Behavioral effects of L-5-hydroxytryptophan after destruction of ascending serotonergic pathways in the rat: the role of catecholaminergic neurons.

Lesions in the medial forebrain bundle of the rat produced a 68 to 74% decrease in telencephalic serotonin (5-HT) and a 30 to 43% decrease in jump threshold. L-5-Hydroxytryptophan (L-5-HTP; 37.5 mg/kg) returned the 5-HT content and jump threshold of lesioned rats to normal levels. These effects of L-5-HTP were also observed after the inhibition of extracerebral decarboxylase activity with Ro 4-4602 (50 mg/kg). Pretreatment with 6-hydroxydopamine (6-OHDA), which selectively destroys catecholamine neurons, had no effect on the jump threshold of nonlesioned rats nor did it further change the 5-HT content or jump threshold of lesioned rats. Lesioned rats pretreated with 6-OHDA demonstrated an increase in 5-HT content after L-5-HTP; however, their jump threshold remained significantly lower than that of controls. This ability of 6-OHDA to block the behavioral effects of L-5-HTP in lesioned rats was also observed after Ro 4-4602. In rats given Ro 4-4602, the accumulation of 5-HT at 90 minutes after injection of L-5-HTP was significantly correlated (r = 0.98) with total monoamine content. Thus, 6-OHDA pretreatment significantly decreased the net accumulation of 5-HT from L-5-HTP in nonlesioned rats. These rats also demonstrated further decreases in norepinephrine and dopamine content after L-5-HTP. It was concluded that L-5-HTP can be decarboxylated to 5-HT in serotonergic and catecholaminergic neurons and that the behavioral effects of L-5-HTP in lesioned rats may be due to the formation of 5-HT in catecholaminergic neurons where it may act as a "false-transmitter."     

Reduction of [3H]6-nitroquipazine-labelled 5-hydroxytryptamine uptake sites in rat brain by 3,4-methylenedioxymethamphetamine

3,4-Methylenedioxymethamphetamine (MDMA; Ecstasy) is a known neurotoxin to 5-hydroxytryptamine (5-HT; serotonin) nerve terminals. It has recently been demonstrated that [3H]6-nitroquipazine is a new radioligand for studying the 5-HT transport system in brain. Therefore, we examined the effects of repeated systemic administration (10 mg/kg ip, twice daily for 3 d) of MDMA on [3H]6-nitroquipazine-labelled 5-HT uptake sites in rat brain. Marked reductions in the concentrations of 5-HT and its major metabolite 5-hydroxyindoleacetic acid (5-HIAA) were observed in the cerebral cortex 1 week after the last injection of MDMA. In addition, the density of [3H]6-nitroquipazine-labelled 5-HT uptake sites was significantly decreased by MDMA. Furthermore, the reduction of 5-HT and 5-HIAA content and the density of [3H]6-nitroquipazine-labelled 5-HT uptake sites by MDMA were significantly prevented by co-administration of 6-nitroquipazine (5 mg/kg), a very potent and selective 5-HT uptake inhibitor. The present results indicate that the 5-HT uptake carrier plays an important role in the neurotoxic action of MDMA.     

The effect of blood ingestion on brain serotonin synthesis in portacaval-shunted rats

In rats with a portacaval shunt (PCS), the effect on the serotonin metabolism in the brain after oral administration of blood, a mixed amino acid solution (Vamin 14; KabiVitrum, Sweden) or a 10% glucose solution was studied. One week after PCS, the animals were fed with a gastric tube for 8 h and thereafter tested for behavioral abnormalities before decapitation at 12 h. The concentration of 5-hydroxytryptophan (5-HTP), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) were analyzed chromatographically (HPLC technique with electrochemical detection) in different regions of the brain. Estimation of synthetic rates of 5-hydroxyindoles was facilitated by aromatic aminoacid decarboxylase inhibition (m-hydroxybenzyl-hydrazine; NSD 1015). The brain concentrations of 5-HTP, 5-HT, and 5-HIAA were increased in all shunted rats as compared with sham-operated animals. Whether animals received blood, glucose, or aminoacid solution made no differences in the brain concentrations of 5-HTP and 5-HT. Concentrations of 5-HIAA were lower in those animals receiving blood as compared with the other shunted groups. No reproducible differences in the behavior of the animals were observed. These results suggest that massive blood administration 1 week after PCS in rats has no influence on the rate of brain indole synthesis. While alterations in serotonin metabolism may play a role in some forms of encephalopathy, this study implies that the behavioral and neurologic disorders which follow gastrointestinal tract hemorrhage in patients with liver failure may have other etiologies.     

