Tryptophan hydroxylase activity and synaptosomal uptake of serotonin in discrete brain regions after midbrain raphe lesions: correlations with serotonin levels and histochemical fluorescence

Serotonergic imaging in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive degeneration of monoaminergic central pathways such as the serotonergic. The degeneration of serotonergic signaling in striatal and extrastriatal brain regions is an early feature of PD and is associated with several motor and non-motor complications of the disease. Molecular imaging techniques with Positron Emission Tomography (PET) have greatly contributed to the investigation of biological changes in vivo and to the understanding of the extent of serotonergic pathology in patients or individuals at risk for PD. Such discoveries provide with opportunities for the identification of new targets that can be used for the development of novel disease-modifying drugs or symptomatic treatments. Future studies of imaging serotonergic molecular targets will better clarify the importance of serotonergic pathology in PD, including progression of pathology, target-identification for pharmacotherapy, and relevance to endogenous synaptic serotonin levels. In this article, we review the current status and understanding of serotonergic imaging in PD.

Design and synthesis of enantiopure 18 F-labelled [18 F]trifluoromethyltryptophan from 2-halotryptophan derivatives via copper(I)-mediated [18 F]trifluoromethylation and evaluation of its in vitro characterization for the serotonergic system imaging

We synthesized [¹⁸ F]trifluoromethyl-l-tryptophan ([¹⁸ F]CF₃ -l-Trp) using Cu(I)-mediated [¹⁸ F]trifluoromethylation to image serotonergic system. Radiochemical yield was 6 ± 1.5% (n = 9), and radiochemical purity was over 99%. The molar activity was 0.44 to 0.76 GBq/μmol. [¹⁸ F]CF₃ -l-Trp was stable for up to 6 hours in mouse and human sera at 37°C. Protein-binding was 0.26 ± 0.03% and 0.34 ± 0.02% in human and mouse serum at 60 minutes, respectively. In conclusion, enantiopure [¹⁸ F]CF₃ -l-Trp was synthesized as a feasible imaging agent for the serotonergic system.

New Trends and Current Status of Positron-Emission Tomography and Single-Photon-Emission Computerized Tomography Radioligands for Neuronal Serotonin Receptors and Serotonin Transporter

The critical role of serotonin (5-hydroxytryptamine; 5-HT) and its receptors (5-HTRs) in the pathophysiology of diverse neuropsychiatric and neurodegenerative disorders render them attractive diagnostic and therapeutic targets for brain disorders. Therefore, the in vivo assessment of binding of 5-HT receptor ligands under a multitude of physiologic and pathologic scenarios may support more-accurate identification of disease and its progression and the patient's response to therapy as well as the screening of novel therapeutic strategies. The present Review aims to focus on the current status of radioligands used for positron-emission tomography (PET) and single-photon-emission computerized tomography (SPECT) imaging of human brain serotonin receptors. We further elaborate upon and emphasize the attributes that qualify a radioligand for theranostics on the basis of its frequency of use in clinics, its benefit to risk assessment in humans, and its continuous evolution, along with the major limitations.

Localization of putative transmitters in the hippocampal formation: with a note on the connections to septum and hypothalamus

Biochemical assays on microdissected samples, denervation studies, subcellular fractionation, and light and electron microscopic autoradiography of high affinity uptake have been performed to study the cellular localization of transmitter candidates in the rat hippocampal formation. High affinity uptake of glutamate and aspartate is localized in the terminals of several excitatory systems, such as the entorhino-dentate fibres (perforant path), mossy fibres (from granular cells) and pyramidal cell axons. Thus, in stratum radiatum and oriens of CA1, 85% of glutamate and asparate uptake and 40% of glutamate and aspartate content are lost after lesions of ipsilateral plus commissural fibres from CA3/CA4. Hippocampal efferents also take up aspartate and glutamate, since these activities are heavily reduced in the lateral septum and mamillary bodies after transection of fimbria and the dorsal fornix. The synthesis (by glutamic acid decarboxylase), content and high affinity uptake of gamma-aminobutyrate (GABA) are not reduced after lesions of these or other projection fibre systems. A localization in intrinsic neurons is confirmed by a selective loss of glutamic acid decarboxylase after local injections of kainic acid. Peak concentrations of the enzyme occur near the pyramidal and granular cell bodies, corresponding to the site of the inhibitory basket cell terminals, and in the outer parts of the molecular layers. Some 85% of glutamic acid decarboxylase is situated in 'nerve ending particles'. Acetylcholine synthesis (by choline acetyltransferase) disappears after lesions of septo-hippocampal fibres. Since 80% of the hippocampal choline acetyltransferase is in 'nerve ending particles', the characteristic topographical distribution of this enzyme should reflect the distribution of cholinergic septo-hippocampal afferents. Serotonin, noradrenaline, dopamine and histamine are located/synthesized in afferent fibre systems. Some monoamine-containing afferents to the hippocampal formation pass via the septal area, others via the amygdala. The hippocampal formation also contains nerve elements reacting with antibodies against neuroactive peptides, such as enkephalin, substance P, somatostatin and gastrin/cholecystokinin.

alpha-[11C]-Methyl-L-tryptophan PET identifies the epileptogenic tuber and correlates with interictal spike frequency

Epilepsy surgery has been successfully performed in patients with tuberous sclerosis complex (TSC) and seizures arising from a restricted epileptogenic area. The outcome of cortical excision depends on accurate pre-surgical identification of the epileptogenic tuber. [11C] alpha-methyl-L-tryptophan (alpha-MTrp) was originally developed to measure serotonin synthesis in vivo with positron emission tomography (PET). However in pathologic conditions its uptake may also depend on the synthesis of quinolinic or kynurenic acid via the kynurenine pathway. Increased levels of serotonin and quinolinic acid have been observed in epileptogenic lesions, raising the possibility that alpha-MTrp PET may localize the epileptogenic area. The aim of this study was to correlate alpha-MTrp PET uptake with the localization of the epileptogenic area and with interictal spike frequency in patients with TSC. alpha-MTrp uptake was measured in 8 patients (2 males, mean age 29.6+/-14.9 years, range 3-50 years) with intractable partial epilepsy due to TSC. All patients underwent scalp EEG monitoring during the PET scan. In four (50%), increased uptake of alpha-MTrp occurred in the epileptogenic area alone. Two (25%) patients showed multifocal abnormalities and the remaining two (25%) did not show focal changes. PET localization was mostly seen in patients with frequent interictal abnormalities on the EEG. Furthermore, there was a significant correlation between alpha-MTrp uptake and the frequency of interictal spikes (r=0.6; P<0.05). alpha-MTrp PET is a promising diagnostic tool in the localization of the epileptogenic area in patients with TSC.

