Efflux of 5-HIAA from 5-HT neurons: a membrane potential-dependent process

Sex and social status modify the effects of fluoxetine on socioemotional behaviors in Syrian hamsters and rhesus macaques

Social subordination increases risk for psychiatric disorders, while dominance increases resilience to these disorders. Fluoxetine, a selective serotonin (5HT) reuptake inhibitor whose actions are mediated in part by the 5HT1A receptor (5HT1AR), has sex- and social status-specific effects on socioemotional behavior and aggressive behavior. However, the impact of social status on these sex-specific effects remains unclear. The current study evaluated the impact of acute fluoxetine treatment and social status on dominance-related behaviors in female and male hamsters, and the impact of chronic fluoxetine treatment on socioemotional behavior and 5HT1AR binding potential (5HT1AR_BP) in female rhesus macaques. We hypothesized that sex differences in the effects of fluoxetine on aggression in hamsters would be diminished in dominant and enhanced in subordinate males and that aggression in female hamsters would be enhanced in dominants and diminished in subordinates. In female rhesus macaques, we hypothesized that chronic fluoxetine would alter socioemotional behaviors and site-specific 5HT1AR_BP in a status-dependent manner. Male (n = 46) and female (n = 56) hamsters were paired with conspecifics for three days to establish social rank. Hamsters received a single dose of 20 mg/kg fluoxetine or vehicle two-hours prior to a test with a non-aggressive intruder. Female rhesus monkeys (n = 14) housed were administered fluoxetine (2.8 mg/kg/day) or vehicle injections chronically for 14-days, separated by a three-week washout period. On Day 15, positron emission tomography neuroimaging for 5HT1AR_BP was conducted. Fluoxetine treatment decreased aggression in subordinate female monkeys and subordinate female hamsters but not in dominant females of either species. Fluoxetine decreased aggression in dominant but not in subordinate male hamsters. Fluoxetine also reduced and increased prefrontal 5HT1AR_BP in dominant and subordinate females, respectively. Taken together, these results provide cross-species evidence that social status and sex impact how increased 5HT modulates agonistic behavior.

A model of dopamine and serotonin-kynurenine metabolism in cortisolemia: Implications for depression

A major factor contributing to the etiology of depression is a neurochemical imbalance of the dopaminergic and serotonergic systems, which is caused by persistently high levels of circulating stress hormones. Here, a computational model is proposed to investigate the interplay between dopaminergic and serotonergic-kynurenine metabolism under cortisolemia and its consequences for the onset of depression. The model was formulated as a set of nonlinear ordinary differential equations represented with power-law functions. Parameter values were obtained from experimental data reported in the literature, biological databases, and other general information, and subsequently fine-tuned through optimization. Model simulations predict that changes in the kynurenine pathway, caused by elevated levels of cortisol, can increase the risk of neurotoxicity and lead to increased levels of 3,4-dihydroxyphenylaceltahyde (DOPAL) and 5-hydroxyindoleacetaldehyde (5-HIAL). These aldehydes contribute to alpha-synuclein aggregation and may cause mitochondrial fragmentation. Further model analysis demonstrated that the inhibition of both serotonin transport and kynurenine-3-monooxygenase decreased the levels of DOPAL and 5-HIAL and the neurotoxic risk often associated with depression. The mathematical model was also able to predict a novel role of the dopamine and serotonin metabolites DOPAL and 5-HIAL in the ethiology of depression, which is facilitated through increased cortisol levels. Finally, the model analysis suggests treatment with a combination of inhibitors of serotonin transport and kynurenine-3-monooxygenase as a potentially effective pharmacological strategy to revert the slow-down in monoamine neurotransmission that is often triggered by inflammation.

