Endogenous Norepinephrine Levels Research Articles

Patterns of extracellular norepinephrine in the paraventricular hypothalamus: Relationship to circadian rhythm and deprivation-induced eating behavior

In order to clarify the physiological role of norepinephrine (NE) in the hypothalamic paraventricular nucleus (PVN), changes in extracellular levels of endogenous NE were measured in 11 freely-moving rats using microdialysis and high pressure liquid chromatography with electrochemical detection. To determine whether there was a circadian pattern of extracellular NE in freely-eating subjects, samples of dialysate from the vicinity of the PVN were collected and assayed for NE every 2 hrs for 48 hrs. The pattern of NE averaged across subjects was similar during both 24-hr periods, with a reliable peak at the beginning of the dark cycle and relatively stable levels at all other times. When these animals were subsequently deprived of food for 24 hrs, a gradual rise in extracellular NE was observed, ultimately increasing to 215% of the predeprivation level. When the animals were refed and NE measurements were continued at more frequent intervals, extracellular levels were found to decline during the first 20 min of eating, as well as over the next 3 hrs as food intake diminished. These patterns of extracellular NE, together with previous evidence, suggest that endogenous NE in the PVN plays a role in the initiation and/or maintenance of normal eating behavior at the beginning of the nocturnal feeding period, as well as after food deprivation.

The perioperative fate of residual gradients after repair of discrete subaortic stenosis and time-related blood levels of catecholamines

The fate of the residual peak systolic left ventricular-aortic gradient was studied perioperatively in 14 patients with membranous discrete subaortic stenosis. In nine (group A) the initial postrepair left ventricular-aortic gradient was greater than 35 mm Hg (mean 56.8 +/- 13.4), and in five (group B) there was no significant postoperative gradient (mean 15.3 +/- 3.2 mm Hg). The operation included membranectomy and myectomy. Peak left ventricular-aortic pressure gradient, endogenous levels of norepinephrine, peak rate of rise of left ventricular pressure, cardiac index, systemic vascular resistance, heart rate, and central venous pressure were recorded at the end of cardiopulmonary bypass and in 3-hour intervals for the next 9 hours. In group A during that period there was a 67% reduction in peak systolic left ventricular-aortic gradient (from 56.8 +/- 13.4 to 18 +/- 14 mm Hg, p less than 0.001). Concomitant reduction in the initial endogenous norepinephrine level was observed (from 982.1 +/- 181 to 422.6 +/- 109 pg/ml, p less than 0.001). A consistent linear relationship between norepinephrine levels and peak systolic left ventricular-aortic gradient was found (r = 0.78). Systolic left ventricular pressure decreased from 174.2 +/- 24.8 to 113.8 +/- 14.7 mm Hg (p less than 0.001). Marked reduction in peak rate of rise of left ventricular pressure (from 3455 +/- 636 to 2161 +/- 680 mm Hg/sec, p less than 0.001) was observed. Cardiac index increased and systemic vascular resistance decreased during the study period (from 2.11 +/- 0.2 to 3.07 +/- 0.26 L/min, p less than 0.001, and from 2172 +/- 331 to 1233 +/- 202 dynes/sec/cm-5, p less than 0.001, respectively). There were no significant changes in heart rate (p = not significant) and central venous pressure p = not significant). Some of the residual perioperative left ventricular-aortic gradients in patients with discrete subaortic stenosis undergoing repairs are dynamic and transient, and are probably related to increased postoperative sympathetic activity.

High NaCl diet reduces hypothalamic norepinephrine turnover in hypertensive rats.

