Dihydroxyphenylglycol Research Articles

Effects of nebivolol stereoisomers on the action of adrenaline on blood pressure, heart rate and blood levels of noradrenaline and DOPEG.

1. Nebivolol (a new beta 1-adrenoceptor blocking drug) has particular effects on the cardiovascular system, i.e. it induces hypotension without affecting cardiac function or increasing peripheral vascular resistances. In this study, we aimed to evaluate the effects of nebivolol and its stereoisomers on the actions of adrenaline (AD) at the cardiovascular level as well as at the prejunctional level (as ascertained by modification of noradrenaline (NA) and dihydroxyphenylglycol (DOPEG) plasma levels in the anaesthetized dog. 2. AD infusion (0.1 micrograms kg-1 min-1) did not induce statistically significant changes in mean blood pressure and heart rate; it caused a pronounced and sustained rise of AD levels, no significant alterations in NA levels and a marked, progressive and sustained increase in DOPEG levels. 3. Mean blood pressure was not affected by any of the nebivolol isomers. d- and dl-nebivolol in the two doses used (0.3 and 0.03 mg kg-1 in 15 min) caused a significant and dose-dependent decrease in heart rate. Plasma levels of AD and NA were not changed by any of the nebivolol isomers tested. However, all of them significantly reduced the increase in plasma levels of DOPEG induced by adrenaline infusion. 4. We conclude (1) that AD infusion in the dog facilitates NA release and that DOPEG is a good index of this effect; and (2) nebivolol appears to act at the prejunctional level, reducing the increase in NA release induced by adrenaline, as shown by the effect on DOPEG plasma levels.

Predominance of oxidative deamination in the metabolism of exogenous noradrenaline by the normal and chemically denervated human uterine artery.

Longitudinal strips were prepared from human uterine arteries obtained at hysterectomy. The artery had a low content of noradrenaline and dopamine, contrasting with a high content of the deaminated catechols, dihydroxyphenylglycol (DOPEG) and dihydroxymandelic acid (DOMA), which together represented 98% of endogenous catechols. When incubated with 3H-noradrenaline (0.1 mumol/l), the uterine artery removed, accumulated and metabolized noradrenaline. Deaminated metabolites predominated, DOMA being the most abundant metabolite. Cocaine markedly reduced the accumulation of 3H-noradrenaline and abolished 3H-DOPEG formation, but did not change 3H-DOMA. Selective monoamine oxidase (MAO) inhibitors (clorgyline, selegiline and 2-amino ethyl carboxamide derivatives) caused a marked decrease in the amounts of 3H-DOPEG, 3H-DOMA and 3H-O-methylated and deaminated metabolites (OMDA) formed by the tissue and an increase in 3H-normetanephrine (NMN) formation. Inhibition of catechol-O-methyltransferase suppressed NMN formation and reduced that of OMDA; hydrocortisone slightly depressed the formation of DOMA and OMDA. Homogenates of the uterine artery deaminated 3H-5-HT, 14C-phenylethylamine and 3H-tyramine; inhibition curves of the deamination of 3H-tyramine by clorgyline and selegiline were compatible with the presence of both MOA A and MOA B. Exposure of the strips to 6-hydroxydopamine (1.5 mmol/l for 20 min; 3 exposure periods followed by washout periods of 15,15 and 30 min) resulted in complete and selective chemical denervation of the arterial tissue. This chemical denervation had effects which were similar to those of cocaine. The 2-amino ethyl carboxyamide derivatives markedly reduced the formation of deaminated metabolites by the denervated strips.(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenergic activation in the paraventricular nucleus during acute and chronic immobilization stress in rats: an in vivo microdialysis study

