Effects of baclofen on dopamine-dependent behaviors in mice

Baclofen, the parachlorophenyl analog of GABA, was found to induce catalepsy and to inhibit the traction response in mice. However, baclofen pretreatment, instead of antagonizing methamphetamine stereotypy and apomorphine-induced cage climbing behavior, was found to potentiate these behaviors, thereby ruling out the possibility of its possessing postsynaptic dopamine (DA) receptor blocking activity. The possible mechanism involved in the induction of catalepsy and in the inhibition of the traction response by baclofen is discussed on the basis that baclofen, by inhibiting the firing of the nigrostriatal and mesolimbic DA neurons, reduces the release of DA and thereby produces a functional lack of DA at postsynaptic DA receptor sites with resultant induction of catalepsy and inhibition of the traction response. Further, the hyper-responsiveness to methamphetamine and apomorphine is explained on the basis that, as the postsynaptic DA receptors are acutely deprived of their transmitter, following baclofen pretreatment, they become supersensitive to the DA agonists.     

Reversal of sibutramine-induced anorexia with a selective 5-HT<Subscript>2C</Subscript> receptor antagonist

Rationale  

The monoamine reuptake inhibitor sibutramine reduces food intake but the receptor subtypes mediating the effects of sibutramine on feeding remain to be clearly identified.     

Effects of naloxone on methamphetamine and apomorphine stereotypy and on haloperidol catalepsy in rats

Pretreatment with the opiate antagonist naloxone, at 1.25–5 mg/kg, increased the intensity of methamphetamine stereotypy, had no effect (over a range of 0.3125–5 mg/kg) on apomorphine stereotypy, and antagonized haloperidol catalepsy in rats at 1.25–5 mg/kg. It is suggested that naloxone, by blocking the opiate receptors located on the nigro-striatal and mesolimbic dopamine (DA) nerve terminals, releases the DA systems from endogenous inhibition, presumably caused by endogenous opiate systems, and thereby potentiates methamphetamine stereotypy and antagonizes haloperidol catalepsy. However, the possibility that naloxone might have affected methamphetamine stereotypy and haloperidol catalepsy by modulating the activity of the central noradrenergic and GABAergic systems, which are reported to influence dopaminergically mediated behaviours, also needs to be considered.     

The effects of the selective 5-HT<Subscript>2C</Subscript> receptor antagonist SB 242084 on learned helplessness in male Fischer 344 rats

Rationale  Rats exposed to an uncontrollable stressor demonstrate a constellation of behaviors such as exaggerated freezing and deficits in shuttle box escape learning. These behaviors in rats have been called learned helplessness and have been argued to model human stress-related mood disorders. Learned helplessness is thought to be caused by hyperactivation of serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN) and a subsequent exaggerated release of 5-HT in DRN projection sites. Blocking 5-HT_2C receptors in the face of an increase in serotonin can alleviate anxiety behaviors in some animal models. However, specific 5-HT receptor subtypes involved in learned helplessness remain unknown. Objectives  The current experiments tested the hypothesis that 5-HT_2C receptor activation is necessary and sufficient for the expression of learned helplessness. Results  The selective 5-HT_2C receptor antagonist SB 242084 (1.0 mg/kg) administered i.p. to adult male Fischer 344 rats prior to shuttle box behavioral testing, but not before stress, blocked stress-induced deficits in escape learning but had no effect on the exaggerated shock-elicited freezing. The selective 5-HT_2C receptor agonist CP-809101 was sufficient to produce learned helplessness-like behaviors in the absence of prior stress and these effects were blocked by pretreatment with SB 242084. Conclusions  Results implicate the 5-HT_2C receptor subtype in mediating the shuttle box escape deficits produced by exposure to uncontrollable stress and suggest that different postsynaptic 5-HT receptor subtypes underlie the different learned helplessness behaviors. Supported by NIMH068283 (MF), American Foundation for Suicide Prevention (BG)     