Anorectic drugs and vascular disease: the role of voltage-gated K+ channels

Anorectic drugs are widely used for the treatment of obesity. They are thought to decrease appetite through their effects on catecholamine or 5-hydroxytryptamine (5-HT) levels in the brain. Their use has been associated with epidemics of pulmonary hypertension and the development of valvular heart disease, hypertension, stroke and digital or mesenteric ischemia. Understanding the mechanism of the cardiovascular toxicity of anorectic drugs is important because of the modern epidemic of obesity and the resulting plethora of new anorexigens, many of which share similar mechanisms with those that have previously caused cardiovascular disease. In addition, the mechanism by which anorexigens cause vascular disease has relevance to the etiology and treatment of pulmonary and systemic hypertension. Recent discoveries have clarified how the anorexigens cause vasoconstriction and hypertension. Most anorexigens directly inhibit voltage-gated K+ (KV) channels in vascular smooth muscle cells (SMCs). This reduced K+ efflux leads to depolarization, the opening of voltage-sensitive Ca2+ channels, an increase in intracellular Ca2+ and vasoconstriction. Endothelial dysfunction appears to be a predisposing factor for the development of anorectic-induced vascular complications. Vasoconstriction is weak at clinically relevant doses of anorectic drugs. However, when nitric oxide synthase is inhibited, vasoconstriction is significantly enhanced. Anorexigens are the only drugs in widespread clinical use that have KV-channel-blocking properties and it is probable that much of their cardiovascular toxicity relates to this mechanism. Investigators need to examine new anorexigens and other therapeutic molecules for inhibitory effects on KV channels, as this effect may be a marker of drugs that will elicit vascular complications.

The effect of acute citalopram on extracellular 5-HT levels is not augmented by lithium: an in vivo microdialysis study

A substantial number of patients do not respond sufficiently to antidepressant drugs and are therefore often co-medicated with lithium as an augmentative strategy. Since lithium has been demonstrated to affect 5-HT neurotransmission, we examined the effect of acute and subchronic lithium on 5-HT levels after a challenge with citalopram. We found that subchronic administration of lithium increases extracellular 5-HT baseline level and decreases the extracellular 5-HIAA baseline. However, we found no evidence that the effect of acute citalopram on extracellular 5-HT levels is augmented by acute or subchronic lithium.

N-methyl-D-aspartate receptor-mediated modulation of monoaminergic metabolites and amino acids in the chick forebrain: an in vivo microdialysis and electrophysiology study

The associative avian forebrain region medio-rostral neostriatum/hyperstriatum ventrale (MNH) is involved in auditory filial imprinting and may be considered the avian analogue of the mammalian prefrontal cortex. In search of the neurochemical and physiological mechanisms which play a role in this learning process, we introduced microdialysis and a combined microdialysis/electrophysiological approach in domestic chicks a few days old. With this technique, we were able to follow changes of the extracellular levels of glutamate, taurine, 5-hydroxyindoleacetic acid (5-HIAA), a metabolite of serotonin, and homovanillic acid (HVA), a metabolite of dopamine, and neuronal activity simultaneously in freely moving animals. We obtained first evidence of a modulatory interaction between glutamatergic and monoaminergic neurotransmission mediated by N-methyl-D-aspartate (NMDA) receptors. During local intracerebral infusion of 300 microM NMDA via reverse microdialysis, an increase of taurine and a decrease of 5-HIAA and HVA were detected, accompanied by enhanced extracellular spike rates. Glutamate was increased only during consecutive infusion of increasing NMDA concentrations, when higher (1 mM) NMDA concentrations were infused. The effects of NMDA were antagonized by D, L-2-amino-5-phosphonovaleric acid (1 mM). Infusion of high potassium induced similar changes in taurine, 5-HIAA, and HVA, as found during infusion of NMDA, but decreased extracellular spike rates, which indicates that different cellular mechanisms may underlie the observed neurochemical changes. Neither urethane anesthesia nor different delays between probe implantation and experiment influenced the neurochemical and electrophysiological results; however, changes of taurine were observed only in chronically implanted, awake animals. In summary, microdialysis in combination with electrophysiology provides a powerful tool to detect changes of neuronal activity and transmitter release in the avian brain, with which the role of transmitter interactions can be followed during and after different learning events.

