Brain Epinephrine Research Articles

Inhibition of brain epinephrine synthesis by 3,4-dichlorophenylethanolamine, a competitive substrate for norepinephrine N-methyltransferase

3,4-Dichlorophenylethanolamine (DCPE) is a substrate for norepinephrine N-methyltransferase (NMT) from rat brain stem or from rabbit adrenal glands and competitively inhibits the methylation of (−)norepinephrine by NMT in vitro. The K i value for inhibition of rat brain NMT by DCPE was 6 × 10 −5 M. When DCPE was injected i.p. into rats at a dose of 50 mg/kg, epinephrine concentration in hypothalamus was reduced, although NMT activity measured in hypothalamic homogenates in vitro was not inhibited. The findings contrast in several ways to those with 8,9-dichloro-2,3,4,5-tetrahydro-1 H-2-benzazepine (LY134046), a competitive inhibitor of rat brain NMT with an in vitro K i of 2.4 × 10 −8 M. Although the potency of DCPE was 1/2500th that of LY134046 in vityro, it lowered hypothalamic epinephrine concentrations at similar doses in vivo. The NMT activity measured in vivo in homogenates of hypothalamic tissue from LY134046-treated rats was reduced by a greater percentage than was epinephrine concentration. The reduction of epinephrine concentration in vivo by DCPE may relate to its ability to be a substrate for NMT, which would result in conversion of S-adenosylmethionine (SAMe) to S-adenosylhomocysteine (SAH) in NMT-containing neurons. The resulting increase in the SAH/SAMe ratio may inhibit norepinephrine N-methylation, since SAH is a competitive inhibitor of NMT when SAMe is the variable substrate. This mechanism is similar to that suggested for the decrease in epinephrine concentration in rat brain following l-dopa injection. l-Dopa injection increases SAH and decreases SAMe concentration measured in hypothalamus, due presumably to extensive O-methylation of dopa and its decarboxylated metabolites. In contrast, SAH and SAMe concentrations in hypothalamus were not measurably altered after DCPE injection, a not-unexpected finding since SAH/SAMe changes would occur only in NMT-containing neurons, which make up a negligibly small percentage by weight of hypothalamic tissue. DCPE and other NMT substrates may be able to inhibit epinephrine synthesis in vivo as effectively as non-substrate, competitive inhibitors having much higher affinity for NMT because their N-methylation elevates SAH/SAMe ratios specifically within NMT-containing neurons.

Epinephrine synthesis inhibitors block naloxone-induced LH release.

Administration of the opiate receptor antagonist, naloxone, evoked rapid rises in plasma LH levels in estrogen, progesterone-primed ovariectomized rats. Pretreatment with a peripherally acting epinephrine (EPI) synthesis inhibitor, SK&F 29661, failed to influence the naloxone-induced LH release. However, two centrally acting EPI synthesis inhibitors, SK&F 64139 and LY 78335, which selectively suppressed hypothalamic EPI levels, blocked stimulation of LH release by naloxone. These results implicate brain EPI neurons in mediation of endogenous opioid peptide influence on LH release.

Role of central epinephrine in regulation of anterior pituitary hormone secretion

Hypothalamic regulation of anterior pituitary hormones is thought to be mediated by the release of stimulatory and/or inhibitory peptides that are, in turn, regulated by catecholaminergic neurons. The recent development of selective epinephrine (EPI) synthesis inhibitors has made it possible to disrupt central EPI neurotransmission without affecting norepinephrine or dopamine. These compounds were used in the present investigation to assess the involvement of brain EPI systems in regulation of GH, LH, and prolactin (PRL) in male and ovariectomized female rats. Inhibition of central EPI synthesis (1) inhibited episodic and morphine-, but not clonidine-induced GH release, and (2) blocked the LH surge induced by estrogen and progesterone, but did not affect episodic LH release in hormonally untreated rats. Inhibition of peripheral (adrenal) EPI synthesis had no effect on these hormones. Results of these studies suggest an excitatory role for EPI in regulation of GH and LH secretion, mediated by stimulation of GH-releasing hormone and LHRH, respectively. EPI does not appear to have a major function in regulation of PRL secretion.