Pharmacological characterisation of the agonist radioligand binding site of 5-HT<Subscript>2A</Subscript>, 5-HT<Subscript>2B</Subscript> and 5-HT<Subscript>2C</Subscript> receptors

In the present study we compared the affinity of various drugs for the high affinity agonist-preferring binding site of human recombinant 5-HT_2A, 5-HT_2B and 5-HT_2C receptors stably expressed in monoclonal mammalian cell lines. To ensure that the agonist-preferring conformation of the receptor was preferentially labelled in competition binding experiments, saturation analysis was conducted using antagonist and agonist radiolabels at each receptor. Antagonist radiolabels ([³H]-ketanserin for 5-HT_2A receptor and [³H]-mesulergine for 5-HT_2B and 5-HT_2C receptor) bound to a larger population of receptors in each preparation than the corresponding agonist radiolabel ([¹²⁵I]-DOI for 5-HT_2A receptor binding and [³H]-5-HT for 5-HT_2B and 5-HT_2C receptor binding). Competition experiments were subsequently conducted against appropriate concentrations of the agonist radiolabels bound to the agonist-preferring subset of receptors in each preparation. These studies confirmed that there are a number of highly selective antagonists available to investigate 5-HT₂ receptor subtype function (for example, MDL 100907, RS-127445 and RS-102221 for 5-HT_2A, 5-HT_2B and 5-HT_2C receptors respectively). There remains, however, a lack of highly selective agonists. (–)DOI is potent and moderately selective for 5-HT_2A receptors, BW723C86 has poor selectivity for human 5-HT_2B receptors, while Org 37684 and VER-3323 display some selectivity for the 5-HT_2C receptor. We report for the first time in a single study, the selectivity of numerous serotonergic drugs for 5-HT₂ receptors from the same species, in mammalian cell lines and using, exclusively, agonist radiolabels. The results indicate the importance of defining the selectivity of pharmacological tools, which may have been over-estimated in the past, and highlights the need to find more selective agonists to investigate 5-HT₂ receptor pharmacology.     

Individual differences in the improvement of cocaine-induced place preference response by the 5-HT<Subscript>2C</Subscript> receptor antagonist SB242084 in rats

Rationale and objectives  

The 5-HT_2A and 5-HT_2C receptors have been shown to be differentially involved in modulating cocaine-induced behaviors. In this study we investigated the effects of the 5-HT_2A antagonist MDL100907 (0.3 mg/kg, i.p.) and the 5-HT_2C antagonist SB242084 (0.5 mg/kg, i.p.) on development, expression, and recall of cocaine-induced conditioned place preference (CPP) in high- (HR) and low-responder (LR) rats to novelty.     

Effects of systemic and intra-nucleus accumbens 5-HT<Subscript>2C</Subscript> receptor compounds on ventral tegmental area self-stimulation thresholds in rats

Rationale  Serotonin 2C (5-HT_2C) receptors may play a role in regulating motivation and reward-related behaviours. To date, no studies have investigated the possible role of 5-HT_2C receptors in ventral tegmental area (VTA) intracranial self-stimulation (ICSS). Objectives  The current study investigated the hypotheses that 5-HT_2C receptors play an inhibitory role in VTA ICSS, and that 5-HT_2C receptors within the nucleus accumbens (NAc) shell may be involved. Methods  Male Sprague–Dawley rats were implanted with a VTA electrode and bilateral NAc shell cannulae for the experiment involving microinjections, and trained to respond for electrical self-stimulation. The systemic effects of the selective 5-HT_2C receptor agonist WAY 161503 (0–1.0 mg/kg), the 5-HT_1A/1B/2C receptor agonist TFMPP (0.3 mg/kg) and the selective 5-HT_2C receptor antagonist SB 242084 (1.0 mg/kg) were compared using rate-frequency threshold analysis. Intra-NAc shell microinjections of WAY 161503 (0–1.5 μg/side) were investigated and compared to amphetamine (1.0 μg/side). Results  WAY 161503 (1.0 mg/kg) and TFMPP (0.3 mg/kg) significantly increased rate-frequency thresholds (M50 values) without altering maximal response rates (RMAX values). SB 242084 attenuated the effects of TFMPP; SB 242084 had no affect on M50 or RMAX values. Intra-NAc shell WAY 161503 had no effect on M50 or RMAX values; intra-NAc amphetamine decreased M50 values. Conclusions  These results suggest that 5-HT_2C receptors play an inhibitory role in regulating reward-related behaviour while 5-HT_2C receptor activation in the NAc shell did not appear to influence VTA ICSS behaviour under the present experimental conditions. This work was funded by the Canadian Institutes of Health Research (CIHR) (A.J.G). D.J.H. was the recipient of a postgraduate scholarship from the Natural Sciences and Engineering Research Council of Canada (NSERC).     

