The role of 5-HT(1B/1D) receptors in the modulation of 5-hydroxytryptamine levels in the frontal cortex of the conscious guinea pig

Clozapine impaired glucose-stimulated insulin secretion partly by increasing plasma 5-HT levels due to the inhibition of OCT1-mediated hepatic 5-HT uptake in mice

Patients taking atypical antipsychotics (AAPs), especially clozapine, are often associated with hyperglycaemia. Here, clozapine served as a representative agent for investigating how AAPs induce hyperglycaemia. In normal mice and mice fed a high fat diet (HFD), clozapine impaired glucose tolerance and glucose-stimulated insulin secretion (GSIS) following intraperitoneal glucose administration and increased plasma 5-HT levels. Intraperitoneal 5-HT administration also impaired glucose tolerance and GSIS in mice. In INS-1 cells, high 5-HT levels impaired GSIS, which was attenuated by the 5-HTR3 antagonist tropisetron or by silencing 5-HTR3a. The 5-HTR2a agonist TCB2 attenuated clozapine-induced GSIS impairment. Silencing 5-HTR2a or the 5-HTR2a antagonist ketanserin impaired GSIS. In mice, 5-HT administration impaired GSIS, which was attenuated by tropisetron but aggravated by clozapine. Clozapine increased plasma [2H]5-HT exposure following intravenous administration to mice. In HEK293-OCT1 cells, clozapine inhibited [2H]5-HT and MPP+ uptake. Clozapine or OCT1 silencing impaired 5-HT metabolism in mouse primary hepatocytes, demonstrating that clozapine increased plasma 5-HT levels via the inhibition of OCT1-mediated hepatic 5-HT uptake. Liver-specific silencing of OCT1 increased plasma [2H]5-HT exposure and 5-HT levels and impaired GSIS and glucose tolerance in mice. In conclusion, clozapine impaired GSIS and glucose tolerance by increasing plasma 5-HT levels via the inhibition of OCT1-mediated hepatic 5-HT uptake. Increased 5-HT impaired GSIS by activating islet 5-HTR3a. The antagonistic effect of clozapine on islet 5-HTR2a also contributed to GSIS impairment. The finding that clozapine-induced GSIS impairment was attributed to increased 5-HT levels via the inhibition of OCT1-mediated hepatic 5-HT uptake may partly explain hyperglycaemia caused by other AAPs.

Altered Serotonin 1B Receptor Binding After Escitalopram for Depression Is Correlated With Treatment Effect

BACKGROUND

Major depressive disorder (MDD) is commonly treated with selective serotonin reuptake inhibitors (SSRIs). SSRIs inhibit the serotonin transporter (5-HTT), but the downstream antidepressant mechanism of action of these drugs is poorly understood. The serotonin 1B (5-HT1B) receptor is functionally linked to 5-HTT and 5-HT1B receptor binding and 5-HT1B receptor mRNA is reduced in the raphe nuclei after SSRI administration in primates and rodents, respectively. The effect of SSRI treatment on 5-HT1B receptor binding in patients with MDD has not been examined previously. This positron emission tomography (PET) study aimed to quantify brain 5-HT1B receptor binding changes in vivo after SSRI treatment for MDD in relation to treatment effect.

METHODS

Eight unmedicated patients with moderate to severe MDD underwent PET with the 5-HT1B receptor radioligand [11C]AZ10419369 before and after 3 to 4 weeks of treatment with the SSRI escitalopram 10 mg daily. Depression severity was assessed at time of PET and after 6 to 7 weeks of treatment with the Montgomery-Åsberg Depression Rating Scale.

RESULTS

We observed a significant reduction in [11C]AZ10419369 binding in a dorsal brainstem (DBS) region containing the median and dorsal raphe nuclei after escitalopram treatment (P = .036). Change in DBS [11C]AZ10419369 binding correlated with Montgomery-Åsberg Depression Rating Scale reduction after 3-4 (r = 0.78, P = .021) and 6-7 (r = 0.94, P < .001) weeks' treatment.

CONCLUSIONS

Our findings align with the previously reported reduction of 5-HT1B receptor binding in the raphe nuclei after SSRI administration and support future studies testing change in DBS 5-HT1B receptor binding as an SSRI treatment response marker.

Multiple facets of serotonergic modulation

The serotonergic system of the central nervous system (CNS) has been implicated in a broad range of physiological functions and behaviors, such as cognition, mood, social interaction, sexual behavior, feeding behavior, sleep-wake cycle and thermoregulation. Serotonin (5-hydroxytryptamine, 5-HT) establishes a plethora of interactions with neurochemical systems in the CNS via its numerous 5-HT receptors and autoreceptors. The facets of this control are multiple if we consider the molecular actors playing a role in the autoregulation of 5-HT neuron activity including the 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_2B, 5-HT₇ receptors as well as the serotonin transporter. Moreover, extrinsic loops involving other neurotransmitters giving the other 5-HT receptors the possibility to impact 5-HT neuron activity. Grasping the complexity of these interactions is essential for the development of a variety of therapeutic strategies for cognitive defects and mood disorders. Presently we can illustrate the plurality of the mechanisms and only conceive that these 5-HT controls are likely not uniform in terms of regional and neuronal distribution. Our understanding of the specific expression patterns of these receptors on specific circuits and neuronal populations are progressing and will expand our comprehension of the function and interaction of these receptors with other chemical systems. Thus, the development of new approaches profiling the expression of 5-HT receptors and autoreceptors should reveal additional facets of the 5-HT controls of neurochemical systems in the CNS.

5-HT Receptor Antagonism Attenuates the Ischemia-Reperfusion Injury After Rabbit Lung Preservation

The success of lung transplantation is threatened by the appearance of ischemia-reperfusion injury, which is characterized by increased vascular permeability. 5-Hydroxytryptamine (5-HT; serotonin) is known to produce microvascular leakage in the systemic circulation, but its possible role in ischemia-reperfusion injury after lung preservation has not been reported. In this work we measured the release of 5-HT during a 24-hour rabbit lung preservation, and the effect of methiothepin (antagonist of the majority of 5-HT receptors) and SB204741 (antagonist of 5-HT2B/2C receptors) on the modified capillary filtration coefficient (mKf,c) was evaluated at the end of this period. Our results showed that the highest release rate of 5-HT occurred during the first 15 minutes after the lung harvesting and progressively decreased in the following time intervals. The baseline mKf,c greatly increased after 24 hours of lung preservation, and this increment was partially reduced by methiothepin and even more by SB204741. We concluded that 5-HT may play an important role in the ischemia-reperfusion process after lung preservation.

