Serotonin in cisternal cerebrospinal fluid of the rat: measurement and use as an index of functionally active serotonin

The effects of 5-hydroxytryptophan on attention and central serotonin neurochemistry in the rhesus macaque

Psychiatric disorders, particularly depression and anxiety, are often associated with impaired serotonergic function. However, serotonergic interventions yield inconsistent effects on behavioral impairments. To better understand serotonin's role in these pathologies, we investigated the role of serotonin in a behavior frequently impaired in depression and anxiety, attention. In this study, we used a quantitative, repeated, within-subject, design to test how L-5-hydroxytryptophan (5-HTP), the immediate serotonin precursor, modulates central serotoninergic function and attention in macaques. We observed that intramuscular 5-HTP administration increased cisternal cerebrospinal fluid (CSF) 5-HTP and serotonin. In addition, individuals' baseline looking duration, during saline sessions, predicted the direction and magnitude in which 5-HTP modulated attention. We found that 5-HTP decreased looking duration in animals with high baseline attention, but increased looking duration in low baseline attention animals. Furthermore, individual differences in 5-HTP's effects were also reflected in how engaged individuals were in the task and how they allocated attention to salient facial features-the eyes and mouth-of stimulus animals. However, 5-HTP constricted pupil size in all animals, suggesting that the bi-directional effects of 5-HTP cannot be explained by serotonin-mediated changes in autonomic arousal. Critically, high and low baseline attention animals exhibited different baseline CSF concentrations of 5-HTP and serotonin, an index of extracellular functionally active serotonin. Thus, our results suggest that baseline central serotonergic functioning may underlie and predict variation in serotonin's effects on cognitive operation. Our findings may help inform serotonin's role in psychopathology and help clinicians predict how serotonergic interventions will influence pathologies.

Cranial dural permeability of inflammatory nociceptive mediators: Potential implications for animal models of migraine

Background Application of inflammatory mediators to the cranial dura has been used as a method to activate and sensitize neurons in the meningeal sensory pathway in preclinical behavioral studies of headache mechanisms. However, the relatively high concentrations and volumes used in these studies raise the question of whether the applied agents might pass through the dura to act directly on central neurons, thus bypassing the dural afferent pathway. Methods We used a radiolabeling approach to quantify the meningeal permeability of two of the inflammatory mediators, 5-HT and PGE2, when applied to the cranial dura as part of an inflammatory mixture used in preclinical headache models. Results Both agents could be detected in samples taken four hours after dural application in the cerebrospinal fluid (CSF) and, in measurements made only for PGE2, in the central nervous system (CNS) as well. Based on our measurements, we made estimates of the CSF and CNS levels that would be attained with the higher concentrations and volumes of 5HT and PGE2 that were exogenously applied in previous pre-clinical headache studies. These estimated levels were comparable to or larger than normal endogenous levels, potentially large enough to have physiological effects. Conclusions The finding that the cranial meninges are permeable to the two tested inflammatory mediators PGE2 and 5-HT raises some uncertainty about whether the behavioral changes observed in prior pre-clinical headache studies with these as well as other agents can be attributed entirely to the activation of dural nociceptors, particularly when the agents are applied at concentrations several orders of magnitude above physiological levels.

Serotonin increases ERK1/2 phosphorylation in astrocytes by stimulation of 5-HT2B and 5-HT2C receptors

We have previously shown that fluoxetine causes ERK(1/2) phosphorylation in cultured mouse astrocytes mediated exclusively by stimulation of 5-HT(2B) receptors (Li et al., 2008b). This raises the question whether this is also the case for serotonin (5-HT) itself. In the present study serotonin was found to induce ERK(1/2) phosphorylation by stimulation of 5-HT(2B) receptors with high affinity (EC(50): 20-30 pM), and by stimulation of 5-HT(2C) receptor with low affinity (EC(50): 1 microM or higher). ERK(1/2) phosphorylation induced by stimulation of either 5-HT(2B) or 5-HT(2C) receptors was mediated by epidermal growth factor (EGF) receptor transactivation (Peng et al., this issue), shown by the inhibitory effect of AG1478, an inhibitor of the EGF receptor tyrosine kinase, and GM6001, an inhibitor of Zn-dependent metalloproteinases, and thus of 5-HT(2B) receptor-mediated EGF receptor agonist release. It is discussed that the high potency of the 5-HT(2B)-mediated effect is consistent with literature data for binding affinity of serotonin to cloned human 5-HT(2B) receptors and with observations of low extracellular concentrations of serotonin in brain, which would allow a demonstrated moderate and modality-dependent increase in specific brain areas to activate 5-HT(2B) receptors. In contrast the relevance of the observed 5-HT(2C) receptors on astrocytes is questioned.

