Inactivation of dopamine beta-hydroxylase by p-cresol: evidence for a second, minor site of covalent modification at tyrosine 357

p-Cresol is a mechanism-based inhibitor of bovine dopamine beta-hydroxylase (3,4-dihydroxyphenethylamine, ascorbate: oxygen oxidoreductase (beta-hydroxylating), EC 1.14.17.1) (DBH) which covalently modifies a tyrosine at position 216 during inactivation (DeWolf, W.E., Jr., Carr, S.A., Varrichio, A., Goodhart, P.J., Mentzer, M.A., Roberts, G.D., Southan, C., Dolle, R.E. and Kruse, L.I. (1988) Biochemistry 27, 9093-9101). Here we report the recovery and characterization of additional minor peptides that are produced during the inactivation of DBH with p-[3H]cresol. Sequence and structural analysis of these peptides indicates tyrosine 357 as a second, minor site of modification.

Inhibition of dopamine beta-hydroxylase by bidentate chelating agents

1-2H-Phthalazine hydrazone (hydralazine; HYD), 2-1H-pyridinone hydrazone (2-hydrazinopyridine; HP), 2-quinoline-carboxylic acid (QCA), 1-isoquinolinecarboxylic acid (IQCA), 2,2'-bi-1H-imidazole (2,2'-biimidazole; BI), and 1H-imidazole-4-acetic acid (imidazole-4-acetic acid; IAA) directly and reversibly inhibit homogeneous soluble bovine dopamine beta-hydroxylase (3,4-dihydroxyphenethylamine, ascorbate:oxygen oxidoreductase (beta-hydroxylating), EC 1.14.17.1). HYD, QCA and IAA show competitive allosteric inhibition of dopamine beta-hydroxylase with respect to ascorbate (Kis = 5.7(+/- 0.9) microM, 0.14(+/- 0.03) mM, 0.80(+/- 0.20) mM; nH = 1.4(+/- 0.1), 1.8(+/- 0.4), 2.8(+/- 0.6), respectively). HYD and IAA show slope and intercept mixed-type allosteric inhibition of dopamine beta-hydroxylase with respect to tyramine. QCA shows allosteric uncompetitive inhibition of dopamine beta-hydroxylase with respect to tyramine. HP, BI and IQCA all show linear competitive inhibition (Kis = 1.9(+/- 0.3) microM, 21(+/- 6) microM, and 0.9(+/- 0.3) microM, respectively) with respect to ascorbate. HP and BI show linear mixed-type while IQCA shows linear uncompetitive inhibition of dopamine beta-hydroxylase with respect to tyramine. In the presence of HP, HYD or IAA intersecting double-reciprocal plots of the initial velocity as a function of tyramine concentration at differing fixed levels of ascorbate are observed. These findings are consistent with a uni-uni-ping-pong-ter-bi kinetic mechanism for dopamine beta-hydroxylase that involves a ternary enzyme-ascorbate-tyramine-oxygen complex. The results for HYD, QCA and IAA are the first examples of allosteric inhibitor interactions with dopamine beta-hydroxylase.

Some benzyl-substituted imidazoles, triazoles, tetrazoles, pyridinethiones, and structural relatives as multisubstrate inhibitors of dopamine beta-hydroxylase. 4. Structure-activity relationships at the copper binding site

Structure-activity relationships (SAR) were determined for novel multisubstrate inhibitors of dopamine beta-hydroxylase (DBH; EC 1.14.17.1) by examining the effects upon in vitro inhibitory potencies resulting from structural changes at the copper-binding region of inhibitor. Attempts were made to determine replacement groups for the thione sulfur atom of the prototypical inhibitor 1-(4-hydroxybenzyl)imidazole-2-thione described previously. The synthesis and evaluation of oxygen and nitrogen analogues of the soft thione group demonstrated the sulfur atom to be necessary for optimal activity. An additional series of imidazole-2-thione relatives was prepared in an effort to probe the relationship between the pKa of the ligand group and inhibitory potency. In vitro inhibitory potency was shown not to correlate with ligand pKa over a range of approximately 10 pKa units, and a rationale for this is advanced. Additional ligand modifications were prepared in order to explore bulk tolerance at the enzyme oxygen binding site and to determine the effects of substituting a six-membered ligand group for the five-membered imidazole-2-thione ligand.

