Different effects of fenfluramine isomers and metabolites on extracellular 5-HIAA in nucleus accumbens and hippocampus of freely moving rats

Hypothalamic serotonergic activity correlates better with brain temperature than with sleep-wake cycle and muscle tone in rats

The activity of the serotonergic system varies in phase with the sleep-wake cycle, which is associated with changes in several physiological functions, including electroencephalographic activity, brain temperature, and locomotion. The aim of the present study was to clarify which of these parameters correlates better with serotonergic activity in spontaneous conditions. Voltammetric recordings by telemetry of serotonergic metabolism in the medial preoptic area and polygraphic recordings of sleep-wake activity (by means of electroencephalographic delta band, brain cortical temperature and neck electromyographic activity recordings) were simultaneously performed in freely moving rats. Univariate analyses of variance revealed that each variable under investigation was statistically correlated with serotonergic metabolism. When the variables were entered into the model simultaneously, both partial correlation and step-wise multiple regression analyses indicated that the highest correlation exists between serotonergic metabolism and brain cortical temperature. The present data show that serotonergic activity in the medial preoptic area is closely linked to physiological changes in brain temperature.

D-Fenfluramine induces serotonin-mediated Fos expression in corticotropin-releasing factor and oxytocin neurons of the hypothalamus, and serotonin-independent Fos expression in enkephalin and neurotensin neurons of the amygdala

The neurotransmitters expressed by neurons activated by D-fenfluramine (5 mg/kg, i.p.) were identified in the hypothalamus, amygdala and bed nucleus of the stria terminalis. Induction of Fos immunoreactivity following D-fenfluramine injection was used as an index of neuronal activation. To test whether D-fenfluramine activated neurons by releasing serotonin from the serotonergic nerve terminals, rats were pretreated with fluoxetine (10 mg/kg, i.p.), a serotonin reuptake inhibitor that prevents the release of serotonin stimulated by D-fenfluramine, 12 h before D-fenfluramine injection. The approximate percentages of peptidergic neurons that contained Fos immunoreactivity after D-fenfluramine administration were 94% of corticotropin-releasing factor and 22% of oxytocin cells in the paraventricular nucleus of the hypothalamus, 6% of oxytocin cells in the supraoptic nucleus of the hypothalamus, 36% of enkephalin and 15% of neurotensin cells in the central amygdaloid nucleus, and 19% of enkephalin and 9% of neurotensin cells in the bed nucleus of the stria terminalis. Fluoxetine pretreatment blocked Fos expression in corticotropin-releasing factor- and oxytocin-expressing cells in the hypothalamus, but not in enkephalin-and neurotensin-expressing cells located in the bed nucleus of the stria terminalis and central amygdaloid nucleus. D-Fenfluramine did not induce Fos immunoreactivity in vasopressin-, thyrotropin-releasing hormone-, somatostatin- and tyrosine hydroxylase-containing cells in the hypothalamus, and corticotropin-releasing factor-expressing cells in the central amygdaloid nucleus and bed nucleus of the stria terminalis. These results show that D-fenfluramine stimulates corticotropin-releasing factor- and oxytocin-expressing cells in the hypothalamus via serotonin release. The enkephalin- and neurotensin-expressing cells in the amygdala are activated by D-fenfluramine via non-serotonergic mechanisms. Induction of Fos expression by D-fenfluramine in restricted populations of cells suggests a selective activation of neuronal circuitry that is likely to be involved in the appetite suppressant effects of D-fenfluramine.

p-Chlorophenylalanine and fluoxetine inhibit D-fenfluramine-induced Fos expression in the paraventricular nucleus, cingulate cortex and frontal cortex but not in other forebrain and brainstem regions

