5-HT2C receptors inhibit and 5-HT1A receptors activate the generation of spike-wave discharges in a genetic rat model of absence epilepsy

The present study was conducted to investigate the role of 5-HT(2C) and 5-HT(1A) receptors in the generation of spike-wave discharges (SWD) in the genetic absence epilepsy model Wistar Albino Glaxo rats from Rijswijk, Netherlands (WAG/Rij rats). We have determined the effects of the 5-HT(2C) receptor preferring agonist m-chlorophenyl-piperazine (m-CPP), the selective 5-HT(2C) receptor antagonist SB-242084, the selective 5-HT(1A) receptor antagonist WAY-100635, two selective serotonin re-uptake inhibitors (SSRI, fluoxetine and citalopram) and their combinations in this model. The 5-HT(2C) agonist m-CPP caused marked, dose-dependent decreases in the cumulative duration and number of SWD administered either intraperitoneally (0.9 and 2.5 mg/kg) or intracerebroventricularly (0.05 and 0.1 mg/kg). Treatment with SB-242084 (0.2 mg/kg, ip) alone failed to cause any significant change in SWD compared to vehicle. Pretreatment with SB-242084 (0.2 mg/kg, ip) eliminated the effects of m-CPP on SWD. Fluoxetine (5.0 mg/kg, ip) alone caused moderate increase in SWD. After pretreatment with SB-242084, the effect of fluoxetine was significantly enhanced. The combination of SB-242084 and citalopram (2.5 mg/kg, ip) caused a similar effect, namely an increase in SWD. In contrast, pretreatment with WAY-100635 significantly attenuated the effect of fluoxetine. In conclusion, these results indicate that the increase in endogenous 5-HT produces a dual effect on SWD; the inhibition of epileptiform activity is mediated by 5-HT(2C), the activation by 5-HT(1A) receptors.     

Effects of mu-CPP and mesulergine on dietary choices in deprived rats: possible mechanisms of their action

Although it has been well established that compounds that stimulate 5-HT(2C) and/or 5-HT(1B) receptors induce hypophagia by promoting satiety process, the relative role of these receptor subtypes in dietary choices remains to be fully determined. m-CPP is considered a useful probe of 5-HT(2C) receptor function in vivo and its administration reduces food intake and appetite in humans and rats. Conversely, the non-selective 5-HT(2C) receptor antagonist mesulergine elicits feeding in rats. Food intake and dietary choices were measured in a food-deprivation experimental protocol employing male Wistar rats. Animals were given access for a 4-h period to a pair of isocaloric diets. These two diets were enriched in protein or carbohydrate proportions, respectively, but fat content was held constant. The mixed 5-HT(2C/1B) receptor agonist, m-CPP, led to a dose-dependent hypophagia, due to substantial reduction in carbohydrate consumption while protein intake was spared (0.62, 1.25 and 2.50 mg/kg i.p., respectively). The non-selective 5-HT(2C) receptor antagonist and also D2 agonist, mesulergine, on its own produced a significant dose-dependent increase in both protein and carbohydrate diets (1.0 and 3.0 mg/kg i.p., respectively). Combined treatment with m-CPP, at its maximum effective dose, and mesulergine dose-dependently reversed m-CPP-induced hypophagia, during the 4-h test period. In order to clarify the effects of mesulergine on dietary choices since it is simultaneously a dopamine agonist besides its antiserotonergic properties, the D2 agonist apomorphine was also used. Apomorphine caused a dose-dependent increase in protein intake while carbohydrate and total food intake remained nearly unchanged (0.5 and 1.0 mg/kg i.p., respectively). It is concluded that the mesulergine-induced hyperphagic response on both diets is the expression of a dual mode of action, due to its 5-HT(2C) antagonist activity together with D2 agonist properties. The results further indicate that the activation of hypothalamic 5-HT(2C) receptors may be involved in both protein sparing and carbohydrate suppressing effects of 5-HT (m-CPP-like effect), whereas an important role in increase of protein consumption seems to have the dopaminergic system probably through D2 receptors (apomorphine-like and mesulergine-like effects, respectively).     