Animal violence demystified

Violence has been observed in humans and animals alike, indicating its evolutionary/biological significance. However, violence in animals has often been confounded with functional forms of aggressive behavior. Currently, violence in animals is identified primarily as either a quantitative behavior (an escalated, pathological and abnormal form of aggression characterized primarily by short attack latencies, and prolonged and frequent harm-oriented conflict behaviors) or a qualitative one (characterized by attack bites aimed at vulnerable parts of the opponent's body and context independent attacks regardless of the environment or the sex and type of the opponent). Identification of an operational definition for violence thus not only helps in understanding its potential differences from adaptive forms of aggression but also in the selection of appropriate animal models for both. We address this issue theoretically by drawing parallels from research on aggression and appeasement in humans and other animals. We also provide empirical evidences for violence in mice selected for high aggression by comparing our findings with other currently available potentially violent rodent models. The following violence-specific features namely (1) Display of low levels of pre-escalatory/ritualistic behaviors. (2) Immediate and escalated offense durations with low withdrawal rates despite the opponent's submissive supine and crouching/defeat postures. (3) Context independent indiscriminate attacks aimed at familiar/unfamiliar females, anaesthetized males and opponents and in neutral environments. (4) Orientation of attack-bites toward vulnerable body parts of the opponent resulting in severe wounding. (5) Low prefrontal serotonin (5-HT) levels upon repeated aggression. (6) Low basal heart rates and hyporesponsive hypothalamus-pituitary-adrenocortical (HPA) axis were identified uniquely in the short attack latency (SAL) mice suggesting a qualitative difference between violence and adaptive aggression in animals.

Characterization of cerebrospinal fluid monoamine metabolites in peripubertal chimpanzees (Pan troglodytes)

BACKGROUND

Assessment of cerebrospinal (CSF) monoamine metabolites 5-hydroxyindoeacetic acid (5-HIAA) and homovanillic acid (HVA), and the serotonin precursor tryptophan (TRP), in chimpanzees may help in understanding the neurobiology underlying aggressive, impulsive behavior in humans and non-human primates.

METHODS

Two CSF samples were obtained from 11 peripubertal chimpanzees 8 months apart and were assayed for monoamine metabolite and TRP concentrations.

RESULTS

Substantial inter-individual stability was observed for 5-HIAA (n = 11; r = 0.83, P < 0.001) and HVA (r = 0.91, P < 0.001). Females had significantly higher concentrations of 5-HIAA compared to males (F(1,8) = 7.31; P < 0.05). Levels of 5-HIAA (r = -0.62, P < 0.05), HVA (r = -0.86, P < 0.001) and TRP levels (r = -0.67; P < 0.05) decreased with age.

CONCLUSION

Close parallels were observed between chimpanzees and humans with respect to absolute levels, sex effects, ontogeny, and 5-HIAA-HVA correlations, supporting the potential utility of the measures in understanding relationships between monoamine functioning and behavior in chimpanzees and humans.

Endocrine and metabolic effects of consuming fructose- and glucose-sweetened beverages with meals in obese men and women: influence of insulin resistance on plasma triglyceride responses

CONTEXT

Compared with glucose-sweetened beverages, consumption of fructose-sweetened beverages with meals elevates postprandial plasma triglycerides and lowers 24-h insulin and leptin profiles in normal-weight women. The effects of fructose, compared with glucose, ingestion on metabolic profiles in obese subjects has not been studied.

OBJECTIVE

The objective of the study was to compare the effects of fructose- and glucose-sweetened beverages consumed with meals on hormones and metabolic substrates in obese subjects.

DESIGN AND SETTING

The study had a within-subject design conducted in the clinical and translational research center.

PARTICIPANTS

Participants included 17 obese men (n = 9) and women (n = 8), with a body mass index greater than 30 kg/m(2).

INTERVENTIONS

Subjects were studied under two conditions involving ingestion of mixed nutrient meals with either glucose-sweetened beverages or fructose-sweetened beverages. The beverages provided 30% of total kilocalories. Blood samples were collected over 24 h.

MAIN OUTCOME MEASURES

Area under the curve (24 h AUC) for glucose, lactate, insulin, leptin, ghrelin, uric acid, triglycerides (TGs), and free fatty acids was measured.

RESULTS

Compared with glucose-sweetened beverages, fructose consumption was associated with lower AUCs for insulin (1052.6 +/- 135.1 vs. 549.2 +/- 79.7 muU/ml per 23 h, P < 0.001) and leptin (151.9 +/- 22.7 vs. 107.0 +/- 15.0 ng/ml per 24 h, P < 0.03) and increased AUC for TG (242.3 +/- 96.8 vs. 704.3 +/- 124.4 mg/dl per 24 h, P < 0.0001). Insulin-resistant subjects exhibited larger 24-h TG profiles (P < 0.03).

