The distribution of opioid binding subtypes in the bovine adrenal medulla

Ontogeny of O2 and CO2//H+ chemosensitivity in adrenal chromaffin cells: role of innervation

The adrenal medulla plays a key role in the physiological responses of developing and mature mammals by releasing catecholamines (CAT) during stress. In rodents and humans, the innervation of CAT-producing, adrenomedullary chromaffin cells (AMCs) is immature or absent during early postnatal life, when these cells possess 'direct' hypoxia- and CO2/H(+)-chemosensing mechanisms. During asphyxial stressors at birth, these mechanisms contribute to a CAT surge that is critical for adaptation to extra-uterine life. These direct chemosensing mechanisms regress postnatally, in parallel with maturation of splanchnic innervation. Here, we review the evidence that neurotransmitters released from the splanchnic nerve during innervation activate signaling cascades that ultimately cause regression of direct AMC chemosensitivity to hypoxia and hypercapnia. In particular, we consider the roles of cholinergic and opioid receptor signaling, given that splanchnic nerves release acetylcholine and opiate peptides onto their respective postsynaptic nicotinic and opioid receptors on AMCs. Recent in vivo and in vitro studies in the rat suggest that interactions involving α7 nicotinic acetylcholine receptors (nAChRs), the hypoxia inducible factor (HIF)-2α signaling pathway, protein kinases and ATP-sensitive K(+) (KATP) channels contribute to the selective suppression of hypoxic chemosensitivity. In contrast, interactions involving μ- and/or δ-opiod receptor signaling pathways contribute to the suppression of both hypoxic and hypercapnic chemosensitivity, via regulation of the expression of KATP channels and carbonic anhydrase (CA I and II), respectively. These data suggest that the ontogeny of O2 and CO2/H(+) chemosensitivity in chromaffin cells can be regulated by the tonic release of presynaptic neurotransmitters.

Mediation of Endogenous β-Endorphin in the Plasma Glucose-Lowering Action of Herbal Products Observed in Type 1-Like Diabetic Rats

Recently, there have been advances in the development of new substances effective in managing diabetic disorders. Opioid receptors couple multiple systems to result in various biological effects, although opioids are best known for analgesia. In the present review, we used our recent data to describe the advance in plasma glucose-lowering action of herbal products, especially the mediation of β-endorphin in glucose homeostasis of insulin-deficient diabetes. In type 1-like streptozotocin-induced diabetic rats, we identified many products purified from herbs that show a dose-dependent plasma glucose-lowering action. Increase in β-endorphin secretion from the adrenal gland may activate peripheral opioid μ-receptors (MOR) to enhance the expression of muscle glucose transporters and/or to reduce hepatic gluconeogenesis at the gene level, thereby leading to improved glucose utilization in peripheral tissues for amelioration of severe hyperglycemia. It has also been observed that stimulation of α(1)-adrenoceptors (α(1)-ARs) in the adrenal gland by some herbal products is responsible for the increase in β-endorphin secretion via a phospholipase C-protein kinase dependent pathway. However, an increase in β-endorphin secretion from the adrenal gland by herbal products can function via another receptor. New insights into the mediation of endogenous β-endorphin activation of peripheral MOR by herbal products for regulation of glucose homeostasis without the presence of insulin have been established. Therefore, an increase in β-endorphin secretion and/or direct stimulation of peripheral MOR via an insulin-independent action might serve as the potential target for development of a therapeutic agent or promising adjuvant in intensive plasma glucose control.

Functional distribution of Ca2+-coupled P2 purinergic receptors among adrenergic and noradrenergic bovine adrenal chromaffin cells

BACKGROUND

Adrenal chromaffin cells mediate acute responses to stress through the release of epinephrine. Chromaffin cell function is regulated by several receptors, present both in adrenergic (AD) and noradrenergic (NA) cells. Extracellular ATP exerts excitatory and inhibitory actions on chromaffin cells via ionotropic (P2X) and metabotropic (P2Y) receptors. We have taken advantage of the actions of the purinergic agonists ATP and UTP on cytosolic free Ca2+ concentration ([Ca2+]i) to determine whether P2X and P2Y receptors might be asymmetrically distributed among AD and NA chromaffin cells.

RESULTS

The [Ca2+]i and the [Na+]i were recorded from immunolabeled bovine chromaffin cells by single-cell fluorescence imaging. Among the ATP-sensitive cells ~40% did not yield [Ca2+]i responses to ATP in the absence of extracellular Ca2+ (Ca2+o), indicating that they expressed P2X receptors and did not express Ca2+- mobilizing P2Y receptors; the remainder expressed Ca2+-mobilizing P2Y receptors. Relative to AD-cells approximately twice as many NA-cells expressed P2X receptors while not expressing Ca2+- mobilizing P2Y receptors, as indicated by the proportion of cells lacking [Ca2+]i responses and exhibiting [Na+]i responses to ATP in the absence and presence of Ca2+o, respectively. The density of P2X receptors in NA-cells appeared to be 30-50% larger, as suggested by comparing the average size of the [Na+]i and [Ca2+]i responses to ATP. Conversely, approximately twice as many AD-cells expressed Ca2+-mobilizing P2Y receptors, and they appeared to exhibit a higher (~20%) receptor density. UTP raised the [Ca2+]i in a fraction of the cells and did not raise the [Na+]i in any of the cells tested, confirming its specificity as a P2Y agonist. The cell density of UTP-sensitive P2Y receptors did not appear to vary among AD- and NA-cells.