Agonist and antagonist properties of serotonergic compounds in pigeons trained to discriminate either quipazine or L-5-hydroxytryptophan

The serotonin (5-HT) receptor-related compounds metergoline, pirenperone, ketanserin, cyproheptadine, pizotyline, methysergide, lysergic acid diethylamide, mianserin and cinanserin were studied in pigeons trained to discriminate l-5-hydroxytryptophan (l-5-HTP) (18.0 mg/kg) from saline and in pigeons trained to discriminate quipazine (1.0 mg/kg) from saline. Metergoline did not generalize to either quipazine or l-5-HTP but did antagonize drug-appropriate responding in both groups. Ketanserin potently blocked the quipazine discriminative stimulus and neither generalized to nor attenuated the l-5-HTP discriminative stimulus. Pirenperone, cinanserin, cyproheptadine, methylsergide, pizotyline and mianserin attenuated the quipazine discriminative stimulus at low doses and, at higher doses, generalized to the l-5-HTP discriminative stimulus. No antagonism of the l-5-HTP-discriminative stimulus or generalization to the quipazine-discriminative stimulus were observed with these compounds. A correlation coefficient of 0.93 was calculated between the potencies of 5-HT compounds to generalize to the l-5-HTP stimulus and the binding affinities of these compounds for a 5-HT1 receptor in rat brain. In addition, a correlation coefficient of 0.78 was calculated between the potencies of 5-HT compounds to attenuate the quipazine stimulus and the binding affinities of these compounds for the 5-HT2 receptor in rat brain. These observations suggest cyproheptadine, pizotyline, methysergide, lysergic acid diethylamide, mianserin and cinanserin are agonists at the 5-HT1 receptor in the l-5-HTP discrimination and antagonists at a 5-HT2 receptor in the quipazine discrimination in pigeons.     

THE EFFECT OF 5-HYDROXYTRYPTAMINE ON THE COTTON PELLET LOCAL INFLAMMATORY RESPONSE IN THE RAT

The cotton pellet technique was used to evaluate the effect of 5-HT and certain related compounds on granuloma formation in the rat. 5-HT (10 mg/kg) significantly decreased granuloma formation, significantly increased adrenal weight, and significantly decreased thymus weight in normal rats, and significantly decreased granuloma formation and thymus weight in sham-operated rats. On the other hand, 5-HT (10 mg/kg) significantly increased granuloma weight in adrenalectomized rats. Methysergide (1 mg/kg) blocked the inhibitory effect of 5-HT on granuloma formation as well as the changes in weight of adrenal and thymus glands. 5-HIAA (11 mg/kg) and 5-HTP (13 mg/kg) failed to alter granuloma formation.     

Discriminative-stimulus effects of quipazine and l-5-hydroxytryptophan in relation to serotonin binding sites in the pigeon

The 5-hydroxytryptamine (serotonin; 5-HT) agonists, RU-24969 [5-methoxy 3-(1,2,3,6-tetrahydro-4-pyridinyl)1H-indole, succinate], ipsapirone [2-(4-[4-(2-pyrimidinyl)-1-piperazinyl]butyl)-1,2- benzisothiazol-3-(2H)one-1,1-dioxidehydrochloride], 8-hydroxy-N,N-dipropyl-2-aminotetralin, lysergic acid diethylamide, fenfluramine and N,N-dimethyltryptamine were studied in pigeons trained to discriminate quipazine (1.0 mg/kg) from saline and in pigeons trained to discriminate I-5-HTP (18.0 mg/kg) from saline. Lysergic acid diethylamide, quipazine and fenfluramine generalized to the training stimulus in both groups of pigeons. N,N-dimethyltryptamine generalized to quipazine in all pigeons tested whereas N,N-dimethyltryptamine generalized to I-5-HTP in most pigeons tested. The natural substrate 5-HT and agonists with affinities for the 5-HT1 receptor and its subtypes (8-hydroxy-N,N-dipropyl-2-aminotetralin, ipsapirone, and RU-24969) only generalized in the I-5-HTP-trained pigeons. Equilibrium binding experiments using the ligands [3H]-5-HT and [3H]ketanserin were performed with six areas of pigeon brain and six homologous areas of rat brain. Two populations of 5-HT binding sites were found in brains of both species; one defined by high-affinity binding of [3H]-5-HT and the other defined by high-affinity binding of [3H]ketanserin. Kd values were similar for the two ligands in brains of both species. 5-HT, RU-24969 and ipsapirone displaced [3H]-5-HT but not [3H]ketanserin from pigeon brain membranes. The present study suggests that, in the pigeon, the 5-HT2 receptor might mediate the discriminative-stimulus effects of quipazine, whereas the 5-HT1 receptor might mediate the effects of I-5-HTP.     

