Carol Routledge

Functional MRI detection of pharmacologically induced memory impairment

To examine alterations in brain activation associated with pharmacologically induced memory impairment, we used functional MRI (fMRI) to study the effects of lorazepam and scopolamine on a face–name associative encoding paradigm. Ten healthy young subjects were scanned on four occasions, 2 weeks apart; they were administered i.v. saline during two placebo-scanning sessions and then alternately administered i.v. lorazepam (1 mg) or scopolamine (0.4 mg) in a double-blind, randomized, cross-over design. Both the extent and magnitude of activation within anatomic regions of interest (ROIs) were examined to determine the reproducibility of activation in the placebo sessions and the regional specificity of the pharmacologic effects. Activation within all ROIs was consistent across the two placebo scans during the encoding of novel face–name pairs (compared with visual fixation). With the administration of either lorazepam or scopolamine, significant decreases were observed in both the extent and magnitude of activation within the hippocampal, fusiform, and inferior prefrontal ROIs, but no significant alterations in activation in the striate cortex were found. Both medications impaired performance on postscan memory measures, and significant correlations between memory performance and extent of activation were found in hippocampal and fusiform ROIs. These findings suggest that pharmacologic effects can be detected with fMRI by using a reproducible experimental paradigm and that medications that impair memory also diminish activation in specific brain regions thought to subserve complex memory processes.

Characterization of SB-271046 : A potent, selective and orally active 5-HT6 receptor antagonist

SB‐271046, potently displaced [3H]‐LSD and [125I]‐SB‐258585 from human 5‐HT6 receptors recombinantly expressed in HeLa cells in vitro (pKi 8.92 and 9.09 respectively). SB‐271046 also displaced [125I]‐SB‐258585 from human caudate putamen and rat and pig striatum membranes (pKi 8.81, 9.02 and 8.55 respectively). SB‐271046 was over 200 fold selective for the 5‐HT6 receptor vs 55 other receptors, binding sites and ion channels. In functional studies on human 5‐HT6 receptors SB‐271046 competitively antagonized 5‐HT‐induced stimulation of adenylyl cyclase activity with a pA2 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 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 (EC50 of 0.16 μM) and brain concentrations of 0.01–0.04 μM at Cmax. These data, together with the observed anticonvulsant activity of other selective 5‐HT6 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‐HT6 receptors. Overall, these studies demonstrate that SB‐271046 is a potent and selective 5‐HT6 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‐HT6 receptors.

Characterization of [125I]-SB-258585 binding to human recombinant and native 5-HT6 receptors in rat, pig and human brain tissue

SB‐258585 (4‐Iodo‐N‐[4‐methoxy‐3‐(4‐methyl‐piperazin‐1‐yl)‐phenyl]‐benzenesulphonamide) is a high affinity ligand at 5‐HT6 receptors. It displays over 100 fold selectivity for the 5‐HT6 receptor over all other 5‐HT receptors tested so far. SB‐258585 has been radiolabelled, to high specific activity, for its characterization as a 5‐HT6 receptor selective radioligand. [125I]‐SB‐258585 bound, with high affinity, to a single population of receptors in a cell line expressing human recombinant 5‐HT6 receptors. Kinetic and saturation binding experiments gave pKD values of 9.01±0.09 and 9.09±0.02, respectively. In membranes derived from rat or pig striatum and human caudate putamen, [125I]‐SB‐258585 labelled a single site with high levels (>60%) of specific binding. Saturation analysis revealed pKD values of 8.56±0.07 for rat, 8.60±0.10 for pig and 8.90±0.02 for human. Bmax values for the tissues ranged from 173±23 and 181±25 fmol mg−1 protein in rat and pig striatum, respectively, to 215±41 fmol mg−1 protein in human caudate putamen. The pKi rank order of potency for a number of compounds, determined in competition binding assays with [125I]‐SB‐258585, at human caudate putamen membranes was: SB‐271046>SB‐258585>SB‐214111>methiothepin>clozapine>5‐Me‐OT>5‐HT>Ro 04‐6790>mianserin>ritanserin=amitriptyline>5‐CT>mesulergine. Similar profiles were obtained from pig and rat striatal membranes and recombinant 5‐HT6 receptors; data from the latter correlated well with [3H]‐LSD binding. Thus, [125I]‐SB‐258585 is a high affinity, selective radioligand which can be used to label both recombinant and native 5‐HT6 receptors and will facilitate further characterization of this receptor subtype in animal and human tissues.

