Biomedical subjects

W R Millington

Publications and source records attributed to W R Millington.

At least 37 records · Page 2Linked to original sources

PubMed1992

The CCK-A and CCK-B receptor antagonists, devazepide and L-365,260, enhance morphine antinociception only in non-acclimated rats exposed to a novel environment.

Devazepide, a potent CCK-A receptor antagonist, and L-365,260, a selective CCK-B receptor antagonist, have been introduced as pharmacologic tools for differentiating the physiologic roles of CCK-A and CCK-B receptor subtypes. In the present study, we tested the effects of devazepide and L-365,260, on morphine antinociception in rats using the thermal sensorimotor tail flick test. Both devazepide and L-365,260 significantly enhanced the antinociceptive action of morphine, but only in rats that had not been acclimated to the laboratory environment or habituated to investigator handling. When tested with fully acclimated animals, devazepide and L-365,260 had no effect whatsoever; they neither enhanced nor attenuated morphine-induced antinociception. These observations indicate that the effects of devazepide and L-365,260, CCK antagonists, on morphine antinociception appear to be dependent on the animal's response to a new environment or to the stress induced by an unaccustomed experimental paradigm.

Adaptation, Physiological↗

PubMed1992

Cholecystokinin type A and type B receptor antagonists produce opposing effects on cholecystokinin-stimulated beta-endorphin secretion from the rat pituitary.

Cholecystokinin octapeptide (CCK-8) is a potent corticotroph secretagogue. Consistent with earlier reports, the present results demonstrate that CCK-8 administration to rats elevates circulating beta-endorphin and adrenocorticotropin, but not alpha-melanocyte-stimulating hormone concentrations. This response was blocked by dexamethasone pretreatment, but not by vagotomy, and it could not be reproduced by i.c.v. CCK-8 injection, evidence that CCK-8 exerts its effects by directly activating cholecystokinin (CCK) receptors localized on anterior pituitary corticotrophs rather than in brain or the vagus nerve. Subsequent experiments demonstrated further that type A CCK receptors primarily mediate the stimulatory effect of CCK-8 on corticotroph secretion. Thus, devazepide, a selective CCK-A receptor antagonist, produced a dose-related inhibition of the CCK-8-stimulated rise in circulating beta-endorphin concentrations. Less selective CCK-A antagonists, including proglumide and lorglumide, produced little or no effect, however. Unexpectedly, the CCK-B receptor antagonist, L-365,260, enhanced the response to CCK-8, an effect diametrically opposite to that produced by CCK-A antagonists. These observations indicate that CCK-A and CCK-B receptors mediate quite different, if not opposing, roles in regulating corticotroph secretion.

Animals↗

PubMed1991

Endoproteolytic conversion of beta-endorphin-1-31 to beta-endorphin-1-27 potentiates its central cardioregulatory activity.

Brain neurons that express the pro-opiomelanocortin gene secrete multiple forms of beta-endorphin (beta E) which subserve diverse bioregulatory processes. beta E-1-31, for example, is a potent analgetic but beta E-1-27 acts as an opioid antagonist and beta E-1-26, as well as the N-acetyl derivatives of all 3 peptides, lack opioid receptor activity. The present study examines the effects of beta-endorphin processing on its central cardioregulatory potency. Consistent with previous reports, intracisternal beta E-1-31 (1.5 nmol) injection lowered mean arterial pressure (MAP); MAP was reduced by 29.7 +/- 3.9 mm Hg at 60 min and returned toward baseline by 120 min. Unexpectedly, beta E-1-27 displayed a 10-fold greater hypotensive potency than beta E-1-31. At 0.15 nmol, it produced a response equivalent to 1.5 nmol beta E-1-31 while 1.5 nmol beta E-1-27 sustained a maximal reduction in MAP (49.2 +/- 3.9 mm Hg) throughout the 120-min test period. In contrast, beta E-1-26 and N-acetyl-beta E-1-26, -1-27 and -1-31 were inactive at 1.5 nmol. Bradycardia accompanied the depressor response to the higher beta E-1-27 dose but not to beta E-1-31. Naloxone pretreatment completely blocked the depressor effects of both beta E-1-31 and beta E-1-27, and reversed the bradycardia produced by beta E-1-27, suggesting that both peptides act through opioid receptors. beta E-1-27 also stimulated catecholamine release from the perfused adrenal gland but beta E-1-31 was inactive. These findings emphasize the importance of regionally selected post-translational processing in defining the functional specificity of beta E peptides.

