The inhibition of tryptophan hydroxylase, not protein synthesis, reduces the brain trapping of alpha-methyl-L-tryptophan: an autoradiographic study

The effects of the tryptophan hydroxylase (TPH) inhibitor p-chlorophenylalanine (PCPA; 200mg/kg; 3 days), and of the protein synthesis inhibitor cycloheximide (CXM, 2mg/kg), on regional serotonin (5-HT) synthesis were studied using the alpha-[14C]methyl-L-tryptophan (alpha-[14C]MTrp) autoradiographic method. The objectives of these investigations were to evaluate the changes, if any, on 5-HT synthesis, as measured with alpha-MTrp method, following the inhibition of TPH by PCPA, or the inhibition of proteins synthesis by CXM. The rats were used in the tracer experiment approximately 24h after the last dose of PCPA was administered, and in the CXM experiments, they were used 30 min following a single injection of CXM. In both experiments, the control rats were injected with the same volume of saline (0.5 ml/kg; s.c.) and at the same times as the drug injections. The results demonstrate that trapping of alpha-MTrp, which is taken to be related to brain 5-HT synthesis, is drastically reduced (40-80%) following PCPA treatment. The inhibition of protein synthesis with CXM did not have a significant effect on the global brain trapping of alpha-MTrp and 5-HT synthesis. These findings suggest that the brain trapping of alpha-[14C]MTrp relates to brain 5-HT synthesis, but not to brain protein synthesis.     

The inhibition of tryptophan hydroxylase, not protein synthesis, reduces the brain trapping of α-methyl-l-tryptophan: an autoradiographic study

The effects of the tryptophan hydroxylase (TPH) inhibitor p-chlorophenylalanine (PCPA; 200mg/kg; 3 days), and of the protein synthesis inhibitor cycloheximide (CXM, 2mg/kg), on regional serotonin (5-HT) synthesis were studied using the alpha-[14C]methyl-L-tryptophan (alpha-[14C]MTrp) autoradiographic method. The objectives of these investigations were to evaluate the changes, if any, on 5-HT synthesis, as measured with alpha-MTrp method, following the inhibition of TPH by PCPA, or the inhibition of proteins synthesis by CXM. The rats were used in the tracer experiment approximately 24h after the last dose of PCPA was administered, and in the CXM experiments, they were used 30 min following a single injection of CXM. In both experiments, the control rats were injected with the same volume of saline (0.5 ml/kg; s.c.) and at the same times as the drug injections. The results demonstrate that trapping of alpha-MTrp, which is taken to be related to brain 5-HT synthesis, is drastically reduced (40-80%) following PCPA treatment. The inhibition of protein synthesis with CXM did not have a significant effect on the global brain trapping of alpha-MTrp and 5-HT synthesis. These findings suggest that the brain trapping of alpha-[14C]MTrp relates to brain 5-HT synthesis, but not to brain protein synthesis.     

Selective 5-HT1B receptor agonist reduces serotonin synthesis following acute, and not chronic, drug administration: results of an autoradiographic study

The effects of acute and chronic administration of the serotonin (5-HT)1B agonist CP-93,129, on 5-HT synthesis rates were evaluated using the alpha-[14C]methyl-L-tryptophan (alpha-MTrp) autoradiographic method. In the acute treatment study, CP-93,129 (7 mg/kg) was injected intraperitoneally 30 min before the alpha-MTrp injection (30 microCi over 2 min). A single dose of CP-93,129 caused a significant increase in the synthesis in the median raphe nucleus (MR) without a significant influence on the dorsal raphe nucleus (DR). There was a reduction in 5-HT synthesis in almost all of the projection areas. In the chronic treatment study, CP-93,129 was administered continuously (7 mg/kg/day) for 14 days using an osmotic minipump implanted subcutaneously. The chronic treatment with CP-93,129 did not produce a significant change in 5-HT synthesis in the raphe nuclei nor in the nerve terminal structures, except for the medial frontal bundle and the visual and sensory-motor cortices. The unaltered 5-HT synthesis rates in the chronic treatment study probably reflect a normalization of the synthesis as a result of the desensitization of 5-HT1B autoreceptors and/or heteroreceptors.     