A role for potassium channels in smooth muscle cells and platelets in the etiology of primary pulmonary hypertension

Plasma serotonin levels are markedly elevated in patients with primary pulmonary hypertension (PPH) and platelet levels of serotonin are low. Furthermore, plasma serotonin levels remain elevated after bilateral lung transplantation, in the absence of any pulmonary hypertension. Dexfenfluramine can cause the anorexigen-induced form of PPH that is clinically and histologically indistinguishable from PPH. We find that dexfenfluramine releases serotonin from platelets and inhibits its reuptake. These observations suggest that serotonin might be involved in, or be a marker for, the mechanism responsible for both forms of PPH. Dexfenfluramine causes inhibition of voltage-sensitive potassium (Kv) channels, membrane depolarization, and calcium entry in pulmonary artery smooth muscle cells and vasoconstriction in isolated perfused rat lungs. We have recently found that dexfenfluramine also inhibits Kv channels in megakaryocytes, the stem cell for platelets. In smooth muscle cells, taken from the pulmonary arteries of PPH patients, Kv channels appear to be dysfunctional. The underlying defect in PPH is likely to be an abnormality of one or more Kv channels in both pulmonary artery smooth muscle cells and platelets. Relatively few patients exposed to dexfenfluramine develop PPH. The factors responsible for susceptibility might be a difference in expression of potassium channels and/or a decrease in the endogenous production of nitric oxide.

The pulmonary hypertensive fawn-hooded rat has a normal serotonin transporter coding sequence

The coding sequence of the serotonin transporter gene was compared in two strains of rat-the Wistar and the fawn-hooded rat (FHR). The FHR has an inherited platelet storage-pool deficiency and a widespread impairment of serotonin storage. It is also susceptible to systemic and pulmonary hypertension. The FHR provides a model to study the genetics in human systemic and pulmonary hypertension. We measured platelet function in these two strains by measuring incorporation of radiolabeled serotonin into a platelet suspension and found significant differences in serotonin uptake and release. The coding sequence for the serotonin transporter in the FHR has yet to be reported. No differences were found in the predicted amino acid sequence between these two strains of rat, either in the platelet or the lung samples or when compared with the published sequence of the brown rat. We conclude that differences in the primary structure of the serotonin transporter gene do not account for the altered serotonin storage in the FHR strain.

Effect of oral feeding of essence of chicken on the level of 5-hydroxyindole acetic acid in the cerebrospinal fluid of the rat

The effect of oral feeding of a commercial preparation of essence of chicken (Brand's Essence of Chicken, BEC) on the level of 5-hydroxyindole acetic acid (5-HIAA) in the cerebrospinal fluid (CSF) of the rat was investigated. BEC, when fed to the rat for a period of 3 days, significantly increased the CSF level of 5-HIAA in seven out of 12 animals studied. As the level of CSF 5-HIAA is taken as an indication of 5-hydroxytryptamine (5-HT) activity in the brain, it is possible that BEC increased brain 5-HT activity. This increase was not due to the ingestion of tryptophan, the primary precursor of 5-HT, because BEC contains undetectable level of tryptophan. The data indicate that by causing an increase in brain 5-HT activity, consumption of BEC may lead to the activation of 5-HT-dependent physiological process like sleep improvement, mood elevation, analgesia, facilitation of motor output and regulation of circadian rhythm. However, such a possibility remains to be further investigated.

Effects of ion channel blockers on rapid postmortem changes in extracellular dopamine and serotonin levels in the rat nucleus accumbens

In the present study, we used in vivo brain microdialysis to examine the effects of ion channel blockers tetrodotoxin (TTX), EGTA-free Ca2+ and verapamil on rapid postmortem changes in extracellular levels of dopamine (DA), serotonin (5-HT) and their metabolites dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) in the ACC of freely moving rats. Extracellular ACC DA levels decreased following the perfusion of the three ion channel blockers in freely moving rats, and then, at death by cervical dislocation, maximum respective 220-, 60- and 90-fold increases were observed in the extracellular output of DA in animals treated with EGTA, verapamil and TTX, respectively. Also, ACC 5-HT decreased following perfusion with the three blockers in the freely moving rats, and then maximum increases of 80-, 30- and 45-fold in the extracellular output of 5-HT were observed at death in animals treated with EGTA, verapamil and TTX, respectively, compared to the baseline. Cervical dislocation-induced rapid postmortem changes were inhibited markedly by perfusion with CSF containing the CA2+ entry blocker verapamil. These observations suggested that rapid postmortem changes in ACC DA and 5-HT release were associated with the action of calcium ion channels and/or voltage gated channels in the CNS.

