Depletion of brain serotonin differently affects behaviors induced by 5HT1A, 5HT1C, and 5HT2 receptor activation in rats

Blunted 5-HT1A receptor-mediated responses and antidepressant-like behavior in mice lacking the GABAB1a but not GABAB1b subunit isoforms

RATIONALE

There is accumulating evidence for a role of GABAB receptors in depression. GABAB receptors are heterodimers of GABAB1 and GABAB2 receptor subunits. The predominant GABAB1 subunit isoforms are GABAB1a and GABAB1b. GABAB1 isoforms in mice differentially influence cognition, conditioned fear, and susceptibility to stress, yet their influence in tests of antidepressant-like activity has not been fully investigated.

OBJECTIVES

Given the interactions between GABAB receptors and the serotonergic system and the involvement of 5-HT1A receptors (5-HT1AR) in antidepressant action, we sought to evaluate 5-HT1AR function in GABAB1a-/- and GABAB1b-/- mice.

METHODS

GABAB1a-/- and GABAB1b-/- mice were assessed in the forced swim test (FST), and body temperature and hypothalamic-pituitary-adrenal (HPA) responses to the 5-HT1AR agonist 8-OH-DPAT were determined. Brain 5-HT1AR expression was assessed by [3H]-MPPF and [3H]-8-OH-DPAT autoradiography and 5-HT1AR G-protein coupling by [35S]GTP-γ-S autoradiography.

RESULTS

As previously described, GABAB1a-/- mice showed an antidepressant-like profile in the FST. GABAB1a-/- mice also demonstrated profoundly blunted hypothermic and motoric responses to 8-OH-DPAT. Furthermore, 8-OH-DPAT-induced corticosterone and adrenocorticotropic hormone (ACTH) release were both attenuated in GABAB1a-/- mice. Interestingly, [3H]-MPPF and [3H]-8-OH-DPAT binding was largely unaffected by genotype. [35S]GTP-γ-S autoradiography suggested that altered 5-HT1AR G-protein coupling only partially contributes to the functional presynaptic 5-HT1AR desensitization, and not at all to the blunted postsynaptic 5-HT1AR-mediated responses, seen in GABAB1a-/- mice.

CONCLUSION

These data demonstrate distinct functional links between 5-HT1ARs and the GABAB1a subunit isoform and suggest that the GABAB1a isoform may be implicated in the antidepressant-like effects of GABAB receptor antagonists and in neurobiological mechanisms underlying depression.

Tryptophan depletion affects compulsive behaviour in rats: strain dependent effects and associated neuromechanisms

RATIONALE

Compulsive behaviour, present in different psychiatric disorders, such as obsessive-compulsive disorder, schizophrenia and drug abuse, is associated with altered levels of monoamines, particularly serotonin (5-hydroxytryptamine) and its receptor system.

OBJECTIVES

The present study investigated whether 5-HT manipulation, through a tryptophan (TRP) depletion by diet in Wistar and Lister Hooded rats, modulates compulsive drinking in schedule-induced polydipsia (SIP) and locomotor activity in the open-field test. The levels of dopamine, noradrenaline, serotonin and its metabolite were evaluated, as well as the 5-HT_2A and 5-HT_1A receptor binding, in different brain regions.

METHODS

Wistar rats were selected as high (HD) or low (LD) drinkers according to their SIP behaviour, while Lister hooded rats did not show SIP acquisition. Both strains were fed for 14 days with either a TRP-free diet (T-) or a TRP-supplemented diet (T+) RESULTS: The TRP depletion diet effectively reduced 5-HT levels in the frontal cortex, amygdala and hippocampus in both strains of rats. The TRP-depleted HD Wistar rats were more sensitive to 5-HT manipulation, exhibiting more licks on SIP than did the non-depleted HD Wistar rats, while the LD Wistar and the Lister Hooded rats did not exhibit differences in SIP. In contrast, the TRP-depleted Lister Hooded rats increased locomotor activity compared to the non-depleted rats, while no differences were found in the Wistar rats. Serotonin 2A receptor binding in the striatum was significantly reduced in the TRP-depleted HD Wistar rats.

CONCLUSIONS

These results suggest that alterations of the serotonergic system could be involved in compulsive behaviour in vulnerable populations.

Synthesis, in vitro and in vivo pharmacological evaluation of serotoninergic ligands containing an isonicotinic nucleus

Isonicotinamide derivatives, linked to an arylpiperazine moiety, were prepared and their affinity to 5-HT1A, 5-HT2A and 5-HT2C receptors were evaluated. The combination of structural elements (heterocyclic nucleus, alkyl chain and 4-substituted piperazine) known to play critical roles in affinity for serotoninergic receptors and the proper selection of substituents led to compounds with high specificity and affinity towards serotoninergic receptors. In binding studies, several molecules showed high affinity in nanomolar and subnanomolar range at 5-HT1A, 5-HT2A and 5-HT2C receptors and moderate or no affinity for other relevant receptors (D1, D2, α1 and α2). N-(3-(4-(bis(4-fluorophenyl)methyl)piperazin-1-yl)propyl)isonicotinamide (4s) with Ki = 0.130 nM, was the most active and selective derivative for the 5-HT1A receptor compared to other serotoninergic, dopaminergic and adrenergic receptors. Compound 4o, instead, showed 5-HT2A affinity values in subnamolar range. Moreover, the compounds having better affinity and selectivity binding profile towards 5-HT1A and 5-HT2A receptors were selected in order to be tested by in vitro and in vivo assays to determine their functional activity.

Effects of repeated exposure to MDMA on 5HT1a autoreceptor function: behavioral and neurochemical responses to 8-OHDPAT

A consistent effect of repeated exposure to 3,4 methylenedioxymethamphetamine (MDMA) is a decrease in the tissue levels of serotonin (5-HT). A variety of behavioural and neurochemical tests were conducted to determine whether the tissue deficits were accompanied by an increased sensitivity of the 5-HT1a autoreceptor. Tests were conducted 2 weeks following MDMA exposure (four injections of 10.0 mg/kg, IP, administered at 2-h intervals in a single day). The response to the 5-HT1a agonist, 8-OHDPAT (0.003-0.5 mg/kg, SC), was assessed using lower lip retraction (LLR), hypoactivity, and 5-hydroxytryptophan (5-HTP) accumulation following decarboxylase inhibition. The 8-OHDPAT produced a dose-dependent increase in LLR and hypoactivity, but these effects were comparable for MDMA and saline pretreated groups. MDMA decreased tissue levels of 5-HT and the accumulation of 5-HTP, but these effects were not reflected in the changes in autoreceptor sensitivity. The data suggest that the decrease in tissue levels of 5-HT produced by MDMA is accompanied by a decrease in tryptophan hydroxylase activity but cannot be explained by supersensitivity of the 5-HT1a autoreceptor.