The current study tested the hypothesis that high NaCl diets elevate blood pressure in NaCl-sensitive spontaneously hypertensive rats (SHR-S) by reducing noradrenergic input to depressor neurons in the anterior hypothalamus. SHR-S were studied at 7 weeks of age, and age-matched salt resistant SHR (SHR-R) and normotensive Wistar-Kyoto rats (WKY) were controls. Rats were fed either high (8%) NaCl or control (1% NaCl) diets for 2 weeks, following which norepinephrine turnover in hypothalamus (anterior, posterior, and ventral regions), brainstem (pons and medulla), and thoracic spinal cord was assessed using the dopamine beta-hydroxylase inhibitor 1-cyclohexyl-2-mercapto-imidazole (CHMI). Regional brain catecholamines were measured by high performance liquid chromatography with electrochemical detection following intraperitoneal injection of CHMI or vehicle. Disappearance of norepinephrine following CHMI was used as an index of noradrenergic neuronal activity. The 8% NaCl diet caused a significant elevation in blood pressure in SHR-S but not in SHR-R or WKY. Endogenous norepinephrine levels and turnover were lower in the anterior hypothalamus of SHR-S fed 8% NaCl than in those fed 1% NaCl but were not significantly different in other groups. Endogenous norepinephrine levels and turnover were greater in pons of 8% NaCl--fed SHR-S than in those fed 1% NaCl but were not significantly different in other groups. These observations support the hypothesis that reduced noradrenergic input to depressor neurons in the anterior hypothalamus and increased noradrenergic input to neurons in the pons are related to NaCl sensitivity in the SHR-S.

Extraction of epinephrine and norepinephrine by the dog pancreas in vivo

This study determined the fractional extraction of epinephrine and norepinephrine by the in situ dog pancreas. Plasma samples for epinephrine measurements were taken simultaneously from the femoral artery and the superior pancreaticoduodenal vein. Pancreatic extraction of epinephrine was 73 ± 5% when basal arterial epinephrine levels were 380 ± 93 pg/mL, 76 ± 4% when arterial levels were 896 ± 123 pg/mL (epinephrine infused intravenously at 20 ng/kg/min), and 84 ± 1% when arterial levels were 2,956 ± 414 pg/mL (epinephrine infused intravenously at 80 ng/kg/min) suggesting that the process of epinephrine extraction by the pancreas is not saturable over this range. During a similar sampling protocol, norepinephrine was infused intravenously at 4 μg/kg/min; pancreatic extraction of norepinephrine was then 65 ± 7% when arterial norepinephrine levels were 107,000 ± 28,000 pg/mL. In separate experiments, lower rates of norepinephrine (12 to 1,200 ng/min) were infused directly into the pancreatic artery and pancreatic norepinephrine extraction was calculated; it ranged between 66% and 75%. Because the pancreas produces as well as extracts norepinephrine, a third technique was required to determine pancreatic norepinephrine extraction at the lower endogenous levels of norepinephrine; 3H-norepinephrine was infused intravenously and the arteriovenous difference of 3H-norepinephrine was measured. Fractional extraction of 3H-norepinephrine was 74 ± 4% both in the basal state (arterial norepinephrine level = 202 ± 44 pg/mL) and during systemic, glucopenic, stress induced by 2-deoxy-glucose (arterial norepinephrine level = 636 ± 70 pg/mL). These data suggest that also the norepinephrine extraction process by the pancreas is not sarurable. Further, since the pancreatic extraction of norepinephrine is so avid, these data suggest that measurement of the arteriovenous concentration difference of endogenous norepinephrine alone will markedly underestimate local norepinephrine spillover. Correcting these measurements for the extraction would, however, provide an index of the endogenous activity of pancreatic adrenergic nerves. Finally, this study also determined whether pancreatic catecholamine extraction is altered by α-or β-adrenoceptor antagonists; phenoxybenzamine was found to reduce the extraction of both epinephrine ( P < .001) and norepinephrine ( P < .05) whereas propranolol did not. Thus, one must consider how changes of pancreatic catecholamine extraction might alter the interpretation of experiments using α-adrenoceptor antagonists to study local norepinephrine release or catecholamine effects on pancreatic hormone secretion. In summary, (1) at steady state 60% to 80% of either epinephrine or norepinephrine is extracted in one pass by the dog pancreas, (2) this process of extraction appears nonsaturable, and (3) the pancreatic extraction of catecholamines is impaired by the α-adrenoceptor antagonist phenoxybenzamine, but not by the β-adrenoceptor antagonist propranolol.