Adrenal Glands↗

PubMed1991

The posttranslational processing of beta-endorphin in human hypothalamus.

beta-Endorphin is posttranslationally processed to six derivatives, which, although structurally similar, produce distinctly different biological effects. beta-Endorphin 1-31 is a potent opioid receptor agonist, but beta-endorphin 1-27 exhibits antagonist properties, and beta-endorphin 1-26 and the alpha-N-acetyl derivatives of all three peptides lack opioid receptor activity. In the present study, we identified the beta-endorphin peptides synthesized in human hypothalamus using cation exchange HPLC. First, we tested whether postmortem changes occur by storing rat hypothalami at 4 degrees C. This demonstrated that relative amounts of the six beta-endorphin forms did not change for up to 24 h, although total beta-endorphin immunoreactivity significantly declined after 6 h. HPLC analysis of human hypothalami revealed that beta-endorphin 1-31 was the principal form, constituting 58.4 +/- 5.4% of total immunoreactivity. Substantial amounts of beta-endorphin 1-27 (13.4 +/- 1.2%) and beta-endorphin 1-26 (13.1 +/- 1.6%) were also present, but alpha-N-acetylated forms were quantitatively minor, each comprising approximately 5% of total beta-endorphin. A similar processing pattern occurred in preoptic and suprachiasmatic areas of the hypothalamus. These results show that, despite differences in primary sequence, beta-endorphin is processed similarly in both rat and human hypothalamus. Opiate-active beta-endorphin 1-31 is the principal form in both species.

Adult↗

PubMed1989

Gonadal steroid and chronic morphine treatment do not change the posttranslational processing of beta-endorphin in the rat brain.

The present study examines whether two treatments known to induce refractoriness to exogenous morphine produce this desensitization through a change in the posttranslational processing of brain beta-endorphin (beta-End). The first experiment examined whether an ovarian steroid regimen which produces a transient desensitization of brain opiate receptor mechanisms alters beta-End processing in the preoptic area (POA), medial basal hypothalamus (MBH), and brainstem (BS). The second experiment monitored the effects of morphine pellet treatment, known to produce morphine dependency, on immunoreactive beta-End forms in the hypothalamus and periaquaductal gray area of the midbrain (PAG). The individual molecular forms of beta-End were separated using ion exchange chromatography and collection fractions were quantitated for beta-End immunoreactivity by RIA. The results show that regional differences occur in the posttranslational processing of beta-End. In the hypothalamus, MBH and POA, beta-End-(1-31) and its non-acetylated C-terminal cleavage products, beta-End-(1-27) and beta-End-(1-16) were the predominant forms of beta-End. The PAG pools produced a beta-End peptide elution profile similar to the hypothalamus, although small amounts of N-acetyl-beta-End-(1-31) were also identified. The BS exhibited the least posttranslational processing of beta-End; beta-End-(1-31) was the primary product with smaller amounts of beta-End-(1-27) and beta-End-(1-26) observed. However, neither ovarian steroid treatment nor chronic morphine produced any changes in posttranslational processing of beta-End or in total beta-End concentration in any of the brain regions examined in these experiments. These data indicate that the refractoriness or tolerance to exogenous morphine associated with steroid or chronic morphine treatment cannot be explained by alterations in the biological activity of beta-End resulting from the differential regulation of its posttranslational processing products.

Animals↗

PubMed1988

Choline as an agonist: determination of its agonistic potency on cholinergic receptors.