Active site labeling of dopamine beta-hydroxylase by two mechanism-based inhibitors: 6-hydroxybenzofuran and phenylhydrazine

6-Hydroxybenzofuran and phenylhydrazine are mechanism-based inhibitors of dopamine beta-hydroxylase (D beta H; EC 1.14.17.1). We report here the isolation and characterization of radiolabeled peptides obtained after inactivation of D beta H with [3H]6-hydroxybenzofuran and [14C]phenylhydrazine followed by digestion with Staphylococcus aureus V8 protease. Inactivation of D beta H with [3H]6-hydroxybenzofuran gave only one labeled peptide, whereas inactivation with [14C]phenylhydrazine gave several labeled peptides. Each inhibitor labeled a unique tyrosine in the enzyme corresponding to Tyr477 in the primary sequence of the bovine enzyme (Robertson, J. G., Desai, P. R., Kumar, A., Farrington, G. K., Fitzpatrick, P. F., and Villafranca, J. J. (1990) J. Biol. Chem. 265, 1029-1035). In addition, [14C]phenylhydrazine also labeled a unique histidine (His249) as well as several other peptides. Examination of the complete peptide profile obtained by high pressure liquid chromatography analysis also revealed the presence of a modified but nonradioactive peptide. This peptide was isolated and sequenced and was identical whether the enzyme was inactivated by 6-hydroxybenzofuran or phenylhydrazine. An arginine at position 503 was missing from the sequence cycle performed by Edman degradation of the modified peptide, but arginine was present in the identical peptide isolated from native dopamine beta-hydroxylase. These data are analyzed based on an inactivation mechanism involving formation of enzyme bound radicals (Fitzpatrick, P. F., and Villafranca, J. J. (1986) J. Biol. Chem. 261, 4510-4518) interacting with active site amino acids that may have a role in substrate binding and binding of the copper ions at the active site.

Pharmacological and functional evidence for extracellular 3,4-dihydroxyphenylacetic acid as an index of metabolic activity of the adrenergic neurons: an in vivo voltammetry study in the rat rostral ventrolateral medulla

Catecholamine metabolism was studied in vivo in the C1 adrenergic area of the rostral ventrolateral medulla oblongata in rats, using differential normal pulse voltammetry coupled with an activated carbon fiber microelectrode. Pharmacological evidence indicates that 3,4-dihydroxyphenylacetic acid, the major dopamine metabolite, is responsible for the electrochemical signal appearance in the C1 group, and that it reflects the catecholamine synthesis rate, as previously reported in the locus coeruleus. Indeed, 3,4-dihydroxyphenylacetic acid was estimated to be formed from 77% of the intracellular dopamine, since its synthesis was increased by only 23%, after blockade of the dopamine-beta-hydroxylase activity. Neuronal activation by retrograde electrical stimulation increased the electrochemical signal, as well as hemorrhage and hypotension, suggesting that the level of extracellular 3,4-dihydroxyphenylacetic acid is a good biochemical index of the C1 adrenergic cellular activity in baseline conditions and during cellular activation.

1-(substituted-benzyl)imidazole-2(3H)-thione inhibitors of dopamine beta-hydroxylase

Molecular shape and quantitative structure-activity relationship (QSAR) analyses of 52 1-(substituted-benzyl)-imidazole-2(3H)-thione inhibitors of dopamine beta-hydroxylase were carried out. QSARs were developed for sets of 45 and sets of 47 analogues. Molecular shape, as represented by common overlap steric volume and the composite charge density on carbons 3, 4, and 5 of the substituted-benzyl ring are the major inhibition-potency descriptors. Five of the 52 compounds were eliminated prior to analyses on the basis of difficulties in characterizing shape and charge state. Two compounds were outliers. The active conformation deduced in the analyses is a low-energy conformer for both active and inactive inhibitors. This suggests that the intrinsic shape of the molecule due to the selection of X is more important than torsion-angle selection for the bonds between the two rings. The QSARs found in this study have only general similarities to one put forth by Kruse et al. using linear free energy descriptors.

Effect of dopamine beta-hydroxylase inhibition on systemic hemodynamics in conscious, spontaneously hypertensive rats

The acute and chronic effects of a potent and selective dopamine beta-hydroxylase inhibitor, SK&F 102698 [1-(3,5-difluorobenzyl)imidazole-2-thiol], on systemic hemodynamic variables were assessed in chronically instrumented spontaneously hypertensive and normotensive Wistar rats. The changes observed in the hypertensive rats were compared with those obtained by vehicle in a different group of rats. Following intragastric administration of SK&F 102698 (75 mg/kg), blood pressure and cardiac output decreased gradually in both strains of rats, while total peripheral vascular resistance remained unchanged. The antihypertensive effect of repeated daily administration of SK&F 102698 in the hypertensive rats was sustained for the duration of treatment. During the acute phase, the decrease in cardiac output was due to bradycardia. In addition, as a result of hemodynamic changes, SK&F 102698 significantly reduced the minute work output of the heart. The results indicate that SK&F 102698 lowers blood pressure primarily by decreasing cardiac output.

