Elevation of plasma corticosterone by swim stress and insulin-induced hypoglycemia in control and fluoxetine-pretreated rats.

Fluoxetine, a drug that inhibits serotonin inactivation by reuptake from the synaptic cleft and thereby enhances serotonin nerve function, was used to study the possible role of serotonin neurons in the activation of the pituitary-adrenal system of rats by swim stress or insulin-induced hypoglycemia. Fluoxetine pretreatment enhanced the elevation of plasma corticosterone produced by injection of L-5-hydroxytryptophan but did not significantly alter the elevation of plasma corticosterone by swim stress or by insulin-induced hypoglycemia, even when the stimulus was shown to be submaximal. The results suggest that serotonin neural pathways postulated to stimulate ACTH secretion are not involved in the activation of adrenocortical function by these stimuli.     

The effect of amperozide, a new antipsychotic drug, on plasma corticosterone concentration in the rat.

Amperozide is an atypical antipsychotic drug with high affinity for the serotonin 5-HT2 receptor but with low affinity for the dopamine D1 and D2 receptors. Amperozide dose-dependently increased the level of plasma corticocorticosterone in the rat. The effect of amperozide on plasma corticosterone was not inhibited by pretreatment with the 5-HT1A receptor antagonist pindolol or the 5-HT2 receptor antagonist ritanserin. Nor was it inhibited by the dopamine D2 receptor antagonist haloperidol. In contrast to ritanserin, amperozide did not antagonize plasma corticosterone elevation elicited by the serotonin receptor agonist MK-212. Similar to the serotonin uptake inhibitor fluoxetine, amperozide (0.5 mg/kg) significantly (p < 0.05) blocked p-chloroamphetamine (PCA) induced corticosterone release 4 and 16 hrs after amperozide administration. However, amperozide significantly increased the plasma corticosterone concentration also in rats pretreated with parachlorophenylalanine (PCPA). These data suggest that other mechanisms than a 5-HT uptake inhibitory effect are involved in the acute stimulation of corticosterone by amperozide.     

Cerebral serotonin and the hypothalamo-hypophyseal-adrenal system of the rat during aging]

The role of brain serotonin (5HT) on the hypothalamus-pituitary-adrenal system (HPAs) under basal condition and after injections of p-chlorophenylalanine (pCPA) and L-5-hydroxytryptophan (L-5HTP) has been studied in 6, 12 and 28 month old male Wistar rats. Four experimental groups were made for each age: control, saline, injected with pCPA (250 mg/kg i.p.) and L-5HTP (200 mg/kg i.p.), the effects being valued 2 hours after L-5HTP administration and 24 hours after pCPA injection. In all groups the plasmatic ACTH, the corticosterone levels as well as the simultaneous changes of the 5TH content tryptophan hydroxylase activity in whole brain were estimated two hours after the L-5HTP injection and 24 hours after that of pCPA. Significant changes are not found in the plasmatic ACTH and corticosterone values with respect to age under basal condition. Nevertheless, the response of HPAs differs with the age after pCPA or L-5HTP injection. The ACTH and corticosterone levels augment by L-5HTP and decrease by pCPA in all age groups, but this corresponding increase or decrease was less marked in the older rats. The 5HT content as tryptophan hydroxylase activity in brain decreased in old animals. pCPA and L-5HTP determine, respectively, high falls and rise of 5TH values, these changes being more intense for pCPA in old rats and for L-5HTP in young and mature animals. The tryptophan hydroxylase activity is decreased by pCPA as L-5HTP injections.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of fluoxetine and metergoline on amphetamine-induced rise in plasma corticosterone

The effect of the serotonin reuptake inhibitor, fluoxetine, and the serotonin antagonist, metergoline, on the rise in plasma corticosterone induced by amphetamine was studied in the conscious, unrestrained rat. Fluoxetine (2.5 mg/kg) did not affect plasma corticosterone. However, this dose of fluoxetine when administered two hours prior to amphetamine (0.1 or 0.5 mg/kg) significantly potentiated the amphetamine-induced rise in plasma corticosterone. Fluoxetine had no effect on the response induced by the highest dose of amphetamine (1.0 mg/kg) utilized in the study. In contrast, metergoline produced a dose-dependent increase in plasma corticosterone over the range 0.1 – 5.0 mg/kg. This response reached maximum 30 minutes after drug administration and had a duration of approximately 120 minutes. Pretreatment of animals with metergoline (5.0 mg/kg) three hours before the administration of amphetamine (1.0 mg/kg) resulted in a significant decrease in the corticosterone rise induced by amphetamine. Lower doses of metergoline were ineffective in reducing the amphetamine-induced response. These observations support the hypothesis that the amphetamine-induced rise in plasma corticosterone is due, in part, to stimulation of serotonergic neurons.     