In vivo microdialysis was used to study the effects of single (2 h) or repeated (2 h for 7 consecutive days) immobilization (IMMO) stress on extracellular fluid concentrations of norepinephrine (NE) and the deaminated metabolites of NE and dopamine, dihydroxyphenylglycol (DHPG) and dihydroxyphhenylacetic acid (DOPAC) in the paraventricular nucleus of conscious rats. During IMMO, NE, DHPG, and DOPAC levels increased markedly, with similar peak values and time courses in the repeatedly stressed and previously unstressed groups. NE levels during a 2-h baseline period were lower in the repeatedly stressed group than in the unstressed group (99 ± 9 pg/ml vs. 167 ± 13 pg/ml, P< 0.05), whereas DHPG (1,697 ± 263 pg/ml vs. 1,424 ± 194 pg/ml) and DOPAC (5,989 ± 863 pg/ml vs. 4,428 ± 1150 pg/ml) levels tended to be higher, so that the NE/DHPG ratio at baseline was significantly lower in the repeatedly stressed group ( P < 0.05). The results indicate that IMMO stress enhances NE release, reuptake, metabolism, and synthesis in the PVN. Repeated exposure to IMMO may decrease the microdialysate NE/DHPG ratio by inhibiting exocytotic release or enhancing neuronal reuptake of NE. In either case, the results suggest that repeated exposure to stress alters the release and disposition of NE in the PVN of conscious animals.

Increased cardiac production of dihydroxyphenylalanine (DOPA) during sympathetic stimulation in anaesthetized dogs

Entry of dihydroxyphenylalanine (DOPA) into plasma from specific organs may reflect regional activity of tyrosine hydroxylase, the enzyme responsible for the immediate synthesis of DOPA and rate-limiting for subsequent formation of catecholamines. Therefore, cardiac spillovers of DOPA, noradrenaline and the intraneuronal metabolite of noradrenaline, dihydroxyphenylglycol (DHPG), were examined during two periods of graded electrical stimulation of the sympathetic nerves to the heart in anaesthetized dogs. Responses were examined before and after neuronal uptake blockade with desipramine. Cardiac spillover of DOPA increased by 1.8- and 4.4-fold during sympathetic stimulation before desipramine and by 1.6- and 3.3-fold after desipramine. Fold increases in cardiac spillover of DOPA were much lower than but positively related with fold increases in noradrenaline spillover (5.9- and 13.8-fold increases before and 9.0- and 15.8-fold increases after desipramine). Increases in cardiac spillover of DHPG (1.5- and 2.3-fold increases) were blocked by desipramine so that fold changes in spillover of DOPA were greater than and poorly related to changes in spillover of DHPG. Fold increases in cardiac spillover of DOPA showed a close one-to-one positive relationship with fold increases in the sum of cardiac spillovers of noradrenaline and dihydroxyphenylglycol before and after desipramine. For a given fold increase in noradrenaline release, transmitter turnover is increased fractionally and noradrenaline synthesis need also only increase fractionally to maintain transmitter stores constant. The close relationship between fold increases in cardiac spillover of DOPA and combined spillovers of noradrenaline and DHPG is consistent with regulation of tyrosine hydroxylase activity to match changes in noradrenaline synthesis with changes in noradrenaline turnover. Changes in cardiac spillover of DOPA appear to reflect local changes in tyrosine hydroxylase activity.

Estimation of noradrenaline concentrations in the axoplasm of noradrenergic neurones in man

The plasma concentrations of noradrenaline (NA) and its primary neuronal metabolite, dihydroxyphenylglycol (DOPEG), were examined during graded orthostasis and NA infusion in 13 healthy subjects to estimate the NA concentration difference between the site of neuronal DOPEG formation and that in plasma. Stimulation of NA release by graded orthostasis resulted in similar absolute increments in plasma NA and DOPEG with both plasma concentrations being dependent on the degree of orthostasis. The mean value of the delta DOPEG/delta NA ratio amounted to 0.999 (0.745; 1.341). NA was infused i.v. during two consecutive 30-min periods at constant rates of 0.43 and 0.86 nmol kg-1 min-1, respectively. This infusion resulted in a delta DOPEG/delta NA ratio of 0.048 (0.036; 0.064) for the first and 0.078 (0.067; 0.090) for the second infusion period (p less than 0.01). For each individual subject, the factor quantifying the NA concentration difference between the site of neuronal DOPEG formation and plasma was calculated from the square root of the ratio of 'delta DOPEG/delta NA during orthostasis to delta DOPEG/delta NA during the low rate of NA infusion'. The average NA concentration at the site of neuronal DOPEG formation (i.e. the axoplasm of noradrenergic neurones) was found to be 4.6-fold higher than that in plasma.

Sympathetic nervous function in human heart as assessed by cardiac spillovers of dihydroxyphenylglycol and norepinephrine.