These experiments examined the potency of choline as a cholinergic agonist at both muscarinic and nicotinic receptors in rat brain and peripheral tissues. Choline stimulated the contraction of isolated smooth muscle preparations of the stomach fundus, urinary bladder and trachea and reduced the frequency of spontaneous contractions of the right atrium at high micromolar and low millimolar concentrations. The potency of choline to elicit a biological response varied markedly among these tissues; EC50 values ranged between 0.41 mM in the fundus to 14.45 mM in the atrium. Choline also displaced [3H]quinuclidinyl benzilate binding in a concentration-dependent manner although, again, its potency varied among different brain regions (Ki = 1.2 to 3.5 mM) and peripheral tissues (Ki = 0.28 to 3.00 mM). Choline exhibited a comparable affinity for nicotinic receptors. It stimulated catecholamine release from the vascularly perfused adrenal gland (EC50 = 1.3 mM) and displaced L-[3H]nicotine binding to membrane preparations of brain and peripheral tissues (Ki = 0.38 to 1.17 mM). However, the concentration of choline required to bind to cholinergic receptors in most tissues was considerably higher than serum levels either in controls (8-13 microM) or following the administration of choline chloride (200 microM). These results clearly demonstrate that choline is a weak cholinergic agonist. Its potency is too low to account for the central nervous system effects produced by choline administration, although the direct activation of cholinergic receptors in several peripheral tissues may explain some of its side effects.

Acetylcholine↗

PubMed1988

Equine Cushing's disease: differential regulation of beta-endorphin processing in tumors of the intermediate pituitary.

Equine Cushing's disease is caused by an adenomatous hyperplasia of the intermediate pituitary which secretes high levels of beta-endorphin, ACTH, and other peptide derivatives of POMC. In the present study we found that plasma and cerebrospinal fluid immunoreactive beta-endorphin (i beta-endorphin) levels were 60- and 120-fold higher than control values in horses with Cushing's disease. There were no significant differences in intermediate lobe i beta-endorphin concentrations, although anterior lobe i beta-endorphin was significantly reduced in Cushing's horses, presumably because high levels of circulating glucocorticoids inhibit POMC biosynthesis in corticotrophs. Although the i beta-endorphin concentration of the tumors was not different from that in normal tissue, the posttranslational processing of beta-endorphin in the two tissues differed significantly. In controls, beta-endorphin-(1-31) was extensively processed to N-acetyl-beta-endorphin-(1-31), -(1-27), and -(1-26) and des-acetyl beta-endorphin-(1-27). N-Acetyl-beta-endorphin-(1-27) was the predominant form, constituting 57% of the total i beta-endorphin, whereas beta-endorphin-(1-31) was quantitatively minor (less than 7% of the total immunoreactivity. In adenomatous pituitaries, the processing of beta-endorphin was restricted, significantly increasing the proportions of beta-endorphin-(1-31) and N-acetyl-beta-endorphin-(1-31) and lowering the amounts of N-acetyl-beta-endorphin-(1-27) and -(1-26). These changes in peptide processing were associated with markedly reduced levels of dopamine, suggesting that the dopaminergic neurons that normally control intermediate lobe secretion no longer innervate the hyperplastic tissue. These findings are consistent with evidence that the dopaminergic innervation of the intermediate pituitary regulates the posttranslational processing and release of beta-endorphin.

3,4-Dihydroxyphenylacetic Acid↗

PubMed1988

Dopaminergic regulation of the biosynthetic activity of individual melanotropes in the rat pituitary intermediate lobe: a morphometric analysis by light and electron microscopy and in situ hybridization.

The primary cell type of the intermediate lobe (IL) of the rat pituitary is a polyhedral secretory cell with a smooth ovoid nucleus. The results of this study demonstrate, however, that IL melanotropes are a heterogeneous cell population. Melanotropes differed in the tinctorial properties of their cytoplasm; some cells appeared distinctly darker, others lighter, and cells staining in intermediate shades were also found. Electron microscopical morphometry revealed that darkly staining melanotropes have a denser cytosol and contain a greater amount of rough endoplasmic reticulum, mitochondria, and secretory vesicles than light cells. In addition, in situ hybridization studies, using a POMC probe, showed that POMC mRNA was distributed unevenly among melanotropes in a pattern comparable to the distribution of light and dark cells. These studies further demonstrated that dopaminergic drug treatments, which are known to alter the secretion of POMC-related peptides from the IL, produced parallel changes in both the histological staining properties and the amount of POMC mRNA per cell. Haloperidol treatment dramatically increased the number of dark melanotropes and the amount of POMC mRNA in each cell and eliminated the cellular heterogeneity in both staining properties and the distribution of POMC mRNA. After bromocriptine treatment the number of light melanotropes increased, and each cell contained reduced levels of POMC mRNA. These findings indicate that individual melanotropes maintain different levels of biosynthetic activity and that treatments that alter the secretion of POMC peptides affect both the rate of POMC synthesis in individual melanotropes and the cellular heterogeneity of the IL.