Pharmacological characterization of mitogen-activated protein kinase activation by recombinant human 5-HT<Subscript>2C</Subscript>, 5-HT<Subscript>2A</Subscript>, and 5-HT<Subscript>2B</Subscript> receptors

The type 2 serotonin (5-HT₂) receptor subfamily is known to couple to phosphoinositide hydrolysis (PI) and the subsequent mobilization of intracellular Ca²⁺, as well as the release of arachidonic acid (AA). Less is known of 5-HT₂-mediated activation of the mitogen-activated protein kinase (MAPK) or extracellular signal-regulated kinase (ERK1/2) signaling. The present study measured the relative efficacies and potencies of 5-HT agonists to activate ERK2 in non-neuronal cells expressing recombinant human 5-HT_2A, 5-HT_2B, and 5-HT_2C(ISV) receptors. 5-HT agonists stimulated ERK2 activity via all three 5-HT₂ subtypes. There were no meaningful differences in the potencies or relative efficacies of these agonists to affect ERK2 activity vs. PI accumulation or Ca²⁺ mobilization, suggesting that these pathways may be sequentially linked. Indeed, ERK2 activity was very sensitive to PKC inhibition and calcium chelation and insensitive to tyrosine kinase and PI-3-kinase inhibition. 5-HT₂ receptors efficiently couple to MAPK activation via sequential PI hydrolysis, and Ca²⁺ mobilization. This profile differs from reports of “agonist-directed trafficking of receptor stimulus” between PI/Ca²⁺ and AA pathways activated by 5-HT₂ receptors.     

Anxiolytic properties of agomelatine,an antidepressant with melatoninergic and serotonergic properties: role of 5-HT<Subscript>2C</Subscript> receptor blockade

Rationale The novel antidepressant agent, agomelatine, behaves as an agonist at melatonin receptors and as an antagonist at serotonin (5-HT)_2C receptors.Objectives To determine whether, by virtue of its antagonist properties at 5-HT_2C receptors, agomelatine elicits anxiolytic properties in rats.Methods Employing a combined neurochemical and behavioural approach, actions of agomelatine were compared to those of melatonin, the selective 5-HT_2C receptor antagonist, SB243,213, and the benzodiazepine, clorazepate.Results In unfamiliar pairs of rats exposed to a novel environment, agomelatine enhanced the time devoted to active social interaction, an action mimicked by clorazepate and by SB243,213. In a Vogel conflict procedure, agomelatine likewise displayed dose-dependent anxiolytic activity with a maximal effect comparable to clorazepate, and SB243,213 was similarly active in this procedure. In a plus-maze procedure in which clorazepate significantly enhanced percentage entries into open arms, agomelatine revealed only modest activity and SB243,213 was inactive. Further, like SB243,213, and in contrast to clorazepate, agomelatine did not suppress ultrasonic vocalizations emitted by rats re-exposed to an environment associated with an aversive stimulus. Whereas clorazepate reduced dialysate levels of 5-HT and noradrenaline in hippocampus and frontal cortex of freely moving rats, agomelatine did not affect extracellular levels of 5-HT and elevated those of noradrenaline. SB243,213 acted similarly to agomelatine. Melatonin, which did not modify extracellular levels of 5-HT or noradrenaline, was ineffective in all models of anxiolytic activity. Furthermore, the selective melatonin antagonist, S22153, did not modify anxiolytic properties of agomelatine in either the social interaction or the Vogel Conflict tests.Conclusions In contrast to melatonin, and reflecting blockade of 5-HT_2C receptors, agomelatine is active in several models of anxiolytic properties in rodents. The anxiolytic profile of agomelatine differs from that of benzodiazepines from which it may also be distinguished by its contrasting influence on corticolimbic monoaminergic pathways.     

