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

Dose-dependent effect and pharmacokinetics of fexinidazole and its metabolites in a mouse model of human African trypanosomiasis

Human African trypanosomiasis (HAT) is a neglected tropical disease, with a population of 70 million at risk. Current treatment options are limited. In the search for new therapeutics, the repurposing of the broad-spectrum antiprotozoal drug fexinidazole has completed Phase III trials with the anticipation that it will be the first oral treatment for HAT. This study used the recently validated bioluminescence imaging model to assess the dose and rate of kill effect of fexinidazole in infected mice, and the dose-dependent effect of fexinidazole on trypanosome infection. Pharmacokinetics of fexinidazole in plasma and central nervous system (CNS) compartments were similar in both infected and uninfected mice. Drug distribution within the CNS was further examined by microdialysis, showing similar levels in the cortex and hippocampus. However, high variability in drug distribution and exposure was found between mice.

Looking back (and in)to the future: A personal reflection on 'Serotonin autoreceptor function and antidepressant drug action' (Hjorth et al., 2000)

Our article in this journal some 15 years ago focussed on the role of serotonin (5-HT) autoreceptors in the mechanism of action of antidepressant drugs. Specifically in this regard, the results were summarised of rat microdialysis studies carried out to examine: (a) the relative importance of 5-HT_1A and 5-HT_1B autoreceptors, including (b) possible regional variation, and (c) potential changes in autoreceptor responsiveness following chronic selective serotonin reuptake inhibitor administration. In the present reflection piece, I recap some of the key findings against a brief background and provide an account of their bearing within the context of subsequent endeavours in the antidepressant drug research and development field. I conclude by shortly commenting on selected topics relevant to novel, interesting advances and avenues for future research.

Evaluation of the pro-cognitive effects of the AMPA receptor positive modulator, 5-(1-piperidinylcarbonyl)-2,1,3-benzoxadiazole (CX691), in the rat

RATIONALE

Positive allosteric modulators of the glutamatergic alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) receptor do not stimulate AMPA receptors directly but delay deactivation of the receptor and/or slow its desensitisation. This results in increased synaptic responses and enhanced long-term potentiation. Thus, it has been suggested that such compounds may have utility for the treatment of cognitive impairment.

OBJECTIVES

The objective of the study was to investigate the effect of an AMPA positive modulator, CX691, (1) in three rodent models of learning and memory, (2) on neurochemistry in the dorsal hippocampus and medial prefrontal cortex following acute administration, and (3) on brain-derived neurotrophic factor (BDNF) messenger RNA (mRNA) expression in the rat hippocampus following acute and sub-chronic administration.

RESULTS

CX691 attenuated a scopolamine-induced impairment of cued fear conditioning following acute administration (0.1 mg/kg p.o.) and a temporally induced deficit in novel object recognition following both acute (0.1 and 1.0 mg/kg p.o.) and sub-chronic (bi-daily for 7 days) administration (0.01, 0.03, 0.1 mg/kg p.o.). It also improved attentional set-shifting following sub-chronic administration (0.3 mg/kg p.o.). Acute CX691 (0.1, 0.3 and 1.0 mg/kg, p.o.) increased extracellular levels of acetylcholine in the dorsal hippocampus and medial prefrontal cortex and dopamine in the medial prefrontal cortex. Sub-chronic administration of CX691 (0.1 mg/kg, p.o.) elevated BDNF mRNA expression in both the whole and CA(1) sub-region of the hippocampus (P < 0.05).

CONCLUSIONS

Collectively, these data support the pro-cognitive activity reported for AMPA receptor positive modulators and suggest that these compounds may be of benefit in treating disorders characterised by cognitive deficits such as Alzheimer's disease and schizophrenia.

Preclinical investigations into the antipsychotic potential of the novel histamine H3 receptor antagonist GSK207040

OBJECTIVES

To test the novel nonimidazole histamine H3 receptor antagonist 5-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazapin-7-yl)oxy]-N-methyl-2-pyrazinecarboxamide (GSK207040) in a series of behavioral and neurochemical paradigms designed to evaluate its antipsychotic potential.