D-Fenfluramine, a putative serotonin releaser and reuptake inhibitor, is commonly prescribed for the treatment of obesity. Brain sites activated by D-fenfluramine have been mapped via the expression of the immediate early gene Fos. However, it is not clear that serotonin release in the brain mediates the effects of D-fenfluramine on Fos expression. The present study determined whether D-fenfluramine induces the expression of Fos in the brain through the release of serotonin. Rats were pretreated either with the serotonin depleting drug p-chlorophenylalanine (PCPA) or with the serotonin reuptake inhibitor fluoxetine. Both drugs inhibited D-fenfluramine-induced Fos expression in the cingulate cortex, frontal cortex, and the parvocellular subdivision of the paraventricular nucleus of the hypothalamus. Neither drug reduced D-fenfluramine-induced Fos responses in several other brain areas, including the caudate-putamen, amygdala, and brainstem regions such as the lateral parabrachial nucleus and nucleus of the solitary tract. These results indicate regional specificity of mechanisms mediating D-fenfluramine-induced Fos expression. It is likely that D-fenfluramine-induced Fos expression at various sites in the brain is mediated via a combination of serotonin release and other, as yet unidentified, neurotransmitters.

Low-level cadmium exposure of lactating rats causes alterations in brain serotonin levels in the offspring

Effects on monoaminergic and cholinergic transmitter systems as well as neurotrophins were characterized in developing Sprague-Dawley rats directly exposed to 5 ppm cadmium in the drinking water or indirectly via exposed dams. Cadmium was given to dams during the lactation period, from parturition to postnatal day 17, and/or to the offspring until postnatal day 42. Cresyl violet staining and glial fibrillary acidic protein immunohistochemistry did not reveal any obvious neuropathology after cadmium exposure. Following high-power microwave fixation, concentrations of acetylcholine (ACh) and monoamines were determined in cerebral cortex, striatum, and hippocampus using HPLC with electro-chemical detection. ACh, dopamine, and noradrenaline levels were not significantly affected after the different cadmium exposures. Cortical levels of serotonin were significantly reduced in rats exposed to cadmium during lactation as well as in rats exposed to cadmium during both lactation and postweaning. A major decrease in 5-hydroxyindoleacetic acid was found in cortex and hippocampus in rats exposed to cadmium during lactation. The regional characteristics of cadmium toxicity as reflected in changes of neurotrophins were studied using in situ hybridization histochemistry with oligonucleotide probes and phosphoimaging evaluation. No significant changes in the mRNA expression of brain-derived neurotrophic factor (BDNF), neurotrophin-3, and the high-affinity tyrosine kinase receptor of BDNF, trkB, were detected. The present results demonstrate that exposure to levels of cadmium as low as 5 ppm in the drinking water leads to neurochemical disturbances of the serotonergic system in the offspring during the lactational period.

Electrically evoked 5-hydroxytryptamine efflux in rat hypothalamus studied using in vivo amperometry

Changes in 5-hydroxytryptamine (5-HT) efflux have been studied for the first time using differential pulse amperometry (DPA). In vitro observations show that the oxidation potential of 5-HT at 37 degrees C is 160 mV. The amperometric signal corresponding to 5-HT was measured in the lateral hypothalamus after brief electrical stimulation of the dorsal raphe nucleus (10 s, 25 or 50 Hz, 300 microA) every 5 or 10 min. Pargyline (100 mg/kg ip) and d-norfenfluramine (5 mg/kg ip) increased the signal to 194 and 243%, respectively. Tetrodotoxin (1 microliter, 100 mM), injected in the proximity of the working electrode, caused the signal to disappear. 8-OH-DPAT (250 micrograms/kg sc) reduced it to 64% for about 1 h and this effect was completely prevented by the 5HT1A antagonist WAY 100635 (1 mg/kg sc). L-Tryptophan (100 mg/kg ip) increased the amperometric signal to 136%. No change was detected when the 5-HT precursor was given to rats pretreated with PCPA (150 mg/kg per day p.o. for 3 days). In conclusion, DPA represents a sensitive and selective approach for studying 5-hydroxytryptaminergic function, offering a good temporal and anatomical specificity.