The influence of modifications in imide fragment structure on 5-HT1A and 5-HT7 receptor affinity and in vivo pharmacological properties of some new 1-(m-trifluoromethylphenyl)piperazines

New, flexible (7, 9, 11 and 13) and rigid (8, 10, 12 and 14) imides with a 1-(m-trifluorophenyl)piperazine fragment and a tetramethylene or a 1e,4e-cyclohexylene spacer, respectively, showed very high affinity (K(i)=0.3-34 nM) and agonistic in vivo activity for 5-HT(1A) receptors. Flexible new compounds and the previously described 5 also bound to 5-HT(7) receptors (K(i)=21-134 nM). Selected glutarimide derivatives, that is, the most potent postsynaptic 5-HT(1A) receptor agonist rigid compound 8 and its flexible analogue 7, as well as the previously described full agonist-rigid compound 6 and the partial agonist-its flexible counterpart 5 exhibited moderate affinity for alpha(1)-adrenoceptors (K(i)=85 - 268 nM), but were practically devoid of any affinity for dopamine D(2) sites. Those glutarimides demonstrated anxiolytic- (5 and 7) and antidepressant-like (5, 6 and 8) activity in the four-plate and the swim tests in mice, respectively; at the same time, however, they inhibited the locomotor activity of mice. The antidepressant-like effect of 8 was significantly stronger than that induced by imipramine used as a reference antidepressant.

7-Arylpiperazinylalkyl and 7-tetrahydroisoquinolinylalkyl derivatives of 8-alkoxy-purine-2,6-dione and some of their purine-2,6,8-trione analogs as 5-HT1A, 5-HT2A, and 5-HT7 serotonin receptor ligands

On the basis of our earlier studies with the serotonin receptor ligands in the group of 1,3-dimethyl-3,7-dihydropurine-2,6-dione derivatives, a series of new arylpiperazinylalkyl and tetrahydroisoquinolinylalkyl analogs of 8-alkoxy-1,3-dimethyl-3,7-dihydropurine-2,6-dione (10-25) and 1,3-dimethyl-7,9-dihydro-3H-purine-2,6,8-trione (26-30) were synthesized and their 5-HT(1A), 5-HT(2A), and 5-HT(7) receptor affinities were determined. The new compounds 17, 18, 20, and 21 were found to be highly active 5-HT(1A) receptor ligands (K(i)=11-19nM) with diversified affinity for 5-HT(2A) receptors (K(i)=15-253nM). Compounds 12, 13, 15, and 19 were moderately potent 5-HT(2A) ligands (K(i)=23-57nM), whereas 17, 18, 24, and 25 showed distinct affinity for 5-HT(7) receptors (K(i)=51-83nM). Purine-2,6,8-triones showed weak affinities for 5-HT(1A) and 5-HT(7) receptors; among them, 27 and 29 were classified as 5-HT(2A) receptor ligands. The selected compounds 17 and 21 were pharmacologically evaluated to determine their functional activities at pre-(hypothermia in mice) and post-(lower lip retraction in rats) synaptic 5-HT(1A) receptors. Compound 17 showed features of a potential agonist of pre- and post-synaptic 5-HT(1A) receptors, whereas 21 was classified as a potential, weak partial agonist of postsynaptic sites. Last of all, the most interesting compound 17 tested in behavioral models showed potential anxiolytic and antidepressant activities.

Novel class of arylpiperazines containing N-acylated amino acids: Their synthesis, 5-HT1A, 5-HT2A receptor affinity, and in vivo pharmacological evaluation

Novel arylpiperazines with N-acylated amino acids, selected on the basis of a preliminary screening of two libraries previously synthesized on SynPhase Lanterns, were prepared in solution and their affinity for 5-HT(1A), 5-HT(2A), and D(2) receptors was evaluated. The compounds bearing (3-acylamino)pyrrolidine-2,5-dione (19-26) and N-acylprolinamide (29-34) moieties showed high affinity for 5-HT(1A) (K(i)=3-47 nM), high-to-low for 5-HT(2A) (K(i)=4.2-990 nM), and low for D(2) receptors (K(i)=0.77-21.19 microM). All the new o-methoxy derivatives of (3-acylamino)pyrrolidine-2,5-diones tested in vivo revealed agonistic activity at postsynaptic 5-HT(1A) receptors, while m-chloro derivatives were classified as antagonists of these sites; similar relations were observed for o-methoxy (29) and m-chlorophenylpiperazine derivatives of N-acylprolinamides. The reported results show that the amino acid-derived terminal fragment modified the in vivo functional profile. Finally, the selected compounds 19 and 20, a 5-HT(1A) partial agonist and a full agonist, respectively, and 26, a mixed 5-HT(1A)/5-HT(2A) antagonist, were evaluated in preclinical animal models of depression and anxiety. The project allowed selecting the lead compound 20 which exhibited an anxiolytic-like effect in the four-plate test in mice and revealed distinct antidepressant-like effects in the forced swimming and tail suspension tests in mice.

Reversal of learned helplessness by selective serotonin reuptake inhibitors in rats is not dependent on 5-HT availability

Serotonin (5-HT) and 5-HT(1A) receptors have been suggested to play a pivotal role in the mechanism of action of antidepressant drugs, particularly in the case of selective serotonin reuptake inhibitors (SSRIs). In the rat learned helplessness (LH) paradigm, a valid animal model of human depression, repeated treatment with the 5-HT(1A) receptor agonist 8-OH-DPAT (0.125 and 0.5mg/kg) and several classes of antidepressants such as the tricyclic agent desipramine (30 and 60mg/kg), the monoamine oxidase inhibitor (MAOI) pargyline (60mg/kg) and the SSRIs fluoxetine (15 and 30mg/kg), paroxetine (15 and 30mg/kg) and sertraline (30mg/kg) improved behavioural deficit in helpless rats. The involvement of serotonergic mechanisms in the antidepressant-like effect of these agents was investigated using the selective 5-HT(1A) receptor antagonist WAY 100,635 and the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA). Pretreatment with WAY 100,635 blocked the 8-OH-DPAT-induced reduction in escape failures, but did not counteract the antidepressant effect of fluoxetine and paroxetine. PCPA given alone did not modify helpless behaviour nor did it affect the behavioural effect of 8-OH-DPAT, fluoxetine and paroxetine. Adaptive changes in 5-HT(1A) receptor function were studied by measuring 8-OH-DPAT-mediated hypothermia and lower lip retraction (LLR) in the animals 24h after LH test session. Fluoxetine and paroxetine treatments caused a marked reduction in agonist-induced responses, an effect completely prevented by WAY 100,635 and PCPA. In conclusion, whereas direct agonist activity at postsynaptic 5-HT(1A) receptors attenuated helpless behaviour, the antidepressant-like effect of SSRIs was found to be independent of their actions on either 5-HT(1A) receptor function or extracellular 5-HT.

