The 5-HT(6) receptor antagonist SB-271046 reverses scopolamine-disrupted consolidation of a passive avoidance task and ameliorates spatial task deficits in aged rats.

The highly potent and selective 5-HT(6) receptor antagonist SB-271046 [5-chloro-N-(4-methoxy-3-piperazin-1-yl-phenyl)-3-methyl-2-benzothiophenesulfonamide] has previously been demonstrated to improve retention significantly in a spatial water maze paradigm in adult rats. However, SB-271046 did not have any effect on task acquisition. As these apparently contradictory findings may be reconciled by a prime influence of SB-271046 on memory consolidation, the ability of this compound to reverse the discrete temporal action of a cholinergic antagonist in the 6-h period following passive avoidance training was investigated. SB-271046, given orally, by gavage, 30 min prior to training Wistar rats in a step-through, light-dark passive avoidance task, was found to reverse significantly the amnesia produced by administering scopolamine (0.8 mg/kg, intraperitoneal) in the 6-h post-training period. The effect was dose-dependent over a range of 3-20 mg/kg. Further, we investigated the cognition-enhancing effects of chronic SB-271046 administration (10 or 20 mg/kg/day; 40 days) on the acquisition and consolidation of a water maze spatial learning task in a population of 20-month-old Wistar rats with age-related learning deficits. Drug treatment progressively and significantly decreased platform swim angle and escape latencies over the five sequential trials on four consecutive daily sessions compared to vehicle-treated controls. SB-271046 also improved task recall as measured by significant increases in the searching of the target quadrant on post-training days 1 and 3, when the animals would have been substantially drug-free. This significant improvement of task recall suggests SB-271046, in addition to inducing symptomatic cognition-enhancing actions, also attenuates age-related decline in neural function.     

Oral administration of the 5-HT6 receptor antagonists SB-357134 and SB-399885 improves memory formation in an autoshaping learning task

In this work we aimed to re-examine the 5-HT6 receptor role, by testing the selective antagonists SB-357134 (1-30 mg/kg p.o.) and SB-399885 (1-30 mg/kg p.o.) during memory consolidation of conditioned responses (CR%), in an autoshaping Pavlovian/instrumental learning task. Bioavailability, half-life and minimum effective dose to induce inappetence for SB-357134 were 65%, 3.4 h, and 30 mg/kg p.o., and for SB-399885 were 52%, 2.2 h, and 50 mg/kg p.o., respectively. Oral acute and chronic administration of either SB-357134 or SB-399885 improved memory consolidation compared to control groups. Acute administration of SB-357134, at 1, 3, 10 and 30 mg/kg, produced a CR% inverted-U curve, eliciting the latter dose a 7-fold increase relative to saline group. Acute injection of SB-399885 produced significant CR% increments, being 1 mg/kg the most effective dose. Repeated administration (7 days) of either SB-357134 (10 mg/kg) or SB-399885 (1 mg/kg) elicited the most significant CR% increments. Moreover, modeling the potential therapeutic benefits of 5-HT6 receptor blockade, acute or repeated administration of SB-399885, at 10 mg/kg reversed memory deficits produced by scopolamine or dizocilpine, and SB-357134 (3 and 10 mg/kg) prevented amnesia and even improved performance. These data support the notion that endogenously 5-HT acting, via 5-HT6 receptor, improves memory consolidation.     

Pharmacological profile of SB-357134: a potent, selective, brain penetrant, and orally active 5-HT(6) receptor antagonist