Inactivation of dopamine beta-hydroxylase by beta-ethynyltyramine: kinetic characterization and covalent modification of an active site peptide

beta-Ethynyltyramine has been shown to be a potent, mechanism-based inhibitor of dopamine beta-hydroxylase (DBH). This is evidenced by pseudo-first-order, time-dependent inactivation of enzyme, a dependence of inactivation on the presence of ascorbate and oxygen cosubstrates, the ability of tyramine (substrate) and 1-(3,5-difluoro-4-hydroxybenzyl)imidazole-2-thione (competitive multisubstrate inhibitor) to protect against inactivation, and a high affinity of beta-ethynyltyramine for enzyme. Inactivation of DBH by beta-ethynyltyramine is accompanied by stoichiometric, covalent modification of the enzyme. Analysis of the tryptic map following inactivation by [3H]-beta-ethynyltyramine reveals that the radiolabel is associated with a single, 25 amino acid peptide. The sequence of the modified peptide is shown to be Cys-Thr-Gln-Leu-Ala-Leu-Pro-Ala-Ser-Gly-Ile-His-Ile-Phe-Ala-Ser-Gln-Leu- His*- Thr-His-Leu-Thr-Gly-Arg, where His* corresponds to a covalently modified histidine residue. In studies using the separated enantiomers of beta-ethynyltyramine, we have found the R enantiomer to be a reversible, competitive inhibitor versus tyramine substrate with a Ki of 7.9 +/- 0.3 microM. The S enantiomer, while also being a competitive inhibitor (Ki = 33.9 +/- 1.4 microM), is hydroxylated by DBH to give the expected beta-ethynyloctopamine product and also efficiently inactivates the enzyme [kinact(app) = 0.18 +/- 0.02 min-1; KI(app) = 57 +/- 8 microM]. The partition ratio for this process is very low and has been estimated to be about 2.5. This establishes an approximate value for kcat of 0.45 min(-1) and reveals that (S)-beta-ethynyltyramine undergoes a slow turnover relative to that of tyramine (kcat approximately 50 s(-1), despite the nearly 100-fold higher affinity of the inactivator for enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Inactivation of dopamine beta-hydroxylase by p-cresol: isolation and characterization of covalently modified active site peptides

Recently, p-cresol has been shown to be a mechanism-based inhibitor of dopamine beta-hydroxylase (DBH; EC 1.14.17.1) [Goodhart, P. J., DeWolf, W. E., Jr., & Kruse, L. I. (1987) Biochemistry 26, 2576-2583]. This inactivation was suggested to result from alkylation of an active site residue by an aberrant 4-hydroxybenzyl radical intermediate. In support of this hypothesis, we report here the isolation and characterization of two modified tryptic peptides from DBH inactivated by p-cresol. Using a combination of automated Edman sequencing, mass spectroscopy (MS), and tandem MS, we have determined the sequence of the putative active site peptides, identified the site of attachment of p-cresol, and defined the chemical nature of the adduct formed. Both modified peptides are the same primary sequence: Ala-Pro-Asp-Val-Leu-Ile-Pro-Gly-Gln-Gln-Thr-Thr-Tyc-Trp-Cys-Tyr-Va l-Thr-Glu- Leu-Pro-Asp-Gly-Phe-Pro-Arg, where Tyc is an amino acid residue with the in-chain mass of a cresol-Tyr adduct (106 + 163 Da). Gas-phase deuterium exchange studies (employing N2H3-DCI MS) of the isolated phenylthiohydantoin (Pth) derivatives of modified residue 13 demonstrate that p-cresol forms two chemically distinct covalent adducts and support the hypothesis that a (4-hydroxyphenyl)methyl radical is generated during catalysis. Rearrangement to a (4-methylphenyl)oxy radical may also occur prior to inactivation.

