Instrumental variables estimation and inference in the presence of many exogenous regressors

Summary We consider a standard instrumental variables model contaminated by the presence of a large number of exogenous regressors. In an asymptotic framework where this number is proportional to the sample size, we study the impact of their ratio on the validity of existing estimators and tests. When the instruments are few, the inference using the conventional 2SLS estimator and associated t and J statistics, as well as the Anderson–Rubin and Kleibergen tests, is still valid. When the instruments are many, the LIML estimator remains consistent, but the presence of many exogenous regressors changes its asymptotic variance. Moreover, the conventional bias correction of the 2SLS estimator is no longer appropriate. We provide asymptotically correct versions of bias correction for the 2SLS estimator, derive its asymptotically correct variance estimator, extend the Hansen–Hausman–Newey LIML variance estimator to the case of many exogenous regressors, and propose asymptotically valid modifications of the J overidentification tests based on the LIML and bias‐corrected 2SLS estimators.     

Identification and inference in a simultaneous equation under alternative information sets and sampling schemes

Summary In simple static linear simultaneous equation models, the empirical distributions of IV and OLS are examined under alternative sampling schemes and compared with their first‐order asymptotic approximations. We demonstrate that the limiting distribution of consistent IV is not affected by conditioning on exogenous regressors, whereas that of inconsistent OLS is. The OLS asymptotic and simulated actual variances are shown to diminish by extending the set of exogenous variables kept fixed in sampling, whereas such an extension disrupts the distribution of IV and deteriorates the accuracy of its standard asymptotic approximation, not only when instruments are weak. Against this background, the consequences for the identification of parameters of interest are examined for a setting in which (in practice often incredible) assumptions regarding the zero correlation between instruments and disturbances are replaced by (generally more credible) interval assumptions on the correlation between endogenous regressor and disturbance. This yields OLS‐based modified confidence intervals, which are usually conservative, as is established by simulation. Often they compare favourably with IV‐based intervals and accentuate their frailty. The latter is demonstrated in an empirical illustration.     

Simulation-based Finite Sample Normality Tests in Linear Regressions

In the literature on tests of normality, much concern has been expressed over the problems associated with residual-based procedures. Indeed, the specialized tables of critical points which are needed to perform the tests have been derived for the location-scale model; hence, reliance on available significance points in the context of regression models may cause size distortions. We propose a general solution to the problem of controlling the size of normality tests for the disturbances of standard linear regressions, which is based on using the technique of Monte Carlo tests. We study procedures based on 11 well-known test statistics: the Kolmogorov–Smirnov, Anderson–Darling, Cramér–von Mises, Shapiro–Wilk, Jarque–Bera and D’Agostino criteria. Evidence from a simulation study is reported showing that the usual critical values lead to severe size problems (over-rejections or under-rejections). In contrast, we show that Monte Carlo tests achieve perfect size control for any design matrix and have good power.     

Standardized LM tests for spatial error dependence in linear or panel regressions

Summary The robustness of the Lagrange Multiplier (LM) tests for spatial error dependence of Burridge (1980) and Born and Breitung (2011) for the linear regression model, and Anselin (1988) and Debarsy and Etur (2010) for the panel regression model with random or fixed effects are examined. While all tests are asymptotically robust against distributional mis‐specification, their finite sample behaviour may be sensitive to the spatial layout. To overcome this shortcoming, standardized LM tests are suggested. Monte Carlo results show that the new tests possess good finite sample properties. An important observation made throughout this study is that the LM tests for spatial dependence need to be both mean‐ and variance‐adjusted for good finite sample performance to be achieved. The former is, however, often neglected in the literature.     

Response error in a transformation model with an application to earnings‐equation estimation*

Summary This paper considers estimation of a transformation model in which the transformed dependent variable is subject to classical measurement error. We consider cases in which the transformation function is known and unspecified. In special cases (e.g. log and square‐root transformations), least‐squares or non‐linear least‐squares estimators are applicable. A flexible approximation approach (based on Taylor expansion) is proposed for a parametrized transformation function (like the Box–Cox model), and a semi‐parametric approach (combining a semi‐parametric linear‐index estimator and non‐parametric regression) is proposed for the case of an unspecified transformation function. The methods are applied to the estimation of earnings equations, using wage data from the Current Population Survey (CPS).     