Distinct activation of monoaminergic pathways in chick brain in relation to auditory imprinting and stressful situations: a microdialysis study

In the forebrain of the domestic chick (Gallus gallus domesticus), an area termed the mediorostral neostriatum/hyperstriatum ventrale is strongly involved in emotional learning paradigms such as acoustic filial imprinting. Furthermore, the involvement of the mediorostral neostriatum/hyperstriatum ventrale in stressful situations, such as social separation, has been demonstrated in 2-deoxyglucose studies. The aim of the present study was to examine whether quantitative changes of dopamine, serotonin and their metabolites occur during auditory filial imprinting and during social separation. Using in vivo microdialysis in tone-imprinted and in naive, control chicks, we compared the extracellular levels of homovanillic acid, a metabolite of dopamine, and 5-hydroxyindoleacetic acid, a metabolite of serotonin, during the presentation of the imprinting tone. A small, but statistically significant, decrease of extracellular homovanillic acid levels was found in the mediorostral neostriatum/hyperstriatum ventrale of imprinted chicks compared to control animals, whereas changes of 5-hydroxyindoleacetic acid were not detected. In a second experiment, we investigated the levels of homovanillic acid and 5-hydroxyindoleacetic acid in the mediorostral neostriatum/hyperstriatum ventrale of socially reared chicks during different stress situations, such as handling or separation from their cage mates. Handling induced a significant increase of homovanillic acid and 5-hydroxyindoleacetic acid, while social separation resulted in a significant increase of 5-hydroxyindoleacetic acid and only a slight increase of homovanillic acid. Despite considerable inter-individual variability, the increase of distress vocalizations (duration of distress calls) after social separation displayed a good correlation to the increased 5-hydroxyindoleacetic acid levels in all animals analysed. These results provide the first evidence that the physiological response of the mediorostral neostriatum/hyperstriatum ventrale related to different emotional conditions after acoustic imprinting and during stressful situations is, at least in part, mediated by dopaminergic and/or serotonergic pathways. Furthermore, the results from the present study indicate a distinct activation of dopaminergic and serotonergic pathways in relation to the behavioural situation and the associated changes of emotional status.

Ischemia increases tissue and decreases extracellular levels of acid dopamine metabolites in the rat striatum: further evidence for active transport of metabolites

During 30 minutes of global forebrain ischemia in rats, striatal intracellular levels of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) increase at the same time extracellular levels, measured by microdialysis, decrease. If DOPAC and HVA were free to diffuse out of cells, these changes in opposite directions should not occur. These data support the theory that a membrane potential-dependent active transport system is required for DOPAC and HVA to leave striatal cells. During ischemia and subsequent to ATP depletion, neurons and glia depolarize and active transport systems fail. DOPAC and HVA, which are still being produced inside neurons and possibly glia, can not be transported out of these cells and accumulate intracellularly.

Depletion and repletion of cortical tissue and dialysate 5-HT after reserpine

Reserpine (5 mg/kg s.c.) was given to rats kept under a reversed light-dark cycle and 5-hydroxytryptamine (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) determined in frontal cortex tissue and dialysate at various times after drug treatment. The decline and return of spontaneous locomotor activity was also measured. Tissue 5-HT was depleted to 16% of control values 24 hr after drug administration and had recovered to 61% of control after 21 days. Locomotion was profoundly reduced by 7 hr after reserpine but had returned to normal at 4 days. Dialysate 5-HT, both basal and its rise on potassium (K+) stimulation, was reduced at 1, 7 and 21 days after reserpine but the K+ stimulated increases (as % of control) did not rise above % tissue repletion, thus providing evidence against increased mobilization of the transmitter from the partially repleted vesicular stores. However, at 1 day after reserpine, basal dialysate 5-HT was proportionately less reduced than tissue 5-HT suggesting that release from a reserpine insensitive (extravesicular) pool was more effective than from the vesicular pool. At this time, the K(+)-stimulated rise of dialysate 5-HT was proportionately more reduced than tissue 5-HT. By 21 days, values converged so that % changes of the 3 compartments were the same suggesting that at this time both basal and K+ stimulated dialysate 5-HT was essentially all derived from the vesicular pool.