Effects of NAD-299, a new,highly selective 5-HT<Subscript>1A</Subscript> receptor antagonist,on bladder function in rats

5-HT is involved in micturition control. In the rat, stimulation of the 5-HT(1A) receptor excites, whereas 5-HT(1A) receptor antagonism inhibits micturition. The present study examined the effects of a new, highly selective, 5-HT(1A) receptor antagonist, NAD-299, on micturition in rats. Comparisons were made with WAY100635, a well-characterised antagonist at the 5-HT(1A) receptor.Female Sprague-Dawley rats, conscious or anaesthetised, were used for cystometric studies. Intravenous (i.v.), intraarterial (i.a.), intrathecal (i.t.) or intracerebroventricular (i.c.v.) catheters were implanted for drug administration. In vitro, rat bladder strips were contracted by carbachol or electrical stimulation of nerves. The effects of NAD-299, WAY100635 and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, a 5-HT(1A) receptor agonist) on cystometric parameters and contraction of bladder strips were recorded.In conscious rats, i.v. NAD-299 and WAY100635 at 1 micro mol kg(-1) increased bladder capacity (24+/-13% and 27+/-19% respectively) and decreased micturition pressure (16+/-8% and 12+/-10% respectively). Given i.a., 5-HT 0.25 micro mol kg(-1) and 8-OH-DPAT 0.37 micro mol kg(-1) stimulated micturition. The effect of 8-OH-DPAT, but not those of 5-HT, was blocked by i.a. NAD-299. 8-OH-DPAT 0.03 micro mol given i.t. or i.c.v. stimulated micturition, an effect blocked by WAY100635 0.1 micro mol, given i.t or i.c.v. NAD-299 or WAY100635 (0.1 micro mol i.t.) were without significant effects, but given i.c.v. at 0.1 micro mol both drugs increased bladder capacity (34+/-12% and 22+/-13% respectively). Neither NAD-299 nor WAY100635 up to 10(-5) M had effects on electrically- or carbachol-induced contractions of rat bladder strips. NAD-299 1 micro mol kg(-1) i.v. suppressed oxyhaemoglobin-induced detrusor over-activity. It is concluded that NAD-299, acting at a supraspinal site, can inhibit micturition in the rat.     

A modified adjusting delay task to assess impulsive choice between isocaloric reinforcers in non-deprived male rats: effects of 5-HT<Subscript>2A/C</Subscript> and 5-HT<Subscript>1A</Subscript> receptor agonists

Rationale  

Existing animal models of impulsivity frequently use food restriction to increase subjects’ motivation. In addition, behavioral tasks that assess impulsive choice typically involve the use of reinforcers with dissimilar caloric content. These factors represent energy-homeostasis limitations, which may confound the interpretation of results and limit the applicability of these models.     

