Differential effect of benserazide (Ro4-4602) on the concentration of indoleamines in rat pineal and hypothalamus

Circadian secretion pattern of melatonin in de novo parkinsonian patients: evidence for phase-shifting properties of l-dopa

Aim of this study was the characterization of the circadian melatonin profile in de novo Parkinson patients (N = 9, age 60.0 +/- 3.2 years, mean +/- SEM) and the comparison of these profiles with those of controls and Parkinson patients treated with l-dopa/decarboxylase inhibitor (l-dopa/DCI). We collected 14 venous blood samples during a period of 24 hours and measured the serum melatonin levels by a radioimmuno assay. De novo Parkinson patients displayed the nocturnal melatonin peak (acrophase) at the same time as controls and significantly later than l-dopa/DCI treated patients (1:54 +/- 15.6 min [average clock time +/- SEM in minutes] vs. 1:45 +/- 15.6 min vs. 0:13 +/- 40.8 min). The amount of secreted melatonin did not differ among the three groups. Stage and duration of Parkinson's disease did not correlate with the amount of secreted melatonin. Patients of the tremor subgroup, however, secreted more melatonin than patients presenting only with rigidity and akinesia. The phase advance in Parkinson patients treated with l-dopa/DCI is possibly due to a central nervous dopaminergic effect elicited by l-dopa administration and not inherent to Parkinson's disease per se.

Pineal indoleamine metabolism in pyridoxine-deficient rats

Pyridoxine deficiency causes physiologically significant decrease in brain serotonin (5-HT) due to decreased decarboxylation of 5-hydroxytryptophan (5-HTP). We have examined the effect of pyridoxine deficiency on indoleamine metabolism in the pineal gland, a tissue with high indoleamine turnover. Adult male Sprague-Dawley rats were fed either a pyridoxine-supplemented or pyridoxine-deficient diet for 8 weeks. Pyridoxine deficiency did not alter the pattern of circadian rhythm of pineal 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), N-acetylserotonin (NAS), and melatonin. However the levels of these compounds were significantly lower in the pineal glands of pyridoxine-deficient animals. Pineal 5-HTP levels were consistently higher in the pyridoxine-deficient animals and a conspicuous increase was noticed at 22.00 h. Increase in pineal NAS and melatonin levels caused by isoproterenol (5 mg/kg at 17.00 h) were significantly lower (P less than 0.05) in the pyridoxine-deficient animals. Treatment of pyridoxine-deficient rats with pyridoxine restored the levels of pineal 5-HT, 5-HIAA, NAS, and melatonin to values seen in pyridoxine-supplemented control animals. These results suggest that 5-HT availability could be an important factor in the regulation of the synthesis of pineal NAS and melatonin.

Monitoring of circadian fluctuations of N-acetylserotonin in the rat pineal body by differential pulse voltammetry

Circadian fluctuations of the electrochemical signal appearing at +270 mV (peak 3) recorded from the pineal body of freely moving rats were first monitored for 24 hours using the in vivo voltammetry technique. The peak 3 height increased after injection of pargyline and S-adenosyl-L-homocysteine but decreased after injection of NSD-1015, while probenecid did not cause any change. Under a 12/12 hours light-dark cycle, the peak 3 height represented circadian fluctuations, similar to those of N-acetyltransferase activity, which were higher in the dark than in the light period. These data suggest that the compound responsible for peak 3 in the pineal body is essentially due to extracellular N-acetylserotonin.

Inhibition of serum melatonin concentration and synthesis of brain indolealkylamines by monofluoromethyldopa in the rat

The simultaneous effect of alpha-monofluoromethyldopa (MFMD) on the synthesis of rat brain indolealkylamines has been investigated both in the daytime and in the dark phase. The effect on serum melatonin concentration has also been determined in the dark period. MFMD inhibits 5-hydroxytryptophan decarboxylase in the pineal gland, hypothalamus, and the cerebral cortex. Simultaneous measurement of 5-hydroxytryptophan (5-HTP), 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) levels in the three brain areas revealed that MFMD caused large increases in pineal 5-HTP concentrations and substantial decreases in pineal 5-HT and 5-HIAA both in the light and dark phases; a significant decrease in daytime hypothalamic 5-HT content after 7-h pretreatment; and a large increase in dark-phase cortical 5-HT concentration after 4-h pretreatment. Serum melatonin levels were also reduced by the action of MFMD in the dark period. The evidence suggests that differences occur in the rate-limiting steps in the indolealkylamine biosynthesis in the three brain areas.

Differential effect of carbidopa on the concentration of rat pineal and hypothalamic indoleamines

Carbidopa, an aromatic acid decarboxylase inhibitor, has been shown to significantly decrease the pineal concentration of melatonin, N-acetyl serotonin (NAS), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) but not the hypothalamic concentrations of these indoles. Increased levels of 5-hydroxytryptophan (5-HTP) indicate that carbidopa directly inhibits 5-HTP decarboxylation, thus limiting 5-HT production. Possible practical implications of selective inhibition of pineal indoleamines by carbidopa are discussed.