Catecholamine content and adenylate cyclase activity in corpora lutea of different ages of the PMSG-treated immature rat

The Catecholamine content and adenylate cyclase response were studied in a well-characterized corpus luteum model, where ovulation was induced by treatment of prepubertal Sprague-Dawley rats with pregnant mare's serum gonadotropin. The luteal content of noradrenaline, determined with HPLC, was constant during the first 7 days of pseudopregnancy, followed by a 3-fold increase in older corpora lutea. No detectable amounts of dopamine were found, while trace amounts of adrenaline were found in a few cases. The increase in noradrenaline content was not associated with a changed sensitivity of luteal adenylate cyclase to catecholamines. The response to adrenaline was maximal in 3-day-old corpora lutea, whereafter a decrease was seen. The significance of the increased endogenous levels of noradrenaline at the end of pseudopregnancy is at present unknown. However, the fact that the increase in noradrenaline occurs a few days before spontaneous luteolysis is of special interest, since it has been suggested that an adrenergic innervation is a prerequisite for the antigonadotropic effect of prostaglandin F 2α in the human corpus luteum.

Chronic salt loading and adrenergic mechanisms in the Sprague-Dawley rat.

The effect of chronic salt loading on adrenergic mechanisms was evaluated in Sprague-Dawley rats (and NMRI mice) maintained on a high sodium (8%) or normal sodium (0.3%) regime for 4 weeks. The basal blood pressure (carotid artery) was not influenced by the high salt diet but the heart rate and blood pressure increases to mental stress (jet air) were larger in the salt loaded rats. There were indications of an increased sympathetic tone in rats on the high salt diet since in these rats sympathoinhibitory treatment with ganglionic blockade or clonidine induced larger falls in blood pressure and heart rate than in the controls. Central catecholamines (brain stem, striatum, hemispheres) were determined spectrofluorimetrically after cation exchange chromatography. The high salt diet influenced neither the endogenous levels of noradrenaline nor central noradrenaline turnover (disappearance of noradrenaline after synthesis inhibition by alpha-methyltyrosine and accumulation of dihydroxyphenylalanine after decarboxylase inhibition by 3-hydroxybenzylhydrazine). There were no changes in central alpha 2-adrenoceptor responsiveness when assessed as clonidine-induced deceleration of noradrenaline turnover in the brain and in central alpha 1-adrenoceptor responsiveness (clonidine-induced increase of flexor reflex in spinalized rats and clonidine-induced increase of motor activity in reserpinized mice). Peripheral sympathetic function was assessed in pithed rats. The pressor responses to intravenously administered noradrenaline (0.01-10 micrograms/kg) and electrical stimulation of the spinal sympathetic nerves (SNS, 0.25-2 Hz) were similar in the two groups, suggesting that salt did not influence vascular alpha-adrenoceptor responsiveness or transmitter release.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of TRH on hypothalamic catecholaminergic and serotoninergic neurons

1. 1. The in vivo and in vitro effects of TRH on tyrosine hydroxylase activity and on endogenous levels of noradrenaline as an index of noradrenergic neurons activity were studied in rat hypothalamus. The hydroxylation of tryptophan and the liberation of 5-hydroxyindole acetic acid were analyzed as an index of serotoninergic activity. 2. 2. TRH had no effect on tyrosine hydroxylase from hypothalamic homogenates in concentrations up to 100 ng/ml. Neither was the hydroxylation of tyrosine in intact hypothalamus. 3. 3. The TRH (4 μg/kg) decreased 5-hydroxytryptophan and 3,4-dihydroxyphenylalanine accumulation after 10 min, and no effect on serotonin and noradrenaline concentrations was observed. 4. 4. TRH decreased endogenous levels of 5-hydroxyindole acetic acid by 35%. 5. 5. Thus, the results show that TRH or TSH might play a role in a short-loop feedback action involving catecholaminergic and serotonergic nerve terminals that regulate the liberation of TRH.