ACP-103, a 5-HT<Subscript>2A/2C</Subscript> inverse agonist,potentiates haloperidol-induced dopamine release in rat medial prefrontal cortex and nucleus accumbens

Rational Atypical antipsychotic drugs (APDs) such as clozapine, olanzapine, quetiapine, risperidone, and ziprasidone are serotonin (5-HT)_2A antagonists and relatively weaker dopamine (DA) D₂ antagonists, with variable 5-HT_2C antagonist properties. The ability of atypical APDs to preferentially increase DA release in the cortex compared to the limbic system is believed to be due in part to their antagonism of 5-HT_2A and D₂ receptors and believed to contribute to their beneficial effects on cognition, negative, and psychotic symptoms. Previous studies from this laboratory using microdialysis have shown that pretreatment of the 5-HT_2A antagonist M100907 with the typical APD and D₂ antagonist haloperidol produced an increase in the medial prefrontal cortex (mPFC), but not in the nucleus accumbens (NAC), DA release. However, pretreatment with the 5-HT_2A/2C receptor antagonist SR46349-B with haloperidol increased both mPFC and NAC DA release, suggesting that both 5-HT_2A and 5-HT_2C properties may be important for atypical APD effects.Objective The purpose of this study was to examine the effects of a novel putative atypical APD ACP-103 on mPFC and NAC DA release using in vivo microdialysis in freely moving rats that are awake. ACP-103 is an inverse agonist at both 5-HT_2A and 5-HT_2C receptors and has intermediate affinities for 5-HT_2C receptors relative to their affinities for 5-HT_2A receptors compared to M100907 and SR46349-B. In addition, the effects of ACP-103 were compared to M100907 and SR46349-B, and ACP-103 was also coadministered with haloperidol.Results ACP-103 10.0 mg/kg, but not 3.0 mg/kg, increased DA release in the mPFC, while neither dose increased DA release in the NAC. Like M100907, ACP-103 (3.0 mg/kg) potentiated 0.1 mg/kg haloperidol-induced DA release in the mPFC while inhibiting that in the NAC. However, ACP-103 (3.0 mg/kg), similar to SR46349-B, potentiated a high dose of haloperidol (1.0 mg/kg)-induced DA release in both regions. The potent 5-HT_2C antagonist SB242084 1.0 mg/kg significantly potentiated 0.1 mg/kg haloperidol-induced DA release in both the mPFC and NAC. However, SB242084, at 0.2 mg/kg, significantly potentiated DA release only in the NAC. Moreover, SB242084 0.2 mg/kg potentiated DA release in the NAC produced by the combination treatment of 3 mg/kg ACP-103 and 0.1 mg/kg haloperidol.Conclusion These data suggest that the relative extent of 5-HT_2A and 5-HT_2C antagonism, as well as the extent of D₂ receptor blockade, has a critical influence on DA release in the mPFC and NAC and may be a determining factor in the action of this class of atypical APDs on these two potentially clinically relevant parameters.     

Paroxetine-induced reduction of sexual incentive motivation in female rats is not modified by 5-HT<Subscript>1B</Subscript> or 5-HT<Subscript>2C</Subscript> antagonists

Rationale  

Clinical data show that paroxetine causes sexual dysfunction in a substantial proportion of women taking this compound.     

Amisulpride is a potent 5-HT<Subscript>7</Subscript> antagonist: relevance for antidepressant actions in vivo

Rationale  Amisulpride is approved for clinical use in treating schizophrenia in a number of European countries and also for treating dysthymia, a mild form of depression, in Italy. Amisulpride has also been demonstrated to be an antidepressant for patients with major depression in many clinical trials. In part because of the selective D₂/D₃ receptor antagonist properties of amisulpride, it has long been widely assumed that dopaminergic modulation is the proximal event responsible for mediating its antidepressant and antipsychotic properties. Objectives  The purpose of these studies was to determine if amisulpride’s antidepressant actions are mediated by off-target interactions with other receptors. Materials and Methods  We performed experiments that: (1) examined the pharmacological profile of amisulpride at a large number of central nervous system (CNS) molecular targets and, (2) after finding high potency antagonist affinity for human 5-HT_7a serotonin receptors, characterized the actions of amisulpride as an antidepressant in wild-type and 5-HT₇ receptor knockout mice. Results  We discovered that amisulpride was a potent competitive antagonist at 5-HT_7a receptors and that interactions with no other molecular target investigated in this paper could explain its antidepressant actions in vivo. Significantly, and in contrast to their wild-type littermates, 5-HT₇ receptor knockout mice did not respond to amisulpride in two widely used rodent models of depression, the tail suspension test and the forced swim test. Conclusions  These results indicate that 5-HT_7a receptor antagonism, and not D₂/D₃ receptor antagonism, likely underlies the antidepressant actions of amisulpride.     