Phenyl benzenesulfonamides are novel and selective 5-HT6 antagonists: identification of N-(2,5-dibromo-3-fluorophenyl)-4-methoxy-3-piperazin-1-ylbenzenesulfonamide (SB-357134)

Substituted N-phenyl-4-methoxy-3-piperazin-1-ylbenzenesulfonamides and conformationally restricted analogues have been identified as high affinity and selective 5-HT6 antagonists. Compounds from this series had a range of pharmacokinetic profiles in rat and in general there was a correlation between clearance and CNS penetration. Based on its overall biological profile 2 (SB-357134) was selected for further pre-clinical evaluation.

Importance of h5-HT1B Receptor Selectivity for 5-HT Terminal Autoreceptor Activity: an In Vivo Microdialysis Study in the Freely-moving Guinea-pig

The importance of h5-HT1B receptor selectivity for 5-HT terminal autoreceptor activity was investigated with the selective h5-HT1B receptor ligands SB 219085, SB 220272, SB 224289 and SB 216641. The studies employed measurement of compound affinity and efficacy in vitro and the measurement of extracellular 5-HT in the frontal cortex of the freely-moving guinea-pig using in vivo microdialysis. All compounds had high affinity and selectivity for the h5-HT1B receptor, with SB 224289 the most selective for h5-HT1B over h5-HT1D receptors. Compounds exhibited a range of efficacies at both receptors: SB 224289 and SB 219085 were inverse agonists, SB 220272 was an antagonist and SB 216641 was a partial agonist. SB 220272, SB 216641 and SB 224289 had no effect on extracellular 5-HT following systemic administration, however, SB 219085 produced a significant increase. The SB 219085-induced increase in extracellular 5-HT was attributed to the compounds non-specific releasing properties as it was also demonstrated to increase basal release of [3H]5-HT from pre-loaded guinea-pig cortical slices. The lack of effect of the above h5-HT1B receptor selective compounds and the decrease in extracellular 5-HT elicited by the non-selective compounds GR 127935, GR125743 and methiothepin suggest that antagonism of 5-HT1D receptors may mediate this decrease in 5-HT levels. It is plausible that blockade of 5-HT1D receptors increases 5-HT levels in the raphe, this activates 5-HTtA receptors which results in an overall decrease in terminal 5-HT release. Definitive proof now awaits elucidation of the action of a selective 5-HT1D receptor antagonist.

Enhancement of 5-HT1B and 5-HT1D receptor antagonist effects on extracellular 5-HT levels in the guinea-pig brain following concurrent 5-HT1A or 5-HT re-uptake site blockade.

