Effect of reserpine on behavioural responses to agonists at 5-HT1A, 5-HT1B, 5-HT2A, and 5-HT2C receptor subtypes.

Rats were given a single dose of reserpine (5 mg/kg s.c.) and behavioural responses to agonists at 5-HT receptor subtypes compared with those of control animals 21 days later. The following effects of activating postsynaptic 5-HT1A receptors by the agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) were significantly increased: tail-flick, reciprocal forepaw treading, flat body posture. The hyperphagic effect of activating presynaptic 5-HT1A receptors by 8-OH-DPAT tended to increase and hypothermia on activating postsynaptic 5-HT1A sites tended to decrease. The hyperlocomotor effect of activating 5-HT1A sites also tended to decrease possibly as a result of a dependence of this response on the known depletion of catecholamines by reserpine. Head shakes on activating 5-HT2A receptors by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and two effects of activating 5-HT2C receptors by 1-(3-chlorophenyl) piperazine (mCPP) were significantly increased (hypophagia, anxiety) and a third effect, hypolocomotion tended to increase but hypophagia on activating postsynaptic 5-HT1B receptors by CP-94, 253 was significantly attenuated. The results are discussed with particular reference to altered 5-HT function in depression.

The serotonergic appetite suppressant fenfluramine. Reappraisal and rejection.

Medical and social pressures have led to increased emphasis on dieting. However, there has been a concurrent world wide increase of obesity. Therefore, much attention has been paid to the development of drugs which decrease appetite. The most extensively used drug of this type over the past three decades has been the serotonergic compound fenfluramine. Recent findings have cast doubt on the previously accepted view that its action requires the release of central 5-HT. Instead, it seems likely that action on specific 5-HT receptors independently of 5-HT stores is involved. It is ironic that these new developments in understanding its mechanism of action have coincided with the recognition of its cardiovascular side-effect apparent especially in patients treated with d-fenfluramine combined with phentermine. This has forced the withdrawal of fenfluramine (both as racemate and d-isomer) from clinical use. The implications of these developments are commented upon.

The effect of D-fenfluramine on brain 5-hydroxytryptamine and 5-hydroxyindoleacetic acid in male and female rats.

Brain regional 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations were determined in freely feeding male and female rats 7 days after giving a single dose of D-fenfluramine (3.8 mg/kg, p.o.) or vehicle. Males showed negligible effects except for a significant decrease of 5-HT in the rest of the cortex, whereas females showed significant decreases of 5-HT and 5-HIAA in the frontal cortex, the rest of the cortex, hippocampus and hypothalamus; 5-HT was also decreased in female midbrain. Females had substantially higher plasma and brain concentrations of fenfluramine and moderately but significantly lower concentrations of norfenfluramine than the males. Plasma fenfluramine + norfenfluramine concentrations of the females were significantly higher than those of the males. Corresponding brain values showed smaller but significant differences. Female brain and plasma areas under the curve for fenfluramine + norfenfluramine (0-24 h after administration of D-fenfluramine) were 20 and 35% higher than male values. However, results suggest that the sex difference in the effect of D-fenfluramine on brain 5-HT metabolism is not due to differences in the metabolism of the drug.

Effects of the (+) and (-) enantiomers of the antidepressant drug tianeptine on 5-HTP-induced behaviour.

The effects of the (+) and (-) enantiomers of the antidepressant drug tianeptine (5, 10, 20 mg/kg, i.p.) on wet dog shakes (WDS) and faecal pellet production induced by concurrently administered 5-hydroxytryptophan (5-HTP, 100 mg/kg, i.p.) given 30 min after carbidopa (25 mg/kg, i.p.) were investigated in rats. WDS scores peaked approximately 1 hr after giving 5-HTP and gradually declined over the next 2 hr. (-)-Tianeptine dose-dependently and significantly inhibited WDS. Inhibition became less marked with time after administration, but remained significant over the 3 hr period after the 10 and 20 mg/kg doses and during the first 40 min after the 5 mg/kg dose. (+)-Tianeptine caused slight inhibition without dose-dependence and slightly increased net inhibition when added to the (-) isomer (10 mg/kg). The induction of faecal pellet production by 5-HTP was significantly and dose-dependently inhibited by (-)-tianeptine. The (+) isomer neither altered this effect of 5-HTP nor its inhibition by (-)-tianeptine. Results show that inhibition of 5-HTP-induced WDS and faecal pellet formation by tianeptine was almost completely dependent on the (-) isomer.