Serotonergic Regulation of Prefrontal Cortical Circuitries Involved in Cognitive Processing: A Review of Individual 5-HT Receptor Mechanisms and Concerted Effects of 5-HT Receptors Exemplified by the Multimodal Antidepressant Vortioxetine

It has been known for several decades that serotonergic neurotransmission is a key regulator of cognitive function, mood, and sleep. Yet with the relatively recent discoveries of novel serotonin (5-HT) receptor subtypes, as well as an expanding knowledge of their expression level in certain brain regions and localization on certain cell types, their involvement in cognitive processes is still emerging. Of particular interest are cognitive processes impacted in neuropsychiatric and neurodegenerative disorders. The prefrontal cortex (PFC) is critical to normal cognitive processes, including attention, impulsivity, planning, decision-making, working memory, and learning or recall of learned memories. Furthermore, serotonergic dysregulation within the PFC is implicated in many neuropsychiatric disorders associated with prominent symptoms of cognitive dysfunction. Thus, it is important to better understand the overall makeup of serotonergic receptors in the PFC and on which cell types these receptors mediate their actions. In this Review, we focus on 5-HT receptor expression patterns within the PFC and how they influence cognitive behavior and neurotransmission. We further discuss the net effects of vortioxetine, an antidepressant acting through multiple serotonergic targets given the recent findings that vortioxetine improves cognition by modulating multiple neurotransmitter systems.

Modifications of plasma 5-HT concentrations during the allergic bronchoconstriction in guinea pigs

Several contractile mediators involved in the antigen-induced airway obstruction have been identified, but the role of 5-HT (5-hydroxytryptamine or serotonin) has been scantily investigated. In this work, the potential role of 5-HT in the allergic bronchoconstriction was evaluated through a pharmacological approach and plasma 5-HT measurement in blood samples from the right and left ventricles of anesthetized guinea-pigs. Intravenous 5-HT caused a dose-dependent increase of the lung resistance in anesthetized, nonsensitized guinea pigs. Likewise, in sensitized animals the antigenic challenge with ovalbumin also caused a transient bronchoconstriction (356 ± 60% the basal value), which was largely inhibited by the blockade of serotonergic receptors with methiothepin plus tropisetron (134 ± 10%, P = .007). Sensitized animals tended to have plasma 5-HT concentrations higher than nonsensitized controls, and shortly after the peak of the allergic bronchoconstriction the 5-HT levels in the left ventricle (blood flowing out from lungs) tended to be higher than in the right ventricle (blood entering the lungs), although data dispersion precluded the obtaining of statistical significance. Interestingly, the degree of bronchoconstriction highly correlated with the concentrations of 5-HT found in the left ventricle and measured either in platelet-rich plasma (r = 0.97 P = .007) or platelet-poor plasma (r = 0.97, P = .006). After the obstructive response subsided these correlations were lost, but now the degree of bronchoconstriction turned to be correlated with 5-HT concentration in platelet concentrate (r = 0.76, P = .03). In conclusion, our results suggested that 5-HT is actively released from lungs during the antigenic challenge and that this autacoid is involved in the generation of the airway obstruction.

Role of 5-HT2A, 5-HT4 and 5-HT7 receptors in the antigen-induced airway hyperresponsiveness in guinea-pigs

BACKGROUND

A possible role of 5-hydroxytryptamine (5-HT) in the origin of antigen-induced airway hyperresponsiveness (AI-AHR) has been scarcely investigated.

OBJECTIVE

To explore the participation of different 5-HT receptors in the development of AI-AHR in guinea-pigs.

METHODS

Lung resistance was measured in anaesthetized guinea-pigs sensitized to ovalbumin (OVA). Dose-response curves to intravenous (i.v.) acetylcholine (ACh) were performed before and 1 h after antigenic challenge and expressed as the 200% provocative dose (PD(200)). Organ bath experiments, confocal microscopy and RT-PCR were additionally used. The 5-HT content in lung homogenates was measured by HPLC.

RESULTS

Antigenic challenge significantly decreased PD(200), indicating the development of AI-AHR. This hyperresponsiveness was abolished by a combination of methiothepin (5-HT(1)/5-HT(2)/5-HT(5)/5-HT(6)/5-HT(7) receptors antagonist) and tropisetron (5-HT(3)/5-HT(4) antagonist). Other 5-HT receptor antagonists showed three different patterns of response. Firstly, WAY100135 (5-HT(1A) antagonist) and ondansetron (5-HT(3) antagonist) did not modify the AI-AHR. Secondly, SB269970 (5-HT(7) antagonist), GR113808 (5-HT(4) antagonist), tropisetron or methiothepin abolished the AI-AHR. Thirdly, ketanserin (5-HT(2A) antagonist) produced airway hyporresponsiveness. Animals with bilateral vagotomy did not develop AI-AHR. Experiments in tracheal rings showed that pre-incubation with LP44 or cisapride (agonists of 5-HT(7) and 5-HT(4) receptors, respectively) induced a significant increase of the cholinergic contractile response to the electrical field stimulation. In sensitized lung parenchyma strips, ketanserin diminished the contractile responses to ACh. Sensitization was associated with a ninefold increase in the 5-HT content of lung homogenates. Confocal microscopy showed that sensitization enhanced the immunolabelling and co-localization of nicotinic receptor and 5-HT in airway epithelium, probably located in pulmonary neuroendocrine cells (PNECs). RT-PCR demonstrated that neither sensitization nor antigen challenge modified the 5-HT(2A) receptor mRNA levels.

CONCLUSIONS

Our results suggested that 5-HT was involved in the development of AI-AHR to ACh in guinea-pigs. Specifically, 5-HT(2A), 5-HT(4) and 5-HT(7) receptors seem to be particularly involved in this phenomenon. Participation of 5-HT might probably be favoured by the enhancement of the PNECs 5-HT content observed after sensitization.

5-HT(1B) receptor regulation of serotonin (5-HT) release by endogenous 5-HT in the substantia nigra

Axonal release of serotonin (5-hydroxytryptamine, 5-HT) in the CNS is typically regulated by presynaptic 5-HT autoreceptors. Release of 5-HT in substantia nigra pars reticulata (SNr), a principal output from the basal ganglia, has seemed an interesting exception to this rule. The SNr receives one of the highest densities of 5-HT innervation in mammalian brain and yet negative feedback regulation of axonal 5-HT release by endogenous 5-HT has not been identified here. We explored whether we could identify autoregulation of 5-HT release by 5-HT(1B) receptors in rat SNr slices using fast-scan cyclic voltammetry at carbon-fiber microelectrodes to detect 5-HT release evoked by discrete stimuli (50 Hz, 20 pulses) paired over short intervals (1-10 s) within which any autoreceptor control should occur. Evoked 5-HT release exhibited short-term depression after an initial stimulus that recovered by 10 s. Antagonists for 5-HT(1B) receptors, isamoltane (1 microM) or SB 224-289 (1 microM), did not modify release during a stimulus train, but rather, they modestly relieved depression of subsequent release evoked after a short delay (< or =2 s). Release was not modified by antagonists for GABA (picrotoxin, 100 microM, saclofen, 50 microM) or histamine-H(3) (thioperamide, 10 microM) receptors. These data indicate that 5-HT release can activate a 5-HT(1B)-receptor autoinhibition of subsequent release, which is mediated directly via 5-HT axons and not via GABAergic or histaminergic inputs. These data reveal that 5-HT release in SNr is not devoid of autoreceptor regulation by endogenous 5-HT, but rather is under modest control which only weakly limits 5-HT signaling.