N-(2,5-Dibromo-3-fluorophenyl)-4-methoxy-3-piperazin-1-ylbenzenesulfonamide (SB-357134) potently inhibited [125I]SB-258585 and [3H]LSD binding in a HeLa cell line expressing human 5-HT(6) receptors (pK(i)=8.6 and 8.54, respectively). Furthermore, SB-357134 inhibited [125I]SB-258585 binding in human caudate--putamen and in rat and pig striatum membranes (pK(i)=8.82, 8.44, and 8.61, respectively). SB-357134 displayed over 200-fold selectivity for the 5-HT(6) receptor versus 72 other receptors and enzymes. 5-HT-stimulated cyclic AMP (cAMP) accumulation in human 5-HT(6) receptors was competitively antagonised by SB-357134 (pA(2)=7.63). SB-357134 inhibited ex vivo [125I]SB-258585 binding in the rat with an ED(50) of 4.9 +/- 1.3 mg/kg po, 4 h postdose. In the rat maximal electroshock seizure threshold (MEST) test, SB-357134 produced a potent and dose-dependent increase in seizure threshold, with a minimum effective dose of 0.1 mg/kg po. At 10 mg/kg po, maximum activity occurred between 4 and 6 h postdose. Good exposure was observed with SB-357134 at 10 mg/kg po, reaching maximal blood and brain concentrations of 4.3 +/- 0.2 and 1.3 +/- 0.06 microM, respectively, 1 h postdose. In addition, SB-357134 (10 mg/kg po) enhanced memory and learning following chronic administration (twice a day for 7 days) in the rat water maze. Overall, these studies demonstrate that SB-357134 is a potent, selective, brain penetrant, and orally active 5-HT(6) receptor antagonist.     

The selective 5-HT6 receptor antagonists SB-271046 and SB-399885 potentiate NCAM PSA immunolabeling of dentate granule cells, but not neurogenesis, in the hippocampal formation of mature Wistar rats

While there is now substantial evidence that 5-HT(6) antagonism leads to significantly improved cognitive ability, the mechanism(s) and/or pathway(s) involved are poorly understood. We have evaluated the consequence of chronic administration of the 5-HT(6) receptor antagonists SB-271046 and SB-399885 on neural cell adhesion molecule polysialylation state (NCAM PSA), a neuroplastic mechanism necessary for memory consolidation. Quantitative analysis of NCAM PSA immunopositive neurons in the dentate gyrus of drug-treated animals revealed a dose-dependent increase in polysialylated cell frequency following treatment with both SB-271046 and SB-399885. These effects could not be attributed to increased neurogenesis, as no difference in the rate of bromodeoxyuridine incorporation was apparent between the control and drug-treated groups. A substantial increase in the frequency of polysialylated cells in layer II of the entorhinal and perirhinal cortices was also observed, brain regions not previously associated with neurogenesis. Chronic treatment with SB-271046 or SB-399885 also significantly increased the activation of dentate polysialylation that is specific to learning. This effect does not occur with other cognition-enhancing drugs, such as tacrine, and this action potentially differentiates 5-HT(6) receptor antagonism as an unique neuroplastic mechanism for cognitive processes which may slow or reverse age/neurodegenerative related memory deficits.     

Memories are made of this (perhaps): a review of serotonin 5-HT(6) receptor ligands and their biological functions

The possible role of 5-HT(6) receptor antagonists in the treatment of learning and memory disorders has stimulated significant recent work in this area. The first selective antagonists of this receptor were identified by Roche (Ro 04-6790 and Ro 63-0563) and SmithKline Beecham (SB-271046), although they only had poor to modest brain penetration, respectively. Recently, several structurally different series of selective antagonists have been reported. Glennon s group and Merck Sharp & Dohme have discovered N,N-dimethyl-1-benzenesulfonyl-5-methoxytrypt amine as a reasonably selective, high affinity antagonist, while Allelix have gone on to find that a 6-bicyclopiperazinyl-1-naphthylsulfonylindole had improved affinity and selectivity. Roche have reported subsequently on more lipophilic analogs of Ro 04-6790 that appear to penetrate the brain better. Reversing the sulfonamide linkage of SB-271046 led to a new series of compounds, producing SB-357134, which also had increased CNS penetration. A series of selective partial agonists containing a 4-piperazinylquinoline system has also been described. Recent studies in the Morris water maze with both Ro 04-6790 and SB-271046 have concluded that 5-HT(6) receptor antagonists improved retention performance, although these results are open to interpretation. Other behavioural studies have also implicated a role for 5-HT(6) in cognition enhancement and this has been supported by in vivo microdialysis studies that showed SB-271046 produced an increase in extracellular glutamate levels in the frontal cortex. However, we have been unable to replicate these effects with either SB-271046 or Ro 04-6790, and clearly further work is required before we can be certain of the functional role of this receptor.     