The lateral hypothalamic area revisited: ingestive behavior

This article discusses the role of the lateral hypothalamic area (LHA) in feeding and drinking and draws on data obtained from lesion and stimulation studies and neurochemical and electrophysiological manipulations of the area. The LHA is involved in catecholaminergic and serotonergic feeding systems and plays a role in circadian feeding, sex differences in feeding and spontaneous activity. This article discusses the LHA regarding dietary self-selection, responses to high-protein diets, amino acid imbalances, liquid and cafeteria diets, placentophagia, "stress eating," finickiness, diet texture, consistency and taste, aversion learning, olfaction and the effects of post-operative period manipulations by hormonal and other means. Glucose-sensitive neurons have been identified in the LHA and their manipulation by insulin and 2-deoxy-D-glucose is discussed. The effects on feeding of numerous transmitters, hormones and appetite depressants are described, as is the role of the LHA in salivation, lacrimation, gastric motility and secretion, and sensorimotor deficits. The LHA is also illuminated as regards temperature and feeding, circumventricular organs and thirst and electrolyte dynamics. A discussion of its role in the ischymetric hypothesis as an integrative Gestalt concept concludes the review.

Biological actions of drugs affecting serotonin and eating

The present status of knowledge on drugs affecting food intake and presumably acting via a serotoninergic mechanism is reviewed. The mechanism of action of these drugs is analyzed at the neurochemical level. All the drugs, to various extents, inhibit the uptake of serotonin (5HT), increase the release of 5HT and decrease brain levels of 5HT and 5HIAA. However, the underlying mechanisms are not identical as exemplified by comparisons made with d-fenfluramine, d-norfenfluramine, fluoxetine, sertraline and paroxetine. An analysis of the role of 5HT in the inhibition of food intake reveals that only d-fenfluramine is inhibited by antiserotonin agents. The role of the different 5HT receptor-subtypes in this antagonism is discussed. More selective 5HT antagonists are needed to establish which 5HT receptor(s) controls food intake.

On the significance of brain extracellular uric acid detected with in-vivo monitoring techniques: a review

The concentration of uric acid [UA] in the extracellular fluid (ECF) estimated with in-vivo voltammetry and microdialysis data is compared for probes of different diameters from the day of implantation (acute) to several days (chronic) or even months after surgery. For small probes (diameter < 160 microns) the acute [UA] of ca. 5 microM decreased significantly to ca. 1 microM under chronic conditions. For larger probes (e.g., 320-microns diameter) the acute [UA] was also ca. 5 microM, but this value significantly increased to ca. 50 microM under chronic conditions. Associated with this difference in [UA], there were parallel differences in the extent of gliosis around the probes. These findings are discussed in terms of possible sources of extracellular UA and their implications for in-vivo monitoring techniques in behaving animals.

8-OH-DPAT attenuates the dexfenfluramine-induced increase in extracellular serotonin: an in vivo dialysis study

Rats with frontocortical microdialysis probes were treated with dexfenfluramine or dexfenfluramine with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) pretreatment. Dexfenfluramine (10 mg/kg i.p.) increased extracellular serotonin (5-hydroxytryptamine, 5-HT) (calculated area under the curve (AUC) for the 0 to 105-min period after dexfenfluramine treatment = 8.22 +/- 2.66 pmol 5-HT). Systemic (0.025 mg/kg i.p.) or local (0.01 microM into the dorsal raphe nucleus) 8-OH-DPAT pretreatement decreased the dexfenfluramine response (AUC: 1.03 +/- 0.07 and 0.44 +/- 0.04 pmol 5-HT, respectively). This result might be explained by the decrease in 5-HT neuronal discharge caused by somatodendritic 5-HT1A autoreceptor activation, and suggests that the 5-HT releasing effect of dexfenfluramine in vivo depends on nerve terminal depolarization.

Serotonergic facilitation of acetylcholine release in vivo from rat dorsal hippocampus via serotonin 5-HT3 receptors