CONCLUSION

Although neither of the major purinoceptor types can be ascribed to a particular cell phenotype, P2X and Ca2+-mobilizing P2Y receptors are preferentially located to noradrenergic and adrenergic chromaffin cells, respectively. ATP might, in addition to an UTP-sensitive P2Y receptor, activate an UTP-insensitive P2Y receptor subtype. A model for a short-loop feedback interaction is presented whereby locally released ATP acts upon P2Y receptors in adrenergic cells, inhibiting Ca2+ influx and contributing to terminate evoked epinephrine secretion.

Fine ultrastructure of chromaffin granules in rat adrenal medulla indicative of a vesicle-mediated secretory process

Observation by transmission electron microscopy, coupled with morphometric analysis and estimation procedure, revealed unique ultrastructural features in 25.94% of noradrenaline (NA)-containing granules and 16.85% of adrenaline (A)-containing granules in the rat adrenal medulla. These consisted of evaginations of the granule limiting membrane to form budding structures having different morphology and extension. In 14.8% of NA granules and 12.0% of A granules, outpouches were relatively short, looked like small blebs emerging from the granule surface and generally contained electron-dense material. A proportion of 11.2% of NA granules and 4.9% of A granules revealed the most striking ultrastructural features. These secretory organelles presented thin, elongated, tail-like or stem-like appendages, which were variably filled by chromaffin substance and terminated with spherical expansions of different electron density. A cohort of vesicles of variable size (30-150 nm in diameter) and content was found either close to them or in the intergranular cytosol. Examination of adrenal medullary cells fixed by zinc iodide-osmium tetroxide (ZIO) revealed fine electron dense precipitates in chromaffin granules, budding structures as well as cytoplasmic vesicles. These data indicate that a common constituent is revealed by the ZIO histochemical reaction in chromaffin cells. As catecholic compounds are the main tissue targets of ZIO complexes, catecholamines are good candidates to be responsible for the observed ZIO reactivity. This study adds further to the hypothesis that release of secretory material from chromaffin granules may be accomplished by a vesiclular transport mechanism typical of piecemeal degranulation.

Opioid receptor stimulation suppresses the adrenal medulla hypoxic response in sheep by actions on Ca(2+) and K(+) channels

Before the preganglionic regulation of the adrenal medulla is established, hypoxia acts directly on the chromaffin cells to evoke the secretion of catecholamines. This direct action of hypoxia is suppressed by the gradual development of the preganglionic innervation and we have proposed that opioid peptides released from the adrenal splanchnic nerves may be responsible for this suppression. The effects of the specific opioid agonists DPDPE (delta-agonist), U-62066 (kappa-agonist) and DALDA (mu-agonist) on the hypoxia-evoked response were investigated in both a whole-gland preparation and in isolated adrenal chromaffin cells using amperometry, whole-cell patch clamping and measurement of cytosolic [Ca(2+)]. The combined application of mu- and kappa-type agonists abolished the hypoxia-evoked catecholamine secretion from whole perfused adrenal gland. In isolated chromaffin cells, mu- and kappa-opioid agonists reduced the rise in [Ca(2+)](i) that results from exposure to hypoxia. Both agonists decreased the voltage-dependent Ca(2+) current in these cells. The mu-agonist increased the conductance through SK-type K(+) channels and this action offset the decrease in K(+) conductance produced by exposure to hypoxia. The kappa-type agonist decreased the conductance through an action on BK-type K(+) channels, a class of channels that are not involved in initiating the direct response to hypoxia. These data suggest that opioids, through their action on SK channels and voltage-dependent Ca(2+) channels, may be responsible for the nerve-induced suppression of the hypoxic response of adrenal chromaffin cells and that these effects of endogenous opioids are mediated via mu- and kappa-type receptors.

Opposite action of beta1- and beta2-adrenergic receptors on Ca(V)1 L-channel current in rat adrenal chromaffin cells

Voltage-gated Ca(2+) channels of chromaffin cells are modulated by locally released neurotransmitters through autoreceptor-activated G-proteins. Clear evidence exists in favor of a Ca(2+) channel gating inhibition mediated by purinergic, opioidergic, and alpha-adrenergic autoreceptors. Few and contradictory data suggest also a role of beta-adrenergic autoreceptors (beta-ARs), the action of which, however, remains obscure. Here, using patch-perforated recordings, we show that rat chromaffin cells respond to the beta-AR agonist isoprenaline (ISO) by either upmodulating or downmodulating the amplitude of Ca(2+) currents through two distinct modulatory pathways. ISO (1 microm) could cause either fast inhibition (approximately 25%) or slow potentiation (approximately 25%), or a combination of the two actions. Both effects were completely prevented by propranolol. Slow potentiation was more evident in cells pretreated with pertussis toxin (PTX) or when beta(1)-ARs were selectively stimulated with ISO + ICI118,551. Potentiation was absent when the beta(2)-AR-selective agonist zinterol (1 microm), the protein kinase A (PKA) inhibitor H89, or nifedipine was applied, suggesting that potentiation is associated with a PKA-mediated phosphorylation of L-channels (approximately 40% L-current increase) through beta(1)-ARs. The ISO-induced inhibition was fast and reversible, preserved in cell treated with H89, and mimicked by zinterol. The action of zinterol was mostly on L-channels (38% inhibition). Zinterol action preserved the channel activation kinetics, the voltage-dependence of the I-V characteristic, and was removed by PTX, suggesting that beta(2)AR-mediated channel inhibition was mainly voltage independent and coupled to G(i)/G(o)-proteins. Sequential application of zinterol and ISO mimicked the dual action (inhibition/potentiation) of ISO alone. The two kinetically and pharmacologically distinct beta-ARs signaling uncover alternative pathways, which may serve the autocrine control of Ca(2+)-dependent exocytosis and other related functions of rat chromaffin cells.