Conformational Restriction in Novel NAN-190 and MP3022 Analogs and Their 5-HT1A Receptor Activity

The newly synthesized analogs of NAN-190 containing m-Cl and m-CF(3) substituents in the arylpiperazine moiety and their conformationally restricted counterparts showed a very high 5-HT(1A )receptor affinity. In the LLR test, the flexible compounds 4a and 5a displayed features of a partial agonist and agonist, respectively. The conformational restriction in the tested structures caused alternations in the observed in vivo effects; compounds 4b and 5b were classified as an inactive agent and an antagonist of postsynaptic 5-HT(1A )receptors, respectively. Rigidification of MP3022 and its 5,6-dimethyl analog structures resulted in cis and trans stereoisomers 6b-9b with a 1- and 2-substituted benzotriazole moiety. In both series, in vitro experiments showed that the cis configurations of the compounds were better tolerated by 5-HT(1A) receptor sites than the trans ones. The conformational analysis revealed various spatial regions that can be explored by terminal benzotriazole fragments in those structures. Like the previously described cis-6b, the new ligand cis-7b, displayed features of a postsynaptic 5-HT(1A) receptor agonist, whereas cis-8b was characterized as a partial agonist of those receptor sites. It was suggested that the nonlinear geometry of the above agents has significant influence on the postsynaptic 5-HT(1A )receptor stimulation.

Structure–intrinsic activity relationship studies in the group of 1-imido/amido substituted 4-(4-arylpiperazin-1-yl)cyclohexane derivatives; new, potent 5-HT1A receptor agents with anxiolytic- like activity

Introduction of 1,4-disubstituted cyclohexane ring in the structure of flexible long chain arylpiperazines resulted in linearly constrained, potent serotonin (5-HT)(1A) ligands. In order to trace structure-intrinsic activity relationships in this group, a new series of 1-substituted 4-(4-arylpiperazin-1-yl)cyclohexane derivatives with different cyclic imide/amide termini, and their flexible, tetramethylene analogues were synthesized and pharmacologically evaluated for 5-HT(1A) receptors. In vitro binding experiments revealed that all the compounds were potent 5-HT(1A) receptor agents (K(i) = 1.9-74 nM). Some derivatives tested additionally showed also high affinity for alpha(1)-adrenergic receptors (K(i) = 2.9-101 nM) and for 5-HT(7) receptors. Functional in vivo examination revealed that rigid ligands with o-OCH(3) group in the aryl moiety and cyclic imide system in the opposite terminal behaved like postsynaptic 5-HT(1A) receptor antagonists. On the other hand, unsubstituted, m-Cl, or m-CF(3) substituted derivatives as well as those with cyclic amide group in the terminal fragment exhibited agonistic or partial agonistic activity. Three out of four derivatives tested, that is, postsynaptic 5-HT(1A) antagonists 9 and 10, and partial agonist 16, showed anxiolytic-like activity in the conflict drinking (Vogel) test in rats.

Serotonin-related enhancement of recovery of hind limb motor functions in spinal rats after grafting of embryonic raphe nuclei

Recently, we demonstrated improvements in hind limb locomotor-like movements following grafting of embryonic raphe nuclei cells into the spinal cord below the level of total transection in adult rats. The purpose of the present study was to clarify whether this improvement was due to newly established serotonergic innervation between the graft and the host. Two months after intraspinal grafting of the embryonic raphe nuclei, the spinalized rats, when put on a treadmill, could be induced to walk with regular alternating hind limb movements with the plantar contact with the ground during the stance phase, and ankle dorsiflexion during the swing phase of each step cycle. In the same situation the spinal rats, that did not receive the graft, were not able to initiate the dorsiflexion of the ankle joint during the swing phase and very often the dorsal surface of the foot was dragged along the ground. Intraperitoneal application of directly acting 5-HT2 antagonist Cyproheptadine (1 mg/kg) impaired reversibly the hind limb locomotor-like movements in grafted rats. This impairment lasted for 2-3 h. The same procedure in control rats did not markedly alter the hind limb locomotor-like movements. The effect of Cyproheptadine in grafted rats was reversed by i.p. injections of the 5-HT2 agonist Quipazine (0.5 mg/kg). These results show that the graft-induced restitution of hind limb locomotor abilities in adult spinal rats is brought about by the new serotonergic innervation of the host spinal cord circuitry from the grafted neurons and is mediated by 5-HT2 receptors.

Reduction in 5-HT1A receptor density, 5-HT1A mRNA expression, and functional correlates for 5-HT1A receptors in genetically defined aggressive rats

The present experiments tested the hypothesis that one of the critical mechanisms underlying genetically defined aggressiveness involves brain serotonin 5-HT1A receptors. 5-HT1A receptor density, the receptor mRNA expression in brain structures, and functional correlates for 5-HT1A receptors identified as 8-OH-DPAT-induced hypothermia and lower lip retraction (LLR) were studied in Norway rats bred for 59 generations for the lack of aggressiveness and for high affective aggressiveness with respect to man. Considerable differences between the highly aggressive and the nonaggressive rats were shown in all three traits. A significant decrease in B(max) of specific receptor binding of [3H]8-OH-DPAT in the frontal cortex, hypothalamus, and amygdala and a reduction in 5-HT1A receptor mRNA expression in the midbrain of aggressive rats were found. 5-HT1A receptor agonist 8-OH-DPAT (0.5 mg/kg, i.p.) produced a distinct hypothermic reaction in nonaggressive rats and did not affect significantly the body temperature in aggressive rats. Similar differences were revealed in 8-OH-DPAT-induced LLR: LLR was expressed much more in nonaggressive than in aggressive animals. Additionally, 8-OH-DPAT (0.5 mg/kg i.p.) treatment significantly attenuated the aggressive response to man. The results demonstrated an association of aggressiveness with reduced 5-HT1A receptor expression and function, thereby providing support for the view favoring the idea that brain HT1A receptor contributes to the genetically defined individual differences in aggressiveness.

1-Aryl-4-(4-succinimidobutyl)piperazines and their conformationally constrained analogues: synthesis, binding to serotonin (5-HT1A, 5-HT2A, 5-HT7), α1-adrenergic, and dopaminergic D2 receptors, and in vivo 5-HT1A functional characteristics

Starting with the structure of potent 5-HT(1A) ligands, that is, MM77 [1-(2-methoxyphenyl)-4-(4-succinimidobutyl)piperazine, 4] and its constrained version 5 (MP349), previously obtained in our laboratory, a series of their direct analogues with differently substituted aromatic ring (R=H, m-Cl, m-CF(3), m-OCH(3), p-OCH(3)) were synthesized. The flexible and the corresponding 1e,4e-disubstituted cyclohexane derivatives were designed in order to investigate the influence of rigidification on 5-HT(1A) affinity, selectivity for 5-HT(2A), 5-HT(7), D(1), and D(2) binding sites and functional profile at pre- and postsynaptic 5-HT(1A) receptors. The new compounds 19-25 were found to be highly active 5-HT(1A) receptor ligands (K(i)=4-44 nM) whereas their affinity for other receptors was: either significantly decreased after rigidification (5-HT(7)), or controlled by substituents in the aromatic ring (alpha(1)), or influenced by both those structural modifications (5-HT(2A)), or very low (D(2), K(i)=5.3-31 microM). Since a distinct disfavor towards rigid compounds was observed for 5-HT(7) receptors only, it seems that the bioactive conformation of chain derivatives at those sites should differ from the extended one. Several in vivo models were used to asses functional activity of 19-25 at pre- (hypothermia in mice) and postsynaptic 5-HT(1A) receptors (lower lip retraction in rats and serotonin syndrome in reserpinized rats). Unlike the parent antagonists 4 and 5, all the new derivatives tested were classified as partial agonists with different potency, however, similar effects were observed within pairs (flexible and rigid) of the analogues. The obtained results indicated that substitution in the aromatic ring, but not spacer rigidification, controls the 5-HT(1A) functional activity of the investigated compounds. Moreover, an o-methoxy substituent in the structure of 5 seems to be necessary for its full antagonistic properties. Of all the new compounds studied, trans-4-(4-succinimidocyclohexyl)-1-(3-trifluoromethylphenyl)piperazine 24 was the most potent 5-HT(1A) receptor ligand in vitro (K(i)=4 nM) and in vivo, with at least 100-fold selectivity for the other receptors tested.