5-HT<Subscript>6</Subscript> receptor antagonists improve performance in an attentional set shifting task in rats

Rationale and objective Performance on the Wisconsin Card Sorting Test (WCST), which requires patients to ‘shift attention’ between stimulus dimensions (sorting categories), is impaired in diseases such as schizophrenia. The rat attentional set shifting task is an analogue of the WCST. Given that 5-HT₆ receptor antagonists improve cognitive performance and influence cortical neurochemistry in rats, the present study investigated the effects of 5-HT₆ receptor antagonists upon attentional set shifting in rats. Methods Rats were tested in this paradigm following sub-chronic SB-399885-T or SB-271046-A (both 10 mg kg⁻¹ bid, p.o. for 8 days prior to testing and either 4 or 2 h prior to testing on day 9, respectively). Rats were trained to dig in baited bowls for a food reward and to discriminate based on odour or digging media (Habituation, day 8). In a single session (day 9), rats performed a series of discriminations, including reversals (REV), intra-dimensional (ID) and extra-dimensional (ED) shifts. Results Neither compound altered performance during Habituation. On the test day, both SB-399885-T and SB-271046-A reduced the total trials to reach criterion and the total errors made when data were collapsed across all discriminations (P<0.05–0.01). Further, both compounds significantly reduced the trials to criterion for REV-1 (P<0.05–0.01) and abolished the ID/ED shift. SB-399885-T, but not SB-271046-A, reduced trials required to complete the ED shift (P<0.05) and the number of errors made during completion of the ID (P<0.05) and ED shifts (P<0.01). Conclusion 5-HT₆ receptor antagonists improved performance in the attentional set shifting task and may have therapeutic potential in the treatment of disorders where cognitive deficits are a feature, including schizophrenia.     

5-HT6 receptor antagonists reverse delay-dependent deficits in novel object discrimination by enhancing consolidation--an effect sensitive to NMDA receptor antagonism

5-HT(6) receptors are expressed in brain regions associated with learning and memory, and blockade of their function increases central cholinergic and glutamatergic neurotransmission and enhances cognitive processes. This study examined the effects of acute systemic administration of two selective 5-HT(6) receptor antagonists Ro 04-6790 and SB-271046 (10 mg kg(-1) i.p.) on acquisition, consolidation, and retrieval in the novel object discrimination (NOD) task, a two-trial test of recognition memory in which rats exposed to two identical objects during a familiarisation trial can discriminate a novel from a familiar object during the subsequent choice trial, following inter-trial delays of up to 3 h. 5-HT(6) receptor antagonist administration 20 min prior to or immediately after the familiarisation trial, but not 20 min prior to the choice trial reversed the deficit in object discrimination produced by a 4 h inter-trial interval. The nootropic effects of the 5-HT(6) receptor antagonists in this task thus appear to involve enhanced consolidation. Pre-treatment with the non-competitive NMDA receptor antagonist MK-801 (0.05 mg kg(-1) i.p.) prevented the effect of Ro 04-6790 on delay-induced deficits in object discrimination. This suggests that the 5-HT(6) receptor antagonist-induced enhancement of consolidation involves increased central glutamatergic neurotransmission.     

Role of 5-HT6 receptors in memory formation

Mice lacking the 5-HT(6) receptor presented neither gross anatomical or behavioral abnormalities nor obvious changes in microscopic brain morphology, and their performance in rotarod, open field and novel object testing paradigms revealed no differences compared with wild-type animals. Nevertheless, an association between the 5-HT(6) receptor polymorphism C267T and Alzheimer's disease has been reported. Interestingly, the 5-HT(6) antisense oligonucleotide decreased 5-HT(6) gene expression and enhanced spatial learning acquisition in the water maze. Similarly, injection of the 5-HT(6) receptor antagonist Ro-04-6790 improved learning consolidation in an autoshaping task, while mCPP, scopolamine and dizocilpine decreased performance. The effect induced by scopolamine or dizocilpine, but not that induced by mCPP, was completely or partially reversed by Ro-04-6790. Ro-04-6790 did not modify the 8-OH-DPAT facilitatory effects on learning consolidation. Since Ro-04-6790 facilitatory effect was unaffected by 5-HT(1A), 5-HT(2A/2B/2C), 5-HT(3), 5-HT(4) or 5-HT(7) receptor blockade, the facilitatory effect induced by Ro-04-6790 involved specifically 5-HT6 receptors. Similarly, the 5-HT(6) receptor antagonist SB-271046 improved retention in the water maze and produced a significant performance improvement in aged rats in an operant-delayed alternation task. A series of Ro-04-6790 analogues that penetrate the brain and specifically bind to 5-HT(6) receptors reversed scopolamine-induced retention deficit in a passive avoidance learning test. Collectively, these data provide further support to the notion that 5-HT systems, via 5-HT(6) receptors, also play a significant role in memory formation under normal and dysfunctional memory conditions.     