Tramadol inhibits norepinephrine transporter function at desipramine-binding sites in cultured bovine adrenal medullary cells

Tramadol is a widely used analgesic, but its mode of action is not well understood. To study the effects of tramadol on norepinephrine transporter (NET) function, we assayed the effect of tramadol on [3H]-norepinephrine ([3H]-NE) uptake and [3H]-desipramine binding to plasma membranes isolated from bovine adrenal medulla. We then characterized [14C]-tramadol binding in cultured bovine adrenal medullary cells. Tramadol inhibited the desipramine-sensitive uptake of [3H]-NE by the cells in a concentration-dependent manner (50% inhibitory concentration = 21.5 +/- 6.0 microM). Saturation analysis revealed that tramadol increased the apparent Michaelis constant of [3H]-NE uptake without changing the maximal velocity, indicating that inhibition occurred via competition for the NET (inhibition constant, K(i) = 13.7 microM). Tramadol inhibited the specific binding of [3H]-desipramine to plasma membranes. Scatchard analysis of [3H]-desipramine binding revealed that tramadol increased the apparent dissociation constant (K(d)) for binding without altering maximal binding, indicating competitive inhibition (K(i) = 11.2 microM). The binding of [14C]-tramadol to the cells was specific and saturable, with a K(d) of 18.1 +/- 2.4 microM. These findings indicate that tramadol competitively inhibits NET function at desipramine-binding sites.

IMPLICATIONS

Tramadol competitively inhibits norepinephrine transporter function at desipramine-binding sites in the adrenal medullary cells and probably the noradrenergic neurons of the descending inhibitory system.

Increase of beta-endorphin biosynthesis in the adrenal gland of streptozotocin-induced diabetic rats

Opioid plays an important role in the regulation of glucose homeostasis in diabetic rats lacking insulin. The present study investigated the changes of beta-endorphin biosynthesis in the adrenal medulla of streptozotocin-induced diabetic rats (STZ-diabetic rats) by determination of the gene expression of pro-opiomelanocortin (POMC) and the amount of beta-endorphin. Expression of the distinct mRNA that encodes proteins of POMC was studied using reverse transcription combined with polymerase chain reaction (RT-PCR). Results of RT-PCR demonstrated that the mRNA level of POMC in the adrenal gland markedly increased in STZ-diabetic rats as compared with that in normal rats. The content of beta-endorphin-like immunoreactivity (BER) in the adrenal medulla, determined by enzyme-linked immunosorbent assay, was actually higher in diabetic rats with insulin deficiency. Normalization of the plasma glucose concentration in STZ-diabetic rats with exogenous insulin or phlorizin, an inhibitor of the renal tubular glucose transport, reversed the mRNA level of POMC in the adrenal gland after 4 days of treatment. A similar decrease of BER amount also observed in the adrenal medulla of STZ-diabetic rats received the same treatment with exogenous insulin or phlorizin. Therefore, correction of hyperglycemia in STZ-diabetic rats could reverse the higher gene expression of POMC in the adrenal gland. These results suggest that hyperglycemia is responsible for the increase of POMC gene expression to enhance beta-endorphin biosynthesis in the adrenal gland of STZ-diabetic rats.

Stimulatory effect of phenylephrine on the secretion of beta-endorphin from rat adrenal medulla in vitro

In an attempt to investigate the role of alpha1-adrenoceptors in the regulation of opioid secretion from adrenal gland, phenylephrine was employed to investigate the effect on secretion of beta-endorphin-like immunoreactivity (BER) from adrenal medulla of rat in vitro. Phenylephrine enhanced the BER from isolated adrenal medulla in a concentration-dependent manner and this action was abolished by the antagonists of alpha1-adrenoceptors, prazosin and tamsulosin. Investigations of signal pathway further support that an activation of alpha1-adrenoceptors is responsible for the stimulatory effect of phenylephrine on BER secretion from adrenal medulla. In the presence of U73312, the specific inhibitor of phospholipase C (PLC), phenylephrine-induced change of BER was reduced in a concentration-dependent manner but it was not affected by U73343, the negative control of U73312. Moreover, chelerythrine and GF 109203X diminished the action of phenylephrine at concentration sufficient to inhibit protein kinase C (PKC). In conclusion, our results suggest that an activation of alpha1-adrenoceptors in adrenal medulla by phenylephrine may enhance the secretion of opioids from adrenal gland of rat via signals of PLC-PKC pathway.

