Effects of fenfluramine on free-operant timing behaviour: evidence for involvement of 5-HT2A receptors

RATIONALE: Temporal differentiation in the free-operant psychophysical procedure is sensitive to the 5-hydroxytryptamine (5-HT)1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) and the 5-HT2 receptor agonist 2,5-dimethoxy-4-iodo-amphetamine (DOI); both drugs shift the psychophysical curve leftwards, reducing the indifference point, T50. We have examined the effect of the 5-HT releasing agent fenfluramine on temporal differentiation. OBJECTIVE: We examined whether fenfluramine's effect on temporal differentiation can be antagonised by the 5-HT1A receptor antagonist N-[2-(4-[2-methoxy-phenyl]-1-piperazinyl)ethyl]-N-2-pyridinylcyclohexane-carboxamide (WAY-100635) and the 5-HT2A receptor antagonist ketanserin, and compared the effects of fenfluramine, DOI and 8-OH-DPAT in intact rats and rats whose 5-HTergic pathways had been destroyed by 5,7-dihydroxytryptamine. METHODS: Rats were trained under the free-operant psychophysical procedure to press levers A and B in 50-s trials in which reinforcers were provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic psychophysical curves were fitted to the data for derivation of timing indices (T50, time corresponding to %B=50%, and Weber fraction). Experiment 1 examined the effects of acute treatment with fenfluramine, and the interaction between fenfluramine and the 5-HT1A and 5-HT2A receptor antagonists WAY-100635 and ketanserin; experiment 2 compared the effects of fenfluramine, 8-OH-DPAT and DOI in intact rats and rats whose 5-HTergic pathways had been destroyed by intra-raphe injection of 5,7-dihydroxytryptamine. Concentrations of 5-HT and catecholamines in the brain were measured by high-performance liquid chromatography. RESULTS: Experiment 1: fenfluramine (2 mg/kg) reduced T50; this effect was attenuated by ketanserin (1.0 mg/kg) but not by WAY-100635 (100 microg/kg). Experiment 2: 8-OH-DPAT (100 microg/kg) and DOI (250 microg/kg) reduced T50 in both groups; fenfluramine reduced T50 only in the sham-lesioned group. Levels of 5-HT were reduced by 80% in the lesioned group; catecholamine levels were not affected. CONCLUSIONS: The results suggest that fenfluramine affects temporal differentiation via the release of endogenous 5-HT which acts mainly on postsynaptic 5-HT2A receptors.     

Effects of 8-hydroxy-2-(di-n-propylamino)tetralin, a selective agonist of 5-HT1A receptors, on the cough reflex in rats.

The effects of the agonist of 5-HT1A receptors, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), on the capsaicin-induced cough reflex in rats were studied. I.p. injection of 8-OH-DPAT, at doses of 0.1 and 0.3 mg/kg, significantly decreased the number of coughs in a dose-dependent manner. The antitussive effect of 8-OH-DPAT (0.3 mg/kg) was blocked by prior injection of methysergide (3 mg/kg i.p.) and spiperone (0.3 mg/kg i.p.), whereas ketanserin (3 mg/kg i.p.) had no effect on the antitussive effect of 8-OH-DPAT. The antitussive effects of dihydrocodeine (1 mg/kg i.p.) and dextromethorphan (3 mg/kg i.p.) were also antagonized by methysergide and spiperone. However, these cough-depressant effects were not reduced by ketanserin. These results suggest that the antitussive action of 8-OH-DPAT may be related to the enhancement of the function of 5-HT1A receptors, and that antitussives interact with the 5-HT1A receptors.     

mCPP-induced hyperactivity in 5-HT2C receptor mutant mice is mediated by activation of multiple 5-HT receptor subtypes