Peripheral nerve stimulation increases serotonin and dopamine metabolites in rat spinal cord

Extracellular serotonin (5-HT), dopamine (DA), and their metabolites, 5-hydroxyindoleacetic acid (5-HIAA), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA), were assessed in the rat lumbar spinal cord (L3-4) by in vivo microdialysis with high performance liquid chromatography and electrochemical detection (HPLC-ECD). Under urethane-chloralose anesthesia, basal levels of 5-HT and DA in the dialysates were approximately 1.0-1.2 pg/22 microliters sample, 5-HIAA, DOPAC, and HVA were constant at 322.6 +/- 14.9, 8.6 +/- 0.7, and 10.4 +/- 0.4 pg/22 microliters sample (mean +/- SE), respectively. Local application of 100 mM KCl via the dialysis probe increased the 5-HT and DA. Peripheral nerve stimulation that selectively excited the large (A-beta) or small (A-delta) myelinated fibres increased the metabolites. Excitation of the A-beta fibers increased the levels of 5-HIAA to 138%, DOPAC to 155%, and HVA to 143% of the controls. Stimulation of the A-delta fibers increased 5-HIAA to 121%, DOPAC to 120%, and HVA to 124% of the controls. The results suggest that non-nociceptive peripheral nerve stimulation may activate the descending 5-HT and DA systems in the spinal cord.

Behavioural and neurochemical evidence for the decrease of brain extracellular 5-HT by the antidepressant drug tianeptine

The effects of the novel antidepressant tianeptine on behaviours induced by the serotonin (5-HT) precursor 5-hydroxytryptophan (5-HTP) and the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) were investigated. Tianeptine (10 mg/kg, i.p.) significantly attenuated wet dog shakes (WDS) induced by 5-HTP (75 mg/kg, i.p.; 30 min after carbidopa 25 mg/kg, i.p.). The effect was most marked when 5-HTP and tianeptine were given together. The main metabolite of tianeptine also attenuated WDS. Components of the 5-HT syndrome (i.e. reciprocal forepaw treading, hind limb abduction, flat body posture) induced by 8-OH-DPAT (0.5 mg/kg, s.c.) were unaffected by tianeptine and 5-HTP given both singly or together. However, tianeptine significantly reduced faecal pellet formation but not cage crossings resulting from 8-OH-DPAT administration. These cage crossings but not the associated faecal pellet formation were reduced by 5-HTP. This reduction was prevented by tianeptine. The increase of extracellular 5-HT in the frontal cortex following administration of 5-HTP was opposed and the concurrent increase of extracellular 5-hydroxyindoleacetic acid (5-HIAA) was enhanced by tianeptine. The above behavioural and neurochemical findings indicate that tianeptine opposes the increase of 5-HT at receptor sites due to 5-HTP administration.

H(+)-ATPase and transport of DOPAC, HVA, and 5-HIAA in monoamine neurons

The effects of N-methylmaleimide (N-MtM), a vacuolar H(+)-ATPase inhibitor, were evaluated in the putamen of the cat to study the in vivo transport mechanisms of dopamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindolacetic acid (5-HIAA), using the brain focal microdialysis technique combined with HPLC. The addition of N-MtM to the perfusate altered invariably the flux of the DOPAC, HVA, and 5-HIAA in a similar pattern, resulting in a decrease of the extracellular levels of such metabolites, its extent being N-MtM concentration dependent, thus indicating that the mechanism(s) of such a decrease is (are) related most likely to decreased transport from the intracellular to the extracellular space as the consequence of the inhibition of the vacuolar H(+)-ATPase of DA and 5-HT neurons by the N-MtM. Furthermore, N-MtM masked the release of DA and 5-HT produced by KCl 120 mmol/l. Indeed, N-MtM increased the extracellular levels of such transmitters to values exceeding 4 to 6 times of those produced by KCl 120 mmol/l alone, which suggests that vacuolar H(+)-ATPase is probably involved also in the retention and/or reuptake process of DA and 5-HT.