CONCLUSIONS

In obese subjects, consumption of fructose-sweetened beverages with meals was associated with less insulin secretion, blunted diurnal leptin profiles, and increased postprandial TG concentrations compared with glucose consumption. Increases of TGs were augmented in obese subjects with insulin resistance, suggesting that fructose consumption may exacerbate an already adverse metabolic profile present in many obese subjects.

Effects of early life stress on [11C]DASB positron emission tomography imaging of serotonin transporters in adolescent peer- and mother-reared rhesus monkeys

Peer-reared (PR) rhesus monkeys with early maternal separation later exhibit aggressiveness, impaired impulse control, alcohol abuse, and low CSF 5-hydroxyindoleacetic acid. This study compared regional brain serotonin transporter (SERT) binding between nine PR and seven mother-reared rhesus monkeys with [11C]DASB positron emission tomography (PET) imaging. Parametric images of binding potential (BP) (which is proportional to Bmax/KD, in which Bmax is transporter density and KD is dissociation constant) and relative blood flow (R1) were generated by the two-parameter multilinear reference tissue model. R1 images were used for coregistration and normalization of PET parametric data to the magnetic resonance imaging template space. Group BP differences were analyzed voxelwise by Student's t test in SPM2. Region of interest-based parameter values were also calculated to obtain the magnitude of regional BP differences between the two groups. For the PR group, SERT BP was decreased by 10-23% across a range of brain areas consisting of the raphe, thalamus, hypothalamus, caudate and putamen, globus pallidum, anterior cingulate gyrus, and medial temporal regions, including amygdala and hippocampus (cluster-level corrected p = 0.002). For the latter three regions, BP was decreased in the right hemisphere. These results agree with the hypothesis that early maternal deprivation affects the development of the serotonergic system and suggest that decreased serotonergic innervations in the critical brain regions may explain some of the behavioral and biochemical abnormalities in PR monkeys.

Time course of the effects of the serotonin-selective reuptake inhibitor sertraline on central and peripheral serotonin neurochemistry in the rhesus monkey

RATIONALE

Fundamental questions remain regarding the actions of the selective serotonin reuptake inhibitors (SSRIs).

OBJECTIVES

To examine the time course of central and peripheral neurochemical effects of sertraline (SER) in non-human primates.

METHODS

SER (20 mg/kg, p.o.) or placebo were administered daily for 4 weeks to two groups of six young adult male rhesus monkeys. Both groups received placebo during a 3-week baseline lead-in period and for 6 weeks after discontinuation. Blood and cisternal cerebrospinal fluid (cCSF) samples were obtained on days -21, -14, -7, 0, +3, +7, +14, +21, +28, +35 and +70.

RESULTS

In animals receiving SER, mean (+/-SD) levels of cCSF serotonin (5-HT) increased from 38.6+/-9.0 pg/ml at baseline to 128+/-46.4 pg/ml during treatment (paired t=4.17, P=0.014). Concentrations of cCSF 5-HT were 290% of baseline on day 0 (+3 h), ranged from 260% to 436% of baseline during treatment, and returned to baseline 7 days after discontinuation. Levels of cCSF 5-hydroxyindoleacetic acid declined to 51+/-2.0% of baseline by day +3 and remained at similarly reduced levels during treatment. Plasma drug levels and decrements in platelet 5-HT were similar to those seen in patients.

CONCLUSIONS

SER rapidly and substantially increases cCSF levels of 5-HT in primates, the extent of elevation is relatively constant during prolonged administration, and values return to baseline shortly after discontinuation. The results suggest that response latency for SSRIs in depression is not due to gradually increasing brain extracellular fluid 5-HT levels and tend not to support theories that posit SSRI response latency as being due to autoreceptor desensitization, transporter downregulation, or drug accumulation.

Peripheral and central neurochemical effects of the selective serotonin reuptake inhibitors (SSRIs) in humans and nonhuman primates: assessing bioeffect and mechanisms of action

It is clear that selective serotonin reuptake inhibitors (SSRIs) act powerfully to inhibit serotonin (5-hydroxytryptamine, 5-HT) uptake centrally and peripherally. However, there are a number of critical unanswered questions concerning the effects of the drugs in adults and children. The influence of age and duration of treatment on the extent of uptake inhibition and on the enhancement of central serotonergic functioning are unclear. In addition, the relationship of these factors and effects to the therapeutic and adverse effects of the SSRIs remain to be clarified. The general clinical utility of platelet 5-HT measurement is reviewed and studies assessing central and peripheral uptake blockade in infants and children and non-human primates are discussed. Recent investigations of central neurochemical effects of the SSRIs in primates assessed through measurement of 5-HT and related compounds in cisternal cerebrospinal fluid (CSF) of the rhesus monkey are presented. In summary, the studies described have found that: human fetal exposure to SSRIs has substantial effects on 5-HT transport in utero; exposure to SSRIs through breastmilk of mothers treated for postpartum depression usually has negligible effects on 5-HT uptake; prescribed SSRIs appear to exert similar effects on 5-HT transporter blockade in children and adults; and rapid and sustained increases are seen in monkey cisternal CSF levels of 5-HT upon initiation of SSRI administration. The implications of the observations in terms of behavioral effects, clinical practice, and underlying mechanisms of action of the SSRIs are discussed.