Measurement of cardiac norepinephrine spillover may indicate the amount of transmitter at neuroeffector sites but does not distinguish neuronal release or reuptake in determining this amount or provide information about other aspects of sympathetic function. This report examines how cardiac spillover of the norepinephrine metabolite dihydroxyphenylglycol (DHPG) provides additional distinct information about cardiac sympathetic function. Arterial and coronary venous blood samples were taken during cardiac catheterization and intravenous infusion of [3H]norepinephrine in 57 subjects. Subjects were given intravenous yohimbine or underwent mental stress, handgrip exercise, and cycling exercise to activate sympathetic nerves or were given intravenous desipramine to block norepinephrine reuptake. Cardiac DHPG spillover (601 +/- 41 pmol/min) was eightfold greater than norepinephrine spillover (78 +/- 10 pmol/min) at rest and increased during sympathetic activation by 65% of the increase of norepinephrine. This and the desipramine-sensitive cardiac production of [3H]-labeled DHPG from [3H]norepinephrine indicated that 10.5 times more endogenous norepinephrine is recaptured than escapes into plasma; that more than 90% of recaptured norepinephrine is sequestered into storage vesicles; and that under resting conditions, most cardiac spillover of DHPG and turnover of norepinephrine are from metabolism of transmitter leaking from vesicles; the latter process is independent of exocytotic transmitter release with a rate at rest over 100-fold that of norepinephrine spillover and over 10-fold that of norepinephrine reuptake. Cardiac spillover of DHPG provides information about processes close to or within sympathetic nerve endings that cannot be provided by measurements of norepinephrine spillover alone. This includes quantitative information about the role of neuronal uptake in terminating the actions of norepinephrine at neuroeffector sites and the importance of vesicular-axoplasmic exchange of norepinephrine as a dynamic process contributing to norepinephrine turnover.

The role of monoamine oxidase in the metabolism of exogenous noradrenaline by the human saphenous vein.

Human saphenous vein segments were obtained from patients subjected to coronary bypass surgery. As determined by HPLC-ED, the veins had a relatively low content of noradrenaline and high content of the deaminated metabolites, dihydroxyphenylglycol (DOPEG) and dihydroxymandelic acid (DOMA). In vein segments which had been incubated with 3H-noradrenaline (0.1 mumol/l), the oxidative deamination pathway predominated over the O-methylating one. Deamination occurred both at the neuronal and extraneuronal level; DOPEG appearing to be a good index of intraneuronal deamination, whereas DOMA and O-methylated and deaminated metabolites were mainly formed extraneuronally. Both MAO type A and MAO type B selective inhibitors reduced the deamination of noradrenaline; deamination was also found to be partially sensitive to semicarbazide. Inhibition of neuronal uptake or of deamination increased O-methylation. The human saphenous vein thus metabolizes exogenous noradrenaline following a pattern which substantially differs from that shown to occur in various blood vessels from other animal species.

Neuronal uptake, metabolism, and release of tritium-labeled norepinephrine during assessment of its plasma kinetics

The extent to which intravenously infused [3H]norepinephrine ([3H]NE) is stored within and released by sympathetic nerves (tracer recycling) was examined by assessment of its plasma concentrations and by those of its intraneuronal metabolite dihydroxyphenylglycol (DHPG). Tracer recycling, as assessed by perturbations to steady-state plasma [3H]NE, was not apparent in humans during sympathetic activation by orthostasis or exercise. During intense electrical stimulation of cardiac sympathetic nerves after an [3H]NE infusion in dogs, plasma [3H]NE was higher in coronary sinus than arterial plasma, consistent with tracer recycling by cardiac sympathetic nerves. The specific activity of released [3H]NE was similar to that of [3H]NE and [3H]DHPG in cardiac tissue, indicating that both released [3H]NE and [3H]DHPG were derived from the same pool of neuronally stored [3H]NE. During intravenous infusion of [3H]NE in humans, plasma [3H]NE reached a steady state within 12 min and remained constant, whereas plasma [3H]DHPG increased progressively, reflecting metabolism of an increasing amount of [3H]NE leaking from sympathetic vesicles. Plasma concentrations of [3H]DHPG approached or exceeded those of [3H]NE after the end of radiotracer infusions and in the venous drainage of the heart and liver. Thus, to avoid error during assessment of plasma [3H]NE kinetics, an analytical step should be employed to separate plasma [3H]DHPG from [3H]NE. Because [3H]DHPG is derived from [3H]NE within the neuron, the specific activity of neuronally stored [3H]NE could be assumed to be no higher than the specific activity of plasma [3H]DHPG. This provided a means to estimate the maximum extent of tracer recycling, thereby indicating that, during intravenous infusion of [3H]NE, tracer recycling contributed negligibly (less than 5%) to steady-state plasma [3H]NE at normal levels of sympathetic activity.