Animals↗

PubMed1987

Histological evaluation of the dopaminergic regulation of proopiomelanocortin gene expression in the intermediate lobe of the rat pituitary, involving in situ hybridization and [3H]thymidine uptake measurement.

The melanotroph, a polyhedral secretory cell with an ovoid smooth nucleus, is the primary cell type of the intermediate lobe (IL) of the rat pituitary. The melanotrophs are not uniform, but differ in the tinctorial properties of their cytoplasm; some cells appear distinctly darker, others lighter, and cells staining in intermediate shades are also found. In addition, in situ hybridization using proopiomelanocortin (POMC) probes shows an uneven distribution of POMC mRNA among melanotrophs, indicating that different cells maintain different levels of biosynthetic activity. Dopaminergic drugs known to alter the secretion of POMC-related peptides from the IL produced parallel changes in histological staining properties and the amount of POMC mRNA per cell, as determined by in situ hybridization. Acute bromocriptine treatment (6 h) produced a dramatic reduction in grain counts over melanotroph cytoplasm (to 10% of the control levels). A similar reduction persisted after chronic treatment. Six hours after a single haloperidol injection, the grain counts were 180% of control levels. After chronic haloperidol treatment, they were further elevated to 300% of control levels. Chronic bromocriptine and haloperidol treatment also changed the thickness of the IL. Bromocriptine reduced and haloperidol treatment increased the number of cell layers in the IL by changing the rate of cell proliferation. Thus, haloperidol treatment significantly increased and bromocriptine treatment significantly decreased the number of melanotrophs labeled by [3H]thymidine. The mitotic index followed the same trend. These results suggest that dopamine regulation of the IL acts by two different mechanisms: POMC gene expression and cellular proliferation. The change in POMC gene expression is the cell's first rapid response. The influence on the cell cycle appears after subchronic and chronic treatment.

Animals↗

PubMed1987

Dopaminergic agents selectively alter the post-translational processing of beta-endorphin in the intermediate pituitary of the rat.

The present study examines whether sustained changes in the biosynthetic activity of the intermediate pituitary, induced pharmacologically by treating rats with dopamine receptor ligands, alters the post-translational processing of beta-endorphin (beta-END). Separation of the individual molecular forms of beta-END using ion exchange chromatography revealed that beta-END is processed to alpha-N-acetylated and nonacetylated forms of beta-END-(1-31), beta-END-(1-27) and beta-END-(1-26) in the neurointermediate lobe (NIL). Chronic treatment with D-2, but not D-1, dopamine receptor antagonists elevated total beta-END immunoreactivity (i beta-END) in the NIL but this increase was predominantly attributable to elevations in the concentrations of N-acetyl-beta-END-(1-31) and N-acetyl-beta-END-(1-27). In contrast, N-acetyl-beta-END-(1-26) was not significantly affected. The dopaminergic agonist, bromocriptine, had the opposite effect; it lowered total i beta-END levels, depleting N-acetyl-beta-END-(1-31) and N-acetyl-beta-END-(1-27) to a greater extent than N-acetyl-beta-END-(1-26). beta-END peptides were released in vitro in the same relative proportion as they were contained in the NIL suggesting that individual molecular forms of beta-END are not released preferentially. i gamma-END levels also were modulated by dopaminergic agents but the processing of gamma-END and alpha-END was not altered. Acute haloperidol treatment depleted i gamma-END levels but did not affect the pattern of beta-END peptides in the NIL. These results indicate that dopaminergic agents influence not only the secretion but also the post-translational processing of pituitary beta-END.

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗

PubMed1986

The size of enzymes acetylating alpha-melanocyte-stimulating hormone and beta-endorphin.

