Lynne E. Rueter

Pharmacological Properties of ABT-239 [4-(2-{2-[(2R)-2-Methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile]: II. Neurophysiological Characterization and Broad Preclinical Efficacy in Cognition and Schizophrenia of a Potent and Selective Histamine H3 Receptor Antagonist

Acute pharmacological blockade of central histamine H3 receptors (H3Rs) enhances arousal/attention in rodents. However, there is little information available for other behavioral domains or for repeated administration using selective compounds. ABT-239 [4-(2-{2-[(2R)-2-methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile] exemplifies such a selective, nonimidazole H3R antagonist with high affinity for rat (pKi = 8.9) and human (pKi = 9.5) H3Rs. Acute functional blockade of central H3 Rs was demonstrated by blocking the dipsogenia response to the selective H3R agonist (R)-α-methylhistamine in mice. In cognition studies, acquisition of a five-trial, inhibitory avoidance test in rat pups was improved with ABT-239 (0.1–1.0 mg/kg), a 10- to 150-fold gain in potency, with similar efficacy, over previous antagonists such as thioperamide, ciproxifan, A-304121 [(4-(3-(4-((2R)-2-aminopropanoyl)-1-piperazinyl)propoxy)phenyl)(cyclopropyl) methanone], A-317920 [N-((1R)-2-(4-(3-(4-(cyclopropylcarbonyl) phenoxy)propyl)-1-piperazinyl)-1-methyl-2-oxoethyl)-2-furamide], and A-349821 [(4′-(3-((R,R)2,5-dimethyl-pyrrolidin-1-yl)-propoxy)-biphenyl-4-yl)-morpholin-4-yl-methanone]. Efficacy in this model was maintained for 3 to 6 h and following repeated dosing with ABT-239. Social memory was also improved in adult (0.01–0.3 mg/kg) and aged (0.3–1.0 mg/kg) rats. In schizophrenia models, ABT-239 improved gating deficits in DBA/2 mice using prepulse inhibition of startle (1.0–3.0 mg/kg) and N40 (1.0–10.0 mg/kg). Furthermore, ABT-239 (1.0 mg/kg) attenuated methamphetamine-induced hyperactivity in mice. In freely moving rat microdialysis studies, ABT-239 enhanced acetylcholine release (0.1–3.0 mg/kg) in adult rat frontal cortex and hippocampus and enhanced dopamine release in frontal cortex (3.0 mg/kg), but not striatum. In summary, broad efficacy was observed with ABT-239 across animal models such that potential clinical efficacy may extend beyond disorders such as ADHD to include Alzheimers disease and schizophrenia.

Pharmacological Properties of ABT-239: II. Neurophysiological Characterization and Broad Preclinical Efficacy in Cognition and Schizophrenia of a Potent and Selective Histamine H3 Receptor Antagonist

Acute pharmacological blockade of central histamine H3 receptors (H3Rs) enhances arousal/attention in rodents. However, there is little information available for other behavioral domains or for repeated administration using selective compounds. ABT-239 [4-(2-{2-[(2R)-2-methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile] exemplifies such a selective, nonimidazole H3R antagonist with high affinity for rat (pKi = 8.9) and human (pKi = 9.5) H3Rs. Acute functional blockade of central H3 Rs was demonstrated by blocking the dipsogenia response to the selective H3R agonist (R)-α-methylhistamine in mice. In cognition studies, acquisition of a five-trial, inhibitory avoidance test in rat pups was improved with ABT-239 (0.1–1.0 mg/kg), a 10- to 150-fold gain in potency, with similar efficacy, over previous antagonists such as thioperamide, ciproxifan, A-304121 [(4-(3-(4-((2R)-2-aminopropanoyl)-1-piperazinyl)propoxy)phenyl)(cyclopropyl) methanone], A-317920 [N-((1R)-2-(4-(3-(4-(cyclopropylcarbonyl) phenoxy)propyl)-1-piperazinyl)-1-methyl-2-oxoethyl)-2-furamide], and A-349821 [(4′-(3-((R,R)2,5-dimethyl-pyrrolidin-1-yl)-propoxy)-biphenyl-4-yl)-morpholin-4-yl-methanone]. Efficacy in this model was maintained for 3 to 6 h and following repeated dosing with ABT-239. Social memory was also improved in adult (0.01–0.3 mg/kg) and aged (0.3–1.0 mg/kg) rats. In schizophrenia models, ABT-239 improved gating deficits in DBA/2 mice using prepulse inhibition of startle (1.0–3.0 mg/kg) and N40 (1.0–10.0 mg/kg). Furthermore, ABT-239 (1.0 mg/kg) attenuated methamphetamine-induced hyperactivity in mice. In freely moving rat microdialysis studies, ABT-239 enhanced acetylcholine release (0.1–3.0 mg/kg) in adult rat frontal cortex and hippocampus and enhanced dopamine release in frontal cortex (3.0 mg/kg), but not striatum. In summary, broad efficacy was observed with ABT-239 across animal models such that potential clinical efficacy may extend beyond disorders such as ADHD to include Alzheimers disease and schizophrenia.