Involvement of 5-HT1B receptors in the anticonflict effect of m-CPP in rats

Summary The anticonflict activity of m-CPP, a non-selective agonist of 5-HT receptors, was studied in the drinking conflict test in rats. m-CPP administered in doses of 0.125–0. 5 mg/kg increased the number of punished licks, the maximum effect having been observed after a dose of 0.25 mg/kg. The anticonflict effect of m-CPP (0.25 mg/kg) was antagonized by the non-selective 5-HT antagonist metergoline (1–4 mg/kg) and by the -adrenoceptor blocker SDZ 21009 (2 and 4 mg/kg) with affinity for 5-HT_1A and 5-HT_1B receptors. On the other hand, the 5-HT_1A receptor antagonist NAN-190 (0.5 and 1 mg/kg), the 5-HT₂ receptor antagonist ritanserin (0.25 and 0.5 mg/kg), and the -blockers betaxolol (8 mg/kg) and ICI 118,551 (8 mg/kg) with no affinity for 5-HT receptors did not affect the effect of m-CPP. The effect of m-CPP was not modified, either, in animals with the 5-HT lesion produced by p-chloroamphetamine.These results suggest that the anticonflict effect of m-CPP described above results from stimulation of 5-HT_1B receptors — most probably these which are located postsynaptically.     

Evidence for serotonin receptor subtypes involvement in agmatine antidepressant like-effect in the mouse forced swimming test

This study investigated the involvement of 5-HT(1) and 5-HT(2) receptors in the antidepressant-like effect of agmatine in the mouse forced swimming test (FST). Pretreatment with p-chlorophenylalanine methyl ester (PCPA; 100 mg/kg, intraperitoneally (i.p.), an inhibitor of serotonin synthesis, for 4 consecutive days), methysergide (5 mg/kg, i.p., a serotonin (5-HT) antagonist), pindolol (32 mg/kg, i.p., a 5-HT(1A/1B) receptor/beta-adrenoceptor antagonist), N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridynyl)cyclohexanecarboxamide (WAY 100635; 0.3 mg/kg, subcutaneously (s.c.), a selective 5-HT(1A) receptor antagonist), 1-(2-methoxyphenyl)-4[-(2-phthalimido)butyl]piperazine) (NAN-190; 0.5 mg/kg, i.p., a 5-HT(1A) receptor antagonist), 1-(2-(1-pyrrolyl)-phenoxy)-3-isopropylamino-2-propanol (isamoltane; 2.5 mg/kg, i.p., a 5-HT(1B) receptor antagonist), cyproheptadine (3 mg/kg, i.p., a 5-HT(2) antagonist) or ketanserin (5 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist), but not with propranolol (2 mg/kg, i.p., a beta-adrenoceptor antagonist), prevented the effect of agmatine (10 mg/kg, i.p.) in the FST. A subeffective dose of agmatine (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with pindolol (32 mg/kg), NAN-190 (0.5 mg/kg, i.p.), WAY 100635 (0.03 mg/kg, s.c.), (+)-8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT; 0.01 mg/kg, i.p., a 5-HT(1A) receptor agonist), R(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI; 1 mg/kg, i.p., a preferential 5-HT(2A) receptor agonist), or fluoxetine (10 mg/kg, i.p., a selective serotonin reuptake inhibitor, SSRI) but not with isamoltane (2.5 mg/kg, i.p.), ritanserin (4 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist) or ketanserin (5 mg/kg, i.p.). Taken together, the results firstly demonstrate that agmatine antidepressant-like effects in the FST seem to be mediated, at least in part, by an interaction with 5-HT(1A/1B) and 5-HT(2) receptors.     

Could the 5-HT1B receptor inverse agonism affect learning consolidation?

Diverse evidence indicates that, the 5-HT system might play a role in learning and memory, since it occurs in brain areas mediating such processes and 5-HT drugs modulate them. Hence in this work, in order to explore further 5-HT involvement on learning and memory 5-HT1B receptors' role is investigated. Evidence indicates that SB-224289 (a 5-HT1B receptor inverse agonist) post-training injection facilitated learning consolidation in an associative autoshaping learning task, this effect was partially reversed by GR 127935 (a 5-HT1B/1D receptor antagonist), but unaffected by MDL 100907 (a 5-HT2A receptor antagonist) or ketanserin (a 5-HT1D/2A/7 receptor antagonist) at low doses. Moreover, SB-224289 antagonized the learning deficit produced by TFMPP (a 5-HT1A/1B/1D/2A/2C receptor agonist), GR 46611 (a 5-HT1A/1B/1D receptor agonist), mCPP (a 5-HT2A/2C/3/7 receptor agonist/antagonist) or GR 127935 (at low dose). SB-224289 did not alter the 8-OH-DPAT (a 5-HT1A/7 receptor agonist) learning facilitatory effect. SB-224289 eliminated the deficit learning produced by the anticholinergic muscarinic scopolamine or the glutamatergic antagonist dizocilpine. Administration of both, GR 127935 (5mg/kg) plus ketanserin (0.01 mg/kg) did not modify learning consolidation; nevertheless, when ketanserin dose was increased (0.1-1.0mg/kg) and SB-224289 dose was maintained constant, a learning facilitation effect was observed. Notably, SB-224289 at 1.0mg/kg potentiated a subeffective dose of the 5-HT1B/1D receptor agonist/antagonist mixed GR 127935, which facilitated learning consolidation and this effect was abolished by ketanserin at a higher dose. Collectively, the data confirm and extend the earlier findings with GR 127935 and the effects of non-selective 5-HT(1B) receptor agonists. Clearly 5-HT1B agonists induced a learning deficit which can be reversed with SB-224289. Perhaps more importantly, SB-224289 enhances learning consolidation when given alone and can reverse the deficits induced by both cholinergic and glutamatergic antagonist. Hence, 5-HT1B receptor inverse agonists or antagonists could represent drugs for the treatment of learning and memory dysfunctions.     

