Biosynthesis of serotonin in Raphé nuclei of rat brain: effect of p-chlorophenylalanine

Synergistic Interaction of 5-HT1B and 5-HT2B Receptors in Cytoplasmic Ca2+ Regulation in Human Umbilical Vein Endothelial Cells: Possible Involvement in Pathologies

The aim of this work was to explore the involvement of 5-HT1B and 5-HT2B receptors (5-HT1BR and 5-HT2BR) in the regulation of free cytoplasmic calcium concentration ([Ca2+]i) in human umbilical vein endothelial cells (HUVEC). We have shown by quantitative PCR analysis, that 5-HT1BR and 5-HT2BR mRNAs levels are almost equal in HUVEC. Immunofluorescent staining demonstrated, that 5-HT1BR and 5-HT2BR are expressed both in plasma membrane and inside the cells. Intracellular 5-HT1BR are localized mainly in the nuclear region, whereas 5-HT2BR receptors are almost evenly distributed in HUVEC. 5-HT, 5-HT1BR agonist CGS12066B, or 5-HT2BR agonist BW723C86 added to HUVEC caused a slight increase in [Ca2+]i, which was much lower than that of histamine, ATP, or SFLLRN, an agonist of protease-activated receptors (PAR1). However, activation of 5-HT1BR with CGS12066B followed by activation of 5-HT2BR with BW723C86 manifested a synergism of response, since several-fold higher rise in [Ca2+]i occurred. CGS12066B caused more than a 5-fold increase in [Ca2+]i rise in HUVEC in response to 5-HT. This 5-HT induced [Ca2+]i rise was abolished by 5-HT2BR antagonist RS127445, indicating that extracellular 5-HT acts through 5-HT2BR. Synergistic [Ca2+]i rise in response to activation of 5-HT1BR and 5-HT2BR persisted in a calcium-free medium. It was suppressed by the phospholipase C inhibitor U73122 and was not inhibited by the ryanodine and NAADP receptors antagonists dantrolene and NED-19. [Ca2+]i measurements in single cells demonstrated that activation of 5-HT2BR alone by BW723C86 caused single asynchronous [Ca2+]i oscillations in 19.8 ± 4.2% (n = 3) of HUVEC that occur with a long delay (66.1 ± 4.3 s, n = 71). On the contrary, histamine causes a simultaneous and almost immediate increase in [Ca2+]i in all the cells. Pre-activation of 5-HT1BR by CGS12066B led to a 3-4 fold increase in the number of HUVEC responding to BW723C86, to synchronization of their responses with a delay shortening, and to the bursts of [Ca2+]i oscillations in addition to single oscillations. In conclusion, to get a full rise of [Ca2+]i in HUVEC in response to 5-HT, simultaneous activation of 5-HT1BR and 5-HT2BR is required. 5-HT causes an increase in [Ca2+]i via 5-HT2BR while 5-HT1BR could be activated by the membrane-permeable agonist CGS12066B. We hypothesized that CGS12066B acts via intracellular 5-HT1BR inaccessible to extracellular 5-HT. Intracellular 5-HT1BR might be activated by 5-HT which could be accumulated in EC under certain pathological conditions.

Mechanism of action of the bimodal antidepressant vilazodone: evidence for serotonin1A-receptor-mediated auto-augmentation of extracellular serotonin output

RATIONALE

The recently approved antidepressant vilazodone, a serotonin (5-HT)1A receptor partial agonist/selective 5-HT reuptake inhibitor offers new possibilities to study the underlying mechanisms of depression pharmacotherapy and of 5-HT augmenting antidepressants.

OBJECTIVE

The role of the 5-HT1A receptor with respect to the regulation of 5-HT output in the mechanism of action of vilazodone.

METHOD

We measured 5-HT levels in two subregions of the rat prefrontal cortex by microdialysis, and 5-hydroxytryptophan (5-HTP) accumulation and tissue 5-HT concentrations ex vivo.

RESULTS

Vilazodone-induced maximal 5-HT levels were similar in the medial and the lateral cortex and were up to sixfold higher than those induced by paroxetine, citalopram, or fluoxetine tested in parallel. Depolarization/autoreceptor-insensitive 5-HT release by vilazodone could be excluded. The citalopram (1 μM, locally infused)-induced increase of 5-HT was further increased by vilazodone (1 mg/kg i.p.), but not by citalopram (10 mg/kg i.p.). Unlike fluoxetine, vilazodone-induced extracellular 5-HT output was not potentiated by cotreatment with the 5-HT1A receptor blocker N-[2-(4-{2-methoxyphenyl}-1-piperazinyl)-ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY 100635). In contrast to fluoxetine, vilazodone exhibited intrinsic 5-HT1A agonist activity: it reduced, similar to (±)-8-hydroxy-2-(dipropylamino)-tetralin (8-OH-DPAT), 5-HTP accumulation in striatum and n. raphe of reserpinized rats. Hence, vilazodone's agonistic actions must be 5-HT1A receptor-related since endogenous 5-HT is lacking in the reserpine-depleted animal.