Effects of buspirone and its metabolite, 1-(2-pyrimidinyl)piperazine, on brain monoamines and their metabolites in rats

Buspirone, an anxiolytic drug with selective affinity for the 5-HT-1A subtype of serotonin receptors, caused a dose-related decrease in 5-hydroxyindole acetic acid (5-HIAA) concentration in rat hypothalamus after doses of 1 to 10 mg/kg s.c. The decrease in 5-HIAA concentration after a 3 mg/kg s.c. dose of buspirone persisted at 4 hr but not at 7 hr. The decrease was due apparently to a reduced turnover of serotonin; the accumulation of 5-hydroxytryptophan after decarboxylase inhibition was also suppressed by buspirone, not only in hypothalamus but also in brain stem, hippocampus and striatum. 1-(2-Pyrimidinyl)-piperazine (1-PP), a major metabolite of buspirone, did not affect hypothalamic 5-HIAA concentration at doses up to 10 mg/kg s.c. Both buspirone and 1-PP increased hypothalamic concentrations of 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) sulfate, the norepinephrine metabolite, the effect being more pronounced with 1-PP but occurring after doses as low as 0.3 mg/kg s.c. with each compound. The increase in MHPG sulfate concentration persisted for at least 4 hr after a 3 mg/kg s.c. dose of each compound. The increase in MHPG sulfate produced by buspirone may have been due partly to 5-HT-1A receptor activation, inasmuch as other serotonin agonists have been found to cause a similar increase. 1-PP is reported to lack affinity for 5-HT-1A receptors so its elevation of MHPG sulfate concentration may have resulted from alpha-2 receptor blockade. The increase in MHPG sulfate concentration after buspirone injection may have been due at least partly to formation of the metabolite, 1-PP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma level of dipropylacetamide and dipropylacetic acid and effect of these drugs on 5-hydroxytryptamine metabolism in mouse brain

After intraperitoneal application of the antiepileptics dipropylacetamide (DPM, 200 mg/kg) and dipropylacetic acid (DPA, 200 mg/kg) in mice their concentration in plasma and their effects on 5-hydroxytryptamine (5-HT) metabolism in the brain were followed during 360 minutes. Peak plasma level of DPA and DPM was observed at 30 minutes, but level of DPM was only about 25% that of DPA. Plasma content of both drugs declined to low levels by 360 minutes. Both drugs increased brain content of the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA). After DPA application 5-HIAA level increased rapidly and had returned to control value by 360 minutes, while following DPM 5-HIAA increase was much more gradual and was more prolonged. Neither DPM nor DPA significantly altered brain 5-HT content. An apparent relationship between DPA plasma level and brain 5-HIAA content was observed while after DPM no association between plasma drug content and effect on cerebral 5-HT metabolism was seen.     

5-HT2 receptor antagonists reverse amphetamine-induced slowing of dopaminergic neurons by interfering with stimulated dopamine synthesis.

