, to 2.4 +/- 0.43 ng/ml 75 min after stimulation (stimulated females). Forty-eight h after stimulation plasma LH returned to baseline levels (post-stimulated females). A significant decrease in the number of perikarya containing LH-releasing hormone (LHRH), detected by immunocytochemistry, was associated with the increase in plasma LH following stimulation. Approximately one half of the number of immunoreactive LHRH neurons (243+/-27) were detected in the forebrain of stimulated females, compared to those detected in the forebrain of post-stimulated animals (436 +/- 88) using antiserum AR 744. Equivalent results were obtained with a different antiserum (RM 1076) capable of detecting the extended decapeptide, or precursor, as well as partially or fully processed decapeptide. We conclude that controlled Vaginocervical stimulation of female ferrets evokes the release of LHRH as well as LH, depleting approximately 50% of the LHRH perikarya of detectable LHRH. Additionally, electron microscopy of LHRH perikarya of stimulated females revealed more Golgi complexes/cell compared to baseline females. We propose that Vaginocervical stimulation also augments the processing of extended precursor forms of LHRH to generate the decapeptide.

Coordinate and differential regulation of proenkephalin A and PNMT mRNA expression in cultured bovine adrenal chromaffin cells: responses to cAMP elevation and phorbol esters

The expression of proenkephalin A (ProEnk A) mRNA and phenylethanolamine N-methyltransferase (PNMT) mRNA in response to cAMP analogues, forskolin and phorbol esters was examined in cultures of bovine adrenal chromaffin cells. Exposure of chromaffin cells to 1 mM dibutyryl cAMP for 24 h increased significantly the levels of ProEnk A mRNA, with no significant effect on the levels of PNMT mRNA. Cells exposed to the tumor promoting phorbol esters (phorbol 12-myristate 13-acetate, phorbol 12,13-dibutyrate or 4-beta-phorbol 12,13-didecanoate) for 12 h differentially activated PNMT mRNA and ProEnk A mRNA expression. The levels of PNMT mRNA were dramatically elevated in response to low concentrations (10(-9) to 10(-8)M) of these phorbol esters, but these increases were diminished at higher concentrations (10(-7) to 10(-6) M) of the phorbol esters. These responses were synergistically potentiated by dexamethasone (1 microM), a synthetic glucocorticoid. None of these effects was seen with the biologically inactive phorbol ester, 4-alpha-phorbol 12,13-didecanoate. By contrast, the expression of ProEnk A mRNA was activated by the tumor promoting phorbol esters in a concentration-dependent manner. The results of this study demonstrate a differential stimulatory effect of second messenger mechanisms in the control of PNMT and ProEnK A mRNA expression and provide further evidence for an independent control for the enkephalin and adrenaline synthesis in these cells.

Effect of secretagogues on chromogranin A synthesis in bovine cultured chromaffin cells. Possible regulation by protein kinase C

Chromogranin A is a major component of storage granules in many different secretory cell types. After [35S]methionine labelling of proteins from cultured bovine chromaffin cells, chromogranin A was immunoprecipitated with specific antibodies, and the radioactivity incorporated into chromogranin A was determined and used as an index of its synthesis rate. Depolarization of cells with nicotine or high K+ evoked a Ca2+-dependent increase in chromogranin A synthesis, whereas muscarine, which does not evoke significant Ca2+ influx from bovine chromaffin cells, had no effect on chromogranin A synthesis. Forskolin, an activator of adenylate cyclase, affected neither the basal nor the nicotine-stimulated rate of chromogranin A synthesis. In contrast, 12-O-tetradecanoylphorbol 13-acetate (TPA), an activator of protein kinase C, significantly enhanced the incorporation of radioactivity into chromogranin A. Sphingosine, an inhibitor of protein kinase C, abolished both nicotine-stimulated and TPA-induced chromogranin A synthesis. In addition, long-term treatment of chromaffin cells with TPA decreased protein kinase C activity and inhibited the nicotine-stimulated chromogranin A synthesis. These results suggest that protein kinase C may play an important role in the control of chromogranin A synthesis.