Simultaneous determination of norepinephrine, dopamine and serotonin in rat brain regions by ion-pair liquid chromatography on octyl silane columns and amperometric detection

A highly sensitive and specific ion-pair liquid chromatography—electrochemical detection method is described for the simultaneous determination of rat brain regional norepinephrine, dopamine and serotonin levels using octylsilane (C 8) columns. These amines were first isolated from tissue homogenates by adsorption on Amberlite CG-50 resin followed by separation on RP-8 columns with a mobile phase consisting of 0.05 M NaH 2PO 4 (pH 3.0) 0.02 m M EDTA, 1 m M heptanesulphonate—methanol (92:8, v/v) at 1.8 ml/min. Using 3,4-dihydroxybenzylamine as the internal standard, tissue recoveries (X̄ ± S.D.) for norepinephrine, dopamine and serotonin were 87.5 ± 2.6%, 61.8 ± 10.5% and 72.9 ± 7.5%, respectively. Assay sensitivities were sufficient for reliable quantitation of at least 200 pg of these compounds in a brain sample. The procedure is readily adaptable to determination of brain epinephrine and normetanephrine levels, as well. Finally, the reversed-phase system employed is highly flexible in that the same column and mobile phase conditions may be used for assay of biogenic amine metabolites.

Influence of harmaline on the ability of pargyline to alter catecholamine metabolism in rats

When pargyline hydrochloride (20 mg/kg, i.p.) was injected into rats 48 hr before the measurement of monoamine oxidase (MAO) activity, the oxidation of [ 14C]phenylethylamine (type B MAO) and of [ 14C]-serotonin (type A MAO) was inhibited. Neither type A nor type B MAO was inhibited 48 hr after the injection of harmaline hydrochloride (30mg/kg, i.p.) a short-acting, reversible, highly selective inhibitor of type A MAO. When harmaline was given just before pargyline, it prevented the inhibition of type A MAO by pargyline but not the inhibition of type B MAO. Pargyline alone elevated epinephrine, norepinephrine, and dopamine concentrations in brain regions and norepinephrine concentration in heart. The concentration of dopamine metabolites (3,4-dihydroxyphenylacetic acid and homovanillic acid) was decreased. Pretreatment with harmaline prevented all of these effects of pargyline. The findings suggest that inhibition of type A MAO is involved in the inhibition of catecholamine metabolism by pargyline, since harmaline pretreatment did not prevent inhibition of type B MAO and would not be expected to alter any other possible actions of pargyline. These findings support the idea that type A MAO is primarily responsible for the oxidation of epinephrine, norepinephrine, and dopamine in rat brain and of norepinephrine in rat heart.

Effects of an epinephrine synthesis inhibitor, SKF64139, on the secretion of luteinizing hormone in ovariectomized female rats

In order to test the involvement of central epinephrine systems in luteinizing hormone (LH) secretion, the epinephrine synthesis inhibitor, SKF64139, was administered to ovariectomized rats treated with ovarian hormones. This agent significantly decreased resting LH concentrations in ovariectomized, hormonally untreated rats and completely suppressed the stimulatory feedback effects of estrogen plus progesterone. In these animals, hypothalamic concentrations of epinephrine, but not norepinephrine or dopamine, were significantly decreased. It is suggested that brain epinephrine may participate in the neural control of LH secretion in rats.

Depletion of epinephrine in rat hypothalamus by Ro 4-1284: Influence of pargyline and harmaline

Ro 4-1284, 2-ethyl-2,3,4,6,7,11b-hexahydro-3 isobutyl-9,10-dimethoxy-2H-benzo[a] quinolizin-2-ol hydrochloride, depleted epinephrine and norepinephrine in rat hypothalamus; the depletion occurred within 1 hr and persisted at 24 hr. The rapid lowering of epinephrine by Ro 4-1284 was antagonized by pargyline, a monoamine oxidase inhibitor, which alone elevated epinephrine concentration. The ability of pargyline to antagonize epinephrine depletion by Ro 4-1284 was prevented by co-treatment with harmaline, which protects against type A but not type B monoamine oxidase inhibition by pargyline. Pargyline also antagonized the Ro 4-1284-induced depletion of norepinephrine in hypothalamus and of norepinephrine and dopamine in cerebral hemispheres, and these effects ofpargyline were also prevented by co-treatment with harmaline. The results suggest that epinephrine in rat brain, like norepinephrine and dopamine, is destroyed primarily by type A monoamine oxidase.