Surveillance of drug abuse in Hong Kong by hair analysis using LC‐MS/MS

The aim of this study is to reveal the habits of drug abusers in hair samples from drug rehabilitation units in Hong Kong. With the application of liquid chromatography–tandem mass spectrometry (LC–MS/MS) technology, a total of 1771 hair samples were analyzed during the period of hair testing service (January 2012 to March 2016) provided to 14 drug rehabilitation units including non‐governmental organizations (NGOs), rehabilitation centers, and medical clinics. Hair samples were analyzed for abused drugs and their metabolites simultaneously, including ketamine, norketamine, cocaine, benzoylecgonine, cocaethylene, norcocaine, codeine, MDMA, MDA, MDEA, amphetamine, methamphetamine, morphine, 6‐acetylmorphine, phencyclidine, and methadone. The results showed that ketamine (77.2%), cocaine (21.3%), and methamphetamine (16.5%) were the frequently detected drugs among those drug abusers, which is consistent with the reported data. In addition, the usage of multiple drugs was also observed in the hair samples. About 29% of drug‐positive samples were detected with multiple drug use. Our studies prove that our locally developed hair drug‐testing method and service can be a valid tool to monitor the use of abused drugs, and which could facilitate rehabilitation program management.     

Determination of amprolium in feed by a liquid chromatography-mass spectrometry method

As a consequence of the finding of veterinarian drugs in food European Community banned several compounds like coccidiostats as amprolium (APL). This antibiotic has been used as a preventive and clinical anticoccidial drug in chicken. The 2005/187/CE, 2005/925/EC Recommendations ban the use of amprolium as additive in chicken feed. For this reason a rapid and sensitive liquid chromatography–mass spectrometry (LC–MS) method was developed to detect amprolium in chicken feed following the European community proposed technique (1999/27/EC) for sample preparation. Cause the validation is required for the analytical methods used in feed official control, this method was validated according to 2004/882/EC Regulation.     

Determination of fatty acid ethyl esters in hair by GC-MS and application in a population of cocaine users

A gas chromatography-mass spectrometry method for the determination of ethyl myristate, ethyl palmitate, ethyl oleate, and ethyl stearate in hair samples was developed, validated and applied to real samples. Ethyl myristate, ethyl palmitate, ethyl oleate, and ethyl stearate are fatty acid ethyl esters (FAEE) which are known to be direct biotransformation products of ethanol. Their presence in the body fluids and tissue is therefore indicative of alcohol intake and, in particular, FAEE concentration in hair higher than 0.5 ng/mg is indicative of excessive chronic alcohol consumption. The method was applied to 80 hair samples formerly found positive for cocaine and FAEE analytical results were compared with the presence of cocaethylene, a cocaine metabolite formed only when alcohol and cocaine are used together. According to our data the two biomarkers (FAEE and cocaethylene in hair) are tools of great value in the assessment of the diagnosis of use of cocaine and ethanol. In fact, discrepancies were noted and might be related to various factors including differences in consumption habits and thus permitting to distinguish the use of both substances non-concurrently or concurrently. Also, the determination of both markers may, in some cases, discriminate the use of moderate or heavy alcohol amounts when associated with cocaine. Finally, in a population of non-cocaine-users our results support FAEE as valuable means in the assessment of excessive alcohol chronic use.     

Different localizations of drugs simultaneously administered in a strand of hair by micro‐segmental analysis

Segmental hair analysis is used to estimate the time of drug intake at monthly precision in drug‐related crimes. Previously, we advanced this analytical method to specify the day of drug intake by cutting a strand of hair into 0.4‐mm segments, which correspond to daily hair growth. Herein, we investigated the distributions of 7 compounds in a strand of hair using micro‐segmental analysis. Several strands of hair were collected 33.1?229.4 days after subjects were administered 4 pharmaceutical products that contained 10 drugs in single doses within 32 hours. The administered drugs and resulting metabolites were extracted from 0.4‐mm hair segments and quantified using liquid chromatography–tandem mass spectrometry. Acidic and neutral compounds were detected at low amounts in any of the hair segments analyzed. Epinastine, fexofenadine, dihydrocodeine, chlorpheniramine, and the chlorpheniramine metabolite, desmethylchlorpheniramine each was localized to 2 regions within a strand of hair. By contrast, methylephedrine and its metabolite, ephedrine, each was localized to only a region. Among 20 individual strands of hair associated with different subjects and head regions, few differences in the shapes of drug concentration–hair segment curves for each compound were detected. Our data indicated that 2 mechanisms for drug uptake into hair can operate depending on drug properties and that co‐administered drugs can be localized to different regions in a strand of hair. Micro‐segmental analysis may aid in the identification of the day of drug intake and help to elucidate the mechanisms of drug uptake into hair.     

