Stimulation and sensitization of the isolated Venus heart by cerebrospinal fluid

5-Hydroxytryptamine was not detected in animal cerebrospinal fluid after amine oxidase inhibitors, reserpine, or both. Both human and animal cerebrospinal fluid can produce a stimulating effect on the clam heart which is not due to 5-hydroxytryptamine. Cerebrospinal fluid also has the property of increasing the sensitivity of the clam heart to 5-hydroxytryptamine.

A method for assessing the effects of drugs on the central actions of 5-hydroxytryptamine

Injection of 5-hydroxytryptophan into mice produces a characteristic head-twitch. For a given period of observation this response may be assessed quantitatively either by observing the proportion of mice showing at least one head-twitch (a quantal response) or by counting the number of head-twitches for each mouse (a graded response). A method, based on the quantal response, of investigating the effect of centrally acting compounds on the head-twitch response is described. Evidence is presented that the head-twitches are due to a central action of 5-hydroxytryptamine formed by decarboxylation of 5-hydroxytryptophan. Head-twitches are potentiated by monoamine oxidase inhibitors and by phenytoin. Antagonists tested include an inhibitor of decarboxylase, antagonists of 5-hydroxytryptamine, some antihistamines, major tranquillizers and analgesic and sympathomimetic drugs. Drugs which neither potentiate nor inhibit the response include barbiturates and minor tranquillizers. The method may be valuable in the preliminary examination of compounds likely to have a central action.

CIRCADIAN RHYTHM IN PINEAL SEROTONIN: EFFECT OF MONOAMINE OXIDASE INHIBITION AND RESERPINE

The pineal gland of the rat shows a circadian rhythm in its serotonin content, the amount of serotonin decreasing at night. This decrease can be prevented by inhibiting the action of monoamine oxidase. Reserpine abolishes the circadian rhythm in pineal serotonin in the same manner as does interruption of the sympathetic nervous connections of the central nervous system and the pineal gland. These observations suggest that circadian changes in release and binding of serotonin may occur in the pineal gland, and that a central mechanism in which monoamines participate may control the circadian pineal-serotonin rhythm.

REVERSAL OF THE EFFECT OF ALPHA-METHYLDOPA BY MONOAMINE OXIDASE INHIBITORS

L-α-Methyldopa, which normally causes sedation, induces a strong central excitation in mice pretreated with a monoamine oxidase inhibitor after a lag of a few hours. It is concluded that this excitation is caused by accumulation of free catecholamines liberated by amines which are slowly formed by decarboxylation of α-methyldopa. The hypotensive and sedative effects of α-methyldopa given alone are attributed to the slow release of catecholamines and subsequent breakdown by monoamine oxidase so that a partial depletion of catecholamines ensues.

MONOAMINE OXIDASE INHIBITORS: AUGMENTATION OF PRESSOR EFFECTS OF PERORAL TYRAMINE

Monoamine oxidase inhibitors markedly enhance the oral pressor potency of tyramine by preventing it from being destroyed by the monoamine oxidase normally present in liver and intestine. Since certain types of cheese contain high concentrations of tyramine, they should not be eaten by patients during treatment with a monoamine oxidase inhibitor.

THE 5-HYDROXYTRYPTAMINE CONTENT OF THE PLACENTA AND FOETUS DURING PREGNANCY IN MICE

5-Hydroxytryptamine (5HT) levels were measured in blood and tissues from pregnant mice. Blood levels remained constant during pregnancy and were the same as those in nonpregnant female mice. Placental levels of 5HT increased throughout pregnancy as did the foetal levels. The maternal blood volume of the placenta also increased with advancing gestation. 5HT levels were measured after treatment of the mother with 5HT, and the critical placental level of 5HT observed at about the time of death of the foetus was determined. The levels of 5HT in the placenta and foetus after treatment of the mother with several monoamine oxidase inhibitors were measured, and found to show no significant increase above the normal levels in these tissues. Treatment with cyproheptadine, a 5HT antagonist, did not delay parturition.