Differential expression of calcium channel subtypes in the bovine adrenal medulla

This study aimed at determining the distribution and expression levels of different subtypes of Ca(2+) channels in the bovine adrenal medulla, and whether individual subtypes were more abundant in chromaffin cells exhibiting an adrenergic or a noradrenergic phenotype. In situ hybridization using riboprobes specific for the pore-forming Ca(2+) channel alpha(1D) (L-type channel), alpha(1B) (N-type channel), and alpha(1A) (P/Q-type channel) subunits of bovine chromaffin cells showed a broad distribution of the three transcripts in adrenal medulla tissue. However, a tissue-specific expression pattern of individual subunits was found; whereas alpha(1B) mRNA was homogeneously distributed throughout the medulla, alpha(1D) and alpha(1A) transcripts were present at higher densities in the internal medullary area, far away from the adrenal cortex. These results were corroborated by comparative analysis of the alpha(1B), alpha(1D), and alpha(1A) products amplified by RT-PCR from total RNA extracted from small pieces of tissue dissected out from external or internal medullary areas. Interestingly, immunohistochemical experiments performed in adrenal gland sections, using antidopamine-beta-hydroxylase and anti-phenylethanolamine-N-methyltransferase antibodies, indicated a higher density of noradrenergic over adrenergic chromaffin cells in the internal medullary region. These results provide direct evidence in favor of a heterogeneous distribution of Ca(2+) channel subtypes in the adrenal medulla, in agreement with previous functional data showing that blockade of the high K+ -elicited responses by dihydropyridines was greater in noradrenergic than in adrenergic chromaffin cells. These differences may be relevant for the differential release regulation of each catecholamine under physiological and pathophysiological conditions.

Loss of plasma glucose lowering response to cold stress in opioid mu-receptor knock-out diabetic mice

Opioid mu-receptor plays an important role in the regulation of glucose homeostasis in diabetic rats lacking insulin. Opioid mu-receptor knockout mice were employed to identify the essential role of this receptor in the present study. Western blotting analysis characterized the deletion of opioid mu-receptor in liver of knockout mice as compared to that of normal (wild-type) mice. We found that the plasma glucose concentration of diabetic mice induced by intraperitoneal injection of streptozotocin was markedly decreased after exposure to cold-stress in a cold room for 1 h. However, this plasma glucose lowering response to cold-stress was disappeared in diabetic mice lacking opioid mu-receptor. The important role of opioid mu-receptor in the plasma glucose lowering response to cold stress can thus be considered. Moreover, bilateral adrenalectomy abolished this plasma glucose lowering response to cold stress in diabetic mice with opioid mu-receptor, as compared to the shamed-operated animals. Therefore, activation of opioid mu-receptor by opioid from adrenal gland appears to be responsible for the plasma glucose lowering response to cold-stress in diabetic mice with insulin deficiency.

Peripheral modulation of learning and memory: enkephalins as a model system

Extensive research on the effects of enkephalins on conditioning is reviewed and used as the basis for a model of peripheral modulation of learning and memory. An overall theme emphasized throughout our discussion is that these peptides can influence the strength with which a memory is acquired and stored by acting outside the blood-brain barrier. This assertion is supported by research on the behavioral effects of systemically administered enkephalins and opioid antagonists, the rapid hydrolysis of circulating enkephalins in vivo, and the limited ability of these peptides to penetrate the blood-brain barrier. A consideration of the extensive distribution of enkephalins throughout peripheral autonomic systems leads to the proposal that enkephalins may act to modulate learning and memory by altering peripheral autonomic function; autonomic afferents may then communicate with the memory trace in the CNS through a central modulatory pathway outlined herein. Evidence that some stressful experiences may lead to increases in circulating enkephalins also is discussed. The sites of action of these circulating enkephalins may involve peripheral autonomic sites, or additionally may involve the circumventricular organs. As a further regulatory mechanism, circulating enkephalin levels may be controlled by experience-dependent alterations of the activity of enzyme systems that participate in their breakdown. Finally, it is emphasized that the mechanisms of enkephalin action postulated herein may be applicable to the actions of other peripheral hormones, peptides, and neurotransmitters that participate in the modulation of learning and memory storage processes.

Characterization of ZFOR1, a putative delta-opioid receptor from the teleost zebrafish (Danio rerio)

ZFOR1 is a putative opioid receptor from zebrafish brain which has 66% homology with the mammalian delta-opioid receptor. When expressed in HEK293 cells ZFOR1 bound the non-selective opioid antagonist [(3)H]diprenorphine with high affinity. However, the binding of this ligand was not readily displaced by opioids selective for mu, delta or kappa opioid receptors (affinities>1000 nM). Rather non-selective ligands showed good affinity, as did the non-peptide delta-ligand BW373U86 (Ki 69 nM), the delta-antagonist naltrindole (Ki 28 nM) and the peptide beta-endorphin (Ki 37 nM). Agonist binding to the receptor encoded by ZFOR1 receptor stimulated the binding of [(35)S]GTPgammaS confirming coupling to G proteins. Study of the receptor should contribute to understanding of the evolution of the opioid system.