Dopamine and noradrenaline content in fish retina: modulation by serotonin.

The presence of noradrenaline (NA) and the possible interaction of serotonin (5-HT) with dopaminergic and noradrenergic neuronal elements was studied in the retina of the teleost Eugerres plumieri. By means of a histofluorescent technique, paired amacrine cells can be visualized after intravitreal injection of NA or 5,6-dihydroxytriptamine, suggesting their probable catecholaminergic and indoleaminergic nature. With a high-performance liquid chromatographic (HPLC) method and after pargyline treatment of the animal, 6.86 ng/mg protein of dopamine (DA) was detected, while NA content was 0.50 ng/mg protein. The NA levels of the retina increased, whereas the DA levels decreased in a significant manner after in vivo treatment with 5-HT. 5-methoxy-N,N-dimethyltryptamine, a 5-HT agonist, was able to mimic this effect partially, while the agonists tryptamine and quipazine did not affect the levels of DA and NA. The antagonists methysergide, metergoline, and cyproheptadine significantly blocked the 5-HT-induced NA increase, whereas only the first two antagonists were effective in inhibiting the 5-HT-induced DA decrease. The 5-HT modulation of NA and DA is apparently receptor mediated and is not due to a hetero-exchange, since imipramine was not able to block the 5-HT effect. These findings support the suggested interaction between serotoninergic and dopaminergic cells [Negishi et al: J Neurosci Res 5:621-635, 1980; Neurosci Lett 25:1-5, 1981]. Furthermore, they demonstrate the possible modulation which 5-HT exerts on the endogenous levels of NA.

Effect of detomidine on the release and turnover of noradrenaline in rat brain.

The effect of detomidine, a novel veterinary sedative/analgesic, on the release and turnover of noradrenaline in rat brain was studied using both in vitro and in vivo techniques. Xylazine was included in the studies for comparison. Rat occipital cortex slices were preloaded with 3H-noradrenaline and the potassium-evoked release of tritium was measured using a superfusion system. Both detomidine and xylazine produced concentration dependent inhibition of stimulation evoked tritium release. A maximal inhibition of 66 and 50% was achieved at detomidine and xylazine concentrations of 1 X 10(-7) and 1 X 10(-6) M, respectively. The dose-response curve of detomidine was shifted to the right in a parallel manner by the selective alpha 2-antagonist idazoxan. Detomidine, while not altering the endogenous levels of noradrenaline, dopamine and 5-hydroxytryptamine, induced a dose-dependent inhibition of noradrenaline turnover as measured by the alpha-methyl-p-tyrosine method. Moreover, detomidine decreased the concentration of MHPG-SO4, the principal metabolite of central noradrenaline, in rat brain. Xylazine was in these tests at least two orders of magnitude less potent than detomidine.

3H-5-hydroxytryptamine accumulation by rat brain synaptic vesicles in a membrane-impermeant medium, and selective reduction by 5,7-dihydroxytryptamine.