5-HT6 receptor blockade differentially affects scopolamine-induced deficits of working memory, recognition memory and aversive learning in mice

Rationale

Blockade of 5-HT6 receptors (5-HT6R) is known to improve cognitive performances in the rodent. This improvement has been hypothesized to be the result, at least in part, of a modulation of the cholinergic neurotransmission.

Objective

We assessed the effects of 5-HT6R blockade on selected types of memory relevant to functional deficits of ageing and neurodegenerative diseases, in mice that present a scopolamine-induced cholinergic disruption of memory.

Method

Following the selection of an adequate dose of scopolamine to induce cognitive deficits, we have studied the effects of the selective 5-HT6R antagonist SB-271046, alone or in combination with scopolamine, on working memory (spontaneous alternation task in the T-maze), recognition memory (place recognition) and aversive learning (passive avoidance).

Results

SB-271046 alone failed to affect working memory, recognition memory and aversive learning performances. In contrast, SB-271046 was able to reverse the scopolamine-induced deficits in working memory (only at 30?mg?kg^?1) and those of acquisition and retrieval of aversive learning (dose-dependent effect); scopolamine-induced deficits in episodic-like memory (acquisition and retrieval) were partially counteracted by 5-HT6R blockade.

Conclusion

The modulation between 5-HT6R and the cholinergic system appears to be predominant for working memory and aversive learning, but not for other types of memory (i.e. episodic-like memory). Interactions between 5-HT6R and alternative neurotransmission systems (i.e. glutamatergic system) should be further studied. The respective involvement of these interactions in the memory disorders related to ageing and neurodegenerative diseases is of pivotal importance regarding the possible use of 5-HT6R antagonists in the treatment of memory disorders in humans.     

Effects of 5-HT6 receptor antagonism and cholinesterase inhibition in models of cognitive impairment in the rat

Background and purpose:

The beneficial effect of 5-HT₆ receptor antagonism in cognition remains controversial. This study has been undertaken to reassess the cognition enhancing properties of acute vs subchronic treatment with the selective 5-HT₆ receptor antagonist SB-271046 in unimpaired rats, as well as against scopolamine (cholinergic-) or MK-801 (glutamatergic-mediated) deficits.

Experimental approach:

The Morris water maze was used, measuring behaviour acquisition and retention, and swim speed. Other behavioural measures included yawning and motor activity. SB-271046 was given acutely before each trial or subchronically for 7 days before the trials. The AChE inhibitor galanthamine was also used alone or in combination with SB-271046.

Key results:

Subchronic treatment with SB-271046 improved acquisition in the Morris water maze, while the acute treatment only improved retention. Neither acute nor subchronic SB-271046 treatment reversed scopolamine-induced learning deficits. MK-801 induced learning impairment associated with a behavioural syndrome, reversed by acute, but not subchronic, SB-271046 treatment. Interestingly, combined treatment with galanthamine and SB-271046 reversed the scopolamine- or MK-801-induced learning impairments. Subchronic treatment with SB-271046 did not modify motor activity or the increased number of yawns, a cholinergic-mediated behaviour, induced by single administration of SB-271046.

Conclusions and implications:

These data suggest a potential therapeutic role of 5-HT₆ receptor antagonists such as SB-271046, alone or in combination with galanthamine, in the treatment of cognitive dysfunction, such as those seen in Alzheimer''s disease and schizophrenia.     