The acute effects of 5HTP, fluoxetine and quipazine on insulin and glucagon release in the intact rat.

5-hydroxytryptophan (5HTP), the immediate precursor of serotonin, induces a release of insulin and glucagon in the intact rat. These effects of 5HTP, which have previously been shown to be blocked by L-aromatic amino acid decarboxylase inhibition, were also prevented by methysergide (a serotonin receptor antagonist). Quipazine (a serotonin receptor agonist) did not alter pancreatic hormone release. Fluoxetine, a serotonin neuronal reuptake blocker did not effect insulin secretion and had a slight glucagon stimulatory effect, however the effects of 5HTP on insulin and glucagon release were not potentiated by fluoxetine pretreatment. Alpha and beta-adrenergic receptor blockade did not alter the pancreatic effects of 5HTP.     

Antihypertensive effects of fluoxetine and L-5-hydroxytryptophan in rats.

The combination of fluoxetine (10 mg/kg) and L-5-hydroxytryptophan (5-HTP) (10 mg/kg) significantly lowered blood pressure in spontaneously hypertensive rats and in rats made hypertensive by treatment with deoxycorticosterone (DOCA) and saline. Fluoxetine alone also had a significant effect on blood pressure in DOCA hypertensive rats, but not as great an effect as the combination. Since fluoxetine is an inhibitor of serotonin reuptake and 5-HTP is the serotonin precursor, the antihypertensive effect of this drug combination strengthens previous evidence that serotonin neurons have a role in the central regulation of blood pressure.     

Effect of brain serotonin on the arterial pressure and plasma renin in normal and hypertensive rats]

The effects of changes in brain serotonin content after injections of p-chlorophenylalanine (p-CPA), L-5-hydroxytryptophan (L-5HTP) and 5-6-dihydroxytryptamine (5-6DHT) on the mean arterial pressure (MAP), plasma renin activity (PRA) and peripheral levels of atrial natriuretic peptide (ANP) have been studied in normal and hypertensive (2K:1C model) male Wistar rats. The p-CPA (250 mg/kg) and L-5HTP (200 mg/kg) were injected i.p., while 5-6 DHT (15 micrograms/animal in 10 mu/animal vehicle) was injected into lateral brain ventricles. The effects were studied 24 h after the p-CPA injection, 2 h after L-5HTP and 10 or 20 days after 5-6DHT administration. The fall in brain serotonin produced by p-CPA and 5-6DHT did not modify the MAP values in the normal and hypertensive rat model, whereas the increase induced after L-5HTP injection only caused a slight decrease in arterial pressure in normotensive animals. The ARP experimented remarkable rises in the normal and hypertensive rats, these values increasing after L-5HTP and falling after p-CPA and 5-6 DHT injections. Similar changes are detected in the normal group after administration of these substances related to serotoninergic brain activity. The ANP levels rose after renal artery constriction, and they are not affected by the above mentioned substances. Only p-CPA and 5-6DHT reduced a low decrease in the ANP levels 10 days after their administration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of fluoxetine on serotonin and dopamine concentration in microdialysis fluid from rat striatum.

Fluoxetine injected i.p. into rats at a dose of 10 mg/kg rapidly increased serotonin concentration in microdialysis fluid from the striatum by at least 4-fold, an increase that was maintained throughout the 3 hr observation period. Dopamine concentration in the microdialysis fluid did not change. The concentration of the two dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, was not changed in the microdialysis fluid, whereas the concentration of the serotonin metabolite, 5-hydroxyindoleacetic acid, was significantly decreased after fluoxetine injection. The increased extracellular concentration of serotonin no doubt resulted from inhibition of the serotonin uptake carrier by fluoxetine, and the lack of change in dopamine is evidence for the specificity of action of this uptake inhibitor.     