In order to examine possible selectivity of amine uptake by synaptic vesicles, the ATP-stimulated accumulation of 3H-5-hydroxytryptamine (5HT) by synaptic vesicles from rat whole brain was examined in a medium comprised largely of membrane-impermeant anions (d-tartrate). Such media have previously been shown to stabilize vesicular accumulation of several neurotransmitters. Accumulation of 3H-5HT did not occur in tartrate medium alone, but was increased biphasically with increasing concentrations of both potassium phosphate and potassium bicarbonate. At optimal concentrations of each anion (10 mM), stable accumulation of 3H-5HT was observed at 37 degrees (26.1 +/- 1.2 pmol/mg protein; Km 6 X 10(-7) M), which was reduced by greater than 95% in the absence of K2ATP, at 4 degrees C, in the presence of 10(-6) M reserpine, or in the presence of the proton ionophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). Uptake was significantly antagonized by millimolar concentrations of Na+, Mg++ or Cl-, but was unaffected by ouabain (10(-5) M). Pretreatment of animals with 5,7-dihydroxytryptamine (5,7-DHT) (200 micrograms, intraventricular) 10 days prior to sacrifice reduced endogenous 5HT levels by 70%, while levels of endogenous norepinephrine (NE) and dopamine (DA) were unaffected. Accumulation of 3H-5HT, examined in the presence of 10(-6) M NE to block 3H-5HT accumulation by vesicles from noradrenergic nerve endings, was reduced by 40% in vesicles from treated animals. Vesicular accumulation of 3H-(-)-NE and 3H-DA was unaffected by 5,7-DHT treatment. The data suggest the possibility of preferential accumulation of 3H-5HT by vesicles arising from serotonergic nerve endings.

Differential blocking actions of idazoxan against the inhibitory effects of 6‐fluoronoradrenaline and clonidine in the rat vas deferens

The prejunctional inhibitory effects of clonidine and 6-fluoronoradrenaline (6-FNA) have been evaluated in the isolated prostatic segment of the rat vas deferens, against the twitch response evoked by low frequency (0.1 Hz) field stimulation. The inhibitory potency of 6FNA was significantly increased in the presence of cocaine (1 microM) or pargyline (10 microM), but was not modified in the vas deferens from rats pretreated with reserpine when the endogenous levels of noradrenaline (NA) were decreased by 97%. Clonidine was significantly more potent than 6-FNA as an inhibitory agonist, and the potency of clonidine was not modified after cocaine, pargyline or reserpine. The alpha 2-adrenoceptor blocking agent idazoxan, was a competitive antagonist against the inhibitory effects of clonidine under all experimental conditions. In contrast, the only antagonism shown by idazoxan against the inhibitory effects of 6-FNA was in the presence of cocaine (1 microM), and this antagonist effect of idazoxan was not concentration-related. Low concentrations of 6-FNA caused concentration-dependent facilitatory effects on the twitch response, which were significantly greater after treatment with idazoxan (1 microM) in reserpine-treated vas deferens. These facilitatory effects of 6-FNA were always observed in the presence of prazosin (300 nM) and also after treatment of the preparations with phenoxybenzamine (10 microM), a concentration which abolished the inhibitory actions of both clonidine and 6-FNA. The facilitatory effects on the twitch response induced by low concentrations of 6-FNA are therefore unlikely to be due to either alpha 1- or alpha 2-adrenoceptor stimulation. In conclusion, the failure of idazoxan to block the inhibitory effects of 6-FNA, while exerting a potent competitive antagonism of clonidine-induced inhibitory effects, supports the proposal that alpha 2-adrenoceptors may in fact be subdivided into two subclasses, involving imidazoline and phenylethylamine recognition sites.

Morphine decreases 3H-norepinephrine release and increases endogenous norepinephrine levels in the isolated cat superior cervical ganglion