The serotonin (5-HT) releaser d-fenfluramine and its active metabolite d-norfenfluramine, or the 5-HT-uptake inhibitor citalopram, by increasing synaptic 5-HT availability, facilitated in vivo release of acetylcholine (ACh) from dorsal hippocampi of freely moving rats as determined by the microdialysis technique. The effects of d-norfenfluramine (7.5 mg/kg i.p.) and citalopram (10 microM, applied by reverse dialysis) were prevented by a 14-day chemical lesion of the raphe nuclei, suggesting mediation by the 5-HT system in the cholinergic action of the drugs. The increase in extracellular ACh content induced by d-norfenfluramine (5 mg/kg i.p.) was antagonized by the 5-HT3 receptor antagonists tropisetron (0.5 mg/kg i.p.) and DAU 6215 (60 micrograms/kg i.p.), but not by the mixed 5-HT1 and 5-HT2 receptor antagonist metergoline (2 mg/kg s.c.). In accordance with an involvement of the 5-HT3 receptor in the ACh facilitation induced by d-norfenfluramine is the finding that the selective 5-HT3 receptor agonist 2-methylserotonin (250 micrograms i.c.v., or 10 microM applied by reverse dialysis) raised ACh release. The effect of the intracerebroventricular drug was prevented by the 5-HT3 antagonists DAU 6215 (60 micrograms/kg i.p.) and ondansetron (60 micrograms/kg s.c.). These antagonists by themselves did not modify the basal ACh release, indicating that 5-HT does not tonically activate the 5-HT3 receptors involved. In conclusion, the overall regulatory control exerted by 5-HT in vivo is to facilitate hippocampal ACh release. This is mediated by 5-HT3 receptors probably located in the dorsal hippocampi.

Evidence of an exocytotic-like release of [3H]5-hydroxytryptamine induced by d-fenfluramine in rat hippocampal synaptosomes

The monoamine releasing activity of d-fenfluramine was investigated with an in vitro model consisting of synaptosomes preloaded with the 3H-neurotransmitter and extensively washed in a superfusion apparatus before a 3-min exposure to d-fenfluramine. With this model, the drug-induced release is real and is not confused by inhibition of reuptake by the drug. d-Fenfluramine (0.5 microM) induced only [3H]5-hydroxytryptamine ([3H]5-HT) release from hippocampal synaptosomes whereas 10 microM also induced some overflow from hippocampal synaptosomes preloaded with [3H]noradrenaline or from striatal synaptosomes preloaded with [3H]dopamine, although the overflow was much lower than from 5-HTergic synaptosomes. We then focused on the [3H]5-HT release induced by 0.5 microM d-fenfluramine, which was previously shown to be Ca2+ dependent. The same finding was confirmed in the present study with other experimental protocols, indicating the requirement for extracellular Ca2+ ions. By measuring [3H]5-HT uptake into rat hippocampal synaptosomes we confirmed that Ca(2+)-ions are not required for the function of the 5-HT uptake carrier or for its interaction with d-fenfluramine. d-Fenfluramine-induced [3H]5-HT release was not altered by 1 microM nitrendipine (blocking the L-type Ca2+ channels) but was slightly decreased (20%) by 0.5 microM omega-conotoxin (blocking the N-type Ca2+ channels). It was also inhibited by 0.5 microM clonidine, interacting with alpha 2-adrenergic heteroreceptors, and by 10 nM tetanus toxin, known to affect the exocytosis of different neurotransmitters including 5-HT. These compounds had very similar effects on the Ca(2+)-dependent, exocytotic release of [3H]5-HT induced by depolarization, i.e. by 15 mM K+.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin uptake inhibition: in vivo effect of sertraline in rats

Sertraline, a potent and selective serotonin uptake inhibitor, was used to analyze the changes occurring in the serotonin system after uptake inhibition in vivo. Sertraline (11 mg/kg) lowered extracellular 5-hydroxyindolacetic acid (5-HIAA), measured in rat hippocampus by in vivo voltammetry, for about 3 h. The interaction between sertraline and drugs known to interfere with the release or uptake of serotonin (L-5-hydroxytryptophan (5-HTP), d-norfenfluramine and tianeptine) was then studied. The sertraline-induced decrease in extracellular 5-HIAA was related to the inhibition of uptake.