Spatial-specific action of serotonin within the leech midbody ganglion

Serotonin is a conspicuous neuromodulator in the nervous system of many vertebrates and invertebrates. In previous experiments performed in the leech nervous system, we compared the effect of the amine released from endogenous sources [using selective serotonin reuptake inhibitors (SSRIs), e.g. fluoxetine] with that of bath-applied serotonin. The results suggested that the amine does not reach all its targets in a uniform way, but produces the activation of an interneuronal pathway that generated specific synaptic responses on different neurons. Taking into account that the release of the amine is often regulated at the presynaptic level, we have investigated whether autoreceptor antagonists mimic the SSRIs effect. We found that methiothepin (100 microM) produced similar effects than fluoxetine. To further test the hypothesis that endogenous serotonin produce its effect by acting locally at specific sites, we analyzed the effect of iontophoretic applications of serotonin. We found a site in the neuropil of the leech ganglia where serotonin application mimicked the effect of the SSRIs and the 5-HT antagonist. The results further support the view that the effect of serotonin exhibits a spatial specificity that can be relevant to understand its modulatory actions.

Characterisation of the selective 5-HT1B receptor antagonist SB-616234-A (1-[6-(cis-3,5-dimethylpiperazin-1-yl)-2,3-dihydro-5-methoxyindol-1-yl]-1-[2′-methyl-4′-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]methanone hydrochloride): In vivo neurochemical and behavioural evidence of anxiolytic/antidepressant activity

The 5-HT1B receptor has attracted significant interest as a potential target for the development of therapeutics for the treatment of affective disorders such as anxiety and depression. Here we present the in vivo characterisation of a novel, selective and orally bioavailable 5-HT1B receptor antagonist, SB-616234-A (1-[6-(cis-3,5-dimethylpiperazin-1-yl)-2,3-dihydro-5-methoxyindol-1-yl]-1-[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]methanone hydrochloride). SB-616234-A reversed the 5-HT1/7 receptor agonist, SKF-99101H-induced hypothermia in guinea pigs in a dose related manner with an ED50 of 2.4 mg/kg p.o. Using in vivo microdialysis in freely moving guinea pigs, SB-616234-A (3-30 mg/kg p.o.) caused a dose-related increase in extracellular 5-HT in the dentate gyrus. Evaluation of antidepressant- and anxiolytic-like effects of this 5-HT1B receptor antagonist was performed in a variety of models and species. SB-616234-A produced a decrease in immobility time in the mouse forced swim test; an effect suggestive of antidepressant activity. Furthermore, SB-616234-A produced dose-related anxiolytic effects in both rat and guinea pig maternal separation-induced vocalisation models with an ED50 of 1.0 and 3.3 mg/kg i.p., respectively (vs fluoxetine treatment ED50 = 2.2 mg/kg i.p. in both species). Also a significant reduction in posturing behaviours was observed in the human threat test in marmosets; an effect indicative of anxiolytic activity. In summary, SB-616234-A is a novel, potent and orally bioavailable 5-HT1B receptor antagonist which exhibits a neurochemical and behavioural profile that is consistent with both anxiolytic- and antidepressant-like activity in a variety of species. Taken together these data suggest that SB-616234-A may have therapeutic efficacy in the treatment of affective disorders.

5-HT(1B) receptors, ventral orbitofrontal cortex, and aggressive behavior in mice

RATIONALE

Systemic injections of 5-HT(1B) receptor agonists have been shown to have specific anti-aggressive effects in aggressive individuals. One site of action for these drugs is the 5-HT(1B) receptors in the ventral orbitofrontal cortex (VO PFC), an area that has been implicated in the inhibitory control of behavior and is a terminal region for 5-HT projections.

OBJECTIVE

To assess the anti-aggressive effects of the 5-HT(1B) receptor agonist CP-94,253 when microinjected into the VO PFC (0.1, 0.56, and 1.0 microg/0.2 microl) or into the infralimbic prefrontal cortex (IL PFC; 1.0 microg/0.2 microl) in separate groups of aggressive resident male mice. To confirm the 5-HT(1B) receptor as the critical site of action for the anti-aggressive effects, the 5-HT(1B/D) antagonist GR-127,935 was microinjected at 10.0 microg/0.2 microl into the VO PFC. After recovery from surgery, the anti-aggressive effects of microinjected CP-94,253 were studied during 5-min resident-intruder confrontations that were recorded and analyzed.

RESULTS

Microinjections of CP-94,253 (0.56 and 1.0 microg/0.2 microl) dose-dependently reduced the frequency of attack bites and sideways threats. This effect was behaviorally specific because non-aggressive motor activities were not significantly altered by the drug. In the IL vmPFC or in an area lateral to the VO PFC, CP-94,253 (1.0 microg/0.2 microl) did not have significant behavioral effects.

CONCLUSIONS

The results highlight the 5-HT(1B) receptors in the VO PFC as a particularly important site for the inhibition of species-typical aggressive behavior in male mice.

Innovative approaches for the development of antidepressant drugs: current and future strategies

Depression is a highly debilitating disorder that has been estimated to affect up to 21% of the world population. Despite the advances in the treatment of depression with selective serotonin reuptake inhibitors (SSRIs) and serotonin and norepinephrine reuptake inhibitors (SNRIs), there continue to be many unmet clinical needs with respect to both efficacy and side effects. These needs range from efficacy in treatment resistant patients, to improved onset, to reductions in side effects such as emesis or sexual dysfunction. To address these needs, there are numerous combination therapies and novel targets that have been identified that may demonstrate improvements in one or more areas. There is tremendous diversity in the types of targets and approaches being taken. At one end of a spectrum is combination therapies that maintain the benefits associated with SSRIs but attempt to either improve efficacy or reduce side effects by adding additional mechanisms (5-HT1A, 5-HT1B, 5-HT1D, 5-HT2C, alpha-2A). At the other end of the spectrum are more novel targets, such as neurotrophins (BDNF, IGF), based on recent findings that antidepressants induce neurogenesis. In between, there are many approaches that range from directly targeting serotonin receptors (5-HT2C, 5-HT6) to targeting the multiplicity of potential mechanisms associated with excitatory (glutamate, NMDA, mGluR2, mGluR5) or inhibitory amino acid systems (GABA) or peptidergic systems (neurokinin 1, corticotropin-releasing factor 1, melanin-concentrating hormone 1, V1b). The present review addresses the most exciting approaches and reviews the localization, neurochemical and behavioral data that provide the supporting rationale for each of these targets or target combinations.