The serotonin receptor agonist mCPP induces hyperlocomotion in 5-HT2C receptor knockout (KO) mice or in the presence of a 5-HT2C receptor antagonist. In the present group of experiments, we evaluate the role of 5-HT1A, 5-HT1B and 5-HT2A receptors in mCPP-induced hyperactivity in 5-HT2C KO mice. We also assess the ability of agonists at these receptors to induce hyperactivity in wildtype (WT) mice pre-treated with a selective 5-HT2C receptor antagonist. As previously reported, mCPP (3 mg/kg) induced hyperactivity in 5-HT2C KO mice. A combination of the 5-HT1B receptor agonist CP-94,253 (20 mg/kg) and the 5-HT1A receptor agonist 8-OH-DPAT (0.5 mg/kg) induced marked hyperactivity in WT but not in 5-HT2C KO mice, nor in mice treated with the selective 5-HT2C receptor antagonist, SB 242084 (1.5 mg/kg). Neither CP-94,253 nor 8-OH-DPAT had any intrinsic effect on locomotion in WTs. mCPP-induced hyperactivity was attenuated in 5-HT2C KO mice by the 5-HT1B receptor antagonist SB 224289 (2.5 mg/kg), and the 5-HT2A receptor antagonists ketanserin (0.3 mg/kg) and M100907 (0.01 mg/kg) but not by the 5-HT1A receptor antagonist WAY 100635 (1 mg/kg). The 5-HT(2A/2B/2C) receptor agonist, Ro 60-0175 (3 mg/kg), induced a modest increase in locomotor activity in WT mice pre-treated with SB 242084. However, the combination of Ro 60-0175 with CP-94,253 induced a substantial increase in activity in 5-HT2C KO mice, an effect comparable to mCPP-induced hyperactivity. Thus, joint activation of 5-HT1A and 5-HT1B receptors stimulates locomotion in WT mice but this response is dependent on a functional 5-HT2C receptor population and hence is absent in 5-HT2C KO mice. By contrast, mCPP-induced hyperactivity depends on the inactivation of a separate 5-HT2C receptor population and is mediated by 5-HT2A and 5-HT1B receptor activation.     

5-HT1A和5-HT2A/2C受体在粉防己碱增强戊巴比妥钠睡眠中的介导作用

前期研究表明粉防己碱增强戊巴比妥钠诱导的催眠作用与5-HT系统相关。本研究采用戊巴比妥钠（45 mg/kg,i.p.）诱导的小鼠翻正反射消失和恢复实验方法,对粉防己碱与不同5-HT受体在增强戊巴比妥钠诱导睡眠中的相互作用进行了探讨。结果表明粉防己碱分别与选择性5-HT1A受体拈抗剂p-MPPI（1 mg/kg,i.p.）,选择性5-HT2A/2C受体拮抗剂ketanserin（1.5 mg/kg,i.p.）合用可以显著增强戊巴比妥钠诱导的催眠作用。选择性5-HT1A受体激动剂8-OH-DPAT（0.1 mg/kg,s.c.）或5-HT2A/2C受体激动剂DOI（0.2mg/kg,i.p.）能够显著减少戊巴比妥钠诱导的小鼠睡眠时间,而粉防己碱（60 mg/kg,i.g.）可以显著拮抗这种睡眠抑制作用。此结果提示,粉防己碱增强戊巴比妥钠诱导的催眠作用可能与5-HT1A受体和5-HT2A/2C受体有关。     

Involvement of dopamine autoreceptors in the hypoactivity induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in mice.

The effect of 8-OH-DPAT, a 5-HT1A receptor agonist, on the locomotor activity was analyzed in Albino Swiss mice. The studied drug (0.5-5 mg/kg) inhibited the spontaneous locomotor activity in mice. The hypoactivity induced by 8-OH-DPAT (1.5 mg/kg) was abolished by the dopamine (D1 and D2) receptor antagonist-haloperidol (0.00125 and 0.0025 mg/kg, but not in higher doses) and by the D2 antagonist with affinity for 5-HT1A and 5-HT2 receptors-spiperone (0.0025 and 0.005 mg/kg, but not in higher doses). The effect of 8-OH-DPAT was slightly reduced by the alpha 2-adrenoceptor antagonists: idazoxan (4 mg/kg), yohimbine (2 and 4 mg/kg) and rauwolscine (4 mg/kg). On the other hand, the non-selective 5-HT antagonist metergoline (0.5-4 mg/kg), the 5-HT1A antagonist NAN-190 (0.5-2 mg/kg), the beta-adrenoceptor blockers with high affinity for 5-HT1A and 5-HT1B receptors: pindolol and SDZ 21009 (2-8 mg/kg) and the agonist/antagonist of 5-HT1A receptors ipsapirone (2.5 and 5 mg/kg) did not affect the 8-OH-DPAT-induced hypoactivity. The obtained results suggest that the reduction of the spontaneous locomotor activity induced by 8-OH-DPAT results from a stimulation of dopamine autoreceptors, but not 5-HT receptors. Involvement of an alpha 2-adrenergic mechanism cannot be excluded.     