The isolated cat superior cervical ganglion (SCG) was labeled in vitro with either 3H-norepinephrine ( 3HNE) or 3H-choline and stimulated through its preganglionic trunk. The release of 3HNE and of 3H-acetylcholine ( 3HACh) elicited by the stimulation was measured under control conditions and in the presence of drugs. The incubation during 30 min with 10 μM morphine lead to a 70 % decrease in the amount of 3HNE released in response to the preganglionic stimulation (10 Hz, 80 V, during 5 min). No further decrease in 3HNE release was produced by a 10 times higher concentration of morphine. The reduction in 3HNE release caused by morphine was coincident with a 60 % increase in the endogenous content of NE. Both effects of morphine were entirely prevented by an antagonist of opioid receptors, 1.0 μM naltrexone. The opioid antagonist did not modify by itself either the stimulation-induced release of 3HNE or the endogenous content of NE. The basal efflux of 3HNE was not altered by morphine. In ganglia labeled with 3H-choline, morphine (10 and 100 μM) did not modify either the basal efflux of 3HACh or the release of 3HACh evoked by stimulation of the preganglionic trunk (5 Hz, 40 V, during 5 min). These observations suggest that in the cat SCG morphine has a direct action on the dendrites of the postganglionic neuron which store and release NE. The effects of morphine in vitro on 3HNE release and on the tissue levels of NE may be mediated through the interaction with dendritic opioid receptors.

Chronic salt loading and central adrenergic mechanisms in the spontaneously hypertensive rat.

The effects of chronic salt loading on central adrenergic mechanisms were evaluated in spontaneously hypertensive rats maintained on tap water or 1.2% sodium chloride drinking water for 4 weeks. Basal blood pressure was increased by 10% in the high salt group. Central catecholamines were measured spectrofluorimetrically after cation exchange chromatography. Endogenous levels of noradrenaline (NA) were not influenced by salt loading but the NA turnover (disappearance of NA following synthesis inhibition by alpha-methyltyrosine) was increased in the hemisperes. Central alpha 2-adrenoceptor sensitivity was assessed as the clonidine-induced reduction in blood pressure and as the clonidine-induced deceleration of NA turnover and locus coeruleus (LC) NA cell firing rate (single unit recording). The results were slightly disparate but the unchanged sensitivity of clonidine to reduce LC NA cell firing suggests that there were no alterations in central alpha 2-adrenoceptor sensitivity following a salt load. There were also no changes in alpha 1-adrenoceptor function, which was assessed semiquantitatively as the clonidine-induced increase in flexor reflex activity in spinalized rats. In salt loaded rats there was an enhanced blood pressure and heart rate reduction following ganglionic blockade which may be interpreted as an increased basal sympathetic tone. In the periphery the pressor responses to phenylephrine were increased whereas the chronotropic response to isoprenaline was unchanged. In conclusion, in the spontaneously hypertensive rat on a high salt intake the aggravated development of hypertension was not associated with major changes in central alpha 1 and alpha 2-adrenoceptor mediated functions or in neuronal activity of brain stem NA neurons. There were indications of an increased basal sympathetic tone and increased blood pressure response to pressor substances.

Endogenous catecholamine levels in chronic heart failure. Relation to the severity of hemodynamic abnormalities

Plasma free epinephrine, norepinephrine, and dopamine concentrations were determined in 48, 63, and 45 patients, respectively, with overt congestive heart failure, and compared with those in 26 patients with stable angina but without heart failure. Systemic hemodynamic values were determined to assess the severity of heart failure. Arterial epinephrine levels were not different between patients with heart failure (73 ± 92 pg/ml) and patients without heart failure (55 ± 73 pg/ml). In patients with congestive heart failure, norepinephrine (665 ± 510 pg/ml, mean ± SD) and dopamine (407 ± 405 pg/ml) levels were significantly higher than in patients with stable angina without heart failure (norepinephrine 184 ± 136 pg/ml, p <0.001, and dopamine 197 ± 259 pg/ml, p <0.02). However, in patients with congestive heart failure, the plasma norepinephrine levels did not correlate with cardiac Index (r = 0.21, p = NS), pulmonary capillary wedge pressure (r = 0.11, p = NS), mean arterial pressure (r = 0.11, p = NS), or systemic vascular resistance (r = 0.18, p = NS). Similarly, there was no correlation between dopamine levels and the hemodynamic abnormalities In patients with congestive heart failure. These findings suggest that although endogenous norepinephrine and dopamine levels are frequently elevated In patients with heart failure, reflecting enhanced sympathetic activity, catecholamine levels do not reflect the severity of heart failure.