Effects of pentadecapeptide BPC157 on regional serotonin synthesis in the rat brain: alpha-methyl-L-tryptophan autoradiographic measurements

A novel pentadecapeptide, BPC157, was recently reported to have a large spectrum of in vivo activities, from anti-ulcer to central action on the brain dopaminergic system. The mechanisms of these actions are not well understood. In this study, the evaluation of the effects of acute and repeated administration of BPC157 on serotonin (5-HT) synthesis in the rat brain is reported. The alpha-[14C]methyl-L-tryptophan (alpha-MTrp) autoradiographic method was used to measure regional 5-HT synthesis rates. In the first series of experiments, a single dose treatment of BPC157 (10 microg/kg) administered intraperitoneally 40 min before the alpha-MTrp tracer injection significantly reduced the regional rate of 5-HT synthesis in the dorsal thalamus, hippocampus, lateral geniculate body and hypothalamus. 5-HT synthesis rates in the substantia nigra reticulate and medial anterior olfactory nucleus in BPC157 treated rats were significantly higher than in the control rats. No significant change in the synthesis rate was observed in the raphe nuclei. In the second series of experiments, following a 7-day treatment with BPC157 (10 microg/kg; s.c.), a significant reduction in the 5-HT synthesis rate was observed in the dorsal raphe nucleus, and significant increases were observed in the substantia nigra, lateral caudate, accumbens nucleus and superior olive. This data suggests that BPC157, a gut peptide, influences brain 5-HT synthesis in rats, but we cannot determine, from this data, the mechanism of this action.     

Acute flesinoxan treatment produces a different effect on rat brain serotonin synthesis than chronic treatment: an alpha-methyl-l-tryptophan autoradiographic study

5-HT(1A) receptor agonists display anxiolytic and anti-depressant properties in clinical studies. In this study, we used the alpha-[(14)C]methyl-l-tryptophan (alpha-MTrp) autoradiographic method to evaluate the effects of the 5-HT(1A) agonist, flesinoxan, on regional 5-HT synthesis in the rat brain, following acute or a 14-day continuous treatment. In the first series of experiments, flesinoxan (5mg/kg; i.p.) was administered 40min before the alpha-MTrp. It resulted in a significant increase of the arterial blood oxygen partial pressure (pO(2)) and a reduction of the regional rate of 5-HT synthesis throughout the brain, with the exception of a few regions (medial geniculate body and thalamus). In the second series of experiments, flesinoxan (5mg/kgday) was administered for 14 days, using an osmotic minipump implanted subcutaneously. When compared to rats treated with saline, there was an overall significant (p<0.05) reduction in the synthesis (one-sample two-tailed t-test). However, there was no significant influence on the 5-HT synthesis rate in the dorsal and median raphe nuclei and the majority of their projection areas. A significant (p<0.05) reduction was observed in the nucleus raphe magnus, medial caudate, ventral thalamus, amygdala, ventral tegmental area, medial forebrain bundle, nucleus accumbens, medial anterior olfactory nucleus and superior olive. The unaltered 5-HT synthesis rates in a large majority of regions following the 14-day treatment of flesinoxan may reflect the normalization (implies to not be different from salne treated control) of synthesis due to a desensitization of 5-HT(1A) autoreceptors on the cell body of 5-HT neurons as well as at postsynaptic sites, which is known to occur following long-term treatment with 5-HT(1A) agonists. It is of some importance to note that the normalization of the synthesis occurred in the majority of the brain limbic structures, the brain areas implicated in affective disorders and the corresponding successful treatments, as well as in the cortical regions, which are implicated in mood. However, there were some terminal regions (e.g., accumbens, anterior olfactory, lateral thalamus, raphe magnus and obscurus) in which the chronic flesinoxan treatment resulted in a significant reduction of synthesis, suggesting that there was not a full desensitization across the brain of the receptors controlling 5-HT synthesis.     

