Irwin J. Kopin

Chronic parkinsonism secondary to intravenous injection of meperidine analogues

Abuse of 4-propyloxy-4-phenyl-N-methylpiperidine, a meperidine congener, produced parkinsonism in a 23-year-old man. Unlike other drug-induced motor disturbances, the syndrome persisted for 18 months and responded to drugs that stimulate dopamine receptors. Biogenic amines and metabolites in the cerebrospinal fluid and microscopic evaluation of the brain at necropsy were consistent with damage to aminergic neurons in the substantia nigra.

Catecholamine Metabolism: A Contemporary View with Implications for Physiology and Medicine

This article provides an update about catecholamine metabolism, with emphasis on correcting common misconceptions relevant to catecholamine systems in health and disease. Importantly, most metabolism of catecholamines takes place within the same cells where the amines are synthesized. This mainly occurs secondary to leakage of catecholamines from vesicular stores into the cytoplasm. These stores exist in a highly dynamic equilibrium, with passive outward leakage counterbalanced by inward active transport controlled by vesicular monoamine transporters. In catecholaminergic neurons, the presence of monoamine oxidase leads to formation of reactive catecholaldehydes. Production of these toxic aldehydes depends on the dynamics of vesicular-axoplasmic monoamine exchange and enzyme-catalyzed conversion to nontoxic acids or alcohols. In sympathetic nerves, the aldehyde produced from norepinephrine is converted to 3,4-dihydroxyphenylglycol, not 3,4-dihydroxymandelic acid. Subsequent extraneuronal O-methylation consequently leads to production of 3-methoxy-4-hydroxyphenylglycol, not vanillylmandelic acid. Vanillylmandelic acid is instead formed in the liver by oxidation of 3-methoxy-4-hydroxyphenylglycol catalyzed by alcohol and aldehyde dehydrogenases. Compared to intraneuronal deamination, extraneuronal O-methylation of norepinephrine and epinephrine to metanephrines represent minor pathways of metabolism. The single largest source of metanephrines is the adrenal medulla. Similarly, pheochromocytoma tumor cells produce large amounts of metanephrines from catecholamines leaking from stores. Thus, these metabolites are particularly useful for detecting pheochromocytomas. The large contribution of intraneuronal deamination to catecholamine turnover, and dependence of this on the vesicular-axoplasmic monoamine exchange process, helps explain how synthesis, release, metabolism, turnover, and stores of catecholamines are regulated in a coordinated fashion during stress and in disease states.

Use of plasma norepinephrine for evaluation of sympathetic neuronal function in man.

Abstract Levels of norepinephrine (NE) in human plasma have been determined by a radioenzymatic technique sufficiently sensitive to measure 0.014 ng NE per ml plasma. Several procedures which raise plasma NE levels have been compared and a standard procedure developed to evaluate sympathetic neuronal function based on the increments in plasma NE produced by postural change and a standard amount of exertion. The mean basal level of NE in plasma of 74 resting, supine, normal subjects ranging in age from 10 to 70 (mean 32.7 years) was 0.292 ± 0.016 (± SEM) ng/ml and ranged from 0.112 to 0.738 ng/ml. There was a significant correlation between age and basal levels of NE (L.R. = 0.33, p

II. Validity and reliability of liquid chromatography with electrochemical detection for measuring plasma levels of norepinephrine and epinephrine in man

Abstract Liquid chromatography with electrochemical detection (LCEC) provides a rapid, sensitive, and specific technique for measuring human plasma norepinephrine (NE) and epinephrine (E) levels. We tested the reliability and validity of this technique against that of the catechol-O-methyl-transferase radioenzymatic (COMT-RE) assay. In healthy, resting humans, mean NE and E values were similar using the LCEC and COMT-RE techniques (311 vs. 300 pg/ml for NE; 57 vs. 52 pg/ml for E). In a series of 25 plasma samples obtained from a variety of sources, the correlation between the two methods was 0.99 for both NE and E. Coefficients of variation were similar for catecholamine levels above 100 pg/ml, but below this, the COMT-RE technique appeared to be more reliable. The advantages of the LCEC method are its speed, simplicity of sample preparation, low cost per assay, lack of use of radionuclides, and ease in trouble-shooting. The COMT-RE technique is preferable for small sample sizes or large numbers of samples. LCEC offers a reasonable alternative to the COMT-RE technique for measuring plasma norepineprhine and epinephrine.