Further characterization of the cardiovascular effects of the dopamine beta-hydroxylase inhibitor SK&F 102698 in conscious hypertensive rats

The dopamine beta-hydroxylase inhibitor SK&F 102698 was characterized by studying its cardiovascular effects in hypertensive rats. The antihypertensive effects of SK&F 102698 (50 mg/kg p.o.) were studied in three different rat models of hypertension. In spontaneously hypertensive and deoxycorticosterone acetate-salt rats SK&F 102698 produced blood pressure reductions of approximately 21 and 23%, respectively. In contrast, SK&F 102698 did not produce a significant decrease in blood pressure in 2-kidney, 1-clip Goldblatt hypertensive rats. The antihypertensive mechanism of action of dopamine beta-hydroxylase inhibition was probed with the selective DA1-receptor antagonist SCH 23390, which produced an attenuation of the antihypertensive effects of SK&F 102698. Experiments were designed to separate the peripheral from the central components of the cardiovascular effects of SK&F 102698. In spinal cord stimulated pithed spontaneously hypertensive rats, SK&F 102698 reduced blood pressure but not heart rate, indicating a peripherally mediated vasodilation and a centrally mediated heart rate effect. Furthermore, when SK&F 102698 was administered directly into the fourth ventricle of conscious spontaneously hypertensive rats, a pronounced bradycardia and lowering of blood pressure was observed. SCH 23390 (200 micrograms/kg i.v.) and l-sulpiride (1 mg/kg i.v.) inhibited the cardiovascular effects of SK&F 102698 administered into the fourth ventricle. These data indicate that inhibition of dopamine beta-hydroxylase with SK&F 102698 results in both peripherally and centrally mediated cardiovascular effects and suggest that central dopamine receptors contribute to the control of systemic blood pressure in hypertensive models associated with an increased sympathetic outflow.

Secretory granules from different glands contain neurocuprein-like protein

Secretory granules obtained from bovine pituitary, atrium and adrenal medulla contain an extremely acidic copper protein resembling by its main physico-chemical and antigenic properties as well as by the ability of its apoform to inhibit dopamine beta-monooxygenase the protein from brain, neurocuprein.

The copper sites of dopamine beta-hydroxylase: an X-ray absorption spectroscopic study

X-ray absorption edge and extended X-ray absorption fine structure (EXAFS) spectra are reported for the Cu(I) and Cu(II) forms of bovine dopamine beta-hydroxylase (DBH; EC 1.14.17.1) and for the Cu(I) form of DBH bound either to tyramine substrate or to a multisubstrate inhibitor [Kruse, L. I., DeWolf, W. E., Jr., Chambers, P. A., & Goodhart, P. J. (1986) Biochemistry 25, 7271-7278]. A significant change in the structure of the copper sites occurs upon ascorbate-mediated reduction of Cu(II) DBH to the Cu(I) form. While the average Cu(II) site most likely consists of a square-planar array of four (N,O)-containing ligands at 1.98 A, the average Cu(I) site shows a reduction in (N,O) coordination number (from approximately 4 to approximately 2) and the addition of a S-containing ligand at 2.30 A. No change in the average Cu(I) ligand environment accompanies binding of tyramine substrate, whereas binding of a multisubstrate inhibitor, 1-(3,5-difluoro-4-hydroxybenzyl)-1H-imidazole-2(3H)-thione, causes an increase in the Cu-S coordination, consistent with inhibitor binding to the Cu(I) site through the S atom. Although excellent signal-to-noise ratio in the EXAFS spectra of ascorbate-reduced DBH facilitated analysis of outer-shell scattering for a Cu..Cu interaction, the presence of a binuclear site could not be proven or disproven due to interference from Cu...C scattering involving the carbons of imidazole ligands.

Effects of the novel dopamine beta-hydroxylase inhibitor SK&F 102698 on catecholamines and blood pressure in spontaneously hypertensive rats

The novel dopamine beta-hydroxylase (D beta H) inhibitor SK&F 102698 was characterized in vitro with soluble enzyme from bovine adrenal medulla and in vivo by measuring the dopamine/norepinephrine (DA/NE) ratio in the mesenteric artery, heart and brains of spontaneously hypertensive rats (SHR). SK&F 102698 was a potent D beta H inhibitor with an IC50 of 1.2 microM on crude enzyme and had a Ki value of 40 nM on purified enzyme. SK&F 102698 produced a dose-dependent fall in NE and a dose-dependent increase in DA in the vasculature of SHR after p.o. administration. Elevation of the vascular DA/NE ratio was observed within 0.5 hr after administration. Peak effects were observed at 12 hr and values were still significantly increased at 18 hr. The rise in the DA/NE ratio of the blood vessels correlated with the fall in blood pressure following the first 4 hr after SK&F 102698. SK&F 102698 inhibited SHR heart D beta H and elevated the myocardial DA/NE ratio approximately 2.4-fold. SK&F 102698 also caused a dose-dependent increase in the whole brain DA/NE ratio of SHR. Catecholamine levels were also studied in six discrete brain regions and SK&F 102698 produced the greatest increase in the DA/NE ratio in the cerebellum, brain stem and midbrain regions, whereas the striatum was the region least affected. No overt sedation was observed in the rats. Further study with SK&F 102698 is warranted to better explore the role of DA and D beta H in pathophysiology, and to determine whether this drug or a congener D beta H inhibitor will be a useful therapeutic agent in humans.