Serotonin-glutamate interaction in rat cerebellum: involvement of 5-HT1 and 5-HT2 receptors

The effects of serotonin (5-HT) on the release of endogenous glutamate (GLU) in rat cerebellum were investigated in slices depolarized with 35 mM K+. The Ca2+-dependent release of GLU was potently inhibited by 5-HT in a concentration-dependent way. Release was also inhibited by the 5-HT1 receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and by the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI). The inhibition by 10 nM 5-HT was partly (35-40%) counteracted by the 5-HT2 receptor antagonist ketanserin but was fully blocked by the mixed 5-HT1/5-HT2 receptor antagonist methiothepin. The effect of 8-OH-DPAT was not affected by ketanserin but was totally antagonized by methiothepin, while the effect of DOI was entirely suppressed by ketanserin. Ketanserin or methiothepin themselves increased (by 23 and 55%, respectively, at 10 nM) the K+-evoked release of GLU. In conclusion the release of endogenous GLU in rat cerebellum can be inhibited by 5-HT through receptors of the 5-HT1 and 5-HT2 type. The enhancement of GLU release by ketanserin or methiothepin could suggest a tonic inhibition. The possible localization of the 5-HT receptors involved in the interaction with the GLU systems is discussed.     

Dose-dependent discriminative stimulus properties of 8-OH-DPAT

Separate groups of rats were trained to discriminate either 0.1mg/kg (low dose; L) or 2.5mg/kg (high dose; H) of 8-OH-DPAT from saline, in a standard operant task. Both cues were found to be dose, time and route dependent and generalized completely to the 5-HT(1A) agonists ipsapirone and flesinoxan. Buspirone substituted completely for 8-OH-DPAT in L and partially in H, whereas the 5-HT(1A/1B) receptor agonist eltoprazine substituted completely for 8-OH-DPAT in H but only partially in L. The 5-HT(1A/1B) receptor agonist RU24969, the 5-HT(1B/2C/1A) receptor agonist TFMPP and the 5-HT reuptake blocker fluvoxamine did not completely mimic the effect of 8-OH-DPAT in either L or H and the 5-HT(1A) mixed agonists/antagonists BMY 7378 and NAN-190 produced partial generalization in L, but no generalization in H. In antagonism tests, NAN-190 and BMY 7378 only partially blocked the 8-OH-DPAT cue in both groups. The non-selective 5-HT receptor antagonist methysergide did not completely block the 8-OH-DPAT cue in in L or H. However, in generalization studies, it completely mimicked the 8-OH-DPAT cue in L and produced partial generalization in H. The beta-adrenergic/5-HT(1A/1B) receptor antagonist pindolol completely blocked the 8-OH-DPAT cue in L and H and did not mimic the 8-OH-DPAT cue in either condition. The alpha(2)-adrenoceptor blocker yohimbine substituted fully for the 8-OH-DPAT cue in L and partially in H. Idazoxan did not substitute for the cue of 8-OH-DPAT in H, but produced nearly 80% generalization in L. The dopamine receptor antagonist pimozide neither blocked nor mimicked the cue of 8-OH-DPAT in either group. A number of other drugs (i.e. m-CPP, S(-)-propranolol, DOI, ketanserin, clonidine and apomorphine) were only tested in H. S(-)-Propranolol blocked the 8-OH-DPAT cue but the other compounds produced neither stimulus generalization nor antagonism. The present study demonstrates that the cues produced by the low and the high training dose of 8-OH-DPAT are quantitatively different and mediated by the agonistic activity of 8-OH-DPAT at 5-HT(1A) receptors. Although the results suggest that the 8-OH-DPAT cue (both L and H) is mediated via postsynaptic 5-HT(1A) receptors, the involvement of presynaptic 5-HT(1A) receptors cannot yet be ruled out.     

Neuroendocrine responses to the serotonin2 agonist DOI are differentially modified by three 5-HT1A agonists.