The role of serotonin in the effect of amphetamine on the firing rate of midbrain dopaminergic neurons was examined using unit recordings of identified A10 dopamine neurons in the chloral hydrate-anesthetized rat. Amphetamine (1 mg/kg, i.v.) reduced the firing rate of these neurons approximately 50 to 60%. This effect was blocked in animals pretreated with the selective serotonin-2 (5-HT2) receptor antagonists, MDL 28,133A (0.2 mg/kg, i.v.) or ritanserin (1 mg/kg, i.v.). Although pretreatment with L-dopa (100 mg/kg, i.v.) plus carbidopa (25 mg/kg, i.p.) alone had no effect on amphetamine-induced slowing of A10 dopamine neurons, when coadministered with the 5-HT2 antagonists, the dopamine precursor completely restored this amphetamine-induced slowing. To verify the role of serotonin in these findings, rats were pretreated with the tryptophan hydroxylase inhibitor, p-chlorophenylalanine (250 mg/kg/day for 2 days) to deplete cortical serotonin levels. Consistent with the results observed with the 5-HT2 receptor antagonists, amphetamine did not produce a significant reduction in the firing rate of A10 neurons in serotonin-depleted rats. These results suggest that, under some conditions, serotonergic input via the activation of 5-HT2 receptors may regulate the availability of the pool of dopamine, which is subject to amphetamine release.     

The effect of liver ischaemia on brain monoamine synthesis in the rat

Subtotal or total liver ischaemia was induced in the rat by dividing the hepatic artery (Expt. I) or by total dearterialisation of the liver (Expt. II) 2 days after porta-caval shunt (PCS). The animals received i.v. a 10% glucose infusion for 5 h after the last operation and were killed by decapitation. At the end of the experiment all animals with liver ischaemia were in Grade III coma. In different regions of the CNS 5-hydroxytryptophan (5-HTP), 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA), were analysed by HPLC-technique with electrochemical detection, while dihydroxyphenylalanine (DOPA), dopamine (DA) and norepinephrine (NE) were analysed with a radio enzymatic method after blocking the decarboxylation of 5-HTP to 5-HT and DOPA to DA by inhibition of the aromatic amino acid decarboxylase enzyme with m-hydroxybenzylhydrazine (NSD 1015) in order to estimate the synthesis rate of 5-hydroxyindoles and catecholamines. In Expt. I concentrations of 5-HTP in animals with PCS were increased as compared to sham operation. In animals with liver ischaemia, 5-HTP concentrations were increased as compared to sham operation but similar to those in animals with PCS alone. These results suggest that ligation of the hepatic artery for 5 h in PCS animals does not further accelerate the rate of brain indole synthesis. In Expt. II, the 5-HTP concentrations were increased in PCS animals as compared to sham operation. Animals with total liver dearterialisation exhibited decreased 5-HTP levels as compared to PCS, suggesting a decreased brain indole synthesis after severe liver ischaemia. In Expt. II, CNS concentrations of DOPA following PCS were unaltered as compared with sham-operated animals. In animals with total liver dearterialisation, DOPA levels were increased, suggesting an augmented catecholamine synthesis. The NE levels were lower than in PCS and in sham-operated animals.     

5-HTP induced diarrhea as a carcinoid syndrome model in mice?

Summary— Serotonin (5-HT) is present in the gastrointestinal tract and is probably one of the compounds responsible for diarrhea in patients presenting with carcinoid syndrome. Intraperitoneal administration of L-5-hydroxytryptophan (L-5-HTP) at doses of 25 to 100 mg/kg dramatically increase defecation in mice. In this new paradigm, counting fecal boli deposited is simple and the appraised or inhibition of diarrhea induced by ip 25 mg/kg of L-5-HTP is very clear, with a good reproducibility of scores. L-5-HTP needs to be metabolized into 5-HT to be active; benserazide, an inhibitor of decarboxylase, antagonized the diarrhea induced by 5-HT. Among the 5-HT antagonists used in interaction with 5-HT, only those of the 5-HT₃ type (ondansetron, granisetron, tropisetron) and, to a lesser extent 5-HT₂ type (ritanserin), decreased the diarrhea induced by 5-HTP. The 5-HT₄ receptor agonists from the benzamide family (metoclopramide and zacopride) increased defecation in mice but the effect failed to reach statistical significance.     

Chronic amphetamine administration to cats: behavioral and neurochemical evidence for decreased central serotonergic function.