Physiological aspects of exocytosis in chromaffin cells of the adrenal medulla

The adrenal medulla is composed principally of groups of adrenergic and noradrenergic chromaffin cells, with minor populations of small intensely fluorescent cells and ganglionic neurones. Different molecular stimuli evoke distinct secretory events in the gland, involving the release of either adrenaline or noradrenaline together with various neuroactive peptides. The nature of the secretory response can be controlled at a central level or regulated locally within the gland. Specific innervation patterns to the different types of chromaffin cell have been implicated in central regulatory mechanisms, while several explanations for regulating secretion locally have been proposed. The differential distribution of various types of receptors between cell phenotypes, such as muscarinic or nicotinic acetylcholine receptors, histamine receptors, angiotensin receptors and different classes of opiate receptors between the two principal chromaffin cell populations could be involved in local control. In addition exocytosis parameters could be modulated differently in adrenergic and noradrenergic cells by phenotype-specific mechanisms, possibly involving molecules like Growth Associated Protein-43, Synaptosomal Associated Protein-25 isoforms or the p11 annexin subunit. The distribution of the various types of calcium channels is also known to vary between chromaffin cell subtypes. This short review examines possible ways in which specific innervation patterns in the adrenal gland could be programmed and discusses exocytosis mechanisms that could differ between chromaffin cell phenotypes. Data reviewed here suggest that the adrenal medulla should no longer be viewed as a homogeneous entity but as consisting of an ensemble of individual cell subpopulations each with a distinct secretory response that could in part reflect its local history.

Kappa1-opioid binding sites are the dominant opioid binding sites in surgical specimens of human pheochromocytomas and in a human pheochromocytoma (KAT45) cell line

The adrenal medulla produces opioids which exert paracrine effects on adrenal cortical and chromaffin cells and on adrenal splanchnic nerves, via specific binding sites. The opioid binding sites in the adrenals are detectable mainly in the medullary part of it and differ in type between species. Thus, the bovine adrenal medulla contains mostly kappa-opioid binding sites and fewer delta- and mu-opioid binding sites while primate adrenals contain mainly delta sites and few kappa-opioid binding sites. Most chromaffin cell tumors, the pheochromocytomas, produce opioids which suppress catecholamine production by the tumor. The aim of the present work was to identify the types of opioid binding sites in human pheochromocytomas. For this purpose, we characterized the opioid binding sites on crude membrane fractions prepared from 14 surgically excised pheohromocytomas and on whole KAT45 cells, a recently characterized human pheochromocytoma cell line. Our data showed that human pheohromocytomas are heterogeneous, as expected, with regard to the production of catecholamines and the distribution and profile of their opioid binding sites. Indeed, only one out of the 14 pheochromocytomas expressed exclusively delta and mu opioid sites, while in the remaining 13 tumors kappa-type binding sites were dominant. The KAT45 cell line possessed a significant number of kappa1 binding sites, fewer kappa2-opioid binding sites and kappa3-opioid binding sites, and minimal binding capacity for delta- and mu-opioid receptor agonists sites. More specifically, the kappa1 sites/cell were approximately 18,000, the kappa2 4500/cell and the kappa3 sites 2000/cell. Our findings for the surgical specimens and the cell line combined with previously published pharmacological data obtained from KAT45 cells suggest that kappa sites appear to be the most prevalent opioid binding sites in pheochromocytomas. Finally, in normal bovine adrenals the profile of opioid binding sites differs in adrenaline and noradrenaline producing chromaffin cells. To test the hypothesis that the type of catecholamine produced by a pheochromocytoma depends on its cell of origin, we compared our binding data with the catecholamine content of each pheochromocytoma examined. We found no correlation between the type of the predominant catecholamine produced and the opioid binding profile of each tumor suggesting that this hypothesis may not be valid.

Ibogaine acts at the nicotinic acetylcholine receptor to inhibit catecholamine release

In an effort to determine mechanisms of action of the putative anti-addictive agent ibogaine, we have measured its effects on catecholamine release in a model neuronal system, cultured bovine chromaffin cells. Various modes of stimulating catecholamine release were used including nicotinic ACh receptor activation, membrane depolarization with elevated K+ and Na+ channel activation with veratridine. In addition, because ibogaine has been reported to interact with kappa opioid receptors, we tested whether kappa receptor antagonists could reverse ibogaine's effects on catecholamine release. Ibogaine, at low concentration (<10 microM) was found to selectively inhibit nicotinic receptor-mediated catecholamine release, while having no significant effect on release evoked by either veratridine or membrane depolarization with elevated K+. The inhibitory actions of ibogaine and the kappa agonists were not reversed by preincubation with the opioid antagonists nor-binaltorphimine or naltrexone, suggesting that these inhibitory effects are not mediated by the kappa opioid receptor. The effects of low dose (10 microM) ibogaine were rapidly reversible, while the inhibitory effects of higher ibogaine doses persisted for at least 19 h following ibogaine washout. The results provide evidence for a mechanism of action ibogaine at the nicotinic ACh receptor. The results are consistent with a model in which the initial high transient brain concentrations (100 microM) of ibogaine act at multiple cellular sites and then have a selective action at the nicotinic ACh receptor cation channel following its metabolism to lower brain concentrations. The present findings are relevant to potential anti-addictive actions of ibogaine and to the development of drugs to combat nicotine addiction.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

The PC12 rat pheochromocytoma cell line expresses the prodynorphin gene and secretes the 8 kDa dynorphin product