Appetite suppression by commonly used drugs depends on 5-HT receptors but not on 5-HT availability.

The widely prescribed appetite suppressants D-fenfluramine and fluoxetine not only decrease feeding and body weight but also increase extracellular brain 5-HT. As central injection of 5-HT also decreases feeding, the drugs are often thought to require an increase of 5-HT at receptors in order to exert their hypophagic effect. However, much evidence now suggests that D-fenfluramine and its metabolite D-norfenfluramine can cause hypophagia by acting directly at unspecified 5-HT receptors and at 5-HT2C receptors, respectively, while fluoxetine may act independently of 5-HT receptors. These hypophagias may involve interference with the hyperphagic action of neuropeptide Y.

A comparison of the effects of 8-OH-DPAT pretreatment of different behavioural responses to 8-OH-DPAT.

The effects of daily pretreatments with the prototypical 5-HT1A receptor agonist 8-hydroxy-(di-n-propylamino) tetralin (8-OH-DPAT) (1.0 mg/kg s.c.) on behavioural responses to challenge by 8-OH-DPAT (0.5 mg/kg s.c.) due to activation of 5-HT1A receptors were determined. The responses had strikingly different susceptibilities to pretreatment. These were not explicable by different effects on pre- and postsynaptic responses. Thus, two components of the 5-HT syndrome due to action at postsynaptic sites (i.e. flat body posture and reciprocal forepaw treading) were substantially attenuated 1 day after a single pretreatment with 8-OH.DPAT, but the tail-flick response, though due to action at postsynaptic 5-HT1A sites, was completely unimpaired by 14 pretreatments while the hypothermic response which also probably involves postsynaptic sites showed progressively increased attenuation on 14 pretreatments. 8-OH-DPAT-induced hyperphagia which depends on activation of presynaptic sites was unimpaired by the pretreatment schedule. The results are discussed in relation to receptor reserve, second messenger changes and effects at NMDA receptors. They imply a need for caution in the use of chronic effects of 5-HTergic drugs on specific 5-HT1A receptor-dependent responses as indices of mechanisms for the therapeutic actions of the drugs.

Dissociation of effects of chronic diazepam treatment and withdrawal on hippocampal dialysate 5-HT and mCPP-induced anxiety in rats.

Acute (10mg/kg, i.p.) and chronic (10mg/kg/day, i.p. for 10 days) diazepam treatments decreased hippocampal dialysate 5-HT (but not 5-HIAA) concentrations in freely moving rats, suggesting decreased availability of 5-HT to receptors. Twenty-four hours after the last chronic diazepam injection, hippocampal dialysate 5-HT did not differ from that in vehicle-treated rats. However, although reduced 5-HT availability often increases postsynaptic 5-HT receptor-mediated responses, the anxiogenic effect of m-chlorophenylpiperazine (mCPP), which is mediated by the activation of postsynaptic 5-HT(2C) receptors, was not increased (as indicated by the elevated plus-maze test) when given 2 days after 10 days of chronic diazepam, in intact rats. Nevertheless, concurrently in that test, significantly increased anxiety occurred after withdrawal from chronic diazepam (10mg/kg/day x 10 days). The results suggest that benzodiazepine withdrawal-induced anxiety is not mediated by changes in 5-HT(2C) receptor sensitivity, and may be independent of the benzodiazepine-induced reduction of 5-HT release in the rat hippocampus.

Effect of p-chlorophenylalanine at moderate dosage on 5-HT and 5-HIAA concentrations in brain regions of control and p-chloroamphetamine treated rats.

