Effects of fusaric (5-butylpicolinic) acid on the monosynaptic reflex neural activity of cat spinal cord

It has been demonstrated that most hypertensive drugs which cause increases in levels of norepinephrine influence the stimulation of monosynaptic reflex (MSR) neural activity. However this report discusses the effects of a hypotensive drug, which causes decreases in levels of norepinephrine, on the MSR amplitude of acute spinal cats. This drug is 5-butylpicolinic acid (fusaric acid: FA) which is an effective hypotensive agent and a potent inhibitor of dopamine beta-hydroxylase (approximately 10 times more potent than disurfiram). Intravenous injections of FA increased MSR neural activity in a dose-dependent manner. The FA-induced neural activity was gradually depressed by treatment with haloperidol, a dopamine and/or a alpha receptor blocker and methysergide, a serotonin receptor blocker, respectively. In addition, this neural activity was potentiated by the sequential administration of L-dopa. FA did not cause increases in the blood pressure but inhibit the synthesis of norepinephrine from dopamine. These results suggest that not only hypertensive but also hypotensive drugs can affect the increase of MSR neural activity, and dopamine plays an important role in FA-induced neural activity.

Inhibition of LH and prolactin release in the cycling rat following inhibition of dopamine-beta-hydroxylase

The effect of fusaric acid, an inhibitor of dopamine-beta-hydroxylase (DBH), on luteinizing hormone (LH) and prolactin levels during the estrous cycle was determined. Fusaric acid was found to cause a selective dose- and time-dependent inhibition of DBH activity in the medial basal hypothalamus without altering tyrosine hydroxylase activity. When DBH was inhibited during the afternoon of diestrus, the proestrous surges of both LH and, to a lesser extent, prolactin were inhibited. These results suggest that noradrenergic neuronal activity in the mediobasal hypothalamus is required during the afternoon preceding proestrus in order for the LH and prolactin surges to occur. It is possible that the rise in serum estrogen during late diestrus 2 is blocked by fusaric acid treatment. This estrogen increase is necessary for LH and prolactin surges to occur during proestrus. When DBH was inhibited during the afternoon of proestrus, the LH and prolactin surges were completely eliminated. This indicates that noradrenergic neuronal activity in the mediobasal hypothalamus during the afternoon of proestrus is important for both the LH and prolactin surges to occur.

The inhibitory effect of serum from diethyldithiocarbamate treated rats on bovine adrenal dopamine-beta-hydroxylase activity

The activity of serum dopamine-beta-hydroxylase does not depend solely on the presence of exogenous inhibitors: its activity also depends on the rate of release into plasma and is influenced by sympathetic activity or pathological conditions. This fluctuation may disguise the effect of an exogenous inhibitor on the enzyme. It has been shown in the present work that boiling of serum obtained from sodium diethyldithiocarbamate treated rats did not eliminate the dopamine-beta-hydroxylase inhibitory effect on a bovine adrenal dopamine-beta-hydroxylase preparation. Thus it is possible to measure the dopamine-beta-hydroxylase inhibitory effect of an exogenous inhibitor of this type without interference from the enzyme present in serum. Serum taken from rats treated with sodium diethyldithiocarbamate depressed in a time and dose dependent way the activity of bovine adrenal dopamine-beta-hydroxylase preparation.

Dopamine beta-hydroxylase. Inactivation by a suicide substrate

Dopamine beta-hydroxylase (EC 1.14.17.1) is inactivated by p-hydroxybenzylcyanide (PHBC) in a manner characteristic of a suicide substrate. The inactivation 1) is first order in inhibitor (Kd = 1.9 mM, k2 = 0.05 min-1, pH 5.0), 2) exhibits saturation kinetics, and 3) is dependent on O2 and ascorbate. Restoration of activity could not be achieved by dialysis. The substrate, p-tyramine, protects the enzyme from inactivation, while in initial velocity kinetic experiments, PHBC is a linear competitive inhibitor (Kis = 2.6 mM) versus p-tyramine. The Kis value for PHBC is in good agreement with the Kd value obtained from analysis of the inactivation reaction. PHBC (in the presence of O2 and ascorbate) is also a substrate for the enzyme, being converted to p-hydroxymandelonitrile (PHMN) at a relative Vmax 13% that of p-tyramine. Under these conditions, the ratio of product formation to inactivation is 8000:1, suggesting that PHMN (or a tautomer of PHMN) is the species responsible for inactivation. Indeed, incubation of the enzyme with PHMN in the absence of ascorbate leads to irreversible inactivation. Since PHMN breaks down to p-hydroxybenzaldehyde and cyanide, experiments were conducted to determine whether these compounds may have been responsible for the inactivation. Incubation of the enzyme with p-hydroxybenzaldehyde did not lead to inactivation, whereas the inactivation produced with cyanide could be reversed by dialysis. Thus, the data point to PHMN as the molecule responsible for time-dependent loss of activity of the enzyme. Together, these experiments demonstrate that the newly discovered inhibitor, PHBC, meets the critieria for a suicide substrate for dopamine beta-hydroxylase.