Cardiac Sympathetic Nerve Function in Congestive Heart Failure

BACKGROUND Increased availability of norepinephrine (NE) for activation of cardiac adrenoceptors (increased cardiac adrenergic drive) and depletion of myocardial NE stores may contribute to the pathophysiology and progression of congestive heart failure. This study used a comprehensive neurochemical approach to examine the mechanisms responsible for these abnormalities. METHODS AND RESULTS Subjects with and without congestive heart failure received intravenous infusions of [(3)H]NE. Cardiac spillover, reuptake, vesicular-axoplasmic exchange, and tissue stores of NE were assessed from arterial and coronary venous plasma concentrations of endogenous and [(3)H]-labeled NE and dihydroxyphenylglycol. Tyrosine hydroxylase activity was assessed from plasma dopa, and NE turnover was assessed from measurements of NE metabolites. NE release and reuptake were both increased in the failing heart; however, the efficiency of NE reuptake was reduced such that cardiac spillover of NE was increased disproportionately more than neuronal release of NE. Cardiac NE stores were 47% lower and the rate of vesicular leakage of NE was 42% lower in the failing than in the normal heart. Cardiac spillover of dopa and NE turnover were increased similarly in congestive heart failure. CONCLUSIONS Increased neuronal release of NE and decreased efficiency of NE reuptake both contribute to increased cardiac adrenergic drive in congestive heart failure. Decreased vesicular leakage of NE, secondary to decreased myocardial stores of NE, limits the increase in cardiac NE turnover in CHF. Decreased NE store size in the failing heart appears to result not from insufficient tyrosine hydroxylation but from chronically increased NE turnover and reduced efficiency of NE reuptake and storage.

Plasma noradrenaline increases with age.

WE measured plasma noradrenaline (NA) levels in about 20 individuals who were to serve as normal control subjects and noted that older subjects tended to have higher NA levels. Extending the study to teenage and elderly subjects revealed that basal levels of plasma NA correlate with age and that the increase in plasma NA in response to stress is similarly related to age. There is considerable evidence that sensitivity to NA and NA metabolism change with increasing age. In rabbits and cats the threshold for cardiovascular response to low levels of NA decreases with old age1. In ageing rats uptake of NA into the heart is greater than in young animals2 and there is a diminished inotropic response of aged rat myocardium to a fixed concentration of NA (ref. 3). Cardiac monoamine oxidase activity increases severalfold during the life span of a rat while dopa decarboxylase decreases during the first year2. In man, propranalol, which blocks β-adrenergic receptors, reduces heart rate and cardiac output during exercise, but this effect is considerably smaller in older subjects4. The response of heart rate to hypoxia and hypercapnia is attenuated in older men5.

METABOLISM OF NORMETANEPHRINE-H3 IN RAT BRAIN—IDENTIFICATION OF CONJUGATED 3-METHOXY-4-HYDROPHENYLGLYCOL AS THE MAJOR METABOLITE

Abstract Normetanephrine-H 3 injected into the cisterna magna of rats is rapidly metabolized and disappears from brain with an initial half-life of about 12 min. Monoamine oxidase inhibition prevents almost completely the conversion of normetanephrine-H 3 to other metabolites and markedly diminishes the rate of disappearance of radioactivity from brain ( T 1 2 = 2·4 hr ) . These data show that normetanephrine is normally metabolized primarily by monoamine oxidase and that unaltered normetanephrine does not readily pass out of the brain. Free 3-methoxy-4-hydroxyphenylglycol (MHPG) and 3-methoxy-4-hydroxymandelic acid (VMA) formed from intracisternally injected normetanephrine-H 3 represent only a small fraction of the radioactivity in brain. The major metabolite was identified as the sulfate conjugate of MHPG. After intracisternal administration of norepinephrine-H 3 , 3-methoxy-4-hydroxyphenylglycol sulfate (MHPG-SO 4 ) was also found to be the major metabolite present in brain. These findings suggest that deamination, reduction, and subsequent conjugation with sulfate is the primary route of metabolism of normetanephrine in rat brain and that norepinephrine is also metabolized to this sulfate conjugate.