The effects of p-chlorophenylalanine (PCPA, 100-150 mg/kg x 1. i.p.), doses which decrease brain 5-hydroxytryptamine (5-HT) by 30-50%, were investigated in both intact rats and 14 days after giving p-chloroamphetamine (PCA, 10 mg/kg/day x 2, i.p.). The PCPA dose-dependently decreased brain regional 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) 24 hr later. As per cent decreases of 5-HIAA were greater than those of 5-HT in cortex, striatum and hippocampus 5-HIAA/5-HT ratios fell, suggesting that partial inhibition of 5-HT synthesis by PCPA increases 5-HT conservation in these terminal regions. In the hypothalamus and brain stem, decreases of the ratio were small or absent. The PCA given without subsequent PCPA treatment decreased 5-HT and 5-HIAA so that 5-HT fell by about 70% in the cortex, striatum and hippocampus, 55% in the brain stem but only by 27% in the hypothalamus. The PCPA given after PCA decreased 5-HT and 5-HIAA further but not the 5-HIAA/5-HT ratios and increased the ratio in the brain stem. The 5-HIAA/5-HT findings imply that the increase of 5-HT conservation after PCPA treatment does not occur after partial depletion of 5-HT by PCA. The increase of the 5-HIAA/5-HT ratio in the brain stem is explicable by the resistance to both PCA and PCPA of 5-HT in cell bodies where the ratio is high. Results are discussed in relation to the question of whether the PCA treatment used destroys axon terminals projecting from the dorsal but not from the median raphe.

Serotonin metabolism and release in frontal cortex of rats on a vitamin E-deficient diet.

Rats were fed a control or vitamin E (all-rac-alpha-tocopheryl acetate)-deficient diet for 3 or 12 weeks. Serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), tryptophan, and alpha-tocopherol concentrations were determined in the frontal cortex using HPLC, alpha-Tocopherol concentrations fell significantly to 27% of control values at 12 weeks. Tissue 5-HT, 5-HIAA, and tryptophan concentrations were not significantly altered by the vitamin E-deficient diet at either time point. In vivo microdialysis revealed normal basal and K(+)-stimulated concentrations of 5-HT and 5-HIAA, but extracellular concentrations of tryptophan were significantly decreased after 3 weeks on the vitamin E-deficient diet, which resulted in an increase in the tissue/extracellular ratio and suggested a change in compartmentation. However, after 12 weeks on the deficient diet these values had returned to normal. Results in general indicate that a prolonged and substantial depletion of brain vitamin E can occur without major disturbance of serotonergic function.

d-Fenfluramine and d-norfenfluramine hypophagias do not require increased hypothalamic 5-hydroxytryptamine release.

d-Fenfluramine (2.5 mg/kg i.p.) caused marked hypophagia in food-deprived rats and significantly increased medial hypothalamic extracellular 5-hydroxytryptamine (5-HT) as indicated by in vivo microdialysis. When the drug was given after the 5-HT synthesis inhibitor p-chlorophenylalanine (150 mg/kg per day x 3) the hypophagic response was unimpaired but dialysate 5-HT concentration no longer rose. The d-fenfluramine metabolite d-norfenfluramine (1.5 mg/kg i.p.) caused slightly greater hypophagia than the parent drug and completely blocked feeding in animals pretreated with p-chlorophenylalanine, but dialysate 5-HT was increased in neither circumstance. The results provide evidence against mediation of the hypophagic effects of d-fenfluramine and d-norfenfluramine by increased availability of 5-HT to receptors.

Effects of d-fenfluramine on feeding and hypothalamic 5-hydroxytryptamine and dopamine in male and female rats.

Male and female rats were given d-fenfluramine and its effects on feeding and on hypothalamic concentrations of the drug, its metabolite norfenfluramine and 5-hydroxytryptamine (5-HT) and dopamine determined. ID50 values (i.p.) for the hypophagic effect of the drug on 30-, 42- and 100-day-old rats measured over 2 h during the light phase after 24 h food deprivation did not vary significantly with sex but tended to decrease with age approximately in parallel with daily percentage increases and (after deprivation) of decreases in body weight. However, male but not female 30-day-old rats showed a rebound of feeding during the subsequent 2 h. ID50 values of 42-day-old rats on a palatable diet or measured during the dark phase when freely feeding also did not vary with sex. Male 30-day-old rats killed at 2-10 h after an ID75 (p.o.) dose of d-fenfluramine had substantially lower hypothalamic concentrations of the drug and comparable or slightly lower concentrations of its metabolite norfenfluramine than 30-day-old females. Similarly treated 100-day-old males also had lower concentrations of fenfluramine but significantly higher norfenfluramine levels than females so that drug plus metabolite concentrations were essentially independent of sex. 100-day-old females killed 2 h, 24 h and 7 days after d-fenfluramine (3.8 mg/kg p.o. = ID75) had larger percentage decreases of hypothalamic 5-HT than identically treated males. Percentage decreases of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) tended to become less marked with time after injection in males but not females.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that mCPP-induced anxiety in the plus-maze is mediated by postsynaptic 5-HT2C receptors but not by sympathomimetic effects.