Comparative in vivo study of iodine-123-labeled beta-CIT and nor-beta-CIT binding to serotonin transporters in rat brain

Both iodine-123-labeled beta-CIT (2beta-carbomethoxy-3beta-(4-iodophenyl)tropane) and nor-beta-CIT (2beta-carbomethoxy-3beta-(4-iodophenyl)nortropane) have shown to be suitable radioligands for imaging serotonin (5-HT) transporters. [(123)I]nor-beta-CIT has the highest in vitro affinity for 5-HT transporters among beta-CIT analogs reported so far. However, no direct comparison-studies of these two radiotracers as to their in vivo binding to 5-HT transporters have been reported so far. Therefore, it is still unclear which of the two radiotracers is more suitable for single photon emission computed tomography (SPECT) imaging of 5-HT transporters. The purpose of this study was to compare directly in a controlled design the in vivo [(123)I]beta-CIT and [(123)I]nor-beta-CIT binding to 5-HT transporters under the same conditions in rats with the focus on brain kinetic characteristics by means of a two-compartment analysis. We observed that [(123)I]beta-CIT has a higher binding potential and faster kinetics for 5-HT transporters than [(123)I]nor-beta-CIT, suggesting that [(123)I]beta-CIT may be a more suitable radioligand than [(123)I]nor-beta-CIT for imaging 5-HT transporters with SPECT.

The ascending serotonergic system in the hamster: comparison with projections of the dorsal and median raphe nuclei

The ascending serotonergic projections are derived largely from the midbrain median and dorsal raphe nuclei, and contribute to the regulation of many behavioral and physiological systems. Serotonergic innervation of the hamster circadian system has been shown to be substantially different from earlier results obtained with other methods and species. The present study was conducted to determine whether similar differences are observed in other brain regions. Ascending projections from the hamster dorsal or median raphe were identified using an anterograde tracer, Phaseolus vulgans leucoagglutinin, injected by iontophoresis into each nucleus. Brains were processed for tracer immunoreactivity, and drawings were made of the median raphe and dorsal raphe efferent projection patterns. The efferents were also compared to the distribution of normal serotonergic innervation of the hamster midbrain and forebrain. The results show widespread, overlapping projection patterns from both the median and dorsal raphe, with innervation generally greater from the dorsal raphe. In several brain regions, including parts of the pretectum, lateral geniculate and basal forebrain, nuclei are innervated by the dorsal, but not the median, raphe. The hypothalamic suprachiasmatic nucleus is the only site innervated exclusively by the median and not by the dorsal raphe. The pattern of normal serotonin fiber and terminal distribution is generally more robust than would be inferred from the anterograde tracer material. However, there is good qualitative similarity between the two sets of data. The oculomotor nucleus and the medial habenula are unusual to the extent that each has a moderately dense serotonin terminal plexus, although neither receives innervation from the median or dorsal raphe. In contrast, the centrolateral thalamic nucleus and lateral habenula have little serotonergic innervation, but receive substantial other neural input from the raphe nuclei. The normal serotonergic innervation of the hamster brain is similar to that in the rat, although there are exceptions. The anterograde tracing of ascending median or dorsal raphe projections reveals a high, but imperfect, degree of correspondence with the serotonin innervation data, and with data from rats derived from immunohistochemical and autoradiographic tract-tracing techniques.

CCAAT displacement protein (CDP/cut) binds a negative regulatory element in the human tryptophan hydroxylase gene

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of serotonin, a neurotransmitter that has been implicated in many psychiatric illnesses. The mechanism of transcriptional regulation of the human TPH gene is largely unknown. We have identified a negative regulatory element located between nucleotides -310 and -220 in the human TPH (hTPH) gene. Electromobility shift analyses performed with the -310/-220 hTPH probe and nuclear extract from P815-HTR (a TPH-expressing cell line) revealed two slow migrating protein-DNA complexes, designated I and II. CCAAT displacement protein (CDP/Cut) is involved in complex I formation as shown in electromobility shift analysis, using consensus oligonucleotide competitor and antibody. Mutations in the CDP/Cut binding site not only disrupted the CDP-DNA complex but also disrupted the second complex, suggesting that the core binding sequences of the two proteins are overlapping. The functional importance of these protein-DNA interactions was assessed by transiently transfecting wild-type and mutant pTPH/luciferase reporter constructs into P815-HTR cells. Mutations in the core CDP/Cut site resulted in an approximately fourfold increase in relative luciferase activities. Because CDP/Cut has been shown to repress transcription of many target genes, we speculate that disruption of the CDP/Cut binding was responsible, at least in part, for the activation of hTPH gene.

[123I]FP-CIT binds to the dopamine transporter as assessed by biodistribution studies in rats and SPECT studies in MPTP-lesioned monkeys

[123I]FP-CIT (N-omega-fluoropropyl-2 beta-carbomethoxy-3 beta-(4-iodophenyl)-tropane), a radioiodinated cocaine analogue, was evaluated as an agent for the in vivo labeling of dopamine (DA) transporters by biodistribution studies in rats and by single photon emission computed tomography (SPECT) studies in unilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys. In rats, intravenous injection of [123I]FP-CIT resulted in high accumulation of radioactivity in the striatum. Less pronounced uptake was seen in brain areas with high densities of serotonergic uptake sites. While striatal uptake of radioactivity after injection of [123I]FP-CIT was displaced significantly by GBR12,909 but not by fluvoxamine, the opposite was observed in brain areas known to be rich of serotonin transporters. Monkeys which were unilaterally treated with neurotoxic doses of MPTP showed severe loss of striatal [123I]FP-CIT uptake at the side of treatment. The results of this study indicate that [123I]FP-CIT, although not being a selective radioligand, binds specifically to the striatal DA transporter in vivo and thus suggest that [123I]FP-CIT promises to be a suitable radioligand for SPECT imaging of DA transporters in humans.

Quantitative subregional distribution of serotonin1A receptors and serotonin transporters in the human dorsal raphe

Subregional distributions of serotonin1A receptors and serotonin transporters within the human dorsal raphe nucleus (DR) were determined by quantitative autoradiographic analyses of radioligand binding in tissue sections. [3H]8-Hydroxy-2-(di-n-propyl)aminotetralin (8-OH-DPAT) and [3H]paroxetine were used to label, respectively, serotonin1A receptors and serotonin transporters in the subnuclei of the DR, which were delineated on the basis of tryptophan hydroxylase (TrpOH) immunoreactivity. [3H]8-OH-DPAT binding was coextensive with the TrpOH-immunoreactive cell bodies and fibers but was distributed unevenly among the subnuclei. In contrast, [3H]paroxetine binding was present throughout the central gray matter, with relatively homogeneous labeling across the subnuclei of the DR. In rostral sections, [3H]8-OH-DPAT binding (fmol/mg protein) in the dorsal subnucleus was lower than that in the ventral or the interfascicular subnucleus. Within the interfascicular subnucleus, [3H]8-OH-DPAT binding decreased progressively in a rostral-to-caudal fashion. The highest levels of [3H]8-OH-DPAT binding were found in the ventrolateral subnucleus at the level of the caudal extent of the trochlear nucleus. The influence of age and postmortem interval on radioligand binding was also examined. These data in the human DR indicate that serotonin1A receptors are differentially distributed among the subnuclei and along the rostro-caudal axis of the midbrain raphe, and serotonin transporters appear to be relatively evenly distributed throughout the DR. Subregional analyses of such serotonergic markers may prove useful in evaluating the role that serotonin may play in depression, schizophrenia, and suicide.