Role of GABAergic antagonism in the neuroprotective effects of bilobalide

Many studies suggest that the 5-HT6 receptors are involved, along with other 5-HT receptors, in the pathophysiology and pharmacotherapy of schizophrenia. It is a putative therapeutic target of atypical antipsychotic drugs, notably clozapine, as well as some other psychotropic agents. Preferential potentiation of dopamine (DA) efflux in the medial prefrontal cortex (mPFC) and hippocampus (HIP) has been suggested to contribute to the ability of atypical antipsychotic drugs (APDs), e.g. clozapine, risperidone, olanzapine and ziprasidone, to improve cognitive function in schizophrenia. The present study demonstrated that SB-399885, a selective 5-HT6 receptor antagonist, at doses of 3 and 10 mg/kg, had no effect on cortical DA release in freely moving rats. However, both doses of SB-399885 slightly but significantly increased DA release in the HIP. Of particular interest, SB-399885, 3 mg/kg, significantly potentiated the ability of a typical antipsychotic drug haloperidol, a D2 receptor antagonist, at a dose of 0.1 mg/kg, to increase DA release in the HIP but not the mPFC. The atypical antipsychotic drug risperidone, a multireceptor antagonist, which lacks 5-HT6 receptor antagonist properties, at doses of 0.1, 0.3 and 1.0 mg/kg, produced a bell-shaped dose response effect on DA efflux in the mPFC and HIP. SB-399885 potentiated risperidone (1.0 mg/kg)-induced DA efflux in both regions. The increase in the HIP, but not the mPFC, DA efflux by 0.3 mg/kg risperidone was also potentiated by SB-399885, 3 mg/kg. These results suggest that the combined blockade of 5-HT6 and D2 receptors may contribute to the potentiation of haloperidol- and risperidone-induced DA efflux in the mPFC or HIP. The present data provides additional evidence in support of a possible therapeutic role for 5-HT6 receptor antagonism, as an addition on therapy, to enhance cognitive function in schizophrenia.     

Role of 5-hT2C receptors in the hypophagic effect of m-CPP, ORG 37684 and CP-94,253 in the rat

Compounds that stimulate 5-HT2C and/or 5-HT1B receptors induce hypophagia, but the relative role of these receptors in the control of feeding behaviour remains to be unequivocally demonstrated. The objectives of the present study were: (a) comparison of the hypophagic effect of the mixed 5-HT2C/1B receptor agonist, m-CPP, with that of ORG 37684 and CP-94,253, a relatively selective 5-HT2C and 5-HT1B receptor agonist, respectively; (b) verification of the contribution of 5-HT2C receptors to the hypophagic effect of these compounds by antagonism experiments; and (c) to test whether cotreatment with ORG 37684 and CP-94,253 leads to a more pronounced reduction of food intake as compared with treatment with either compound alone. Food intake was measured in a free feeding experimental protocol employing female Wistar rats. m-CPP was more potent in suppressing food intake than ORG 37684 and CP-94,253 (ED50 values for the first hour of access: 0.45, 1.84 and 3.48 mg/kg ip, respectively). The 5-HT2C receptor antagonists, metergoline and SB 242.084, completely reversed the hypophagic effect of ORG 37684, but not that of CP-94,253 and m-CPP. The hypophagic effect of ORG 37684 was potentiated by a low (inactive) dose of CP-94,253 (ED50: 4.95 and 2.44 mg/kg ip after vehicle and CP-94,253 pretreatment, respectively) and vice versa (ED50 values: 4.02 and 0.62 mg/kg ip). It is concluded that the hypophagic effect of ORG 37684-but not that of m-CPP and CP-94,253--is exclusively mediated by activation of 5-HT2C receptors. The results further indicate that simultaneous activation of 5-HT2C and 5-HT1B receptors underlies the higher potency of m-CPP in reducing food intake, as compared with other, more selective, compounds.     