Dopamine-beta-hydroxylase and its apparent endogenous inhibitory activity in the plasma of some psychiatric patients

The "apparent" and "absolute" levels of dopamine-beta-hydroxylase (DBH) in 62 psychiatric patients and 20 healthy control subjects have been investigated. No significant differences could be detected between control subjects and patients diagnosed as schizophrenic, unipolar depressive and bipolar depressive, when DBH was assayed in the presence of the anti-inhibitors Cu++ and N-ethylmaleimide (i.e., "absolute" DBH activity). In the absence of these anti-inhibitors, however, the levels of plasma DBH (i.e., "apparent" DBH activity) were considerably reduced in all cases, with the schizophrenic group also being significantly reduced (p less than 0.02) in comparison with the control group.

Inhibition of dopamine-beta-hydroxylase by alternative electron donors

The alternative electron donors ferrocyanide and hydroquinone have been shown to also act as inhibitors of dopamine-beta-hydroxylase (3,4-dihydroxyphenylethylamine, ascorbate:oxygen oxidoreductase (beta-hydroxylating), EC 1.14.17.1). Hydroquine shows uncompetitive inhibition with respect to ascorbate and competitive inhibition with respect to tyramine. Ferrocyanide shows uncompetitive inhibition with respect to ascorbate and mixed type inhibition with respect to tyramine. Inhibition by ferrocyanide at concentrations at or above 2.5 . 10(-5) M was prevented by 2.5 . 10(-6) M cupric ion. These results indicate that the inhibitory action of these alternative electron donors is due to their interaction with a reduced enzyme species. The potency of inhibition of dopamine-beta-hydroxylase by both ferrocyanide and hydroquinone is dependent on the degree of protonation of a group in the enzyme having a pKa of 5.3.

Inhibition of dopamine-beta-hydroxylase, A copper enzyme, by bleomycin

Bleomycin was found to be one of the most potent inhibitors of dopamine-beta-hydroxylase. Bleomycin-A2 at 8 X 10(-8) M inhibited the enzyme activity by 50%. Kinetic studies showed that the inhibition by bleomycin-A2 was of the competitive type with both the substrate and the cofactor, ascorbate, and was not affected by fumarate, a stimulator for the enzyme. The inhibition mechanism is possibly due to chelating action of bleomycin toward the copper atom at the active site of the enzyme together with some other kinds of binding, for the addition of the cupric ions or extensive dialysis completely reversed the inhibition and bleomycin Cu(II)-complex did not inhibit the enzyme.

24-hour rhythm of human plasma noradrenaline and the effect of fusaric acid, a dopamine-beta-hydroxylase inhibitor

A 24-hour of basal noradrenaline (NA) levels in normal human plasma in the supine position was observed; higher NA levels during daytime and lower NA levels at night, especially early in the morning. This pattern of the 24-hour rhythm of plasma NA levels was similar to that of serum dopamine-beta-hydroxylase (DBH) levels. After oral administration of fusaric acid, a DBH inhibitor, the rhythm changed, and plasma NA levels increased at 3 h and returned to the initial level at 23 h. In contrast, plasma DBH activity was markedly inhibited during 23 h after fusaric acid administration.