Inhibition of rat brain norepinephrine N-methyltransferase by 2,3,4,5-tetrahydro-1H-indeno [1,2-c]-pyridine hydrochloride (LY87130)

2,3,4,5-Tetrahydro-1H-indeno[1,2-c]pyridine hydrochloride (LY87130) was a potent inhibitor of rat brain stem norepinephrine N-methyltransferase (NMT) in vitro. The inhibition was competitive with l-norepinephrine as the variable substrate and non-competitive with S-adenosyl- l-methionine as the variable substrate. The K i value for LY87130 calculated from a Dixon plot was 2 × 10 −7 M. In rats, LY87130 inhibited NMT in vivo when injected i.p. at 10–40 mg/kg doses. Epinephrine concentration in hypothalamus was lowered within 1 h after LY87130 injection; the lowering was maximum at 6 h and persisted at 24 but not 48 h after a 40 mg/kg dose of LY87130. The lowering of epinephrine at 6 h was dose-related over a 10–40 mg/kg dose range. Norepinephrine and dopamine concentrations were increased transiently; by 6 h the concentrations of these other catecholamines were normal whereas epinephrine was maximally depleted. MOPEG sulfate, a major metabolite of norepinephrine in rat brain, was markedly elevated by LY87130. The effects of low doses of LY87130 (5 and 10 mg/kg) were enhanced by iprindole, which increased LY87130 levels in brain, suggesting that ring hydroxylation is a major pathway of LY87130 metabolism. The ability of LY87130 to produce selective depletion of epinephrine in rat brain makes it a useful tool for manipulating epinephrine-forming neurons.

Properties of norepinephrine N-methyltransferase from pigeon brain

Epinephrine cocentration in pigeon hypothalamus was higher on an absolute basis and as a percentage of total catecholamine concentration than in other regions of pigeon brain or in rat hypothalamus. Norepinephrine N-methyltransferase (NMT; the epinephrine-forming enzyme) from pigeon brain had Km values for S-adenosyl-L-methionine and L-norepinephrine of 24 ± 1 and 101 ± 4 μM, respectively. The enzyme was inhibited by excess L-norepinephrine and by several arylalkylamines that earlier had been identified as NMT inhibitors with the enzyme from mammalian brain and adrenal medulla. These results indicate that pigeon brain NMT is similar to that in mammalian brain and adrenal glands and can be inhibited by agents previously used to deplete brain epinephrine selectively in rats. The use of pigeons as experimental subjects in studies on the possible role of epinephrine-forming neurons in behavior is suggested.

Developmental changes in rat brain epinephrine levels

Developmental changes in rat brain epinephrine levels

Epinephrine distribution in human brain

The distribution of epinephrine (E) in human brain has been analyzed by a previously described [11] liquid chromatographic method. E, though detectable in most areas sampled, was found to be highly localized in those areas previously known to contain highest norepinephrine (NE) concentrations [4]. The regions found to be highest in E concentration were the ventromedial, dorsomedial, supraoptic and paraventricular nuclei of the hypothalamus.

6-hydroxydopa depletes both brain epinephrine and norepinephrine: Interactions with antidepressants

Intravenous administration of the neurotoxic agent, 6-hydroxydopa, to mice lowered both epinephrine and norepinephrine concentrations in the brainstem. Pretreatment with the tricyclic antidepressant drugs, imipramine, iprindole, and protriptyline, differentially blocked these depletions. Imipramine and protriptyline blocked both the effects on epinephrine and norepinephrine, although higher doses were required to protect epinephrine. Iprindole selectively blocked epinephrine depletions.

Effect of phenylethanolamine N-methyltransferase and dopamine-β-hydroxylase inhibition on epinephrine levels in the brain

The effects of phenylethanolamine N-methyltransferase (PNMT) and dopamine-β-hydroxylase (DβH) inhibition on the epinephrine content in specific regions of the brain were studied. SKF 64139, a potent PNMT inhibitor, is effective in lowering brain epinephrine levels. The time course of PNMT inhibition by SKF 64139 parallels the lowering of epinephrine levels in the brain. Diethyldithiocarbamate (DDC), a potent inhibitor of DβH, is effective in lowering norepinephrine and epinephrine levels and in elevating dopamine levels in the analyzed regions of the brain. The epinephrine levels in the brain appear to be under similar biosynthetic control as in the adrenal glands.