Differential effects of 5-HT1 and 5-HT2 receptor agonists on hindlimb movements in paraplegic mice

The effects induced by serotonergic (5-HT) agonists of the 5-HT1 and 5-HT2 subclasses were examined on hindlimb movement generation in adult mice completely spinal cord transected at the low thoracic level. One week postspinalization, intraperitoneal injection (0.5-10 mg/kg) of meta-chlorophenylpiperazine (m-CPP; 5-HT(2B/2C) agonist) or trifluoromethylpiperazine (TFMPP; 5-HT(1B) agonist) failed to induce locomotor-like movements. However, dose-dependent nonlocomotor movements were induced in air-stepping condition or on a motor-driven treadmill. In contrast, hindlimb locomotor-like movements were found after the injection of quipazine (5-HT(2A/2C) agonist; 1-2 mg/kg). Combined with L-DOPA (50 mg/kg, i.p.), low doses of quipazine but not of m-CPP and TFMPP produced locomotor-like and nonlocomotor movements in air-stepping condition or on the treadmill. Subsequent administration of m-CPP or TFMPP significantly reduced and often completely abolished the hindlimb movements induced by quipazine and L-DOPA. Altogether, these results demonstrate that 5-HT(2A/2C) receptor agonists promote locomotion while 5-HT(1B) and 5-HT(2B/2C) receptor agonists interfere with locomotor genesis in the hindlimbs of complete paraplegic mice. These results suggest that only subsets of spinal 5-HT receptors are specific to locomotor rhythmogenesis and should be activated to successfully induce stepping movements after spinal cord injury.     

Effects of the serotonin2A/2C receptor agonist and antagonist on phencyclidine-induced dopamine release in rat medial prefrontal cortex

1. Systemic administration of PCP (7.5 mg/kg, i.p.) produced a greater increase in extracellular DA levels in the mPFC than in the STR and NAC, as determined by in vivo microdialysis of awake, freely moving rats. Preferential activation by PCP of prefrontal DA neurons may be, at least in part, the basis for the pathophysiology of PCP-induced psychosis as well as schizophrenia. 2. Recent studies suggest a possible involvement of 5-HT2A receptors in the pathophysiology and treatment of schizophrenia. This study was designed to examine whether and how 5-HT2A receptors modulate PCP-induced DA release in the mPFC. 3. The 5-HT2A/2C receptor agonist (+/-)-DOI (2.5 mg/kg, but not 0.75 mg/kg, i.p.), administered 60 min prior to PCP, significantly attenuated the PCP-induced increase in extracellular DA levels. Pretreatment of the 5-HT2A/2C receptor antagonist ritanserin (1.0 and 5.0 mg/kg, i.p.), administered 60 min prior to PCP, did not influence the PCP-induced increase. When administered alone, neither DOI (2.5 mg/kg) nor ritanserin (1.0 mg/kg) affected basal extracellular DA levels in the mPFC. 4. The NMDA receptor antagonist MK-801 (1.0 mg/kg, i.p.) also increased extracellular DA levels in the mPFC, but this effect was unaffected by pretreatment with DOI (2.5 mg/kg). 5. These results suggest that the stimulation of 5-HT2A/2C receptors may inhibit DA release in the mPFC when it is facilitated by PCP. Other than the NMDA receptor-mediated mechanism may also be involved in the neurochemical interaction between 5-HT2A receptors and PCP in the mPFC.     

Involvement of 5-HT1A receptors in the antidepressant-like effect of adenosine in the mouse forced swimming test