Neuronal reuptake of norepinephrine and production of dihydroxyphenylglycol by cardiac sympathetic nerves in the anesthetized dog.

Reuptake of norepinephrine by cardiac sympathetic nerves before and during two levels of electrical stimulation of the left ansa subclavia was estimated in anesthetized dogs from the cardiac production of dihydroxyphenylglycol (DHPG), the intraneuronal metabolite of norepinephrine. The method depended on the effects of neuronal uptake blockade with desipramine on the cardiac production of [3H]DHPG from intravenously infused [3H]norepinephrine. The ratio of the desipramine-induced decrease in the cardiac extraction of [3H]norepinephrine to the production of [3H]DHPG was used to transform the cardiac production of DHPG from recaptured norepinephrine into a rate for norepinephrine reuptake. Cardiac spillover of norepinephrine into plasma increased from 49 +/- 12 to 205 +/- 40 and 451 +/- 118 pmol/min during sympathetic activation. Cardiac DHPG production increased from 108 +/- 18 to 166 +/- 34 and 240 +/- 47 pmol/min. Desipramine decreased resting cardiac DHPG production by 20% and completely blocked the stimulation-induced increase. Thus, most (80%) cardiac DHPG produced at rest was derived from norepinephrine leaking from storage vesicles. This amount remained constant, and that derived from recaptured norepinephrine increased during sympathetic activation. The cardiac extraction of [3H]norepinephrine (126,000 dpm/min) and production of [3H]DHPG (3,790 dpm/min) were decreased by 55-57% after desipramine. Thus, only 3% of the norepinephrine recaptured by cardiac sympathetic nerves appeared in plasma as DHPG. The remainder was sequestered into storage vesicles (more than 94%) or ultimately formed metabolites other than DHPG (less than 3%). Reuptake of norepinephrine by cardiac sympathetic nerves was 1,188 +/- 476 pmol/min and increased in parallel with cardiac norepinephrine spillover to 4,182 +/- 1,982 and 6,594 +/- 2,241 pmol/min during sympathetic stimulation. Of the norepinephrine released by cardiac sympathetic nerves, 16-fold more was recaptured than entered plasma. Combined estimation of norepinephrine reuptake and spillover offers an approach to assess the efficiency of neuronal reuptake in disorders of cardiac function.

Sympathoneural and skeletal muscle contributions to plasma DOPA responses in pithed rats

Dihydroxyphenylalanine (DOPA) in plasma has been thought to originate from sympathetic nerve endings and to reflect catecholamine biosynthesis, because changes in DOPA levels follow pharmacologically- or environmentally-induced manipulations that alter turnover of the sympathetic neurotransmitter, norepinephrine (NE). Skeletal muscle may be an additional, non-neural source of circulating DOPA. In the present study we examined sympathoneuronal and skeletal muscle contributions to DOPA in arterial plasma in pithed rats. Electrical stimulation of the spinal cord causes discharges of sympathetic post-ganglionic neurons, with attendant release of NE into the bloodstream, and discharges of spinal motoneurons, which causes diffuse contraction of skeletal muscle. Stimulation of the spinal cord rapidly elevated arterial plasma concentrations of NE, dihydroxyphenylglycol (DHPG), and DOPA. Pre-treatment with curare, a skeletal muscle relaxant, did not affect the NE and DHPG responses but attenuated the DOPA responses by about 50%. Administration of chlorisondamine, a ganglionic blocker, abolished NE and DHPG responses to cord stimulation, and DOPA responses were decreased by about 90%. Adrenal-demedullation did not affect the stimulation-induced DOPA responses. The results demonstrate that in pithed rats undergoing spinal cord stimulation, DOPA is released into the bloodstream. Since this response is markedly inhibited after ganglionic blockade and also attenuated after skeletal muscle paralysis, the results provide indirect evidence that DOPA formed in sympathetic neurons can be stored in a non-neuronal pool and released during skeletal muscle contraction.