Dissolution enhancement of the anti-HIV drug UC 781 by formulation in a ternary solid dispersion with TPGS 1000 and Eudragit E100

The present research deals with the improvement of the dissolution properties of the anti-HIV drug UC 781. A ternary solid dispersion consisting of a high amount of TPGS 1000 and exhibiting good powder properties with respect to flowability was developed. Eudragit E100 was selected as a polymer based on supersaturation studies. DSC analysis of solid dispersions containing drug doses from 0 to 80% w/w revealed eutectic phase behaviour of the ternary TPGS 100–Eudragit E100–UC 781 mixture. The release of UC 781 in a medium simulating the gastrointestinal lumen was markedly enhanced, reaching a release of 70% w/w after 4 h. XRD results pointed to the presence of crystalline drug in the solid dispersion. The presence of UC 781 in the dispersion had an influence on the TPGS 1000–Eudragit E100 carrier, favoring folding of the polyethylene glycol chains in TPGS 1000. Moreover, the addition of UC 781 to the binary polymer–surfactant mixture was physically expressed by an increase in fluidity of the samples up to a drug load of 50% w/w. NMR was used to investigate this phenomenon, revealing a shielding and/or deshielding effect of the carrier on aromatic C atoms and methyl groups in UC 781. Polyethylene glycol chains present in TPGS 1000 seemed to play a role in this process. In addition, combining UC 781 with the TPGS 1000–Eudragit E100 mixture led to the appearance of TPGS 1000 clusters with a glass transition temperature well below the T_g’s of the pure compounds.     

β‐turn formation by a six‐residue linear peptide in solution

Abstract: A model peptide AAGDYY‐NH₂ (B1), which is found to adopt a β‐turn conformation in the TEM‐1 β‐lactamase inhibitor protein (BLIP) in the TEM‐1/BLIP co‐crystal, was synthesized to elucidate the mechanism of its β‐turn formation and stability. Its structural preferences in solution were comprehensively characterized using CD, FT‐IR and ¹H NMR spectroscopy, respectively. The set of observed diagnostic NOEs, the restrained molecular dynamics simulation, CD and FT‐IR spectroscopy confirmed the formation of a β‐turn in solution by the model peptide. The dihedral angles [(φ₃, ?₃) (φ₄, ?₄)] of [(?52°, ?32°) (?38°, ?44°)] of Gly‐Asp fragment in the model peptide are consistent with those of a type III β‐turn. In a conclusion, the conformational preference of the linear hexapeptide B1 in solution was determined, and it would provide a simple template to study the mechanism of β‐turn formation and stability.     

Determination of mianserin in human plasma by high performance liquid chromatography-electrospray ionization mass spectrometry (HPLC-ESI/MS): application to a bioequivalence study in Chinese volunteers

This study aims to develop a standard protocol for the bioequivalence study of mianserin hydrochloride tablets--a tetracyclic antidepressant drug. For this purpose, a rapid, convenient and selective method using high performance liquid chromatography coupled with electrospray ionization mass spectrometry (HPLC-ESI/MS) has been developed and validated to determine mianserin in human plasma. Mianserin and the internal standard (I.S.), cinnarizine were extracted from plasma by N-hexane:dimethylcarbinol (98:2, v/v) after alkalinized with sodium hydroxide. LC separation was performed on a Thermo Hypersil-Hypurity C18 (5 microm, 150 mm x 2.1 mm) with the mobile phase consisting of 10mM ammonium acetate (pH 3.4)-methanol-acetonitrile (35:50:15, v/v/v) at 0.22 ml/min. The retention time of mianserin and cinnarizine was 3.4 and 2.1 min, respectively. Quadrupole MS detection and quantitation was done by monitoring at m/z 265 [M+H]+ for mianserin and m/z 369 [M+H]+ for cinnarizine. The method was validated over the concentration ranges of 1.0-200.0 ng/ml for mianserin. The recovery was 81.3-84.1%, intra- and inter-day precision of the assay at three concentrations were 9.6-11.4% with accuracy of 97.5-101.2% and the lower limit of quantitation (LLOQ) detection was 1.0 ng/ml for mianserin. The stability of compounds was established in a battery of stability studies, i.e., short-term and long-term storage stability as well as freeze-thaw cycles. This method proved to be suitable for the bioequivalence study of mianserin hydrochloride tablets in healthy human male volunteers.     

