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.     

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.     

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 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.     

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.     

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.     

Brain 5-HT synthesis in the Flinders Sensitive Line rat model of depression: an autoradiographic study

Alterations of serotonin (5-HT) levels and serotonergic transmission have been associated with depression. 5-HT synthesis is an important factor of serotonergic neurotransmission that may also be altered in depression. Many studies of the relationships between brain serotonergic functions and affective disorders have been performed in different animal models. In this study, brain regional 5-HT synthesis was examined using the alpha-[(14)C]methyl-L-tryptophan (alpha-MTrp) autoradiographic method in a genetic rat model of depression, Flinders Sensitive Line (FSL) rats, and was compared to both the Flinders Resistant Line (FRL) rats and the control Sprague-Dawley (SD) rats. The plasma concentration of free tryptophan in the FSL rats was not significantly different (p > 0.05; ANOVA and post-hoc Bonferroni correction) when compared to that of the FRL and SD rats. The FSL rats had significantly lower 5-HT synthesis (one sample two-tailed t-test on the ratio) than both the FRL and SD rats (the mean ratios were 0.78 +/- 0.12 and 0.73 +/- 0.15, respectively). Overall, the 5-HT synthesis in the FRL rats was not significantly different (p > 0.05) from that in the SD rats (one sample two-tailed t-test on the ratio and the mean ratio was 0.93 +/- 0.13). Studies of individual brain structures, such as the raphe nuclei and their many terminal areas, including the nucleus accumbens, cingulate and frontal cortex, hippocampus, amygdala, and thalamus revealed significant reductions (typically 25-50%) in 5-HT synthesis in the FSL rats compared to the non-depressive FRL and SD rats. These results suggest that significantly reduced 5-HT synthesis in the raphe nuclei and limbic areas in FSL rats may contribute to their depressive features.     

Administration of 8-Hydroxy-2-(Di-n-Propylamino)tetralin in Raphe Nuclei Dorsalis and Medianus Reduces Serotonin Synthesis in the Rat Brain: Differences in Potency and Regional Sensitivity

Following administration of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.04-5.0 micrograms/0.5 microliter) in the raphe nucleus dorsalis (DR) or medianus (MR), the synthesis of serotonin (5-HT), as assessed by the accumulation of 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition, was measured in various regions of the rat CNS. At all doses, 8-OH-DPAT in the DR significantly reduced 5-HTP accumulation in the striatum, nucleus accumbens, cortex, and prefrontal cortex, whereas even the highest dose had no effect in the hippocampus, hypothalamus, and spinal cord. One microgram of 8-OH-DPAT in the MR significantly reduced 5-HTP accumulation in the nucleus accumbens and prefrontal cortex, and 5 micrograms had an effect in all the areas except the striatum and spinal cord. One and 5 micrograms of 8-OH-DPAT, administered in either the DR or MR, did not significantly modify the accumulation of dihydroxyphenylalanine in the striatum and nucleus accumbens. The results confirm that DR and MR have different sensitivities to 5-HT1A receptor agonists, and that activation of 5-HT1A receptors in these nuclei produces different effects on 5-HT synthesis in different brain regions.     

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.     

Flesinoxan Dose-Dependently Reduces Extracellular 5-Hydroxytryptamine (5-HT) in Rat Median Raphe and Dorsal Hippocampus Through Activation of 5-HT1A Receptors

Abstract: The effects of systemic administration of the serotonin (5-hydroxytryptamine) 5-HT_1A receptor agonists flesinoxan and 8-hydroxy-2-(di- n -propylamino)tetralin on extracellular 5-HT were measured using microdialysis probes in both median raphe nucleus and dorsal hippocampus. Both 5-HT_1A agonists dose-dependently decreased dialysate 5-HT levels from both brain regions. The effects of flesinoxan in the median raphe (0.3 mg/kg) and dorsal hippocampus (1.0 mg/kg) could be blocked by the 5-HT_1A receptor antagonist N -[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]- N -(2-pyridyl)cyclohexane carboxamide trihydrochloride (WAY 100,635) at a dose of 0.05 mg/kg s.c. The antagonist itself had no effect at this dosage. Local perfusion of flesinoxan for 30 min through the dialysis probe into the median raphe region at concentrations of 20, 100, and 1,000 n M resulted in a significant decrease in dialysate 5-HT content from both regions. The effect of 100 n M flesinoxan could be blocked by coperfusion of 1,000 n M WAY 100,635. The data indicate that flesinoxan is a potent 5-HT_1A receptor agonist and also support the notion that somatodendritic 5-HT_1A autoreceptors regulate both terminal and somatodendritic 5-HT release.     