Extracellular dopamine and serotonin in the rat striatum during transient ischaemia of different severities: a microdialysis study

Generalised neurotransmitter overflow into the extracellular space on cerebral ischaemia has been widely reported and implicated in events leading to subsequent neuronal death. As little is known about the effect of depth of ischaemia on these changes, we have subjected anaesthetised rats to a sequence of four challenges [high K+ stimulus, moderate (penumbral) ischaemia, severe ischaemia, cardiac arrest] and have concurrently monitored both electrophysiological parameters and changes in extracellular dopamine, serotonin, and their metabolites in the striatum. Of particular relevance to human stroke therapy was penumbral ischaemia, where ionic homeostasis was maintained even though electrical function was lost. All challenges increased extracellular monoamines, although levels were significantly greater when ischaemia was severe enough to produce sustained anoxic depolarisation. Baseline levels were rapidly restored during recovery phases. Acidic monoamine metabolites decreased significantly during each insult, returning to basal levels during reperfusion after moderate ischaemia, and to significantly higher levels after severe ischaemia. Results indicate that sustained anoxic depolarisation may be a critical factor in determining outcome after ischaemia, being associated with significantly greater release of monoamines, and impairment of electrical function recovery.

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.

Effect of p-chlorophenylalanine on release of 5-hydroxytryptamine from the rat frontal cortex in vivo

1. Rats were given p-chlorophenylalanine (PCPA, 150 mg kg-1, i.p.) to inhibit partially 5-hydroxytryptamine (5-HT) synthesis so that its concentration in the frontal cortex fell by about half. The effects of this treatment on frontal cortex dialysate 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) concentrations were determined before and after stimulation by increasing K+ concentration in the perfusion fluid by 100 mM for 20 min. Rates of 5-HT synthesis as indicated by the effects of 3-hydroxybenzylhydrazine (NSD 1015, 150 mg kg-1, i.p.) on frontal cortex tissue and dialysate 5-hydroxytryptophan (5-HTP) and dialysate 5-HIAA were also measured in rats that had not been stimulated with K+. 2. Dialysate 5-HT and 5-HIAA concentrations of both vehicle- and PCPA-treated rats fell into major (group 1) and minor (group 2) populations statistically distinguishable from each other by the high 5-HT and low 5-HIAA values of the latter group. 3. In group 1 animals, PCPA decreased both the dialysate 5-HT concentration and its rise following stimulation by K+ in proportion with the decrease of 5-HT in frontal cortex tissue. 5-HIAA fell more markedly than 5-HT and in similar proportion in both tissue and dialysate. The fall of dialysate 5-HIAA on stimulation by K+ was also attenuated to the same degree. The elevated 5-HT/5-HIAA ratios after PCPA treatment imply increased conservation of the depleted 5-HT stores. 4. PCPA decreased the above 5-HIAA values and the effects of NSD 1015 on tissue 5-HTP or dialysate 5-HIAA concentrations in similar proportion. However, PCPA had little effect on corresponding dialysate 5-HTP values. 5. The results are discussed with respect to relationships between synthesis, storage and release of 5-HT. They indicate that (under the conditions of the present study) the availability of 5-HT to receptors is directly proportional to total vesicular stores under both basal conditions and during neuronal firing.

Active transport pumps of HVA and DOPAC in dopaminergic nerve terminals

The effect of the membrane potential on the efflux of HVA and DOPAC from DA neurons was studied in anesthetized (1% halothane in gas mixture of 70% N2O and 30% O2) cats. Extracellular DA, HVA and DOPAC were measured continuously from the putamen, the hypothalamus, the thalamus, the raphe nuclei and the cortex using brain microdialysis technique combined with HPLC-ED monoamine measurements. HVA and DOPAC concentrations were highest in the putamen and lowest in the cerebral cortex. Extracellular HVA levels exceed those of the DOPAC. Increases in the extracellular potassium from 4 to 120 mM invariably produced decreases of the extracellular HVA and DOPAC in all the tested brain regions. These decreases were inversely proportional to the extracellular potassium concentration. Thus, it is concluded that the HVA and the DOPAC are extruded from inside the cell to the extracellular space by active mechanisms of transport similar to that reported for 5-HIAA in serotonergic neurons.