Chronic administration of amphetamine to cats (twice daily, in doses increasing from 5 to 15 mg/kg over a 10-day period) elicited a number of behaviors, e.g., limb flick and abortive groom, characteristic of the action of hallucinogenic drugs and dependent on a depression of central serotonergic neurotransmission. This drug treatment produced large decreases (-40 to -60%) in central nervous system serotonin (5-HT) and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA), when measured either 6 or 24 hr after the last amphetamine injection. The rate of limb flicking returned to a predrug level approximately 5 days after drug withdrawal, at which time 5-HT and 5-HIAA levels had returned to within 30 to 40% of base line. Both 5-HT and 5-HIAA returned to base-line levels within 14 days after drug withdrawal. Norepinephrine (NE), dopamine (DA) and DA metabolites were decreased 60 to 95% by chronic amphetamine treatment and showed little recovery within the 14 days after drug withdrawal. A second experiment examined the latency to onset of the behavioral and neurochemical changes with a constant dose of amphetamine (7.5 mg/kg, twice daily). Limb flicking was significantly increased above base-line levels following 3 days of amphetamine administration, at which time 5-HT and 5-HIAA levels were decreased 30 to 40%. NE, DA and DA metabolites were decreased approximately 50 to 90% by this treatment regimen. A third experiment examined the effects of a low dose of amphetamine (3.75 mg/kg), injected more frequently (every 6 hr for 6 days), to approximate the administration pattern in human amphetamine abuse. This treatment produced significant increases in limb flicking and abortive grooming on days 5 and 6 and resulted in 30 to 40% depletions of 5-HT and 5-HIAA. NE, DA and DA metabolites were decreased by approximately 50 to 90%. These data are discussed in relation to a role for serotonin in amphetamine psychosis and schizophrenia.     

Brain and peripheral metabolism of 5-hydroxytryptophan-14C following peripheral decarboxylase inhibition.

Following selective peripheral decarboxylase inhibition, a greater fraction of peripherally measured monoamine metabolites should derive from central nervous system monoamine metabolism. In rats pretreated with the peripheral decarboxylase inhibitor L-alpha-(3,4-dihydroxybenzyl)-alpha-hydrazinopropionic acid (MK-486) (50 mg/kg i.p. or i.v.), the peripheral formation of 5-hydroxytryptamine-14C (5-HT-14C) from the precursor DL-5-hydroxytryptophan-14C (5-HTP-14C) was reduced by 82 to 100% over a 4-hour interval. There was a marked increase in the penetration of 5-HTP-14C into brain and in the appearance in brain of 5-HT-14C and 5-hydroxyindoleacetic acid-14C (5-HIAA-14C). The cerebral metabolism of 5-HTP-14C in vehicle-pretreated animals appeared to occur mainly in the brain vascular compartment. Following MK-486 pretreatment, the in vivo metabolism of 14C-5-hydroxyindoles in the extracerebral compartment obeys linear kinetics as does the uptake and decarboxylation of 5-HTP-14C in brains. The cerebral turnover of 5-HT-14C appeared to increase as a function of the dose of 5-HTP injected. This phenomenon may be a result of either saturation of brain 5-HT storage sites or nonspecific metabolism of 5-HTP in brain catecholamine neurons. A highly signigificant correlation was found between the brain and blood 5-HIAA-14C levels following MK-486. The adrenal formation of 5-HT, which was not inhibited by MK-486, did not appear to contribute significantly to the blood 5-HIAA-14c levels. These results suggest the possibility that following peripheral decarboxylase inhibition a larger fraction of peripherally measured 5-HIAA may derive from central nervous system 5-HT metabolism.     

Neurochemical actions of ethylketocyclazocine and ketocyclazocine in different regions of rat brain

The effects of ethylketocyclazocine (EKC) and ketocyclazocine (KC), benzomorphan derivatives proposed as kappa opioid receptor agonists, were studied by measuring changes in the levels of dopamine (DA), noradrenaline (NA), 5-hydroxytryptamine (5-HT), and their major metabolites, DOPAC, HVA, MHPG-SO4, 5-HIAA, in different regions of rat brain. Doses ranging from 1 to 10 mg/kg were tested. EKC decreased the levels of DOPAC and HVA in striatum, and increased DA concentrations, EKC markedly increased the levels of MHPG-SO4 in hypothalamus, but not in cortex, midbrain and pons-medulla. There was a non-significant decrease in NA concentrations. EKC increased the levels of 5-HIAA in hypothalamus and also in cortex, midbrain and pons-medulla, while the levels of 5-HT were increased. On the whole, similar neurochemical effects were observed after KC administration. These data were discussed in relation to the behavioral actions caused in rats by EKC and KC, including the increase in food intake, and they raise the possibility that the hypothalamic noradrenergic system participate in feeding behavior of these drugs.     