Cerebrospinal fluid monoamine and metabolite concentrations and aggression in rats

In humans and other primates low cerebrospinal fluid (CSF) levels of the major serotonin (5-HT) metabolite 5-hydroxyindoleacetic acid (5-HIAA) have been correlated to high aggressiveness. This finding forms the basis of the 5-HT deficiency hypothesis of aggression. Surprisingly, this correlation has not been confirmed in rodents so far, while manipulation studies aimed to investigate the link between 5-HT and aggressive behaviour are mostly carried out in rodents. In this study the relation between aggression and CSF monoamine and metabolite concentrations was investigated in male Wildtype Groningen rats. In sharp contrast to the hypothesis and our expectation, a clear positive correlation was found between the individual level of trait-like aggressiveness and CSF concentrations of 5-HT, 5-HIAA, norepinephrine (NE), dopamine (DA), and 3,4-dihydroxyphenylacetic acid (DOPAC). Shortly after the acute display of aggressive behaviour (as a state-like phenomenon), decreased 5-HT levels and an increase in 5-HIAA/5-HT ratio and NE concentrations were found. Surprisingly, pharmacological challenges known to influence 5-HT transmission and aggressive behaviour did not affect CSF 5-HT and 5-HIAA concentrations, only the NE level was increased. Lesioning 5-HT terminals by 5,7-dihydroxytryptamine (5,7-DHT) administration caused a decrease in CSF 5-HT and 5-HIAA, but without affecting aggressive behaviour. The observed positive correlation between CSF 5-HIAA and trait aggressiveness makes it questionable whether a direct extrapolation of neurobiological mechanisms of aggression between species is justified. Interpretation of CSF metabolite levels in terms of activity of neural substrates requires a far more detailed knowledge of the dynamics and kinetics of a neurotransmitter after its release.

Reissner fiber binds and transports away monoamines present in the cerebrospinal fluid

The subcommissural organ (SCO) is a brain gland that secretes glycoproteins into the cerebrospinal fluid (CSF), where they subsequently aggregate to form Reissner fiber (RF). By addition of newly released glycoproteins to its cephalic end, RF grows constantly through the Sylvian aqueduct, fourth ventricle and central canal of the spinal cord. Disaggregation of RF-material and passage to blood occur when RF reaches the terminal ventricle at the filum. The present investigation was designed to test the hypothesis that RF binds, transports and clears away monoamines present in the CSF. Four experimental protocols were applied: (i) in vivo binding of labeled monoamines to the rat RF, studied by pulse and chase, and after perfusion for 7 days; (ii) identification of monoamines, by high-performance liquid chromatography (HPLC), naturally occurring in the bovine RF; (iii) in vitro binding of labeled and unlabeled monoamines to the isolated bovine RF; and (iv) tentative identification of the amine binding site(s) in RF-proteins by use of specific antibodies. The results obtained indicate that RF participates in the regulation of the CSF concentration of monoamines either by binding and transporting them away throughout the central canal of the spinal cord (L-DOPA, noradrenaline, adrenaline), or by transiently binding them and releasing them back to the CSF (serotonin). Furthermore, evidence was obtained that (i) adrenaline and noradrenaline share the same binding site, and that this site would correspond to a repeated sequence present in the SCO-spondin, the major protein component of RF; and (ii) serotonin has its own binding site in RF.

Serotonin elevates intracellular Ca2+ in rat choroid plexus epithelial cells by acting on 5-HT2C receptors

The effects of serotonin (5-HT) on intracellular calcium activity ([Ca2+]i) in epithelial cells from rat choroid plexuses were examined. Experiments were performed on isolated cells which had been maintained in primary culture. ([Ca2+]i) was measured using micro-spectrofluorimetric techniques and the fluorescent indicator Fura-2. 5-HT was found to increase [Ca2+]i in a dose dependent manner. The [Ca2+]i response was biphasic, with an initial peak of [Ca2+]i (due to release from intracellular stores), followed by an elevated plateau phase (the result of calcium influx). The effect of 1 microM 5-HT was inhibited by mesulergine and mianserin (50 nM), which are antagonists of the 5-HT2C receptor. Spiperone and ketanserin (200 nM), less specific 5-HT2 receptor blockers, caused only a slight reduction in the response to 1 microM 5-HT. The [Ca2+]i response decreased upon repeated challenges with 1 microM 5-HT, probably as a result of receptor desensitisation. Taken together, the data suggest that 5-HT acts at 5-HT2C receptors to increase [Ca2+]i in choroid plexus epithelial cells, both by liberating Ca2+ from intracellular stores and by activating a Ca2+ influx pathway.