Dexfenfluramine. A review of its pharmacological properties and therapeutic potential in obesity

Dexfenfluramine stimulates serotoninergic activity by inhibiting serotonin reuptake into presynaptic neurons and by enhancing its release into brain synapses. Based on the serotonin hypothesis of appetite control these effects would be expected to reduce food intake and thus body-weight. Studies in animal models and severely overweight patients have confirmed the effectiveness of dexfenfluramine as a weight-reducing agent which appears to be well tolerated. Permanent weight loss is the goal of weight-reducing strategies and, based on current clinical evidence, dexfenfluramine appears to exert a weight reducing effect over periods of up to 12 months without development of tolerance, a problem that has limited the long term use of other pharmacological agents used in the treatment of this disorder. Dexfenfluramine facilitated weight loss in patients who had not responded satisfactorily to other weight-reducing strategies, prevented relapse in those patients who had achieved weight reduction by other methods, and corrected disturbed eating patterns (and therefore reduced weight gain) in small studies involving patients with premenstrual syndrome, seasonal affective disorder and nicotine withdrawal syndrome. Follow-up of the longest study reported with dexfenfluramine suggests that continued therapy is required in severely overweight patients if weight loss is to be maintained. Dexfenfluramine has not been directly compared with nonpharmacological measures of weight control such as behaviour modification or exercise programmes. The decision that pharmacological means are indicated in overweight patients must be highly individualised, and must consider the many complex factors that often contribute to overweight states, as well as the anticipated magnitude of drug effect. Despite such a cautionary note, and the expected need (at this stage of its development) for an expanded clinical study programme in certain areas, dexfenfluramine is a clear advance in the pharmacological approach to improved management of overweight individuals.

In vivo studies on the enhancement of serotonin reuptake by tianeptine

The present study investigates the in vivo effects of the serotonin uptake enhancer tianeptine. The serotonin metabolite, 5-hydroxy-indolacetic acid (5-HIAA) was measured by in vivo voltammetry and carbon fiber electrodes chronically implanted in different brain areas of freely moving rats. Tianeptine (10 mg/kg i.p.) increased extracellular 5-HIAA in the hippocampus and hypothalamus. The interaction between tianeptine and drugs known to interfere with the uptake or release of serotonin (sertraline, buspirone, D-norfenfluramine) was then studied and, to ascertain the in vivo pharmacological relevance of tianeptine's effects, its ability to reduce the serotoninergic syndrome was evaluated. Both the biochemical and behavioral data indicate that in vivo tianeptine's effects on the serotoninergic system are likely to be due to serotonin uptake enhancement.

In Vivo Neurochemical Evaluation of Striatal Serotonergic Hyperinnervation in Rats Depleted of Dopamine at Infancy

Destruction of nigrostriatal dopamine (DA) neurons with 6-hydroxydopamine (6-OHDA) early in development results in hyperinnervation of striatum by the serotonergic afferents deriving from the dorsal raphe nucleus. We have used in vivo microdialysis to investigate the degree to which serotonergic neurotransmission in striatum is altered by this increase in the density of serotonin (5-HT) terminals. The effects of several manipulations known to influence 5-HT function on extracellular 5-HT and 5-hydroxyindoleacetic acid in striatum were compared in adult rats treated neonatally with 6-OHDA and in intact adult rats. Basal levels of 5-HT in extracellular fluid (ECF) of striatum were similar in neonatally DA-depleted rats and in intact rats. Perfusion with the 5-HT reuptake blocker, fluoxetine (100 microM), increased 5-HT in striatal ECF of neonatally DA-depleted rats to levels that were threefold greater than those achieved in intact rats. Likewise, K(+)-depolarization of the 5-HT terminals (100 mM in perfusate) or systemic administration of the 5-HT releaser, (+/-)-fenfluramine (10 mg/kg i.p.), increased the concentration of 5-HT in striatal ECF of neonatally DA-depleted rats to levels approximately threefold greater than those observed in striatum of intact rats. These findings indicate that the 5-HT hyperinnervation of striatum that takes place in rats depleted of DA at infancy is associated with an increased capacity for neurotransmitter release in this system. Concomitant increased in high-affinity 5-HT uptake may prevent the occurrence of any measurable changes in the resting concentration of 5-HT in striatal ECF.