Molecular and cellular regulation of neuropeptide expression: the bag cell model system

The bag cell neuroendocrine system of Aplysia californica has been under intensive investigation for nearly two decades. The favorable morphology and hardiness in organ culture of this preparation have permitted a wide range of electrophysiological, cellular, and molecular studies. In this review we have focused our attention on the biochemical and physiological processes that serve the principle function of the bag cells: the synthesis and secretion of the neuropeptide egg-laying hormone. Although these cells were at first considered a model system for the most elementary neuroendocrine mechanisms, increasing knowledge has disclosed a surprising degree of complexity in both neuropeptide biosynthesis and the electrophysiological processes responsible for secretion. Not only may various components of the prohormone be sorted into different classes of neurosecretory granules, which may in turn have different probabilities of secretion, but biosynthesis itself appears to be regulated by the same intracellular messengers that mediate the electrophysiological discharge cycle. Hence, the bag cells, and presumably other peptidergic neurons, appear to possess an array of regulatory processes that can modulate the amount and character of their secretory output. The interactions of these processes may confer a degree of plasticity to the functional expression of peptidergic neurons unanticipated in studies of other neuron types.

Possible induction of fatty acid cyclooxygenase in mouse osteoblastic cells (MC3T3-E1) by cAMP

Prostaglandin E2 (PGE2), a bone-resorption factor, was essentially the sole arachidonate metabolite in an osteoblastic cell line cloned from mouse calvaria (MC3T3-E1). When the cells were cultured in the presence of 2% newborn bovine serum, 1 microM epinephrine markedly stimulated PGE2 synthesis from endogenous arachidonic acid. The PGE2 synthesis commenced after a lag phase of 1-2 h, and reached a maximum at about 3 h after the addition of epinephrine. The effect of epinephrine was inhibited by propranolol, and epinephrine could be replaced by isoproterenol, suggesting beta-adrenergic stimulation of PGE2 production. A rapid increase in intracellular cAMP was observed upon the addition of epinephrine. When the intracellular cAMP level was raised using cholera toxin or forskolin, the PGE2 synthesis was also stimulated. The enhanced PGE2 synthesis was attributed to an increased level of cyclooxygenase, which was shown by immunoprecipitation of the enzyme using anti-cyclooxygenase antibody. Inhibitors of transcription and translation suppressed the epinephrine-dependent increase in cyclooxygenase activity. These findings suggest induction of cyclooxygenase involving cAMP via an as yet unclarified mechanism.

Ontogeny of enkephalin- and VIP-containing neurons in dissociated cultures of embryonic mouse spinal cord and dorsal root ganglia

The ontogeny of vasoactive intestinal polypeptide (VIP), and Met-enkephalin in primary cultures of spinal cord/dorsal root ganglia from 12-day mouse embryos was examined by radioimmunoassay and immunohistochemistry. Met-enkephalin levels rose from less than 5 to 700 pg/culture over 26 days and were half maximal by day 16-18 in culture. VIP levels rose from less than 1 to 30 pg/culture over the same period, but were already half maximal by day 9. Met-enkephalin immunoreactivity was localized in multipolar medium sized neurons while VIP immunoreactivity was visualized both in neurons with extensively branched processes and in bipolar cells some of which appeared to be dorsal root ganglion cells. Tetrodotoxin (TTX)-sensitive spontaneous release of both peptides developed in parallel with the ability to stimulate peptide release with elevated potassium. Factors affecting the ontogeny of neuropeptide expression in, and release from, spinal cord neurons can now be examined in vitro in a strictly defined neurochemical environment.