This study investigated the involvement of 5-HT1 and 5-HT2 receptors in the antidepressant-like effect of adenosine in the mouse forced swimming test (FST). The pre-treatment of mice with PCPA (100mg/kg, i.p., an inhibitor of serotonin synthesis, for four consecutive days), NAN-190 (0.5mg/kg, i.p., a 5-HT1A receptor antagonist), pindolol (32 mg/kg, i.p., a 5-HT1A/1B receptor/beta-adrenoceptor antagonist) or WAY100635 (0.1 and 0.3mg/kg, s.c., a selective 5-HT1A receptor antagonist), but not with ketanserin (5mg/kg, i.p., a 5-HT2A/2C receptor antagonist), prevented the antidepressant-like effect of adenosine (10mg/kg, i.p.) in the FST. Moreover, the pre-treatment of animals with WAY100635 (0.1mg/kg, s.c.) blocked the decrease in immobility time in the FST elicited by adenosine (5 or 10mg/kg, i.p.), but produced a synergistic effect with a sub-effective dose of adenosine (1mg/kg, i.p.) and did not cause any alteration at the highest dose of adenosine administered (50mg/kg, i.p.). Adenosine (1mg/kg, i.p.) produced a synergistic antidepressant-like effect with pindolol (32 mg/kg), NAN-190 (0.5mg/kg, i.p.), WAY100635 (0.03 mg/kg, s.c.), 8-OH-DPAT (1mg/kg, i.p., a 5-HT1A receptor agonist), but not with DOI (1mg/kg, i.p., a preferential 5-HT2A receptor agonist) or ketanserin. The pre-treatment of mice with DPCPX (2mg/kg, i.p., a selective adenosine A1 receptor antagonist) or ZM241385 (1mg/kg, i.p., a selective adenosine A2A receptor antagonist) did not prevent the effect of fluoxetine (32 mg/kg, i.p., a preferential serotonin reuptake inhibitor) in the FST. Besides that, adenosine (1mg/kg, i.p.) did not produce a synergistic antidepressant-like effect with fluoxetine (10mg/kg, i.p.). Taken together, the results indicate that the antidepressant-like effect of adenosine in the FST appears to be mediated, at least in part, by an interaction with 5-HT1A receptors.     

Evidence for a 5-HT2A receptor mode of action in the anxiolytic-like properties of DOI in mice

DOI [(+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane] displays a high affinity for the rat 5-HT2A, 5-HT2B and 5-HT2C receptors (pKi 7.3, 7.4 and 7.8, respectively) and acts as an agonist. DOI (0.5-4 mg/kg, i.p. 30 min pre-test) increased the number of punished passages in the mouse four plates test (FPT). The anti-punishment action of DOI (1 mg/kg, i.p. 30 min pre-test) was abolished by prior treatment with the selective 5-HT2A receptor antagonist SR 46949B (0.1 and 1 mg/kg, i.p. 45 min pre-test) but not by the selective 5-HT2C receptor antagonist RS 10-2221 (0.1 and 1 mg/kg, i.p. 45 min pre-test) nor the selective 5-HT2C/2B receptor antagonist SB 206553 (0.1 and 1 mg/kg, i.p. 45 min pre-test). An anxiolytic-like action was also observed for DOI (1 mg/kg) in the elevated plus maze (EPM). The anxiolytic-like action of DOI (1 mg/kg, i.p. 30 min pre-test) was antagonised by pre-treatment with SR 46949B (0.125 and 0.5 mg/kg, i.p. 45 min pre-test) but not by RS 10-2221 (0.1 and 1 mg/kg, i.p. 45 min pre-test) nor SB 206553 (0.1 and 1 mg/kg, i.p. 45 min pre-test). In conclusion, DOI produced an anxiolytic-like profile in the mouse FPT and EPM. These effects are likely to be 5-HT2A receptor mediated.     

Activation of 5-HT2 receptors enhances the release of acetylcholine in the prefrontal cortex and hippocampus of the rat

The role of 5-HT2 receptors in the regulation of acetylcholine (ACh) release was examined in the medial prefrontal cortex and dorsal hippocampus using in vivo microdialysis. The 5-HT(2A/2C) agonist +/-1-(2,5-dimethoxy-4-iodophenyl) -2- aminopropane hydrochloride (DOI) (1 and 2 mg/kg, i.p.) significantly increased the extracellular concentration of ACh in both brain regions, and this response was attenuated in rats treated with the 5-HT(2A/2B/2C) antagonist LY-53,857 (3 mg/kg, i.p.). Treatment with LY-53,857 alone did not significantly alter ACh release in either brain region The 5-HT(2C) agonist 6-chloro-2-(1-piperazinyl)-pyrazine) (MK-212) (5 mg/kg, i.p.) significantly enhanced the release of ACh in both the prefrontal cortex and hippocampus, whereas the 5-HT2 agonist mescaline (10 mg/kg, i.p.) produced a 2-fold increase in ACh release only in the prefrontal cortex. Intracortical, but not intrahippocampal, infusion of DOI (100 microM) significantly enhanced the release of ACh, and intracortical infusion of LY-53,857 (100 microM) significantly attenuated this response. These results suggest that the release of ACh in the prefrontal cortex and hippocampus is influenced by 5-HT2 receptor mechanisms. The increase in release of ACh induced by DOI in the prefrontal cortex, but not in the hippocampus, appears to be due to 5-HT2 receptor mechanisms localized within this brain region. Furthermore, it appears that the prefrontal cortex is more sensitive than the dorsal hippocampus to the stimulatory effect of 5-HT2 agonists on ACh release.     

