Biosynthesis of enkephalins by chromaffin cells of bovine adrenal medulla

No detectable analgesic effects in the formalin test even with one million bovine adrenal chromaffin cells

The present experiments were conducted to identify analgesic agents for transfection into immortalized adrenal chromaffin cell lines to maximize their analgesic potential. Analgesic agents known to be produced by adrenal chromaffin cells were infused intrathecally at a low dose (0.2 microg) which might conceivably be attained by adrenal chromaffin cell transplants. Numerous agents, administered individually and in two-factor combinations, produced significant analgesic effects in the formalin test. Before assessing the potential additive or synergistic effects of these analgesic agents with adrenal chromaffin cells, studies were conducted to demonstrate analgesic effects with adrenal chromaffin cells alone. Analgesic effects were previously reported in the literature with 80-100k intrathecal bovine adrenal chromaffin (BAC) cells; but in the present study 500k purified BAC cells failed to produce detectable analgesic effects. One million purified BAC cells also failed to produce analgesic effects in the formalin test. In a final study, even nicotine-stimulated release from one million purified BAC cells failed to produce analgesic effects in the formalin test. The fact that even one million nicotine-stimulated BAC cells failed to demonstrate therapeutic potential in these blinded experiments under conditions which were clearly sensitive to the analgesic agents produced by BAC cells, raises serious questions about the clinical utility of this experimental treatment.

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.

Analgesia induced by isolated bovine chromaffin cells implanted in rat spinal cord

Chromaffin cells synthesize and secrete several neuroactive substances, including catecholamines and opioid peptides, that, when injected into the spinal cord, induce analgesia. Moreover, the release of these substances from the cells can be stimulated by nicotine. Since chromaffin cells from one species have been shown to survive when transplanted to the central nervous system of another species, these cells are ideal candidates for transplantation to alter pain sensitivity. Bovine chromaffin cells were implanted into the subarachnoid space of the lumbar spinal region in adult rats. Pain sensitivity and response to nicotine stimulation was determined at various intervals following cell implantation. Low doses of nicotine were able to induce potent analgesia in implanted animals as early as one day following their introduction into the host spinal cord. This response could be elicited at least through the 4 months the animals were tested. The induction of analgesia by nicotine in implanted animals was dose related. This analgesia was blocked by the opiate antagonist naloxone and partially attenuated by the adrenergic antagonist phentolamine. These results suggest that the analgesia is due to the stimulated release of opioid peptides and catecholamines from the implanted bovine chromaffin cells and may provide a new therapeutic approach for the relief of pain.

Influence of delta-opioid receptors on production of labeled methionine5-enkephalin in murine neuroblastoma cells

Synthesis of methionine5-enkephalin by intact cells of murine neuroblastoma clone N1E-115 has been demonstrated both immunocytochemically and biochemically. In addition, N1E-115 cells possess homogeneous enkephalin (delta) receptors which inhibit prostaglandin E1-induced intracellular cyclic AMP formation. An assay was developed for measuring de novo synthesis of methionine5-enkephalin by pulsing cells in culture with radioactive methionine and isolating this pentapeptide to radiochemical purity by a procedure that included immunoaffinity chromatography specific for oxidized methionine5-enkephalin. This assay indicated that production of radiolabeled-methionine5-enkephalin was increased upon lengthy exposure of intact N1E-115 cells in the late logarithmic phase of growth to a nonproteolyzable analog of methionine5-enkephalin. This increase in synthesis of intracellular methionine5-enkephalin relative to control cells was prevented by prior incubation of the clone with naloxone, indicating that the response was mediated by the delta receptor.

Use of isolated chromaffin cells to study basic release mechanisms

An account is given of the authors' work with isolated adrenal chromaffin cells to study the synthesis, storage and release of catecholamines and of a number of neuropeptides endogenous to the adrenal medulla. A review of other studies in the literature with the isolated chromaffin cell system is included. It is seen that the isolated chromaffin cells are a convenient in vitro system well-suited to studies of basic release mechanisms. The isolated adrenal chromaffin cells maintain high levels of catecholamines and opiates and release them by exocytosis. The cells have both nicotinic and muscarinic receptors but only the nicotinic are involved in the agonist-evoked release of catecholamines (EC50 nicotine 5 X 10(-6) M: ACh 5 X 10(-5) M). The cells can synthesize AChE and selectively release the 10S molecular form by a mechanism different from exocytosis. Substance P (SP) modulates the secretion of catecholamines and ATP evoked by ACh or nicotine but not that evoked by K+ or veratridine. SP appears to interact with the nicotinic receptor-ionophore complex to regulate Na+ entry. SP receptors on the chromaffin cells show similar structural requirements to SP receptors in other SP responsive tissues. Binding studies on isolated chromaffin cell membranes with [4-3H-Phe]SP have shown specific binding in the nM range. In addition, at high concentrations of ACh, SP protects against nicotinic receptor desensitization. Since SP is contained in the splanchnic nerve terminals that innervate the medulla, the demonstration of SP action and SP receptors on the chromaffin cells suggests a physiological role for SP in the regulation of secretion from the adrenal medulla. Somatostatin (SS) and a number of SS analogues also inhibit release, but are approximately 15-fold less potent than SP. Leu- and Met-enkephalin, which are co-stored with adrenaline in the bovine adrenal medullary cells produce a non-specific inhibition of the nicotine-evoked release of CA, but enhance the basal release of endogenous catecholamines by a mechanism that is Ca2+-dependent, stereospecific and reversible by naloxone and naltrexone. The implication of these peptide-amine interactions for physiological processes regulating homeostasis in the adrenal are discussed.