Neural and humoral factors separately regulate neuropeptide Y, enkephalin, and chromogranin A and B mRNA levels in rat adrenal medulla

The influence of neurogenic versus humoral factors on mRNA levels of several secretory proteins of rat adrenal medulla was studied in vivo. Increased splanchnic nerve activity was generated (reflexly) with insulin treatment. Twenty-four hours after insulin injection, levels of mRNAs encoding neuropeptides (enkephalin and neuropeptide Y) were increased 6.5-fold, whereas those of mRNAs for the major secretory proteins (chromogranins A and B) were unchanged. Bilateral transection of the splanchnic nerves completely prevented this increase. Hypophysectomy decreased levels of chromogranin A mRNA to 32% of control, suggesting a dependence on hormones of the pituitary-adrenal axis. Treatment of hypophysectomized rats with dexamethasone restored chromogranin A mRNA to basal levels. Chromogranin B mRNA levels were not changed by either insulin treatment or hypophysectomy. These results demonstrate (i) that different classes of secretory proteins present in chromaffin granules are regulated by different mechanisms, (ii) that this regulation occurs at a pretranslational site, and (iii) that the relative concentration of secretory constituents of chromaffin granules may vary. The significance of an altered composition of secretory-granule constituents, which may be important in hypotension or stress, is discussed.

Mechanisms of vasoactive intestinal peptide release in short-term culture of vasoactive intestinal peptide-producing tumor

Vasoactive intestinal peptide-producing tumor tissue fragments obtained at surgery were maintained in short-term culture. Functional cellular integrity of vasoactive intestinal peptide-producing tumor tissue was reflected by progressive protein synthesis and the ability of tumor tissue to release vasoactive intestinal peptide when stimulated by the intracellular second messengers cyclic adenosine monophosphate and calcium. Studies with verapamil and ethyleneglycol-bis (beta-aminoethylether)-N,N'-tetraacetic acid suggest that cyclic nucleotide- and ionophore A23187-mediated vasoactive intestinal peptide release are dependent, at least in part, upon the availability and transmembrane transport of extracellular calcium.

Vasoactive intestinal peptide elevates cyclic AMP levels and potentiates secretion in bovine adrenal chromaffin cells

Vasoactive intestinal peptide (VIP) elevates the levels of cyclic AMP in primary cultures of highly purified chromaffin cells isolated from the bovine adrenal medulla, an effect that is potentiated by inhibitors of cyclic nucleotide phosphodiesterases. This elevation occurs within minutes and elevated levels of cyclic AMP are observed for as long as 24 hr in culture. The half-maximally effective concentration of VIP was approximately 1 microM. The VIP-induced elevation in chromaffin cell cyclic AMP levels was potentiated by simultaneous addition of secretagogues which alone also elevated cellular cyclic AMP levels. VIP alone failed to evoke catecholamine secretion from chromaffin cells, but potentiated potassium-, veratridine-, and nicotine-evoked secretion. Because VIP is found in nerve terminals innervating the adrenal medulla, this neuropeptide appears to be a modulator of chromaffin cell function, possibly via regulation of cyclic AMP levels.

Calcium requirements for barium stimulation of enkephalin and vasoactive intestinal peptide biosynthesis in adrenomedullary chromaffin cells

The divalent cation barium was used to study the role of calcium in coupling neuropeptide secretion and biosynthesis following secretagogue stimulation of bovine chromaffin cells. Barium chloride (0.1-2.5 mM) stimulated in a dose-dependent manner the secretion of met-enkephalin (up to 20% of intracellular peptide content) and increased the total amount (cell plus medium content) of met-enkephalin and vasoactive intestinal polypeptide (VIP) 2- to 3-fold after 72 hours. A greater than six-fold increase in proenkephalin mRNA (mRNA(enk)) was observed by 24 hours following barium stimulation. The voltage-sensitive calcium channel blocker D600 inhibited the barium-stimulated secretion of enkephalin and blocked the stimulation of VIP biosynthesis and mRNA(enk). Reducing calcium in the medium resulted in an enhancement of barium-stimulated release of both peptides, but blocked the induction of their biosynthesis. The data indicate that calcium targets involved in secretion can be activated by barium or calcium while calcium targets involved in biosynthesis specifically require calcium. It is therefore proposed that pathways leading to peptide secretion and biosynthesis in the adrenal diverge just after secretagogue-stimulated calcium influx.