Regional extraction of circulating norepinephrine, DOPA, and dihydroxyphenylglycol in humans

Dihydroxyphenylglycol (DHPG) is the main intraneuronal metabolite of the sympathetic neurotransmitter, norepinephrine (NE), and dihydroxyphenylalanine (DOPA) the immediate product of the rate-limiting step in catecholamine biosynthesis. Simultaneous measurements of regional rates of appearance (spillovers) of NE, DOPA, and DHPG in plasma have the potential to provide unique information about aspects of sympathoneural function but have not actually been measured in humans. In the present study, spillovers of DHPG, DOPA, and NE in the heart, head, leg, and lungs, were estimated from regional extraction fractions of infused [ 3H]-1-NE, DHPG, and [ 13C 6]DOPA or unlabelled DOPA in humans during cardiac catheterization. There was little cardiac extraction of DHPG (7 ± SEM 2%) or DOPA (8 ± 4%) but substantial extraction of NE (69 ± 4%). Values for cardiac spillover of DHPG and DOPA therefore were similar to values for the arteriovenous increment times plasma flow (arteriovenous production rate), whereas the cardiac spillover of NE averaged about 7-times the NE arteriovenous production rate. Cardiac DHPG spillover (28 ± 3 ng/min) exceeded the spillovers of NE (9 ± 2 ng/min) and DOPA (15 ± 4 ng/min). In contrast, cranial DOPA spillover (159 ng/min) exceeded those of NE and DHPG by 8- and 2-fold and accounted for about 1/10 of the total spillover of DOPA into arterial plasma. In the femoral vascular bed, arteriovenous production rates of NE and DHPG were unrelated to femoral spillovers of NE and DHPG. Arterial and regional clearances of [ 13 C 6]DOPA were similar to those of unlabelled DOPA. The results suggest that (1) endogenous NE, DOPA, and DHPG all are released into the bloodstream by the heart, head, and limbs of humans; (2) DHPG and DOPA are not co-released with NE; (3) cardiac arteriovenous production rates of DOPA and DHPG can be used to indicate cardiac spillover of these catechols, whereas the cardiac NE arteriovenous production rate substantially underestimates cardiac NE spillover; and (4) estimates of limb spillover of NE and DHPG require concurrent measurements of the corresponding regional clearances.

Plasma noradrenaline in cirrhosis: a study of kinetics and temporal relationship to ascites formation

The kinetics of plasma noradrenaline (NA) were studied in 14 patients with cirrhosis and ascites and 13 normal subjects. [3H]noradrenaline ([3H] NA) was infused intravenously to steady state and the spillover of NA into plasma and its clearance from plasma calculated. The increase in plasma NA in the cirrhotic patients was due to an increase in NA spillover (14.5 vs 3.9 nmol min-1m-2; P less than 0.001). NA plasma clearance was also increased in the cirrhotic patients (3.5 vs 2.11 min-1m-2; P less than 0.01). Plasma NA and dihydroxyphenylglycol (DHPG), a metabolite of NA of which a portion is formed after re-uptake of NA into sympathetic nerve endings, were then measured in 23 patients with cirrhosis and ascites, 17 patients with cirrhosis who had never had ascites, and 34 normal subjects. Both plasma NA and DHPG were significantly increased in the patients with ascites (NA 4.7, DHPG 14.7 nmol l-1 and in the patients with cirrhosis but no ascites (NA 3.8, DHPG 12.0 nmol l-1) compared with normal subjects (NA 1.9, DHPG 8.8 nmol 1-1). Therefore, the increase in plasma NA in cirrhosis is due to increased activity of the sympathetic nervous system rather than interference with the metabolism of NA or impaired neuronal uptake of NA. This increase appears to precede the development of ascites.