SB-616234-A (1-[6-(cis-3,5-dimethylpiperazin-1-yl)-2,3-dihydro-5-methoxyindol-1-yl]-1-[2'methyl-4'-(5-methyl-1,2,3-oxadiazol-3-yl)biphenyl-4-yl]methanone hydrochloride): a novel, potent and selective 5-HT1B receptor antagonist

SB-616234-A possesses high affinity for human 5-HT1B receptors stably expressed in Chinese hamster ovary (CHO) cells (pKi 8.3+/-0.2), and is over 100-fold selective for a range of molecular targets except h5-HT1) receptors (pKi 6.6+/-0.1). Similarly, affinity (pKi) for rat and guinea pig striatal 5-HT1B receptors is 9.2+/-0.1. In [35S]-GTPgammaS binding studies in the human recombinant cell line, SB-616234-A acted as a high affinity antagonist with a pA2 value of 8.6+/-0.2 whilst providing no evidence of agonist activity in this system. In [35S]-GTPgammaS binding studies in rat striatal membranes, SB-616234-A acted as a high affinity antagonist with an apparent pKB of 8.4+/-0.5, again whilst providing no evidence of agonist activity in this system. SB-616234-A (1 microM) potentiated electrically stimulated [3H]-5-HT release from guinea pig and rat cortical slices (S2/S1) ratios of 1.8 and 1.6, respectively). SB-616234-A (0.3-30 mg kg(-1) p.o.) caused a dose-dependent inhibition of ex vivo [3H]-GR125743 binding to rat striatal 5-HT1B receptors with an ED50 of 2.83+/-0.39 mg kg(-1) p.o. Taken together these data suggest that SB-616234-A is a potent and selective 5-HT(1B) autoreceptor antagonist that occupies central 5-HT1B receptors in vivo following oral administration.

Escalated aggressive behavior: Dopamine, serotonin and GABA

The ethical dilemma in aggression research is how to reconcile two divergent objectives, namely to avoid harm and injury as much as possible and, at the same time, how to study behavioral phenomena that validly represent the essence of the neurobiology of aggression. Clinical and preclinical aggression research focuses on different types of aggression. Preclinical studies are usually stimulated by an ethological approach and focus on the phylogeny, ontogeny, survival value and neural mechanisms of ritualized displays and signals. On the other hand, clinical studies focus on violent individuals and pathologically excessive forms of aggressive behavior. This review emphasizes research on escalated forms of aggression in animals and humans and their pharmacotherapy. The current experimental models to generate escalated levels of aggressive behavior in laboratory rely on social instigation, frustrative non-reward and alcohol drinking. These types of aggression are modulated by canonical neurotransmitters like dopamine, serotonin (5-HT) and GABA. It continues to be a main goal of much neurobiological research to find potential targets of pharmacological agents that interact with dopaminergic, GABAergic and serotonergic systems and have high efficacy and selectivity to reduce excessive levels of aggressive and violent behaviors without side-effects. While the mesocorticolimbic dopamine system is implicated in the initiation, execution, termination and consequences of aggressive behavior, drugs with a high affinity for dopamine D2 receptors lack specificity for reducing aggressive behavior. Current investigations point to 5-HT(1B) receptor subtypes as particularly relevant. First, they are differentially expressed in aggression-prone individuals relative to those who are not excessively aggressive. Second, these and also other 5-HT receptor subtypes emerge to be significant targets for anti-aggressive interventions. Positive modulators of GABA(A) receptors with specific subunit configuration may be relevant for heightening aggression, and these sites may be targets for intervention. A prerequisite for rational pharmacotherapies will be adequate characterization of serotonergic and GABAergic receptor regulation in individuals exhibiting escalated aggression.

Synthesis and SAR of highly potent dual 5-HT1A and 5-HT1B antagonists as potential antidepressant drugs

Novel 5-HT(1) autoreceptor ligands based on the N-4-aryl-piperazinyl-N'-ethyl-5,6,7,8-tetrahydropyrido[4', 3':4,5]thieno[2,3-d]pyrimidin-4(3H)-one core are described. Aiming at antidepressants with a novel mode of action our objective was to identify potent antagonists showing balanced affinities and high selectivity for the 5-HT(1A) and 5-HT(1B) receptors. Strategies for the development of dual 5-HT(1A) and 5-HT(1B) antagonists based on 1 and 2 as leads and the corresponding results are discussed. Isoquinoline analogue 33 displayed high affinity and an antagonistic mode of action for the 5-HT(1A) and the 5-HT(1B) receptors and was characterized further with respect to selectivity, electrically stimulated [(3)H]5-HT release and in vivo efficacy.

Differential autoreceptor control of extracellular 5-HT in guinea pig and rat: species and regional differences

RATIONALE

Central 5-hydroxytryptamine (5-HT) release is regulated by inhibitory 5-HT autoreceptors, including 5-HT(1A) and 5-HT(1B) receptors.

OBJECTIVES

The purpose of this study was to use combinations of selective autoreceptor antagonists to elucidate the role of these receptors in controlling extracellular 5-HT in terminal areas. METHODS. Microdialysis was carried out in awake rats and guinea pigs to measure extracellular 5-HT in the frontal cortex and dentate gyrus. Using the selective 5-HT(1A) receptor antagonist, WAY-100635, and the selective 5-HT(1B) receptor antagonist, SB-224289, we have compared the roles of 5-HT(1A) and 5-HT(1B) autoreceptors in controlling extracellular 5-HT.

RESULTS

SB-224289 (4 mg/kg i.p.) alone produced a significant 50% increase in extracellular 5-HT in the dentate gyrus of guinea pigs, but not in the frontal cortex of the same animals. Co-administration of WAY-100635 (0.3 mg/kg s.c.), did not change the SB-224289-induced increase in dentate gyrus 5-HT but did produce a significant augmentation (60% increase) of guinea pig frontal cortex 5-HT. In contrast, neither autoreceptor antagonist, alone or in combination, affected extracellular 5-HT in the frontal cortex or dentate gyrus of rats.

CONCLUSIONS

These data indicate that there is a species difference in the autoreceptor control of 5-HT release. Furthermore, in the guinea pig there is a divergence between dorsal and median raphe innervated brain regions. On the basis that antagonism of 5-HT(1A) and 5-HT(1B) receptors produced an immediate increase in extracellular 5-HT in multiple brain regions in the guinea pig, it is suggested that this might be a novel mechanism for achieving antidepressant efficacy.