Novel, flexible, and conformationally defined analogs of gepirone: synthesis and 5-HT1A, 5-HT2A, and D2 receptor activity

Novel, flexible arylpiperazine gepirone analogs (1a-3a) with a mixed 5-HT1A/5-HT2A receptor profile, low D2 receptor affinity, and agonistic (2a) or partial agonistic (1a, 3a) activity toward 5-HT1A receptor sites were synthesized. Their conformationally restricted counterparts (1b-3b) were selective 5-HT1A ligands (over 5-HT2A and D2 receptors), which turned out to be agonists (2b, 3b), or partial agonist (1b) of 5-HT1A receptors.

Spinal neurons activated with the urethrogenital reflex in the male rat

The urethrogenital (UG) reflex is a spinal ejaculatory-like reflex. The location of spinal neurons activated by the UG reflex was examined in the male rat using the immediate early gene, c-fos. In addition, co localization of neurons containing galanin and choline acetyl transferase (ChAT) and serotonin fibers with fos-immunoreactive (fos-I) nuclei was examined. Activation of the UG reflex resulted in a significant increase in fos positive nuclei in segments T13-S1, compared to controls in which the UG reflex was not activated. Spinal circuits involved in the UG reflex include neurons relaying afferent information from the pudendal sensory nerve, in the dorsal horn and medial cord of L5-S1. Interneurons specifically activated with the UG reflex were identified in the medial, intermediate and lateral gray. A small proportion of parasympathetic and sympathetic preganglionic neurons in the intermediolateral cell column (IML) of L5-S1 and IML and medial gray of T13-L2, respectively, was activated with the UG reflex. A significant increase in the number of galanin containing neurons expressing c-fos in the medial gray of L3-L4 was also observed with the UG reflex. Serotonin fibers and varicosities were found throughout the spinal cord and were especially dense in the ventral horn, IML and medial gray. Fos activated neurons were found in close apposition to serotonin fibers in the IML and medial gray. These studies demonstrate the multisegmental intraspinal circuitry responsible for ejaculatory-like responses and demonstrate the potential involvement of galanin, acetylcholine and serotonin in mediation of the UG reflex.

Intrinsic activity and comparative molecular dynamics of buspirone analogues at the 5-HT1A receptors

In CNS, the 5-hydroxytryptamine(1A) (5-HT(1A)) receptors exist in two different populations with different behavioural and physiological effects: (1) somatodendritic autoreceptors located pre-synaptically of 5-HT containing neurons and (2) receptors located post-synaptic to 5-HT containing neurons. Clinical studies have shown that 5-HT(1A) partial agonists have anxiolytic properties, while antagonists of pre-synaptical autoreceptors shorten the onset time of selective serotonin reuptake inhibitors (SSRIs). In the present study, the pre- and post-synaptic activity of structural analogues of buspirone was evaluated in animal models. A three dimensional model of the 5-HT(1A) receptor was used to study their interaction modes and helical displacements upon receptor binding. The predicted receptor-ligand interactions indicated similarities in the receptor binding modes for all buspirone analogues, and no clear relationship between receptor contact residues and activity at pre- and post-synaptic receptors. Comparative molecular dynamics (MD) simulations for 650ps indicated that pre-synaptic antagonistic behaviour is connected to large displacements of transmembrane helix (TMH) 7 upon binding, while pre-synaptic agonistic behaviour is connected to large displacements of TMH2 and small displacements of TMH7. Post-synaptic partial agonist behaviour is connected to large displacements of TMH4 and TMH5 upon binding, while post-synaptic antagonists only slightly displace these helices.

Prepulse inhibition and habituation of acoustic startle response in male MDMA ('ecstasy') users, cannabis users, and healthy controls

Chronic administration of 3,4-methylenedioxymethamphetamine (MDMA) is associated with long-term depletion of serotonin (5-HT) and loss of 5-HT axons in the brains of rodents and nonhuman primates. Despite the broad database concerning the selective serotonergic neurotoxicity of recreational MDMA consumption by humans, controversy still exists with respect to the question of whether the well-known functional consequences of these neurotoxic effects, such as memory impairment, were caused by chronic 5-HT deficiency. Habituation and prepulse inhibition (PPI) of the acoustic startle response (ASR) can be used as a marker of central serotonergic functioning in rodents and humans. Thus, we investigated the functional status of the central serotonergic system in chronic but abstinent MDMA users by measuring PPI and habituation of ASR. PPI and habituation of ASR were measured in three groups. The first group (MDMA group) included 20 male drug-free chronic users of MDMA; the second group (cannabis group) consisted of 20 male drug-free chronic users of cannabis; and the third group (healthy controls) comprised 20 male participants with no history of illicit drug use. Analysis revealed significantly increased PPI of MDMA users compared to those of cannabis users and healthy controls. Cannabis users and healthy controls showed comparable patterns of PPI. There were no differences in habituation among the three groups. These results suggest that the functional consequences of chronic MDMA use may be explained by 5-HT receptor changes rather than by a chronic 5-HT deficiency condition. Use of cannabis does not lead to alterations of amplitude, habituation, or PPI of ASR.

New Arylpiperazine 5-HT1A Receptor Ligands Containing the Pyrimido[2,1-f]purine Fragment: Synthesis, in Vitro, and in Vivo Pharmacological Evaluation

New 1H,3H-pyrimido[2,1-f]purine-2,4-dione derivatives of arylpiperazine (11-22) were prepared and evaluated in vitro for their affinity for 5-HT(1A), 5-HT(2A), alpha(1), and D(2) receptors. The tested compounds showed high affinity for 5-HT(1A) and alpha(1) receptors (K(i) = 1.1-87 and 10-62 nM, respectively) and moderate to low affinity for 5-HT(2A) (K(i) = 56-881 nM) and D(2) receptors (K(i) = 94-1245 nM). Compounds 14, 15, 18, 19, and 21, mostly 3'-chlorophenylpiperazine derivatives, can be classified as mixed 5-HT(1A)/5-HT(2A)/alpha(1) ligands. Compound 13, which showed the highest 5-HT(1A) receptor affinity (K(i) = 1.1 nM), was 50-fold selective in relation to alpha(1) adrenoceptors and at least 250-fold over 5-HT(2A) and D(2) sites. On the basis of in vivo functional tests, 8-phenylpiperazinoethylamino (11), 8-(2'-methoxyphenylpiperazino)ethylamino (13), and 8-phenylpiperazinopropylamino (14) derivatives of 1,3-dimethyl-1H,3H-pyrimido[2,1-f]purine-2,4-dione were identified as potent pre- and postsynaptic 5-HT(1A) receptor antagonists. 1,3-Dimethyl-7-bromo-8-(phenylpiperazinopropylamino)-1H,3H-pyrimido[2,1-f]purine-2,4-dione (20) behaved like an agonist of presynaptic and as a partial agonist of postsynaptic 5-HT(1A) receptors and resembled ipsapirone in terms of functional intrinsic activity. It revealed marked anxiolytic-like activity in the Vogel test in rats, comparable to that of the reference drug diazepam, and exhibited antidepressant-like activity in the Porsolt test in rats. The sedative effect of 20, evaluated in the open field test in rats, appeared at doses twice as high as those inducing a minimal anxiolytic-like effect and was similar to the effects of diazepam.