The effects of selective serotonin re-uptake inhibitor (SSRI), paroxetine, and 5-HT1A, 5-HT1B and 5-HT1B/1D receptor antagonists on in vivo extracellular 5-HT levels in the guinea-pig frontal cortex and dorsal hippocampus were investigated using the technique of microdialysis. The aim of the study was to further investigate the autoreceptor roles of the 5-HT1A, 5-HT1B and 5-HT1D receptors in the median vs dorsal raphe nuclei. In the frontal cortex, 5-HT1A (WAY 100635, 1 mg/kg i.p.) or 5-HT1B (SB-224289, 4 mg/kg i.p.) receptor antagonists had no effect on extracellular levels of 5-HT, whilst the mixed 5-HT1B/1D receptor antagonist (GR 127935, 0.3 mg/kg i.p) produced a significant decrease in extracellular 5-HT levels. Paroxetine (10 microM) significantly increased extracellular 5-HT levels when perfused locally into the cortex. Administration of SB-224289, followed 120 min later by WAY 100635, had no effect on extracellular 5-HT levels. In contrast, sequential administration of either WAY 100635 and GR 127935, or SB-224289 and paroxetine significantly increased extracellular 5-HT levels. In the dorsal hippocampus, whilst 5-HT1A receptor antagonism elicited by administration of WAY 100635 had no effect, both 5-HT1B and mixed 5-HT1B/1D receptor blockade significantly increased extracellular 5-HT levels. Administration of SB-224289 followed 120 min later with WAY 100635, or WAY 100635 followed 30 min later with GR 127935, potentiated the effect of the three compounds alone, significantly increasing extracellular 5-HT levels. These data demonstrate that to simultaneously increase extracellular 5-HT in both frontal cortex and dorsal hippocampus of the guinea-pig brain concurrent 5-HTA1A, 5-HT1B and 5-HT1D receptor blockade is required. Whereas in the dorsal hippocampus, 5-HT1B receptor blockade is sufficient to elicit an increase in extracellular 5-HT levels.

Evidence to Suggest that Agonist Modulation of Hyperlocomotion is Via Post-synaptic Dopamine D2 or D3 Receptors

It has been suggested that a sub-population of dopamine D3 receptors is located pre-synaptically and these serve as autoreceptors in dopamine projection areas such as the nucleus accumbens/ventral striatum. To study further the physiological role and synaptic location of the dopamine D3 receptor, we have investigated the in vivo effect of the D3/D2 receptor agonist quinelorane on amphetamine-induced hyperactivity and extracellular dopamine release from the nucleus accumbens of the conscious rat. Amphetamine increased dopamine release to 202 +/- 34% of pre-injection control values, but quinelorane at 2.5 micrograms/kg, a dose which effectively blocked amphetamine-induced hyperlocomotion, had no significant effect on amphetamine-induced dopamine release. These data suggest that hyperlocomotion is mediated via post-synaptic rather than pre-synaptic dopamine receptors. Since quinelorane has significant affinity for the dopamine D3 receptor, these effects may be via post-synaptic D3 receptors; however, D2 receptor effects cannot be disregarded. In summary, these data indicate that the quinelorane effect on amphetamine-stimulated hyperlocomotion is not mediated via D3 or D2 autoreceptors, but rather a population of receptors located post-synaptically, which appear to mediate the inhibition of rat locomotor activity.

Novel (4-piperazin-1-ylquinolin-6-yl) arylsulfonamides with high affinity and selectivity for the 5-HT6 receptor

The discovery of (4-piperazin-1-ylquinolin-6-yl) arylsulfonamides and their binding affinities for a selection of 5-HT and dopamine subreceptors is described. Many compounds show high affinity (pK(i)>8) for the 5-HT(6) receptor and >100-fold selectivity against a range of other receptors. Structure-activity relationships of these compounds are discussed.

Bicyclic piperazinylbenzenesulphonamides are potent and selective 5-HT6 receptor antagonists.

The synthesis of novel 3-(octahydropyrido[1,2-a]pyrazin-2-yl)- and 3-(hexahydropyrrolo[1,2-a]pyrazin-2-yl)phenyl-2-benzo[b]thiophene sulphonamide derivatives 3, (S)-4 and (R)-4 is described. The compounds show high affinity for the 5-HT6 receptor, excellent selectivity against a range of other receptors and good brain penetration.

Methylphenidate but not sulpiride modulates cognitive activation in healthy elderly volunteers

Introduction: Functional MRI can measure pharmacological modulation of cognitive activation by dopaminergic drugs in humans [l]. The safety and acceptability of the technique encourages its application to study age-related changes in dopaminergic function that are hypothetically anticipated by studies of aged monkeys [2]. A clearer understanding of normative developmental change in human dopaminergic function would be relevant to many neuropsychiatric disorders and might also help to explain normal age-related cognitive decline.