THE EFFECT OF PRENYLAMINE ON THE METABOLISM OF CATECHOL AMINES AND 5-HYDROXYTRYPTAMINE IN BRAIN AND ADRENAL MEDULLA

Effect of drugs used in psychoses on cerebral dopamine metabolism

1. Chlorpromazine 15 mg/kg, given daily to cats for 2 weeks, produced a rise in homovanillic acid (HVA) content of the caudate nucleus, whereas the same dose of thioridazine lacked this effect. Of these two drugs, only chlorpromazine causes a high incidence of drug-induced Parkinsonism in man.2. In the mouse, chlorpromazine, thioridazine and haloperidol increased striatal concentrations of HVA and accelerated the disappearance of dopamine (DA) after inhibition of catecholamine synthesis with alpha-methyltyrosine. Low doses of the three compounds increased, whereas high doses reduced, the concentration of DA in the striatum. In their effects on the DA metabolism of the mouse, chlorpromazine and thioridazine had the same potency, but haloperidol was between 10 and 100 times more active than the other two drugs. In producing hypothermia and sedation, the three compounds were equiactive.3. Oxypertine, another drug apt to produce Parkinsonism in man, caused a severe reduction in striatal DA and hypothalamic noradrenaline (NA). Though the clinical signs produced in the mouse were indistinguishable from those seen after the same dose of chlorpromazine, the biochemical changes in the brain were thus quite different.4. Though all the drugs used caused temporary motor disabilities in animals, these bore no resemblance to human Parkinsonism, even when treatment was continued for 7 weeks or more as it was in cats and monkeys. The latter were treated with chlorpromazine 7.5 mg/kg daily, a dose chosen to avoid loss of weight and which may have been too small to produce toxic side-effects. It caused no changes in striatal DA turnover.5. Even at the high dose of 50 mg/kg, phenoxybenzamine did not increase DA turnover in mouse brain, but it sedated the mice as did the tranquillizers.6. Atropine sulphate, 25 mg/kg, reduced the HVA content of mouse striatum and partially antagonized the rise in HVA produced by phenothiazines. The effect was surmountable. Possible modes of action of atropine are discussed.7. At present we know of two types of biochemical changes which may occur in the brain of animals after treatment with drugs apt to cause Parkinsonism in man: a loss of cerebral catecholamines, as seen after reserpine or oxypertine, or an increase in turnover of DA as after phenothiazines and butyrophenones.

The distribution of dopamine in the brain of a tortoise, Geochelone chilensis (Gray)

1. Dopamine was found in the tortoise brain; the highest concentration (3.7 mug/g) occurred in a region of the telencephalon containing the nucleus basalis and cortex olfactoria. This region may be considered similar to the mammalian and avian corpus striatum with respect to dopamine.2. The administration of reserpine or prenylamine substantially decreased the concentration of dopamine in the tortoise brain. This effect appeared later and lasted longer than in birds or mammals. When these drugs were given in doses which nearly depleted brain dopamine the characteristic signs observed in birds or mammals were not present.

Brain monoamines and adrenocortical activation

1. The effect of a pharmacologically induced increase or depletion of brain monoamines (5-hydroxytryptamine, noradrenaline, dopamine) was investigated with respect to adrenocortical function.2. No strict correlation was found between the depletion of brain amine stores induced by prenylamine or p-chlorophenylalanine and adrenocortical activation, even at times in which the peak effect of brain amine depletion occurs.3. Restraint stress causes a manifest increase in brain 5-hydroxytryptamine while decreasing the cerebral noradrenaline and dopamine content. This stress strongly stimulates corticosterone secretion by the adrenals.4. A monoamine oxidase inhibitor, nialamide, at a dose which causes an evident increase in brain amine concentrations, does not modify plasma corticosterone. At the same dose it was, furthermore, unable to prevent the adrenocortical stimulation induced by restraint stress.5. Brain amine content does not seem to play an important part in the control of corticotrophin releasing factor in corticotrophin secretion by the pituitary gland. The relationship between hypothalamic monoamines and other neurohumours is discussed.

Monoamines and their metabolites in the avian brain

1. In the avian brain, a high concentration of dopamine was found in a sharply contoured region of the nucleus basalis which may or may not have included the nucleus entopeduncularis, and therefore lay within the palaeostriatum of the nomenclature of Crosby and Huber. This was thus the only region which may be considered biochemically homologous to the mammalian corpus striatum. For purposes of macroscopic identification only, the region is described here as the ;anterior part of the nucleus basalis'. The concentration of dopamine was 3 mug/g in the pigeon, about the same in the duck and chicken, and 7.5 mug/g in the finch. In the pigeon this region also contained some noradrenaline; the quantity of 5-hydroxytryptamine (1.4 mug/g) and 5-hydroxyindolylacetic acid (0.6 mug/g) was larger than in any other part of the brain.2. In the brain of the pigeon and the chicken, the highest concentrations of noradrenaline (1.5 and 1.4 mug/g) were found in the hypothalamus.3. The concentration of adrenaline was higher in the avian than in the mammalian brain. In the hypothalamus, it ranged from 0.4 mug/g in the pigeon to 1 mug/g in the chicken.4. Fluorescence microscopy, using the formaldehyde condensation method, showed, in the anterior part of the nucleus basalis, a large area of diffuse green-yellow fluorescence, similar in appearance to the fluorescence of the striatum of the rat. In addition this part of the brain contained a small region of fluorescent fibres and varicosities. It is suggested that the diffuse fluorescence was produced by dopamine. It was absent from brains of reserpine-treated pigeons.5. In the pigeon, reserpine, tetrabenazine and prenylamine produced a decrease in the concentration of brain monoamines, an effect which was comparable to that seen in mammals. Yet, none of these drugs raised the concentration of homovanillic acid, but they increased that of 5-hydroxyindolylacetic acid; these drugs raise the concentration of both acids in mammalian brain.6. In the pigeon beta-tetrahydronaphthylamine decreased the concentration of all monoamines and their metabolites, an action quite different from that produced in the mammalian brain.7. The main effect of morphine and of M 99 (6,14-endoetheno-7-(2-hydroxy-2-pentyl)-tetrahydro-oripavine hydrochloride) was a lowering of the noradrenaline concentration.8. As in mammals, chlorpromazine affected only the dopamine metabolism.9. In the guinea-pig and the pigeon, the administration of alpha-methyl-DOPA led to a substitution of much of the cerebral noradrenaline by alpha-methyl-noradrenaline, sometimes in excess of the lost noradrenaline. However, although the loss of dopamine was severe in both pigeon and guinea-pig, only little alpha-methyl-dopamine accumulated in the pigeon brain, so that it did not consitute a replacement for the lost dopamine; in the guinea-pig, alpha-methyl-dopamine was found in quantities similar to, or exceeding those, of the lost dopamine.