The effects of tianeptine on wet-dog shakes, fore-paw treading and a flexor reflex in rats are consistent with enhancement of 5-hydroxytryptamine uptake

Tianeptine is a novel antidepressant which uniquely facilitates 5-hydroxytryptamine (5-HT) uptake. When given in a dose of 10 mg/kg to rats pretreated with either carbidopa or phenelzine, it markedly reduced the frequency of wet-dog shakes, fore-paw treading, tremor and hind-limb abduction evoked by L-5-hydroxytryptophan (L-5-HTP) given 30 or 60 min later. This effect of tianeptine was opposite to that of paroxetine, a selective 5-HT uptake inhibitor, which greatly increased the 5-HTP-induced behavioural syndrome. In contrast, tianeptine did not affect behaviours elicited by the 5-HT receptor agonists, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) or (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI), which are not substrates for the 5-HT uptake process. In spinal animals, tianeptine attenuated an ipsilateral flexor reflex, an effect opposite to that of citalopram, a selective 5-HT uptake inhibitor. These effects of tianeptine are consistent with its ability to increase 5-HT reuptake.

Comparative studies on the anorectic activity of d-fenfluramine in mice, rats, and guinea pigs

The present study compares the anorectic activity of d-fenfluramine and its metabolite d-norfenfluramine in three animal species. d-Fenfluramine and d-norfenfluramine show anorectic activity at increasing doses (ED50) in rats, guinea pigs, and mice, d-norfenfluramine being more active than d-fenfluramine in all three species. Equiactive anorectic activities are reached with different brain levels of d-fenfluramine and d-norfenfluramine, guinea pigs being the most sensitive species, followed by rats then mice. The metabolite most probably plays a major role in the anorectic effect of d-fenfluramine in guinea pigs, contributes to the anorectic activity in rats, but adds little to the action of the parent drug in mice. The different sensitivity to d-fenfluramine and d-norfenfluramine in these three species does not appear to be explained by a number of biochemical parameters, including serotonin uptake or release, receptor subtypes, or 3H-d-fenfluramine binding and uptake.

Hypothalamic monoamines measured by microdialysis in rats treated with 2-deoxy-glucose or d-fenfluramine

The effects on brain monoamines (norepinephrine, serotonin and the metabolite of dopamine) following administration of d-fenfluramine (10 mg/kg IP) and 2-deoxy-d-glucose (500 mg/kg IP) have been measured by microdialysis from the ventromedial hypothalamus, lateral hypothalamic area and dorsomedial hypothalamus of conscious, unrestrained rats. Following administration of d-fenfluramine there was a significant increase in the concentration of serotonin in the ventromedial hypothalamus and lateral hypothalamic area, but no significant increase in the DMH. 5-HIAA (5-hydroxy-3-indoleacetic acid), the metabolite of serotonin, was increased in the DMH, but not in the other two regions. DOPAC (3,4-dihydroxyphenylacetic acid) was increased following fenfluramine treatment in all three regions examined. An increase in norepinephrine was observed in the VMH, but not in the other two regions, while the concentration of the 3-methoxy-4-hydroxyphenylglycol (MHPG) was increased in both areas. Treatment with 2-deoxy-D-glucose (2DG) was associated with fewer changes. In the lateral hypothalamic area there was a decrease in 5-HIAA and an increase in DOPAC. In the VMH there was an increase in norepinephrine and a decrease in MHPG in the DMH, but otherwise no significant alterations were observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Fenfluramine administered systemically or locally increases extracellular serotonin in the lateral hypothalamus as measured by microdialysis

Microdialysis was used to monitor serotonin, 5-hydroxyindoleacetic acid (5-HIAA) and the metabolites of dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the perifornical lateral hypothalamus (PFH) of freely moving rats. Systemically administered d-fenfluramine (d-FEN) increased extracellular serotonin, DOPAC and HVA, while decreasing serotonin's metabolite 5-HIAA. Local application of d-FEN directly to the hypothalamic terminal region caused large increases in extracellular serotonin and had a tendency to decrease all 3 metabolites. This effect was confirmed when d-FEN was infused locally by reverse dialysis. These results provide direct evidence that the anorectic drug d-FEN can increase extracellular serotonin in the hypothalamus in vivo and suggest a serotonergic action in the perifornical region. This finding is consistent with our report that a meal also increases serotonin in this region.