The effects of an inhibitor of tryptophan 2,3-dioxygenase and a combined inhibitor of tryptophan 2,3-dioxygenase and 5-HT reuptake in the rat

The effects of a novel inhibitor 680C91 ((E)-6-fluoro-3-[2-(3-pyridyl)vinyl]-1H-indole) of the key enzyme of tryptophan catabolism tryptophan 2,3-dioxygenase (TDO), and a novel inhibitor 709W92 ((E)-6-fluoro-3-[2-(4-pyridyl)vinyl]-1H-indole), of both TDO and 5-hydroxytryptamine (5-HT) reuptake, were examined on tryptophan catabolism, cerebrospinal fluid (CSF) concentrations of tryptophan and 5-HT and serotonergic-mediated physiology and behaviour in the rat. The catabolism of L-[ring-2-14C]tryptophan in vivo was completely inhibited by prior administration of 709W92. 709W92, but not 680C91, potentiated head-twitch produced by 5-hydroxytryptophan, prevented head-twitch and whole brain 5-HT depletion produced by p-chloroamphetamine and rapidly decreased dorsal raphe firing. Both 709W92 and 680C91 elevated CSF tryptophan by up to 260% of basal concentration. A maximally effective dose of 680C91 elevated a global measure of brain extracellular 5-HT (CSF 5-HT) to concentrations similar to those seen maximally after exogenous tryptophan administration (approx 170% of basal). Maximally effective doses of 709W92 increased CSF 5-HT to concentrations comparable to those seen after tryptophan and 5-HT reuptake inhibitor coadministration (approx 900% of basal) and to concentrations greater than those achieved maximally with serotonergically active antidepressant monotherapy (approx 500% of basal). 709W92 did not elevate CSF 5-HT to concentrations associated with the serotonin syndrome (approx 3000% of basal). The combined TDO inhibitor/5-HT reuptake inhibitor, 709W92, showed anxiolytic activity in the rat-pup vocalization model of anxiety. These results show that 709W92 (a novel inhibitor of both TDO and 5-HT reuptake), can produce an elevation of CSF 5-HT similar to that achieved with a serotonin reuptake inhibitor/tryptophan combination therapy but with a more sustained timecourse; such compounds may therefore have superior antidepressant efficacy in the clinic.

Serotonin activates Cl- channels in the apical membrane of rat choroid plexus epithelial cells

The effects of serotonin on ion channel activity in epithelial cells from rat choroid plexus were examined. Serotonin (50 nM, 500 nM and 1 microM) stimulated channel activity in cell-attached patches. The current-voltage (I-V) relationship for the serotonin-activated channel gave a conductance of 26.6 +/- 1.5 pS and the current reversed at an applied electrode potential (-Vp) = 15.3 +/- 3.3 mV with a KCl-rich electrode solution (n = 8). Similar I-V relationships were obtained using electrode solutions in which K+ was replaced by other cations (Na+ and N-methyl-D-glucamine), suggesting that the serotonin-activated channels are selective to Cl-. The effect of 1 microM serotonin on channel activity was inhibited by ritanserin (30 and 100 nM) which has a high affinity for serotonin 5-HT1C receptors and 5-HT2 receptors. Spiperone (30 nM), which binds weakly to 5-HT1C receptors but has a high affinity for 5-HT2 receptors, did not inhibit the actions of serotonin. These data suggest that serotonin increases Cl- channel activity by acting on the 5-HT1C receptors on the epithelium.

Possible link between brain serotonin metabolism and methionine sulfoximine-induced hypothermia and associated behavior in the rat

L-Methionine-D,L-sulfoximine (MSO) intraperitoneally or intracerebroventricularly (third ventricle) injected at convulsant doses induced a hypothermia, primarily associated with a syndrome of ataxia, in the restrained rat maintained at an ambient temperature of 23 degrees C. Depletion of brain serotonin (5-HT) by pretreatment with p-chlorophenylalanine (PCPA), p-chloroamphetamine (PCA), and d-fenfluramine (FFA) did not significantly modify the time course and magnitude of MSO-induced developing hypothermia but it enhanced abnormal motor behavior. Enhancement of 5-HT synthesis in MSO-submitted rats pretreated with 5-hydroxytryptophan (5-HTP) (200 mg/kg, IP) alone or 5-HTP (100 mg/kg, IP) preassociated with carbidopa (10 mg/kg, IP) suppressed significantly hypothermia, but it did not greatly modify motor disturbances. In conclusion, the neurocytochemical processes initiating hypothermia following administration of MSO to the rat appear to be linked to a slowdown of the rate of brain 5-HT turnover, maybe at the level of the midbrain raphe nuclei.