CONCLUSIONS

In spite of high intrinsic 5-HT1A activity in reserpinized rats, the net effect of vilazodone at release-regulating 5-HT1A autoreceptors must be inhibitory, leading to markedly increased 5-HT output. Another possibility is that vilazodone rapidly desensitizes autoinhibitory 5-HT1A receptors by an unknown mechanism.

Neither serotonin nor adenosine-dependent mechanisms preserve ventilatory capacity in ALS rats

In rats over-expressing SOD1G93A, ventilation is preserved despite significant loss of respiratory motor neurons. Thus, unknown forms of compensatory respiratory plasticity may offset respiratory motor neuron cell death. Although mechanisms of such compensation are unknown, other models of respiratory motor plasticity may provide a conceptual guide. Multiple cellular mechanisms give rise to phrenic motor facilitation; one mechanism requires spinal serotonin receptor and NADPH oxidase activity whereas another requires spinal adenosine receptor activation. Here, we studied whether these mechanisms contribute to compensatory respiratory plasticity in SOD1G93A rats. Using plethysmography, we assessed ventilation in end-stage SOD1G93A rats after: (1) serotonin depletion with parachlorophenylalanine (PCPA), (2) serotonin (methysergide) and A2A (MSX-3) receptor inhibition, (3) NADPH oxidase inhibition (apocynin), and (4) combined treatments. The ability to increase ventilation was not decreased by individual or combined treatments; thus, these mechanisms do not maintain breathing capacity at end-stage motor neuron disease. Possible mechanisms giving rise to enhanced breathing capacity with combined treatment in end-stage SOD1G93A rats are discussed.

TPH2 in the ventral tegmental area of the male rat brain

This study surveyed the distribution of tryptophan hydroxylase 2 (TPH2) mRNA, protein, and enzymatic activity throughout the male Sprague-Dawley rat brain. TPH2 is the genetic isoform of TPH that catalyzes the rate-limiting step in serotonin biosynthesis within the central nervous system. Although cell bodies of serotonergic neurons are located mainly in the raphe, serotonin-containing axons innervate many regions of the brain. In the present study, we assessed the levels of mRNA, protein expression, and enzyme activity of TPH2 in the rat raphe, ventral tegmental area (VTA), substantia nigra, hippocampus, cerebellum, dorsal striatum, nucleus accumbens, amygdala, and medial prefrontal cortex to more fully understand the distribution of this enzyme throughout the central nervous system. The pineal gland was used as a control tissue that expresses TPH1 (the peripheral enzyme), but not TPH2. As expected, the raphe showed the highest brain TPH2 activity and protein expression. In the contrast to other reports, however, the VTA followed the raphe as the region with the second-highest amount of TPH2 activity, mRNA and protein expression. There were significantly lower TPH activities and levels of TPH2 protein in the other regions. In addition, TPH2 immunocytochemistry demonstrated the presence of TPH-positive cell bodies within the VTA. The results of this study indicate that TPH2 and serotonergic signaling may play an important role in the mesolimbic/mesocortical reward pathway.

Prenatal administration of para-chlorophenylalanine results in suppression of serotonergic system and disturbance of swimming movements in newborn rats

Postnatal development of the spinal cord serotonergic (5-HT) system and of swimming movements were studied in newborn Sprague-Dawley rats, in which the serotonin level in the central nervous system was lowered in the prenatal period. For this purpose, para-chlorophenylalanine (PCPA) (300 mg/kg) was administered intraperitoneally to pregnant mother rats on day 8 of gestation, followed by a daily injection of PCPA (80 mg/kg) from day 9 of gestation to delivery. The postnatal development of the 5-HT system in the spinal cord of the pups (PCPA-treated pups) born from the PCPA-administered mothers was markedly delayed during the period between PND 1 and PND 10 in comparison to that in the control pups born from healthy mothers. Postnatally, the control pups developed their swimming movements regularly through three distinct phases: forelimb dominant, forelimb and hindlimb well coordinated, hindlimb dominant. In contrast, in the PCPA-treated pups, swimming movements were disorganized during the period in which the development of 5-HT system was delayed. However, between PND 17 and 22 in which the 5-HT system developed to that extent observed in the control pups, the pups eventually developed swimming movements as observed in the control pups. These results suggest that the disorganized developmental process of swimming movements in the PCPA-treated pups is due to the possible failure in the prenatal and postnatal development of the 5-HT system and its target system in the brain stem and the spinal cord.