Characterization of SB-271046: a potent, selective and orally active 5-HT(6) receptor antagonist

SB-271046, potently displaced [(3)H]-LSD and [(125)I]-SB-258585 from human 5-HT(6) receptors recombinantly expressed in HeLa cells in vitro (pK(i) 8.92 and 9.09 respectively). SB-271046 also displaced [(125)I]-SB-258585 from human caudate putamen and rat and pig striatum membranes (pK(i) 8.81, 9.02 and 8.55 respectively). SB-271046 was over 200 fold selective for the 5-HT(6) receptor vs. 55 other receptors, binding sites and ion channels. In functional studies on human 5-HT(6) receptors SB-271046 competitively antagonized 5-HT-induced stimulation of adenylyl cyclase activity with a pA(2) of 8.71. SB-271046 produced an increase in seizure threshold over a wide-dose range in the rat maximal electroshock seizure threshold (MEST) test, with a minimum effective dose of < or =0.1 mg kg(-1) p.o. and maximum effect at 4 h post-dose. The level of anticonvulsant activity achieved correlated well with the blood concentrations of SB-271046 (EC(50) of 0.16 microM) and brain concentrations of 0.01-0.04 microM at C(max). These data, together with the observed anticonvulsant activity of other selective 5-HT(6) receptor antagonists, SB-258510 (10 mg kg(-1), 2-6 h pre-test) and Ro 04-6790 (1-30 mg kg(-1), 1 h pre-test), in the rat MEST test, suggest that the anticonvulsant properties of SB-271046 are likely to be mediated by 5-HT(6) receptors. Overall, these studies demonstrate that SB-271046 is a potent and selective 5-HT(6) receptor antagonist and is orally active in the rat MEST test. SB-271046 represents a valuable tool for evaluating the in vivo central function of 5-HT(6) receptors.     

SB-699551-A (3-cyclopentyl-N-[2-(dimethylamino)ethyl]-N-[(4'-{[(2-phenylethyl)amino]methyl}-4-biphenylyl)methyl]propanamide dihydrochloride), a novel 5-ht5A receptor-selective antagonist, enhances 5-HT neuronal function: Evidence for an autoreceptor role for the 5-ht5A receptor in guinea pig brain

This study utilised the selective 5-ht(5A) receptor antagonist, SB-699551-A (3-cyclopentyl-N-[2-(dimethylamino)ethyl]-N-[(4'-{[(2-phenylethyl)amino]methyl}-4-biphenylyl)methyl]propanamide dihydrochloride), to investigate 5-ht5A receptor function in guinea pig brain. SB-699551-A competitively antagonised 5-HT-stimulated [35S]GTPgammaS binding to membranes from human embryonic kidney (HEK293) cells transiently expressing the guinea pig 5-ht5A receptor (pA2 8.1+/-0.1) and displayed 100-fold selectivity versus the serotonin transporter and those 5-HT receptor subtypes (5-HT(1A/B/D), 5-HT2A/C and 5-HT7) reported to modulate central 5-HT neurotransmission in the guinea pig. In guinea pig dorsal raphe slices, SB-699551-A (1 microM) did not alter neuronal firing per se but attenuated the 5-CT-induced depression in serotonergic neuronal firing in a subpopulation of cells insensitive to the 5-HT1A receptor-selective antagonist WAY-100635 (100 nM). In contrast, SB-699551-A (100 or 300 nM) failed to affect both electrically-evoked 5-HT release and 5-CT-induced inhibition of evoked release measured using fast cyclic voltammetry in vitro. SB-699551-A (0.3, 1 and 3 mg/kg s.c.) did not modulate extracellular levels of 5-HT in the guinea pig frontal cortex in vivo. However, when administered in combination with WAY-100635 (0.3 mg/kg s.c.), SB-699551-A (0.3, 1 or 3 mg/kg s.c.) produced a significant increase in extracellular 5-HT levels. These studies provide evidence for an autoreceptor role for the 5-ht5A receptor in guinea pig brain.     

Analysis of the role of the 5-HT1B receptor in spatial and aversive learning in the rat.