Effect of aging on the response of the cerebral cortex to noradrenergic denervation.

Cyclic AMP generation was measured in isolated minces of the ipsilateral and contralateral cortex of young (3-4 months) and aged (28-29 months) Fischer-344 rats, 2 weeks after unilateral lesion of the nucleus locus ceruleus (LC). Measurements were conducted under basal conditions and in the presence of 0.1 mM isoproterenol. Despite marked norepinephrine (NE) depletion of about 90% in the ipsilateral cerebral cortex in both age-groups, cyclic AMP generation under basal conditions was unaffected. However, whereas isoproterenol-stimulated cyclic AMP generation was significantly higher in the NE-depleted ipsilateral cerebral cortex of young rats, such 'denervation supersensitivity' was not apparent in aged rats. Also, endogenous levels of NE were 31% higher, and isoproterenol-stimulated cyclic AMP generation was 33% lower in the contralateral 'control' cortex of aged rats than in young rats. These results suggest an age-related defect in the postsynaptic expression of noradrenergic mechanisms in the rat cerebral cortex. Further research using intermediate ages is needed to establish whether these age-related findings are caused by 'senescence' or are 'developmental' phenomena.

Cyclazocine and Norepinephrine Uptake, Metabolism, and Turnover in the Rat Brain

The effects of cyclazocine, a narcotic agonist/antagonist, on the uptake, metabolism, and turnover of norepinephrine in rat brains were studied. Cyclazocine significantly depressed the accumulation of intracisternally administered [3H]norepinephrine and the formation of 3,4-[3H]dihydroxymandelic acid but did not alter the formation of [3H]-normetanephrine. Comparatively, imipramine reduced the uptake of [3H]norepinephrine, decreased the formation of 3,4-[3H]dihydroxymandelic acid, and increased the formation of [3H]normetanephrine. Moreover, cyclazocine increased the formation of endogenous 3-methoxy-4-hydroxyphenylethylene glycol and produced a fall in endogenous norepinephrine. It is suggested that cyclazocine reduces the accumulation of [3H]norepinephrine, decreases endogenous norepinephrine levels, and increases the formation of 3-methoxy-4-hydroxyphenylethylene glycol by interacting with brain adrenergic nerves, presumably by inhibiting neuronal uptake and facilitating norepinephrine turnover.

GM 1 ganglioside enhances regrowth of noradrenaline nerve terminals in rat cerebral cortex lesioned by the neurotoxin 6-hydroxydopamine

The effect of exogenous GM 1 ganglioside on selectively noradrenaline-denervated rat cerebral cortex was investigated by measuring the spatial distribution of endogenous noradrenaline levels and by fluorescence histochemical analysis. A local noradrenaline denervation was produced by intracortical infusion of the selective catecholamine neurotoxin 6-hydroxydopamine for 3 or 7 days. The neurotoxin infusion caused an almost complete noradrenaline denervation in a restricted area around the infusion point as reflected by an almost complete long-term disappearance of noradrenaline nerve terminals and reduction of noradrenaline levels. There was with time a slow recovery of the levels, most likely related to a spontaneous noradrenaline nerve terminal regeneration. Post-treatment for 1 week with GM 1 had very small effects on the 6-hydroxydopamine-induced reduction of the noradrenaline levels, while pretreatment with GM 1 for 3 days before the neurotoxin infusion and continuing the GM 1 administration for another 7–14 days significantly enhanced noradrenaline recovery, as observed both bio- and histochemically. GM 1 had no effect on the 6-hydroxydopamine-induced noradrenaline depletion acutely, indicating that GM 1 does not interfere with the direct neurotoxic actions of 6-hydroxydopamine. The present results thus indicate that exogenous GM 1 enhances regrowth of noradrenaline nerve terminals which may be due to a regrowth stimulatory effect (regeneration/collateral sprouting) and/or related to protective actions of GM 1 against retrograde degeneration of noradrenaline axons following the neurotoxin-induced lesion.