Evidence suggests that activation of 5-HT1A receptors leads to inhibition of 5-HT2-mediated behavior. The purpose of this study was to investigate the interaction between 5-HT1A and 5-HT2 receptor-mediated hormone secretion. Rats were pretreated with the 5-HT1A agonists buspirone (0, 0.5, 2.0 mg/kg, i.p.), 8-OH-DPAT (0, 0.05, 0.2 mg/kg, s.c.) or ipsapirone (0, 1.0, 2.5 mg/kg, i.p.), 45 min before decapitation. The 5-HT2 agonist DOI was administered (0-10 mg/kg, i.p.) 15 min after injection of the 5-HT1A agonists. The three 5-HT1A agonists differentially altered the DOI-induced increase of concentrations of hormone in plasma. None of the three 5-HT1A agonists influenced the basal levels of renin, ACTH and prolactin but 8-OH-DPAT and buspirone increased the basal level of corticosterone in plasma. Also, 8-OH-DPAT increased the effects of DOI on the concentration of ACTH in plasma but ipsapirone and buspirone did not. None of the 5-HT1A agonists significantly affected DOI-induced increase of concentration of corticosterone in plasma. Buspirone and 8-OH-DPAT potentiated the effect of DOI on prolactin in plasma, but ipsapirone did not. Ipsapirone potentiated the effect of DOI on the concentration of renin in plasma but this effect was not observed in 8-OH-DPAT- and buspirone-pretreated rats. The results do not support the hypothesis for a functional interaction between 5-HT1A and 5-HT2 receptors, since the three 5-HT1A agonists did not have the same influence on the hormonal effects of DOI.     

Modulation of 8-OH-DPAT-induced hypothermia by imipramine in rats

The effects of imipramine on 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), the 5-hydroxytryptamine (5-HT)(1A)-receptor full agonist, -induced hypothermia was examined in rats. Single administration of imipramine (30 mg/kg, i.p.) attenuated 8-OH-DPAT-induced hypothermia. This effect of imipramine was blocked by the 5-HT(2A)-receptor antagonist ketanserin. 8-OH-DPAT-induced hypothermia was not altered 24 h after repeated administration of imipramine (1 - 10 mg/kg per day) for 14 days. However, 8-OH-DPAT-induced hypothermia was significantly enhanced in repeated imipramine (10 mg/kg per day)-treated rats that received 8-OH-DPAT plus imipramine 24 h after the final imipramine administration for 14 days. The 5-HT(2A)-receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ((+/-)-DOI) attenuated the 8-OH-DPAT-induced hypothermia in drug naive rats. The inhibitory effect of (+/-)-DOI (0.3 mg/kg, s.c.) on 8-OH-DPAT-induced hypothermia was attenuated by repeated administration of imipramine (10 mg/kg per day) for 14 days. These findings suggest that enhancement of the 5-HT(1A) receptors by repeated administration of imipramine may be due to reduction of the inhibitory effects from the 5-HT(2A) receptors to the 5-HT(1A) receptors.     

Selective cross-tolerance to 5-HT1A and 5-HT2 receptor-mediated temperature and corticosterone responses

The repeated administration of 5-methoxy-N,N-dimethyltryptamine (5-MeODMT, 3 mg/kg, twice daily for 14 days) significantly diminished hypothermia and corticosterone secretion induced by an acute challenge with the 5-HT1A agonist 8-OH-DPAT (0.1 mg/kg) when compared to the responses in animals treated chronically with the solvent vehicle. In contrast, the chronic administration of 5-MeODMT did not alter the magnitude of hyperthermia or corticosterone secretion induced by the acute administration of MK-212 (1.0 mg/kg). The repeated administration of the 5-HT2 agonist DOI (1.0 mg/kg, daily for 7 days) significantly reduced the increase in corticosterone, but not body temperature, produced by MK-212. Chronic treatment with DOI did not alter the hypothermia or increase in corticosterone secretion elicited by 8-OH-DPAT. These data are consistent with other evidence that these physiological effects of 8-OH-DPAT and MK-212 are mediated by 5-HT1A and 5-HT2 receptors, respectively. Thus, data presented in these studies are suggestive that the chronic administration of 5-MeODMT diminishes the responsiveness of 5-HT1A receptor-mediated changes in body temperature and corticosterone secretion without altering the responses mediated by 5-HT2 receptors. In contrast, the chronic administration of DOI selectively diminishes the magnitude of 5-HT2 receptor-mediated changes in corticosterone secretion without affecting the responsiveness of those receptors involved in thermoregulatory responses. These selective changes in receptor responsiveness following the chronic administration of these 5-HT agonists further establishes the independence of 5-HT1A and 5-HT2 receptor-mediated pharmacological effects.     