Most adrenal chromaffin cells synthesize opioids derived from proenkephalin but not from prodynorphin. However, human pheochromocytomas and the PC12 rat pheochromocytoma cell line synthesize dynorphins. The aim of this study was to confirm the presence of the authentic prodynorphin transcript and its dynorphin product in PC12 cells. We have found that the sequence of a 458 bp cDNA fragment derived from RT-PCR amplification of total PC12 RNA was in complete accordance with the published sequence of the equivalent region of the prodynorphin gene. It encodes the potent endogenous kappa opioid agonists alpha-neo-endorphin, dynorphin A and dynorphin B. Furthermore, immunoaffinity-purified PC12 cell extracts were subjected to RP-HPLC. Most of its IR-dynorphin eluted on a peak exhibiting the retention time of similarly treated rat anterior pituitary. The expression of the prodynorphin gene in pheochromocytomas can be explained as either the result of (a) the process of dedifferentiation of chromaffin cells to pheochromocytoma which may thus cause the expression of a previously unexpressed prodynorphin or that (b) those pheochromocytomas expressing the prodynorphin gene derive from the few, centrally located chromaffin cells, which express this gene even under normal conditions.

Kappa opioids exert a strong antiproliferative effect on PC12 rat pheochromocytoma cells

Pheochromocytomas synthesize several types of opioids and their receptors. Opioids affect the proliferation rate of normal and tumoral cells. We have previously shown that the PC12 rat pheochromocytoma cells synthesize multiple opioids. The aim of the present work was to study the effect of opioids on the proliferation of these pheochromocytoma cells. Thus, the effect of several opioid agonists and antagonists was examined on basal and EGF-induced PC12 cell proliferation. The kappa opioid agonists dynorphin A, U-69593, and U-50488 suppressed basal proliferation in a dose-dependent manner. The effect of kappa opioids was blocked by the general opioid antagonist naloxone and the selective kappa antagonist nor-binaltorphimine. Furthermore, both opioid antagonists given alone had a strong stimulatory effect, a findings suggesting that the proliferation of PC12 cells is under tonic inhibition by locally produced kappa opioids. Finally, the mu-opioid agonist DAGO and the delta and mu agonists DADLE and DSLET were ineffective.

Opioids inhibit dopamine secretion from PC12 rat pheochromocytoma cells in a naloxone-reversible manner

Opioids inhibit the release of catecholamines in the nervous system. Normal adrenal chromaffin cells produce delta opioids and they respond to them by suppressing the release of their catecholamines. Chromaffin cell tumors, the pheochromocytomas, produce mainly kappa opioids. The aim of this work was: (a) to test if pheochromocytomas retain the response of normal chromaffin cell catecholamines to delta opioids and to naloxone (a general opioid antagonist), and (b) to test if kappa opioids exert any specific effect on catecholamine release from these tumors. Since we have previously shown that, in common with human pheochromocytomas, the PC12 rat pheochromocytoma cells express the prodynorphin gene and secret its kappa opioid products, we used these cells to examine the effect of several opioid agonists and of naloxone on basal, nicotine-, and KCl-induced dopamine release. Dopamine is the main PC12 catecholamine. We have found that the specific kappa opioid agonist U-69593 inhibited the release of dopamine in a dose-dependent manner (IC50=0.5 x 10(-8)M). Under basal conditions the mean concentration of dopamine in the culture media was 11.25 +/- 0.57 ng/mg of total cellular protein (n=13). A 30 min exposure to U-69593 at 10(-6) M suppressed basal dopamine release to 58 +/- 2% (n=7) of controls. A 12 hr pre-incubation with U-69593 caused the same degree of suppression. The effect of the synthetic kappa opioid agonist dynorphin A was indistinguishable from that of U-69593. DADLE (a mu and delta synthetic opioid agonist) was significantly less effective in suppressing dopamine release (IC50=10(-7)M). The concentration of dopamine following exposure to 10-6 M of DADLE for 30 min was 74 +/- 5% of the controls (n=4). The mu opioid agonist DAGO was ineffective. The suppressive effect of all opioid agonists was blocked by naloxone suggesting that conventional opioid receptors were involved.

The sigma compounds 1,3-di-o-tolylguanidine and N-allylnormetazocine inhibit agonist-stimulated inositol phospholipid metabolism in bovine adrenal medullary cells

Muscarine stimulated a concentration-dependent accumulation of [3H]inositol phosphates in bovine adrenal medullary cells preloaded with [3H]inositol. This muscarinic activation of inositol phospholipid metabolism was fully inhibited by the sigma-ligand 1,3-di-o-tolylguanidine (DTG) with an IC50 of approximately 45 microM. Higher concentrations (100 microM) of (+) N-allylnormetazocine (SKF-10047) also partially inhibited this response. A concentration of DTG sufficient to fully inhibit the muscarinic response also produced a significant partial inhibition of [3H]inositol phosphate accumulation in response to histamine but not to angiotensin II. These data demonstrate that sigma-compounds inhibit agonist-stimulated inositol phospholipid metabolism in bovine adrenal medullary cells, with a degree of selectivity towards the muscarinic response.