Sensitive quantification of the somatostatin analog AP102 in plasma by ultra‐high pressure liquid chromatography–tandem mass spectrometry and application to a pharmacokinetic study in rats

AP102 is a di‐iodinated octapeptide somatostatin agonist (SSA) designed to treat acromegaly and neuroendocrine tumors. A sensitive and selective method was validated for the quantification of AP102 in plasma following the European Medicines Agency (EMA) and Food and Drug Administration (FDA) guidelines. Sample preparation was performed using solid‐phase extraction microplates. Chromatographic separation was achieved on an ultra‐high pressure liquid chromatography (UHPLC) C18 column in 6.0 minutes. The compounds were quantified using multiple reaction monitoring on a tandem quadrupole mass spectrometer with ¹³C,¹⁵N‐labeled AP102 as internal standard. Calibration ranged from 50 to 10000 pg/mL. The lower limit of quantification (LLOQ) was measured at 20 pg/mL, and robust analytical performances were obtained with trueness at 99.2%–100.0%, intra‐assay imprecision at 2.5%–4.4%, and inter‐assay imprecision at 8.9%–9.7%. The accuracy profiles (total error) built on the 3 concentrations levels showed accuracy within the 70%–130% range. AP102 is remarkably stable since no proteolytic fragments were detected on plasma samples analyzed by Orbitrap‐MS. Pharmacokinetic studies were conducted in rats, after single dose (1, 3, and 10 μg/kg, sc) and continuous subcutaneous administration (osmotic minipumps for 28 days, 3.0 or 10.0 μg/kg/h). AP102 showed a rapid absorption by the subcutaneous route (T_max: 15–30 minutes) and a fast elimination (t_1/2: 33–86 minutes). The PK profile of AP102 exhibited a mean clearance of 1.67 L/h and a mean distribution volume at steady state of 7.16 L/kg, about 10‐fold higher than those observed with other SSA or non‐ and mono‐iodinated AP102. LogD_7.4 determination confirmed the lipophilic properties of AP102 that might influence its distribution in tissues.     

Determination of GHB in human hair by HPLC‐MS/MS: Development and validation of a method and application to a study group and three possible single exposure cases

Gamma‐hydroxybutyrate (GHB) over the last two decades has generated increased notoriety as a euphoric and disinhibiting drug of abuse in cases of drug‐related sexual assault and for this reason it is considered a ‘date rape’ drug. The first aim of this paper was to develop and fully validate a method for the detection of GHB in human hair by high performance liquid chromatography‐tandem mass spectrometry (HPLC‐MS/MS) after liquid‐liquid extraction (LLE). The second aim was the application of the method to hair samples of 30 GHB‐free users in order to determine the basal level. The results obtained showed no significant differences in endogenous concentrations (p = 0.556) between hair samples of the three groups (black, blonde, and dyed hair) and the age and sex of the subjects did not affect the endogenous levels. Another 12 healthy volunteers, with no previous history of GHB use, were selected and a single dose (25 mg/Kg) was orally administered to all of them and hair samples were collected before the administration of the single dose and other two samples were collected one month and two months later, respectively. The segmental analysis of the latter two samples allowed us to calculate two ratios: 4.45:1 (95% C.I. 3.52–5.63) and 3.35:1 (95% C.I. 2.14–5.18), respectively, which can be recommended as reasonable values for a positive identification of GHB intake. Finally the method was applied to three real cases where a GHB single exposure probably occurred. Copyright © 2014 John Wiley & Sons, Ltd.     