Synthesis of serotonin from 5-hydroxytryptophan in the post-crush retina: inhibition of in vitro outgrowth by the intraocular administration of the precursor

Serotonin is present in the retina of many species, in which plays roles as a neurotransmitter, as a modulator of regeneration, and as the precursor of melatonin. The turnover of serotonin in the goldfish retina is modified by the lesion of the optic nerve and, in postcrush goldfish retinal explants, serotonin inhibits the outgrowth. In the present study, the modification of the serotonergic system of the retina induced by the process of regeneration was explored. The addition of the precursor of serotonin, 5-hydroxytryptophan, to retinal explants, increased the levels of serotonin in a concentration-dependent manner. The concentration of serotonin differentially increased in control and postcrush explants cultured in the presence of 5-hydroxytryptophan for various periods of time, indicating a greater accumulation of the indoleamine at early periods of time in the control than in the postcrush tissue culture. This observation, together with the fact that serotonin concentration in postcrush retina cultured in the absence of 5-hydroxytryptophan and exposed to the precursor for 60 min increased less than in the control, indicates a saturation of the serotonergic system produced by the lesion. The addition of imipramine or citalopram, serotonin uptake blockers, did not significantly change the concentration of serotonin in the cultures, thus, the elevation of serotonin accumulation, especially in the post-crush tissue, might not be due to the transport from the medium. The intraocular injection of 5-hydroxytryptophan after the crush of the optic nerve resulted in a decrease in the outgrowth of retinal explants, supporting the in vivo role of serotonin during the regenerating process in situ. The lesion of the optic nerve did not affect the specific cells, since the number of serotonin-immunoreactive neurons in the retina were not modified by the crush. Taken together, retinal serotonin system is regulated after producing a lesion of the optic nerve, a modulation which has been demonstrated in vivo and in vitro. Thus, there is a reciprocal interaction, since serotonin influences outgrowth in the postcrush retina and the serotonergic system is modulated by the crush, indicating a mechanism of feed-back regulation.

Postnatal development of locomotor movements in normal and para-chlorophenylalanine-treated newborn rats

The contribution of the serotonergic system to postnatal development of locomotor movements in newborn Sprague-Dawley rats was studied from PND 1 to PND 15. To deplete serotonin (5-HT), p-chlorophenylalanine (PCPA) (200 mg/kg) was intraperitoneally administered at PND 1, 4, 7, 10 and 13, and the results obtained from PCPA-treated pups (n = 20) and control pups (n = 20) were compared. Locomotor movements of the pups on the surface of a digitizer table were measured by attaching a miniature coil to the abdomen of the pups. Video-recordings were made to study the locomotor movements. From PND 1 to PND 5, no significant difference in body weights and locomotor movements was observed across control and PCPA-treated pups. After PND 6, the development of PCPA-treated pups was slightly retarded although both groups of pups opened their eyes at PND 14. Throughout PND 6 to PND 15, the total distance and mean velocity of locomotor movements were significantly lower in PCPA-treated pups than in the control pups. In addition, forelimb and hindlimb movements were not well coordinated in the former. These results suggest that retardation of locomotor movements in the PCPA-treated pups is due to a possible failure in the postnatal development of 5-HT and other neural mechanisms related to postural and locomotor control, and not due to that of general development of the pups themselves.

Evidence for postsynaptic mediation of the hypothermic effect of 5-HT1A receptor activation