The present study examined the role of the 5-HT1B receptor in learning and memory. The ability of the 5-HT1B receptor agonist anpirtoline and the selective 5-HT1B receptor antagonist NAS-181 to affect spatial learning in the water maze (WM) and aversive learning in the passive avoidance (PA) task were examined in the rat. Anpirtoline (0.1-1.0 mg/kg, s.c.) caused a dose-dependent impairment of learning and memory in both the WM and PA tasks. NAS-181 (1.0-10 mg/kg, s.c.) failed to alter performance of the WM task, but produced a dose-dependent (0.1-20 mg/kg) facilitation of PA retention. Furthermore, treatment with NAS-181 (10 mg/kg) fully blocked the impairment of the WM and PA performance caused by anpirtoline (1.0 mg/kg). In contrast, NAS-181 (3.0-10 mg/kg) did not attenuate the spatial learning deficit and the impairment of PA retention caused by scopolamine (0.1 mg/kg in WM task, 0.3 mg/kg in PA task, s.c.), a nonselective muscarinic antagonist. Moreover, a subthreshold dose of scopolamine (0.1 mg/kg) blocked the facilitation of PA retention induced by NAS-181 (1.0-10 mg/kg). In addition, the behavioral disturbances (eg thigmotaxic swimming and platform deflections) induced by anpirtoline and scopolamine were analyzed in the WM task and correlated with WM performance. These results indicate that: (1) 5-HT1B receptor stimulation and blockade result in opposite effects in two types of cognitive tasks in the rat, and that (2) the 5-HT1B antagonist NAS-181 can facilitate some aspects of cognitive function, most likely via an increase of cholinergic transmission. These results suggest that 5-HT1B receptor antagonists may have a potential in the treatment of cognitive deficits resulting from loss of cholinergic transmission.     

The 5-HT(6) receptor antagonist SB-271046 selectively enhances excitatory neurotransmission in the rat frontal cortex and hippocampus.

Preclinical evidence has suggested a possible role for the 5-HT(6) receptor in the treatment of cognitive dysfunction. However, currently there is little neurochemical evidence suggesting the mechanism(s) which may be involved. Using the selective 5-HT(6) antagonist SB-271046 and in vivo microdialysis, we have evaluated the effects of this compound on the modulation of basal neurotransmitter release within multiple brain regions of the freely moving rat. SB-271046 produced no change in basal levels of dopamine (DA), norepinephrine (NE) or 5-HT in the striatum, frontal cortex, dorsal hippocampus or nucleus accumbens. Similarly, this compound had no effect on excitatory neurotransmission in the striatum or nucleus accumbens. Conversely, SB-271046 produced 3- and 2-fold increases in extracellular glutamate levels in both frontal cortex and dorsal hippocampus, respectively. These effects were completely attenuated by infusion of tetrodotoxin but unaffected by the muscarinic antagonist, atropine. Here we demonstrate for the first time the selective enhancement of excitatory neurotransmission by SB-271046 in those brain regions implicated in cognitive and memory function, and provide mechanistic evidence in support of a possible therapeutic role for 5-HT(6) receptor antagonists in the treatment of cognitive and memory dysfunction.     

The 5-HT₆ serotonin receptor antagonist SB-271046 attenuates the development and expression of nicotine-induced locomotor sensitisation in Wistar rats

5-HT(6) receptors are almost exclusively expressed in the central nervous system, particularly in areas relevant for addictive behaviour. Based on this, together with other data, this receptor may be a viable target for the control of drug abuse. The present study tested the ability of the 5-HT(6) receptor antagonist SB-271046 to attenuate the development and expression of nicotine-induced behavioural sensitisation. Rats were habituated to the test apparatus prior to experimentation (day 0) and locomotor activity recorded. On days 1 and 5, animals were placed in locomotor test apparatus and after 30?min injected with SB-271046 (1, 3, and 6?mg/kg, intraperitoneally IP) or vehicle. Thirty minutes later, nicotine (0.4?mg/kg, subcutaneously SC) or saline were administered and activity recorded for 60?min. On days 2, 3 and 4 treatments were performed in the home cage. After 17 days of withdrawal (day 23), a challenge test was performed with nicotine (0.4?mg/kg SC) or saline. In a separate experiment of similar design the effects of SB-271046 (1, 3, and 6?mg/kg IP) was tested for its ability to reduce the expression of behavioural sensitisation (day 23). SB-271046 dose dependently reduced the development and expression of nicotine sensitisation vs respective controls. In conclusion, the 5-HT(6) receptor antagonist SB-271046 reduced both the development and expression of nicotine sensitisation, suggesting that the 5-HT(6) receptor may be a viable target for the control of nicotine abuse. Further studies are warranted to substantiate this conclusion and further understand the role of 5-HT(6) receptors in addiction.     