1-(3-Chlorophenyl)piperazine (mCPP) (0.125-1.0 mg/kg i.p.), previously shown to inhibit social interaction, dose-dependently reduced exploration of the open arms of an elevated plus-maze. These findings suggest anxiogenic properties. The effect of mCPP was more potently inhibited by 1-(1-naphthyl)piperazine than by ketanserin, indicative of its mediation via activation of 5-HT2C rather than 5-HT2A receptors. The 5-HT1B receptor agonist CGS 12066B did not antagonise the anxiety-like response to mCPP, and further reduced exploration at the highest dose tested (10 mg/kg i.p.). Depletion of serotonin (5-HT) by p-chlorophenylalanine (PCPA, 150 mg/kg/day x 3) did not prevent the response, although PCPA itself increased open arm exploration. The 5-HT1A/B and beta-adrenoceptor antagonist 1-propanolol (5 mg/kg i.p.) and the peripheral beta 1-receptor antagonist atenolol (20 mg/kg i.p.) showed no significant activity on the plus-maze either alone or against the anxiogenic effect of mCPP. These results indicate that mCPP induces anxiety in the rat in the elevated plus-maze primarily by stimulation of postsynaptic 5-HT2C receptors, and suggest that sympathomimetic effects of mCPP are not involved.

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.

A previous potassium stimulation enhances the increases of striatal extracellular dopamine and 5-hydroxytryptamine during global ischaemia under simulated penumbral conditions.

The effect of a previous K+ stimulation on striatal extracellular monoamine levels during global ischaemia, under simulated penumbral conditions, was investigated. Rats were implanted with microdialysis probes in both striata, monoamine release was stimulated unilaterally by adding K+ (100 mM, 20 min) to the artificial CSF perfused through one probe, and bilateral partial ischaemia was imposed after monoamine levels had returned to basal values or below. Resultant increases in dialysate levels of dopamine and 5-hydroxytryptamine were markedly and significantly greater on the side previously exposed to K+, even though electrophysiological measurements indicated similarly severe ischaemia on both sides. Associated monoamine metabolite changes did not differ significantly between the two sides. There was no evidence of greater neuronal loss in the K(+)-stimulated striata 7 days after ischaemia. However, striatal tissue probably exposed to the highest concentrations of K+ could not be examined because of extensive gliosis around the probe.

Effect of global ischaemia, under simulated penumbral conditions, on brain monoamine neurochemistry and subsequent neurological and histological deficits.

We have measured changes in the levels of dopamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites in striatal dialysates during 30 min of global ischaemia under simulated penumbral conditions, and compared these with neurological assessments over the following 7 days and histological damage at the end of this period. On the basis of dialysate DA levels during ischaemia, the animals fell into two subgroups; group I, with little or no DA increase (less than three times basal); and group II, with a much larger increase (greater than 30 times basal). Changes in 5-HT, though of lesser magnitude, showed a similar pattern. These findings may indicate that the amine changes depend on a critical reduction of blood flow within the range obtained by our experimental procedure. Levels of deaminated metabolites fell in all ischaemic animals, with comparable decreases of 3,4-dihydroxyphenylacetic acid plus homovanillic acid in both groups. Decreases of 5-hydroxyindoleacetic acid were greater in group II than in group I, but the relative differences between the groups were much less marked than those of 5-HT. These neurochemical findings suggest that moderate ischaemia affects extracellular amine and deaminated metabolite levels by different mechanisms. Only one of the ischaemic rats (a member of group II) showed a marked neurological deficit, but histological damage, as indicated by neuronal loss and gliosis in vulnerable structures, was apparent in all ischaemic animals. Although damage tended to be greater in animals with marked increases in extracellular monoamines, differences were not significant.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

d-Fenfluramine- and d-norfenfluramine-induced hypophagia: differential mechanisms and involvement of postsynaptic 5-HT receptors.