Ipsilateral alterations in tryptophan hydroxylase activity in rat brain after hypothalamic 5,7-di-hydroxytryptamine lesion

The in vivo relationship between the amounts of tryptophan hydroxylase (TPH) protein and its intrinsic synthetic activity, measured by quantifying the amounts of alpha-[3H]methyl-5-hydroxytryptamine (alpha-[3H]M5-HT), is reported in cell body and terminal areas of intact and disturbed serotonergic neurons following a unilateral 5,7-dihydroxytryptamine (5,7-DHT) lesion of the dorsolateral hypothalamus. Five days after the lesion, the relationships between TPH and its synthetic product 5-HT were evaluated on adjacent brain sections in serotonergic cells bodies of the dorsal raphe nucleus (DRN) and nerve fibres of the medial forebrain bundle (MFB). On the side contralateral to the lesion, TPH and alpha-[3H]M5-HT levels in the intact hemi-DRN exhibited a caudo-rostral distribution and were positively and significantly correlated (p < or = 0.001); the calculated TPH-specific activity was 0.76 nCi of alpha-[3H]M5-HT formed per U TPH. In the MFB, quantitative measurements of TPH and alpha-[3H]M5-HT showed no correlation between enzyme and product and no specific activity for TPH could be determined. On the side ipsilateral to the lesion, the density of TPH-immunoreactive fibers was drastically decreased in the dorsolateral hypothalamus where a significant reduction in TPH content (45.5% of control side, P < 0.001) was found. In the overall ipsilateral hemi-DRN, TPH and alpha-[3H]M5-HT levels, their correlation as well as TPH-specific activity were unaltered by the lesion but a significant increase in alpha-[3H]M5-HT and TPH contents was observed in the lateral wings of the DRN. The lesion also induced a significant increase in alpha-[3H]M5-HT and TPH levels (136% and 93.8%, P < 0.001, respectively) in the ipsilateral MFB, which resulted in a positive and significant correlation between these two markers and yielded a TPH-specific activity of 1.0 nCi of alpha-[3H]M5-HT formed per U TPH. TPH topological area was also significantly increased in the lateral aspect of the ipsilateral MFB 5 days post lesion. These results show that 5-HT synthesis in the intact DRN is proportional to and dependent on TPH activity while in the MFB, 5-HT accumulation appears unrelated to TPH content which is most likely in an inactive enzymatic form. Moreover, the data show that a local disruption of serotonergic terminals in the dorsolateral hypothalamus does not affect 5-HT synthesis in the overall ipsilateral DRN neurons but results in local activation of TPH within the serotonergic projection neurons and the ipsilateral MFB, as evidenced by active de novo synthesis of 5-HT. Altogether the results point to circumscribed activation of compensatory mechanisms in 5-HT synthesis after selective destruction of serotonergic terminals.

In vivo-synthesized radioactively labelled alpha-methyl serotonin as a selective tracer for visualization of brain serotonin neurons

To investigate the use of alpha-[3H]methyl tryptophan (alpha-[3H]MTrp) as a tracer for the in vivo study of brain serotonergic neurons, we examined whether alpha-[3H]MTrp and its metabolite alpha-[3H]methyl serotonin (alpha-[3H]M5-HT) selectively label serotonergic neurons and whether once accumulated in these neurons, the radioactive metabolite behaves like endogenous serotonin. Rats received a systemic injection of 1-5 mCi of alpha-[3H]MTrp and 24 h later their brains were immediately removed or fixed by perfusion before removal. Tissue sections in which serotonergic neurons had been immunostained for 5-HT or its synthesizing enzyme, tryptophan hydroxylase, were processed for radioautography at the light and electron microscopic level. In another group of rats, the release of radioactivity from different brain areas was studied both under basal and depolarizing conditions. In the dorsal raphe nucleus, the light microscopic examination revealed almost complete colocalization between serotonergic neurons and those that accumulated radioactivity, with a heterogeneity in the content of alpha-[3H]M5-HT among the various cells. At the ultrastructural level, immunoidentified serotonergic perikarya and dendritic processes in the dorsal raphe nucleus, as well as nerve terminals in the cerebral cortex were also found to contain alpha-[3H]M5-HT. Under basal conditions, radioactivity was released from the brainstem raphe region and from projection areas such as the striatum and hippocampus. The basal output of alpha-[3H]M5-HT increased approximately twofold after a depolarizing 50 mM KCl solution was added to the perfusion fluid. These findings suggest that newly synthesized alpha-[3H]M5-HT can be released both at somatodendritic and terminal sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Occupancy of the serotonin transporter by fluoxetine, paroxetine, and sertraline: in vivo studies with [125I]RTI-55

[125I]RTI-55 was used tracer doses to label serotonin (5-HT) transporters in vivo in the mouse brain. Fluoxetine, paroxetine, and sertraline, potent antidepressants and selective inhibitors of serotonin transporter sites, were administered in various doses and at various times. The doses and times that result in significant binding of the drugs to transporters correspond to doses and times where they are reported to have physiological effects. Estimates of occupancy rate and duration of binding to serotonin transporters were made. The rate of occupancy of the 5-HT transporter site was fastest for sertraline, intermediate for paroxetine and slowest for fluoxetine. Similarly, the duration of occupancy was significantly shorter for sertraline and paroxetine (approximately 10 h) than for fluoxetine (approximately 50 h). The results indicate that in competition studies, [125I]RTI-55 can be used to identify doses of drugs that are physiologically effective, to determine their relative rate of occupancy, and most importantly, to measure the residency time on the central serotonin transporter in vivo.

Development of the hamster serotoninergic system: cell groups and diencephalic projections

Nuclei of the circadian visual system are extensively innervated by serotoninergic neurons and rhythmicity is modulated by the serotoninergic system. This study investigated the temporal relationships between prenatal origins of serotoninergic cell groups and perinatal innervation of structures in the hamster circadian visual system as well as in the remaining diencephalon. Serotonin-immunoreactive (5-HT-IR) neurons of the B4-B9 complex were first seen on embryonic day 8 (E8). The number of neurons increases sharply by E10 when the first 5-HT-IR cells are evident in the medulla (B1-B3 complex). The distribution of serotoninergic neurons in the hamster brainstem is generally adult-like by E14. Thick 5-HT-IR fibers arch around the mesencephalic flexure at E10 and reach more rostral mesencephalic areas at E11. A branch of the medial forebrain bundle (MFB) projects ventrally toward the retrochiasmatic area; a second branch ascends along the fasciculus retroflexus. Fibers cross the midline in the supraoptic commissure by E12, other arrive in the lateral geniculate region, and a branch of the MFB extends toward the mammillary area. At E13, a periventricular medial thalamic branch of the MFB is seen, axons appear in the supramammillary commissure, and a fine fasciculus between the medial thalamus and intergeniculate leaflet is visible. Lateral, paraventricular, and retrochiasmatic hypothalamic areas and centro- and dorsomedial thalamus are densely innervated at E14. The mammillary area and lateral geniculate body are moderately innervated, and the first fibers appear in the deep laminae of the superior colliculus. The innervation of the suprachiasmatic nuclei, periventricular hypothalamus, and superficial layers of the superior colliculus occurs postnatally. The results are consistent with serotoninergic system development in other species.