Non-cholinergic nervous control of catecholamine secretion from perfused bovine adrenal glands

1. Field stimulation of adrenal nerves was used to study nervous control of adrenal catecholamine secretion in isolated, retrogradely perfused, bovine adrenal glands. 2. Secretion of both adrenaline and noradrenaline was maximal at 10 Hz. Secretion at 2 Hz was < 10% of maximum. Stimulating with trains of pulses at ten times the average frequency for 1 s out of every 10 s gave 2-fold greater secretion at 2 Hz average frequency, similar release at 5 Hz, and only half the secretion at 10 Hz, compared to continuous stimulation at the average frequency. 3. At 10 Hz, adrenaline and noradrenaline secretion was virtually abolished by tetrodotoxin (1 microM), but was only reduced by 75% by prolonged perfusion with a combination of mecamylamine (5 microM) and atropine (1 microM). Mecamylamine and atropine completely abolished the secretory response to 2 Hz stimulation. Tetrodotoxin had no significant effect on secretion induced by perfusing glands with nicotine (5 microM), while mecamylamine abolished this response. Mecamylamine and atropine had no effect on secretion induced by K+ depolarization. 4. The secretion of adrenaline and noradrenaline induced by 10 Hz stimulation was not inhibited by naloxone at either 1 or 30 microM. 5. The results suggest that bovine adrenal chromaffin cells, like those in the rat, receive a significant non-cholinergic secretomotor innervation. In contrast to the rat, however, the non-cholinergic component in the bovine adrenal is negligible at low-frequency nerve stimulation and substantial at higher frequencies, and is not antagonized by naloxone. The identity of the non-cholinergic transmitter remains to be determined.

Biochemical, autoradiographic and functional studies on a unique glutamate binding site in adrenal gland

L-Glutamate is known to function as a major excitatory neurotransmitter in the mammalian central nervous system, and recent reports suggest the existence of receptors for glutamate in several peripheral tissues. In the present study, the characteristics of the binding of [3H]L-glutamate to sections of bovine adrenal gland were studied, and the localisation of this binding was investigated in adrenal glands from cow, dog, rat and guinea pig. In addition, the effects of glutamate on catecholamine release from the perfused isolated bovine adrenal gland were investigated. Binding of [3H]L-glutamate to slide-mounted sections of bovine adrenal gland was of high affinity (Kd 0.4 microM), rapid, saturable, reversible, stereospecific and to a single population of sites. The pharmacological profile of this binding site appeared to be unique, and did not correspond to any of the central receptor subtypes for glutamate so far identified. In the adrenal gland of the cow, rat and guinea pig, the binding density of [3H]L-glutamate was higher in cortex than medulla, while this pattern was reversed in the canine adrenal gland. Glutamate had no effect on the basal secretion of noradrenaline or adrenaline from the perfused isolated bovine adrenal gland, and neither glutamate nor the glutamate receptor antagonist kynurenate altered the nicotine-stimulated release of these catecholamines. These results suggest the existence of a novel peripheral binding site for glutamate in the adrenal gland. The differential autoradiographic localisation of this binding site in the adrenal glands of the various species studied may reflect different functional properties of glutamate in these species, and suggests possible roles for glutamate in the modulation of adrenal function.

Localisation and pharmacological characterisation of [3H]bremazocine binding in the bovine adrenal medulla

[3H]Bremazocine (5 nM), in the presence of excess unlabelled mu and delta opioid ligands labelled two anatomically distinct populations of binding sites in the bovine adrenal medulla; a high density over the peripheral adrenaline-containing region of the medulla and a lower density over the central noradrenaline-containing region. This non-mu, non-delta opioid binding was specific (diprenorphine sensitive) but did not appear to involve classical kappa (kappa 1), sigma or PCP binding sites being insensitive to high concentrations of dynorphin (1-13), 3-PPP or MK-801. A significant proportion of the binding at both locations was however sensitive to competition by U50,488H or metorphamide. These data provide further evidence to support the existence of multiple opioid binding sites in the bovine adrenal medulla.

Opioid inhibition of nicotine-induced 45Ca2(+)-uptake into cultured bovine adrenal medullary cells

The ability of a number of opioid agonists and antagonists to affect nicotine-induced 45Ca2(+)-uptake into cultured bovine adrenal medullary cells has been investigated. High (10 microM) concentrations of the opioid agonist bremazocine produced a significant inhibition of nicotine-induced 45Ca2(+)-uptake throughout the 15 min time course examined. The opioid subtype-selectivity of this inhibition was investigated; mu and delta selective agonists produced only minor effects whereas the kappa selective agonist U50-488H and the endogenous opioid peptides dynorphin(1-13) and metorphamide almost abolished nicotine-induced 45Ca2(+)-uptake. The U50-488H inhibition was significant at 10 nM concentrations with an IC50 of approximately 1 microM. U50-488H inhibition could not be reversed or reduced by the opioid antagonists naxolone, diprenophine or Mr2266. Furthermore, Mr2266 and its optical isomer Mr2267 also produced marked inhibition of 45Ca2(+)-uptake. The inhibition was specific to nicotine-induced 45Ca2(+)-uptake in that a similar level of uptake evoked by potassium depolarization was unaffected by high concentrations of U50-488H. These data indicate that opioid inhibition of nicotine-induced 45Ca2(+)-uptake does not involve classical, stereospecific opioid receptors and suggests the involvement of a pharmacologically distinct opioid recognition site. It is speculated that this may be associated with the nicotine receptor-ionophore complex.