Stress-Induced Norepinephrine Release in the Hypothalamic Paraventricular Nucleus and Pituitary-Adrenocortical and Sympathoadrenal Activity: In Vivo Microdialysis Studies

The hypothalamic-pituitary-adrenocortical (HPA) axis and the autonomic nervous system are major effector systems that serve to maintain homeostasis during exposure to stressors. In the past decade, interest in neurochemical regulation and in pathways controlling activation of the HPA axis has focused on catecholamines, which are present in high concentrations in specific brain areas--especially in the hypothalamus. The work described in this review has concentrated on the application of in vivo microdialysis in rat brain regions such as the paraventricular nucleus (PVN) of the hypothalamus, the central nucleus of the amygdala (ACE), the bed nucleus of the stria terminalis (BNST), and the posterolateral hypothalamus in order to examine aspects of catecholaminergic function and relationships between altered catecholaminergic function and the HPA axis and sympathoadrenal system activation in stress. Exposure of animals to immobilization (IMMO) markedly and rapidly increases rates of synthesis, release, and metabolism of norepinephrine (NE) in all the brain areas mentioned above and supports previous suggestions that in the PVN NE stimulates release of corticotropin-releasing hormone (CRH). The role of NE in the ACE and the BNST and most other areas possessing noradrenergic innervation remains unclear. Studies involving lower brainstem hemisections show that noradrenergic terminals in the PVN are derived mainly from medullary catecholaminergic groups rather than from the locus ceruleus, which is the main source of NE in the brain. Moreover, the medullary catecholaminergic groups contribute substantially to IMMO-induced noradrenergic activation in the PVN. Data obtained from adrenalectomized rats, with or without glucocorticoid replacement, and from hypercortisolemic rats suggest that glucocorticoids feedback to inhibit CRH release in the PVN, via attenuation of noradrenergic activation. Results from rats exposed to different stressors have indicated substantial differences among stressors in eliciting PVN noradrenergic responses as well as of responses of the HPA, sympathoneural, and adrenomedullary systems. Finally, involvement of other areas that participate in the regulation of the HPA axis such as the ACE, the BNST, and the hippocampus and the importance of stress-induced changes in expression of immediate early genes such as c-fos are discussed.

L-Dopa-Induced Release of Cerebral Monoamines

L-Dopa markedly increased the efflux of tritiated dopamine and tritiated serotonin from rat brain slices. This action appeared contingent on the decarboxylation of L-dopa to dopamine, since it could be blocked by an inhibitor of L-amino acid decarboxylase. Selective destruction of catecholamine-containing nerve terminals by 6-hydroxydopamine significantly decreased the uptake and release of tritiated dopamine but not that of tritiated serotonin. These observations support the hypothesis that a portion of exogenously administered L-dopa may enter central serotonin terminals and undergo decarboxylation to the amine with resultant displacement of the endogenous indoleamine from vesicular stores.

Hemiparkinsonism in monkeys after unilateral internal carotid artery infusion of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

Infusion of MPTP (0.2-0.8 mg/kg) into the right internal carotid artery of monkeys produces toxin-induced injury to the right nigrostriatal pathway with sparing of other dopaminergic neurones on the infused side and with negligible or little injury to the opposite, untreated side. There are contralateral limb dystonic postures, rigidity, and bradykinesia, but the animals are able to eat and maintain health without drug treatment. Spontaneous motor activity is attended by circling towards the injured side, whereas treatment with L-DOPA/-carbidopa or apomorphine stimulates circling towards the intact side. Dopamine and dopamine metabolite levels are normal in the left caudate and putamen, but markedly depressed on the right (MPTP-treated) side. This animal hemiparkinsonian model will be useful in studies of volitional movement control, drug treatments of Parkinsons disease, and functional efficacy of brain tissue implants.