MATERIALS AND METHODS

Acute orally administered GSK207040 was investigated for its capacity to reverse a 24-h-induced deficit in novel object recognition memory, deficits in prepulse inhibition (PPI) induced by isolation rearing, and hyperlocomotor activity induced by amphetamine. The acute neurochemical effects of GSK207040 were explored by analyzing rat anterior cingulate cortex microdialysates for levels of dopamine, noradrenaline, and acetylcholine and by c-fos immunohistochemistry. The potential for interaction with the antipsychotic dopamine D2 receptor antagonist haloperidol was explored behaviorally (spontaneous locomotor activity and catalepsy), biochemically (plasma prolactin), and via ex vivo receptor occupancy determinations.

RESULTS

GSK207040 significantly enhanced object recognition memory (3 mg/kg) and attenuated isolation rearing-induced deficits in PPI (1.0 and 3.2 mg/kg) but did not reverse amphetamine-induced increases in locomotor activity. There was no evidence of an interaction of GSK207040 with haloperidol. GSK207040 (3.2 mg/kg) raised extracellular concentrations of dopamine, noradrenaline, and acetylcholine in the anterior cingulate cortex and c-fos expression in the core of the nucleus accumbens was increased at doses of 3.2 and 10.0 mg/kg.

CONCLUSIONS

The behavioral and neurochemical profile of GSK207040 supports the potential of histamine H3 receptor antagonism to treat the cognitive and sensory gating deficits of schizophrenia. However, the failure of GSK207040 to reverse amphetamine-induced locomotor hyperactivity suggests that the therapeutic utility of histamine H(3) receptor antagonism versus positive symptoms is less likely, at least following acute administration.

In vitro and in vivo characterization of the non-peptide NK3 receptor antagonist SB-223412 (talnetant): potential therapeutic utility in the treatment of schizophrenia

Neurokinin-3 (NK3) receptors are concentrated in forebrain and basal ganglia structures within the mammalian CNS. This distribution, together with the modulatory influence of NK3 receptors on monoaminergic neurotransmission, has led to the hypothesis that NK3 receptor antagonists may have therapeutic efficacy in the treatment of psychiatric disorders. Here we describe the in vitro and in vivo characterization of the highly selective NK3 receptor antagonist talnetant (SB-223412). Talnetant has high affinity for recombinant human NK3 receptors (pKi 8.7) and demonstrates selectivity over other neurokinin receptors (pKi NK2 = 6.6 and NK1<4). In native tissue-binding studies, talnetant displayed high affinity for the guinea pig NK3 receptor (pKi 8.5). Functionally, talnetant competitively antagonized neurokinin B (NKB)-induced responses at the human recombinant receptor in both calcium and phosphoinositol second messenger assay systems (pA2 of 8.1 and 7.7, respectively). In guinea pig brain slices, talnetant antagonized NKB-induced increases in neuronal firing in the medial habenula (pKB = 7.9) and senktide-induced increases in neuronal firing in the substantia nigra pars compacta (pKB = 7.7) with no diminution of maximal agonist efficacy, suggesting competitive antagonism at native NK3 receptors. Talnetant (3-30 mg/kg i.p.) significantly attenuated senktide-induced 'wet dog shake' behaviors in the guinea pig in a dose-dependent manner. Microdialysis studies demonstrated that acute administration of talnetant (30 mg/kg i.p.) produced significant increases in extracellular dopamine and norepinephrine in the medial prefrontal cortex and attenuated haloperidol-induced increases in nucleus accumbens dopamine levels in the freely moving guinea pigs. Taken together, these data demonstrate that talnetant is a selective, competitive, brain-penetrant NK3 receptor antagonist with the ability to modulate mesolimbic and mesocortical dopaminergic neurotransmission and hence support its potential therapeutic utility in the treatment of schizophrenia.