Simultaneous effects of p-chloroamphetamine, d-fenfluramine, and reserpine on free and stored 5-hydroxytryptamine in brain and blood

The effects of acute treatment with p-chloramphetamine, d-fenfluramine, and reserpine on intracellular (brain tissue and whole blood) and extracellular (CSF and platelet-free plasma) compartments of 5-hydroxytryptamine (5-HT) in the brain and blood of the same rats have been examined. These treatments affected 5-HT in brain tissue and whole blood similarly (r = 0.823). Reserpine significantly reduced both intracellular pools at 2 and 24 h. p-Chloroamphetamine and d-fenfluramine were more effective on brain tissue 5-HT. The concentration of 5-HT in CSF was significantly increased by all treatments. p-Chloroamphetamine induced a dramatic 70-fold increase of CSF 5-HT, paralleling a 42% decrease in brain tissue. d-Fenfluramine significantly increased CSF 5-HT to 212% of controls and reduced whole brain 5-HT (-23%). The effects of p-chloroamphetamine and d-fenfluramine on 5-HIAA in brain, CSF, and plasma were nonsignificant. Individual values of 5-hydroxyindoleacetic acid (5-HIAA) in CSF and brain were highly correlated (r = 0.855), indicating that CSF 5-HIAA reflects well the concentration of 5-HIAA in brain tissue. Yet the intra- and extracellular concentrations of 5-HIAA were unrelated to the 5-HT changes. This indicates that CSF 5-HIAA does not reflect the active (extracellular) compartment of 5-HT in brain.

Neuroendocrine regulatory mechanisms in the choroid plexus-cerebrospinal fluid system

The CSF is often regarded as merely a mechanical support for the brain, as well as an unspecific sink for waste products from the CNS. New methodology in receptor autoradiography, immunohistochemistry and molecular biology has revealed the presence of many different neuroendocrine substances or their corresponding receptors in the main CSF-forming structure, the choroid plexus. Both older research on the sympathetic nerves and recent studies of peptide neurotransmitters in the choroid plexus support a neurogenic regulation of choroid plexus CSF production and other transport functions. Among the endocrine substances present in blood and CSF, 5-HT, ANP, vasopressin and the IGFs have high receptor concentrations in the choroid plexus and have been shown to influence choroid plexus function. Finally, the choroid plexus produces the growth factor IGF-II and a number of transport proteins, most importantly transthyretin, that might regulate hormone transport from blood to brain. These studies suggest that the choroid plexus-CSF system could constitute an important pathway for neuroendocrine signalling in the brain, although clearcut evidence for such a role is still largely lacking.

Effect of tryptophan on extracellular concentrations of tryptophan and 5-hydroxyindoleacetic acid in the striatum and cerebellum

The effects of L-tryptophan (50 mg/kg i.p.) on extracellular concentrations of tryptophan and the 5-hydroxytryptamine (5-HT) metabolite 5-hydroxyindoleacetic acid (5-HIAA) were determined in the rat striatum and cerebellum, regions with rich and poor 5-HT innervation, respectively. Determinations were on perfusates from dialysis probes in the brains of conscious, freely moving rats. The pharmacokinetic profiles of dialysate tryptophan after tryptophan load (peak concentration, time to peak concentration, area under curve, and half-life) in the two regions did not differ significantly. The dialysate 5-HIAA concentration in the striatum rose two- to threefold after the administration of tryptophan. Therefore, as 5-HIAA was undetectable in the cerebellum either before or after the administration of tryptophan, the increase of 5-HIAA in the striatum is unlikely to depend appreciably on its production within the cerebral vasculature or outside the brain or on its entering the striatum through a blood-brain barrier damaged by placement of the dialysis probe. Overall, the findings strengthen previous evidence that extracellular 5-HIAA concentrations determined by cerebral dialysis are a valid measure of the metabolism of 5-HT of brain neuronal origin.

Signal-to-noise optimization of HPLC-fluorometric systems and their application to the analysis of indoles

The signal-to-noise optimization of high performance liquid chromatographic (HPLC) flow-cell fluorometric systems is described and the possibilities for further improving limits of detection for indoles is discussed. Application of HPLC-fluorometry to analyses in rat cerebrospinal fluid (CSF), human lumbar CSF, and human blood is presented. Finally, the intriguing clinical finding of hyperserotonemia in autism is discussed.