1. The 5-HT1A ligand BMY 7378 (8-[2[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]8-azaspirol [4,5]-decane-7,9-dione dihydrochloride, 0.032-2 mg kg-1, s.c.) caused hyperphagia, a response to the activation of presynaptic 5-HT1A receptors. 2. BMY 7378 (8 mg kg-1, s.c.) and the 5-HT1A agonist (8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), 0.10 and 0.25 mg kg-1 s.c.) also caused hypothermia. This was inhibited by (-)-pindolol (1-mg kg-1, i.p.) and not prevented by pretreatments with p-chlorophenylalanine which grossly depleted 5-hydroxytryptamine (5-HT) from terminal regions. The hypothermic effects are explicable by activation of postsynaptic 5-HT1A receptors. Infusion of BMY 7378 (8-64 micrograms) into the dorsal raphe was without convincing hypothermic effect. 3. BMY 7378 (8 mg kg-1, s.c.) inhibited another effect of activation of postsynaptic 5-HT1A receptors, i.e., the induction of components of the 5-HT syndrome by 8-OH-DPAT (0.5, 1.0 mg kg-1, s.c.) which suggests that BMY 7378 has antagonistic as well as agonistic effects at these sites. 4. Partial agonist properties of BMY 7378 at postsynaptic sites were also indicated by doses for hypothermia being much greater than those for hyperphagia i.e., ED50 (hypothermia) greater than 2 mg kg-1, ED50 (hyperphagia) = 0.010 mg kg-1. This contrasts with the similar ED50 values for both the hypothermic (ED50 = 0.08-0.10 mg kg-1) and hyperphagic (ED50 = 0.06-0.10 mg kg-1) effects of 8-OH-DPAT.5. The evidence obtained for mediation of the hypothermic response to 5-HTIA agonists by postsynaptic sites is relevant to the interpretation of the effects on it of antidepressant treatments and depressive illness.

Biochemical and functional studies on EMD 49,980: a potent, selectively presynaptic D-2 dopamine agonist with actions on serotonin systems

EMD 49,980 proved to be a potent and selectively presynaptic D-2 dopamine receptor agonist in biochemical studies with rats. Thus, the gamma-butyrolactone-induced accumulation of dihydroxyphenylalanine, used as a presynaptic model, was antagonized with ED50 values of 0.29 and 0.09 mumol/kg in striatum and t. olfactorium, respectively, with high maximal effects. In contrast, striatal acetylcholine concentrations, reflecting actions at normosensitive postsynaptic D-2 receptors, were only moderately increased by about 30% over a dose range of 2.3-68 mumol/kg. In rats with unilateral nigrostriatal lesions, EMD 49,980 induced long-lasting contralateral turning, indicative of agonistic actions at denervated postsynaptic D-2 receptors. In addition, EMD 49,980 potently inhibited serotonin (5-HT) uptake in vitro and in vivo. Binding studies confirmed D-2 activity in the nM range but, similarly potent effects were observed at 5-HT1A binding sites. Measurement of 5-hydroxytryptophan (5-HTP) accumulation in the n. raphe revealed that, in vivo, the net effect of EMD 49,980 on 5-HT systems is an agonistic one. Control experiments indicate that inhibition of 5-HTP accumulation by EMD 49,980 is induced mainly via direct activation of 5-HT1A receptors, although some contribution due to 5-HT uptake inhibition is likely. Furthermore, results with various reference compounds make it unlikely that there are indirect effects, also via alpha 2-receptors in the models used and support the view that D-2 agonistic, 5-HT uptake inhibiting and 5-HT1A agonistic actions are independent properties of EMD 49,980.

Differential effects of p-chlorophenylalanine on indoleamines in brainstem nuclei and spinal cord of rats. I. Biochemical and behavioral analysis

The involvement of endogenous serotonergic pathways in the mediation of antinociception has been indicated by electrophysiological, pharmacological and behavioral experiments. However, manipulation of the indole pathway, either by lesioning of raphe nuclei or drug intervention, often produces disparate results. In particular, serotonin (5-HT) synthesis inhibition with p-chlorophenylalanine (PCPA) has been reported to produce either hyperalgesia or analgesia, depending upon the type of pain measurement examined. In the present study, we sought to evaluate the effects of PCPA on (1) behavioral responses to noxious stimulation, and (2) levels of serotonin, tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) in raphe nuclei (pallidus, obscurus, magnus and dorsalis) and spinal cord regions by HPLC with electrochemical detection. Treatment of rats with 400 or 600 mg/kg of PCPA for 3 consecutive days resulted in significant elevations in pain thresholds assessed by tail withdrawal from radiant heat as well as vocalization to electric shock of the tail. The effect of PCPA on vocalization threshold was particularly striking, for the majority of animals showed a nociceptive-specific attenuation of this response. Although the PCPA induced changes in indole content of the various raphe nuclei were not unequivocally dose-dependent, differential reductions of serotonin and 5-HIAA were clearly detected in the various raphe regions. Nuclei raphe pallidus and obscurus were depleted of 5-HT and 5-HIAA to the greatest extent, whereas levels detected in nuclei raphe magnus and dorsalis were reduced by 30-40% from control values. Metabolism of 5-HT and 5-HIAA appeared unaffected by PCPA in all regions examined except the dorsal portion of the spinal cord. These findings collectively suggest that the effects of PCPA are not uniform throughout the central nervous system and raise the possibility that discrepancies in the behavior literature may be attributed to drug-induced changes in some, but not all serotonergic pathways.