5-HT<Subscript>2A/2C</Subscript> receptor signaling via phospholipase A<Subscript>2</Subscript> and arachidonic acid is attenuated in mice lacking the serotonin reuptake transporter

Subjects The serotonin reuptake transporter (SERT) helps to regulate brain serotonergic transmission and is the target of some antidepressants. To further understand SERT function, we measured a marker of regional brain phospholipase A₂ (PLA₂) activation in SERT knockout mice (SERT–/–) and their littermate controls (SERT+/+).Methods Following administration of 1.5 mg/kg s.c. (±)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), a 5-HT_2A/2C receptor agonist, to unanesthetized mice injected intravenously with radiolabeled arachidonic acid (AA), PLA₂ activation, represented as the regional incorporation coefficient k* of AA, was determined with quantitative autoradiography in each of 71 brain regions.Results In SERT+/+ mice, DOI significantly increased k* in 27 regions known to have 5-HT_2A/2C receptors, including the frontal, motor, somatosensory, pyriform and cingulate cortex, white matter, nucleus accumbens, caudate putamen, septum, CA1 of hippocampus, thalamus, and hypothalamus. In contrast, DOI did not increase k* significantly in any brain region of SERT–/– mice. Head twitches following DOI, which also were measured, were robust in SERT+/+ mice but were markedly attenuated in SERT–/– mice.Conclusions These results show that a lifelong elevation of the synaptic 5-HT concentration in SERT–/– mice leads to downregulation of 5-HT_2A/2C receptor-mediated PLA₂ signaling via AA and of head twitches, in response to DOI.     

8-OH-DPAT, a 5-HT1A agonist and ritanserin, a 5-HT2A/C antagonist, reverse haloperidol-induced catalepsy in rats independently of striatal dopamine release

In this study, both catalepsy and changes in extracellular levels of striatal dopamine (DA) and dihydroxyphenyl acetic acid (DOPAC) induced by the typical neuroleptic haloperidol (HAL) were simultaneously assessed, using intracerebral microdialysis in freely moving rats, in the presence of either the 5-HT_1A agonist 8-OH-DPAT or the 5-HT_2A/C antagonist ritanserin. HAL (1 mg/kg, SC) elicited a strong cataleptic state, reaching its maximal intensity (about 240 s) 2 h after the drug administration. This effect was paralleled by a long-lasting enhancement of striatal DA and DOPAC extracellular levels, reaching 230 and 350% of basal values, respectively. 8-OH-DPAT (0.1 mg/kg, SC) given 2.5 h after, and ritanserin (0.63 and 1.25 mg/kg, IP), given 15 min prior to HAL, significantly reduced the neuroleptic-induced catalepsy. However, both 5-HT agents failed to modify basal DA and DOPAC striatal outflow as well as the stimulatory effect of HAL on these parameters. It can thus be concluded that the anticataleptic effect of these compounds is not related to an alteration of DA release within the striatum. Received: 29 August 1996/Final version: 25 November 1996     

Differential regulation of rat peripheral 5-HT<Subscript>2A</Subscript> and 5-HT<Subscript>2B</Subscript> receptor systems: influence of drug treatment