The enkephalin-containing cell: strategies for polypeptide synthesis and secretion throughout the neuroendocrine system

1. Enkephalinergic cells are found throughout the diffuse neuroendocrine system, in the adrenal medulla, brain, spinal cord, peripheral and enteric nervous systems, and endocrine pancreas. 2. In each of these diverse cell types, the enkephalin phenotype is (i) established during development, (ii) modified by the particular environment in which the cell is located, and (iii) maintained by ongoing biosynthesis at a rate consistent with loss of enkephalins from the cell during periods of secretion. 3. Enkephalin expression and biosynthesis have been studied in several neuroendocrine cell types and tumor cell lines. Transcriptional, translational, and posttranslational factors can play a role at all three stages (establishment, modification, and maintenance) in the regulation of enkephalin expression during the lifetime of the cell. 4. Cyclic nucleotides, glucocorticoids, and calcium may all act to control the overall level of enkephalin biosynthesis pretranslationally, while regulation of posttranslational processing of proenkephalin seems to be important in determining the pattern of proenkephalin-derived opiate peptides produced in a given tissue. 5. The themes (and variations) of cell regulation that apply to enkephalin expression may be similar for other bioactive peptides produced in neural and endocrine tissues.

Reserpine increases chromaffin cell enkephalin stores without a concomitant decrease in other proenkephalin-derived peptides

Reserpine increases the levels of enkephalins in adrenal medullary chromaffin cells; however, the origin of the newly apparent pentapeptides has been the subject of debate, because no increase in the levels of proenkephalin mRNA has been observed. The present study was performed for determining if the reserpine-induced increase in context of enkephalins was derived from processing of preexisting fragments of proenkephalin. Bovine chromaffin cell enkephalins and larger enkephalin-containing peptides were separated by reversed-phase HPLC and identified by approximate molecular weight, elution with peptide standards, and enkephalin sequences contained. Treatment of the cells with reserpine increased the levels of enkephalins and of enkephalin-containing peptides of up to approximately 3 kilo-daltons without reducing the levels of larger enkephalin-containing peptides. Similar results were obtained with another catecholamine-depleting drug, tetrabenazine. In contrast, treatment of chromaffin cells with theophylline or forskolin increased the levels of both enkephalins and enkephalin-containing peptides of all sizes. The results suggest that new synthesis of proenkephalin is required for the effects of reserpine, although proenkephalin processing is also altered by this drug.

Thyroid hormones regulate levels of thyrotropin-releasing-hormone mRNA in the paraventricular nucleus

Cellular levels of messenger RNA encoding thyrotropin-releasing hormone (TRH) were measured in the paraventricular nucleus of the hypothalamus and the reticular nucleus of the thalamus in male rats after chemical thyroidectomy and thyroid hormone replacement. TRH mRNA levels were measured by quantitative in situ hybridization histochemistry using a 35S-labeled synthetic 48-base oligodeoxynucleotide probe and quantitative autoradiography. Chemical thyroidectomy, produced by the administration of 6-(n-propyl)-2-thiouracil (PrSur), reduced plasma thyroxine below detection limits and significantly increased TRH mRNA in the paraventricular nucleus. Treatment with exogenous L-triiodothyronine (T3) reduced TRH mRNA to the same level in both hypothyroid and euthyroid animals. Neither PrSur treatment nor T3 replacement influenced TRH mRNA levels in the reticular nucleus of the thalamus. Blot hybridization analysis of electrophoretically fractionated total RNA from pituitaries of these animals indicated that thyrotropin-beta mRNA levels were elevated after thyroidectomy and reduced by T3 treatment, showing that the pituitary-thyroid axis was indeed stimulated by PrSur treatment. These results suggest that thyroid hormones are involved, either directly or indirectly, in regulating the biosynthesis of TRH in the thyrotropic center of the hypothalamus.