Effects of serotine receptors agonists, TFMPP and CGS12066B, on regional serotonin synthesis in the rat brain: an autoradiographic study

The effects of acute and repeat administration of the serotonin (5-HT)(1) agonists TFMPP [N -(3-trifluoromethyl)phenylpiperazine hydrochloride] and CGS12066B [7-trifluoromethyl-4- (4-methyl-1-piperazinyl)pyrrolo[1,2-a ]-quinoxaline dimaleate] were evaluated on 5-HT synthesis rates using the alpha-[(14) C]methyl-l-tryptophan (alpha-MTrp) autoradiographic method. In the acute treatment study, TFMPP (10 mg/kg) and CGS12066B (5 mg/kg) were injected intraperitoneally 30 min before an alpha-MTrp injection. In an acute study TFMPP reduced overall brain 5-HT synthesis, in the dorsal and median raphe, and in almost all of their projection areas, with the exception of the parietal, sensory-motor, and frontal cortices, the accumbens nucleus, and the caudate. Acute CGS12066B treatment did not have overall significant effect, but the rates did decrease in the cell body areas of 5-HT neurons. In a 7-day treatment with TFMPP (10 mg/kg/day) or CGS12066B (5 mg/kg/day), the 5-HT synthesis rates (24 h after last dose) decrease, with both compounds, in almost all of the nerve terminal structures. TFMPP reduced the synthesis in the dorsal and median raphe, while CGS12066B reduced it only in the dorsal raphe. This data suggests that after a 7-day treatment with TFMPP and CGS12066B, the rate of 5-HT synthesis in the dorsal raphe is restored and is reduced in many projection areas. The observed effects in the 7-day treatment could also be related to actions through the postsynaptic 5-HT(1B) sites and/or other 5-HT receptors since this compounds have limited selectivity.     

Acute and Chronic D-Fenfluramine Treatments Have Different Effects on Serotonin Synthesis Rates in the Rat Brain: An Autoradiographic Study

The effects of acute and chronic treatments with D-fenfluramine on the regional rates of serotonin (5-hydroxy-tryptamine; 5-HT) synthesis were investigated using the -[¹⁴C]methyl-L-tryptophan (-[¹⁴C]MTrp) autoradiographic method. In the first series of experiments, acute D-fenfluramine treatment (5 mg/kg; i.p.) given 20 min before the tracer injection significantly (p < 0.05) decreased 5-HT synthesis in the dorsal raphe, and significantly (p < 0.05) increased the rates in the cerebral cortices and caudate nucleus, when compared to the rates in the control rats (saline treated). In a second series of experiments, following a 7-day treatment with D-fenfluramine (5 mg/kg/day; i.p.), a significant (p < 0.05) decrease of 5-HT synthesis, in the dorsal raphe was observed, and significant (p < 0.05) increases were observed in the hypothalamus, the dorsal thalamus, the medial and lateral geniculate body and some brain stem regions (locus ceruleus, inferior and superior colliculus). No significant changes were observed in the cerebral cortices.     

Effects of desipramine on regional serotonin synthesis in the rat brain: acute and chronic autoradiographic studies

Various studies have implicated the involvement of noradrenaline (NA) and/or serotonin (5-hydroxytryptamine (5-HT)) in the pathogenesis and treatment of depression. The aim of the present study was to investigate the effects of acute and 7 days of administration of desipramine, a NA re-uptake inhibitor, on the rate of 5-HT synthesis in the rat brain. The study was done by an autoradiographic method using alpha-[14C]-methyl-L-tryptophan as a tracer. The acute (10mg/kg, i.p., 2h before i.v. infusion of the tracer) or 7 days of desipramine (10mg/kg per day, i.p.) did not affect plasma tryptophan (Trp) concentrations, as compared to control (saline treated) rats. Acute treatment with desipramine decreased the rate of 5-HT synthesis in the brain regions that contain 5-HT cell bodies between 19 and 28%, and increased the rate of 5-HT synthesis in the majority of areas containing 5-HT terminals between 21 and 65%. In contrast to the acute treatment, a 7-day administration increased 5-HT synthesis rates in the dorsal raphe (24%), but decreased it in raphe magnus (35%), superior olive (45%), caudate (31%), superior (38%) and inferior (53%) colliculus, and in the auditory cortex (35%). This suggests that the effect of desipramine on 5-HT synthesis rate is time-dependent and differs in the cell bodies and structures containing 5-HT nerve terminals.     