A microdialysis examination of serotonin release in the rat forebrain induced by behavioral/environmental manipulations.

Serotonin (5-HT) has been implicated in a variety of behavioral and physiological processes mediated by the central nervous system. However, the exact nature of 5-HT release under naturalistic or physiologic conditions remains unclear. The present study investigated this issue by employing in vivo microdialysis to examine 5-HT release in the hippocampus, corpus striatum, amygdala, and prefrontal cortex of the rat during manipulations that induced varying behavioral and physiological responses. In each rat, two sites were examined simultaneously during two of the following manipulations: tail pinch, tail pinch with food present, feeding, cat exposure, floating, and swimming. Results indicate that 5-HT levels increased 20-65% above baseline levels in response to each manipulation in all forebrain areas examined. The only significant difference found across manipulations was that tail pinch with food present produced a greater increase in 5-HT than cat exposure, which may be attributable to differences in the degree of general activation induced by these manipulations rather than to specific physiological or behavioral aspects. Furthermore, there was a strong positive correlation between 5-HT release and time spent in alert waking. Finally, there was an overall significantly smaller increase in 5-HT release in the corpus striatum compared to the other three sites. These data suggest that the release of 5-HT in the forebrain is closely related to behavioral state, not to specific behaviors, and that the magnitude of 5-HT release may reflect the degree of activation. In addition, this study suggests there can be some degree of differential pattern of release of 5-HT in the forebrain.

A critical review of 5-HT brain microdialysis and behavior.

Serotonin (5-HT) has been implicated in many central nervous system-mediated functions including sleep, arousal, feeding, motor activity and the stress response. In order to help establish the precise role of 5-HT in physiology and behavior, in vivo microdialysis studies have sought to identify the conditions under which the release of 5-HT is altered. Extracellular 5-HT levels have been monitored in more than fifteen regions of the brain during a variety of spontaneous behaviors, and in response to several physiological, environmental, and behavioral manipulations. The vast majority of these studies found increases (30-100%) in 5-HT release in almost all brain regions studied. Since electrophysiological studies have shown that behavioral arousal is the primary determinant of brain serotonergic neuronal activity, we suggest that the increase in 5-HT release seen during a wide variety of experimental conditions is largely due to one factor, namely an increase in behavioral arousal/motor activity associated with the manipulation.

Behavioral profile of P2X7 receptor knockout mice in animal models of depression and anxiety: Relevance for neuropsychiatric disorders

The purinergic P2X(7) receptor is a ligand-gated ion channel found on peripheral macrophages and microglia in the nervous system. Activation of P2X(7) receptors results in the rapid release of interleukin-1 beta (IL-1 beta). Cytokines like IL-1 beta are suggested to be involved in the pathophysiology of depression. The aim of this study was to behaviorally profile P2X(7) receptor knockout (KO) mice in behavioral models of depression- and anxiety-like behaviors. P2X(7) receptor KO and wild type (WT) mice were tested in multiple models including; forced swim test, tail suspension test, elevated plus maze, novelty suppressed feeding, spontaneous locomotor activity, and food intake. P2X(7) receptor KO mice exhibited an antidepressant-like profile in tail suspension test and forced swim test; an effect that was not associated with changes in spontaneous locomotor activity. In addition, P2X(7) receptor KO mice showed higher responsivity to a subefficacious dose of the antidepressant drug imipramine (15 mg/kg) in forced swim test. No significant differences between genotypes were observed in models of anxiety. These data support the relevance of pro-inflammatory cytokines in depressive-like states, and suggest that P2X(7) receptor antagonists could be of potential interest for the treatment of affective disorders.