8-OH-DPAT acts on both 5-HT1A and 5-HT7 receptors to induce hypothermia in rodents

Studies using selective drugs and knockout mice have demonstrated that the 5-HT(7) receptor plays an instrumental role in serotonin-induced hypothermia. There is also evidence supporting an involvement of the 5-HT(1A) receptor, although mainly from studies using 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT(1A/7) receptor agonist. Here we studied the effects of 8-OH-DPAT and selective antagonists for the 5-HT(1A) and 5-HT(7) receptors on body temperature in rats, wild-type (5-HT(7)(+/+)) mice and knockout (5-HT(7)(-/-)) mice. At lower doses (0.3-0.6 mg/kg, i.p.), 8-OH-DPAT decreased body temperature in 5-HT(7)(+/+) mice but not in 5-HT(7)(-/-) mice. At a higher dose (1 mg/kg, i.p.) 8-OH-DPAT induced hypothermia in both 5-HT(7)(-/-) and 5-HT(7)(+/+) mice. The 5-HT(1A) receptor antagonist (S)-N-tert-butyl-3-(4-(2-methoxyphenyl)piperazine-1-yl)-2-phenylpropanamide (WAY-100135) (10 mg/kg, i.p.) inhibited the effect of 8-OH-DPAT at all doses in rats and mice. In 5-HT(7)(+/+) mice the selective 5-HT(7) receptor antagonist (R)-3-(2-(2-(4-methylpiperidin-1-yl)-ethyl)pyrrolidine-1-sulfonyl)phenol (SB-269970) (10 mg/kg, i.p.) fully inhibited the hypothermia induced by 0.3 mg/kg 8-OH-DPAT, but not that of higher doses. In rats, SB-269970 caused a 60% inhibition of the hypothermia induced by 0.3 mg/kg 8-OH-DPAT. Thus, both 5-HT(7) and 5-HT(1A) receptors are involved in a complex manner in thermoregulation, with the 5-HT(7) receptor being more important at lower, possibly more physiological, concentrations.     

Effects of cold stress on some 5-HT1A, 5-HT1C and 5-HT2 receptor-mediated responses.

The purpose of the present study was to analyze the influence of stress (24-h cold exposure) on presynaptic 5-HT1A receptors, and on postsynaptic 5-HT1A, 5-HT1C and 5-HT2 receptors. Cold exposure for 24 h affected neither pargyline-induced decreases in 5-hydroxyindoleacetic acid (5-HIAA) levels in midbrain and rest of brain, nor plasma glucose and corticosterone levels. Treatment with the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.5-1 mg/kg), 3-5 h after the end of cold exposure triggered less intense flat body posture and forepaw treading in cold-exposed rats than in controls. On the other hand, 15- and 30-min plasma glucose responses to 8-OH-DPAT (0.25-0.5 mg/kg, 3-5 h after cold) or to the alpha 2-adrenoceptor agonist, clonidine (0.025 mg/kg), were not affected by cold, while the 15-min, but not the 30 min, plasma corticosterone response to 8-OH-DPAT was slightly amplified in cold-exposed rats. Cold exposure affected neither the inhibitory effect of 8-OH-DPAT (0.25-0.5 mg/kg, 3-5 h after cold) on midbrain 5-HIAA levels, nor the hypothermic effect of 8-OH-DPAT (0.5-1 mg/kg, 3-5 h after cold). Lastly, the hypoactivity elicited by the 5-HT1C receptor agonist, m-chlorophenyl-piperazine (1.5-3 mg/kg, 3-5 h after cold), or head shakes elicited by the 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (1-2 mg/kg, 3-5 h after cold), were of similar intensities in control and in cold-exposed rats.     

The effect of serotonin and serotonin receptor antagonists on motion sickness in Suncus murinus

In the present study, we investigated the effect of 5-hydroxytryptamine (5-HT) and 5-HT receptor agonists and antagonists on motion sickness in Suncus murinus, and the possibility that the emetic stimulus of 5-HT can alter the sensitivity of the animals to the different emetic stimulus of motion sickness. 5-HT (1.0, 2.0, 4.0 and 8.0 mg/kg ip) induced emesis and that was antagonised by methysergide (1.0 mg/kg ip), the 5-HT(4) receptor antagonist sulphamate[1-[2-[(methylsulphonyl)amino]ethyl]-4-piperidinyl]methyl-5-fluoro-2-methoxy-1H-indole-3-carboxylate (GR125487D; 1.0 mg/kg ip) and granisetron (0.5 mg/kg ip). Pretreatment with 5-HT caused a dose-related attenuation of the emetic response induced by a subsequent motion stimulus, which was not significantly modified by methysergide, granisetron or GR125487D pretreatment. (+)-1-(2,5-Dimethoxy-4-iodophenyl)-2-amino-propane (DOI; 0.5 and 1.0 mg/kg ip), 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT; 0.1 mg/kg ip) but not methysergide, GR125487D or granisetron, attenuated motion-induced emesis, and that was not affected by pretreatment with ketanserin (2.0 mg/kg, ip) or N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide trihydrocholoride (WAY-100635; 1.0 mg/kg ip), respectively. Indeed, ketanserin alone (0.1, 0.3, 1.0 and 2.0 mg/kg ip) attenuated motion sickness. These data indicate that 5-HT(1/2), 5-HT(3) and 5-HT(4) receptors are involved in the induction of 5-HT-induced emesis. However, agonist action at the 5-HT(1A/7) and 5-HT(2) receptors, and antagonist action at the 5-HT(2A) receptors can attenuate motion sickness in S. murinus.     