Ventricular cerebrospinal fluid monoamine transmitter and metabolite concentrations reflect human brain neurochemistry in autopsy cases

Concentrations of dopamine (DA), its metabolites 3-methoxytyramine and homovanillic acid (HVA), noradrenaline (NA), its metabolites normetanephrine (NM) and 3-methoxy-4-hydroxyphenylglycol (MHPG), 5-hydroxytryptamine (5-HT, serotonin), and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were measured in 14 brain regions and in CSF from the third ventricle of 27 human autopsy cases. In addition, in six cases, lumbar CSF was obtained. Monoamine concentrations were determined by reversed-phase liquid chromatography with electrochemical detection. Ventricular/lumbar CSF ratios indicated persistence of rostrocaudal gradients for HVA and 5-HIAA post mortem. Ventricular CSF concentrations of DA and HVA correlated positively with striatal DA and HVA. CSF NA correlated positively with NA in hypothalamus, and CSF MHPG with levels of MHPG in hypothalamus, temporal cortex, and pons, whereas CSF NM concentration showed positive correlations with NM in striatum, pons, cingulate cortex, and olfactory tubercle. CSF 5-HT concentrations correlated positively with 5-HT in caudate nucleus, whereas the concentration of CSF 5-HIAA correlated to 5-HIAA levels in thalamus, hypothalamus, and the cortical areas. These data suggest a specific topographic origin for monoamine neurotransmitters and their metabolites in human ventricular CSF and support the contention that CSF measurements are useful indices of central monoaminergic activity in man.

Simultaneous high-performance liquid chromatographic determination of norepinephrine, serotonin, acetylcholine and their metabolites in the cerebrospinal fluid of anaesthetized normotensive rats

A high-performance liquid chromatographic method with electrochemical detection was developed for the simultaneous determination of the levels of norepinephrine (NE), serotonin (5-HT), acetylcholine (ACh) and their metabolites in the cerebrospinal fluid (CSF) of anaesthetized rats. The response curve for each compound was linear for the concentration way of interest. The within- and between-day coefficients of variation (C.V.) for NE, 5-HT and their metabolites were less than 7.85% and 15.67%, respectively, and those for ACh and choline were less than 3.08% and 6.27%, respectively. This simultaneous determination should be useful for elucidating the noradrenergic, serotonergic and cholinergic nerve activity in the central nervous system.

Relationship between serotoninergic measures in blood and cerebrospinal fluid simultaneously obtained in humans

The relationships between the concentration of serotonin (5-HT) and related metabolites in human blood and CSF have been studied. Plasma tryptophan (TP), 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), and indoleacetic acid (IAA), whole-blood 5-HT, and CSF TP, 5-HT, 5-HIAA, IAA, homovanillic acid, and 3-methoxy-4-hydroxyphenylethylene glycol were determined in 35 unmedicated outpatients who underwent minor surgical operations and had no history of psychiatric or neurological illnesses. Significant correlations were found between the serotoninergic parameters analyzed in blood and CSF. Plasma free 5-HT correlated significantly with CSF 5-HT (r = 0.411, p less than 0.02), and plasma 5-HIAA correlated with the CSF 5-HIAA/5-HT ratio (r = 0.508, p less than 0.004). The concentration of 5-HIAA in CSF correlated with the plasma 5-HIAA/5-HT ratio (r = 0.405, p less than 0.026) (which can be taken as an index of monoamine oxidase type A activity in peripheral tissues) and with the platelet 5-HT/plasma 5-HT ratio (r = 0.375, p less than 0.05). The concentrations of IAA in CSF and plasma were strongly correlated (r = 0.899, p less than 0.001). The significance of these results and their relationship to the use of "in vivo" measures of 5-HT and related metabolites in plasma and platelets as an index of serotoninergic function in affective disorders are discussed.

Serotonin in human lumbar cerebrospinal fluid: a reassessment

An inter-laboratory comparison study was carried out in order to ascertain mean levels of serotonin (5-HT) in human lumbar cerebrospinal fluid (CSF). Analyses were performed using high performance liquid chromatography (HPLC) coupled with either electrochemical (LC-EC) or fluorometric (LC-F) detection. With the detection limits obtained (7-8 pg/ml for LC-EC, 7-15 pg/ml for LC-F) 5-HT was not usually detected in human lumbar CSF. The findings indicate that the true mean concentration of CSF 5-HT is less than 10 pg/ml. This upper limit is substantially lower than all previous reports of 5-HT concentrations in normal human lumbar CSF. The extremely low concentrations of 5-HT present in CSF make it unlikely that CSF 5-HT will be of clinical utility in assessing central serotonergic function.