Acetylation of alpha-melanocyte-stimulating hormone and beta-endorphin in a granule fraction from the rat pituitary neurointermediate lobe was examined by radiation inactivation. Target sizes obtained were 71 +/- 12 and 76 +/- 9 kDa, respectively. These data suggest that a single enzyme species may be responsible for acetylation of the two substrates. A similar analysis of alpha-melanocyte-stimulating hormone acetylation in the rat lens revealed a complex inactivation curve which could be explained by the presence of at least two different enzyme molecules, one of 86 and the other greater than 200 kDa.

Animals↗

PubMed1986

A diurnal rhythm in proopiomelanocortin messenger ribonucleic acid that varies concomitantly with the content and secretion of beta-endorphin in the intermediate lobe of the rat pituitary.

The present study investigated whether diurnal variations in the secretion of alpha MSH and beta-endorphin from the intermediate lobe (IL) of the pituitary are associated with parallel changes in the synthesis of mRNA specifically encoding proopiomelanocortin (POMC). The results demonstrate that concomitant diurnal variations occur in both plasma and IL concentrations of immunoreactive beta-endorphin and alpha MSH. Plasma and IL peptide levels were relatively constant during daylight hours (0600-1800 h), but increased after the onset of darkness and reached maximal concentrations at 0200 h. To examine the possibility that this diurnal rhythm in the content and secretion of POMC-derived peptides resulted from diurnal changes in the biosynthesis of POMC, the concentration and rate of synthesis of POMC mRNA were examined. POMC mRNA levels were elevated during the dark period, reaching a maximum level at 0200 h that was 2-fold higher than that occurring during the light period. POMC mRNA synthesis, determined by measuring the number of RNA polymerase II complexes transcribing the POMC gene in isolated cell nuclei, also varied diurnally, with maximum transcription rates occurring at 1800 h, thus preceding maximal increases in POMC mRNA content and POMC peptide secretion by 8 h. Together, these data indicate that diurnal variations in the content and secretion of POMC-derived peptides are associated with parallel changes in POMC mRNA concentrations and are preceded by similar changes in POMC gene transcription.

Animals↗

PubMed1986

Coordinate regulation of peptide acetyltransferase activity and proopiomelanocortin gene expression in the intermediate lobe of the rat pituitary.

Proopiomelanocortin (POMC) is posttranslationally processed in the intermediate lobe of the pituitary to both N-terminally acetylated and nonacetylated forms of alpha MSH and beta-endorphin (beta END). N-Acetylation substantially modifies the physiological responses produced by both peptides, suggesting that the activity of the peptide acetyltransferase, which posttranslationally acetylates beta END and des-acetyl-alpha MSH, may play an important role in defining the biological activity of the secretory products of the intermediate pituitary lobe. The present results demonstrate that peptide acetyltransferase activity is induced by treating rats chronically with the dopamine receptor antagonist haloperidol. Haloperidol administration produced parallel and essentially equivalent increases in acetyltransferase activity, POMC mRNA levels, and the content and secretion of POMC-derived peptides in the neurointermediate pituitary. Time-course and dose-response studies further demonstrated that acetyltransferase activity covaried with POMC mRNA and peptide levels. Chronic treatment with the dopamine receptor agonist bromocriptine had the opposite effects; it lowered acetyltransferase activity, POMC mRNA levels, and alpha MSH and beta END immunoreactivities. Subcellular fractionation showed that acetyltransferase activity was localized in three subcellular compartments corresponding in density to secretory vesicles, rough endoplasmic reticulum and Golgi apparatus, and cytosol. Haloperidol treatment significantly increased the specific activity of the secretory vesicle-associated acetyltransferase without affecting the specific activity of the enzymes present in the endoplasmic reticulum or cytosol. Together, these data indicate that peptide acetyltransferase activity and POMC biosynthesis are coregulated.

Acetylation↗

PubMed1986

Principles of irrational drug therapy.

Drugs are extensively used in medicine when they are unlikely to produce a benefit to the patient. In most instances this does no apparent harm to the patient but sometimes (for example, the extensive use of thorotrast) the results are tragic. Even when the patient is not injured, overuse of medicines is an undesirable and money-wasting behavioral pattern. Several factors relating to the social process of drug use which encourage overprescribing are discussed and the Principles of Irrational Drug Therapy are derived. These principles are presented as negative role models for the use of medicines in developing countries.