Regional brain monoamines and their metabolites after portacaval shunting

Disturbances in brain monoamine neurotransmitter metabolism have been implicated in the development of hepatic encephalopathy produced by portacaval shunting or liver disease. We have measured the content of serotonin, norepinephrine and dopamine, as well as their metabolites 5-hydroxyindoleacetic acid, dihydroxyphenylacetic acid and homovanillic acid in nine selected brain areas of rats with portacaval shunts and sham-operated control rats. All substances were measured in single samples using high performance liquid chromatography with electrochemical detection, after a simple extraction procedure. In shunted rats serotonin content was 26% higher in the raphe nuclei area, and 5-hydroxyindoleacetic acid throughout the brain (by 51 to 137%), suggesting increased serotonin turnover. Norepinephrine content was higher by 26% in the frontal cortex. Dopamine content was unaffected; however its metabolites were higher in a few areas including the caudate and ventral tegmentum. Brain content of the monoamine precursor amino acids tryptophan, tyrosine and phenylalanine was higher throughout the brain in the shunted rats. The results suggest that serotonin metabolism is altered throughout the brain after portacaval shunting, which could be related to some of the characteristic behavioral abnormalities found in this condition. Catecholamine metabolism appears to be more selectively and less extensively affected.

Serotonin metabolism in the CNS in cerebellar ataxic mice

The metabolism of 5-hydroxytryptamine (5-HT) in the CNS was investigated in four kinds of morphologically different ataxic mice; reeler, staggerer, weaver and Purkinje cell degeneration mutants, and in hypocerebellar mice experimentally produced by injection of cytosine arabinoside. 5-HT and 5-hydroxyidoleacetic acid concentrations and tryptophan hydroxylase (TrpOH) activity were measured in the cerebrum, cerebellum and brain stem, respectively. TrpOH activity was significantly reduced only in the reeler mouse. The enhancements of the cerebellar 5-HT metabolism observed in the ataxic mice other than the reeler were supposed to be pseudo-enhancements subsequent to the cerebellar hypoplasia.

Reserpine and the monoaminergic regulation of adrenal dopamine beta-hydroxylase activity

The systemic administration of reserpine to rats increases adrenal dopamine beta-hydroxylase activity, but is without significant effect on the Km(tyramine). This induction is partially blocked by hemisplanchnicotomy and by impairment of translation. The combined administration of (a) alpha-methyl-p-tyrosine and p-chlorophenylalanine; (b) 6-hydroxydopamine and p-chlorophenylalanine; or (c) alpha-methyl-p-tyrosine and 5,7-dihydroxytryptamine increases adrenal dopamine beta-hydroxylase activity. These results suggest that the simultaneous depletion of central serotonin and catecholamines, as achieved by reserpine alone or by conjoint action of two specific drugs, is necessary for the induction. p-Chlorophenylalanine (intraperitoneal) or 5,7-dihydroxytryptamine (intracerebroventricular or injected into the medial raphe nucleus) increases the effect of reserpine, but the use of a catecholamine-depleting agent with reserpine does not alter the increase of adrenal dopamine beta-hydroxylase obtained with reserpine alone. The potentiation by 5,7-dihydroxytryptamine is abolished by hemisplanchnicotomy, a result that demonstrates neural mediation of its effect. Although intravenous administration of 6-hydroxydopamine alone increases the activity of adrenal dopamine beta-hydroxylase, the combination of this treatment with p-chlorophenylalanine does not elevate it further, as occurs with intracerebroventricular injections; this suggests a specific role of central catecholamine depletion. The serotonin agonists 5-hydroxytryptophan, fenfluramine and 5-methoxy-N,N-dimethyltryptamine abolish the inducing effect of reserpine. This work sheds light on the action of reserpine as inducer and provides evidence for the role of monoaminergic pathways, with net inhibitory effects, that are involved in the regulation of the activity of an adrenal enzyme.