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.     

Effect of the 5-HT1A receptor agonist 8-OH-DPAT on the release of 5-HT in dorsal and median raphe-innervated rat brain regions as measured by in vivo microdialysis.

Recent electrophysiological studies, measurements of 5-HT synthesis and in vivo voltammetry recordings of 5-HT metabolism have suggested that serotoninergic neurones in the median raphe (MR) are less sensitive to 5-HT1A autoreceptor stimulation relative to those in the dorsal raphe (DR). To further study the putative differences in regulation between ascending 5-HT projections from the raphe nuclei we have used microdialysis to measure the release of 5-HT in ventral hippocampus, globus pallidus, dorsal hippocampus, frontal cortex, nucleus accumbens and medial septum, following systemic administration of the specific 5-HT1A agonist 8-OH-DPAT. The results show that the baseline output of 5-HT was similar in each of the areas studied. While 8-OH-DPAT decreased dialysate levels of 5-HT in all areas, the inhibition of 5-HT release seen in globus pallidus was significantly less marked compared to that observed in the other five regions. The results indicate that 5-HT1A autoreceptor-mediated control of 5-HT release is functional in all of the brain areas studied, including those receiving a preferential 5-HT innervation from the DR and MR. We find little evidence in support of the idea that brain 5-HT neuronal projections are heterogenous with respect to 5-HT1A autoreceptor regulation of 5-HT release; the globus pallidus, however representing a possible exception to this.     

Altered serotonin and dopamine metabolism in the CNS of serotonin 5-HT(1A) or 5-HT(1B) receptor knockout mice

Measurements of serotonin (5-HT), dopamine (DA), and noradrenaline, and of 5-HT and DA metabolites, were obtained by HPLC from 16 brain regions and the spinal cord of 5-HT(1A) or 5-HT(1B) knockout and wild-type mice of the 129/Sv strain. In 5-HT(1A) knockouts, 5-HT concentrations were unchanged throughout, but levels of 5-HT metabolites were higher than those of the wild type in dorsal/medial raphe nuclei, olfactory bulb, substantia nigra, and locus coeruleus. This was taken as an indication of increased 5-HT turnover, reflecting an augmented basal activity of midbrain raphe neurons and consequent increase in their somatodendritic and axon terminal release of 5-HT. It provided a likely explanation for the increased anxious-like behavior observed in 5-HT(1A) knockout mice. Concomitant increases in DA content and/or DA turnover were interpreted as the result of a disinhibition of DA, whereas increases in noradrenaline concentration in some territories of projection of the locus coeruleus could reflect a diminished activity of its neurons. In 5-HT(1B) knockouts, 5-HT concentrations were lower than those of the wild type in nucleus accumbens, locus coeruleus, spinal cord, and probably also several other territories of 5-HT innervation. A decrease in DA, associated with increased DA turnover, was measured in nucleus accumbens. These changes in 5-HT and DA metabolism were consistent with the increased aggressiveness and the supersensitivity to cocaine reported in 5-HT(1B) knockout mice. Thus, markedly different alterations in CNS monoamine metabolism may contribute to the opposite behavioral phenotypes of these two knockouts.     

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.     

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.     

Lack of CB1 receptor activity impairs serotonergic negative feedback

Serotonergic and endocannabinoid systems are important substrates for the control of emotional behaviour and growing evidence show an involvement in the pathophysiology of mood disorders. In the present study, the absence of the activity of the CB₁ cannabinoid receptor impaired serotonergic negative feedback in mice. Thus, in vivo microdialysis experiments revealed increased basal 5-HT extracellular levels and attenuated fluoxetine-induced increase of 5-HT extracellular levels in the prefrontal cortex of CB₁ knockout compared with wild-type mice. These observations could be related to the significant reduction in the 5-HT transporter binding site density detected in frontal cortex and hippocampus of CB₁ knockout mice. The lack of CB₁ receptor also altered some 5-HT receptors related to the 5-HT feedback. Extracellular recordings in the dorsal raphe nucleus (DRN) revealed that the genetic and pharmacological blockade of CB₁ receptor induced a 5-HT_1A autoreceptor functional desensitization. In situ hybridization studies showed a reduction in the expression of the 5-HT_2C receptor within several brain areas related to the control of the emotional responses, such as the DRN, the nucleus accumbens and the paraventricular nucleus of the hypothalamus, whereas an over-expression was observed in the CA3 area of the ventral hippocampus. These results reveal that the lack of CB₁ receptor induces a facilitation of the activity of serotonergic neurons in the DRN by altering different components of the 5-HT feedback as well as an increase in 5-HT extracellular levels in the prefrontal cortex in mice.     