The A11 catecholamine cell group: another origin of the dopaminergic innervation of the amygdala

A combination of fluorescent retrograde tracing and immunofluorescence histochemistry for tyrosine hydroxylase was employed to re-examine the origin of the dopaminergic innervation of the amygdala in the rat. The present data show that the major input source of this innervation includes the subparafascicular thalamic nucleus as well as the substantia nigra pars compacta and ventral tegmental area, but not the substantia nigra pars lateralis.

Paroxetine as an in vivo indicator of 3,4-methylenedioxymethamphetamine neurotoxicity: a presynaptic serotonergic positron emission tomography ligand?

The present study sought to determine whether [3H]paroxetine, a potent and selective inhibitor of serotonin uptake in vitro, could be used to label the serotonin transporter in the rat brain in vivo such that it might be employed to develop a presynaptic serotonergic positron emission tomography ligand. Tritium labeled paroxetine was administered intravenously to rats by means of tail vein injection. Four hours later, specific [3H]paroxetine binding was determined by subtracting non-specific binding in the cerebellum from total binding in other brain regions of interest. The distribution of specific [3H]paroxetine binding paralleled the distribution of serotonin uptake sites in all brain regions examined. Pretreatment with serotonin re-uptake inhibitors (citalopram or sertraline) reduced in vivo specific [3H]paroxetine binding by as much as 99%. Specific in vivo [3H]paroxetine binding was further characterized through the use of 5,7-dihydroxytryptamine (5,7-DHT), a known serotonergic neurotoxin. 5,7-DHT (200 micrograms, i.c.v.) caused a marked reduction in specific [3H]paroxetine binding, and induced a prolonged depletion of regional brain serotonin. In a final study, the feasibility of using in vivo [3H]paroxetine binding as an indicator of serotonergic damage induced by another neurotoxin (3,4-methylenedioxymethamphetamine, MDMA) was tested. MDMA-treated rats showed a profound reduction in in vivo [3H]paroxetine binding, along with a lasting depletion of regional brain serotonin. These results demonstrate that [3H]paroxetine can be used to label serotonin uptake sites in the rat brain in vivo, and that the damage induced by serotonergic neurotoxins can be detected using in vivo [3H]paroxetine binding as an indicator. Paroxetine (or one of its derivatives) therefore holds promise as a PET ligand for studying serotonergic neurons in the living human brain in health as well as after neurotoxic injury.

Reduction of in vivo binding of [3H]paroxetine in mouse brain by 3,4-methylenedioxymethamphetamine

The effects of 3,4-methylenedioxymethamphetamine (MDMA) on the in vivo binding of [3H]paroxetine, a potent and selective 5-hydroxytryptamine (5-HT; serotonin) uptake inhibitor, in the brain of the mouse were studied. The distribution of radioactivity in the brain of the mouse, after intravenous administration of [3H]paroxetine, was significantly altered by pretreatment with MDMA (15 mg/kg, i.p., 3 hr before). The hypothalamus/cerebellum and cerebral cortex/cerebellum ratios, as a function of time, were significantly decreased after the pretreatment with MDMA, indicating that the in vivo binding of [3H]paroxetine to uptake sites for 5-HT in the brain of the mouse was significantly decreased by MDMA. These ratios could reflect those of the total binding, to the non-specific binding and free ligand, since the cerebellum has very low levels of binding for [3H]paroxetine. Furthermore, these ratios decreased after pretreatment with MDMA, in a dose-dependent manner. However, the binding of [3H]paroxetine to membranes from the brain of the mouse in vitro was not altered by treatment with MDMA. The discrepancy between the in vivo binding and in vitro binding of [3H]paroxetine in the brain of the mouse is discussed.

Quantified distribution of the serotonin innervation in adult rat hippocampus

To quantify the serotonin innervation in adult rat hippocampus, serotonin axon terminals (varicosities) were uptake-labeled for light microscope radioautography in whole hemisphere slices incubated with 1 microM [3H]serotonin. The labeled varicosities were visualized as small aggregates of silver grains and counted with the aid of an image analysis system across all layers in representative sectors of subiculum, Ammon's horn (CA1, CA3-a, CA3-b) and dentate gyrus (medial blade, crest and lateral blade). Counts were obtained in six rats at three equidistant horizontal levels from the ventral two-thirds of the hippocampus. After double correction for duration of radioautographic exposure and section thickness, and measurement of the mean diameter of labeled varicosities in electron microscope radioautographs, the results were expressed in number of varicosities per mm3 of tissue. The overall density of hippocampal serotonin innervation was thus evaluated at 2.7 x 10(6) varicosities per mm3, and appeared significantly higher in subiculum (3.6 x 10(6)) and Ammon's horn (3.1 x 10(6)) than in dentate gyrus (2.2 x 10(6)). Subiculum and dentate gyrus-crest (2.0 x 10(6)) had the highest and lowest regional densities. There was a marked heterogeneity also in terms of laminar distribution. For example, the stratum moleculare of subiculum and CA1, and the stratum oriens of CA3 (5.2 x 10(6)) varicosities in CA3-a), showed much higher values than the pyramidal cell layer (0.7, 1.1 and 0.7 x 10(6) in CA1, CA3-a and CA3-b, respectively). Similarly, the granular layer of dentate gyrus had a much lower density (1.1 x 10(6)) than did the molecular (2.8 x 10(6)) and the polymorph layer (2.4 x 10(6)). From these data, it was possible to evaluate the mean endogenous amine content per hippocampal serotonin varicosity (0.05-0.07 fg), and the average number of serotonin varicosities per hippocampal neuron in both CA3 (130) and dentate gyrus (20-35). In the context of current data on the distribution of serotonin receptors and diverse actions of serotonin at the cellular level in hippocampus, such quantified information provides new insights on some basic properties of serotonin in this part of the brain.

A new method to measure brain serotonin synthesis in vivo. I. Theory and basic data for a biological model

We describe here an autoradiographic method to measure the in vivo rate of serotonin synthesis in rat brain. The method is based on the use of the L-tryptophan analogue alpha-methyl-L-tryptophan (alpha-MTrp), which is converted in vivo into alpha-methylserotonin (alpha-M5HT). Since alpha-M5HT is not a substrate for monoamine oxidase, it is accumulated in the brain tissue. Data are presented to confirm time-dependent conversion of alpha-MTrp into alpha-M5HT in the dorsal raphe nucleus and also in the pineal body, an organ outside the blood-brain barrier. It has also been shown that washing brain slices in 10% trichloroacetic acid results in less than 3% incorporation of alpha-MTrp into brain proteins. The rates of synthesis are calculated in several grossly dissected brain structures by using tracer kinetics and a three-compartment biological model. The half-life of the precursor pool is estimated to be approximately 20 min. The rate of serotonin synthesis is highest in the pineal body.