Autism: review of neurochemical investigation

The neurochemistry of autism, the most well-validated childhood neuropsychiatric disorder, has been studied extensively over the past three decades. Autism is of interest neurochemically because it represents a relatively homogeneous disorder with a triad of social, communicative, and intellectual developmental disturbance. Because a sufficient animal model has been lacking and relatively few diagnosed people with autism have died, most investigation has been of peripheral fluids and tissues. The most consistent finding has been that over 25% of autistic children and adolescents are hyperserotonemic. However, after 29 years of investigation, the mechanism of hyperserotonemia has not been determined. Hyperserotonemia has been found to be familial. Elevated plasma norepinephrine has also been a replicated finding. Cerebrospinal fluid (CSF) opiate activity has been found to be elevated in two studies. Plasma cyclic adenosine monophosphate (cAMP) has been found to be elevated in autistic children. A high rate of nonsuppression after dexamethasone and blunted or delayed growth hormone response to L-dopa have been found. Abnormal cell-mediated immunity has been replicated consistently in autism. Although several pharmacological trials have been conducted and shown promise in initial open trials, only "typical" antipsychotic drugs have shown replicable chronic ameliorating effects in double-blind trials. However, chronic neurotoxicity (tardive dyskinesia) has also been revealed. Findings of morphological changes in the cerebellum have been replicated. Findings in need of replication include diminished platelet function, increased baseline CSF homovanillic acid, decreased nerve cell adhesion molecule serum fragment, blunted prolactin response to fenfluramine, amelioration of symptoms by naltrexone and bromocriptine, reduced electroretinographic (ERG) b-wave amplitude, and morphological changes in the hippocampus, amygdala, and septal nuclei. In addition to refining and replicating past findings, future directions that may be fruitful include investigation of neurochemical aspects of platelet function, of interactions between monoaminergic systems, of phosphatidylinositides, and of pharmacological response to "atypical" antipsychotic agents and relatively selective serotonin receptor subtype agonists or antagonists.

Tryptophan availability modulates serotonin release from rat hypothalamic slices

Application of a novel in vitro experimental system has allowed us to describe the relationship between tryptophan availability and serotonin release from rat hypothalamic slices. Superfusing hypothalamic slices with a physiologic medium containing l-tryptophan (1, 2, 5, or 10 microM) caused dose-dependent elevations in tissue tryptophan levels; the magnitude of the elevations produced by supplementing the medium with less than 5 microM tryptophan was within the physiologic range for rat brain tryptophan levels. Slice serotonin levels rose biphasically as the tryptophan concentration in the medium was increased. Superfusing the slices with medium supplemented with a low tryptophan concentration (1 or 2 microM) caused proportionally greater incremental changes in serotonin levels than the increases caused by further elevating the tryptophan concentration (5 or 10 microM). The spontaneous release of serotonin from the slices exhibited a dose-dependent relationship with the tryptophan concentration of the superfusion medium. Electrically evoked serotonin release, which was calcium-dependent and tetrodotoxin-sensitive, also increased in proportion to the medium tryptophan concentration. These data suggest that the rate at which serotonin is released from hypothalamic nerve terminals is coupled to brain tryptophan levels. Accelerations in hypothalamic serotonin synthesis, caused by elevating brain tryptophan levels, result in proportionate increases in the rates of serotonin release during rest and with membrane depolarization.

Neuroactive metabolites of L-tryptophan, serotonin and quinolinic acid, in striatal extracellular fluid. Effect of tryptophan loading

Extracellular fluid levels of the neurotoxin quinolinic acid in the corpus striatum of rats, measured by in vivo microdialysis, were increased in a dose-dependent manner following the intraperitoneal administration of tryptophan. The lowest dose of tryptophan (12.5 mg/kg), equivalent to about 5% of the normal daily intake, increased peak quinolinic acid levels nearly 3-fold. At higher doses of tryptophan (up to 250 mg/kg), concentrations of quinolinic acid increased over 200-fold and exceeded potentially neurotoxic levels (10 microM). In contrast, the increase in extracellular serotonin following even the highest tryptophan dose was small (less than 2-fold). These data indicate that quinolinic acid is present in the extracellular fluid where it may function as a neuromodulator and that it is very responsive to physiological changes in precursor availability.

Tryptophan availability, 5HT synthesis and 5HT function

1. Tryptophan increases 5HT synthesis, but the extent to which it increases 5HT release and therefore 5HT function is unclear. 2. The possibility that increased 5HT levels will lead to increased 5HT release is enhanced when 5HT neurons are firing at a higher rate. The rate of firing of 5HT neurons is increased as the level of behavioral arousal increases. Thus, altered tryptophan levels will be more likely to influence brain function at higher levels of arousal. 3. In the rat, tryptophan administration increased CSF 5HT appreciably when the animals were aroused by being put in the dark, but not when they were left in a lighted room. 4. In monkeys, the level of behavioral arousal does seem to influence the effect of altered tryptophan levels on aggression. This is consistent with the fact that altered tryptophan levels had no effect on aggression in normal subjects, but that tryptophan had a therapeutic effect in pathologically aggressive patients. 5. The confusing literature on the antidepressant effect of tryptophan can, to some extent, be explained by considering the circumstances in which tryptophan administration will lead to increases in 5HT release as well as increases in 5HT synthesis. 6. Although in some circumstances tryptophan can decrease pain perception by activation of spinal 5HT pathways, when it was given to postoperative patients it attenuated morphine analgesia by activation of a 5HT pathway in the brain. 7. The effect of altered tryptophan levels depend critically on the circumstances in which it is given.