Noradrenaline reuptake and plasma dihydroxyphenylglycol during sustained changes in sympathetic activity in rabbits

The effects of sustained changes in sympathetic activity, produced by intracisternal (i.c.) infusion of yohimbine or clonidine, on the formation of the intraneuronal noradrenaline metabolite, dihydroxyphenylglycol (DHPG), and on the efficiency of noradrenaline reuptake were examined in conscious rabbits. Noradrenaline spillover was estimated by radiotracer dilution analysis of i.v. infused [ 3H]noradrenaline. Noradrenaline reuptake was estimated from the amount of DHPG derived from recaptured neurotransmitter and the effects of desipramine-induced neuronal uptake blockade on noradrenaline clearance and plasma [ 3H]DHPG. The efficiency of neuronal reuptake was assessed from relationships between noradrenaline reuptake and spillover. Sustained sympathetic activation with i.c. yohimbine increased the amount of plasma DHPG that was derived from recaptured noradrenaline as well as that derived from other sources. Acute administration of desipramine decreased both components so that the decrease in plasma DHPG overestimated the amount derived from recaptured noradrenaline. Thus, estimation of the component of plasma DHPG that was derived from recaptured noradrenaline was most accurately achieved by examination of relationships between plasma noradrenaline and DHPG. Noradrenaline reuptake and spillover into plasma were decreased by i.c. infusion of clonidine and increased by i.c. infusion of yohimbine. Neither i.c. infusion of clonidine nor yohimbine altered relationships between noradrenaline reuptake and spillover indicating that the efficiency of neuronal reuptake was unaltered by sustained changes in sympathetic activity.

Calcium antagonists and cardiac noradrenaline release in ischemia

The effects of the calcium antagonists verapamil, gallopamil, nifedipine, felodipine and diltiazem on noradrenaline release during ischemia were studied in isolated perfused rat hearts. Endogenous levels of noradrenaline and its intraneuronal metabolite dihydroxyphenylethyleneglycol (DOPEG) were determined by high pressure liquid chromatography. Global isothermic ischemia of 20 min caused a release of endogenous noradrenaline amounting to 180 ± 15 pmol/g. The calcium antagonists tested significantly suppressed ischemia-induced noradrenaline release (IC 50 in μmol/l: verapamil 1, gallopamil 0.3, nifedipine 1, felodipine 3). Noradrenaline release during ischemia and the inhibitory effect of the calcium antagonists, were independent of extracellular calcium, indicating a nonexocytotic release mechanism. Interaction of the calcium antagonists with the major components of nonexocytotic release, intraneuronal storage and carrier-mediated transport, was tested in normoxic rat hearts. Vesicular storage was not stabilized by the calcium antagonists. In fact, verapamil, gallopamil, diltiazem and felodipine disturbed storage function, as indicated by an increased DOPEG release. Direct interaction with the noradrenaline carrier (uptake 1) was demonstrated for verapamil, gallopamil, and diltiazem in a model of 3H-noradrenaline uptake. In conclusion, the calcium antagonists investigated inhibit noradrenaline release in ischemia by a mechanism which is different from blockade of neuronal calcium influx, and is rather due to an interaction with carrier-mediated transport of noradrenaline across the plasma membrane.

Simultaneous measurement of plasma and brain extracellular fluid concentrations of catechols after yohimbine administration in rats

The present study examined whether systemic injection of the α 2 adrenoceptor blocker, yohimbine, affects concentrations of norepinephrine (NE) and its metabolites in extracellular fluid in the brain and in blood. Microdialysis probes were inserted into the posterior hypothalamus, medulla, and caudate/putamen in rats. Microdialysate and arterial blood were sampled after intravenous administration of yohimbine. In the hypothalamus yohimbine produced significant increases in extracellular fluid concentrations of NE, its intraneuronal metabolite, dihydroxyphenylglycol (DHPG), and methoxyhydroxyphenylglycol (MHPG), a major neuronal and extraneuronal metabolite of NE. The increases in these levels were small or absent in the caudate/putamen, where dopamine is the primary catecholamine transmitter. During systemic infusion of tracer amounts of [ 3H]NE, little if any radioactive NE or DHPG appeared in the microdialysate, whereas substantial levels of [ 3H]MHPG were present and increased as plasma [ 3MHPG levels rose. The results support the view that α 2 adrenoceptor blockade in the brain increases hypothalamic and medullary release, reuptake, and metabolism of NE. The findings cannot be explained by disruption of the blood-brain barrier for catecholamines by insertion of the microdialysis probes. Enhanced sympathetic outflow and peripheral release of NE when α 2 adrenoceptors are blocked appears to be attended by enhanced central NE release, presumably as a results of presynaptic α 2 adrenoceptor blockade at noradrenergic terminals in the brain. This is consistent with the hypothesis that central noradrenergic NE release is regulated by presynaptic α 2 adrenoceptors.