5-HT7 receptor subtype as a mediator of the serotonergic regulation of luteinizing hormone release in the zona incerta

5-Hydroxytryptamine (5-HT) and the 5-HT(1A/7) receptor agonist (+)-8-hydroxy-2-(di-n-propylamino) tetralinHBr (8-OH-DPAT), injected into the zona incerta (an area in the dorsal hypothalamus) of the female rat, inhibit the release of luteinizing hormone (LH) and the effects of both are blocked by the 5-HT(2/7) receptor antagonist, ritanserin. As both 8-OH-DPAT and ritanserin have moderate activity at the 5-HT7 receptor subtype, the possibility that this subtype might mediate their effects in the zona incerta has been investigated. Ovariectomised rats were primed with 5 microg oestradiol benzoate followed at 48 h by 0.5 mg progesterone, which induces an LH surge. 5-Carboxamidotryptamine (5-CT), a potent but non-selective agonist at 5-HT7 receptors, like 5-HT and 8-OH-DPAT, inhibited the LH surge at 5 and 1.25 nmol injected bilaterally into the zona incerta. The non-selective 5-HT(2/7) receptor antagonist ritanserin and the selective 5-HT7 receptor antagonist, (R)-3-(2-(2-(4-methyl-piperidin-1-yl)-pyrrolidine-1-sulfonyl)-phenol (SB-269970-A) at 0.5 microg/side blocked all three receptor agonists when injected concurrently into the zona incerta. However, lower (0.2 microg) and higher doses (2 and 5 microg) of SB-269970-A were less effective, indicating a bell-shaped dose-response curve. SB-269970-A was also inhibitory when administered systemically (1 mg/kg intraperitoneally (i.p.)). When LH release was suppressed by 5 microg oestradiol benzoate, SB-269970-A (0.5 and 2 microg) did not elevate levels, indicating it is unlikely that 5-HT7 receptors mediate a tonic inhibition on release but rather are involved in terminating the pre-ovulatory LH surge. These data demonstrate that 5-HT7 receptors play a role in the regulation of LH by the zona incerta in rat brain.     

GABAergic modulation of 5-HT7 receptor-mediated effects on 5-HT efflux in the guinea-pig dorsal raphe nucleus

5-HT(7) receptor mRNA and protein are localised in the dorsal raphe nucleus (DRN) on non-serotonergic neurones. The effect of 5-HT(7) receptor antagonism on 5-HT efflux was measured from guinea-pig DRN slices, using the technique of fast cyclic voltammetry. The 5-HT(7) receptor antagonist, SB-269970-A, significantly inhibited 5-HT efflux. The GABA(A) receptor agonist, muscimol, significantly inhibited 5-HT efflux, to a similar degree as SB-269970-A. In contrast, the GABA(A) receptor antagonist, bicuculline, significantly increased 5-HT efflux and attenuated the muscimol-induced inhibition. The muscimol and SB-269970-A effects were not additive and in the presence of bicuculline the SB-269970-A-induced inhibition of 5-HT efflux was attenuated. These data suggest that 5-HT(7) receptor antagonist-induced inhibition of 5-HT efflux occurs indirectly via activation of GABA(A) receptors. That is, 5-HT(7) receptors may be located on GABA interneurones and when activated decrease GABA release and hence decrease the inhibitory tone on 5-HT neurones, increasing 5-HT efflux in the DRN. Therefore, in the presence of GABAergic tone 5-HT(7) receptor antagonists would decrease 5-HT release from the DRN.     

The 5-HT6 serotonin receptor antagonist SB-271046 attenuates the development and expression of nicotine-induced locomotor sensitisation in Wistar rats

5-HT₆ receptors are almost exclusively expressed in the central nervous system, particularly in areas relevant for addictive behaviour. Based on this, together with other data, this receptor may be a viable target for the control of drug abuse.The present study tested the ability of the 5-HT₆ receptor antagonist SB-271046 to attenuate the development and expression of nicotine-induced behavioural sensitisation.Rats were habituated to the test apparatus prior to experimentation (day 0) and locomotor activity recorded. On days 1 and 5, animals were placed in locomotor test apparatus and after 30 min injected with SB-271046 (1, 3, and 6 mg/kg, intraperitoneally IP) or vehicle. Thirty minutes later, nicotine (0.4 mg/kg, subcutaneously SC) or saline were administered and activity recorded for 60 min. On days 2, 3 and 4 treatments were performed in the home cage. After 17 days of withdrawal (day 23), a challenge test was performed with nicotine (0.4 mg/kg SC) or saline. In a separate experiment of similar design the effects of SB-271046 (1, 3, and 6 mg/kg IP) was tested for its ability to reduce the expression of behavioural sensitisation (day 23).SB-271046 dose dependently reduced the development and expression of nicotine sensitisation vs respective controls.In conclusion, the 5-HT₆ receptor antagonist SB-271046 reduced both the development and expression of nicotine sensitisation, suggesting that the 5-HT₆ receptor may be a viable target for the control of nicotine abuse. Further studies are warranted to substantiate this conclusion and further understand the role of 5-HT₆ receptors in addiction.     

