LTP but not seizure is associated with up-regulation of AKAP-150

We have used differential display to profile and compare the mRNAs expressed in the hippocampus of freely moving animals after the induction of long-term potentiation (LTP) at the perforant path-dentate gyrus synapse with control rats receiving low-frequency stimulation. We have combined this with in situ hybridization and have identified A-kinase anchoring protein of 150 kDa (AKAP-150) as a gene selectively up-regulated during the maintenance phase of LTP. AKAP-150 mRNA has a biphasic modulation in the dentate gyrus following the induction of LTP. The expression of AKAP-150 was 29% lower than stimulated controls 1 h after the induction of LTP. Its expression was enhanced 3 (50%), 6 (239%) and 12 h (210%) after induction, returning to control levels by 24 h postinduction. The NMDA receptor antagonist CPP blocked the tetanus-induced modulation of AKAP-150 expression. Interestingly, strong generalized stimulation produced by electroconvulsive shock did not increase the expression of AKAP-150. This implies that the AKAP-150 harbours a novel property of selective responsiveness to the stimulation patterns that trigger NMDA-dependent LTP in vivo. Its selective up-regulation during LTP and its identified functions as a scaffold for protein kinase A, protein kinase C, calmodulin, calcineurin and ionotropic glutamate receptors suggest that AKAP-150 encodes is an important effector protein in the expression of late LTP.

Seizure-induced gene expression in area CA1 of the mouse hippocampus

Synaptic plasticity in the hippocampus requires activity-dependent gene expression. We have therefore profiled gene expression in area CA1 following the induction of an electroshock-evoked maximal seizure. Using cDNA microarrays, the differential expression of approximately 9000 cDNAs was examined. In situ hybridization on 14 transcripts that showed strongest modulation in the microarray screen (1.8-2-fold) confirmed the differential expression of a single gene that encodes for the nuclear hormone receptor NGFI-B (Nur77, N10). Although this gene is only modestly up-regulated (approximately 2-fold) in area CA1, in situ hybridization revealed that maximal seizures induce a marked (approximately 12-fold) up-regulation of NGFI-B in the dentate gyrus. These data support the notion [French et al. (2001) Eur. J. Neurosci., 13, 968-976] that CA1 pyramidal neurons are more refractory than granule cells of the dentate gyrus with respect to activity-dependent gene transcription. Furthermore, our results argue against a large cohort of activity-dependent genes in area CA1.

SB-243213; a selective 5-HT2C receptor inverse agonist with improved anxiolytic profile: lack of tolerance and withdrawal anxiety

SB-243213 (5-methyl-1-[[-2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-6-trifluoromethylindoline hydrochloride) is a new, selective 5-hydroxytryptamine (5-HT)2C receptor inverse agonist. SB-243213 has high affinity for the human 5-HT2C receptor (pK(i) 9.37) and greater than a 100-fold selectivity over a wide range of neurotransmitter receptors, enzymes and ion channels. In in vitro functional studies, SB-243213 acted as an inverse agonist at the human 5-HT2C receptor with a pK(b) of 9.8. In in vivo studies, SB-243213 was a potent inhibitor of central 5-HT2C receptor-mediated function in rats, blocking meta-chlorophenylpiperazine-induced hypolocomotion with an ID50 of 1.1 mg/kg p.o. and a long duration of action (>8 h). In rats, SB-243213 exhibited anxiolytic-like activity in both the social interaction and Geller-Seifter conflict tests. Importantly, unlike diazepam, chronic administration of SB-243213 did not result in the development of either tolerance to the anxiolytic-like effects or withdrawal anxiogenesis. Furthermore, in rodents, SB-243213 did not affect seizure threshold, did not increase body weight or induce catalepsy, but attenuated the haloperidol-induced catalepsy. SB-243213 did not affect amphetamine-, MK-801- or phencyclidine-induced hyperactivity. In conclusion, SB-243213 may possess an improved anxiolytic profile compared to benzodiazepines. SB-243213 also modulates dopaminergic transmission, lacks pro-psychotic properties and may have utility in the treatment of schizophrenia and motor disorders.