Fluvoxamine suppresses the long-term potentiation in the hippocampal CA1 field of anesthetized rats: an effect mediated via 5-HT1A receptors

A selective 5-HT reuptake inhibitor, fluvoxamine (10 and 30 mg/kg, i.p.) suppressed long-term potentiation (LTP) in the hippocampal CA1 field of anesthetized rats. Fluvoxamine (30 mg/kg, i.p.)-induced suppression of LTP was completely reversed by the 5-HT(1A) receptor antagonist NAN-190 (0.5 mg/kg, i.p), but not by the 5-HT(4) receptor antagonist GR 113808 (20 microg/rat, i.c.v.) and the 5-HT(7) receptor antagonist DR 4004 (10 microg/rat, i.c.v.). These data suggest that the inhibitory effect of fluvoxamine on LTP induction is mediated via 5-HT(1A) receptors.     

The effects of the 5-HT(6) receptor agonist EMD and the 5-HT(7) receptor agonist AS19 on memory formation

Growing evidence indicates that 5-hydrohytryptamine (5-HT) receptors mediate learning and memory. Particularly interesting are 5-HT(6) and 5-HT(7) receptors, which are localized in brain areas involved in memory formation. Interestingly, recently selective 5-HT(6) and 5-HT(7) receptor agonists and antagonists have become available. Previous evidence indicates that 5-HT(6) or 5-HT(7) receptors antagonists had no effects, improved memory formation and/or reversed amnesia. Herein, the effects of EMD (a 5-HT(6) receptor agonist) and AS19 (a 5-HT(7) receptor agonist) in the associative learning task of autoshaping were studied. Post-training systemic administration of EMD (1-10 mg/kg) or AS19 (1-10 mg/kg) were tested in short-term memory (STM) and long-term memory (LTM). Results showed that only EMD 5.0mg/kg impaired both STM and LTM. AS19 at 1-10 mg/kg significantly impaired STM but not LTM. In those groups used to test only LTM, EMD impaired it; while AS19 improved LTM. Moreover, in the interaction experiments, the STM EMD-impairment effect was partially reversed by the selective 5-HT(6) receptor antagonist SB-399885 (10 mg/kg). The STM AS19-impairment effect (5.0 mg/kg) was not altered by the selective 5-HT(1A) antagonist WAY 100635 (0.3 mg/kg) but reversed by the selective 5-HT(7) receptor antagonist SB-269970 (10.0 mg/kg). The AS19-SB-269970 combination impaired LTM. Taken together these data suggest that the stimulation of 5-HT(6) impaired both STM and LTM. 5-HT(7) receptors stimulation impaired STM but improved LTM. And these results are discussed in the context of their possible neural bases.     

Fenfluramine evokes 5-HT2A receptor-mediated responses but does not displace [11C]MDL 100907: small animal PET and gene expression studies

The in vivo binding of the 5-HT(2A) receptor-selective positron emission tomography (PET) ligand [(11)C]MDL 100907 and its sensitivity to endogenous 5-HT were quantified in rat brain using quad-HIDAC, a novel high-resolution PET camera for small animals. Specific binding of [(11)C]MDL 100907, estimated using volume of interest (VOI) to cerebellum ratios, corresponded well with both the known distribution of 5-HT(2A) receptors and tissue:cerebellum ratios obtained using ex vivo dissection. Specific binding was blocked by predosing with either nonradioactive MDL 100907 (0.2 or 0.4 mg/kg i.v.) or the 5-HT(2A/2C) receptor antagonist ketanserin (2 mg/kg i.v.), but was unaffected in rats pretreated with the 5-HT releasing agent, fenfluramine (10 mg/kg i.p.). In parallel studies, the same dose of fenfluramine was shown to be sufficient to cause an increase in the expression of the immediate early genes (IEG) c-fos and Arc mRNA in cortical regions with high 5-HT(2A) receptor density. This increase was blocked by MDL 100907 (0.2 mg/kg i.v.), confirming a 5-HT(2A) receptor-mediated effect. The results demonstrate that PET with [(11)C]MDL 100907 is insensitive to an increased concentration of synaptic 5-HT, implying that the ligand can be used clinically to monitor 5-HT(2A) receptor function or dysfunction in disease or during therapy, without the need to consider concomitant changes in neurotransmitter concentration.     