The relationship between age and venous plasma concentrations of noradrenaline, catecholamine metabolites, DOPA and neuropeptide Y-like immunoreactivity in normal human subjects

Forearm venous plasma concentrations of noradrenaline (NA), catecholamine metabolites (dihydroxyphenylglycol (DHPG), dihydroxyphenylacetic acid (DOPAC], dihydroxyphenylalanine (DOPA) and neuropeptide Y-like immunoreactivity (NPY-LI) were studied in 22 men aged 20 to 81 years in the supine position and after 30 min of standing posture. Venous plasma NA, DHPG and NPY-LI increased significantly in the standing position, whereas venous plasma DOPAC and venous plasma DOPA remained unchanged. Increments in venous plasma NA and DHPG upon standing up, as well as venous plasma NA in the standing-up position, increased significantly with age. The ratio between increments in venous plasma DHPG and venous plasma NA did not change with age. Venous plasma NPY-LI and venous plasma DOPA did not change with age, whereas venous plasma DOPAC decreased significantly. Changes in venous plasma NPY-LI were negatively correlated with changes in pulse pressure. These results confirm previous studies that sympathetic activity increases with age. The unchanged ratio between increments in venous plasma DHPG and venous plasma NA suggests that intra-neuronal deamination of recaptured NA is unchanged in the elderly. The lower basal venous plasma DOPAC concentration may suggest a reduced basal intraneuronal synthesis of NA in old age.

Direct determination of homovanillic acid release from the human brain, and indicator of central dopaminergic activity

The plasma concentration of the dopamine (DA) metabolite, homovanillic acid (HVA), is used as an indicator of central nervous system dopaminergic activity. Using percutaneously inserted catheters we were able to obtain blood samples simultaneously from the right and left internal jugular veins. Veno-arterial HVA plasma concentration differences combined with adjusted organ plasma flows were used, according to the Fick Principle, to determine the HVA overflow from the brain. The HVA overflow from the liver was also measured. HVA overflow from the brain represented 12% of the total body HVA production. A similar amount was released from the liver, illustrating the limited validity of peripheral plasma HVA measurements as an indicator of central dopaminergic activity. HVA release from the human brain displayed a degree of asymmetry, the overflow into the left internal jugular vein being 36% greater than that into the right. Cerebral venous blood flow scans indicated that cortical cerebral regions drained preferentially into the right internal jugular; by inference the higher HVA overflow on the left originated from dopamine-rich subcortical brain areas. Since HVA in plasma may arise from the metabolism of DA existing either as a neurotransmitter or a norpinephrine (NE) precursor we measured the internal jugular vein plasma concentrations of NE, and its metabolite dihydroxyphenylglycol (DHPG), to determine whether they displayed a similar pattern of release to HVA. The overflow of both NE and DHPG into the right internal jugular vein was approximately double that on the left. Since the overflow of HVA did not parallel that of NE and DHPG it may be inferred that the origin of much of the subcortically produced HVA is from dopaminergic neurons and not from the metabolism of precursor DA in noradrenergic neurones or cerebrovascular sympathetic nerves.

Medial basal hypothalamic monoamine activity associated with intracerebroventricular p-chlorophenylalanine-induced hyperphagia