In vivo labeling of serotonin uptake sites with [3H]paroxetine

Previous work has shown that [3H]paroxetine is a potent and selective in vitro label for serotonin uptake sites in the mammalian brain. In the present study, [3H]paroxetine was tested in mice as an in vivo label for serotonin uptake sites. Maximum tritium concentration in the whole brain (1.4% of the intravenous dose) was reached 1 h after injection into a tail vein. Distribution of the tracer at 3 h after injection followed the distribution of serotonin uptake sites known from previous in vitro binding studies (r = 0.85). The areas of highest [3H]paroxetine concentration, in decreasing order, were: hypothalamus greater than frontal cortex greater than olfactory tubercles greater than thalamus greater than upper colliculi greater than brainstem greater than hippocampus greater than striatum greater than cerebellum. Preinjection of carrier paroxetine (1 mg/kg) significantly decreased [3H]paroxetine concentration in all areas except in the cerebellum, which is known to contain a relatively low number of specific binding sites. Kinetic studies showed highest specific [3H]paroxetine binding (tissue minus cerebellum) at 2 h after injection and slow clearance of activity thereafter (half-time of dissociation from the hypothalamus, 215 min). The specificity of in vivo [3H]paroxetine binding was studied by preinjecting monoamine uptake blockers or receptor antagonists 5 min before administration of [3H]paroxetine. Serotonergic or muscarinic cholinergic receptor antagonists and dopamine or norepinephrine uptake blockers did not reduce the in vivo binding of [3H]paroxetine. In contrast, there was an excellent correlation (r = 0.99) between the in vivo inhibitory potencies of serotonin uptake blockers in this study and previously published in vitro data on inhibition of [3H] serotonin uptake in brain synaptosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

5-HT2 receptor binding and 5-HT uptake in mouse brain: developmental changes and the relationship to audiogenic seizure susceptibility in DBA/2J mice

Abnormal function of serotonergic neurones may be involved in the age-related susceptibility of DBA/2J mice to generalised convulsions induced by auditory stimulation. We have measured 5-HT2 receptor binding sites and synaptosomal 5-HT uptake in 5 brain regions of DBA/2J mice at ages before, during and after their maximal susceptibility to audiogenic seizures and in age-matched C57 B1/6 mice, a strain resistant to audiogenic seizures at all ages. The number of 5-HT2 binding sites was 20% higher in the cerebral cortex of DBA/2J than C57 B1/6 mice at the time of maximal susceptibility of DBA/2J mice to audiogenic seizures but did not differ at other ages. The number of 5-HT2 binding sites did not differ between the two strains at the ages studied in forebrain, mid-brain, hippocampus and pons-medulla. A marked reduction in the number of 5-HT2 binding sites was apparent in the mid-brain, hippocampus and pons-medulla of both strains of mice between 13-15 days of age and 21-23 days of age. Synaptosomal 5-HT uptake did not differ significantly between DBA/2J and C57 B1/6 in any of the brain regions at the ages studied. The higher density of cortical 5-HT2 binding sites in DBA/2J mice may contribute to their susceptibility to sound-induced seizures.

Ultrastructural relationships of serotonin and GABA terminals in the rat suprachiasmatic nucleus. Evidence for a close interconnection between the two afferent systems

Serotonin (5-HT) and gamma-aminobutyric acid (GABA) nerve endings were identified in the rat suprachiasmatic nucleus (SCN) by combined [3H]5-HT uptake radioautography and glutamate decarboxylase (GAD) immunocytochemistry at the electron microscope level. In areas of good overlap between radioautographic and immunocytochemical labellings, there were no axonal varicosities exhibiting both labellings, indicating that 5-HT and GABA are not co-localized in the SCN. The systematic survey in these areas of all profiles that had accumulated [3H]5-HT and of all GAD-immunoreactive varicosities allowed the analysis of 247 of the former and 896, i.e. an almost four-fold greater number, of the latter. This seems concordant with the view that GABA endings would be the most numerous of all classes of nerve terminals so far identified in the SCN. More than 22% of the [3H]5-HT labelled profiles showed the membrane specialization typically associated with synaptic junctions. Thereby, it was possible to evaluate that about 45% of the 5-HT terminals actually form a synapse in the SCN. Some 37% of the GAD-positive varicose profiles which could be formally interpreted also showed well differentiated synaptic contacts, suggesting that the GABAergic innervation of the SCN could be entirely junctional. Whereas 5-HT terminals usually innervated only one dendritic or somatic element, a convergence of several GABAergic terminals onto the same postsynaptic target also receiving a 5-HT input was frequently observed.(ABSTRACT TRUNCATED AT 250 WORDS)

An immunocytochemical study on the GABA-ergic and serotonin-ergic neurons in rat locus ceruleus with special reference to possible existence of the masked indoleamine cells

Alternate consecutive cryostat sections of the rat locus ceruleus (LC) were immunostained for either GABA or serotonin by the unlabelled peroxidase-antiperoxidase method. By identifying the same neurons in adjacent sections of this series, we confirmed that the GABA- and serotonin-ergic neurons are different cell groups. Serial sections of the LC were alternately stained either by the immunocytochemical method for serotonin or with cresyl violet for neuroanatomical observations, under normal and various experimental conditions using pharmacological manipulations. By using both a monoamine oxidase (MAO) inhibitor (pargyline) and a serotonin precursor, 5-hydroxytryptophan rather than L-tryptophan, with or without an inhibitor of serotonin synthesis, p-chlorophenylalanine, we have demonstrated that many LC neurons have the capacity to accumulate serotonin and not other indoleamines. It is suggested that the LC has numerous masked indoleamine cells that contain large amounts of MAO in the cytoplasm, so that under physiological conditions, the serotoninimmunoreactivity of these cells is masked.

Serotonin in involuntary movement disorders

Several recent studies have emphasized that serotonergic pathways in the CNS are intimately involved in the modulation of motor behavior, and in the pathophysiology of human involuntary movement disorders. These observations are supported by recent reports demonstrating large serotonergic innervation of the striatum and substantia nigra, and a close interaction between the activity of serotonergic neurons with the dopamine system in the striatum and nigra. In the following communication we summarize evidence demonstrating defective serotonergic functions in a number of human movement disorders and discuss their management with serotonergic drugs.

Evidence for a projection from the B9 serotonergic cell group to the median raphe nucleus

Following injections of several fluorescent retrograde tracers into the median raphe nucleus large numbers of retrogradely labeled cells were observed in the ventral mesencephalic tegmentum. Subsequent immunocytochemical processing for serotonin-like immunoreactivity suggested that a large component of this projection originates from serotonergic B9 cells. Although tracer injections into the dorsal raphe did not result in similar labeling of B9 cells, evidence was obtained suggesting the existence of a small serotonergic projection from the median to the dorsal raphe.