Validated bioanalytical method for the quantification of RGB-286638, a novel multi-targeted protein kinase inhibitor, in human plasma and urine by liquid chromatography/tandem triple-quadrupole mass spectrometry

A rapid and sensitive liquid chromatography/tandem mass spectrometry (LC–MS/MS) method has been developed and validated for the quantitative determination of RBG-286638, a novel multi-targeted protein kinase inhibitor, in 200 μl aliquots of human potassium EDTA plasma with deuterated RGB-286638 as internal standard. The sample extraction and cleaning-up involved a simple liquid–liquid extraction with 100 μl aliquots of acetonitrile and 1 ml aliquots of n-butylchloride. Urine was accurately 5- and 10-fold diluted in blank plasma prior to extraction. Chromatographic separations were achieved on a reversed phase C₁₈ column eluted at a flow-rate of 0.250 ml/min on a gradient of 0.2 mM ammonium formate and acetonitrile both acidified with 0.1% formic acid. The overall cycle time of the method was 7 min, with RGB-286638 eluting at 1.9 min. The multiple reaction monitoring transitions were set at 546 > 402 (m/z), and 549 > 402 (m/z) for RGB-286638 and the internal standard, respectively. The calibration curves were linear over the range of 2.00 to 1000 ng/ml with the lower limit of quantitation validated at 2.00 ng/ml. The within-run and between-run precisions were within 7.90%, while the accuracy ranged from 92.2% to 99.7%. The method was successfully applied to samples derived from a clinical study.     

Simultaneous detection and quantitation of organic impurities in methamphetamine by ultra‐high‐performance liquid chromatography–tandem mass spectrometry,a complementary technique for methamphetamine profiling

The analysis of organic impurities plays an important role in the impurity profiling of methamphetamine, which in turn provides valuable information about methamphetamine manufacturing, in particular its synthetic route, chemicals, and precursors used. Ultra‐high‐performance liquid chromatography – tandem mass spectrometry (UHPLC – MS/MS) is ideally suited for this purpose due to its excellent sensitivity, selectivity, and wide linear range in multiple reaction monitoring (MRM) mode. In this study, a dilute‐and‐shoot UHPLC – MS/MS method was developed for the simultaneous identification and quantitation of 23 organic manufacturing impurities in illicit methamphetamine. The developed method was validated in terms of stability, limit of detection (LOD), lower limit of quantification (LLOQ), accuracy, and precision. More than 100 illicitly prepared methamphetamine samples were analyzed. Due to its ability to detect ephedrine/pseudoephedrine and its high sensitivity for critical target markers (eg, chloro‐pseudoephedrine, N‐cyclohexylamphetamine, and compounds B and P), more impurities and precursor/pre‐precursors were identified and quantified versus the current procedure by gas chromatography – mass spectrometry (GC – MS). Consequently, more samples could be classified by their synthetic routes. However, the UHPLC – MS/MS method has difficulty in detecting neutral and untargeted emerging manufacturing impurities and can therefore only serve as a complement to the current method. Despite this deficiency, the quantitative information acquired by the presented UHPLC – MS/MS methodology increased the sample discrimination power, thereby enhancing the capacity of methamphetamine profiling program (MPP) to conduct sample‐sample comparisons.     

Optimization and validation of simultaneous analyses of ecgonine,cocaine, and seven metabolites in human urine by gas chromatography–mass spectrometry using a one‐step solid‐phase extraction

The presence of ecgonine in urine has been proposed as an appropriate marker of cocaine use. Only a few methods have been published for their determination along with cocaine and the rest of its metabolites. Due to their high polarity and consequent solubility in water, these have low recoveries, which is why it is necessary to increase the sensitivity, by the formation of hydrochloric salts or multiderivatization of the analytes or by performing two solid‐phase extractions (SPEs), considerably increasing the time and cost of the analysis. This work describes a fast and fully validated procedure for the simultaneous detection and quantification of ecgonine, ecgonine‐methyl‐ester, benzoylecgonine, nor‐benzoylecgonine, m‐hydroxybenzoylecgonine, cocaethylene, cocaine, norcocaine, and norcocaethylene in human urine (500 μL) using one SPE and simple derivatization. Separation and quantification were achieved by gas chromatography–electron ionization–mass spectrometry (GC–EI–MS) in selected‐ion monitoring mode. Quantification was performed by the addition of deuterated analogs as internal standards. Calibration curves were linear in the adopted ranges, with determination coefficients higher than 0.99. The lower limits of quantification ranged from 2.5 to 10 ng/mL. The intra‐ and inter‐day precision, calculated in terms of relative standard deviation, were 1.2%–14.9% and 1.8%–17.9%, respectively. The accuracy, in terms of relative error, was within a ± 16.4% interval. Extraction efficiency ranged from 84% to 103%. Compared with existing methods, the procedure described herein is fast, since only one SPE is required, and cost‐effective. In addition, this method provides a high recovery for ecgonine, resulting in a better alternative to the previously published methods.     