Effects of norlaudanosolinecarboxylic acids on enzymes of catecholamine metabolism

Enzymes involved in catecholamine metabolism were assayed in the presence of a new class of naturally occurring tetrahydroisoquinoline alkaloids, the norlaudanosolinecarboxylic acids (NLCAs). NLCAs inhibited tyrosine hydroxylase noncompetitively with respect to its substrate, tyrosine and the cofactor, 6-methyltetrahydropterin (NLCA Kj = 4 x 10(-4) M; 3',4'-deoxynorlaudanosolinecarboxylic acid (DNLCA) Kj = 1.5 x 10(-4) M). Adrenal dopamine-beta-hydroxylase was also inhibited by NLCAs [3'O-methylnorlaudanosolinecarboxylic acid (MNLCA) Kj = 1.2 x 10(-4) M] and NLCA is a competitive inhibitor of norepinephrine methylation by hepatic catechol-O-methyltransferase (NLCA Kj = 5.6 x 10(-5) M). While a slight reduction of rat adrenal monoamine oxidase by MNLCA was also observed, NLCA did not affect the oxidation of tyrosine by D-amino acid oxidase. Kinetic patterns of tyrosine aminotransferase and aromatic amono acid decarboxylase from rat liver were not altered by addition of 1 to 10 x 10(-5) M NLCA or its 3'-O-methyl ether (MNLCA). In vivo studies of brain tyrosine metabolism in mouse neonates corroborated results on the in vitro effect of DNLCA on tyrosine hydroxylase. The potential of high-pressure liquid chromatography was demonstrated in both enzyme assays and radiometric studies of in vivo metabolism.

Reinforcement with intragastric infusions of ethanol: blocking effect of FLA 57

Suppression of oral intake of ethanol by FLA 57 has been reported for rats and was attributed to an inhibition of dopamine beta-hydroxylase. We have demonstrated the ability of FLA 57 (50 mg/kg, IP) to suppress bar-pressing for intragastric (IG) delivery of doses of ethanol (25 mg/kg). This indicates that the effect on oral intake of ethanol may not be attributed to a taste factor, e.g., a decreased palatability of the ethanol solution. The same dose of FLA 57 did not suppress responding for IG doses of sweet milk. Thus, there was not an impairment of appetitive behavior in general through some nonspecific depressant or toxic action. Furthermore, the primary reinforcing action of ethanol, when used to establish a buzzer as a conditioned reinforcer through repeated pairings, was blocked if FLA 57 was given before pairings. This was evidenced by a failure of such rats to bar-press above the baseline level in a later test of conditioned reinforcement, which contrasted with the increased responding seen for rats receiving saline instead of FLA 57 before ethanol. These data support the previous findings on oral ethanol and confirm that FLA 57 can impair the mechanism by which ethanol produces positive reinforcement in rats.

Blood pressure response to ethanol in relation to acetaldehyde levels and dopamine-beta-hydroxylase activity in rats pretreated with disulfiram, cyanamide and coprine

The blood pressure response after ethanol administration was studied in relation to blood acetaldehyde levels, aldehyde-dehydrogenase (ALDH)--and dopamine-beta-hydroxylase (DBH) activities in rats pretreated with the ethanol-sensitizing compounds disulfiram, cyanamide and coprine and the DBH-inhibitor FLA-57. Disulfiram, cyanamide and coprine, but not FLA-57, inhibited the low-Km ALDH in the liver and caused an increased acetaldehyde level in blood. Disulfiram and FLA-57, but not cyanamide and coprine, decreased the DBH-activity in the heart and the levels of norepinephrine in the heart and the brain. In disulfiram-treated rats with a low DBH-activity, a fall in blood pressure was observed at acetaldehyde levels being slightly higher than those found in control rats. In disulfiram-treated rats with a DBH-activity close to control activity and in rats pretreated with cyanamide or coprine, a fall in blood pressure were observed in rats pretreated with FLA-57. In rats pretreated with coprine + FLA-57, the fall in blood pressure was similar, or even lower, than in rats pretreated with coprine alone. The results suggest that acetaldehyde is the main determinant of the hypotension elicited by ethanol in rats pretreated with ALDH-inhibitors, and that the role of DBH in the disulfiram-ethanol reaction has been over-estimated in previous studies.

Effect of dopamine-beta-hydroxylase inhibitor (disulfiram) on the response of adenohypophysis, serum ceruloplasmin and hypothalamic ascorbic acid to estradiol treatment

The administration of tetraethylthiuramdisulfide (disulfiram, Antabus--inhibitor of dopamine-beta-hydroxylase) resulted in an inhibition of the response of anterior pituitary to estradiol (i. e. increase of weight and of binding capacity of anterior pituitary proteins to thyroxine). At the same time there was no inhibition of an increase of ceruloplasmin (polyphenoloxidase) level after estradiol, while a decrease of ascorbic acid in hypothalamus was fully prevented. It was concluded that disulfiram may inhibit dopamine-beta-hydroxylase activity and thus presumably protect the hypothalamic dopamine which further inhibits the response of anterior pituitary to estradiol either directly or through a dopamine dependent growth inhibiting factor.