Role of Catecholamines in Photoperiodically-Induced Gonadal Development in Coturnix Quail1

The involvement of central biogenic amines in the mechanisms controlling photoperiodically-induced testicular development in Coturnix quail were studied employing pharmacological approaches. Daily administration of alpha-methyltyrosine (MT), which blocks catecholamine (CA) biosynthesis, to 6-week-old quail exposed to long daily photoperiods markedly depleted brain dopamine (DA), norepinephrine (NE) and epinephrine (E). This was associated with a partial suppression of testicular growth 7 days after light stimulation. Combined treatment with MT and L-dihydroxyphenylalanine (L-DOPA), a precursor of DA, NE and E, elevated the monoamine levels in the brain and prevented the blocking effect of MT on testicular growth. Selective blockade of NE and E biosynthesis with diethyl-dithiocarbamate (DDC) depleted both NE and E, while it elevated DA level. Furthermore, such treatment reduced testicular weight. In DDC-treated birds, D,L-dihydroxyphenylserine (DL-DOPS) administration, which bypasses the DDC block, restored only brain NE level and reversed the blocking action of DDC on testicular weight. It is concluded that central NE plays a role in transformation of the photoperiodic information affecting gonadal development in Coturnix quail.

Brain epinephrine and norepinephrine in the neonate chick: Phylogenetic implications

1. 1. Average norepinephrine and epinephrine levels in whole brain of 112 neonate chicks, Gallus domesticus, were found to be 0·522 and 0·277 μg. per g. respectively, a ratio of 2 : 1. 2. 2. In contrast, norepinephrine greatly predominates over epinephrine in fish and mammalian brain, whereas the reverse is true for amphibians. 3. 3. The ratio of catecholamines in chick brain most resembles that of reptiles and this is consistent with the established phylogenetic position of Aves.

Catecholamines in rat tissues and serum determined by gas chromatographic method.

Nanogram order determination of catecholamines in rat tissues and serum was achieved. The recovery of added catecholamines to the acid homogenates was about 50%. The presence of epinephrine in brain was confirmed. The amount of dopamine was unexpectedly low in lung.

Inhibition of dopamine β-hydroxylase by fusaric acid (5-butylpicolinic acid) in vitro and in vivo

Fusaric acid (5-butylpicolinic acid) was found to be a potent inhibitor of dopamine β-hydroxylase in vitro and in vivo. Fusaric acid inhibited the enzyme by 50 per cent at a concentration of 3 × 10 −8 M. Kinetic studies with purified dopamine β-hydroxylase showed that the inhibition by fusaric acid was of the uncompetitive type with the substrate and of the mixed type with a cofactor, ascorbic acid. Fusaric acid lowered endogenous levels of norepinephrine and epinephrine in brain, heart, spleen and adrenal glands. Maximum depletion of norepinephrine and epinephrine was observed between 3 and 6 hr after the administration of fusaric acid. Since dopamine β-hydroxylase activity in adrenal medulla was found to be inhibited in vivo after the administration of fusaric acid, the decrease in the catecholamine levels is attributed to the inhibition of norepinephrine synthesis at the dopamine β-hydroxylase stage.

Seasonal variations of brain epinephrine, norepinephrine and 5-hydroxytryptamine associated with changes in the EEG of the toad, Bufo arenarum hensel

1. 1. Observations concerning the concentrations of epinephrine (E), norepinephrine (NE) and 5-hydroxytryptamine (serotonin, 5-HT) in the brain of the toad and the EEG pattern according to the seasons of the year were simultaneously performed. 2. 2. Brain E showed marked seasonal changes, with its lowest value near the onset of the mating period. From this moment it began to increase, reaching its maximum at the middle of estivation. Thereafter a progressive and significant lowering was noted. 3. 3. Conversely, 5-HT brain content showed its highest during hibernation, whereas a marked fall coinciding with mating was evident. 4. 4. Accordingly, the 5-HT/E ratio varied in the course of the year from a maximum of 18 during hibernation to 1·3 in the middle of the estivating period. 5. 5. Our results concerning these seasonal and opposite displacements of brain levels in E and 5-HT in toads are discussed in relation to simultaneously occuring changes in the EEG and to the hibernation-estivation cycle in amphibians.

Changes in brain epinephrine and norepinephrine induced by afferent electrical stimulation in the isolated toad head

Changes in brain epinephrine and norepinephrine induced by afferent electrical stimulation in the isolated toad head