The distribution of 5-hydroxytryptamine in the gastrointestinal tract of reptiles, birds and a prototherian mammal. An immunohistochemical study

The distribution of 5-hydroxytryptamine in the gut of several species of birds and reptiles, and of a prototherian mammal, the platypus, was studied using a monoclonal antibody. 5-Hydroxytryptamine-like immunoreactivity was found in enterochromaffin cells and, in birds, in thrombocytes. Immunoreactivity was not found in enteric neurons fixed immediately after dissection. A detailed study was made on one avian species, the budgerigar. Following incubation of intestine in physiological solution, immunoreactivity was found in nerve fibres in the gut wall that was more marked after incubation with the monoamine oxidase inhibitor pargyline. These fibres took up exogenous 5-hydroxytryptamine. Similar fibres were found in the intestinal nerves and in perivascular plexuses on mesenteric arteries. Both the uptake of 5-hydroxytryptamine and the appearance of neuronal immunoreactivity after incubation were inhibited by the amine uptake inhibitors desmethylimipramine or fluoxetine. Fibres taking up 5-hydroxytryptamine were damaged by pretreatment with 6-hydroxydopamine. It was concluded that the fibres showing immunoreactivity after incubation were adrenergic fibres that had taken up 5-hydroxytryptamine released in vitro from enterochromaffin cells or thrombocytes. These, and more limited observations made on the other species, suggest that birds, reptiles and prototherian mammals lack enteric neurons that use 5-hydroxytryptamine as a transmitter substance.

Distribution of catecholamines, serotonin, and their major metabolites in the rat cingulate, piriform-entorhinal, somatosensory, and visual cortex: a biochemical survey using high-performance liquid chromatography

The catecholamines noradrenaline (NA), dopamine (DA), adrenaline (AD), the indoleamine 5-hydroxytryptamine (5-HT; serotonin), as well as some of their major metabolites were assayed by high-performance liquid chromatography (HPLC) with electrochemical detection, in four well-defined areas of the rat cerebral cortex: anterior cingulate (CIN;Cg1 and Cg3), piriform and entorhinal (PiEn), hind-limb primary somatosensory (SSC;HL) and primary visual (VIS; Oc1M and Oc1B). The concentrations of NA and that of its main metabolite 3-methoxy-4-hydroxyphenylglycol were highest in PiEn, had intermediate values in CIN and were lowest for SSC and VIS cortices. The DA levels were also highest in PiEn, intermediate in CIN, while the lowest values were in SSC and VIS cortices. The different DA/NA ratios support the hypothesis that they are indeed independent neurotransmitters. In addition, the levels of 3,4-dihydroxyphenylacetic acid, homovanillic acid and 3-methoxytyramine paralleled the distribution of DA, thus confirming the presence of release sites, even in regions in which the low levels of this catecholamine could be interpreted simply as the precursor of NA. Traces of AD were detected in all the regions examined. The 5-HT contents, as well as that of its precursor 5-hydroxy-1-tryptophan and that of its metabolite 5-hydroxyindole-3-acetic acid were also found to be non-homogenous, with the highest levels measured in the PiEn and CIN regions.

The development of differential mabQ113-immunoreactivity in the rat habenular complex

Monoclonal antibody mabQ113 selectively labels a subset of Purkinje cells which are arranged in parasagittal bands throughout the vermis and hemispheres of the rat cerebellar cortex. No other cerebellar cell types are immunoreactive. By contrast, in the remainder of the brain the mabQ113 epitope is located primarily in glial cells. In general, the glial immunoreactivity is not differentially distributed. An exception is that mabQ113 densely and uniformly stains the lateral habenula (LHb) but gives no labelling of the medial habenula (MHb). During cerebellar development, the mabQ113 epitope is expressed in three stages. Before postnatal day 7 (P7) all Purkinje cells are negative. Secondly, all Purkinje cells become mabQ113+ between P7 and P12. The parasagittal bands are created between P12 and P30 by selective suppression of epitope expression. To explore whether epitope suppression is also responsible for differential staining patterns in other brain regions the ontogenic development of mabQ113 immunoreactivity has been mapped in the habenular complex. Neither the MHb nor the LHb express the mabQ113 epitope prenatally. P1 is the first age at which the LHb is stained. During the next few days the intensity of staining within the LHb steadily increases until the adult pattern is attained at P6. At no time is there expression of the mabQ113 antigen in the MHb. This also confirms that the two classes of habenular astrocytes, mabQ113-/GFAP+ and mabQ113+/GFAP+, are intrinsically different throughout postnatal life.

Autoradiographic localization of 3H-paroxetine-labeled serotonin uptake sites in rat brain

Paroxetine is a potent and selective inhibitor of serotonin uptake into neurons. Serotonin uptake sites have been identified, localized, and quantified in rat brain by autoradiography with 3H-paroxetine; 3H-paroxetine binding in slide-mounted sections of rat forebrain was of high affinity (KD = 10 pM) and the inhibition affinity constant (Ki) values of various drugs in competing 3H-paroxetine binding significantly correlated with their reported potencies in inhibiting synaptosomal serotonin uptake. Serotonin uptake sites labeled by 3H-paroxetine were highly concentrated in the dorsal and median raphe nuclei, central gray, superficial layer of the superior colliculus, lateral septal nucleus, paraventricular nucleus of the thalamus, and the islands of Calleja. High concentrations of 3H-paroxetine binding sites were found in brainstem areas containing dopamine (substantia nigra and ventral tegmental area) and norepinephrine (locus coeruleus) cell bodies. Moderate concentrations of 3H-paroxetine binding sites were present in laminae I and IV of the frontal parietal cortex, primary olfactory cortex, olfactory tubercle, regions of the basal ganglia, septum, amygdala, thalamus, hypothalamus, hippocampus, and some brainstem areas including the interpeduncular, trigeminal, and parabrachial nuclei. Lower densities of 3H-paroxetine binding sites were found in other regions of the neocortex and very low to nonsignificant levels of binding were present in white matter tracts and in the cerebellum. Lesioning of serotonin neurons with 3,4-methylenedioxyamphetamine caused large decreases in 3H-paroxetine binding. The autoradiographic distribution of 3H-paroxetine binding sites in rat brain corresponds extremely well to the distribution of serotonin terminals and cell bodies as well as with the pharmacological sites of action of serotonin.