Differential effects of 5-HT agonists and antagonists on the repeated acquisition and performance of response sequences in monkeys

As a means of characterizing the role of 5-HT1A and 5-HT2A receptors in learning, 5-hydroxytryptamine (5-HT) agonists and antagonists with selective affinities for each receptor subtype (i.e. 8-hydroxy-dipropylaminotetralin (8-OH-DPAT), (-)-4-(dipropylamino)-1,3,4,5-tetrahydrobenz-(c,d,)indole-6-carboxamide (LY228729), (+/-)-1-(4-iodo-2,5-dimeth-oxyphenyl)-2-aminopropane hydrochloride (DOI), 4-iodo-N-[2- [4-(methoxyphenyl)-1-piperazinyl] ethyl]-N-2-pyridinyl-benzamide hydrochloride (p-MPPI), N-[2- [4- (2-methoxyphenyl)-1-piperazinyl] ethyl] -N-2-pyridinyl-cyclohexanecarboxamide maleate (WAY-100635), 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyllpiperazine hydrobromide (NAN-190) and ritanserin) were administered to monkeys responding under a multiple schedule of repeated acquisition and performance. In addition, a selective 5-HT1A receptor agonist (8-OH-DPAT) was administered in combination with a 5-HT2A receptor antagonist (ritanserin) to examine any potential interactions between the two 5-HT receptor subtypes. When administered alone, 8-OH-DPAT (0.1-3.2mg/kg), LY228729 (0.32-3.2 mg/kg) and DOI (0.018-3.2 mg/kg) dose-dependently decreased overall response rate in both schedule components, and generally increased the percentage of errors in the acquisition components at doses lower than those that increased the percentage of errors in the performance components. At the doses of each drug tested (i.e. 0.1 or 0.32 mg/kg), both p-MPPI and WAY-100635 antagonized the disruptive effects of 8-OH-DPAT, by shifting the dose-effect curves for overall response rate and the percentage of errors to the right. In contrast, ritanserin (0.32 or 1mg/kg) had little or no effect on the disruptions produced by 8-OH-DPAT, but it effectively antagonized the rate-decreasing and error-increasing effects produced by the 5-HT2A agonist DOI. Administration of the 5-HT1A antagonists WAY-100635 and NAN-190 alone produced dose-dependent rate-decreasing effects, but the effects on accuracy of responding in the acquisition components differed from those of the 5-HT1A agonists (8-OH-DPAT and LY228729), in that they did not produce an increase in the percentage of errors. Together, these results suggest that 5-HT is capable of disrupting learning in monkeys through actions at both the 5-HT1A and 5-HT2A receptors, and that 5-HT2A receptor antagonism does not unilaterally modify the effects produced by 5-HTA1A receptor activation.     

Agonist action at 5-HT1C receptors facilitates 5-HT1A receptor-mediated spontaneous tail-flicks in the rat

In rats lightly restrained in plastic cylinders, subcutaneous administration of the selective, high efficacy 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), induced spontaneous tail-flicks, that is, tail-flicks in the absence of extraneous stimulation. The putative 5-HT1B receptor agonist, CGS 12066B, the mixed 5-HT1B/1C receptor agonists, 1-((3-(trifluoromethyl)phenyl]piperazine (TFMPP) and 1-(3-chlorophenyl)piperazine (mCPP), the 5-HT1C/2 receptor agonist, [+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the 5-HT1B/1C/2 receptor agonist, quipazine, did not, in contrast, elicit tail-flicks when applied alone. However, TFMPP, mCPP, DOI and quipazine, but not CGS 12066B, each potentiated the action of 8-OH-DPAT. Further, in the presence of TFMPP, mCPP and DOI, the dose-response curve for the induction of tail-flicks by 8-OH-DPAT was both steeper and shifted to the left. Tail-flicks induced by another high efficacy 5-HT1A receptor agonist, lisuride, were also enhanced by TFMPP, mCPP and DOI. The 5-HT1A receptor partial agonists, buspirone and (+/-)-flesinoxan, evoked tail-flicks only in the presence of TFMPP, mCPP or DOI. The mixed 5-HT1C/2 receptor antagonists, ritanserin and ICI 169,369, did not modify the action of 8-OH-DPAT alone but abolished the potentiation of 8-OH-DPAT-induced tail-flicks by DOI and TFMPP. Further, the selective 5-HT1A receptor antagonist, BMY 7378, blocked tail-flicks induced by both 8-OH-DPAT alone and 8-OH-DPAT plus DOI or TFMPP. A common property of those drugs potentiating 8-OH-DPAT-induced tail-flicks is an agonist action at 5-HT1C receptors and the data indicate that it is this mechanism which underlies the facilitation of tail-flicks.     