Pharmacological characterization of MP349, a novel 5-HT1A-receptor antagonist with anxiolytic-like activity, in mice and rats

The purpose of this study was to further characterize the pharmacological effects of MP349 (trans-1-(2-methoxyphenyl)-4-(4-succinimidocyclohexyl)piperazine), a new serotonin 5-HT(1A) postsynaptic receptor antagonist, using several biochemical and behavioural assays. The silent 5-HT(1A)-receptor antagonist WAY 100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide) was used as a reference compound in in-vivo tests, and diazepam served as standard anxiolytic drug in animal models of anxiety. In this study we showed that MP349 bound with moderate affinity (K(i) = 234 nM) for alpha(1)-adrenoceptors, and with very low affinity (K(i) > 2600 nM) for 5-HT(2A), dopamine D(1), D(2) and benzodiazepine receptors. The effects of MP349 on presynaptic 5-HT(1A) receptors were studied in two models (mice and rats). Like WAY 100635, MP349 antagonized the hypothermia induced by the 5-HT(1A)-receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin(8-OH-DPAT) in mice. Neither MP349 nor WAY 100635 administered alone induced hypothermia. In a rat microdialysis study, MP349 (like WAY 100635) did not affect 5-HT dialysate level in the prefrontal cortex; however, when given before 8-OH-DPAT, it inhibited the decrease in 5-HT release induced by the 5-HT(1A )agonist. The data demonstrated that MP349 behaved like a functional antagonist of presynaptic 5-HT(1A) receptors. The potential anxiolytic activity of MP349 and reference drugs was examined in a conflict drinking test in rats, a plus-maze test in rats and a four-plate test in mice. MP349 and WAY 100635 produced anxiolytic-like effects, though somewhat weaker than those induced by diazepam, and only in the case of diazepam the anxiolytic-like effects were dose-dependent. Moreover, MP349 administered in doses inducing anxiolytic-like effects did not disturb the locomotor activity (open field test) or locomotor coordination (rota-rod test) of rats. These and earlier results indicated that MP349 was an antagonist of 5-HT(1A) receptors which exhibited anxiolytic-like activity in an animal model of anxiety.

Interaction of 1,2,4-substituted piperazines, new serotonin receptor ligands, with 5-HT1A and 5-HT2A receptors

In the present paper, we describe affinities to 5-HT1A and 5-HT2A receptors of several new 1,2,4-trisubstituted piperazine derivatives. The affinities were compared with those described earlier for 1,4-disubstituted piperazines and the influence of the third (methyl) substituent on the affinity to both receptors is discussed. The difference between two- and three-substituted derivatives was rationalised in terms of molecular modelling of the respective ligand-receptor complexes. Additionally, the functional activity of some 1,2,4-trisubstituted piperazines for 5-HT1A receptor was examined in behavioural and biochemical models. The obtained results have shown that some trisubstituted compounds exhibited a higher affinity to 5-HT2A receptors than their respective disubstituted analogues (with the affinity to 5-HT1A receptors remaining the same or somewhat improving). The molecular dynamics simulations suggested that the presence of the third substituent in the piperazine ring of those compounds may induce stabilising effect on the ligand-receptor complexes. The results of the in vivo studies have shown that some of the examined trisubstituted piperazines (10-13, 16, 17) exhibited properties of postsynaptic 5-HT1A partial agonists. Moreover, compounds 13 and 16 exhibited features of 5-HT1A presynaptic agonists in in vitro test, and compound 16 also in in vivo tests.

The influence of substitution at aromatic part of 1,2,3,4-tetrahydroisoquinoline on in vitro and in vivo 5-HT(1A)/5-HT(2A) receptor activities of its 1-adamantoyloaminoalkyl derivatives

Further structure-activity relationship (SAR) studies with the 1,2,3,4-tetrahydroisoquinoline (THIQ) class of 5-HT(1A) ligands led to the synthesis of new 1-adamantoyloaminoalkyl derivatives. The impact of substituent variations in the aromatic part of THIQ moiety on 5-HT(1A) and 5-HT(2A) receptor affinities, as well as in vivo functional properties of the investigated compounds were discussed. It was found that those modifications reduced the binding affinity for 5-HT(1A) receptors (in comparison with unsubstituted THIQ derivatives); however, the majority of new compounds still remained potent 5-HT(1A) ligands (K(i)=4.9-46 nM) and most of them showed features of partial agonists of postsynaptic 5-HT(1A) receptors. At the same time, their 5-HT(2A) receptor affinity was slightly increased (K(i)=40-1475 nM), which resulted in a loss of 5-HT(2A)/5-HT(1A) selectivity. 5-Br,8-OCH3 derivative-the most potent, mixed 5-HT(1A)/5-HT(2A) ligand-produced activation of presynaptic 5-HT(1A) receptors and showed properties of a 5-HT(2A) receptor antagonist.

MDMA and fenfluramine alter the response of the circadian clock to a serotonin agonist in vitro

The substituted amphetamine drugs, 3,4-methylenedioxymethamphetamine (MDMA or 'Ecstasy') and fenfluramine, are known to damage 5-HT neurons in the brain of animals. However, little is known about the drugs' effects on circadian rhythmicity which is known to be influenced by serotonergic input to the suprachiasmatic nuclei. In the present study, we tested the ability of MDMA and fenfluramine treatment to alter the ability of the circadian clock to reset in response to an agonist of the 5-HT1A and 5-HT7 receptor subtypes soon after treatment with the drugs, and then again at 20 weeks. Coronal hypothalamic slices containing the suprachiasmatic nuclei (SCN) were prepared from rats and 3-min recordings of the firing rate of individual cells were performed throughout a 12-h period. The ability of the 5-HT agonist, 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT), to cause a phase advance in the firing pattern of SCN neurons was assessed in slices from control animals and those pretreated with MDMA or fenfluramine (10, 15 and 20 mg/kg administered on successive days) 6-10 days or 20 weeks previously. Phase advances to 8-OH-DPAT in the slice were attenuated by pretreatment with MDMA or fenfluramine at both drug-test intervals. Our study demonstrates that repeated exposure to MDMA or fenfluramine may interfere with the ability of serotonin to phase shift the circadian clock in the rat. It is possible that such an effect may be responsible for some of the clinical changes, such as sleep disorders and mood changes, sometimes reported by human users of the substituted amphetamines.