Enhancement of prepulse inhibition of startle in mice by the H3 receptor antagonists thioperamide and ciproxifan.

Histamine H3 receptor antagonists/inverse agonists have been proposed as potential therapeutic agents for the treatment of a number of neurological disorders ranging from attention deficit hyperactivity disorder and Alzheimers disease to narcolepsy and schizophrenia. With respect to the latter, schizophrenic patients typically exhibit impaired prepulse inhibition (PPI) of startle, a reflex that can be modeled in many animal species. Certain strains of mice naturally display poor PPI and it was recently suggested that these mice might offer a new way to screen for novel antipsychotic compounds. To examine whether H3 receptor antagonists might enhance PPI in mice with naturally occurring deficits, DBA/2 and C57BL/6 were tested in a startle paradigm with three prepulse intensities: 5, 10 and 15 dB above background. Both thioperamide and ciproxifan enhanced PPI in the DBA/2 strain; thioperamide also showed a trend towards enhancing PPI in C57BL/6. Risperidone, an atypical antipsychotic, enhanced PPI in both the DBA/2 and the C57BL/6 strain. These data confirm previous reports describing a natural deficit in PPI in some mouse strains that is amenable to enhancement with known antipsychotics. Further, these data suggest that H3 receptor antagonists/inverse agonists have anti-psychotic potential for disorders such as schizophrenia.

Neuronal release of serotonin in the cerebellum of behaving rats : An in vivo microdialysis study

Abstract: Release of endogenous serotonin [5‐hydroxy‐tryptamine (5‐HT)] in the cerebellum of awake rats was characterized using in vivo microdialysis. 5‐HT output was increased (∼70%) by local application of KCl (100 mM) and was reduced (∼60%) by both tetrodotoxin (0.5 µM) and omission of Ca2+ from the perfusion fluid. 5‐HT release was decreased (∼70%) by the selective 5‐HT1A agonist 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin (0.25 mg/kg, s.c.), and this effect was rapidly reversed by a selective 5‐HT1A antagonist, N‐[2‐[4‐(2‐methoxyphenyl)‐1‐piperazinyl]ethyl]‐N‐(2‐pyridinyl)cyclohexane‐carboxamide trihydrochloride (WAY‐100635; 0.1 mg/kg, i.p.). These results indicate that a large portion of the measurable 5‐HT output in the cerebellum is of neuronal origin, is dependent on impulse flow, and is sensitive to 5‐HT1A autoreceptor activation. Further studies examined the relationship between 5‐HT levels and general activity of the animals across the light‐dark transition and during behavioral manipulations. Both 5‐HT levels and behavioral activity were significantly elevated during the dark period, with changes in 5‐HT efflux closely paralleling changes in activity. Similar increases (∼40%) in 5‐HT output were observed during both feeding and feeding in the presence of a stressor (tail pinch). These findings suggest that behavioral state is an important factor determining neuronal 5‐HT release in cerebellum under physiological conditions.

Changes in forebrain serotonin at the light-dark transition : correlation with behaviour

IN vivo microdialysis in the rat was used to determine whether changes in forebrain serotonin (5-HT) levels seen during the light-dark transition differ by area examined and whether these changes could be accounted for by complementary changes in behaviour. 5-HT levels increased significantly during the first half hour of dark phase in the hippocampus (45.9%), striatum (13.4%), amygdala (19.7%) and prefrontal cortex (18.4%), the increase in the hippocampus being significantly larger than those in the other areas. 5-HT levels co-varied significantly with changes in the levels of alert waking, a behavioural measure of the time spent in active waking, suggesting that this factor might account for the changes in 5-HT across the light-dark transition.