3,4-Methylenedioxymethamphetamine ("ecstasy") selectively destroys brain serotonin terminals in rhesus monkeys

3,4-Methylenedioxymethamphetamine (MDMA, "Ecstasy"), an amphetamine analog, is a "designer drug" which is being increasingly abused. The potential neurotoxic hazard of MDMA in humans was assessed by examining the effects of repeated systemic administration of MDMA on selected neurochemical and behavioral measures in rhesus monkeys. In the first study, MDMA (2.5 or 10 mg/kg twice daily for 4 days) produced selective and significant neurochemical decreases in cerebrospinal fluid (CSF) concentrations of 5-hydroxyindoleacetic acid (5-HIAA) and brain concentrations of serotonin and 5-HIAA. At the high dose of MDMA, a selective decrease in serotonin uptake sites (reflecting destruction of brain serotonin terminals) was observed. To determine if these changes after high dose MDMA were pharmacologic or truly neurotoxic, in a subsequent study monkeys were treated with MDMA (10 mg/kg twice daily for 4 days) and then monitored for 14 weeks. Throughout this period, CSF 5-HIAA was decreased in MDMA-treated animals but not in saline-injected controls. At the end of this period, significant decreases in the concentration of serotonin, 5-HIAA and serotonin uptake sites were observed in cerebral cortex and striatum but not in hypothalamus or spinal cord. In contrast to these widespread alterations in serotonin markers, comparable noradrenergic and dopaminergic measures in CSF and brain appeared generally unaffected. These data demonstrating potent and selective effects of MDMA on various brain serotonin parameters in rhesus monkeys suggest that the drug may produce similar effects in humans.     

Serotonin formation in nonblood-perfused rat kidneys

Infusion of the aromatic L-amino acid decarboxylase substrate L-5-hydroxytryptophan (L-5-HTP) at 1.5, 3.0 and 15 micrograms/min into isolated Krebs-Henseleit-perfused rat kidneys was associated with serotonin output in the urinary and venous effluents. Serotonin was measured by high-performance liquid chromatography using electrochemical detection. Infusion at the two higher doses of L-5-HTP caused marked increases in renal vascular resistance (RVR) of over 80 and 490%, respectively. Administration of the aromatic L-amino acid decarboxylase inhibitor carbidopa (20 micrograms) decreased serotonin output and RVR to base-line levels despite continued infusion of L-5-HTP. Infusion of the D-isomer of 5-HTP at 3 micrograms/min did not significantly alter RVR and produced minimal increases in serotonin output relative to L-5-HTP. These results are consistent with the stereospecific formation of serotonin from its amino acid precursor 5-HTP by whole rat kidney.     

Effects of serotonin receptor agonists and antagonists on schedule-controlled behavior of squirrel monkeys

The behavioral effects of the serotonin (5-HT) precursor l-5-hydroxytryptophan (l-5-HTP) and the phenylpiperazine 5-HT agonists 6-chloro-2-(1-piperazinyl)pyrazine (MK-212), 1-(m-trifluromethylphenyl) piperazine (TFMPP), 1-(m-chlorophenyl)piperazine (CPP) and 2-(1-piperazinyl)quinoline (quipazine) were compared with those of the putative 5-HT antagonists metergoline, methysergide, cyproheptadine, cinanserin and ketanserin under a multiple 5-min fixed-interval schedule of food or electric shock presentation in squirrel monkeys. Intramuscular administration of l-5-HTP (0.3-17 mg/kg), MK-212 (0.01-1.0 mg/kg), TFMPP and CPP (0.03-10 mg/kg) produced dose-related decreases in responding under both the food- and shock-presentation schedules. Quipazine differed from the other 5-HT agonists in that it increased shock-maintained behavior at doses (0.1-1.0 mg/kg) that decreased responding maintained by food. The 5-HT antagonists produced mixed behavioral effects. Metergoline (0.03-1.0 mg/kg), cyproheptadine (0.1-1.0 mg/kg) and cinanserin (1.0-10 mg/kg) produced dose-related increases in responding maintained by food, whereas only metergoline and methysergide increased behavior maintained by shock presentation. The prototype 5-HT2-receptor ligand ketanserin (0.3-10 mg/kg) differed from the other 5-HT antagonists in that it decreased behavior maintained by either event. Thus, performances maintained by food or shock presentation reveal both qualitative and quantitative differences in the behavioral effects of 5-HT receptor agonists and antagonists.     