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.     

Effects of chronic paroxetine treatment on dialysate serotonin in 5-HT1B receptor knockout mice

The role of serotonin (5-HT)1B receptors in the mechanism of action of selective serotonin re-uptake inhibitors (SSRI) was studied by using intracerebral in vivo microdialysis in conscious, freely moving wild-type and 5-HT1B receptor knockout (KO 5-HT1B) mice in order to compare the effects of chronic administration of paroxetine via osmotic minipumps (1 mg per kg per day for 14 days) on extracellular 5-HT levels ([5-HT]ext) in the medial prefrontal cortex and ventral hippocampus. Basal [5-HT]ext values in the medial prefrontal cortex and ventral hippocampus, approximately 20 h after removing the minipump, were not altered by chronic paroxetine treatment in both genotypes. On day 15, in the ventral hippocampus, an acute paroxetine challenge (1 mg/kg i.p.) induced a larger increase in [5-HT]ext in saline-pretreated mutant than in wild-type mice. This difference between the two genotypes in the effect of the paroxetine challenge persisted following chronic paroxetine treatment. Conversely, in the medial prefrontal cortex, the paroxetine challenge increased [5-HT]ext similarly in saline-pretreated mice of both genotypes. Such a challenge produced a further increase in cortical [5-HT]ext compared with that in saline-pretreated groups of both genotypes, but no differences were found between genotypes following chronic treatment. To avoid the interaction with raphe 5-HT1A autoreceptors, 1 micro m paroxetine was perfused locally through the dialysis probe implanted in the ventral hippocampus; similar increases in hippocampal [5-HT]ext were found in acutely or chronically treated wild-type mice. Systemic administration of the mixed 5-HT1B/1D receptor antagonist GR 127935 (4 mg/kg) in chronically treated wild-type mice potentiated the effect of a paroxetine challenge dose on [5-HT]ext in the ventral hippocampus, whereas systemic administration of the selective 5-HT1A receptor antagonist WAY 100635 did not. By using the zero net flux method of quantitative microdialysis in the medial prefrontal cortex and ventral hippocampus of wild-type and KO 5-HT1B mice, we found that basal [5-HT]ext and the extraction fraction of 5-HT were similar in the medial prefrontal cortex and ventral hippocampus of both genotypes, suggesting that no compensatory response to the constitutive deletion of the 5-HT1B receptor involving changes in 5-HT uptake capacity occurred in vivo. As steady-state brain concentrations of paroxetine at day 14 were similar in both genotypes, it is unlikely that differences in the effects of a paroxetine challenge on hippocampal [5-HT]ext are due to alterations of the drug's pharmacokinetic properties in mutants. These data suggest that there are differences between the ventral hippocampus and medial prefrontal cortex in activation of terminal 5-HT1B autoreceptors and their role in regulating dialysate 5-HT levels. These presynaptic receptors retain their capacity to limit 5-HT release mainly in the ventral hippocampus following chronic paroxetine treatment in mice.     