The effects of selective serotonin reuptake inhibitors on extracellular 5-HT levels in the hippocampus of 5-HT(1B) receptor knockout mice

The effects of two selective serotonin reuptake inhibitors on 5-hydroxy-tryptamine (5-HT) in the hippocampus were studied in wildtype and in 5-HT(1B) receptor knockout mice using in vivo microdialysis. Basal 5-HT levels in the hippocampus were not different between the two genotypes. The functional absence of 5-HT(1B) receptors was examined in the knockout mice by local infusion of the 5-HT(1B) receptor agonist, 1,4-Dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one (CP93129) into the hippocampus. CP93129 (1 microM) decreased 5-HT levels in wildtype mice, but not in 5-HT(1B) knockout mice. Systemic administration of the selective 5-HT reuptake inhibitor paroxetine (5 mg/kg, i.p.) increased extracellular 5-HT levels. The increase of 5-HT in 5-HT(1B) knockout mice was almost twofold higher than in wildtype mice. Systemic administration of selective 5-HT reuptake inhibitors stimulates both terminal 5-HT(1B) autoreceptors and somatodendritic 5-HT(1A) autoreceptors. Therefore, the selective 5-HT reuptake inhibitor, fluvoxamine, was applied locally into the hippocampus to investigate the role of the terminal 5-HT(1B) autoreceptors. Local administration of 0.3 microM fluvoxamine resulted in comparable increases in extracellular 5-HT in both genotypes, whereas 1.0 microM fluvoxamine produced a twofold greater increase in 5-HT levels in 5-HT(1B) knockout as compared to wildtype mice. In conclusion, the differences in hippocampal 5-HT output between wildtype and 5-HT(1B) knockout mice after local or systemic administration of selective 5-HT reuptake inhibitors show that 5-HT(1B) autoreceptors play a significant role in the inhibition of 5-HT release at serotonergic nerve terminals. In addition, the different dose-response to fluvoxamine suggests that 5-HT(1B) knockout mice have possible adaptations of 5-HT transporters in order to compensate for the loss of the terminal 5-HT(1B) autoreceptor.

SB-236057-A: a selective 5-HT1B receptor inverse agonist

5-HT1B autoreceptors are involved in the control of extracellular 5-HT levels from both the terminal and cell body regions of serotonergic neurons. In this manuscript we review the pharmacological and pharmacokinetic data available for the selective and potent 5-HT1B receptor inverse agonist, SB-236057-A (1'-ethyl-5-(2'-methyl-4'-(5-methyl-1,3,4-oxadiazolyl-2-yl)biphenyl-4-carbonyl)-2,3,6,7-tetrahydrospiro (furo[2,3-f]indole-3,4'-piperidine) hydrochloride). SB 236057-A has been shown to have high affinity for human 5-HT1B receptors (pK(i) = 8.2) and displays 80 or more fold selectivity for the human 5-HT1B receptor over other 5-HT receptors and a range of additional receptors, ion channels and enzymes. In functional studies at human 5-HT1B receptors SB-236057-A displayed inverse agonism (pA(2) = 8.9) using [(35)S]GTPgammaS binding, and silent antagonism (pA(2) = 9.2) using cAMP accumulation. SB-236057-A also acted as an antagonist at the 5-HT terminal autoreceptor as measured by [3H]5-HT release from electrically stimulated guinea pig and human cortical slices. In the guinea pig, pharmacokinetic analysis demonstrated that SB-236057-A was bioavailable and according to in vivo pharmacodynamic assays it enters brain and has a long duration of action. Importantly no side effect liability was evident at relevant doses from anxiogenic, cardiovascular, sedative or migraine viewpoints. In vivo microdialysis studies demonstrated that SB-236057-A is an antagonist in the guinea pig cortex but has no effect on extracellular 5-HT levels per se. In contrast, SB-236057-A increased extracellular 5-HT levels in the guinea pig dentate gyrus. This increase in 5-HT release was comparable to that observed after 14 days of paroxetine administration. SB-236057-A has been a useful tool in confirming that, in either guinea pigs or humans, the terminal 5-HT autoreceptor is of the 5-HT1B subtype. It appears that acute 5-HT1B receptor blockade, by virtue of increased 5-HT release in the dentate gyrus, may provide a rapidly acting antidepressant.

LY393558, a 5-hydroxytryptamine reuptake inhibitor and 5-HT(1B/1D) receptor antagonist: effects on extracellular levels of 5-hydroxytryptamine in the guinea pig and rat

The stimulation of terminal 5-HT(1B/1D) autoreceptors limits the effects of selective serotonin reuptake inhibitors on extracellular levels of 5-hydroxytryptamine (5-HT, serotonin) in vivo. Microdialysis studies show that acute oral administration of LY393558-a 5-HT reuptake inhibitor and antagonist at both the human 5-HT(1B) and 5-HT(1D) receptor-in the dose range 1-20 mg/kg, increases extracellular levels of 5-HT in both the guinea pig hypothalamus and rat frontal cortex. In both species, the levels of 5-HT that were attained were higher than following an acute, maximally effective dose of fluoxetine (20 mg/kg orally), reaching approximately 1500% in the guinea pig hypothalamus and 700% in the rat frontal cortex. In both species, the response to LY393558 (10 mg/kg p.o.) was impulse dependent, being absent in the presence of tetrodotoxin delivered at 1 microM via the microdialysis probe. The sensitivity to tetrodotoxin contrasted with the effects seen with DL-fenfluramine. Studies in rats showed that the microdialysate 5-HT concentration achieved in the frontal cortex after an acute challenge with LY393558 (5 mg/kg p.o.) was significantly greater than following a chronic regime of fluoxetine treatment (10 mg/kg/day orally for 21 days). Moreover, in rats chronically treated with LY393558 (5 mg/kg/day orally for 21 days), the mean basal concentration, 24 h after the final pretreatment dose, was of the same magnitude as that following chronic fluoxetine. However, in contrast to the response seen in fluoxetine-pretreated animals, a challenge dose of LY393558 still elicited a further increase in extracellular 5-HT in LY393558-pretreated animals. LY393558 is a potent 5-HT reuptake inhibitor and 5-HT(1B/1D) receptor antagonist. Microdialysis studies show that acute oral administration increases extracellular levels of 5-HT, by an impulse-dependent mechanism, above those produced by a maximally effective dose of fluoxetine, and in rats to levels only achieved following chronic fluoxetine treatment. Its neurochemical profile in vivo suggests that it may be a more effective antidepressant with the potential for producing an earlier onset of clinical activity than selective serotonin reuptake inhibitors.