GSK189254, a novel H3 receptor antagonist that binds to histamine H3 receptors in Alzheimer's disease brain and improves cognitive performance in preclinical models

6-[(3-Cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy]-N-methyl-3-pyridinecarboxamide hydrochloride (GSK189254) is a novel histamine H(3) receptor antagonist with high affinity for human (pK(i) = 9.59 -9.90) and rat (pK(i) = 8.51-9.17) H(3) receptors. GSK189254 is >10,000-fold selective for human H(3) receptors versus other targets tested, and it exhibited potent functional antagonism (pA(2) = 9.06 versus agonist-induced changes in cAMP) and inverse agonism [pIC(50) = 8.20 versus basal guanosine 5'-O-(3-[(35)S]thio)triphosphate binding] at the human recombinant H(3) receptor. In vitro autoradiography demonstrated specific [(3)H]GSK189254 binding in rat and human brain areas, including cortex and hippocampus. In addition, dense H(3) binding was detected in medial temporal cortex samples from severe cases of Alzheimer's disease, suggesting for the first time that H(3) receptors are preserved in late-stage disease. After oral administration, GSK189254 inhibited cortical ex vivo R-(-)-alpha-methyl[imidazole-2,5(n)-(3)H]histamine dihydrochloride ([(3)H]R-alpha-methylhistamine) binding (ED(50) = 0.17 mg/kg) and increased c-Fos immunoreactivity in prefrontal and somatosensory cortex (3 mg/kg). Microdialysis studies demonstrated that GSK189254 (0.3-3 mg/kg p.o.) increased the release of acetylcholine, noradrenaline, and dopamine in the anterior cingulate cortex and acetylcholine in the dorsal hippocampus. Functional antagonism of central H(3) receptors was demonstrated by blockade of R-alpha-methylhistamine-induced dipsogenia in rats (ID(50) = 0.03 mg/kg p.o.). GSK189254 significantly improved performance of rats in diverse cognition paradigms, including passive avoidance (1 and 3 mg/kg p.o.), water maze (1 and 3 mg/kg p.o.), object recognition (0.3 and 1 mg/kg p.o.), and attentional set shift (1 mg/kg p.o.). These data suggest that GSK189254 may have therapeutic potential for the symptomatic treatment of dementia in Alzheimer's disease and other cognitive disorders.

Differentiating antidepressants of the future: Efficacy and safety

There have been significant advances in the treatment of depression since the serendipitous discovery that modulating monoaminergic neurotransmission may be a pathological underpinning of the disease. Despite these advances, particularly over the last 15years with the introduction of selective serotonin and/or norepinephrine reuptake inhibitors (SNRI), there still remain multiple unmet clinical needs that would represent substantial improvements to current treatment regimens. In terms of efficacy there have been improvements in the percentage of patients achieving remission but this can still be dramatically improved and, in fact, issues still remain with relapse. Furthermore, advances are still required in terms of improving the onset of efficacy as well as addressing the large proportion of patients who remain treatment resistant. While this is not well understood, collective research in the area suggests the disease is heterogeneous in terms of the multiple parameters related to etiology, pathology and response to pharmacological agents. In addition to efficacy further therapeutic advances will also need to address such issues as cognitive impairment, pain, sexual dysfunction, nausea and emesis, weight gain and potential cardiovascular effects. With these unmet needs in mind, the next generation of antidepressants will need to differentiate themselves from the current array of therapeutics for depression. There are multiple strategies for addressing unmet needs that are currently being investigated. These range from combination monoaminergic approaches to subtype selective agents to novel targets that include mechanisms to modulate neuropeptides and excitatory amino acids (EAA). This review will discuss the many facets of differentiation and potential strategies for the development of novel antidepressants.