Autoradiographic localization of insulin-like growth factor I binding sites in the bovine adrenal gland and on cultured bovine adrenal chromaffin cells

Abstract Previous studies have reported the presence of binding sites for insulin-like growth factor I (IGF-I) in membranes prepared from isolated bovine adrenal medullary cells, and IGF-I was found to regulate the secretory function of bovine chromaffin cells. In the present study, binding sites for IGF-I have been localized in sections of bovine adrenal gland and on cultured bovine adrenal medullary cells, using [(125) l][Thr(59)]-IGF-l as the ligand in conjunction with qualitative autoradiographic techniques. Binding sites were present throughout the adrenal gland and were distributed evenly over all cortical cell layers and over both adrenaline and noradrenaline cell types in the medulla. They were also present at lower density over blood vessels and nerve bundles and over the capsule. The binding of radioligand was to a single class of sites with K(d) 0.61 nM, and was completely displaced by excess unlabelled [Thr(59)]-IGF-l and by insulin (Actrapid, K(d) 1.04muM). Binding sites were also identified on single cells in primary monolayer cultures of bovine adrenal medullary cells. More than 96% of the cells possessed binding sites, although only 85% of such cells were chromaffin cells, as previously determined from dopamine beta-hydroxylase immunohistochemical staining. The results suggest that IGF-I may affect the maturation, growth or function not only of adrenal chromaffin cells but also of many others cell types in this tissue.

Receptor stimulated formation of inositol phosphates in cultures of bovine adrenal medullary cells: the effects of bradykinin, bombesin and neurotensin

The ability of a number of drugs and neuropeptides to stimulate phosphoinositide metabolism in cultured bovine adrenal medullary cells has been assessed. Low concentrations (10 nM) of angiotensin II, bradykinin, histamine, arginine-vasopressin, and bombesin, and high (10 microM) concentrations of oxytocin, prostaglandins E1, and E2, beta-endorphin, and neurotensin stimulated significant accumulation of [3H]inositol phosphates in adrenal medullary cells preloaded with [3H)]inositol. Bradykinin stimulated a significant response at concentration as low as 10pM, with an EC50 of approximately 0.5 nM. The response was markedly inhibited by the bradykinin B2 antagonist [Thi5,8,D-Phe7] bradykinin but not the B1 antagonist [Des-Arg9,Leu8] bradykinin. Higher concentrations of bombesin and neurotensin were required to elicit a response (10 nM and 10 microM respectively). The bombesin response was sensitive to inhibition by the bombesin antagonist [D-Arg1,D-Pro2,D-Trp7,9Leu11]-substance P. In contrast, the neurotensin response was not reduced by the NT1 antagonist [D-Trp11]-neurotensin. These results indicate there are a number of agents that can stimulate phosphatidylinositide hydrolysis in the adrenal medullary cells by acting on different classes of receptors. Such a range of diverse agonists that stimulate inositol phosphate formation will facilitate further analysis of the phosphatidylinositide breakdown in chromaffin cell function.

Co-localization of RNAs coding for phenylethanolamine N-methyltransferase and proenkephalin A in bovine and ovine adrenals

A 29-mer oligodeoxyribonucleotide probe, complementary to the coding region of bovine phenylethanolamine N-methyltransferase (PNMT) mRNA was synthesized. Characterization of this probe by Northern blot hybridization showed that it hybridized to a single band in RNA extracted from bovine and ovine adrenal medullae. The molecular size of this hybridized band was approximately 1.0-1.2 kb which is consistent with recently reported data on the molecular weight of bovine PNMT mRNA. In situ hybridization histochemistry was carried out with this probe on bovine and ovine adrenal sections and results compared on adjacent sections with a probe against proenkephalin A (ProEnk A) mRNA synthesized previously. Both showed a similar localization to the outer margin of cells in the adrenal medulla. The results of this study provide strong evidence at the level of mRNA expression that ProEnk A mRNA is expressed preferentially in the adrenaline synthesizing cells within the adrenal medulla. Further, it demonstrates the usefulness of a synthetic oligodeoxyribonucleotide probe for the study of PNMT gene expression.

Localization of angiotensin II binding sites in the bovine adrenal medulla using a labelled specific antagonist

Angiotensin II binding sites have been localized in sections of bovine adrenal glands and on living cultured bovine adrenal medullary cells using [125I]-[Sar1,Ile8]-angiotensin II and autoradiographic techniques. Binding sites were observed over both adrenaline and noradrenaline chromaffin cells. However, they were present in higher density over adrenaline cells, as determined by the distribution of phenylethanolamine N-methyltransferase mRNA by in situ hybridization histochemistry and of glyoxylic acid-induced fluorescence of noradrenaline. Binding sites were also observed in low density over nerve tracts within the bovine adrenal gland. Living cultured bovine adrenal medullary cells possessed angiotensin II binding sites. Not all cells were labelled. At least 73% of identified dispersed chromaffin cells in these cultures were labelled. Some chromaffin cells were not labelled with the ligand, and at least some non-chromaffin cells in the cultures did possess angiotensin II binding sites. The results provide direct anatomical support for the known ability of angiotensin II to elicit catecholamine secretion from perfused adrenal glands and from cultured adrenal chromaffin cells. They also suggest that some of the effects of angiotensin II on calcium fluxes and second messenger levels measured in cultured adrenal medullary cell preparations may be due to angiotensin II acting on non-chromaffin cells present in these cultures.