Increased m-CPP-induced oral dyskinesia after lesion of serotonergic neurons

Peripheral administration of the 5-hydroxytryptamine (5-HT)(2C/1B) agonist 1-(m-chlorophenyl)piperazine (m-CPP) produces abnormal orofacial movements in rats. We have previously shown that this behavior is mediated by 5-HT(2C) receptors in the subthalamic nucleus [Neuroscience 72 (1996) 117]. The present studies examined this effect after serotonin depletion to determine whether removal of endogenous serotonin affected this behavioral response and/or subthalamic 5-HT(2C) receptors. Rats received an intraventricular infusion of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT, 100 mg/10 ml) or vehicle after desipramine pretreatment (25 mg/kg ip). The efficacy of serotonin depletion was confirmed by a decrease in serotonin uptake sites measured by autoradiography. Oral dyskinesia induced by peripheral administration of m-CPP (1.0 mg/kg ip) was markedly increased in lesioned rats compared to sham-operated controls 4 and 8 but not 12 days after the lesion. A subset of lesioned rats that displayed transient seizures after m-CPP injection did not prevent the measurement of oral dyskinesia during the observation period. No differences in 5-HT(2C) receptor levels were found with ligand-binding autoradiography in the subthalamic nucleus, or in other brain regions that express this receptor, in rats sacrificed 5 days following 5,7-DHT lesions. The data indicate that lesion of serotonergic neurons in adult rats induces a transient increase in motor responses mediated by subthalamic 5-HT(2C) receptors. These data suggest that functional alterations in serotonergic transmission in the subthalamic nucleus may be involved in the pathophysiology of hyperkinetic movement disorders.

Central neural regulation of penile erection

Penile erection is caused by a change of the activity of efferent autonomic pathways to the erectile tissues and of somatic pathways to the perineal striated muscles. The spinal cord contains the cell bodies of autonomic and somatic motoneurons that innervate the peripheral targets. The sympathetic outflow is mainly antierectile, the sacral parasympathetic outflow is proerectile, and the pudendal outflow, through contraction of the perineal striated muscles, enhances an erection already present. The shift from flaccidity to erection suggests relations among these neuronal populations in response to a variety of informations. Spinal neurons controlling erection are activated by information from peripheral and supraspinal origin. Both peripheral and supraspinal information is capable of eliciting erection, or modulating or inhibiting an erection already present. One can hypothesize a spinal network consisting of primary afferents from the genitals, spinal interneurons and sympathetic, parasympathetic and somatic nuclei. This system is capable of integrating information from the periphery and eliciting reflexive erections. The same spinal network, eventually including different populations of spinal interneurons, would be the recipient of supraspinal information. Premotor neurons that project directly onto spinal sympathetic, parasympathetic or somatic motoneurons, are present in the medulla, pons and diencephalon. Several of these premotor neurons may in turn be activated by sensory information from the genitals. Aminergic and peptidergic descending pathways in the vicinity of spinal neurons, exert complex effects on the spinal network that control penile erection. This is caused by the potential interaction of a great variety of receptors and receptor subtypes present in the spinal cord. Brainstem and hypothalamic nuclei (among the latter, the paraventricular nucleus and the medial preoptic area) may not necessarily reach spinal neurons directly. However they are prone to regulate penile erection in more integrated and coordinated responses of the body, such as those occurring during sexual behavior. Finally, the central and spinal role of regulatory peptides (oxytocin, melanocortins, endorphins) has only recently been elucidated.

Effect of a long-term low tryptophan diet on the prolactin responses to the 5-HT1A and 5-HT2C agonists, 8-OH-DPAT and mCPP in the male rat

The study was undertaken to assess the long term effects of tryptophan (TRP) depletion through diet on the prolactin (PRL) responses to the serotonin (5-hydroxytryptophan, 5-HT) agonists m-chlorophenyl-piperazine (mCPP) and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) in the male rat. The low TRP diet caused significant reductions in both plasma total TRP and brain cortical 5-HT content together with a significant increase in the PRL responses to mCPP. In contrast the PRL responses to 8-OH-DPAT in animals on the low TRP diet for 1 week and 6 weeks were similar to control rats. However, a small but significant increase in PRL was observed at 2 min after dosing in the 1-week group. At the same time the 3H-8-OH-DPAT binding parameters, Kd and Bmax, were similar in both control and TRP depleted animals. The results confirm that long-term TRP depletion causes a deficiency of brain TRP and a subsequent reduction of brain 5-HT. This is associated with an enhanced PRL response to mCPP probably resulting from a functional up-regulation of post-synaptic 5-HT2C receptors. The small or transient changes brought about by long-term TRP depletion on post-synaptic 5-HT1A receptors, suggests that these receptors may be less susceptible to 5-HT depleting effects than the 5-HT2C subtype.

1,2,3,4-tetrahydroisoquinoline derivatives: a new class of 5-HT1A receptor ligands

Three series of new N-substituted 1,2,3,4-tetrahydroisoquinolines with 2-, 3-, and 4-membered alkyl chains (a, b, and c, respectively) were synthesized, and the effect of some structural modifications on their 5-HT1A receptor affinities and functional properties was discussed. It was found that the volume of the terminal amide substituent was a crucial parameter which determined 5-HT1A receptor affinities of the tested compounds, while the in vivo activity seemed to depend on both the R-volume and the length of a hydrocarbon chain. It was demonstrated that the most active ligands behaved like agonists or partial agonists at postsynaptic 5-HT1A receptors.

Serotonin, aging and cognitive functions of the hippocampus

The cholinergic hypothesis of senile dementia /18/ does not provide a sufficient explanation for age-dependent spatial learning deficits; these are observed before an appreciable reduction of cholinergic markers can be detected. Behavioral deficits similar to those observed in old rats cannot be induced in young rats by comparable cholinergic lesions but do occur following combined cholinergic/serotonergic lesions. Serotonergic raphe grafts in the hippocampus (but not in the entorhinal cortex or hypothalamus) prevent such combined lesion-induced spatial learning deficits. The behavioral deficits are associated with a reduction of hippocampal commissure feed-forward inhibition. Similar reduced inhibition is found in old rats, deficient in their performance of a spatial learning water-maze task. Finally, treating old rats with the serotonergic precursor 5-hydroxytryptophan (5-HTP) reduces the age-dependent spatial learning deficits and restores hippocampal commissure feed-forward inhibition. Serotonin may act in parallel to the cholinergic innervation of the hippocampus by affecting inhibitory interneurons but in addition it may act by modulating acetylcholine release. Acetylcholine release is modulated by serotonin and the enhancing effects of serotonin releasing drugs on dentate granule cell excitability are mediated by acetylcholine. We thus propose that a reduction of serotonergic modulation of hippocampal interneuron activity and impaired modulation of cholinergic effects in the hippocampus contribute to age-dependent cognitive deficits.