Acute flesinoxan treatment produces a different effect on rat brain serotonin synthesis than chronic treatment: An α-methyl-l-tryptophan autoradiographic study

5-HT_1A receptor agonists display anxiolytic and anti-depressant properties in clinical studies. In this study, we used the α-[¹⁴C]methyl-l-tryptophan (α-MTrp) autoradiographic method to evaluate the effects of the 5-HT_1A agonist, flesinoxan, on regional 5-HT synthesis in the rat brain, following acute or a 14-day continuous treatment. In the first series of experiments, flesinoxan (5 mg/kg; i.p.) was administered 40 min before the α-MTrp. It resulted in a significant increase of the arterial blood oxygen partial pressure (pO₂) and a reduction of the regional rate of 5-HT synthesis throughout the brain, with the exception of a few regions (medial geniculate body and thalamus). In the second series of experiments, flesinoxan (5 mg/kg day) was administered for 14 days, using an osmotic minipump implanted subcutaneously. When compared to rats treated with saline, there was an overall significant (p < 0.05) reduction in the synthesis (one-sample two-tailed t-test). However, there was no significant influence on the 5-HT synthesis rate in the dorsal and median raphe nuclei and the majority of their projection areas. A significant (p < 0.05) reduction was observed in the nucleus raphe magnus, medial caudate, ventral thalamus, amygdala, ventral tegmental area, medial forebrain bundle, nucleus accumbens, medial anterior olfactory nucleus and superior olive. The unaltered 5-HT synthesis rates in a large majority of regions following the 14-day treatment of flesinoxan may reflect the normalization (implies to not be different from salne treated control) of synthesis due to a desensitization of 5-HT_1A autoreceptors on the cell body of 5-HT neurons as well as at postsynaptic sites, which is known to occur following long-term treatment with 5-HT_1A agonists. It is of some importance to note that the normalization of the synthesis occurred in the majority of the brain limbic structures, the brain areas implicated in affective disorders and the corresponding successful treatments, as well as in the cortical regions, which are implicated in mood. However, there were some terminal regions (e.g., accumbens, anterior olfactory, lateral thalamus, raphe magnus and obscurus) in which the chronic flesinoxan treatment resulted in a significant reduction of synthesis, suggesting that there was not a full desensitization across the brain of the receptors controlling 5-HT synthesis.     

Regional brain serotonin synthesis is increased in the olfactory bulbectomy rat model of depression: an autoradiographic study

Serotonin synthesis rates were evaluated using alpha-[14C]methyl-l-tryptophan (alpha-MTrp) autoradiographic methods in olfactory bulbectomized (OBX) rats. They were significantly (p < 0.05) increased in the frontal (50%) and parietal (40%) cortices, superior olive (over 30%), and the substantia nigra (30%) in the OBX rats as compared to the sham operated animals. There were also increases in 5-hydroxytryptamine (5-HT) synthesis in some limbic areas: the cingulate (32%), the medial forebrain bundle (58%), the hippocampus (13-25%) and the thalamus (22-40%). The largest increase in 5-HT synthesis after OBX was observed in the sensory-motor cortex (67%). 5-HT synthesis rates were significantly decreased in the dorsal and medial raphe nuclei, but there was no significant change the ventral tegmental area and the locus coeruleus following OBX. These results indicate that olfactory bulbectomy causes an imbalance in 5-HT synthesis in some projection areas by disproportionally increasing 5-HT synthesis rates in specific brain regions and making more 5-HT available for neurotransmission. This imbalance in 5-HT synthesis and the subsequent elevation of tissue 5-HT may be responsible for the creation of non-physiological circuitry which may, in part, be reflected in the symptoms resembling human depression.     

Flesinoxan challenge suggests that chronic treatment with paroxetine in rats does not desensitize receptors controlling 5-HT synthesis