5-HT1A receptor activity disrupts spontaneous alternation behavior in rats

Agonists selective for three different serotonin (5-HT) receptor subtypes were tested for the ability to disrupt spontaneous alternation behavior (SAB) in the CD strain of rats. Rats were scored for alternation or repetition in their choice of arms of a T-maze equally baited with chocolate milk. Compared with vehicle controls, the 5-HT(1A) agonist 8-hydroxy-dipropylaminotetraline (8-OH-DPAT; 2 mg/kg) significantly (P<.0001) increased repetitive choices (disrupted SAB). In contrast, intraperitoneal injections with the 5-HT(2) agonist R-(-)-dimethoxyiodophenylaminoethane (DOI; 1 mg/kg) or the 5-HT(3) agonist N-methyl quipazine (NMQ; 3 mg/kg) had no significant effect on SAB in CD rats. Onset of vicarious trial and error (VTE) behavior prolonged the time required for each rat to select an arm of the T-maze when injected with either 8-OH-DPAT (P<.0001) or buspirone (1-2 mg/kg), a 5-HT(1A) partial agonist. The disruption of SAB and the induction of VTE behavior were reversible with behavioral scores returning to preinjection levels within 48 h after injections. The disruption of SAB by 8-OH-DPAT was also seen with the Long-Evans rat strain. The results extend the use of the SAB model and point to a specific role of 5-HT(1A) receptors in the induction of repetitive behavioral patterns.     

Effects of a 5-HT2 receptor agonist, DOI (2,5-dimethoxy-4-iodoamphetamine), and antagonist, ketanserin, on the performance of rats on a free-operant timing schedule

This experiment examined the effect of a 5-HT2 receptor agonist DOI (2,5-dimethoxy-4-iodoamphetamine), and antagonist, ketanserin, on temporal differentiation performance. Twelve rats were trained under the free-operant psychophysical procedure to press two levers (A and B) in 50-s trials in which sucrose reinforcement (0.6 mol/l, 50 microl) was provided intermittently for responding on A during the first half, and on B during the second half of the trial. Psychometric curves were derived from percent responding on B (%B), recorded in successive 5-s epochs of the trials; logistic functions were fitted to these data for the derivation of timing indices (T50 [time corresponding to %B=50%], epsilon [slope of the logistic curve], Weber fraction). Cumulative probability of switching in successive 5-s epochs was used to estimate the mean switching time, S50. DOI (0.0625, 0.125 and 0.25 mg/kg, s.c.) dose-dependently reduced T50 and S50. These effects of DOI (0.25 mg/kg) were antagonized by ketanserin (1.0 mg/kg). The results show that DOI alters temporal differentiation in the free-operant psychophysical procedure. The antagonistic effect of ketanserin indicates that the effect of DOI was probably mediated by 5-HT2A rather than 5-HT2C receptors, since ketanserin is relatively selective for 5-HT2A receptors. Comparison of these results with our previous findings with a 5-HT1A receptor agonist indicates that 5-HT1A and 5-HT2A receptors mediate qualitatively similar effects on temporal differentiation.     