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 -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.     

Changes of Biogenic Amines and Their Metabolites in Postmortem Brains from Patients with Alzheimer-Type Dementia

Noradrenaline (NA), 3,4-dihydroxyphenylethylamine (dopamine, DA), 5-hydroxytryptamine (serotonin, 5-HT), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) were measured in 22 regions of postmortem brains from four histologically verified cases with Alzheimer-type dementia (ATD) and nine histologically normal controls. Compared with the controls, concentrations of 5-HT and 5-HIAA in the ATD brains were significantly reduced in nine regions (superior frontal gyrus, insula, cingulate gyrus, amygdala, putamen, medial and lateral segments of globus pallidus, substantia nigra, lateral nucleus of thalamus) and in eight regions (amygdala, substantia innominata, caudate, putamen, medial and lateral segments of globus pallidus, medial and lateral nuclei of thalamus), respectively. NA concentrations of the ATD brains were significantly reduced in six regions (cingulate gyrus, substantia innominata, putamen, hypothalamus, medial nucleus of thalamus, raphe area). In contrast, significant reductions of DA and HVA concentrations in the ATD brains were found only in putamen and amygdala, respectively. The 5-HIAA/5-HT ratio in the ATD brains decreased significantly in locus coeruleus, while the HVA/DA ratio increased significantly in putamen and medial segment of globus pallidus. These findings suggest that the serotonergic and noradrenergic systems are affected, while the dopaminergic system is relatively unaffected in ATD brains.     

Regulation of release processes in central serotoninergic neurons

Different technical, physiological and biochemical aspects concerning the study of the release of 5-HT are discussed herein. Isotopic methods are the most suitable techniques since these allow the release of 3H-5-HT to be measured after having determined the identity of the labelled compounds formed from 3H-tryptophan by co-chromatography. Under these conditions, the 3H-amine released in the superfusates comes from serotoninergic nerve endings, since tryptophan hydroxylase is exclusively localized in serotoninergic neurons. Moreover, it appears that newly synthesized 5-HT is preferentially released. The release of 5-HT is dependent on neuronal activity, but is not always linked to the synthesis of 5-HT. The increase in the firing rate of serotoninergic cell bodies by a local application of glutamate in the area of the nucleus raphe dorsalis induces a marked increase n the release of 5-HT in the caudate nucleus; an opposite effect is observed after cooling this region. The local depolarization of serotoninergic terminals located in the caudate nucleus increases the release of this amine. This effect is blocked by TTX. LSD reduces the stimulating effect of KCl, thus indicating that the release of 5-HT can be controlled at a presynaptic level. In addition, the release of the amine is dependent on the presence of calcium. Serotoninergic neuronal activity can be controlled at the preterminal or at the cell body levels by the activity of other neuronal systems. The effects of the release of dopamine from dendrites, and that of GABA in the substantia nigra are reported herein. Furthermore, changes in the activity of the dopaminergic, gabaergic and serotoninergic systems innervating the nucleus raphe dorsalis modulate the release of 5-HT, measured both in the caudate nucleus and in the nucleus raphe magnus. Finally, it has been reported that the release of 5-HT can be estimated in the raphe nuclei dorsalis and magnus. It has been shown that the amounts of 3H-5-HT continuously formed from 3H-TRP and released in the nucleus raphe dorsalis are much greater than those estimated in the caudate nucleus or in the substantia nigra. Although the quantities of endogenous 5-HT measured in the nucleus raphe dorsalis are the highest in the brain, this structure presents only a few serotoninergic nerve endings. This raises the question of the origin of the 5-HT released in serotoninergic nuclei. A possible dendritic release of 5-HT is discussed.