Dopamine beta-hydroxylase from bovine adrenal medulla contains covalently-bound pyrroloquinoline quinone

Treatment of homogeneous dopamine beta-hydroxylase (DBH) preparations from bovine adrenals with the inhibitor phenylhydrazine (PH) changed the structureless absorption spectrum of DBH into spectra with a maximum at 350 nm. A product with this absorption spectrum could be detached with pronase, enabling its isolation. It appeared to be the C(5) hydrazone of pyrroloquinoline quinone (PQQ) and PH, as judged from its properties and the fact that it could be transformed into PQQ itself. From the yield obtained a ratio of 0.85 PQQ per enzyme subunit was calculated. In contrast to copper-quinoprotein amine oxidases (EC 1.4.3.6), hydrazone formation in DBH did not require saturation of the mixture with O2. DBH is the first copper-quinoprotein hydroxylase found so far. The implications of this finding for the current views on mechanism of action and inhibition by hydrazines are discussed. The success of the recently developed 'hydrazine method' [(1987) FEBS Lett. 221, 299-304] for all different types of amine oxidoreductases, suggest that the method could also be applied to other enzymes for which hydrazines are inhibitors and where the identity of the cofactors has not been established or the presence of PQQ is suspected.

Epinephrine mediates the growth hormone-releasing effect of galanin in infant rats

The mechanism underlying the GH-releasing effect of galanin (GAL), a novel 29-amino acid peptide, was investigated in the neonatal rat. The effect of galanin was compared to that of clonidine (CLO), a drug known to release GH via endogenous GHRF. GAL administration (5-25 micrograms/kg BW, sc) induced in 10-day-old pups a clear-cut and dose-related rise in plasma GH 15 min postinjection. CLO (50-450 micrograms/kg BW, sc) induced a marked rise in plasma GH, but no dose-related effect was evident. Inhibition of hypothalamic norepinephrine and epinephrine biosynthesis by DU-18288 (6 mg/kg BW, ip) or selective inhibition of epinephrine biosynthesis by SKF-64139 (50 mg/kg BW, ip) completely abolished the GH-releasing effect of GAL (25 micrograms/kg, sc), but left unaltered the GH rise induced by CLO (150 micrograms/kg, sc). Passive immunization with an anti-GHRF serum decreased basal GH levels and prevented the GH-releasing effect of either GAL or CLO, whereas in pups pretreated with an antisomatostatin serum, CLO, but not GAL, increased the already elevated plasma GH titers. In all these data indicate that in the infant rat 1) GAL is a potent GH secretagogue; 2) the action of GAL is not exerted directly on GHRF- or somatostatin-secreting structures, but requires the intervention of catecholaminergic neurons; 3) the GH-releasing effect of GAL is ultimately exerted via GHRF release, although a mechanism operating to inhibit hypothalamic somatostatin release cannot be ruled out; and 4) differently from GAL, CLO releases GH via postsynaptic stimulation of GHRF-secreting neurons.

Assay for dopamine beta-hydroxylase in human plasma and rat serum by high-performance liquid chromatography with fluorimetric detection

A highly sensitive assay for dopamine beta-hydroxylase activity in human plasma and rat serum by high-performance liquid chromatography with fluorimetric detection is described. Norepinephrine, formed enzymatically from the substrate dopamine, and epinephrine (internal standard), after clean-up with a cation-exchange cartridge (Toyopak IC-SP M), are converted into the corresponding fluorescent compounds by reaction with 1,2-diphenylethylenediamine. The derivatives are separated by reversed-phase chromatography on TSK gel ODS-80TM. The detection limit for norepinephrine formed enzymatically is 5 pmol per assay tube.