Relation between brain 5-HIAA levels and the release of serotonin into brain synapses

Our findings in experiments using reserpine, an amine releaser, and fluoxetine, a serotonin uptake blocker, indicate that the reuptake of serotonin from brain synapses precedes its transformation to 5-hydroxyindoleacetic acid (5-HIAA). Male rats were injected with reserpine or fluoxetine alone, or with fluoxetine one hour before reserpine; control animals received diluents. Reserpine lowered brain serotonin and raised brain 5-HIAA levels. Fluoxetine alone did not change serotonin levels but lowered 5-HIAA. Fluoxetine completely antagonized the reserpine-induced increase in 5-HIAA, and significantly enhanced its depletion of serotonin. In order to determine whether the ability of fluoxetine to block the rise in 5-HIAA after reserpine resulted from its effect on serotonin reuptake or from suppression of impulse flow along serotoninergic neurons, we also examined the effects of the drugs on serotonin metabolism in distal portions of acutely transected neurons (which, presumably, were no longer able to conduct impulses). No differences were noted between the responses of intact and lesioned serotoninergic neurons, indicating that fluoxetine's blockade of the rise in brain 5-HIAA results from its effect on serotonin reuptake.     

The effect of d-fenfluramine on anterior pituitary hormone release in the rat: in vivo and in vitro studies

Administration of d-fenfluramine, a serotonin-releasing drug, to male rats induced a dose-dependent increase in both serum prolactin and corticosterone concentrations. Serum growth hormone levels increased, but not significantly, at a dose of 1.25 mg/kg i.p. and decreased significantly at higher doses. When rats were pretreated with the serotonin uptake inhibitor fluoxetine (10 mg/kg i.p.) 30 min prior to injection of d-fenfluramine (5 mg/kg i.p.), the serum prolactin response to d-fenfluramine was partially inhibited, whereas the growth hormone response was not significantly modified. Fluoxetine pretreatment increased the serum corticosterone to the same level as did d-fenfluramine. d-Fenfluramine's effect on prolactin and growth hormone release was further tested in a hypothalamic-pituitary in vitro system. The addition of d-fenfluramine (5-500 ng/mL) for 30 min to rat hypothalami resulted in an enhancement of prolactin and growth hormone-releasing activities. These were expressed as the ability of the media in which the hypothalami had been incubated to stimulate prolactin and growth hormone release by cultured pituitary cells. The data suggest that the effect of d-fenfluramine on prolactin secretion is exerted through the hypothalamus and is probably mediated, at least partially, by a serotoninergic mechanism. The mechanism of d-fenfluramine's effect on corticosterone and growth hormone release needs further evaluation.     

Fluoxetine, a selective inhibitor of serotonin uptake, potentiates morphine analgesia without altering its discriminative stimulus properties or affinity for opioid receptors

The analgesic effect of morphine in the rat tail jerk assay was enhanced by the serotonin uptake inhibitor, fluoxetine. Tail jerk latency was not affected by fluoxetine alone. Morphine's affinity for opioid receptors labeled in vitro with 3H-naloxone or 3H-D-Ala2-D-Leu5-enkephalin was not altered by fluoxetine, which has no affinity for these sites at concentrations as high as 1000 nM. In rats trained to discriminate morphine from saline, fluoxetine at doses of 5 or 10 mg/kg were recognized as saline. Increasing the fluoxetine dose to 20 mg/kg did not result in generalization to either saline or morphine. The dose response curve for morphine generalization was not significantly altered by fluoxetine doses of 5 or 10 mg/kg. Those rats treated with the combination of morphine and 20 mg/kg of fluoxetine did not exhibit saline or morphine appropriate responding. Fluoxetine potentiates the analgesic properties of morphine without enhancing its affinity for opioid receptors or its discriminative stimulus properties.     

The effect of fluoxetine on warfarin metabolism in the rat and man.

Fluoxetine, a selective blocker of serotonin uptake, inhibits the metabolism of warfarin in rats. In contrast, after a single dose or seven daily doses of fluoxetine to human subjects, no inhibition of warfarin metabolism was observed.     

Involvement of striatal serotonin in fluoxetine effects on adrenocortical function and behaviour

Treatment of the adult rats with selective serotonin (5-HT) reuptake inhibitor: fluoxetine and its complexes with glycyrrizhinic acid during 2 weeks (25 mg/kg/day) significantly increased plasma corticosterone levels that were measured after 5-min plus-maze. All the drugs decreased the content of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the striatum as well as 5-HT in the hippocampus. There was a significant negative correlation between 5-HT in the striatum and corticosterone levels. These data suggest that fluoxetine induces serotoninergic changes in the striatum that might be related to neuroendocrine and behavioural effects of the drug.     