Ligands for the investigation of 5-HT autoreceptor function

The existence of multiple 5-HT autoreceptors in the central nervous system is now firmly established and they have been pharmacologically identified as belonging to the 5-HT(1A), 5-HT(1B), and 5-HT(1D) receptor subtypes. In addition, 5-HT(1F), 5-HT(5A), and 5-HT(7) receptors remain as potential candidates for additional autoreceptors. The emergence of selective ligands, such as SB-224289 (5-HT(1B) receptor antagonist), BRL 15572 (5-HT(1D) receptor antagonist), GR 127935 (a mixed 5-HT(1B/1D) receptor antagonist), LY 334370 (5-HT(1F) receptor agonist), and SB-269970 (5-HT(7) receptor antagonist), has aided the characterisation of 5-HT autoreceptors and has highlighted the complexity of mechanisms which modulate the release of 5-HT.

The role of 5-HT1B receptors in the regulation of serotonin cell firing and release in the rat brain

The release of 5-HT in terminal areas of the rodent brain is regulated by 5-HT1B receptors. Here we examined the role of 5-HT1B receptors in the control of 5-HT output and firing in the dorsal raphe nucleus (DR), median raphe nucleus (MnR) and forebrain of the rat in vivo. The local perfusion (30-300 microM) of the selective 5-HT1B receptor agonist CP-93,129 to freely moving rats decreased 5-HT release in the DR and more markedly in the MnR. Likewise, 300 microM CP-93,129 reduced 5-HT output in substantia nigra pars reticulata, ventral pallidum, lateral habenula and the suprachiasmatic nucleus. The effect of CP-93,129 was prevented by SB-224289, but not by WAY-100635, selective 5-HT1B and 5-HT1A receptor antagonists, respectively. SB-224289 did not alter dialysate 5-HT in any raphe nuclei. The intravenous administration of the brain-penetrant selective 5-HT1B receptor agonist CP-94,253 (0.5-2.0 mg/kg) to anesthetized rats decreased dialysate 5-HT in dorsal hippocampus and globus pallidus, increased it in MnR and left it unaltered in the DR and medial prefrontal cortex. SB-224289, at a dose known to block 5-HT1B autoreceptor-mediated effects (5 mg/kg), did not prevent the effect of CP-94,253 on MnR 5-HT. The intravenous administration of CP-94,253 (0.05-1.6 mg/kg) to anesthetized rats increased the firing rate of MnR, but not DR-5-HT neurons. The local perfusion of CP-94,253 in the MnR showed a biphasic effect, with 5-HT reductions at 0.3-3 microM and increase at 300 microM. These results suggest that 5-HT cell firing and release in midbrain raphe nuclei (particularly in the MnR) are under control of 5-HT1B receptors. The activation of 5-HT1B autoreceptors (possibly located on 5-HT nerve endings and/or varicosities within DR and MnR) reduces 5-HT release. The effects of higher concentrations of 5-HT1B receptor agonists seem more compatible with the activation of 5-HT1B heteroreceptors on inhibitory neurons.

Serotonergic effects and extracellular brain levels of eletriptan, zolmitriptan and sumatriptan in rat brain

In vivo microdialysis was used to assess the central serotonergic effects and extracellular brain levels of the 5-HT(1B/1D) receptor agonists eletriptan, zolmitriptan and sumatriptan in rats after intravenous and intracerebral administration, while their binding affinities and functional potencies were determined at 5-HT(1B), 5-HT(1D) and 5-HT(1A) receptors. In vitro studies showed that all three triptans are high affinity, full agonists at 5-HT(1B/1D) receptors, but that sumatriptan is functionally less potent as a 5-HT(1B/1D) agonist than zolmitriptan and eletriptan. Local intracortical perfusion with the compounds via the dialysis probe decreased cortical 5-HT (5-hydroxytryptamine, serotonin) release with ED(50) values of approximately 0.1 microM for eletriptan and zolmitriptan and 0.5 microM for sumatriptan. At 3.2 mg/kg i.v., both eletriptan and zolmitriptan decreased 5-HT levels by about 35%, while sumatriptan had no effect, despite the fact that maximal sumatriptan concentrations in cortical dialysates were higher (8.8 nM at 20 min) than those of zolmitriptan (5.9 nM at 20 min) and eletriptan (2.6 nM at 40 min). The observation that eletriptan and zolmitriptan produce almost identical central serotonergic effects, after intracerebral as well as after systemic administration, is in agreement with their comparable functional 5-HT(1B/1D) receptor agonist potencies and their free levels in cortical dialysates after 3.2 mg/kg i.v. On the other hand, the lack of central serotonergic effects of 3.2 mg/kg i.v. sumatriptan is likely due to its weaker functional 5-HT(1B/1D) receptor agonist potency than eletriptan and zolmitriptan, rather than lower brain levels, consistent with sumatriptan's fivefold lower potency after intracerebral administration.

Control of dorsal raphé 5-HT function by multiple 5-HT(1) autoreceptors: parallel purposes or pointless plurality?

The serotonergic cells of the dorsal raphé nucleus innervate much of the forebrain and are thought to be involved in the mechanism of action of antidepressants. Dysfunction of these cells might be involved in the neural mechanisms underlying depression and suicide. The traffic in pathways emanating from the dorsal raphé nucleus is controlled by 5-HT(1) autoreceptors. Until recently it was thought that the autoreceptors in the dorsal raphé nucleus were solely of the 5-HT(1A) subtype. In this article, we discuss evidence that the situation is more complex and that multiple 5-HT(1) subtypes govern different aspects of 5-HT function in the dorsal raphé nucleus presenting new therapeutic opportunities.

The possible role of 5-HT(1B/D) receptors in psychiatric disorders and their potential as a target for therapy

Serotonin (5-hydroxytryptamine, 5-HT) is implicated in several psychiatric diseases. Is this also true for 5-HT(1B/D) receptors? These receptors are found in high density in substantia nigra, globus pallidus, striatum and basal ganglia and in other brain regions. This ubiquity makes 5-HT(1B/D) receptors responsible for many physiological and behavioural functions. This review focuses on the role of 5-HT(1B) receptors in the regulation of 5-HT release and synthesis. Microdialysis experiments performed on freely moving animals are an interesting in vivo model to study the function of the terminal 5-HT(1B) autoreceptor. Synthesis of 5-HT, estimated by the measurement of the accumulation of 5-hydroxytryptophan (5-HTP) ex vivo or in vitro, is modulated by the 5-HT(1B) autoreceptors. Many reports have shown that chronic administration with selective serotonin reuptake inhibitors leads to the desensitisation of the terminal 5-HT(1B) autoreceptors. With the help of some animal models of depression and anxiety and with some data from clinical studies it has been hypothesised that 5-HT(1B) receptors may be supersensitive in depression, anxiety and obsessive compulsive disorder. Thus, since the dysfunction of 5-HT(1B) receptors may be involved in some pathological states, particularly in the psychiatric field, these receptors represent important potential targets for drugs to treat mental diseases.