Serotonin in body fluids: characterization of human plasmatic and cerebrospinal fluid pools by means of a new HPLC method

A new HPLC technique for the analysis of picomolar amounts of serotonin (5HT) in plasma and cerebrospinal fluid (CSF) is described. Bufotenin is used as internal standard. Detection is achieved electrochemically or fluorimetrically. The detection limit can be estimated as 50 pg 5HT/mL of either fluid (0.3 picomolar). The method is used to characterize a non-particulate pool of 5HT which is clearly distinct of the platelet pool. Administration of parachlorophenylalanine (PCPA) 300 mg/kg to rats leads to a 90% reduction in the plasmatic pool whereas platelet 5HT is only slightly decreased (3rd day after PCPA) or even increased (7th day after PCPA). Human concentration (n = 15) of 5HT in plasma is 2.6 +/- 0.9 ng/mL (X +/- S.D.). The application of the method to CSF of neurological patients reveals 5HT concentrations ranging from 93 to 962 pg/mL.

Epinephrine accumulation in rat brain after chronic administration of pargyline and LY 51641--comparison with other brain amines

Concentrations of biogenic amines and metabolites were measured in regions of rat brain following administration of monoamine oxidase (MAO) inhibitors for 21 days. Epinephrine concentrations were increased from 350 to 500% following chronic administration of LY 51641, a selective inhibitor of MAO type A. Norepinephrine, dopamine and serotonin showed much less relative accumulation. The marked relative accumulation of epinephrine may be related to the efficacy of inhibitors of MAO type A in the treatment of depression.

Decreased serotonin content of embryonic raphe neurons following maternal administration of p-chlorophenylalanine: a quantitative immunocytochemical study

Previous studies from this laboratory have suggested that serotonergic (5-HT) neurons may influence the differentiation of their embryonic target cells in the developing rat brain. The present study was designed to determine whether or not maternal p-chlorophenylalanine (pCPA) administration could deplete serotonin (5-HT) in developing 5-HT neurons during embryonic days 13-15, when the effects of pCPA on neuronal genesis have been observed previously. For this study, pCPA was administered to timed-pregnant rats and embryos were sacrificed at two different gestational ages, embryonic days 13-14 (E13-14) and 14-15 (E14-15). Immunotitration experiments were carried out on tissue sections, using an antiserum to 5-HT-hemocyanin conjugates to obtain a relative estimate of the amount of 5-HT contained within individual 5-HT neurons of embryos from pCPA-treated and control mothers. Diminished immunoreactivity as a consequence of addition of increasing amounts of antigen was then quantitated on a relative scale by comparison with the amount of immunoreactivity present when no antigen was added to the primary antiserum. Two major findings resulted from this study: maternal pCPA treatment depleted 5-HT by approximately 50% in developing 5-HT neurons at embryonic ages E13-14 and E14-15, but depletion appeared to be greatest in the youngest embryos; developing 5-HT neurons increased their content of neurotransmitter by approximately 10-fold during this one day of embryonic development, an effect which could be observed in both pCPA-treated and control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyrotropin releasing hormone increases 5-hydroxytryptamine1 receptors in the limbic brain of the rat

The effect of TRH on 5-HT1 receptors in the rat brain was investigated. A crude membrane preparation was incubated at 37 degrees C for 15 min with or without TRH prior to [3H]5-HT binding assay. TRH at 100 nM increased the number of 5-HT1 receptors significantly (approximately 20%) in the limbic forebrain and the hippocampus without altering their affinity. As this concentration of TRH is close to its dissociation constant (2 nM and 51 nM in the limbic forebrain, 11 nM in the hippocampus), this effect is probably of physiological relevance. This finding seems to support a pharmacological finding of others that the anti-convulsion effect of TRH may be related to increased serotonergic transmission.

Time course analysis of para-chlorophenylalanine induced suppression of self-stimulation behavior

The hypothesis of a serotonergic basis of reward rests partly on data showing that serotonin (5-HT) depletion by para-chlorophenylalanine (p-CPA) causes depression of self-stimulation (SS) rates. These data do not clearly demonstrate a time course relationship between 5-HT depletion and SS suppression. The present study shows that SS behavior has fully recovered from p-CPA induced suppression when 5-HT levels are still maximally depleted. The data reveal that the effects of electrical brain SS on p-CPA induced reductions of 5-HT cannot explain the temporal dissociation between 5-HT depletion and SS suppression. As a whole the data suggest that midbrain 5-HT neurons are not critically involved in SS behavior.

