Effects of L-dopa and L-tryptophan on the cerebral blood flow in the dog

Subcutaneous apomorphine increases regional cerebral blood flow in parkinsonian patients via peripheral mechanisms

1. We have measured regional cerebral blood flow (rCBF) and motor function before and after the subcutaneous (s.c.) injection of apomorphine in parkinsonian patients deprived of their usual treatment for at least 48 h. 2. Nineteen patients, pretreated with domperidone (20 mg three times daily for 48 h), received a mean dose of 5.8 mg s.c. apomorphine. All patients switched 'on'. The mean motor score was significantly improved (-65%, P less than 0.01) but no significant change in rCBF was observed. 3. Seven other patients, not pretreated with domperidone, received a lower dose (0.3 mg) of s.c. apomorphine. No change in motor score was observed while the mean rCBF significantly increased (+12%, P less than 0.05). 4. We conclude that s.c. apomorphine increases rCBF in parkinsonian patients. This effect is independent of the central therapeutic effects of the drug. It is mediated by the stimulation of dopaminergic receptors of the cerebral vessels. These receptors are located outside the cerebral blood brain barrier and can be considered as 'peripheral' ones.

Cerebral circulatory effects of a dopaminergic agonist (apomorphine) in the dog

Dopaminergic influence upon cerebral circulation was studied in 12 adult mongrel dogs, local cerebral blood flow (lCBF) being measured by the microspheres method. The intravenous injection of apomorphine (0.1 mg/kg) produced heterogeneous effects on lCBF; an increase in lCBF reaching the level of statistical significance was observed in frontal and sensory motor cortices, whereas blood flow was not significantly altered in the other regions examined. When the animals are considered individually, a widespread response was generally observed. Local cerebral blood flow increased from 16% to 85% (depending on the regions examined) in 4 animals, and tended to decrease (particularly in the caudate nucleus) in the 8 others. These opposite effects cannot be related with changes in general metabolic or hemodynamic parameters (PaCO2, PaO2, pH, and arterial pressure). It is thus concluded that the effect of dopaminergic stimulation upon cerebral circulation is more complex than previously reported. These effects could only be partly mediated via vascular receptors and could be the result of modifications of the local metabolic level.

Barrier mechanisms for neurotransmitter monoamines and their precursors at the blood-brain interface

The integrity of the endothelial cell lining of the cerebrovascular bed constitutes a morphological blood-brain barrier mechanism to neurotransmitter monoamines. Circulating monoamines are prevented from entering the brain primarily at the luminal membrane of the endothelial ling. The small percentage of amines that may pass this membrane is deaminated within the endothelial cells and pericytes of brain microvessels (capillaries, venules, and small veins) and, in the case of large parenchymal and pial vessels, in the smooth muscle layers, where O-methylation also takes place. In the choroid plexus a corresponding deamination and O-methylation takes place in the epithelial cells. The presence of these enzymes constitutes a further, enzymatic, blood-brain barrier in the brain vessels for these monoamines. The monoamine precursors L-3,4-dihydroxyphenylalanine (L-dopa) and L-5-hydroxytryptophan readily pass from the luminal endothelial cell membrane but are trapped by another enzymatic barrier mechanism. Within the endothelial cells and pericytes of the microvasculature, these compounds are decarboxylated to their corresponding amines and then immediately deaminated. One clinical implication of these enzymatic barrier mechanisms is the use of decarboxylase and monoamine oxidase inhibitors as adjuncts to L-dopa treatment of Parkinson disease; these substances facilitate the entry of L-dopa into brain and thus increase the amount of dopamine available at receptor sites. A brief hypertensive or hypertonic stimulus can transiently open the blood-brain barrier through an effect on endothelial cell linings. High circulating concentrations of monoamines can also open the morphological barrier, but probably only indirectly by inducing an acute rise in systemic blood pressure. Once the barrier is open, systemically administered monoamines enter the brain parenchyma, where they can induce pronounced changes in cerebral blood flow and metabolism.

Regional cerebral blood flow in parkinsonism. Measurement before and after levodopa

Data from recent studies in experimental animals suggest that central dopaminergic mechanisms may be involved in regulation of the regional cerebral blood flow (rCBF) in the mammalian brain. In order to evaluate possible effects of dopaminergic stimulation on human cerebral circulation, rCBF was measured by the 133Xenon inhalation technique in 26 patients with Parkinson's disease before and after the chronic oral administration of levodopa combined with a peripheral Dopa-decarboxylase inhibitor. Treatment with levodopa did not induce significant alterations in the mean brain, mean hemispheric or regional flow values in these patients. These findings suggest that the dopaminergic system does not play an important role in modulation of the rCBF in man.