Amplification of cortical serotonin release: a further neurochemical action of the vigilance-promoting drug modafinil

The present in vitro and in vivo studies examined the effects of modafinil on serotonergic transmission in the rat frontal cortex. In the in vitro study modafinil (0.3-30 microM) increased electrically-evoked, but not spontaneous, serotonin ([(3)H]5-HT) efflux from cortical slices in a concentration-dependent manner while the indirect serotonin agonist dl-fenfluramine (1-15 microM) enhanced both spontaneous and evoked [(3)H]5-HT efflux. The effects of modafinil were more pronounced when the 5-HT reuptake was blocked by paroxetine. Contrary to paroxetine (0.3-3 microM) and dl-fenfluramine (1-5 microM), modafinil failed to influence the [(3)H]5-HT uptake. In the in vivo study modafinil (3-100 mg/kg i.p.) increased 5-HT dialysate levels, the maximal effect being already reached at the 30 mg/kg dose. dl-fenfluramine (5 mg/kg) induced an increase in 5-HT levels which was significantly higher than that displayed by modafinil at 30 mg/kg. In the presence of paroxetine (3 microM), the effect of modafinil at 30 mg/kg was higher than that observed in the absence of 5-HT reuptake inhibition. Finally, in the presence of the selective 5-HT(1A) receptor agonist 8-OH-DPAT, modafinil at 100 mg/kg failed to affect 5-HT dialysate levels. These results demonstrate that modafinil regulates cortical serotonergic transmission and suggest that the drug preferentially acts by amplifying the electro-neurosecretory coupling mechanisms and via mechanisms which do not involve the reuptake process.

Enhanced 5-HT metabolism and synthesis rate by the new selective r5-HT1B receptor antagonist, NAS-181 in the rat brain

NAS-181 ((R)-(+)-2-(3-morpholinomethyl-2H-chromen-8-yl) oxymethyl-morpholine methanesulfonate) is a novel rat 5-hydroxytryptamine1B, (r5-HT1B) receptor antagonist with high selectivity. The in vivo effects of NAS-181 on 5-HT metabolism and synthesis in the rat brain were examined. 5-HT metabolism, measured as the ratio 5-hydroxyindoleacetic acid (5-HIAA)/5-HT, was dose-dependently increased in all four brain regions analysed (hypothalamus, hippocampus, frontal cortex and striatum) at doses 0.1 to 20 mg/kg s.c. NAS-181. The enhancement of 5-HT metabolism at the dose 20 mg/kg s.c. was maximal one hour after the injection and was still significant eight hours but not 24 hours after the injection. 5-HT synthesis rate measured as the accumulation of 5-hydroxytryptophan (5-HTP) after inhibition of the aromatic amino acid decarboxylase activity was also elevated by NAS-181 at doses 0.3 to 20 mg/kg s.c. NAS-181 competitively antagonised the decrease in 5-HT metabolism evoked by the r5-HT1B receptor agonist, anpirtoline, in hypothalamus, hippocampus and frontal cortex. Anpirtoline had no effect on 5-HT metabolism in striatum. However, anpirtoline antagonised the enhancement of 5-HT metabolism induced by NAS-181 in striatum. Combined treatment of rats with NAS-181 and the 5-HT1A receptor antagonist, WAY-100635, increased 5'-HT metabolism considerably more than when the compounds were given alone.

Characterization of endogenous serotonin-mediated regulation of dopamine release in the rat prefrontal cortex

Endogenous serotonin (5-hydroxytryptamine, 5-HT)-mediated regulation of dopamine release in the rat prefrontal cortex was pharmacologically characterized using in vivo microdialysis. To increase synaptic 5-HT availability, a selective 5-HT uptake inhibitor fluoxetine was applied via the dialysis probe. Local perfusion of fluoxetine (30 and 100 microM) increased dopamine levels in a concentration-dependent manner. The fluoxetine (100 microM)-induced increases in dopamine release were abolished by pretreatment with the 5-HT(1B/1D) receptor antagonist GR 127935 (N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5- methyl-1,2,4-oxadiazol-3-yl)-[1,1-biphenyl]-4-carboxamide] ) (10 and 100 microM). The facilitation of dopamine release was also prevented by selective inactivation of the mRNA encoding 5-HT(6) receptors using antisense oligonucleotides techniques. These findings suggest that not only 5-HT(1B) receptors but also 5-HT(6) receptors are associated with the endogenous 5-HT-mediated facilitation of dopamine release. In other words, 5-HT(6) receptors may play, in part, a significant role in the functional interaction between the dopaminergic and serotonergic neuronal system in the rat prefrontal cortex.

Diurnal variation in 5-HT1B autoreceptor function in the anterior hypothalamus in vivo: effect of chronic antidepressant drug treatment

1. Intracerebral microdialysis was used to examine the function of the terminal 5-hydroxytryptamine (5-HT) autoreceptor in the anterior hypothalamus of anaesthetized rats at two points in the light phase of the light-dark cycle. 2. Infusion of the 5-HT1A/1B agonist 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridyl)-1H-indole (RU24969) 0.1, 1.0 and 10 microM through the microdialysis probe led to a concentration-dependent decrease (49, 56 and 65% respectively) in 5-HT output. The effect of RU24969 (1 and 5 microM) was prevented by concurrent infusion of methiothepin (1 and 10 microM) into the anterior hypothalamus via the microdialysis probe. Infusion of methiothepin alone (1.0 and 10 microM) increased (15 and 142% respectively) 5-HT output. 3. Infusion of RU24969 (5 microM) through the probe at mid-light and end-light resulted in a quantitatively greater decrease in 5-HT output at end-light compared with mid-light. 4. Following treatment with either paroxetine hydrochloride (10 mg kg(-1) i.p.) or desipramine hydrochloride (10 mg kg)(-1) i.p.) for 21 days the function of the terminal 5-HT1B autoreceptor was more markedly attenuated at end-light. 5. The data show that, as defined by the response to RU24969, the function of the 5-HT1B receptors that control 5-HT output in the anterior hypothalamus is attenuated following chronic desipramine or paroxetine treatment in a time-of-day-dependent manner.

Role of somatodendritic 5-HT autoreceptors in modulating 5-HT neurotransmission

A very important element controlling serotonin (5-HT) release throughout the brain is the 5-HT1A autoreceptor present on the soma and dendrites of 5-HT neurons since it exerts a negative feedback influence on their firing activity. This 5-HT1A autoreceptor receives an increased activation by endogenous 5-HT at the beginning of a treatment with a selective 5-HT reuptake inhibitor (SSRI) and, consequently, a decreased 5-HT neuronal firing activity is obtained. As the SSRI treatment is prolonged, the 5-HT1A autoreceptor desensitizes and firing activity is restored in the presence of the SSRI. That this adaptive change underlies, at least in part, the delayed therapeutic effect of SSRI in major depression is supported by the acceleration of the antidepressant response by the concomitant administration of the 5-HT1A autoreceptor antagonist pindolol with SSRIs.