Effects of ovarian steroids on serotonin metabolism within grossly dissected and microdissected brain regions of the ovariectomized rat

Brain serotonin (5-HT) levels and accumulation rate within the dorsal raphe nucleus (DR) were elevated in ovariectomized (OVX) rats after ovarian steroid treatment, which consisted of estradiol benzoate (EB) followed by progesterone (P) 48 hr later. Some animals were also given the monoamine oxidase (MAO) inhibitor pargyline at designated times 10-40 min prior to sacrifice. Decapitation occurred 5 hr following the second steroid injection and 4-6 hr into the dark phase of a 14:10 light/dark cycle. Basal 5-HT levels and the rate of 5-HT accumulation (5-HTr) following MAO inhibition were estimated by radioenzymatic assay of brain tissue punched from frozen sections or dissected freehand. Within the DR, 5-HT levels and 5-HTr rates were respectively 13.8% (p less than 0.001) and 23.2% (p less than 0.025) higher in EB+P treated females compared with oil controls. Differences in basal 5-HT were found within the median raphe nucleus, but these were inconsistent from experiment to experiment. No differences were found in tissue immediately adjacent to these nuclear regions or within other brain regions including the ventral medial nucleus and suprachiasmatic nucleus of the hypothalamus. Increased levels of 5-HT were also found in grossly dissected tissue containing the mesencephalic raphe nuclei.

Roles for serotonin in neuroembryogenesis

Possible non-transmitter roles for 5-HT in different phases of early neuroembryogenesis have been discussed based upon experimental evidence from the rat and chick. Fluorescence histochemical studies have demonstrated sites of uptake and synthesis of 5-HT in the chick embryo during the first few days of incubation. These sites are located in discrete regions of the notochord and floor plate of the neural tube as well as in extra-neural regions such as the somites and primitive gut. The 5-HT patterns are distinctly different from those observed for the uptake and synthesis of norepinephrine in embryos of the same age. Spatio-temporal changes in the distribution of these sites during closure of the neural tube suggest a role for 5-HT in various aspects of neural tube development. Moreover, the non-overlapping localization of 5-HT and norepinephrine raises the possibility that these two amines may exert different and perhaps cooperative influences on early neurogenic processes in the chick. In the rat, autoradiographic and biochemical studies concerning the consequences of 5-HT depletion in the embryo for development of different brain regions have provided evidence that 5-HT acts as a "differentiation signal" regulating the time of neuronal genesis in those cell populations which will eventually receive 5-HT innervation. Although the details of this system are as yet unknown, these studies suggest that 5-HT (and possibly the other monoamine transmitters) may actually "mold" the construction of their own circuitry during neurogenesis. Further, the ability of drugs and stress to interact with this process during that period of gestation when the monoamines are required as humoral signals suggests that maternal influences can interfere with ontogeny of this circuitry during pre- and possibly postnatal development. It is not yet clear whether the data in chicks and rats can be directly analogized from the one species to the other. Nevertheless, the evidence that sites of 5-HT uptake and/or synthesis are present during the earliest phases of neurogenesis in the chick and the observation that 5-HT depletion can alter the time of genesis of 5-HT target cells in the rat provide a new context for the consideration of possible actions of 5-HT prior to its role as a neurotransmitter substance.

Determination of tryptophan and metabolites in rat brain and pineal tissue by reversed-phase high-performance liquid chromatography with electrochemical detection

Tryptophan and many of its indole metabolites were separated using reversed-phase high-performance liquid chromatography (HPLC) and determined using electrochemical detection. This was accomplished isocratically using an acetate--citric acid eluent with various amounts of methanol. Brain and pineal tissue was analyzed for several tryptophan metabolites. Tissue preparation required only homogenization in acidic solution and centrifugation prior to application to the HPLC column. Detection limits in the low picogram range were found for those indoles separated.

Open field activity and avoidance behavior following serotonin depletion: a comparison of the effects of parachlorophenylalanine and electrolytic midbrain raphe lesions

Three experiments were performed in order to compare the behavioral effects of electrolytic destruction of the dorsal and median mesencephalic raphe nuclei (MR lesion) and parachlorophenylalanine (pCPA; 300 mg/kg, IP) administration. Forebrain 5-hydroxytryptamine (5-HT) was measured in all animals following completion of behavioral testing. In the first experiment open field behavior (one 50 min session) and two-way (shuttle) conditioned avoidance acquisition (50 massed trials) were examined 68-72 hr after vehicle or pCPA administration in rats which had received control operations or MR lesions two weeks earlier. Only the MR lesion and the MR lesion + pCPA groups evidenced increased open field activity and facilitated two-way avoidance learning. Although the reduction in forebrain 5-HT of the pCPA group (85%) was greater than in the MR lesion group (55%), the pCPA treated animals did not differ from the control group. In the second experiment animals were tested in the open field 24, 48 or 72 hr after pCPA treatment to determine its effects on activity level as a function of the time after injections. No differences between the vehicle and pCPA groups, however, were found. In the third experiment, the effects of pCPA (72 hr postinjection) on the acquisition of an unsignalled one-way avoidance response was examined. MR lesion rats tested in the same apparatus and with the same procedure repeatedly have been shown to be impaired in this task. The pCPA and vehicle animals, however, did not differ. Reduction in 5-HT following electrolytic MR lesions and pCPA administration, thus, produce different behavioral effects. MR lesions, but not pCPA treatment, result in (1) increased activity in a novel environment, (2) facilitated two-way conditioned avoidance learning, and (3) impaired acquisition of an unsignalled one-way avoidance response. These data support earlier studies suggesting that the behavioral effects of electrolytic MR lesions are not due primarily to their disruption of ascending 5-HT pathways. The role of 5-HT in avoidance conditioning and the regulation of activity level, furthermore, remains to be elucidated.