Epileptogenesis and enhanced prepulse inhibition in GABA(B1)-deficient mice

The recent cloning of two GABA(B) receptor subunits, GABA(B1) and GABA(B2), has raised the possibility that differences in GABA(B) receptor subunit composition may give rise to pharmacologically or functionally distinct receptors. If present, such molecular diversity could permit the selective targeting of GABA(B) receptor subtypes specifically involved in pathologies such as drug addiction, spasticity, pain, and epilepsy. To address these issues we have developed a GABA(B1) subunit knockout mouse using gene targeting techniques. In the brains of GABA(B1) null mice, all pre- and postsynaptic GABA(B) receptor function was absent demonstrating that the GABA(B1) subunit is essential for all GABA(B) receptor-mediated mechanisms. Despite this, GABA(B1) null mice appeared normal at birth, although by postnatal week four their growth was retarded and they developed a generalized epilepsy that resulted in premature death. In addition, GABA(B1) heterozygote animals showed enhanced prepulse inhibition responses compared to littermate controls, suggesting that GABA(B1) deficient mice exhibit increased sensorimotor gating mechanisms. These data suggest that GABA(B) receptor antagonists may be of benefit in the treatment of psychiatric and neurological disorders in which attentional processing is impaired.

Subfield-specific immediate early gene expression associated with hippocampal long-term potentiation in vivo

It is not known whether NMDA receptor-dependent long-term potentiation (LTP) is mediated by similar molecular mechanisms in different hippocampal areas. To address this question we have investigated changes in immediate early gene and protein expression in two hippocampal subfields following the induction of LTP in vivo and in vitro. In granule cells of the dentate gyrus, LTP induced in vivo by tetanic stimulation of the perforant path was followed by strong induction of the immediate early genes (IEGs) Zif268, Arc and Homer. The increase in Zif268 mRNA was accompanied by an increase in protein expression. In contrast, we were unable to detect modulation of the IEGs Zif268, Arc, Homer and HB-GAM following induction of LTP by high-frequency stimulation of the commissural projection to CA1 pyramidal cells in vivo. In this pathway, we also failed to detect modulation of Zif268 protein levels. Zif268, Arc and Homer can be modulated in CA1 pyramidal cells approximately twofold after electroshock-induced maximal seizure, which demonstrates potential responsiveness to electrical stimuli. When LTP was induced in vitro neither CA1 pyramidal cells nor granule cells showed an increase in Zif268, Arc or Homer mRNA. However, in the slice preparation, granule cells have a different transcriptional state as basal IEG levels are elevated. These results establish the existence of subfield-specific transcriptional responses to LTP-inducing stimulation in the hippocampus of the intact animal, and demonstrate that in area CA1-enhanced transcription of Zif268, Arc and Homer is not required for the induction of late LTP.

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.

Characterization of SB-269970-A, a selective 5-HT(7) receptor antagonist

The novel 5-HT(7) receptor antagonist, SB-269970-A, potently displaced [(3)H]-5-CT from human 5-HT(7(a)) (pK(i) 8.9+/-0.1) and 5-HT(7) receptors in guinea-pig cortex (pK(i) 8.3+/-0.2). 5-CT stimulated adenylyl cyclase activity in 5-HT(7(a))/HEK293 membranes (pEC(50) 7.5+/-0.1) and SB-269970-A (0.03 - 1 microM) inhibited the 5-CT concentration-response with no significant alteration in the maximal response. The pA(2) (8.5+/-0.2) for SB-269970-A agreed well with the pK(i) determined from [(3)H]-5-CT binding studies. 5-CT-stimulated adenylyl cyclase activity in guinea-pig hippocampal membranes (pEC(50) of 8.4+/-0.2) was inhibited by SB-269970-A (0.3 microM) with a pK(B) (8.3+/-0.1) in good agreement with its antagonist potency at the human cloned 5-HT(7(a)) receptor and its binding affinity at guinea-pig cortical membranes. 5-HT(7) receptor mRNA was highly expressed in human hypothalamus, amygdala, thalamus, hippocampus and testis. SB-269970-A was CNS penetrant (steady-state brain : blood ratio of ca. 0.83 : 1 in rats) but was rapidly cleared from the blood (CLb=ca. 140 ml min(-1) kg(-1)). Following a single dose (3 mg kg(-1)) SB-269970 was detectable in rat brain at 30 (87 nM) and 60 min (58 nM). In guinea-pigs, brain levels averaged 31 and 51 nM respectively at 30 and 60 min after dosing, although the compound was undetectable in one of the three animals tested. 5-CT (0.3 mg kg(-1) i.p.) induced hypothermia in guinea-pigs was blocked by SB-269970-A (ED(50) 2.96 mg kg(-1) i.p.) and the non-selective 5-HT(7) receptor antagonist metergoline (0.3 - 3 mg kg(-1) s.c.), suggesting a role for 5-HT(7) receptor stimulation in 5-CT induced hypothermia in guinea-pigs. SB-269970-A (30 mg kg(-1)) administered at the start of the sleep period, significantly reduced time spent in Paradoxical Sleep (PS) during the first 3 h of EEG recording in conscious rats.