Blockade of 5-HT1A receptors by (+/-)-pindolol potentiates cortical 5-HT outflow, but not antidepressant-like activity of paroxetine: microdialysis and behavioral approaches in 5-HT1A receptor knockout mice

Selective serotonin reuptake inhibitors like paroxetine (Prx) often requires 4-6 weeks to achieve clinical benefits in depressed patients. Pindolol shortens this delay and it has been suggested that this effect is mediated by somatodendritic 5-hydroxytryptamine (5-HT) 1A autoreceptors. However clinical data on the beneficial effects of pindolol are conflicting. To study the effects of (+/-)-pindolol-paroxetine administration, we used genetical and pharmacological approaches in 5-HT1A knockout mice (5-HT1A-/-). Two assays, in vivo intracerebral microdialysis in awake mice and the forced swimming test (FST), were used to assess the antidepressant-like effects of this drug combination. Basal levels of extracellular serotonin, 5-HT ([5-HT]ext) in the frontal cortex (FCX) and the dorsal raphe nucleus (DRN) did not differ between the two strains of mice, suggesting a lack of tonic control of 5-HT1A autoreceptors on nerve terminal 5-HT release. Prx (1 and 4 mg/kg) dose-dependently increased cortical [5-HT]ext in both genotypes, but the effects were greater in mutants. The selective 5-HT1A receptor antagonist, WAY-100635 (0.5 mg/kg), or (+/-)-pindolol (5 and 10 mg/kg) potentiated the effects of Prx (4 mg/kg) on cortical [5-HT]ext in 5-HT1A+/+, but not in 5-HT1A-/- mice. Similar responses were obtained following local intra-raphe perfusion by reverse microdialysis of either WAY-100635 or (+/-)-pindolol (100 microM each). In the FST, Prx administration dose-dependently decreased the immobility time in both strains of mice, but the response was much greater in 5HT1A-/- mice. In contrast, (+/-)-pindolol blocked Prx-induced decreases in the immobility time while WAY-100635 had no effect in both genotypes. These findings using 5-HT1A-/- mice confirm that (+/-)-pindolol behaves as an antagonist of 5-HT1A autoreceptor in mice, but its blockade of paroxetine-induced antidepressant-like effects in the FST may be due to its binding to other neurotransmitter 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.

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.

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.

5-HT(1A) and 5-HT(2A) receptors in the rat dorsal periaqueductal gray mediate the antipanic-like effect induced by the stimulation of serotonergic neurons in the dorsal raphe nucleus

RATIONALE

It has been proposed that the serotonergic pathway that connects the dorsal raphe nucleus (DRN) to the dorsal periaqueductal gray (DPAG) is implicated in the regulation of escape, a behavior that has been related to panic.

OBJECTIVES

We further evaluated this hypothesis by investigating whether intra-DRN injection of the 5-HT(1A) receptor antagonist WAY-100635 changes the escape response of rats submitted to the elevated T-maze. This test also measures inhibitory avoidance, which has been associated with generalized anxiety disorder. We also investigated whether the 5-HT(1A) and 5-HT(2A) receptors in the DPAG mediate the behavioral consequences induced by the injection of WAY-100635 into the DRN.

RESULTS

Intra-DRN injection of WAY-100635 facilitated inhibitory avoidance, while impairing escape. The same effect was obtained after intra-DRN injection of the glutamate receptor agonist kainic acid. Preadministration of WAY-100635 into the DPAG counteracted the effect induced by intra-DRN injection of WAY-100635 and of kainic acid on escape, but not on inhibitory avoidance. Preadministration of the preferential 5-HT(2A) receptor antagonist ketanserin into the DPAG abolished the effects of intra-DRN injection of WAY-100635 on both elevated T-maze tasks.

CONCLUSION

The results are indicative that 5-HT(1A) autoreceptors in the DRN are under tonic inhibitory influence by endogenous 5-HT. The effects of 5-HT release in the DPAG after intra-DRN injection of WAY-100635 and kainic acid on inhibitory avoidance and escape involve different 5-HT receptor subtypes. Whereas 5-HT(2A) receptors in the DPAG seem to mediate the effect of 5-HT on both behaviors, 5-HT(1A) receptors are only involved in the regulation of escape.