Dorotea Muck-Šeler

Influence of Fluoxetine on Regional Serotonin Synthesis in the Rat Brain

Abstract: The aim of the present study was to test the hypothesis that there should be a difference between the effects of an acute and an 8‐day (chronic) administration of fluoxetine (10 mg/kg) on the rate of serotonin [5‐hydroxytryptamine (5‐HT)] synthesis. The 5‐HT synthesis rate was measured in discrete regions of the rat brain using the α‐[14C]methyl‐l‐tryptophan autoradiographic method. The results show that the acute and chronic fluoxetine treatments influence the 5‐HT synthesis rate in different ways. A single dose of fluoxetine induced a significant increase in 5‐HT synthesis in the visual, auditory, and parietal cortices, substantia nigra, hypothalamus, ventral thalamus, and dorsal hippocampus. In contrast, after a chronic treatment a decrease was observed in the substantia nigra, caudate, and nucleus accumbens, the auditory, parietal, sensorimotor, and frontal cortices, and ventral tegmental area. A significant decrease in the rate of 5‐HT synthesis was observed in the dorsal raphe after both the single and chronic treatments. The results suggest that extracellular 5‐HT has a delayed influence on the brain 5‐HT synthesis rate in structures with serotonergic terminals. The findings from the acute study could be important for patients who have just started receiving fluoxetine treatment, as an increase in the 5‐HT synthesis rate might occur in the acute phase of their treatment. In addition, the findings from the chronic treatment study might give us a better understanding of how the brain serotonergic system adapts during a prolonged exposure to extracellular 5‐HT.

dl-Fenfluramine increases the 5-HT synthesis rate in the terminals while decreasing it in the cell bodies of the rat brain

The rate of 5-HT synthesis in discreet rat brain regions was determined using the alpha-[14C]methyl-L-tryptophan autoradiographic method. DL-Fenfluramine (10 mg/kg, i.p.), given 20 min before tracer injection, decreased the rate of 5-HT synthesis in the serotonergic cell bodies (-32% in dorsal and -23% in median raphe nuclei) but increased the rate in almost all the terminal areas investigated when compared to the rate in the control (saline treated) rats. The most pronounced increase was observed in the cortex (% difference of control between +22% and +49% in auditory and parietal-sensory-motor cortex, respectively), striatum (+32% in globus pallidus; +17% median part of caudatus-putamen), superior olive (+36%), dorsal hippocampus (+33%) and ventral thalamus (+29%). Our results suggest that axon terminals respond by increasing 5-HT synthesis, after enhanced release of 5-HT from terminals induced by fenfluramine. This increase in 5-HT synthesis in the terminals probably occurs as part of the compensatory mechanisms that replenish the loss of neurotransmitter from the terminal releasible pool. At the same time our data suggests that the fenfluramine-induced release of 5-HT in the cell bodies inhibits synthesis of the 5-HT through an autoreceptor.

Autoradiographic evaluation of the influence of hypothalamic 5,7-dihydroxytryptamine lesion on brain serotonin synthesis

The influence of a unilateral stereotaxically induced 5,7-dihydroxytryptamine (5,7-DHT) lesion in the dorsolateral hypothalamus on brain serotonin synthesis was evaluated by an autoradiographic method, using labelled alpha-methyl-L-tryptophan (alpha-MTrp). The hypothalamus was selected as the lesion site because it receives well defined and relatively large projections from the raphe nuclei. Data suggest that the unilateral lesion in the dorsolateral hypothalamus had a significant influence (an increase) on the rate of serotonin synthesis in the large majority of ipsilateral brain structures examined. It seems that the effect was the greatest in the hippocampal structures, the thalamus, and the parietal and sensory motor cortices. The average increase in the rate of serotonin synthesis on the lesion side when compared with the contralateral side was between 3% (amygdala) and 52% (dorsal hippocampus; CA3 layer of hippocampus). Since in the sham-injected rats (same volume of saline) there was no obvious injection-contralateral side asymmetry observed (except for two structures, probably affected by the injection needle, which showed a significant difference), we concluded that the effect observed in the present study was most likely related to the 5,7-DHT-induced lesion on the serotonergic terminals in the hypothalamus. Comparison of the rate of synthesis in the dorsal and medial raphe and the pineal body with the rates reported earlier for these structures led us to conclude that either the 5,7-DHT lesion in the hypothalamus did not influence the rates in these structures in their entirety, or the method used was not sensitive enough to reveal this influence. Data reported here also demonstrate how a highly specific tracer (alpha-MTrp), in conjunction with a specific and localized lesion, could aid our understanding of the brain serotonergic system.

The effect of MDMA (3,4-methylenedioxymethamphetamine) on the 5-HT synthesis rate in the rat brain: an autoradiographic study

The effect of MDMA (3,4-methylenedioxymethamphetamine), a psychotropic amphetamine derivative, treatment on the rate of serotonin (5-hydroxytryptamine; 5-HT) synthesis in the rat brain was studied by autoradiography using alpha-[14C]-methyl-l-tryptophan method. Three different treatment protocols were compared to the control (saline) treated rats: (1) rats treated twice with 10 mg/kg every 12 h (20 mg/kg total) and injected tracer for the synthesis measurements 15 h later; (2) rats treated with four injections of 5 mg/kg every 12 h (20 mg/kg total) and injected tracer for the synthesis measurement 17 h after the last dose; and (3) rats given eight injections of 5 mg/kg every 12 h for four days (40 mg/kg) and used in the synthesis study 14 days after the last dose. Results showed a significant decrease in the rate of synthesis in the majority of cerebral structures examined in the 10 mg/kg group. In contrast the group receiving the same total amount (20 mg/kg) of MDMA but over two days (4x5 mg/kg) showed a significant increase in 5-HT synthesis in comparison to controls. The 5-HT synthesis rates measured 14 days after the last dose (four days, 8x5 mg/kg) were significantly reduced. The findings suggest that MDMA can produce either an increase or a decrease in the 5-HT synthesis a short time after a total dose of 20 mg/kg depending on the dose fractionation. However, 14 days after total dose of 40 mg/kg given over four days the synthesis rate was significantly reduced in many brain structures. The latter suggests a possible effect of the MDMA neurotoxicity on the serotonergic neurons, in addition to a possible influence on 5-HT synthesis via a feedback mechanism.

Serotonin Synthesis Increased in Terminals Four Days after Reserpine Treatment: An Autoradiographic Study in Rat Brain

The rate of 5-HT synthesis was determined in discrete rat brain regions 4 days after a single dose of reserpine (10 mg/kg) or reserpine carrier (controls), using an autoradiographic method with labelled α-methyl-L-tryptophan as a tracer. The results show that the rate of 5-HT synthesis was unchanged in the dorsal and median raphe, significantly decreased in the raphe magnus, and significantly increased in areas rich in serotonergic nerve terminals (i.e., hypothalamus, hippocampus, median geniculate body, parietal and visual cortices). An increase in tryptophan hydroxylase activity could account for the increase in the rate of serotonin synthesis seen in some regions. Since the 5-HT synthesis rate showed regional variability there seems to be a need for regional studies of the effect of drugs on the 5-HT synthesis. In addition, the 5-HT synthesis rate was not significantly different from that in controls in many of the brain regions.