[Correction of arterial pressure in rats with hereditary hypertension by altering catecholamine metabolism in early ontogeny]

In a newly developed rat strain with inherited stress-sensitive arterial hypertension (ISSAH) an attempt was made to reduce arterial blood pressure by L-DOPA injections during a short time period of the early ontogenesis. It was shown that L-DOPA injections to rats on days 7-9 or 14-16 of life had no effect. The same procedure performed on 21-23 or 21-25-day-old rats was followed by a decrease in the basal and stress-induced arterial blood pressure levels, measured in adulthood. Injections of dopamine-beta-hydroxylase inhibitor (FLA-59) with parallel L-DOPA administration completely blocked the blood pressure decreasing effect. It can be suggested that injections of L-DOPA in the 4th week of post-natal life reduce the blood pressure level in ISSAH rats by enhancing the rate of brain noradrenaline biosynthesis.

Mechanism-based inhibitors of dopamine beta-hydroxylase

The copper-containing monooxygenase dopamine beta-hydroxylase catalyzes the hydroxylation of dopamine at the benzylic position to form norepinephrine. Mechanism-based inhibitors for dopamine beta-hydroxylase have been used as probes of the mechanism of catalysis. The variety of such inhibitors that have been developed for this enzyme can be divided into three groups: (i) those in which the inactivating species is formed by abstraction of a hydrogen atom to form a radical intermediate; (ii) those in which the inactivating species is formed by abstraction of an electron to form an epoxide-like intermediate; and (iii) those in which the product is the inactivating species. A mechanism consistent with inactivation by all three groups of inhibitors which proposes that hydroxylation of dopamine by dopamine beta-hydroxylase involves formation of a benzylic radical has been developed. The benzylic radical is formed by abstraction of a hydrogen atom from the substrate by a high-potential copper-oxygen species.

Inhibitors of dopamine beta-hydroxylase. 3. Some 1-(pyridylmethyl)imidazole-2-thiones

The 1-benzylimidazole-2-thione moiety has been previously shown by Kruse et al. to be broadly associated with dopamine beta-hydroxylase (DBH) inhibitory activity both in vitro and in vivo in spontaneously hypertensive rats (SHR). An extension of structure-activity studies to 1-(pyridylmethyl)- and 1-(oxypyridylmethyl)imidazole-2-thiones is reported here in an attempt to exploit the pH differential that exists across the chromaffin vesicle membrane. We hypothesized that the weakly basic pyridyl compounds would diffuse into the acidic vesicles in their neutral forms where protonation and concentration would occur to enhance their in vivo effectiveness as inhibitors. To test this hypothesis, isomeric 2-, 3- and 4-(1-pyridylmethyl)imidazole-2-thiones were synthesized from the appropriate pyridinecarboxaldehydes by reductive alkylation of aminoacetaldehyde dialkyl acetal followed by imidazole-2-thione formation using acidic potassium thiocyanate. Related oxypyridyl compounds were synthesized by first preparing the appropriate aldehyde intermediate followed by conversion to the imidazole-2-thione by the same procedure. The unsubstituted pyridylmethyl compounds showed modest DBH inhibition in vitro but, consistent with a transport-mediated increase in observed potency, showed significant effects in vivo to increase the vascular ratio of dopamine to norepinephrine and to lower blood pressure.

Nucleus tractus solitarius: an evaluation by in vivo voltammetry

Nucleus tractus solitarius (NTS) is a brainstem nucleus known to play an important role in baroreceptor mediated cardiovascular regulation. As part of our study of the role of monoamines in the function of NTS, we have characterized pharmacologically the in vivo electrochemical signal recorded from the nucleus using carbon paste electrodes and linear sweep voltammetry with semiderivative signal processing in awake, freely moving rats. Two peaks were recorded by these techniques, one at 0.14 V and a second at 0.28 V. The tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine led to a significant reduction in the peak recorded at 0.14 V whereas it had no effect on the higher potential peak at 0.28 V. The dopamine-beta-hydroxylase inhibitor fusaric acid resulted in a large reduction in the 0.14 V peak and led to a 30% increase in the 0.28 V peak height. Pargyline, a monoamine oxidase inhibitor, did not change the low potential peak but did significantly reduce the 0.28 V peak. Tissue assays provided further support for the interpretation of in vivo electrochemical recordings. Norepinephrine concentration was reduced with fusaric acid. Tissue serotonin was not affected by any of the drugs while the 5-HIAA content was increased with fusaric acid and reduced with pargyline. These experimental findings lead to the conclusion that the first peak in the voltammogram most likely represents norepinephrine with a possible contribution by dopamine but not by DOPAC. The second peak appears to be 5-HIAA.