Spinal mechanisms underlying A-85380-induced effects on acute thermal pain

Systemic administration of nicotinic receptor (nAChR) agonists is antinociceptive in models of acute pain whereas their intrathecal (i. t.) administration has been reported to be antinociceptive, nociceptive or without effect. It has been hypothesized that the action induced is dependent upon the subtype and location of the nAChR activated. In addition, there is considerable evidence that nAChR ligand-induced antinociception is mediated by other neurotransmitter systems via descending pathways from the brainstem to the spinal cord. The present study investigated the effects of i. t. and systemic administration of A-85380, a novel nAChR agonist, in the paw withdrawal model of acute thermal pain in the rat. Given i.t. , A-85380 (1 and 10 nmol/rat) decreased the latency to paw withdrawal by 2-4 s. This pronociception was accompanied by a spontaneous flinching behavior. Both of these effects were differentially blocked by i.t. pretreatment with the nAChR antagonists mecamylamine (10 nmol)>MLA (100 nmol)>DHbetaE (50% with 1000 nmol) but not by alpha-bungarotoxin (0% at 0.63 nmol). Given systemically, A-85380 (0.56 micromol/kg, i.p.) induced antinociception as indicated by an increased latency to paw withdrawal, an effect differentially altered by i.t. pretreatment with monoaminergic antagonists (100 nmol/rat). While mecamylamine and prazosin had no effect, scopolamine, methysergide and MDL 72222 partially antagonized and idazoxan completely antagonized A-85380-induced antinociception. Finally, as measured by in vivo microdialysis, levels of 5-HT, but not NE, in the i.t. space of the lumber region of the spinal cord were significantly increased following the systemic administration of A-85380. Together these data suggest that the nociceptive properties of spinally administered nAChR agents are not mediated by either an alpha(4)beta(2) or an alpha(7) subtype nAChR, whereas the antinociceptive properties of systemically-administered nAChR agents are mediated by descending noradrenergic, serotonergic and muscarinic inhibitory pathways.

Lack of cataleptogenic potentiation with non-imidazole H3 receptor antagonists reveals potential drug-drug interactions between imidazole-based H3 receptor antagonists and antipsychotic drugs.

Since H3 receptor (H3R) antagonists/inverse agonists can improve cognitive function in animal models, they may have the potential to be used as add-on therapy in the treatment of schizophrenia, a disease with significant cognitive deficits. However, a recent study showed potentiation of haloperidol-induced catalepsy by ciproxifan, an imidazole-containing H3R antagonist/inverse agonist, suggesting there is a potential risk of exacerbating extrapyramidal symptoms (EPS) if H3R antagonists were used as adjunctive treatment [Pillot, C., Ortiz, J., Heron, A., Ridray, S., Schwartz, J.C. and Arrang, J.M., Ciproxifan, a histamine H3-receptor antagonist/inverse agonist, potentiates neurochemical and behavioral effects of haloperidol in the rat, J Neurosci, 22 (2002) 7272-80]. In order to clarify the basis of this finding, we replicated this result and extended the work with another imidazole and two non-imidazole H3R antagonists. The results indicate that ciproxifan significantly augmented the effects of haloperidol and risperidone on catalepsy. Another imidazole H3R antagonist, thioperamide, also potentiated the effect of risperidone on catalepsy. In contrast, no catalepsy-enhancing effects were observed when selective non-imidazole H3R antagonists, ABT-239 and A-431404, were coadministered with haloperidol and/or risperidone. As ciproxifan and thioperamide are inhibitors of cytochrome P450 enzymes, responsible for metabolizing risperidone and haloperidol, the possibility that the augmentation of antipsychotics by imidazoles resulted from drug-drug interactions was tested. A drug metabolism study revealed that an imidazole, but not a non-imidazole, potently inhibited the metabolism of haloperidol and risperidone. Furthermore, ketoconazole, an imidazole-based CYP 3A4 inhibitor, significantly augmented risperidone-induced catalepsy. Together, these data suggest the potentiation of antipsychotic-induced catalepsy may result from pharmacokinetic drug-drug interactions and support the potential utility of non-imidazole H3R antagonists in treatment of cognitive impairment in schizophrenia without increased risk of increased EPS in patients.