Fluoxetine Partly Exerts its Actions Through GABA: A Neurochemical Evidence

Fluoxetine, as a serotonin re-uptake inhibitor augments serotonin concentration within the synapse by inhibiting the serotonin transporter. The contribution of amino acids has also been shown in depression. We hypothesized that fluoxetine exerts its actions at least in part by intervening brain signaling operated by amino acid transmitters. Therefore the aim of this study is to supply neurochemical evidence that fluoxetine produces changes in amino acids in cerebrospinal fluid of rats. Sprague-Dawley rats were anesthetized and concentric microdialysis probes were implanted stereotaxically into the right lateral ventricle. Intraperitoneal fluoxetine (2.5 or 5 mg/kg) or physiological saline was administered and the probes were perfused with artificial cerebrospinal fluid at a rate of 1 μl/min. In the chronic fluoxetine group, the rats were treated daily with oral fluoxetine solution or inert syrup for 3 weeks. The microdialysis probes were placed on the 21st day and perfused the next day. Fluoxetine was ineffective in changing the cerebrospinal fluid GABA levels at the dose of 2.5 mg/kg but produced a significant increase in the perfusates following injection of 5 mg/kg of fluoxetine (P < 0.05). Oral fluoxetine administration (5 mg/kg) for 21 days also elevated the CSF GABA levels by approximately 2-fold (P < 0.05). l-glutamic acid levels were not affected in all groups. These neurochemical findings show that fluoxetine, a selective serotonin re-uptake inhibitor affects brain GABA levels indirectly, and our results suggest that acute or chronic effects may be involved in beneficial and/or adverse effects of the drug.     

Effect of long-term fluoxetine treatment on the human serotonin transporter in Caco-2 cells

Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) broadly used in the treatment of human mood disorders and gastrointestinal diseases involving the serotoninergic system. The effectiveness of this therapy depends on repeated long-term treatment. Most of the long-term studies in vivo of SSRI effects on serotoninergic activity have focused on their effects on autoreceptors or postsynaptic receptors. The chronic effect of SSRIs on the activity of the serotonin transporter (SERT) has been less studied and the results have been contradictory. The aim of this study was to determine the specific effect of long-term fluoxetine treatment on human serotonin transporter (hSERT) in vitro, by using the human enterocyte-like cell line Caco-2. Results show that fluoxetine diminished the 5-HT uptake in a concentration-dependent way and that this effect was reversible. Fluoxetine affected mainly the hSERT transport rate by reducing the availability of the transporter in the membrane with no significant alteration of either the total hSERT protein content or the hSERT mRNA level. These results suggest that the effect of fluoxetine on the expression of hSERT is post-translational and has shown itself to be independent of PKC and PKA activity. This study may be useful to clarify the effect of the long-term fluoxetine therapy in both gastrointestinal and central nervous system disorders.     

Pharmacological profile of a model for central serotonin receptor activation

The head shake response in rats after systemic administration of the serotonin (5HT) precursor 5-hydroxytryptophan (5HTP) was pharmacologically characterized and shown to be a useful animal model to quantify brain 5HT receptor activation. The behavior occurred in a dose-dependent manner after injection of 5HTP and the 5HT agonist quipazine. Head shakes were also observed after injection of L-tryptophan, 5-methoxydimethyltryptamine and fenfluramine. The 5HT antagonists cyproheptadine and metergoline were potent blockers of the response. Xylamidine, a peripheral 5HT antagonist, had no effect on head shaking. Inhibition of 5HT uptake with fluoxetine potentiated the head shake response after 5HTP. Manipulation of central cholinergic or GABAergic mechanisms did not alter 5HTP-induced shakes. Alpha-noradrenergic receptor blockade had no significant effect on head shakes. However, desmethylimipramine was equipotent with methysergide as an antagonist of the behavior. Beta-noradrenergic receptor blockade had no specific effect on 5HTP head shakes. Concomitant dopamine receptor activation with SK&;F 38393 did not affect head shakes but the neuroleptics chlorpromazine and pimozide reduced the number of head shakes after 5HTP. The H₁ receptor antagonist pyrilamine had no effect on head shakes. It is concluded that 5HTP-induced head shakes in rats is a quantitative model of brain 5HT receptor activation which is particularly sensitive to 5HT antagonists.     