Role of 5-HT3 and 5-HT2C receptors located within the medial amygdala in the control of salt intake in sodium-depleted rats

In the present study, we investigated the role of 5-HT(3) and 5-HT(2C) receptors located within the medial amygdala (MeA) in the control of water and salt intake in sodium-depleted rats. Pharmacological activation of 5-HT(3) receptors located in the medial amygdala by the selective 5-HT(3) receptor agonist m-CPBG significantly reduced salt intake in sodium-depleted rats, an effect that is reverted by pretreatment with the selective 5-HT(3) receptor antagonist ondansetron. In addition, the injection of ondansetron alone into the medial amygdala had no effect on salt intake in sodium-depleted and in sodium-repleted rats. Pharmacological stimulation of 5-HT(2C) receptors located in the medial amygdala by the selective 5-HT(2C) receptor agonist m-CPP failed to modify salt intake in sodium-depleted rats, whereas the blockade of these receptors by the selective 5-HT(2C) receptor antagonist SDZ SER 082 significantly reduced salt intake in this same group of animals. These results lead to the conclusion that the pharmacological activation of 5-HT(3) receptors located within the MeA inhibits salt intake in sodium-depleted rats and that, in this same brain region, the functional integrity of 5-HT(2C) receptors is required to achieve the full expression of sodium appetite in sodium-depleted rats.     

Effect of acute and chronic administration of the selective 5-HT2C receptor antagonist SB-243213 on midbrain dopamine neurons in the rat: an in vivo extracellular single cell study

In this study, we examined the effect of the acute and chronic administration of the selective 5-HT2C receptor antagonist SB-243213 (SB) on the activity of spontaneously active dopamine (DA) cells in the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) in anesthetized, albino, male Sprague-Dawley rats. This was accomplished using the technique of in vivo extracellular single cell recording. The acute i.v. administration of SB-243213 (0.025-3.2 mg/kg) did not significantly alter the basal firing rate or pattern of either spontaneously active SNC or VTA DA neurons compared to vehicle-treated controls. The acute i.p. administration of either 1 or 10 mg/kg of SB-243213 did not significantly alter the number of spontaneously active DA cells in the SNC or VTA compared to vehicle-treated controls, whereas the 3 mg/kg dose only significantly decreased the number of spontaneously active VTA DA neurons. Overall, the 1 mg/kg dose of SB-243213 did not significantly alter the firing pattern of either SNC or VTA DA neurons compared to vehicle-treated controls. In contrast, the 3 mg/kg dose significantly altered the firing pattern of SNC DA neurons, whereas the 10 mg/kg dose altered the firing pattern of DA neurons in both the SNC and VTA. The repeated i.p. administration (21 days) of 1, 3, and 10 mg/kg of SB-243213 or 20 mg/kg of clozapine produced a significant decrease in the number of spontaneously active DA cells in the VTA compared to vehicle-treated controls. The decrease in the number of spontaneously active VTA DA cells was not reversed by the i.v. administration of (+)-apomorphine (50 microg/kg). The repeated administration of either 1 or 3 mg/kg of SB-243213 had minimal effects on the firing pattern of either SNC or VTA DA neurons. In contrast, the firing pattern of VTA DA neurons was significantly altered by 10 mg/kg dose of SB-243213. Overall, our results indicate that antagonism of the 5-HT2C receptor alters the activity of midbrain DA neurons in anesthetized rats and suggest that SB-243213 has an atypical antipsychotic profile following chronic administration.     

Inhibition of hippocampal 5-HT synthesis by fluoxetine and paroxetine: evidence for the involvement of both 5-HT1A and 5-HT1B/D autoreceptors