It has been proposed that the desensitization of 5-HT_1A (5-hydroxytryptamine; serotonin) receptors following chronic therapy with selective serotonin reuptake inhibitors (SSRIs) is necessary for their therapeutic efficacy. Stimulation of the 5-HT_1A receptors decreases serotonin (5-HT) synthesis and release, but it is not clear if the receptors are fully desensitized following chronic SSRI treatment. The main objective of this study was evaluation of ability of 5-HT_1A receptors to modulate 5-HT synthesis after 14-day paroxetine treatment. 5-HT_1A receptor sensitivity following chronic administration of the SSRI paroxetine was assessed by the ability of an acute challenge with the 5-HT_1A agonist, flesinoxan, to modulate 5-HT synthesis in the rat brain. The rates of 5-HT synthesis were measured using the α-[¹⁴C]methyl-l-tryptophan autoradiographic method. The rats were treated for 2 weeks with paroxetine (10 mg/(kg day), s.c., delivered by osmotic minipump). After this treatment, the rats received an acute challenge with flesinoxan (5 mg/kg, i.p.), while the control rats were injected with the vehicle. Forty minutes following the flesinoxan injection, the tracer, α-[¹⁴C]methyl-l-tryptophan, was injected over 2 min. 5-HT synthesis rates were calculated from autoradiographically measured tissue tracer concentrations and plasma time–activity curves. The results demonstrated that the acute flesinoxan challenge produced a significant decrease in 5-HT synthesis rates throughout the rat brain. The greatest decrease was observed in the ventral hippocampus, somatosensory cortex and the ascending serotonergic cell bodies. In comparison with data reported on an acute challenge with flesinoxan in naïve rats (rats without any other treatment), the results presented here suggest a greater effect of flesinoxan on synthesis reduction in rats chronically treated with paroxetine. The results also suggest that the 5-HT receptors were not fully desensitized by paroxetine treatment, and that the stimulation of 5-HT_1A receptors with an agonist is still capable of reducing 5-HT synthesis.     

Nicotine-induced changes in the metabolism of specific brain proteins

The effect of acute and chronic nicotine on the metabolism of specific brain proteins was examined by measuring incorporation of labeled valine into protein, with densitometric scanning of proteins resolved by gel electrophoresis. Acute and chronic administration of nicotine (0.4 mg/kg per 30 min for 2 hours, s.c., or 0.5 mg/kg per 30 min for 5 days (Alzet mini-pump implanted subcutaneously) reduced incorporation of [¹⁴C]valine administered by approximately 6–7%. The results with chronic nicotine administration indicated a lack of tolerance for this effect of nicotine. Mecamylamine, a nicotinic ganglionic antagonist, does not seem to block the inhibition of protein synthesis. Small increases in protein content were observed in a high- and a low-molecular-weight region of SDS-polyacrylamide gel, used to separate proteins from newborn brain. In adult brain after chronic nicotine administration, selective increases and a decrease were seen in selective bands. Results are consonant with selective effects of nicotine on the synthesis or degradation of specific brain proteins.Special Issue Dedicated to Dr. Abel Lajtha.     

Flesinoxan challenge suggests that chronic treatment with paroxetine in rats does not desensitize receptors controlling 5-HT synthesis

It has been proposed that the desensitization of 5-HT_1A (5-hydroxytryptamine; serotonin) receptors following chronic therapy with selective serotonin reuptake inhibitors (SSRIs) is necessary for their therapeutic efficacy. Stimulation of the 5-HT_1A receptors decreases serotonin (5-HT) synthesis and release, but it is not clear if the receptors are fully desensitized following chronic SSRI treatment. The main objective of this study was evaluation of ability of 5-HT_1A receptors to modulate 5-HT synthesis after 14-day paroxetine treatment. 5-HT_1A receptor sensitivity following chronic administration of the SSRI paroxetine was assessed by the ability of an acute challenge with the 5-HT_1A agonist, flesinoxan, to modulate 5-HT synthesis in the rat brain. The rates of 5-HT synthesis were measured using the α-[¹⁴C]methyl-l-tryptophan autoradiographic method. The rats were treated for 2 weeks with paroxetine (10 mg/(kg day), s.c., delivered by osmotic minipump). After this treatment, the rats received an acute challenge with flesinoxan (5 mg/kg, i.p.), while the control rats were injected with the vehicle. Forty minutes following the flesinoxan injection, the tracer, α-[¹⁴C]methyl-l-tryptophan, was injected over 2 min. 5-HT synthesis rates were calculated from autoradiographically measured tissue tracer concentrations and plasma time–activity curves. The results demonstrated that the acute flesinoxan challenge produced a significant decrease in 5-HT synthesis rates throughout the rat brain. The greatest decrease was observed in the ventral hippocampus, somatosensory cortex and the ascending serotonergic cell bodies. In comparison with data reported on an acute challenge with flesinoxan in naïve rats (rats without any other treatment), the results presented here suggest a greater effect of flesinoxan on synthesis reduction in rats chronically treated with paroxetine. The results also suggest that the 5-HT receptors were not fully desensitized by paroxetine treatment, and that the stimulation of 5-HT_1A receptors with an agonist is still capable of reducing 5-HT synthesis.     