Effects of acute or chronic administration of anti-migraine drugs sumatriptan and zolmitriptan on serotonin synthesis in the rat brain

Triptans are 5-HT1 receptor agonists used as anti-migraine drugs. They act primarily on meningeal blood vessels and on trigeminovascular afferents, but they may also exert central effects. We studied the regional effects of acute and chronic treatment with sumatriptan or zolmitriptan on the rate of serotonin (5-HT) synthesis in the rat brain, using the alpha-14C-methyl-L-tryptophan quantitative autoradiographic method. Sumatriptan at low (300 microg/kg, s.c.) and high (1 mg/kg) doses, as well as zolmitriptan (100 microg/kg), acutely decreased (15-40%, P < 0.05-0.001) 5-HT synthetic rate in many brain regions, including the dorsal raphe nucleus. Chronically, sumatriptan (21 days, approximately 300 microg/kg per day via osmotic minipumps) induced significant increases in the 5-HT synthesis rate in many projection areas but had no effect in the dorsal raphe nucleus. The acute effects on 5-HT synthesis rate would be compatible with activation of 5-HT1 autoreceptors that inhibit serotonin release. In contrast, the increased 5-HT synthesis rate observed after chronic sumatriptan might possibly result from a down-regulation/desensitization of 5-HT1 receptors and/or unmasking of excitatory triptan-sensitive 5-HT receptors. Overall, the present findings indicate that not only zolmitriptan but also sumatriptan affect brain serotonergic neurotransmission.     

Biochemical and electrophysiological properties of SR 57746A, a new, potent 5-HT1A receptor agonist

Summary— SR 57746A (1-[2-(naphth-2-yl) ethyl]-4-(3-trifluoromethylphenyl)-1, 2, 5, 6 tetra-hydropyridine hydrochloride) binds competitively, and with high affinity (K_i = 2.0 ± 0.7 nM) to 5-HT_1A receptors from rat hippocampus in vitro , but has much less affinity for other 5-HT receptor subtypes (IC₅₀ > 650 nM). SR 57746A produces a concentration-dependent inhibition of forskolin-stimulated adenylate cyclase activity in rat hippocampal homogenates, with a maximal effect identical to that of 8-OH-DPAT, suggesting that SR 57746A behaves as a full agonist in tthis experimental model. SR 57746A potently displaces [³H]8-OH-DPAT binding to rat hippocampal membranes ex vivo , with an ID₅₀ of 11.1 mg/kg po, 30 min after administration, and 2.8 mg/kg po, 2 h after administration. This effect of SR 57746A is long-lasting (at least 24 hours at 10 mg/kg po). SR 57746A does not modify the levels of 5-HT or DA in various brain areas, but decreases the concentrations of 5-HIAA, and increases those of DOPAC, HVA and 3-MT. Following iv administration, SR 57746A (0.095 to 0.25 mg/kg) inhibits the spontaneous firing of dorsal raphe neurones, but does not modify the activity of DA neurones in the substantia nigra or ventral tegmental area. Thus, SR 57746A is a potent, selective and full agonist at 5-HT_1A receptors in vitro and vivo.     

Interaction of cholecystokinin and diazepam: effects on brain monoamines

An antagonism between cholecystokinin (CCK) peptides and benzodiazepines (BZD) has been described in various paradigms. We sought to determine whether CCK and BZD are also antagonistic in their effects on brain neurotransmitter levels in the rat. No effect on the noradrenergic system was induced in any brain area by CCK 8 S and diazepam alone or in combination. Administered alone, sulfated CCK octapeptide (CCK 8 S) (5 micrograms/kg ip) and diazepam (5 mg/kg ip) were found to decrease DOPAC levels in the cortex and to induce 5-hydroxy-tryptamine accumulation in the hippocampus. When administered together, these variations were no longer observed. However, a slight tendency by each substance to decrease 3-methoxy-tyramine levels in the striatum, became significant when given in association. The differences in CCK-BZD interactions observed in the striatum, cortex and hippocampus suggest that different mechanisms of action are involved. The addition of the effects occurring in the striatum might involve a GABA-ergic mechanism.