A comparison of biochemical indices of 5-hydroxytryptaminergic neuronal activity following electrical stimulation of the dorsal raphe nucleus

The activity of 5-hydroxytryptaminergic neurons has been estimated from measurements of: concentrations of 5-hydroxyindoleacetic acid; the ratio of the concentrations of 5-hydroxyindoleacetic acid to 5-hydroxytryptamine; the rate of accumulation of 5-hydroxytryptophan following the administration of an aromatic L-amino acid decarboxylase inhibitor (e.g., NSD 1015); the rate of accumulation of 5-hydroxytryptamine, and the rate of decline of 5-hydroxyindoleacetic acid following the administration of a monoamine oxidase inhibitor (e.g., pargyline). The purpose of the present study was to compare these different methods under conditions of changing neuronal impulse traffic produced by electrical stimulation of 5-hydroxytryptaminergic neurons. Male rats anesthetized with chloral hydrate were killed following 0, 15, or 30 min of electrical stimulation of the dorsal raphe nucleus at a frequency of 0, 5, or 10 Hz. The concentrations of 5-hydroxytryptamine, 5-hydroxyindoleacetic acid, and 5-hydroxytryptophan in nucleus accumbens, amygdala, suprachiasmatic nucleus, and dorsomedial nucleus were measured using HPLC coupled to an electrochemical detector. In each brain region, stimulation elicited an increase in the concentration of 5-hydroxyindoleacetic acid and the 5-hydroxyindoleacetic acid/5-hydroxytryptamine concentration ratio in saline-treated animals and an increase in 5-hydroxytryptophan accumulation in NSD 1015-treated animals, but did not alter the concentration of 5-hydroxytryptamine or 5-hydroxyindoleacetic acid in pargyline-treated rats. The results o f this study indicate that although the first three methods serve as valid indices of 5-hydroxytryptaminergic neuronal activity, the pargyline-dependent techniques are not responsive to changes in the rate of 5-hydroxytryptamine nerve firing.

Effects of p-chlorophenylalanine on cortical monoamines and on the activity of noradrenergic neurons

The catecholamines noradrenaline, dopamine, adrenaline, the indoleamine 5-hydroxy-tryptamine (5-HT; serotonin), and some of their major metabolites were assayed, using high performance liquid chromatography (HPLC), in the neocortex of normal rats as well as in animals in which 5-HT synthesis had been inhibited with p-chlorophenylalanine. Besides important depletions in serotonin and in 5-hydroxyindole-3-acetic acid, noradrenaline levels were significantly reduced, but the content in 3-methoxy-4-hydroxyphenylglycol was increased, indicating an augmented utilization of this amine. The levels of dopamine and 3-methoxytyramine were also reduced, although homovanillic acid and 3,4-dihydroxyphenylacetic acid levels remained constant. The spontaneous unitary activity of identified noradrenergic neurons in the Locus coeruleus was increased, indicating an hyperactivity of this system. These results can be interpreted in relation to functional interactions between the catecholamines and serotonin; i.e.: a decrease in endogenous serotonin results in the loss of a negative feedback control of noradrenaline release.

A survey of MabQ113 immunoreactivity in the adult rat brain: differential staining of the lateral and medial habenular nuclei

Monoclonal antibody mabQ113 recognizes a polypeptide antigen which, in rat cerebellum, is confined exclusively to a subset of Purkinje cells which are organized into parasagittal bands. In this report we have explored the distribution of mabQ113 immunoreactivity in some other regions of the rat brain. The most interesting result was a dramatic differential staining of the habenular complex in which mabQ113 densely and uniformly stained the lateral habenula but did not stain the medial habenula. Within the lateral habenula reaction product is localized primarily in the cellular processes of astrocytes but there is also staining of neighboring neuronal dendritic and axonal profiles. The afferent and efferent tracts of the habenular nuclei are not immunoreactive and there was no systematic difference in staining between the afferent and efferent nuclear groups of the two habenular nuclei. The pattern of mabQ113 immunoreactivity in rat brain is distinct from previously described biochemical differentiation markers of the two nuclei and thus may serve as a useful probe to study habenular anatomy, development and function.

Increased tryptophan hydroxylase activity in serotonergic nerve terminals spared by 5,7-dihydroxytryptamine

Adult rats received intraventricular injections of 5,7-dihydroxytryptamine (5,7-DHT) to destroy serotonin (5-HT)-containing nerve terminals throughout the brain. When the animals were killed 3 or 21 days later, we observed a marked decrease in 5-HT content in septum and hippocampus and a parallel decline in in vitro high affinity 5-HT uptake. 5-Hydroxyindoleacetic acid (5-HIAA) concentrations also were reduced but by a much smaller extent, resulting in significant increases in the ratio of 5-HIAA to 5-HT. These changes were accompanied by similar increases in the ratio of tryptophan hydroxylase (TPH) activity to 5-HT content. The relative increases in TPH activity resulted from two temporally distinct processes, the first of which appeared to be an activation that could be mimicked in vitro by Ca2+-dependent phosphorylation. We conclude that, after partial damage to 5-HT neurons, there is a compensatory increase in the synthesis and release of 5-HT from those terminals that remain.

Changes in the serotonergic, noradrenergic and dopaminergic levels in the brain of scrapie-infected rats

Levels of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylethyleneglycol (DHPG) were determined by high-performance liquid chromatography in different brain areas of scrapie-infected rats, 8.5 months after intracerebral inoculation of a rat-adapted strain from mice brain (C 506). At this time, rats developed early clinical signs of the disease. Scrapie-infected rats showed a reduction in the levels of 5-HT and 5-HIAA (frontal cortex, hippocampus, mesolimbic structure). Concentrations of DHPG decreased in the frontal and parietal cortices but remained unchanged in the hippocampus. DOPAC levels decreased in the striatum but not in the mesolimbic structure. These results confirm that the serotonergic, noradrenergic and dopaminergic systems are altered in the brain of scrapie-infected rats. They can partly account for clinical signs of scrapie and are in agreement with the scarce data provided by the postmortem analysis of Creutzfeldt-Jakob disease brains.

VIP neurons as prime synaptic targets for serotonin afferents in rat suprachiasmatic nucleus: a combined radioautographic and immunocytochemical study

Cellular relationships between serotonin (5-HT) axon terminals and neurons containing vasoactive intestinal peptide (VIP) were characterized by combined radioautography and immunocytochemistry in rat suprachiasmatic nucleus (SCN). Light microscopic immunoradioautographs showed significant overlap between (3H)5-HT uptake sites and VIP-immunoreactive elements in the ventral half of the SCN. Of the 255 (3H)5-HT-labelled axonal profiles detected in a systematic electron microscopic survey of single thin sections from this area, 75 (30%) were directly apposed to VIP-immunoreactive nerve cell bodies and/or dendrites. Radioautographically labelled 5-HT varicosities often showed well-differentiated, symmetrical or asymmetrical synaptic junctions, 60% of which were established on VIP-immunoreactive nerve cell bodies or dendrites. In a separate sampling of 198(3H)5-HT-labelled terminals seen in apposition with VIP-immunoreactive elements, 50 showed a junctional complex at the site of contact. Postsynaptic immunoreactive elements were mostly dendrites but also included nerve cell bodies. Despite the methodological limitations inherent to the present double labelling approach, these data strongly support the view that VIP neurons are prime synaptic targets for 5-HT afferents in the SCN. VIP/5-HT interactions are thus likely to play an important functional role in this nucleus and may in particular subserve the 5-HT mediated regulation of certain circadian rhythms, including that of pituitary hormone secretion.