Studies on the role of 5-HT2C and 5-HT2B receptors in regulating generalised seizure threshold in rodents

The present studies were conducted to investigate the role of 5-HT2C and 5-HT2B receptors in the generation of pentylenetetrazol and electroshock-evoked seizures. The 5-HT2C/2B receptor-preferring agonist 1-(m-chlorophenyl)-piperazine (mCPP; 2.5-7 mg/kg i.p.) weakly elevated seizure threshold in the mouse (but not the rat) electroshock test and also provided appreciable protection against pentylenetetrazol-induced myoclonic and/or tonic seizures in mice and rats, an action that was inhibited by the 5-HT2C/2B receptor antagonist 5-methyl-1-(3-pyridylcarbomoyl)-1,2,3,5-tetrahydropyrrolo[2, 3-f]indole (SB-206553; 10-20 mg/kg p.o.). In contrast, the 5-HT2B receptor agonist 1-[5-(2-thienylmethoxy)-1H-3-indoyl]propan-2-amine hydrochloride (BW-723C86; 3-30 mg/kg s.c.) had no effect on the threshold for generalised seizures in any of the models employed. These results indicate that the observed anticonvulsant effects of mCPP are likely to be mediated by activation of 5-HT2C receptors. However, blockade of these receptors in mice (or rats) by SB-206553 (5-20 mg/kg p.o.) did not result in the reduced seizure threshold characteristic of mutant mice deficient of 5-HT2C receptors, suggesting that in normal adult animals this receptor subtype may usually be subjected to only a low level of 5-hydroxytryptamine tone.

SB 242084, a selective and brain penetrant 5-HT2C receptor antagonist

SB 242084 has a high affinity (pKi 9.0) for the cloned human 5-HT2C receptor and 100- and 158-fold selectivity over the closely related cloned human 5-HT2B and 5-HT2A subtypes respectively. SB 242084 had over 100-fold selectivity over a range of other 5-HT, dopamine and adrenergic receptors. In studies of 5-HT-stimulated phosphatidylinositol hydrolysis using SH-SY5Y cells stably expressing the cloned human 5-HT2C receptor, SB 242084 acted as an antagonist with a pKb of 9.3, which closely resembled its corresponding receptor binding affinity. SB 242084 potently inhibited m-chlorophenylpiperazine (mCPP, 7 mgkg i.p. 20 min pre-test)-induced hypolocomotion in rats, a model of in vivo central 5-HT2C receptor function, with an ID50 of 0.11 mg/kg i.p., and 2.0 mg/kg p.o. SB 242084 (0.1-1 mg/kg i.p.) exhibited an anxiolytic-like profile in the rat social interaction test, increasing time spent in social interaction, but having no effect on locomotion. SB 242084 (0.1-1 mg/kg i.p.) also markedly increased punished responding in a rat Geller-Seifter conflict test of anxiety, but had no consistent effect on unpunished responding. A large acute dose of SB 242084 (30 mg/kg p.o.) had no effect on seizure susceptibility in the rat maximal electroshock seizure threshold test. Also, while SB 242084 (2 and 6 mg/kg p.o. 1 hr pre-test) antagonized the hypophagic response to mCPP, neither acute nor subchronic administration of the drug, for 5 days at 2 or 6 mg/kg p.o. twice daily, affected food intake or weight gain. The results suggest that SB 242084 is the first reported selective potent and brain penetrant 5-HT2C receptor antagonist and has anxiolytic-like activity, but does not possess either proconvulsant or hyperphagic properties which are characteristic of mutant mice lacking the 5-HT2C receptor.

The novel anticonvulsant SB 204269 binds to a stereospecific site in the mouse brain

The novel compound SB 204269 (trans-(+)-6-acetyl-4S-(4-fluorobenzoylamino)- 3,4-dihydro-2,2-dimethyl-2H-benzo[b]pyran-3R-ol, hemihydrate) shows potent anticonvulsant activity in the mouse maximal electroshock seizure threshold test. The binding of [3H]SB 204269 to mouse forebrain membranes is saturable (Bmax 217 fmol/mg protein, Kd 32 nM) and stereospecific. The excellent anticonvulsant profile of SB 204269, combined with the identification of a unique binding site for the compound, suggest that it has potential clinical utility as a novel treatment for epilepsy.