An investigation of the mechanism responsible for fluoxetine-induced hypophagia in rats

The effect of fluoxetine on feeding in p-chlorophenylalanine (PCPA) pretreated rats and the nature of its interaction with 5-HT2C receptors have been investigated. Animals that received 3 days PCPA (150 mg/kg i.p.) pretreatment and vehicle on the test day consumed a similar amount as control, saline pretreated animals under the test paradigm used in this study. Fluoxetine (20 and 30 mg/kg p.o.) significantly reduced food intake in PCPA and control pretreated animals to a similar extent, despite an approximately 90% reduction in the levels of brain 5-hydroxytryptamine (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) in the PCPA-pretreated animals. Thus, hypophagia is unlikely to be caused by inhibition of 5-HT reuptake. In the pig choroid plexus in vitro, fluoxetine and norfluoxetine inhibited specific [3H] mesulergine binding with pKI's (+/- S.E.M.) of 6.45 +/- 0.09 (n = 4) and 6.05 +/- 0.05 (n = 3), and slope factors (+/- S.E.M.) of 1.06 +/- 0.14 and 0.99 +/- 0.13, respectively. In slices of piglet choroid plexus fluoxetine (1, 10 and 33 microM) caused a rightward shift in the dose-response curve produced by 5-HT with no effect on the maximal response, and a mean pKB of 5.94 +/- 0.09. Norfluoxetine (10 microM) also produced a rightward shift in the 5-HT dose-response curve with no effect on the maximal response, and a pKB of 6.20. Thus, both compounds acted as surmountable antagonists with no agonist efficacy at 5-HT2C receptors present in choroid plexus. The hypophagic effect of fluoxetine (20 mg/kg p.o.) was also unaffected by the non-specific 5-HT2C receptor antagonist metergoline (2 and 5 mg/kg i.p.). These findings suggest that the hypophagic effect of fluoxetine is not likely to be dependent upon intact brain 5-hydroxytryptaminergic presynaptic function, nor is it mediated by an agonist action at the 5-HT2C receptor, but by an additional, unknown mechanism.

5-HT2C receptor-mediated phosphoinositide turnover and the stimulus effects of m-chlorophenylpiperazine

The present study was designed to investigate the hypothesis that agonist interactions at 5-HT2C receptors mediate the discriminative stimulus properties of m-chlorophenylpiperazine (mCPP). Three structural classes of compounds have been described to stimulate increases in phosphoinositide (PI) hydrolysis at the 5-HT2C receptor site: phenylpiperazines, phenylalkylamines, and indolamines. Four representative phenylpiperazines, mCPP, TFMPP, MK-212 and quipazine, one phenylalkylamine, (-)DOM, and one indolamine, LSD, were employed in the present study. The efficacies of these compounds were defined (1) in vitro, with respect to their abilities to stimulate increases in PI hydrolysis in the choroid plexus, and (2) in vivo with respect to their abilities to substitute for the mCPP discriminative stimulus. In vitro intrinsic activity at the 5-HT2C site was expressed as a fraction of the maximal PI hydrolysis response elicited by serotonin (5-HT). MK-212 (fractional efficacy = 1.1) and (-)DOM (0.77) were full agonists, while mCPP (0.72), LSD (0.27), quipazine (0.24), and TFMPP (0.22) were partial agonists with respect to the stimulation of PI hydrolysis at the 5-HT2C receptor. In vivo, each of the phenylpiperazines fully substituted for the mCPP stimulus, while (-)DOM (75%), and LSD (67%) elicited only partial substitution. While compounds with agonist activity at the 5-HT2C receptor in vitro substitute for the mCPP stimulus in vivo, no clear relationship exists between in vitro intrinsic activity at the 5-HT2C receptor with respect to the stimulation of PI turnover and maximal substitution for the mCPP stimulus in vivo. The present data suggest that mCPP elicits a compound stimulus which is mediated by agonist interactions at the 5-HT2C receptor and possibly additional interactions with 5-HT2A, 5-HT3, and/or 5-HT1B receptors.

The role of the 5-HT2A and 5-HT2C receptors in the stimulus effects of hallucinogenic drugs. III: The mechanistic basis for supersensitivity to the LSD stimulus following serotonin depletion

The present study was designed to determine the effects of p-chlorophenylalanine (PCPA) and p-chloroamphetamine (PCA) administration on (1) the levels of serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in rat brain, (2) the sensitivity of LSD-trained rats to the stimulus effects of LSD, and (3) the maximal levels of 5-HT2A and 5-HT2C receptor mediated phosphoinositide (PI) hydrolysis in rat brain. PCA and PCPA both produced a significant depletion of whole brain 5-HT and 5-HIAA concentrations. The depletion of serotonin with PCPA, but not PCA, resulted in supersensitivity of LSD-trained subjects to the stimulus effects of LSD. Neither PCPA nor PCA treatment altered the maximal level of 5-HT2A receptor-mediated PI hydrolysis. However, PCPA, but not PCA, treatment resulted in a significant upregulation (46%, P < 0.05) of the maximal level of 5-HT2C receptor mediated PI hydrolysis. These data suggest that upregulation of the 5-HT2C receptor mediates the supersensitivity to LSD discriminative stimulus which follows the depletion of central nervous system serotonin by PCPA.

Lesion of descending 5-HT pathways increases zimeldine-induced waking in rats

Sleep, waking, and EEG power spectra were investigated in rats with spinal 5,6-dihydroxytryptamine (5,6-DHT) lesions, following 20 mg/kg zimeldine or vehicle IP injections. 5,6-DHT selectively lesioned the descending serotonergic pathways. Lesion alone did not change sleep and waking stages compared to baseline, except for a reduction in REM sleep. Consistent with earlier findings, zimeldine in nonlesioned rats increased waking the first 2 h of recording. Zimeldine treatment in lesioned rats gave a significant additional 50% increase in waking the first 2 h and a corresponding decrease in total slow wave sleep, suggesting a potentiation of these effects. Zimeldine gave no significant changes in waking EEG power spectral density. Lesion gave a tendency to reduction between 4.0 and 15.5 Hz compared with baseline, and between 10.0 and 16.5 compared to the independent control group. In both comparisons, the combined treatment strengthened this effect, again suggesting a potentiating effect of lesion. In sleep, zimeldine reduced power over the whole spectrum (0.5-20.0 Hz), less in the lower frequencies than in the higher frequencies.

Interactions between 5-hydroxytryptamine receptor subtypes: is a disturbed receptor balance contributing to the symptomatology of depression in humans?

The purpose of this review is to describe the consequences of antidepressant treatment on the behaviour of rodents after activation of serotonin (5-hydroxytryptamine, 5-HT) receptor subtypes. In a summary table, the involvement of 5-HT receptors in inducing behavioural changes are described. It is emphasized that these effects are not always only exclusively linked to serotonergic functions nor that they are only initiated by central 5-HT receptors. Hereafter, the complex mutual inhibitory effects of 5-HT receptor subtype-mediated processes are discussed by interpreting effects of antagonists and describing the different effects of low and high doses of mixed 5-HT1C/5-HT2 receptor agonists. Mutual influences are seen particularly with 5-HT1A, 5-HT1C and 5-HT2, but not with 5-HT1B, 5-HT1D or 5-HT3 receptor-mediated effects. It is shown that the behavioural consequences of 5-HT1A, 5-HT1C and 5-HT2 receptor stimulation may be changed by brain lesions or chronic treatment with drugs. Among these drugs are the antidepressants. Finally, 5-HT receptor function in depressed patients is discussed, and the hypothesis is proposed that an important function of antidepressants is to restore a disturbed balance between 5-HT1A, 5-HT1C and 5-HT2 receptors in depressed patients.