Methamidophos,dichlorvos, O‐methoate and diazinon pesticides used in Turkey make a covalent bond with butyrylcholinesterase detected by mass spectrometry

Organophosphorus pesticides used most commonly in Turkey include methamidophos, dichlorvos, O‐methoate and diazinon. These toxic chemicals or their metabolites make a covalent bond with the active site serine of butyrylcholinesterase. Our goal was to identify the adducts that result from the reaction of human butyrylcholinesterase with these pesticides. Highly purified human butyrylcholinesterase was treated with a 20‐fold molar excess of pesticide. The protein was denatured by boiling and digested with trypsin. MS and MSMS spectra of HPLC‐purified peptides were acquired on a MALDI‐TOF‐TOF 4800 mass spectrometer. It was found that methamidophos added a mass of +93, consistent with addition of methoxy aminophosphate. A minor amount of adduct with an added mass of +109 was also found. Dichlorvos and O‐methoate both made dimethoxyphosphate (+108) and monomethoxyphosphate adducts (+94). Diazinon gave a novel adduct with an added mass of +152 consistent with diethoxythiophosphate. Inhibition of enzyme activity in the presence of diazinon developed slowly (15 h), concomitant with isomerization of diazinon via a thiono‐thiolo rearrangement. The isomer of diazinon yielded diethoxyphosphate and monoethoxyphosphate adducts with added masses of +136 and +108. MSMS spectra confirmed that each of the pesticides studied made a covalent bond with serine 198 of butyrylcholinesterase. These results can be used to identify the class of pesticides to which a patient was exposed.     

The outward transport of axoplasmic noradrenaline induced by a rise of the sodium concentration in the adrenergic nerve endings of the rat vas deferens

Summary The adrenergic nerve endings of the rat vas deferens were loaded with ³H-(–)-noradrenaline; COMT was inhibited by the presence of 10 mol/l U-0521, and all experiments were carried out with calcium-free solution. After 100 min of wash-out a neuronal efflux of tritium was obtained which remained constant with time (when expressed as fractional rate of loss; FRL); it contained more DOPEG than noradrenaline.The in vitro administration of reserpine-like drugs (reserpine and Ro 4-1284) increased the FRL of tritium, presumably because of an increase in the leakage of noradrenaline from storage vesicles; the efflux of DOPEG increased more than that of noradrenaline, and the ratio NA/DOPEG declined.Inhibition of the membrane ATPase (by omission of potassium from the medium or by the presence of 3 mmol/l ouabain) increased the FRL of tritium, presumably because of an increase in the net leakage of noradrenaline from the storage vesicles (as a consequence of the fall in the concentration of free axoplasmic noradrenaline; see below).Veratridine also increased the FRL of tritium, partly because of its known reserpine-like effect (Bönisch et al. 1983); in the presence of 1 mol/l veratridine, the efflux of DOPEG increased.Irrespective of the presence or absence of reserpine or Ro 4-1284, inhibition of the membrane ATPase or the presence of veratridine (agents or procedures which increase the axoplasmic sodium concentration) always resulted in a brisk increase of the efflux of noradrenaline that was accompanied by a simultaneous decrease in the efflux of DOPEG (see above for one exception). In all experiments the rise in internal sodium caused the ratio NA/DOPEG to increase.These results indicate that—as long as the sodium gradient is normal—the axonal membrane functions as a barrier that largely prevents any outward movement of axoplasmic noradrenaline. Consequently, the axoplasmic amine is largely deaminated, and the ratio NA/DOPEG is low. However, when the axoplasmic sodium concentration rises, axoplasmic noradrenaline is transported out of the nerve ending at such high rates that the axoplasmic noradrenaline concentration falls; the fall in the efflux of DOPEG is indicative of a fall in the intraneuronal formation of DOPEG. The results show that changes in the efflux of DOPEG (i.e., of a highly lipophilic metabolite that easily leaves adrenergic nerve endings) can serve as an index of changes in axoplasmic noradrenaline levels.Supported by the Deutsche Forschungsgemeinschaft (Tr.96)