Studies on the up regulation of alpha-adrenoceptors on rat hippocampal perikarya by chemical lesion of the median raphe nucleus

Neurotoxin-induced lesion of the serotonergic raphe-hippocampal pathway produced about a 50% increase in the density of a nM affinity alpha-adrenergic binding site for (3H)WB-4101 in rat hippocampus 18 days postlesion without altering the specific binding of (3H)5-HT to serotonergic receptors. The chronic i.c.v. infusion of serotonin by minipump started at the appropriate time averted or reverted the effect. The dynamics of noradrenergic neurotransmission in the hippocampus was not impaired by lesion of the median raphe nucleus as determined by the uptake and turnover of noradrenaline as well as its release - as reflected by the normetanephrine concentration. In addition, neurotoxin-induced lesion of the dorsal noradrenergic bundle failed to alter either the Bmax or the Kd of (3H)WB-4101 binding to the nM site. Kainic acid-induced destruction of perikarya depressed the nM (3H)WB-4101 binding sites by 60% and completely prevented the up regulation caused by lesion of the median raphe nucleus. Thus, the supersensitivity-like response of the adrenoceptors to the lack of serotonin appears to be localized on kainate-sensitive cells within the hippocampus.

The effects of dopamine and dopamine agonists on the release of 3H-GABA and 3H-5HT from rat nigral slices

Only high micromolar concentrations of dopamine and dopamine agonists altered spontaneous and KCl-evoked release of 3H-GABA and 3H-5HT from rat nigral slices in vitro. Apomorphine (100 microM) and dopamine (100 microM) enhanced the spontaneous release of 3H-5HT but the effect of dopamine was not reversed by haloperidol (1 microM). Both apomorphine (100 microM) and dopamine (100 microM) enhanced the KCl-evoked release of 3H-5HT but these effects were not reversed by haloperidol (1 microM). Apomorphine (10-250 microM) and dopamine (10-250 microM) inhibited 3H-5HT uptake into nigral synaptosomal preparations in a concentration-dependent manner. Accordingly, a major portion of the apparent effect of these drugs on 3H-5HT release may be due to inhibition of 3H-5HT uptake. Dopamine (100 and 1000 microM), amphetamine (100 microM), apomorphine (100 microM) and 2-amino-6,7-dihydroxytetralin (ADTN; 100 microM) were without effect on the spontaneous release of 3H-GABA from nigral slices. Apomorphine (100 microM) and ADTN (100 microM) reduced the KCl-evoked release of 3H-GABA from substantia nigra, an effect antagonized by haloperidol (1 microM). However, amphetamine (100 microM) and dopamine (100-1000 microM) were without effect on KCl-evoked 3H-GABA release. These results suggest that only high concentration of some dopamine agonists can modulate 3H-5HT and 3H-GABA release in substantia nigra. However, dopamine either had no effect, or its actions were not reversed by dopamine receptor blockade, so it appears unlikely that dendritic dopamine release will influence GABA and 5HT release in substantia nigra.

Organization of raphe-cortical projections in rat: a quantitative retrograde study

Retrograde transport of a fluorescent dye was employed to study the projections from raphe nuclei to neocortex in the rat. The spatial distributions of labeled raphe cells were analyzed quantitatively to determine whether the nuclei are topographically organized with respect to different cortical targets. The dorsal raphe nucleus (DRN), exclusive of the lateral wing regions, has a predominantly (3:1) ipsilateral projection with decreasing numbers of cells projecting to frontal, parietal, and occipital cortex. Overlapping cell groups within the DRN project differentially to these three cortical areas: DRN cells innervating frontal cortex extend more rostrally and laterally than those to either parietal or occipital cortex. The medium raphe and B9 projections are bilaterally symmetric, with equal cell numbers projecting to frontal, parietal, and occipital cortex. The rostro-caudal distributions of cells that project to disparate cortical areas differ in B9 but not in MR. The percentage of cortically projecting cells that are serotonergic is 80% for the DRN, 60% in the MR and 33% in the B9 cell group. The dorsal raphe nucleus and the B9 cell group are organized heterogeneously, and overlapping sets of neurons project differentially upon particular areas of neocortex. In contrast, the median raphe nucleus projects uniformly upon the neocortex and does not exhibit topographic organization. The three rostral raphe nuclei (DR, MR and B9) are each organized according to different rules with regard to their efferent projections to cortex. The differential organization of the raphe nuclei suggests that groups of cells within these three raphe nuclei are likely to innervate different combinations of cortical targets and thus to have different functional effects.

In vivo increase in hypothalamic cyclic AMP following 5-hydroxytryptophan administration in rats

The administration of 5-hydroxytryptophan (5-HTP, 100 mg/kg, i.p.) consistently increased hypothalamic cyclic AMP levels in rats treated 10 days earlier with the serotonin neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), to produce 5-HT receptor supersensitivity. However 5-HTP (100 mg/kg), failed to cause an increase in hypothalamic cyclic AMP in rats not pretreated with 5,7-DHT. The 5-HTP-induced increase in cyclic AMP was blocked by the decarboxylase inhibitor, benserazide (RO 4-4602, 800 mg/kg) and by the 5-HT antagonist metergoline (5 mg/kg). Other treatments that caused a significant elevation of hypothalamic cyclic AMP included: (a) L-Tryptophan plus the monoamine oxidase inhibitor, tranylcypromine, and (b) the serotonin agonist, 1-(m-trifluromethylphenyl)-1-piperazine. The 5-HT antagonist, methysergide, blocked the serotonin receptor mediated behavioral syndrome, but failed to prevent the increase in hypothalamic cyclic AMP. Moreover, the 5-HT agonist, 5-methoxy-N, N-dimethyltryptamine, (5-Me-DMT), induced a strong behavioral syndrome but failed to significantly increase hypothalamic cyclic AMP. These findings suggest that activation of 5-HT receptors somewhere in the brain causes an increase in hypothalamic cyclic AMP, but further studies will be needed to determine whether this is a direct result of activation of the 5-HT receptors in the hypothalamus.

Serotonin-mediated inhibition from dorsal raphe nucleus of neurons in dorsal lateral geniculate and thalamic reticular nuclei

Electrophysiological studies using rats anesthetized with chloral hydrate were performed to determine whether or not serotonin originating in the dorsal raphe nucleus (DR) acts as an inhibitory transmitter or neuromodulator on neurons of the dorsal lateral geniculate nucleus (LGN) and neurons located in the thalamic reticular nucleus (TRN) immediately rostral to the dorsal LGN. In the LGN, conditioning stimuli applied to the DR preceding test stimulus to the optic tract and visual cortex inhibited orthodromic and antidromic spikes in about one-third of the relay neurons and in more than half of the intrageniculate interneurons. Conditioning stimulation of the DR also produced an inhibition of the spikes elicited by stimulation of the optic tract and visual cortex of at least three-quarters of the TRN neurons. Iontophoretic application of serotonin (25 nA) inhibited the orthodromic spikes of the LGN relay neuron and TRN neuron. A close correlation was observed between the effects of DR conditioning stimulation and iontophoretic serotonin in the same neurons. The inhibition with DR conditioning stimulation and iontophoretically applied serotonin was antagonized during iontophoretic application of methysergide (15-40 nA), a serotonin antagonist. These results strongly suggest that serotonin derived from the DR acts on the LGN and TRN neurons as an inhibitory transmitter or neuromodulator to inhibit transmission in these nuclei.