Regulation of 5-HT2 receptors in rat cortex. Studies with a putative selective agonist and an antagonist

The phenylisopropylamine derivative 1-(2,5-dimethoxy-4-iodo-phenyl)-2-aminopropane (DOI) has been suggested recently as a selective serotonin2 (5-HT2) receptor agonist. Because of the potential importance of such a tool for investigations of 5-HT2 receptor regulation, receptor binding studies were performed in rats after acute and chronic treatment with DOI, the selective 5-HT2 antagonist ketanserin, or vehicle. Single injections of 5 or 10 mg/kg DOI reduced the Bmax of cortical sites labeled with [3H]1-(2,5-dimethoxy-4-bromo-phenyl)-2-aminopropane and [3H]ketanserin (9-32 or 32-46%, respectively). Chronic daily treatment with DOI (3-9 mg/kg) further down-regulated 5-HT2 sites in cortex identified with either [3H]ketanserin (-60%) or with [3H]DOB (-75%), without altering Kd values or affecting 5-HT1 sites. In vitro addition to the [3H]ketanserin or [3H]DOB binding assay of 10 nM to 1 microM DOI resulted in competitive inhibition, suggesting that down-regulation found in vivo was not secondary to residual drug. Chronic treatment with ketanserin (10 mg/kg) also down-regulated both [3H]ketanserin (-38%) and [3H]DOB (-58%) sites in cortex without charges in 5-HT1 sites. In naive cortex, competition experiments revealed a Ki (nM) for ( +/- )-DOI of 1.7 +/- 0.02 at sites labeled by [3H]DOB, and a KH and KL of 4.8 +/- 1.5 and 53 +/- 2 nM at sites labeled by [3H]ketanserin. These data indicate that in chronic treatment, DOI, like ketanserin, is highly selective for 5-HT2 vs 5-HT1 sites at behaviorally useful doses. However, a representative putative 5-HT2 selective agonist and antagonist have similar effects on 5-HT2 receptors labeled by agonist or antagonist radioligands.     

2,5-Dimethoxy-4-iodoamphetamine (DOI) inhibits Delta9-tetrahydrocannabinol-induced catalepsy-like immobilization in mice

The effect of the serotonin 5-HT(2A/2C)-receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) on Delta(9)-tetrahydrocannabinol (THC)-induced catalepsy-like immobilization was studied in mice. DOI (0.3 and 1 mg/kg, i.p.) significantly inhibited the catalepsy-like immobilization induced by THC (10 mg/kg, i.p.). In contrast, the selective 5-HT(2C)-receptor agonist 8,9-dichloro-2,3,4,4a-tetrahydro-1H-pyrazino[1,2-a]quinoxalin-5(6H)-one (WAY 161503) had no effect on this catalepsy-like immobilization. Moreover, the 5-HT(2A)-receptor antagonist ketanserin (0.3 mg/kg, i.p.) reversed the inhibition of THC-induced catalepsy-like immobilization caused by DOI (1 mg/kg), whereas the selective 5-HT(2C)-receptor antagonist 6-chloro-2,3-dihydro-5-methyl-N-[6-(2-methyl-3-pyridinyl)oxyl]-3-pyridinyl]-1H-indole-1-carboxyamide (SB 242084) did not affect this inhibitory effect of DOI. On the other hand, ketanserin (0.3 and 1 mg/kg, i.p.) enhanced the catalepsy-like immobilization induced by THC (6 mg/kg, i.p.). Thus, on the basis of these results, it appears that 5-HT(2A)-receptor mechanisms might be responsible for the inhibitory effect of DOI on THC-induced catalepsy-like immobilization.     

Different role of 5-HT1A and 5-HT2 receptors in spinal cord in the control of nociceptive responsiveness.

The effects of the 5-hydroxytryptamine type-2 (5-HT2) receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the 5-HT1A agonist (+)-8-hydroxy-2-(di-n-propylamino)-tetralin [(+)-8-OH-DPAT] on nociceptive responsiveness were compared in mice. Intrathecal administration of DOI (5-20 micrograms) produced a dose-dependent behavioural syndrome, consisting of biting or licking, directed towards the caudal part of the body and reciprocal hindlimb scratching. However, (+)-8-OH-DPAT (5-20 micrograms) did not produce the biting and scratching behaviour. The response to DOI (20 micrograms) was reversed by treatment with the substance P receptor antagonist, [D-Arg1, D-Trp7,9, Leu11]-SP (Spantide) (5 micrograms). The tail-flick reflex was markedly depressed 5-20 min after administration of (+)-8-OH-DPAT; DOI did not change the tail-flick reflex after 5 min but significantly inhibited the reflex response 10-20 min after injection. The data show that stimulation of 5-HT2 receptors, but not 5-HT1A receptors, induced a behavioural syndrome, which may reflect activation of nociceptive pathways. The tail-flick reflex was more markedly inhibited by stimulation of 5-HT1A than 5-HT2 receptors. Accordingly, 5-HT2 and 5-HT1A receptors seem to have a different function in the modulation of nociceptive responsiveness in the mouse.