Met-enkephalin [Arg6, Phe7] immunoreactivity in bovine caudate and bovine adrenal medulla

A radioimmunoassay specific for the COOH-terminus of Met-enkephalin [Arg6, Phe7] and a separate assay specific for the COOH-terminus of Met-enkephalin are described. Immunoreactivity by these two assays was compared in bovine caudate and bovine chromaffin granule preparation after Sephadex G75 chromatography in 50% acetic acid. When the assays were applied to the chromatography fractions of the bovine caudate extract, the majority of the immunoreactivity was found in the fractions corresponding to the heptapeptide and the pentapeptide respectively. When the chromaffin granule chromatography fractions were assayed, both of the radioimmunoassays showed that most reactivity was in several peaks in the larger molecular weight fractions. The major peak for the Met-enkephalin [Arg6, Phe7] assay had an apparent molecular weight of 2800, while with the Met-enkephalin assay the dominant peak of immunoreactivity had an apparent molecular weight of 10,000. The presence of authentic Met-enkephalin [Arg6, Phe7] in both caudate and chromaffin granule extracts was confirmed by reverse-phase chromatography of the previously sized fractions. It appears then that the processing of precursors of opioid peptides is directed, in the caudate, to the synthesis and storage of the enkephalins and of Met-enkephalin [Arg6, Phe7]; in the adrenal medulla the major products of precursor processing are a variety of polypeptides of larger sizes.

Naloxone inhibits and morphine potentiates the adrenal steroidogenic response to ACTH

The administration of morphine to hypophysectomized rats potentiated the steroidogenic response of the adrenal cortex to exogenous adrenocorticotrophic hormone (ACTH) in a dose-dependent fashion. Conversely, the opiate antagonist naloxone inhibited the adrenal response to ACTH. Naloxone pretreatment also antagonized the potentiating effect of morphine on ACTH-induced steroidogenesis in a dose-dependent manner. Neither morphine nor naloxone, administered to hypophysectomized rats, had any direct effect on adrenal steroidogenesis. These adrenal actions were stereospecific since neither the (+)-stereoisomer of morphine, nor that or naloxone, had any effect on the adrenal response to ACTH. The administration of human beta-endorphin to hypophysectomized rats had no effect on the adrenal corticosterone concentration nor did it alter the response of the adrenal gland to ACTH. These results indicate that morphine can potentiate the action of ACTH on the adrenal by a direct, stereospecific, dose-dependent mechanism that is prevented by naloxone pretreatment and which may involve competition for ACTH receptors on the corticosterone-secreting cells of the adrenal cortex.

Opioid peptide synthesis in bovine and human adrenal chromaffin cells

Addition of inhibitors of chromaffin vesicle catecholamine uptake, reserpine or tetrabenazine, to cultures of bovine adrenal chromaffin cells led to a depletion of cellular catecholamine stores and to an increased opioid peptide content. Addition of monoamine oxidase inhibitors, catecholamines or catecholamine receptor agonists to the cultures produced similar increases in the opioid peptide content without altering catecholamine stores, suggesting that an extravesicular catecholamine pool influences chromaffin cell opioid peptide content. This elevated peptide content resulted from increased synthesis by the cells and led to enhanced stimulus-evoked secretion of opioid peptides. Dissociated cells obtained from a human pheochromocytoma tumor also increased their opioid peptide content following treatment with reserpine or a secretagogue. The results obtained with cultured chromaffin and pheochromocytoma cells suggest that a mechanism for replenishing and increasing adrenal medullary opioid peptide stores and secretory capacity may function in vivo. The observed regulation of opioid peptide stores may also be important in the clinical pathology of opioid peptide containing tumors.