Formation of serotonin by rat kidneys in vivo

Renal formation of serotonin by decarboxylation of its amino acid precursor L-5-hydroxytryptophan (L-5-HTP) has been demonstrated with renal tissue homogenates and isolated perfused rat kidneys. Our objective in the present study was to determine whether the conversion of L-5-HTP to serotonin was associated with functional changes by kidneys in vivo. Renal clearance studies were conducted in anesthetized, volume-expanded male Sprague-Dawley rats receiving either saline (n = 9) or L-5-HTP (15 and 75 micrograms/min iv, n = 9). No change in mean arterial pressure was measured during infusions of L-5-HTP at either dose, whereas glomerular filtration rate (GFR), as measured by the clearance of inulin, and effective renal plasma flow (CPAH) decreased by 34 +/- 5% (mean +/- SE, P less than 0.001) and 26 +/- 7% (P greater than 0.07), respectively. Urine flow and sodium excretion decreased by 41 +/- 9% (P less than 0.01). Serotonin and 5-HTP were determined in urine and plasma using HPLC. High levels of 5-HTP were present in plasma, but not urine. Urinary serotonin increased in the rats receiving L-5-HTP without concomitant increases in plasma serotonin. More than 20% of the infused L-5-HTP was recovered in the urine as serotonin. The decarboxylase inhibitor carbidopa (20 micrograms/min) markedly reduced urinary serotonin excretion in the rats which received L-5-HTP and reversed the changes in GFR, CPAH, urine flow, and sodium excretion. Infusions of the amino acid precursor of L-5-HTP, L-tryptophan (n = 7), did not alter kidney function or increase plasma or urinary 5-HTP or serotonin levels. These results are consistent with the intrarenal formation of serotonin by renal decarboxylase with attendant alterations in renal hemodynamics and salt and water excretion.     

Decreased plasma leptin levels in lean and obese Zucker rats after treatment with the serotonin reuptake inhibitor fluoxetine.

Leptin inhibits feeding, stimulates thermogenesis and decreases body weight. Serotonin reduces food intake when injected into the hypothalamus and may interact with other neurotransmitters in the control of appetite. We therefore investigated the effects of the serotonergic drug fluoxetine, which inhibits serotonin reuptake, on food intake and plasma leptin levels in lean and obese Zucker rats. Fluoxetine significantly reduced food intake in lean and obese rats, both acutely after a single injection (10 mg/kg) and during continuous subcutaneous infusion (10 mg/kg/day for 7 days). Plasma leptin levels were reduced after both 4 hours and 7 days of fluoxetine administration in lean and after 7 days in fatty rats (all p<0.01). We have previously suggested that serotonin may decrease food intake by inhibiting neuropeptide Y neurones, and we further suggest that it also inhibits leptin, possibly by an effect on white adipose tissue.     

Fluoxetine increases extracellular levels of 3-methoxy-4-hydroxyphenylglycol in cultured COLO320 DM cells

Fluoxetine (Prozac) is a serotonin reuptake inhibitor. It increases extracellular levels of serotonin and is used in relieving the depressive symptoms of cancer patients. It has been reported that the drug may enhance the growth of certain cancer cells. This study investigates whether fluoxetine enhances the growth of a human colon cancer cell line (COLO320 DM) and if it affects the extracellular levels of serotonin or its metabolite, 5-hydroxyindole-3-acetic acid (5-HIAA) and other monoamines and metabolites at two cell densities. The extracellular levels of serotonin, 5-HIAA and other monoamines and metabolites were measured simultaneously by high performance liquid chromatography from cell-culture media after incubation of cells both with and without fluoxetine for 3 days. The viability of COLO320 DM cells was evaluated using 3-(4,5-cimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT). At low cell densities (1.25x10(5) cells ml-1), fluoxetine at 1-10 microM significantly increased the extracellular levels of serotonin (p<0.005), 5-HIAA (p<0.005), and 3-methoxy-4-hydroxyphenylglycol (MHPG; p<0.001) as compared to the controls. Fluoxetine at 10-100 microM significantly inhibited the growth of COLO320 DM (p<0.005). At high cell densities (2x10(6) cells ml-1), fluoxetine at 1-10 microM significantly increased the extracellular levels of MHPG (p<0.01), and at 10 microM it significantly increased the extracellular levels of 5-HIAA (p<0.05). Fluoxetine at 100 microM significantly inhibited the growth of the cells (p<0.0001). These results suggest that fluoxetine at 1 microM of effective concentration may increase the extracellular levels MHPG, in addition to serotonin and 5-HIAA levels, yet not inhibit the growth of COLO320 DM.     

Effect of fluoxetine hydrochloride (Lilly 110140), a specific inhibitor of serotonin uptake, on morphine analgesia and the development of tolerance

Fluoxetine, a specific inhibitor of the re-uptake of serotonin in the brain, was found to potentiate the analgesic effect of morphine as measured by the tail-flick method in rats. One dose of fluoxetine thirty minutes prior to analgesic testing in morphine pellet implanted rats was shown to inhibit the analgesic effect of acute challenges of morphine to the same degree as in rats treated daily with fluoxetine during the development of tolerance to morphine. These data indicate that serotonin may play a role in the analgesic effect of morphine, but not in the development of tolerance to narcotic analgesia.