In vivo effects of the 5-HT(6) antagonist SB-271046 on striatal and frontal cortex extracellular concentrations of noradrenaline, dopamine, 5-HT, glutamate and aspartate

Although the 5-HT(6) receptor subtype was identified some 5 years ago, very little is known about its function within the brain. Here we demonstrate, for the first time, the neurochemical effects of a selective 5-HT(6) receptor ligand. Using in vivo microdialysis in the freely moving rat, we evaluated the effects of the selective 5-HT(6) receptor antagonist SB-271046 by simultaneous measurement of 5-hydroxytryptamine (5-HT), dopamine (DA), noradrenaline (NA), glutamate and aspartate from the striatum and frontal cortex. SB-271046 did not alter basal levels of 5-HT, DA and NA in either brain region. Similarly, there was no change basal levels of either of the excitatory amino acids within the striatum. In contrast, administration of SB-271046 (10 mg kg(-1) s.c.) produced a significant (P<0.05), tetrodotoxin-dependent, increase in extracellular levels of both glutamate and aspartate within the frontal cortex, reaching maximum values of 375.4+/-82.3 and 215. 3+/-62.1% of preinjection values, respectively.     

SB-656104-A,a novel selective 5-HT7 receptor antagonist,modulates REM sleep in rats

1 (6-((R)-2-[2-[4-(4-Chloro-phenoxy)-piperidin-1-yl]-ethyl]-pyrrolidine-1-sulphonyl)-1H-indole hydrochloride) (SB-656104-A), a novel 5-hydroxytryptamine (5-HT(7)) receptor antagonist, potently inhibited [(3)H]-SB-269970 binding to the human cloned 5-HT(7(a)) (pK(i) 8.7+/-0.1) and 5-HT(7(b)) (pK(i) 8.5+/-0.2) receptor variants and the rat native receptor (pK(i) 8.8+/-0.2). The compound displayed at least 30-fold selectivity for the human 5-HT(7(a)) receptor versus other human cloned 5-HT receptors apart from the 5-HT(1D) receptor ( approximately 10-fold selective). 2 SB-656104-A antagonised competitively the 5-carboxamidotryptamine (5-CT)-induced accumulation of cyclic AMP in h5-HT(7(a))/HEK293 cells with a pA(2) of 8.5. 3 Following a constant rate iv infusion to steady state in rats, SB-656104 had a blood clearance (CL(b)) of 58+/-6 ml min(-1) kg(-1) and was CNS penetrant with a steady-state brain : blood ratio of 0.9 : 1. Following i.p. administration to rats (10 mg kg(-1)), the compound displayed a t(1/2) of 1.4 h with mean brain and blood concentrations (at 1 h after dosing) of 0.80 and 1.0 micro M, respectively. 4 SB-656104-A produced a significant reversal of the 5-CT-induced hypothermic effect in guinea pigs, a pharmacodynamic model of 5-HT(7) receptor interaction in vivo (ED(50) 2 mg kg(-1)). 5 SB-656104-A, administered to rats at the beginning of the sleep period (CT 0), significantly increased the latency to onset of rapid eye movement (REM) sleep at 30 mg kg(-1) i.p. (+93%) and reduced the total amount of REM sleep at 10 and 30 mg kg(-1) i.p. with no significant effect on the latency to, or amount of, non-REM sleep. SB-269970-A produced qualitatively similar effects in the same study. 6 In summary, SB-656104-A is a novel 5-HT(7) receptor antagonist which has been utilised in the present study to provide further evidence for a role for 5-HT(7) receptors in the modulation of REM sleep.