Feedback control of rat brain 5-hydroxytryptamine synthesis

Abstract— The effect of increased levels of 5-hydroxytryptamine (5-HT) on the synthesis of [³H]5-HT from intracisternally injected tracer doses of [³H]tryptophan was studied in the rat brain stem. The [³H]5-HT which accumulated in the first 15 min after [³H]tryptophan injection was measured at various times after the acute intraperitoneal administration of the monoamine oxidase inhibitors Catron or Pargyline. The 5-HT levels reached two and three times control values respectively at 20 min and 180 min after monoamine oxidase inhibitor administration but [³H]5-HT accumulation was decreased (40 per cent) at 180 min when compared with 20 min. These data as well as those obtained after chronic treatment with monoamine oxidase inhibitors revealed that there is an inverse relationship between [³H]5-HT accumulation and the endogenous 5-HT level. Monoamine oxidase activity was undetectable during all the intervals in which [³H]5-HT accumulation was measured. No inhibition of [³H]5-HT accumulation was detected when [³H]5-hydroxytryptophan was injected instead of [³H]tryptophan. The results are consistent with a negative feedback of 5-HT synthesis at the rate-limiting tryptophan hydroxylation step.     

Propranolol and serotonin removal in lung injury

Indicator dilution technique was used to study effects of reduced vascular volume or acute injury on removal of low doses of [3H]propranolol and [14C]serotonin (5-hydroxytryptamine, 5-HT) by perfused rabbit lung. Glass-bead (500 micron) embolization doubled pulmonary arterial pressure (Ppa) at flow rates of 20, 50, and 100 ml/min, decreased volume of distribution by approximately 50%, and increased pulmonary vascular resistance by at least 60%. Before embolization, (flow rate 20 ml/min) removal of [3H]propranolol and [14C] 5-HT was 89 +/- 2 and 75 +/- 5%, respectively, and was unaltered by changes in flow rate. However, after embolization, [3H]propranolol and [14C]5-HT removal decreased in a flow-dependent manner, reaching 28 +/- 4 and 1 +/- 3% (P less than 0.05), respectively, at a flow rate of 100 ml/min. When phorbol myristate acetate (PMA, 200 nM) was perfused (50 ml/min) through the lungs for 15 min, Ppa increased from 13 +/- 1 to 25 +/- 2 cmH2O (P less than 0.05), whereas [3H]propranolol removal decreased from 92 +/- 1 to 75 +/- 5% (P less than 0.05) and [14C]5-HT removal decreased from 73 +/- 3 to 46 +/- 8% (P less than 0.05). The PMA also caused vasoconstriction, which could be partially blocked by adding papaverine (500 microM) to the perfusion medium. Under the latter conditions, Ppa increased to 19 +/- 1 cmH2O and [3H]propranolol removal was unaffected. However, the combination of PMA and papaverine reduced [14C]5-HT removal from 64 +/- 4 to 19 +/- 3%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Both acute and chronic buspirone treatments have different effects on regional 5-HT synthesis in Flinders Sensitive Line rats (a rat model of depression) than in control rats