Increased angiotensin II receptors in neuronal cultures from hypertensive rat brain.

Binding of 125I-angiotensin II (ANG II) to neuronal cultures made from the brains of 1-day-old normotensive (Wistar-Kyoto, WKY) and spontaneously hypertensive (SH) rats was time dependent, saturable, reversible, and 90-95% specific. Neuronal cultures from SH rats bound 50-70% more 125I-ANG II compared with their WKY controls. Scatchard analysis revealed that the increase in the specific binding of ANG II to SH rat neuronal cultures was due to an increase in the number of binding sites per cell rather than change in the affinity of receptors for ANG II. Light-microscopic autoradiographic analysis showed that ANG II specific binding sites were located on neuronal cell bodies and neurites. Treatment of neuronal cultures from both strains of rats with alpha-methyl-p-tyrosine caused a 50-60% decrease in the endogenous levels of norepinephrine (NE) and dopamine (DA). This decrease was associated with increases in the specific binding of 125I-ANG II in neuronal cultures from WKY rat brain. In contrast, ANG II binding in neuronal cultures from SH rat brain failed to respond to changes in NE and DA levels. These observations suggest that ANG II specific receptors are increased and that they are not under a negative-feedback control by catecholamines in SH rat brain neuronal cultures.

Depression of Pulmonary Metabolic Function by Cardiopulmonary Bypass Procedures Increases Levels of Circulating Norepinephrine

We measured plasma levels of endogenous norepinephrine radioenzymatically in mixed venous and arterial blood simultaneously sampled from anesthetized dogs before, during, and after 1 to 4 hours of total cardiopulmonary bypass (CPB) or 2 hours of left heart bypass. Prior to bypass, arterial levels of norepinephrine were 0.41 ± 0.04 ng/ml and pulmonary extraction of norepinephrine was 25 ± 3% (N = 20). During bypass, norepinephrine levels significantly increased from control to 1.41 ± 0.15 (1 or 2 hours of CPB; N = 8) and 1.97 ± 0.30 (3 or 4 hours of CPB; N = 8) or 0.97 ± 0.29 (2 hours of left heart bypass) ng/ml. Restoration of normal pulmonary blood flow was associated with a rapid and significant decrease in arterial levels of norepinephrine, which, after 1 or 2 hours of CPB or 2 hours of left heart bypass, returned to levels obtained before bypass. However, arterial levels of norepinephrine remained higher than 1 ng/ml in the 3-hour recovery period after prolonged bypass. In these animals, pulmonary extraction of norepinephrine was significantly less than control. These data suggest that the lung's ability to remove norepinephrine is altered by CPB and that the severity of the alterations is directly related to pump time. The impairment in extraction allows higher than normal concentrations of norepinephrine to enter the arterial circulation and may contribute to systemic hypertension after bypass.

Effects of methyldopa metabolites on amine transmitters and adrenergic receptors in rat brain.

Studies of catecholamine concentrations in defined nuclei from the anterior hypothalamic-preoptic regions and the medulla oblongata, known to contribute to cardiovascular control, were measured following acute or chronic methyldopa administration. These studies indicated that methyldopa was enzymatically converted to methyldopamine and methylnorepinephrine, and in some areas to methylepinephrine which replaced endogenous epinephrine. The predominant metabolite was methylnorepinephrine, which accumulated in concentrations higher than endogenous norepinephrine levels. (-)Methylnorepinephrine was found to be 6 times more potent and 75 times more selective for alpha 2-adrenergic receptors than (-) norepinephrine, and it is suggested that this alpha 2-adrenergic receptor action, particularly in the nucleus tractus solitarius, contributes to a major part of the antihypertensive effect of methyldopa.