Clonidine reverses the amnesia induced by dopamine beta hydroxylase inhibition

The role of noradrenergic (NE) mechanisms in amnesia induced by the dopamine-beta-hydroxylase (DBH) inhibitor, diethyldithiocarbamate (DEDTC) was examined by studying the antiamnestic characteristics of the alpha-NE receptor stimulator clonidine. DEDTC (250 mg/kg) administered 3 hr prior to training to C57BL/6J mice resulted in marked deficits when retention of a multiple trial food motivated spatial discrimination task was measured 24 hr after learning. Investigation of the temporal aspects of recovery indicated that the agonist was an effective antiamnestic agent when administered 0, 1, 3, 21 and 23 hr after training. No recovery was observed when the drug was administered 6 and 18 hr posttraining. A dose response study of the effectiveness of clonidine administered 1 hr prior to testing indicated recovery of memory at doses ranging from 10-500 microgram/kg. The clonidine induced recovery was not a result of general performance facilitation, but specific to the memory tested. In addition, the clonidine effect was pharmacologically specific to its actions on NE receptors, as recovery was blocked by pre-treatment with the alpha-NE antagonist, phentolamine. No recovery from DEDTC induced amnesia was seen with post-training or pre-test injection of d-amphetamine.

Adrenergic mechanisms in postvagal tachycardia

The adrenergic mechanisms involved in the sinus tachycardia which follows the termination of vagal stimulation ("postvagal tachycardia") were studied in the anesthetized dog. The following results were obtained. Postvagal tachycardia: 1) is not affected by ventricular or atrial drive; 2) is enhanced by the application of either norepinephrine or dopamine on the sinus node area; 3) is increased in magnitude and duration by the catecholamine uptake blockers desipramine and phenoxybenzamine; 4) is significantly reduced by beta blockade with propranolol; 5) is not affected by alpha blockade with phentolamine or dopaminergic blockade with haloperidol; 6) is not affected by Disulfiram, a drug which inhibits dopamine beta-hydroxylase and thereby increases endogenous dopamine; 7) rises more slowly to a smaller peak after bretylium; and 8) is enhanced by phenoxybenzamine, but not by desipramine, after destruction of sympathetic nerves by 6-hydroxydopamine. It is concluded that release of catecholamines by vagally liberated acetylcholine is involved in postvagal tachycardia. The released catecholamine acts on beta but not alpha or dopamine receptors. Catecholamines released from sympathetic nerves may account for an initial component in postvagal tachycardia, but catecholamine(s) released from extraneuronal stores account for most of postvagal tachycardia.

Modification of cerebrovascular CO2 reactivity by inhibition of dopamine beta-hydroxylase

The influence of sympathetic nervous activity on cerebral circulation and cerebrovascular CO2 reactivity was investigated through inhibition of dopamine beta-hydroxylase (DBH). A PO2 electrode, a PCO2 electrode and a plate-type thermocouple-flowmeter were placed on the pial surface of the cat brain. Cerebrocortical PO2, PCO2, cerebrocortical blood flow and arterial blood pressure were continuously recorded before, during and after intracarotid infusion of 10 mg/kg of fusaric acid, a potent DBH inhibitor. The effects of 5% CO2 inhalation and hyperventilation were measured before and after the inhibition of DBH. Following the intracarotid infusion of fusaric acid, cerebrocortical PO2 and cerebrocortical blood flow increased significantly. After the inhibition of DBH, the degree of the increase in cerebrocortical PO2 during 5% CO2 inhalation was enhanced while the degree of the decrease in cerebrocortical PO2 during hyperventilation did not show any significant change. The cerebral vasodilatation caused by fusaric acid suggests that the sympathetic nervous system takes part in the resting tone of cerebral blood vessels. The increase in the cerebrovascular CO2 reactivity produced by the inhibition of DBH suggests that the sympathetic nervous system modifies cerebrovascular CO2 reactivity.