The main objective of this investigation was to evaluate the effects of buspirone, a 5-HT_1A agonist with some partial agonist properties and also an antidepressant, on regional 5-HT synthesis in Flinders Sensitive Line (FSL) rats (“depressed”), and to compare the effects to the Flinders Resistant Line (FRL) control rats (not “depressed”). In addition results were compared to those previously reported in normal Sprague–Dawley (SPD) rats (normal control). Serotonin synthesis in both FSL and FRL rats was measured following acute and chronic treatments with buspirone. Both of these strains were derived from the SPD rats. No direct comparison was done between the FSL saline and FRL saline groups, or the FSL buspirone and FRL buspirone groups, because the objective of the studies was to evaluate effects of buspirone in these two strains. The results show that acute treatment with buspirone elevates 5-HT synthesis throughout the brain in the FRL rats. In the FSL rats, there were reductions in some brain regions (e.g., dorsal and median raphe, amygdala, anterior olfactory nucleus, substantia nigra reticulate), while in other regions, there were increases in the synthesis observed (e.g., frontal, parietal, visual and somatosensory cortices, ventral hippocampus). In 20 out of the 30 brain regions investigated in the FSL rats, there was no significant change in the synthesis following acute buspirone treatment. During the chronic treatment, buspirone produced a significant reduction of 5-HT synthesis in 15 out of 30 brain regions in the FRL rats. In the FSL rats, buspirone produced a significant elevation of the synthesis in 10 out of 30 brain regions. In both the FSL and FRL rats, buspirone produced rather different effects than those reported previously for SPD (normal) rats. The acute effect in the FSL rats was somewhat similar to the effect reported previously for the SPD rats, while in the FRL rats, the acute buspirone treatment produced an effect observed previously in treatments with 5-HT_1A antagonists suggesting an action of buspirone as partial agonist in FRL rats. The data suggest that with respect to 5-HT synthesis, FRL rats differ from SPD rats (a natural control; normal rats) and, as such, indicate that when the effects related to the serotonergic system (e.g., influence of serotonergic drugs) are studied in the FSL rats and compared to those in the FRL rats, any conclusions drawn may not reflect differences relative to a normal rat.     

Synthesis and pharmacological study of radioiodinated serotonin derivative specific of 5-HT1B and 5-HT1D binding sites of the central nervous system

We describe here the synthesis of a new serotonin conjugate, S-CM-GTNH2, and its radioiodinated derivative. Quantitative autoradiographic studies on rat and guinea pig brain sections incubated with 2 nM [3H]5-HT showed a preferential affinity of S-CM-GTNH2 for 5-HT1B and 5-HT1D sites. Autoradiograms from brain sections incubated with 0.02 nM S-CM-G[125I]TNH2 showed a heterogeneous anatomical distribution of the labelling with high densities in regions rich in 5-HT1B or 5-HT1D binding sites, and with no labelling of those rich in 5-HT1A or 5-HT1C sites. The pharmacological profiles of the binding sites corresponded to those of 5-HT1B and 5-HT1D receptor subtypes. The radioligand S-CM-G[125I]TNH2 is a good probe for the study of these sites and will be used for their subcellular localization in electron microscopy.     

Effects of acute and chronic administrations of phencyclidine on the levels of serotonin and 5-hydroxyindoleacetic acid in discrete brain areas of mouse

The effects of phencyclidine (PCP) on the levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in discrete brain areas of mouse were investigated. Following a single administration, PCP significantly increased at 60 min the level of 5-HT but not 5-HIAA in the cortex. However, acute administration of PCP induced no changes of 5-HT and 5-HIAA levels in other brain areas investigated. On the other hand, chronic treatment of PCP produced a significant increase the striatal 5-HT and 5-HIAA levels by about 30% and 20%, respectively. These increased levels were gradually returned to the control levels, and there was no difference of these levels between the control group and the 48 hr withdrawal group. The changes of 5-HT level in the hypothalamus were similar to those in the striatum. These results suggest that the pharmacological actions of PCP and tolerance development to PCP may be related to the functional changes of serotonergic neuronal activity.