Most studies of 5-HT₂ receptor regulation have been carried out on the central nervous system (CNS) (which expresses 5-HT_2A and 5-HT_2C receptors); very few in vitro studies have addressed the peripheral receptors 5-HT_2A and 5-HT_2B. The aim of this investigation was to compare the possible short- and long-term processes regulating these peripheral receptors in the rat.The in vitro contractile response elicited by serotonin (5-HT, 10 µM) in the rat gastric fundus (5-HT_2B receptor system) was rapid and followed by a partial fade to a steady state, in contrast with the rat thoracic aorta response (5-HT_2A receptor system), which was more stable, slower and sustained. To characterize drug-receptor interactions, cumulative concentration/response curves (CCRCs) for 5-HT were constructed ex vivo for rat tissues treated with drugs acting at these receptors. Rats were examined 4 or 24 h after a single, i.p. administration of (±)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(±)DOI, 1 or 2.5 mg/kg], clozapine, cyproheptadine or rauwolscine (10 mg/kg), 48 h after a single i.p. administration of (±)DOI (2.5 mg/kg), clozapine or cyproheptadine (10 mg/kg) or 24 h after the last of with 15 daily i.p. administrations of (±)DOI (1 or 2.5 mg/kg), clozapine, cyproheptadine or rauwolscine (10 mg/kg). In the aorta, E_max (the maximum response elicited by 5-HT) was unchanged 4 h after a single dose of any of the drugs tested. However, 24 h after a single dose, E_max was lower in animals treated with (±)DOI (2.5 mg/kg), clozapine or cyproheptadine than in controls, whilst 48 h after a single dose of (±)DOI (2.5 mg/kg), clozapine or cyproheptadine there was no difference in E_max between experimental and control animals. After chronic treatment with (±)DOI (2.5 mg/kg), clozapine and cyproheptadine, E_max was lower than in controls. In the gastric fundus, E_max 4 h after a single dose of each drug was lower than in controls, and the response recovered by 24 or 48 h. Following chronic treatment, E_max was significantly lower than in controls for each drug used.These findings suggest first, that regulation of peripheral 5-HT₂ receptors (5-HT_2A and 5-HT_2B) is a functionally significant phenomenon in vivo, and occurs after administration of both agonists and antagonists. Second, the kinetics of peripheral 5-HT₂ receptor regulation were similar in both in vivo and ex vivo experiments. The 5-HT_2B receptors in rat gastric fundus are more sensitive to drug-induced regulation than the 5-HT_2A rat aortic receptors. Finally, long-term regulation of both receptors stabilizes short-term desensitization for longer.     

Effect of 5-HT<Subscript>3</Subscript> antagonists and a 5-HT<Subscript>1A</Subscript> agonist on fluoxetine-induced conditioned gaping reactions in rats

Rationale  The effect of manipulation of the serotonin (5-HT) system on conditioned gaping (presumably reflective of nausea in rats) was evaluated. Objective  The potential of the selective serotonin reuptake inhibitor (SSRI), fluoxetine (which produces nausea in the clinic), to produce conditioned gaping in rats and of the 5-HT₃ antagonists (ondansetron and palonosetron) and the 5-HT_1A autoreceptor agonist (8-OH-DPAT) to reverse this effect were evaluated. Materials and methods  In each of four experiments, rats received three pairings of intraorally delivered 17% sucrose solution and fluoxetine (0, 2, 10 or 20 mg/kg) and 72 h later were given a drug-free test trial. In experiment 2, rats were pretreated with the 5-HT₃ antagonists, ondansetron (0, 0.1 or 1.0 mg/kg) or the longer acting palonosetron (0.1 mg/kg), 30 min before each of three sucrose–fluoxetine (20 mg/kg) pairings. In experiment 3, rats were injected with palonosetron (0.1 mg/kg) 2 h before each of three sucrose–fluoxetine (20 mg/kg) or sucrose–lithium chloride (LiCl, 25 mg/kg) pairings. In experiment 4, rats were pretreated with the 5-HT_1A autoreceptor agonist, 8-OH-DPAT (DPAT, 0.1 mg/kg) 30 min before each of three sucrose–fluoxetine (20 mg/kg) pairings. Results  After two sucrose–fluoxetine pairings, the highest dose of fluoxetine tested (20 mg/kg) produced conditioned gaping reactions. These conditioned gaping reactions were prevented by pretreatment with DPAT, but not with the 5-HT₃ antagonists. On the other hand, palonosetron administered 2 h prior to sucrose–LiCl pairings attenuated conditioned gaping reactions. Conclusions  These results suggest that the conditioned nausea produced by SSRIs, but not LiCl, may be resistant to treatment with 5-HT₃ antagonists, but not 5-HT_1A autoreceptor agonists.     