URINARY EXCRETION OF CATECHOL AMINES IN THE RAT AFTER THEIR LIBERATION BY RESERPINE OR DEXAMPHETAMINE

Daily urinary excretion of catechol amines in normal rats and in rats from which the adrenal medullae had been removed has been determined by a photofluorimetric method. In both groups, reserpine (2 mg/kg, intraperitoneally) produces: (1) A decrease in the urinary excretion of noradrenaline which persists for more than 3 weeks; this action is not influenced by monoamine oxidase inhibitors and mecamylamine. (2) An increase, within 20 to 68 hr, in the urinary excretion of adrenaline, even though the urine of rats without adrenal medullae does not usually contain adrenaline. These effects are prevented by monoamine oxidase inhibitors and, in the normal animals, are reduced by mecamylamine. In both groups, dexamphetamine (6 mg/kg, intraperitoneally) produces an increase in the excretion of adrenaline and noradrenaline, the adrenaline appearing in the urine of the rats without adrenal medullae within 20 to 44 hr. Mecamylamine prevents the effect of dexamphetamine on the excretion of noradrenaline. Dexamphetamine, administered within a week of reserpine treatment, produces its usual effects on the urinary excretion of catechol amines in normal rats, but has no effect in rats without adrenal medullae. The results are discussed with regard to both the mechanism by which reserpine and dexamphetamine influence the peripheral stores of adrenaline and noradrenaline, and the significance of the adrenal and extra-adrenal chromaffin system.

EFFECTS OF CHLORPROMAZINE AND BROMOLYSERGIC ACID DIETHYLAMIDE ON GASTRIC SECRETION OF ACID INDUCED BY HISTAMINE IN RATS

In anaesthetized rats in which the lumen of the stomach was perfused with 0.001 to 0.00025 N-sodium hydroxide solution and the pH of effluent fluid was recorded continuously, intravenous administration of chlorpromazine caused transient inhibition of acid secretion. After acid secretion had returned to the control level the responses to histamine were greater than those before chlorpromazine was given. Aminoguanidine, iproniazid and bromolysergic acid diethylamide also potentiated the effect of histamine on acid secretion but the initial inhibition was absent. Indirect evidence from experiments in which mixtures of aminoguanidine with chlorpromazine or bromolysergic acid diethylamide and of iproniazid with chlorpromazine or bromolysergic acid diethylamide were given, suggests that chlorpromazine and bromolysergic acid diethylamide enhance responses to histamine by inhibition of imidazole-N-methyl transferase.

THE INCREASE IN THE TOXICITY OF YOHIMBINE INDUCED BY IMIPRAMINE AND OTHER DRUGS IN MICE

In mice, yohimbine appears to accentuate the normal "alarm" reactions (alerting, flight) to external stimuli. Imipramine increases this effect and at the same time converts a non-lethal dose of yohimbine into a lethal one. The effect of imipramine is greatly reduced by adrenalectomy or by treatment with reserpine, syrosingopine, ganglion-blocking drugs or adrenaline antagonists acting on sympathetic beta-receptors. Hypnotic, anti-convulsant or anaesthetic agents, tetrabenazine or antagonists of 5-hydroxytryptamine do not reduce the imipramine effect. A variety of drugs which, like imipramine, are known to interfere with the tissue binding of noradrenaline also increase the toxicity of yohimbine. Yohimbine significantly reduces brain noradrenaline content; adrenal catechol amines are slightly reduced. The results suggest that yohimbine releases noradrenaline from stores or nerves as a consequence of increased central sympathetic activity. Imipramine increases the actions and toxicity of yohimbine by increasing the effects of the released noradrenaline on beta-receptors. The lethal effects of a high dose of yohimbine alone are not reduced by any of the treatments tested, and appear not to result from activation of sympathetic mechanisms.