Differential effect of benserazide on catecholamine concentrations in the rat pineal, cerebral cortex and hypothalamus

The effect of benserazide, an aromatic L-amino acid decarboxylase inhibitor, has been investigated simultaneously on the noradrenaline content of the pineal, hypothalamus and cerebral cortex of the rat. The dopamine concentration of the hypothalamus was investigated in the same animals. Benserazide had no effect on pineal noradrenaline content throughout the light phase and the early part of the dark phase but caused a drastic reduction in the latter part of the dark period. The drug had no significant effect on noradrenaline or dopamine contents of the hypothalamus. Benserazide treatment caused a large reduction in the noradrenaline content of the cerebral cortex throughout 24 hr. However, this effect is unlikely to be due to a differential penetration of benserazide into the brain areas as a similar degree of decarboxylase inhibition was observed in all three tissues.

Ageing, opioid analgesia and the pineal gland

The effects of ageing on day-night rhythms of analgesia was examined with young (1-2 months), mature (8-12 months) and old (20-30 months) mice. Significant age-related declines were observed both in the absolute levels and diel rhythms of morphine analgesia, with the most pronounced changes occurring at night. Administration of the pineal hormone, melatonin, augmented day-time levels of analgesia in all age classes and reversed the age-related decline in nocturnal morphine analgesia in old mice. Inhibition of pineal function in young mice by either exposure to light pulses or treatment with benserazide mimicked the effects of ageing on nocturnal morphine analgesia. These findings suggest that the pineal gland and melatonin are involved in modulating diel rhythms of analgesia and have an influential role on age-related changes in opioid responses.

The effect of chronic antidepressant administration on beta-adrenoceptor function of the rat pineal

1 The beta-adrenoceptor agonist, isoprenaline (1.5-3.0 mg kg(-1) intravenously), produced a dose-related increase in rat pineal melatonin content. This increase was prevented by pretreatment with the selective beta(1)-adrenoceptor antagonist, atenolol (2 mg kg(-1)), but not by the beta(2)-adrenoceptor antagonist, butoxamine (2 mg kg(-1)). The beta(2)-adrenoceptor agonist, terbutaline (5.0 mg kg(-1)), produced a moderate increase in pineal melatonin content.2 Repeated daily administration of desmethylimipramine (10 mg kg(-1) for 10 days) and maprotiline (10 mg kg(-1) for 10 days), antidepressants predominantly inhibiting noradrenaline (NA) uptake, reduced the isoprenaline-induced increase in pineal melatonin content. Amitriptyline (20 mg kg(-1) for 14 days), a drug which inhibits both NA and 5-hydroxytryptamine (5-HT) uptake, had a similar effect. The beta-adrenoceptor agonist, clenbuterol (5 mg kg(-1) for 14 days), also attenuated the increase in pineal melatonin produced by isoprenaline.3 In contrast, chronic administration of the selective 5-HT uptake inhibitor, fluoxetine (10 mg kg(-1) for 10 days), or the antidepressants, iprindole and mianserin (both 20 mg kg(-1) for 14 days), which do not inhibit monoamine uptake, failed to reduce the increase in pineal melatonin following isoprenaline. Repeated electroconvulsive shock was similarly without effect.4 Ten hours after the final dose of desmethylimipramine (10 mg kg(-1)) once daily for 10 days there was no change in the usual dark phase increase in pineal melatonin.5 The data suggest that repeated administration of certain antidepressant drugs results in reduced pineal beta-adrenoceptor sensitivity. However the lack of change in the dark phase increase in pineal melatonin following repeated desmethylimipramine, implies that the reduced ss-adrenoceptor sensitivity may be part of an adaptive process which maintains normal pineal function. Therefore the decrease in beta-adrenoceptor number in the brain reported after chronic antidepressant administration may not be associated with a change in overall synaptic function.

Effect of benserazide on the levels of pineal 5-hydroxytryptamine, melatonin synthesising enzymes and serum melatonin

The effect of benserazide, an aromatic L-amino acid decarboxylase inhibitor, has been investigated on pineal 5-hydroxytryptamine content, melatonin synthesising enzyme activities and serum melatonin concn. Increasing doses of benserazide caused increasing reductions in the pineal content of 5-hydroxytryptamine. Dark phase serum melatonin concns were also greatly reduced. The drug abolished the diurnal rhythm of hydroxyindole-O-methyltransferase by increasing the light period activities and decreasing the night levels of this enzyme. Pineal N-acetyltransferase was unaffected. It is concluded that benserazide probably inhibits melatonin synthesis by preventing the formation of the substrate, 5-hydroxytryptamine.