Simultaneous blockade of 5-HT<Subscript>1A/B</Subscript> receptors and 5-HT transporters results in acute increases in extracellular 5-HT in both rats and guinea pigs: in vivo characterization of the novel 5-HT<Subscript>1A/B</Subscript> receptor antagonist/5-HT transport inhibitor SB-649915-B

Rationale The delay in onset and treatment resistance of subpopulations of depressed patients to conventional serotonin reuptake inhibitors has lead to new drug development strategies to produce agents with improved antidepressant efficacy. Objectives We report the in vivo characterization of the novel 5-HT_1A/1B autoreceptor antagonist/5-HT transporter inhibitor (6-[(1-{2-[(2-methyl-5-quinolinyl)oxy]ethyl}-4-piperidinyl)methyl]-2H-1,4-benzoxazin-3(4H)-one), SB-649915-B. Materials and methods Ex vivo binding was used to ascertain 5-HT_1A receptor and serotonin transporter occupancy. 8-OH-DPAT-induced hyperlocomotion and SKF-99101-induced elevation of seizure threshold were used as markers of central blockade of 5-HT_1A and 5-HT_1B receptors, respectively. In vivo electrophysiology in the rat dorsal raphe and microdialysis in freely moving guinea pigs and rats were used to evaluate the functional outcome of SB-649915-B. Results SB-649915-B (1–10 mg/kg p.o.) produced a dose-related inhibition of 5-HT_1A receptor radioligand binding and inhibited ex vivo [³H]5-HT uptake in both guinea pig and rat cortex. SB-649915-B (0.1–10 mg/kg p.o.) reversed both 8-OH-DPAT-induced hyperlocomotor activity and SKF-99101-induced elevation of seizure threshold in the rat, demonstrating in vivo blockade of both 5-HT_1A and 5-HT_1B receptors, respectively. SB-649915-B (0.1–3 mg/kg i.v.) produced no change in raphe 5-HT neuronal cell firing per se but attenuated the inhibitory effect of 8-OH-DPAT. Acute administration of SB-649915-B resulted in increases (approximately two- to threefold) in extracellular 5-HT in the cortex of rats and the dentate gyrus and cortex of guinea pigs. Conclusions Based on these data, one may speculate that the 5-HT autoreceptor antagonist/5-HT transport inhibitor SB-649915-B will have therapeutic efficacy in the treatment of affective disorders with the potential for a faster onset of action compared to current selective serotonin reuptake inhibitors.     

Effects of SR 48692, a selective non-peptide neurotensin receptor antagonist,on two dopamine-dependent behavioural responses in mice and rats

One major mechanism underlying the central action of neurotensin is an interaction with the function of dopamine (DA)-containing neurons. In addition, direct or indirect DA agonists have been reported to promote neurotensin release. We have found that SR 48692, a non-peptide neurotensin receptor antagonist (0.04 – 0.64 mg/kg orally), antagonizes (50–65%) yawning induced by apomorphine (0.07 mg/kg SC) or bromocriptine (2 mg/kg IP) in rats, and turning behaviour induced by intrastriatal injection of apomorphine (0.25 µg), (+) SKF 38393 (0.1 µg), bromocriptine (0.01 ng) or (+) amphetamine (10 µg) in mice. Other apomorphine-induced effects in mice and rats such as climbing, hypothermia, hypo- and hyper-locomotion, penile erections and stereotypies were not significantly modified by SR 48692. Taken together, these data suggest that neurotensin may play a permissive role in the expression of some but not all behavioural responses to DA receptor stimulation.