Possible serotonergic mechanisms underlying the antidepressant and anti-obsessive-compulsive disorder responses

Considerable evidence is now available to support the pivotal role of the serotonin (5-HT) system is exerting the antidepressant response in humans. Different type of antidepressant treatments enhance 5-HT neurotransmission via different pre- or postsynaptic mechanisms. The time course for the occurrence of these adaptive changes in the brain of laboratory animals is consistent with the delayed onset of the antidepressant response in humans. The drugs effective in obsessive-compulsive disorder (OCD) also enhance 5-HT neurotransmission in brain regions involved in mediating OCD symptoms but with a more prolonged delay, consistently with the larger time necessary to obtain therapeutic effect in OCD than in depression. The elucidation of these mechanisms of action lead to the development of new pharmacologic strategies to potentiate the therapeutic effect of the drugs currently available and the identification of novel targets to accelerate and further improve treatment response in depression and OCD.

Differential effects of 5-HT1B/1D receptor antagonists in dorsal and median raphe innervated brain regions

The effect of SB-224289 (2,3,6.7-tetrahydro-1'-methyl-5-¿2'-methyl-4'-[(5-methyl-1,2,4-oxadiazol e-3-yl)biphenyl-4-yl]carbonyl¿Furo[2,3-F]-indole-3-spiro-4'-piperidine oxalate) (4 mg/kg i.p., 5-HT1B receptor antagonist), GR 127935 (N-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-2'-methyl-4'-(5-me thyl-1,2,4-oxadiazole-3-yl)[1,1'-biphenyl]-carboxamide) (0.3 mg/kg i.p., 5-HT1B/1D receptor antagonist), and paroxetine (10 mg/kg p.o.) were investigated on extracellular 5-hydroxytryptamine (5-HT) levels in the frontal cortex, striatum and dentate gyrus of the freely moving guinea-pig with microdialysis. In the frontal cortex and striatum (dorsal raphe innervated areas), GR 127935 evoked a significant decrease in extracellular 5-HT, reaching minima of 41+/-12% and 32+/-6% of basal, respectively. This decrease may be explained by antagonism of inhibitory 5-HT1B/1D receptors on raphe cell bodies, leading to a local increase in 5-HT, which, in turn, stimulated 5-HT1A receptors to decrease cell firing, and hence 5-HT release from terminals. In contrast, SB-224289 had no effect on 5-HT levels in either region. In the dentate gyrus (median raphe innervated area), GR 127935 and SB-224289 significantly increased extracellular 5-HT, reaching maxima of 146+/-11% and 151+/-19% of basal, respectively. The ability of both compounds to increase 5-HT levels in the dentate gyrus suggests a lack of 5-HT1B/1D receptors in the median raphe nucleus. Paroxetine produced a small but non-significant increase in extracellular 5-HT in the frontal cortex, and a small decrease in the dentate gyrus. The lack of effect of paroxetine in terminal areas may be due to the limiting effects of cell body 5-HT autoreceptors. In summary, the above data demonstrate that 5-HT1B/1D receptor antagonists increase 5-HT levels in the dentate gyrus, implying that acute administration of 5-HT1B/1D receptor antagonists will achieve a similar effect to chronic selective serotonin re-uptake inhibitor treatment in median raphe innervated areas. This, in turn, suggests that such compounds may be efficacious in the treatment of depression.

Pharmacological analysis of G-protein activation mediated by guinea-pig recombinant 5-HT1B receptors in C6-glial cells: similarities with the human 5-HT1B receptor

1. The guinea-pig recombinant 5-hydroxytryptamine1B (gp 5-HT1B) receptor stably transfected in rat C6-glial cells was characterized by monitoring G-protein activation in a membrane preparation with agonist-stimulated [35S]-GTPgammaS binding. The intrinsic activity of 5-HT receptor ligands was compared with that determined previously at the human recombinant 5-HT1B (h 5-HT1B) receptor under similar experimental conditions. 2. Membrane preparations of C6-glial/gp 5-HT1B cells exhibited [3H]-5-carboxamidotryptamine (5-CT) and [3H]-N-[4-methoxy-3,4-methylpiperazin-1-yl) phenyl]-3-methyl-4-(4-pyridinyl)benzamide (GR 125743) binding sites with a pKd of 9.62 to 9.85 and a Bmax between 2.1 to 6.4 fmol mg(-1) protein. The binding affinities of a series of 5-HT receptor ligands determined with [3H]-5-CT and [3H]-GR 125743 were similar. Ligand affinities were comparable to and correlated (r2: 0.74, P<0.001) with those determined at the recombinant h 5-HT1B receptor. 3. [35S]-GTPgammaS binding to membrane preparations of C6-glial/gp 5-HT1B cells was stimulated by the 5-HT receptor agonists that were being investigated. The maximal responses of naratriptan, zolmitriptan, sumatriptan, N-methyl-3-[pyrrolidin-2(R)-ylmethyl]-1H-indol-5-ylmethyl sulphonamide (CP 122638), rizatriptan and dihydroergotamine were between 0.76 and 0.85 compared to 5-HT. The potency of these agonists showed a positive correlation (r2: 0.72, P=0.015) with their potency at the recombinant h 5-HT1B receptor. 1-naphthylpiperazine, (+/-)-cyanopindolol and (2'-methyl-4'-(5-methyl[1,2,4] oxadiazole-3-yl)biphenyl-4-carboxylic acid [4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]amide (GR 127935) elicited an even smaller response (Emax: 0.32 to 0.63). 4. The ligands 1'-methyl-5-(2'-methyl-4'-(5-methyl-1,2,4-oxadiazole-3-yl) biphenyl-4-carbonyl)-2,3,6,7tetrahydrospiro [furo[2,3-f]indole-3-spiro-4'-piperidine] (SB224289), methiothepin and ritanserin displayed inhibition of basal [35S]-GTPgammaS binding at concentrations relevant to their binding affinity for the gp 5-HT1B receptor. Methiothepin and SB224289 behaved as competitive antagonists at gp 5-HT1B receptors; pA2 values were 9.74 and 8.73, respectively when 5-HT was used as an agonist. These estimates accorded with the potencies measured in antagonism of zolmitriptan-mediated inhibition of forskolin-stimulated cyclic AMP formation. Ketanserin acted as a weak antagonist (pK(B): 5.87) at gp 5-HT1B receptors. 5. In conclusion, the recombinant gp 5-HT1B receptor shares important pharmacological similarities with the recombinant h 5-HT1B receptor. The finding that negative activity occurs at these receptors further suggests that SB224289, methiothepin and ritanserin are likely to be inverse agonists.