Electrophysiological evidence against negative neuronal feedback from the forebrain controlling midbrain raphe unit activity

The hypothesis that the activity of serotonin (5-HT)-containing neurons of the midbrain raphe is subject to negative neuronal feedback regulation was examined. This hypothesis is based primarily on the observation that the administration of drugs which increase the synaptic availability of 5-HT depress midbrain raphe neuron discharge. Since the preponderance of midbrain raphe efferents are ascending, transections which interrupt both the main efferent outflow, as well as all inputs from anterior levels, ought to disrupt the functional integrity of a neuronal feedback loop. The effect of complete transections of the neuraxis placed just rostral to the midbrain raphe nuclei on the efficacy of two drugs which elevate synaptic serotonin, chlorimipramine and p-chloroamphetamine, was investigated in the chloral hydrate anesthetized rat. Such transections neither blocked nor attenuated the depressive effect of intravenously administered chlorimipramine (0.33 or 0.15 mg/kg) or p-chloroamphetamine (1.25 mg/kg) on midbrain raphe unit discharge. These results suggest that neuronal feedback involving the forebrain dose not mediate the depressive effect of drugs which elevate synaptic serotonin on midbrain raphe neuronal activity. An action at serotonergic synapses intrinsic to the midbrain raphe is suggested as an explanation for the persistence of drug effects in transected animals.

Brain tryptophan hydroxylase: purification of, production of antibodies to, and cellular and ultrastructural localization in serotonergic neurons of rat midbrain

Tryptophan hydroxylase [EC 1.14.16.4; L-tryptophan, tetrahydropteridine:oxygen oxidoreductase (5-hydroxylating)], the enzyme catalyzing the rate-limiting step in the biosynthesis of serotonin, was purified 79-fold from the region of the raphe nucleus of rat midbrain by sequential column chromatography and disc-gel electrophoresis. In electrophoresis three bands were distinguished, A, B, and C, which, when separated and submitted individually to electrophoresis, reproduced the same three bands. Bands A and C were enzymatically active and inhibited by para-chlorohenylalanine. Antibodies produced to each of the three bands crossreacted by immuno double diffusion and electrophoresis with each other and homogenates of raphe nuclei; they completely inhibited enzyme activity only of tryptophan hydroxylase. Tryptophan hydroxylase was localized by light and electron immunohistochemistry to serotonin neutrons of the raphe. Ultrastructurally, in cell bodies, the enzyme was distributed in cytoplasm and in association with endoplasmic reticulum and Golgi apparatus. In dendrites and axons, it was associated with microtubules. Tryptophan hydroxylase in brain is only neuronal and cytoplasmic, exists in multiple forms, and is associated with microtubules, suggesting it may be transported from sites of synthesis in cell body into axons.

[Hypersomnia by isthmic lesion in cat. Study of the metabolism of cerebral monamines]

(1) A significant increase of 5-HT synthesis is observed in several areas of the cat brain 24 h after the bilateral destruction of the dorsal noradrenergic bundle in the isthmus. This stimulation of the synthesis was simultaneously observed at the level fo serotoninergic cell bodies (anterior part of the raphe system) and of 5-HT terminals (cortex, thalamus, mesencephalon, medulla oblongata). Conversely, a significant decrease of 5-HT synthesis was found in the caudal part of the raphe and in the hypothalamus. The possibility of a catecholaminergic control of 5-HT synthesis by neurons passing through the isthmus is discussed. (2) in the same experimental conditions, an important decrease of endogenous dopamine content without any subsequent change of noradrenaline concentration was observed in the thalamus, the geniculate body and the pons. This important decrease could be due to a greater utilization of dopamine into hypothetical dopaminergic terminals localized in these structures.