Severe depletion of 5-hydroxytryptamine (5-HT) by para-chlorophenylalanine (pCPA, 150 mg/kg per day x3) did not alter the hypophagic effect of d-fenfluramine (1-3 mg/kg i.p.) 1 h after food presentation in 24-h food-deprived rats, and moderately and comparably increased the hypophagic effects of its metabolite, d-norfenfluramine (0.35-1.0 mg/kg i.p.), and of the 5-HT1C receptor agonist, 1-(3-chlorophenyl)piperazine (mCPP; 1.5, 2.0 mg/kg i.p.). Chronic treatment with mCPP (2.5 mg/kg i.p. x 14) attenuated the hypophagia induced by d-norfenfluramine (1, 1.5 mg/kg) but not d-fenfluramine (1, 3 mg/kg). 1-(1-Naphthyl)piperazine (3, 8 mumol/kg s.c.), which has greater affinity for 5-HT1C than for 5-HT2 receptors, had no effect on the hypophagia induced by d-fenfluramine (1.25, 2.0 mg/kg), but 1.3 and 3 mumol/kg 1-(1-naphthyl)piperazine largely and comparably attenuated the substantial hypophagic effect of d-norfenfluramine (0.75 mg/kg). The essentially complete hypophagic action of d-norfenfluramine (1.25 mg/kg) was inhibited by 1-(1-naphthyl)piperazine with ID50 = 2.13 mumol/kg. Ketanserin, which binds more weakly than 1-(1-naphthyl)piperazine to 5-HT1C receptors and more strongly to 5-HT2 receptors, attenuated weaker but not stronger hypophagic effects of d-fenfluramine (1.25, 2.0 mg/kg) when given at high dosage (8, 16 mumol/kg s.c.). Ketanserin (16 mumol/kg) also weakly attenuated the hypophagia due to d-norfenfluramine (0.75 mg/kg), but not the essentially complete hypophagia due to d-norfenfluramine (1.25 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

The history of neurochemistry as revealed by the Journal of Neurochemistry.

Analyses of samples of articles in the Journal of Neurochemistry between 1956 (the year of its foundation) and 1990 were used to obtain numerical indices of the history of neurochemistry. Data suggest that the acceleration of neurochemical research did not merely reflect the increase of biochemical research in general and that it involved progressive decreases and increases of interest in major constituents and transmitters, respectively, as indicated by both numbers and citations of papers. Papers on all classes of transmitters increased steadily and in the order of amines > amino acids, acetylcholine > peptides. Within the field of brain metabolism, papers on energy metabolism decreased markedly. Use of techniques other than those of biochemistry/neurochemistry altered strikingly with decreases of histological, electrophysiological, and pharmacological methods and increases of chemical, immunological, and tissue culture methods. Citations by neuroscience core journals between 1975 and 1988 suggest that the relative prominence of neurochemistry within neuroscience has remained constant. Analyses indicate that the influence of the U.S.A. relative to that of other regions has remained fairly steady between 1956 and 1990, but that number of papers from the U.K. has declined, whereas the influences of Western Europe and other areas appear to have recently increased substantially. Sociological changes have been the virtual disappearance of single-author papers, an increase of multiauthorship (> 3), and a recent striking increase of assertive sentence titles.

Extracellular neuroactive amino acids in the rat striatum during ischaemia: comparison between penumbral conditions and ischaemia with sustained anoxic depolarisation.

Changes in the extracellular levels of excitatory and inhibitory amino acid transmitters were studied in the rat striatum during penumbral ischaemia using intracerebral microdialysis. Effects of penumbral forebrain ischaemia were compared with those of ischaemia with sustained anoxic depolarisation and K+ (100 mM). Comparisons were also made between different groups of animals at 2 and 24 h after dialysis probe implantation. The K+ stimulus did not provoke any release of excitatory amino acids in the 24-h group, probably reflecting a decrease of functional synapses adjacent to the probe. During 30 min of penumbral ischaemia, excitatory amino acids did not reach critical concentrations in the extracellular fluid, and increases in levels of inhibitory/modulatory amino acids were similar. On the other hand, severe transient ischaemia resulted in massive synchronous release of many neuroactive excitatory and inhibitory compounds, in both the 2- and 24-h groups. These and other data suggest that changes during severe ischaemia may arise from both neurotransmitter and metabolic pools. It is concluded that ischaemic damage in the penumbra may not be related to extracellular neuroactive amino acid changes generated within this region.