There is evidence for reciprocal interactions between the brain monoamine neurotransmitters serotonin and noradrenaline which may play a critical role in homeostasis. The aim of the present study was to establish the effect of drug-induced damage to the serotoninergic system on noradrenergic activity in the hypothalamus. Bilateral intracerebroventricular injections of p-chlorophenylalanine (PCPA; 3 mg/kg in 2 × 6 μl) were made to induce destruction in the serotoninergic system. Relative to saline-injected controls, PCPA-injected rats began overeating by 3 days postinjection. On day 10, when the experimental rats were consuming approximately 120% that of controls, animals were 4-h food deprived, sacrificed and the medial basal hypothalamus was removed for later analysis (by gas chromatography/mass spectrometry) of noradrenaline (NA), serotonin (5-HT) and dopamine (DA) and their principal metabolites dihydroxyphenylethyleneglycol (DHPG), 5-hydroxyindoleacetic acid (5-IAAA) and 3,4-dihydroxyphenylacetic acid (DOPAC), respectively. The ratio of metabolite to monoamine provided an index of functional activity. Trunk blood was collected for analysis of serum insulin and glucose. PCPA-injected animals had higher levels of DHPG ( P < 0.05), an increase in the DHPG/NA ratio ( P <0.02), lower serum insulin ( P < 0.05) and increased serum glucose ( P < 0.05). There were significant correlations between noradrenergic activity (DHPG/NA ratio) and: (1) food intake (day 9 and 10 average; r = 0.62, P < 0.05); and (2) serum glucose ( r = 0.59, P < 0.05). In contrast, PCPA treatment only marginally lowered levels of 5-HT and 5-HIAA with no change in the 5-HIAA/5-HT ratio and no changes in DA or DOPAC or the DOPAC/DA ratio. Neither serotoninergic nor dopaminergic activity was related to any of the intake, body weight, insulin or glucose measures. We conclude that intracerebroventricular PCPA chronically elevates hypothalamic noradrenergic activity and this, rather than decreased serotoninergic activity, may lead to the observed hyperphagia, hyperglycemia and hypoinsulinemia.

Parallel increases in noradrenaline reuptake and release into plasma during activation of the sympathetic nervous system in rabbits

Rates of noradrenaline reuptake and spillover into plasma were examined in conscious rabbits before and during activation of the sympathetic nervous system to determine whether neuronal reuptake varies disproportionately or in parallel with increases in noradrenaline release. The sympathetic nervous system was stimulated by nitroprusside-induced hypotension, 2-deoxyglucose-induced glucopenia or intravenous infusion of isoprenaline before and after administration of desipramine to block neuronal uptake. Spillover of noradrenaline into plasma was estimated from the dilution of intravenously infused 3H-noradrenaline with endogenous plasma noradrenaline. The amount of dihydroxyphenylglycol (DHPG) in plasma that was derived from metabolism of recaptured noradrenaline, together with the desipramine-induced decreases in clearance from plasma of 3H-noradrenaline and appearance in plasma of 3H-DHPG, were used to estimate the rate of neuronal reuptake of noradrenaline. The mean (+/- SEM) resting noradrenaline reuptake rate (n = 28) was 0.62 +/- 0.04 nmol kg-1 min-1, 5-fold greater than the rate of its spillover into plasma (0.12 +/- 0.02 nmol kg-1 min-1). Intravenous infusion of nitroprusside at 3 rates titrated to cause graded increases in heart rate caused 74%, 129% and 240% increases in noradrenaline spillover into plasma and 66%, 104% and 198% increases in noradrenaline reuptake. At 15-30 min after intravenous injection of 2-deoxyglucose (500 mg/kg) there was a 106% increase in noradrenaline spillover and a 93% increase in noradrenaline reuptake. Infusion of isoprenaline (0.25 micrograms kg-1 min-1) caused a 102% increase in noradrenaline spillover and a 130% increase in noradrenaline reuptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve impulse-induced release of endogenous noradrenaline and adrenaline from the perfused cod spleen

Release of endogenous catecholamines (CA) by electrical nerve stimulation (NS) was studied in the isolated perfused spleen of the Atlantic cod, Gadus morhua. An HPLC-system for the analysis of endogenously released CA is described. Cocaine (COC) was used to block neuronal re-uptake of endogenous CA released by NS. Splanchnic NS at frequencies of 1-40 Hz for 20 s resulted in release of noradrenaline (NA) and adrenaline (A) with a maximal total overflow at 20 Hz for both amines. The release of CA was frequency-dependent. COC (0.1 mmol.l-1) increased NS-evoked (40 Hz) overflow of NA and A by 4.8 and 2.2 times, respectively. and reduced the overflow of dihydroxyphenylglycol (DOPEG) to spontaneous efflux levels or less. It can be concluded that the HPLC-technique used was adequate for measurement of NS-evoked release of endogenous CA and DOPEG from the isolated perfused cod spleen, and the model presented can therefore be used when studying adrenergic mechanisms in fish spleen.