The effects of a low tryptophan diet on brain 5 -HT metabolism and 5-HT-mediated neuroendocrine responses in the male rat

The study was undertaken to assess the effects of periods of dietary tryptophan (TRP) depletion on (i) plasma total and free TRP together with brain TRP, 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5- HIAA) and (ii) the prolactin (PRL) responses to the 5-HT releasing agent, D-fenfluramine (FEN), and the 5-HT agonist, m-chlorophenylpiperazine (mCPP). The TRP-deficient diet caused significant reductions in plasma total and free TRP for a short-lived period; however, it caused longer lasting and significant reductions in brain TRP, 5-HT and 5-HIAA content. Following periods of dietary TRP depletion, plasma PRL responses to FEN were significantly reduced for 14 days but had normalised by day 21. In contrast, the PRL responses to mCPP were significantly enhanced after 6 and 21 days, the response at 14 days being similar to control. The result suggests that periods of dietary invoked TRP depletion cause a deficiency of brain TRP content which subsequently leads to a fall in brain 5-HT. This is associated with a reduced PRL response to FEN and an enhanced PRL response to mCPP, the latter possibly resulting from functional up-regulation of post-synaptic 5-HT receptors.

Spinal supersensitivity to 5-HT1, 5-HT2 and 5-HT3 receptor agonists following 5,7-dihydroxytryptamine

The present study examined functional supersensitivity to 5-hydroxytryptamine (5-HT) and 5-HT ligands selective for 5-HT1, 5-HT2 and 5-HT3 receptors in two tests for nociception following the spinal administration of 5,7-dihydroxytryptamine (5,7-DHT). Intrathecal pretreatment with 5,7-DHT 30-100 micrograms (following desipramine) produced a selective depletion of spinal cord 5-HT levels of > 80% and augmented the antinociceptive action of 5-HT in the tail flick and hot plate tests. The tail flick test was the more sensitive test for expression of this action. Supersensitivity was observed with the 5-HT1 receptor ligands CGS 12066B (7-trifluoromethyl-4-(4-methyl-1-piperazinyl-pyrrolo[1,2-a] quinoxalinedimaleate), RU 24969 (5-methoxy-3-(1,2,4,6-tetrahydro-4-pyridinyl)1H indole succinate), TFMPP (m-trifluoromethylphenyl-piperazine HCl), mCPP (1-(3-chlorophenyl)piperazine dihydrochloride) and 5-Me-ODMT (5-methoxy-N,N-dimethyltryptamine hydrogen oxalate) but not with the 5-HT2 receptor ligand DOI ((+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane HCl) or the 5-HT3 receptor ligand 2-Me-5-HT (2-methyl-5-hydroxytryptamine maleate) in the tail flick test. In the hot plate test, supersensitivity was observed only with 5-Me-ODMT. Intrathecal pretreatment with fluoxetine, a 5-HT uptake inhibitor, potentiated the action of 5-HT but not any of the other 5-HT1 receptor ligands examined. These results indicate that supersensitivity occurs with 5-HT and 5-HT1 receptor ligands but not with 5-HT2 or 5-HT3 receptor ligands. Both the loss of uptake sites and receptor upregulation may contribute to enhanced activity of 5-HT, but for other ligands, only the latter mechanism appears to occur.

Involvement of 5-HT2C receptors in mediating the discriminative stimulus properties of m-chlorophenylpiperazine (mCPP)

Rats were trained to discriminate the 5-HT receptor agonist m-chlorophenylpiperazine (mCPP; 1 mg/kg) from saline using a two-lever, water-reinforced drug discrimination task. The antidepressant trazodone (1-8 mg/kg), the 5-HT1B/2C receptor agonists 1-(m-trifluoromethylphenyl)piperazine (TFMPP; 0.25-1 mg/kg) and MK 212 (0.125-1 mg/kg), and the mixed 5-HT1A/B receptor agonist RU 24969 (0.25-2 mg/kg) substituted fully for mCPP. The 5-HT2A/2C receptor agonists 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 0.25-1 mg/kg) and d-lysergic acid diethylamide (LSD; 0.02-0.08 mg/kg) and the 5-HT releaser fenfluramine (0.5-2 mg/kg) also mimicked mCPP. Agonists selective for the 5-HT1A or 5-HT3 receptor or the 5-HT reuptake site produced saline-lever responding. The ergoline derivative mesulergine (0.5-4 mg/kg) produced a partial agonist/antagonist profile. The 5-HT1/2 receptor antagonist metergoline (0.125-1 mg/kg) completely blocked the mCPP cue whereas the 5-HT2A/2C receptor antagonists ketanserin and LY 53857 as well as all other 5-HT receptor antagonists failed to block the mCPP cue. The dopamine receptor antagonists SCH 23390 and haloperidol were also ineffective mCPP antagonists. Following pretreatment with the 5-HT synthesis inhibitor p-chlorophenylalanine (pCPA; 100 mg/kg/day) for 3 consecutive days, the discriminability of low doses of mCPP increased, whereas the effects of fenfluramine decreased. The present results suggest that the discriminative stimulus effects of mCPP in rats are mediated primarily by postsynaptic 5-HT2C receptors.

Fluoxetine potentiates nitrazepam-induced behavioral sleep in young chicks

Nitrazepam (0.5-10 mg/kg, IP) dose dependently induced behavioral sleep in day-old chicks. Fluoxetine (0.1-1 mg/kg) did not produce sleep in the young birds, but the 5-HT reuptake inhibitor (0.1 and 0.5 mg/kg) potentiated nitrazepam (0.5 and 1.0 mg/kg)-induced hypnosis. Doses (0.5 and 1 mg/kg) of the benzodiazepine that did not produce sleep in any or all the chicks, when administered alone, induced sleep in some or all the chicks in the presence of fluoxetine (0.1 and 0.5 mg/kg). Ketanserin (0.5 and 2.5 mg/kg) effectively antagonized the effect of fluoxetine on nitrazepam-induced behavioral sleep. These results suggest that enhancement of 5-HT level by fluoxetine may be the mechanism involved in the potentiation of nitrazepam-induced sleep in the young chicks.

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

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

Differential changes in serotonin 5-HT1A and 5-HT2 receptor binding in patients with chronic schizophrenia

Serotonin 5-HT1A and 5-HT2 receptors were examined in the postmortem brains of controls and patients with chronic schizophrenia. In the prefrontal cortex from patients with schizophrenia, 5-HT1A receptor binding was increased, while 5-HT2 receptor binding was decreased, when compared to controls. The increased 5-HT1A receptor binding or the decreased 5-HT2 receptor binding was observed in both the patients who had been medicated with neuroleptics at time of death and those who had not, at least 2 months prior to death. Thus, abnormalities of 5-HT receptor subtypes seem to exist in the brains of patients with chronic schizophrenia. 5-HT related agents might be beneficial for the treatment of schizophrenia.