Barium distinguishes separate calcium targets for synthesis and secretion of peptides in neuroendocrine cells

The effect of barium and potassium on the secretion and biosynthesis of enkephalin in bovine chromaffin cells, and prolactin and beta-endorphin in rat anterior pituitary cells, was examined to determine whether calcium-dependent secretion and biosynthesis are mediated by the same or by different calcium targets within the neuroendocrine cell. In the presence of 1.8 mM calcium, barium and potassium stimulated the secretion of all three peptides over 30 min, and increased the levels of proenkephalin and prolactin mRNA in 24 hr. These effects were inhibited by the calcium channel blocker D600. When the extracellular calcium concentration was lowered to 0.1 mM or less, secretion elicited by potassium was blocked, whereas secretion elicited by barium was enhanced, indicating that barium wholly substitutes for extracellular calcium in mediating peptide secretion. On the other hand, stimulation of proenkephalin and prolactin mRNA by both potassium and barium was inhibited when the extracellular calcium concentration was reduced. We conclude that calcium acts at two different intracellular targets to activate secretion versus biosynthesis of both enkephalin and prolactin. This appears to be the first report in which two different calcium-dependent processes in the intact cell are distinguished by a calcium ion agonist. Calcium-dependent processes such as protein phosphorylation, protein translocation, and enzyme activation may thus be related to events in the intact cell such as peptide synthesis and secretion on the basis of selective stimulation by barium.

Secretion of [Met]enkephalyl-Arg6-Phe7-related peptides and catecholamines from bovine adrenal chromaffin cells: modification by changes in cyclic AMP and by treatment with reserpine

Investigations into the effects of culturing bovine adrenal chromaffin cells in the presence (72 h) of dibutyryl cyclic AMP, forskolin, and reserpine on the level and release of [Met]enkephalyl-Arg6-Phe7 immunoreactivity, noradrenaline, and adrenaline are reported. The assay for [Met]enkephalyl-Arg6-Phe7 immunoreactivity recognises both peptide B, the 31-amino acid carboxy-terminal segment of proenkephalin, and its heptapeptide fragment, [Met]enkephalyl-Arg6-Phe7. Treatments that elevate cyclic AMP increase the amount of peptide immunoreactivity in these cells; this is predominantly peptide B-like immunoreactivity in both control cells and cyclic AMP-elevated cells. Treatment with reserpine gives no change in total immunoreactivity levels, but does not result in increased accumulation of the heptapeptide [Met]enkephalyl-Arg6-Phe7 at the expense of immunoreactivity that elutes with its immediate precursor, peptide B. Cyclic AMP treatment causes either no change or a decrease in levels of accumulated noradrenaline and adrenaline. However, the release of [Met]enkephalin-Arg6-Phe7 immunoreactivity, noradrenaline, and adrenaline is increased by 72-h pretreatment with forskolin or dibutyryl cyclic AMP, whether release is stimulated by nicotine or elevated potassium. In each case the molecular form of [Met]enkephalyl-Arg6-Phe7 immunoreactivity that is released approximately reflects the cell content. Pretreatment with reserpine has no effect on the total [Met]enkephalyl-Arg6-Phe7 immunoreactivity released, but does result in an increased release of the heptapeptide and a decrease in release of peptide B-like immunoreactivity. The studies suggest that the levels of [Met]enkephalyl-Arg6-Phe7 and peptide B available for release are controlled both at the level of proenkephalin synthesis and at the level of double-basic residue proteolysis.