Hippocampal serotonin (5-hydroxytryptamine, 5-HT) synthesis, as determined by the accumulation of 5-hydroxytryptophan (5-HTP) following inhibition of L-aromatic amino acid decarboxylase with NSD 1015, was inhibited by systemic administration of the selective serotonin reuptake inhibitors fluoxetine (10 mg/kg i.p.) and paroxetine (3 mg/kg i.p.). Pretreatment of rats with the selective 5-HT1A receptor antagonist WAY 100635 for a period of 7 days using subcutaneously implanted osmotic minipumps (1 mg/kg/day) was sufficient to block the inhibition of 5-HT synthesis following the 5-HT 1A receptor agonist 8-OH-DPAT (0.3 mg/kg s.c.), but failed to inhibit the decrease of hippocampal 5-HT synthesis by fluoxetine (10 mg/kg i.p.) or paroxetine (3 mg/kg i.p.). Similarly, pretreatment of rats with GR 127935 (5 mg/kg i.p.), an antagonist with high affinity for 5-HT1B/D receptors, blocked the reduction of hippocampal 5-HT synthesis following the 5-HT receptor agonist TFMPP (3 mg/kg s.c.) without affecting the reduction of hippocampal 5-HT synthesis by either fluoxetine or paroxetine. In contrast, pretreatment with WAY 100635 (1 mg/kg/day, for 7 days s.c. in osmotic minipumps) in combination with GR 127935 (5 mg/kg i.p.) significantly attenuated the decrease of hippocampal 5-HT synthesis by both fluoxetine and paroxetine. These results indicate that both 5-HT1A and 5-HT1B/1D receptors, which function in the rat as inhibitory somatodendritic and nerve terminal autoreceptors, independently regulate hippocampal 5-HT synthesis and must be simultaneously blocked to prevent the inhibition of 5-HT synthesis by selective serotonin reuptake inhibitors which increase 5-HT availability at both nerve terminals in hippocampus and 5-HT cell bodies in the raphe nuclei.     

3,4-Methylenedioxymethamphetamine (MDMA) enhances the release of acetylcholine by 5-HT4 and D1 receptor mechanisms in the rat prefrontal cortex

3,4-Methylenedioxymethamphetamine (MDMA), an amphetamine analog, has been shown recently to increase the release of acetylcholine (ACh) in the prefrontal cortex (PFC). The present study further characterizes the stimulatory effect of MDMA on cortical ACh release and examines the role of serotonin (5-HT) and dopamine (DA) receptors in this response. The extracellular concentration of ACh was increased dose-dependently and similarly by the (+) and (-) enantiomers of MDMA (5 and 20 mg/kg, i.p.). The systemic administration of the 5-HT(4) antagonist SDZ 205,557 (1 mg/kg, i.p.), but not the 5-HT(2A/2B/2C) antagonist LY-53,857 (3 mg/kg, i.p.), significantly decreased cortical ACh release induced by MDMA. The MDMA-induced increase in the extracellular concentration of ACh also was significantly blunted in rats treated with the D(1) receptor antagonist SCH 23390 (0.5 mg/kg, i.p.). The extent to which the coadministration of SDZ 205,557 and SCH 23390 suppressed the MDMA-induced release of ACh in the PFC was no greater than that produced by either antagonist alone. These results suggest that the 5-HT(4) and D(1) receptor subtypes contribute to the mechanism by which MDMA increases ACh release in the PFC.     

Combined treatment with citalopram and buspirone: effects on serotonin 5-HT2A and 5-HT2C receptors in the rat brain

INTRODUCTION: We wanted to elucidate whether the proposed advantages of citalopram-buspirone combination treatment are related to changes in 5-HT(2A/C) receptor-mediated neurotransmission. METHODS: The affinity of buspirone to 5-HT2A and 5-HT2C receptors was measured in vitro, and the influence of buspirone on 5-HT2C receptor-mediated phosphoinositide hydrolysis was estimated. Four groups of rats received citalopram (10 mg/kg), buspirone (6 mg/kg), citalopram-buspirone combination, or saline once a day s.c. for 14 days. Treatment effects on 5-HT2A and 5-HT2C receptors were investigated by receptor autoradiography with antagonist and agonist radioligands. RESULTS: Buspirone was found to be a weak 5-HT2C receptor antagonist, with a low affinity for 5-HT2A and 5-HT2C receptors. Repeated buspirone-citalopram combination treatment markedly decreased [3H]ketanserin and [125I]DOI binding to 5-HT2A receptors. Repeated administration of buspirone and buspirone-citalopram combination increased the affinity of [3H]mesulergine toward 5-HT2C receptors, and buspirone-citalopram combination also decreased [125I]DOI binding to 5-HT2C receptors. DISCUSSION: We suggest that downregulation of brain 5-HT2A receptors and possibly of 5-HT2C receptor agonist sites is involved in the beneficial clinical effects of buspirone-SSRI augmentation treatment. Furthermore, a conversion of brain 5-HT2C receptors from high- to low-affinity state may provide an additional mechanism for the anti-anxiety effects of buspirone.