Attitude of Health Personnel↗

PubMed1985

Diisopropylfluorophosphate inhibits choline efflux from the perfused rat hemidiaphragm.

Acetylcholine (ACh) synthesis by motor nerve terminals requires an adequate supply of its precursor, choline. The results reported here show that diisopropylfluorophosphate (DFP), an acetylcholinesterase inhibitor commonly used in ACh release studies, reduces the rate of endogenous choline efflux from the perfused rat hemidiaphragm. Perfusion of the isolated hemidiaphragm with 10 microM or 100 microM DFP reduced choline efflux by 39% and 69% respectively. DFP administration to rats (6 mg/kg) also lowered the in vitro release of choline by 33%. The rate of ACh release from hemidiaphragm preparations perfused with DFP was significantly lower than the rate of release from preparations perfused with physostigmine, an acetylcholinesterase inhibitor which had no effect on choline efflux. The addition of choline (10-30 micron) to the perfusion medium restored the rate of ACh release from DFP-treated hemidiaphragms but did not further elevated ACh release from physostigmine-treated preparations. These results demonstrate that DFP inhibits choline efflux from the isolated hemidiaphragm and further suggest that, by limiting the availability of choline for ACh synthesis, DFP reduces the rate of ACh release.

Acetylcholine↗

PubMed1985

Axonal transport of alpha-bungarotoxin binding sites in rat sciatic nerve.

[125I]alpha-Bungarotoxin (alpha-BuTX) binding sites accumulate both proximal and distal to a ligature positioned around the sciatic nerve of rats. [125I]alpha-BuTX binding sites, localized using quantitative receptor autoradiography, were found to accumulate at nerve ligatures at a relatively constant rate which suggests that they undergo both anterograde and retrograde axonal transport. [125I]alpha-BuTX binding to sections of ligated sciatic nerve was saturable with apparent dissociation constants of 0.97 nM proximal and 0.53 nM distal to the ligature. D-Tubocurarine, nicotine, decamethonium and atropine displaced [125]alpha-BuTX from sciatic nerve sections with affinities comparable to those previously reported for the toxin binding component of rat brain. These data indicate that [125I]alpha-BuTX binding sites pharmacologically similar to those of rat brain are transported in sciatic nerve. Axonally transported toxin binding sites may correspond to those previously localized to the plasma membrane of peripheral nerve axons and on the terminals of motor neurons.

Animals↗

PubMed1985

Long-term haloperidol treatment elevates beta-endorphin levels in the intermediate pituitary but not in rat brain.

Long-term treatment of rats with haloperidol, a dopamine receptor antagonist, produced a dose-dependent increase in immunoreactive beta-endorphin (i beta-END) concentrations in the neurointermediate lobe of the pituitary (NIL). In contrast, chronic haloperidol treatment had no significant effect on i beta-END levels in the hypothalamus, the midbrain or in discrete, microdissected brain nuclei even when administered at a dose (5 mg/kg) ten-fold higher than that which elevated i beta-END levels in the NIL. Chronic treatment with bromocriptine, a dopamine receptor agonist, had the opposite effect on the NIL, lowering i beta-END levels to approximately one-third of control values, but it did not affect hypothalamic i beta-END concentrations. These data are consistent with prior evidence that the synthesis of beta-END by IL melanotrophs is reciprocally modulated by dopaminergic ligands. The results indicate, however, that beta-END-releasing neurons are not similarly regulated.

Animals↗

PubMed1984

Diethyldithiocarbamate depresses the acoustic startle response in rats.

The relationship between norepinephrine (NE) content in cortex and spinal cord and acoustic startle amplitude was investigated in two experiments. Administration of diethyldithiocarbamate (DDC) depressed startle amplitude at the same time and dose that it most severely depleted NE content. These results support the conclusion that NE facilitates the normal elaboration of the acoustic startle reflex and also support evidence that NE activity in the spinal cord may be of particular importance in the maintenance of normal startle amplitude.

Acoustic Stimulation↗