Chromogranin A synthesis and secretion in chromaffin cells

A sensitive and selective radioimmunoassay for chromogranin A (Chrg A) has been developed to quantitate content, release, and biosynthesis of this secretory protein in neuroendocrine tissues. An antiserum raised against Chrg A from bovine adrenal medulla was found to detect predominantly only the Mr 70-75 kilodalton Chrg A in its native form, allowing the use of this antiserum as a quantitatively specific probe for Chrg A in cell-free extracts of the adrenal medulla and chromaffin cells. Chrg A comprises about 10% of the total protein of the chromaffin cell. It is released in parallel with Met-enkephalin and catecholamines from the bovine chromaffin cell in primary culture in response to nicotine and nicotinic cholinergic agonists. From 14 to 22% of total Chrg A is released from the cell during a 15-min exposure to a maximally stimulatory dose of nicotine (10-100 microM). Chrg A release on nicotinic stimulation is blocked by D-600 and hexamethonium to the same extent as Met-enkephalin and catecholamine release. The parallel time course and percent release of Chrg A and Met-enkephalin indicate that these secretory polypeptides are contained in, and released from, functionally identical cellular compartments. Chrg A and Met-enkephalin pentapeptide sequences are present in the chromaffin cell at a ratio of about 2:1, although Chrg A is far more abundant on a mass basis. Chrg A and Met-enkephalin biosynthesis appear to be differentially regulated within the chromaffin cell, since chronic treatment of cells with nicotine and forskolin causes an elevation of Met-enkephalin pentapeptide without a concomitant elevation of intracellular levels of Chrg A.

The use of a tyrosine-hydroxylase cDNA probe to study the neurotransmitter plasticity of rat sympathetic neurons in culture

We have compared quantitatively the effects of muscle-conditioned medium (CM) and elevated K+ concentration (40 mM) on the enzymatic activity of tyrosine hydroxylase (TH) and on TH-mRNA levels in primary cultures of rat sympathetic neurons. Northern blot analysis of RNA from cultured neurons with a 32P-labeled rat TH-cDNA probe was performed. The probe hybridized strongly with a single RNA species of 1.9 kb, similar in size to the TH-mRNA from PC12 pheochromocytoma cells. In agreement with earlier data both CM and a partially purified factor from CM increased choline acetyltransferase activity up to 200-fold and depressed TH activity by 2- to 7-fold in cultured sympathetic neurons. These effects were accompanied by a decrease in TH-mRNA level, which correlated with the decrease in TH activity. On the other hand, a culture medium supplemented with 40 mM KCl caused a 1.5- to 5-fold increase in TH activity, which was accompanied by an increase in TH-mRNA level of the same order of magnitude. As a working hypothesis, we suggest that CM and neuronal depolarization control the transcription of the TH gene in an antagonistic manner.

Met-enkephalyl-Arg6-Phe7 immunoreactivity in a human neuroblastoma cell line: effect of dibutyryl 3':5'-cyclic AMP and reserpine

The carboxy terminal part of the proenkephalin A sequence is the 31 amino acid peptide B, which has as its final seven amino acids the sequence of the opioid peptide Met-enkephalyl-Arg6-Phe7. Using a radioimmunoassay which recognises both these peptides we have investigated the relative amounts of peptide B and Met-enkephalyl-Arg6-Phe7 in a human neuroblastoma cell line. We show that these cells contain peptide B-like immunoreactivity but not its heptapeptide fragment. This may be due to lack of proteolytic activity cleaving Met-enkephalyl-Arg6-Phe7 from its precursor, peptide B. On treatment with dibutyryl cyclic AMP the level of immunoreactivity approximately doubles, due to increased amounts of peptide B-like immunoreactivity. Treatment with reserpine, which increases conversion of peptide B to the heptapeptide in bovine chromaffin cells in culture does not stimulate the accumulation of Met-enkephalyl-Arg6-Phe7 in the human neuroblastoma cells. The results are discussed with respect to peptide processing.