Preclinical evaluation of non-imidazole histamine H3 receptor antagonists in comparison to atypical antipsychotics for the treatment of cognitive deficits associated with schizophrenia.

Cognitive deficits associated with schizophrenia (CDS) are implicated as a core symptom cluster of the disease and are associated with poor daily life functioning. Unfortunately, current antipsychotic agents provide little alleviation of CDS, representing a critical unmet therapeutic need. Here we investigated the effects of ABT-239 and A-431404, non-imidazole histamine H(3) receptor (H(3)R) antagonists, in animal models with relevance to CDS. As N-methyl-d-aspartate receptor hypofunction is considered an important factor in the pathogenesis of schizophrenia, acute administration of ketamine or MK-801 was used to induce cognitive impairments. The assays employed in the current studies were spontaneous alternation in cross-maze, used as an indication of working memory, and inhibitory avoidance (IA), used to assess long-term memory retention. Risperidone and olanzapine were also tested to directly compare the effects of H(3)R antagonists to two widely used antipsychotics. ABT-239 and A-431404, but not risperidone and olanzapine, attenuated ketamine-induced deficits on spontaneous alternation in cross-maze, while none of these compounds affected alternation performance on their own. ABT-239 and A-431404 also attenuated MK-801-induced impairments in IA; no effects were observed when given alone. Risperidone and olanzapine, however, failed to attenuate MK-801-induced deficits in IA and produced dose-dependent impairments when given alone. ABT-239 was also investigated in methylazoxymethanol acetate (MAM) treated rats, a neurodevelopmental model for schizophrenia. Chronic, but not acute, treatment with ABT-239 significantly improved spontaneous alternation impairments in MAM rats tested in cross-maze. In summary, these results suggest H(3)R antagonists may have the potential to ameliorate CDS.

Acute inhibition of 11beta-hydroxysteroid dehydrogenase type-1 improves memory in rodent models of cognition.

Mounting evidence suggests excessive glucocorticoid activity may contribute to Alzheimer's disease (AD) and age-associated memory impairment. 11ß-hydroxysteroid dehydrogenase type-1 (HSD1) regulates conversion of glucocorticoids from inactive to active forms. HSD1 knock-out mice have improved cognition, and the nonselective inhibitor carbenoxolone improved verbal memory in elderly men. Together, these data suggest that HSD1 inhibition may be a potential therapy for cognitive deficits, such as those associated with AD. To investigate this, we characterized two novel and selective HSD1 inhibitors, A-918446 and A-801195. Learning, memory consolidation, and recall were evaluated in mouse 24 h inhibitory avoidance. Inhibition of brain cortisol production and phosphorylation of cAMP response element-binding protein (CREB), a transcription factor involved in cognition, were also examined. Rats were tested in a short-term memory model, social recognition, and in a separate group cortical and hippocampal acetylcholine release was measured via in vivo microdialysis. Acute treatment with A-801195 (10-30 mg/kg) or A-918446 (3-30 mg/kg) inhibited cortisol production in the ex vivo assay by ∼ 35-90%. Acute treatment with A-918446 improved memory consolidation and recall in inhibitory avoidance and increased CREB phosphorylation in the cingulate cortex. Acute treatment with A-801195 significantly improved short-term memory in rat social recognition that was not likely due to alterations of the cholinergic system, as acetylcholine release was not increased in a separate set of rats. These studies suggest that selective HSD1 inhibitors work through a novel, noncholinergic mechanism to facilitate cognitive processing.

Pharmacological properties and procognitive effects of ABT-288, a potent and selective histamine H3 receptor antagonist.

Blockade of the histamine H(3) receptor (H(3)R) enhances central neurotransmitter release, making it an attractive target for the treatment of cognitive disorders. Here, we present in vitro and in vivo pharmacological profiles for the H(3)R antagonist 2-[4'-((3aR,6aR)-5-methyl-hexahydro-pyrrolo[3,4-b]pyrrol-1-yl)-biphenyl-4-yl]-2H-pyridazin-3-one (ABT-288). ABT-288 is a competitive antagonist with high affinity and selectivity for human and rat H(3)Rs (K(i) = 1.9 and 8.2 nM, respectively) that enhances the release of acetylcholine and dopamine in rat prefrontal cortex. In rat behavioral tests, ABT-288 improved acquisition of a five-trial inhibitory avoidance test in rat pups (0.001-0.03 mg/kg), social recognition memory in adult rats (0.03-0.1 mg/kg), and spatial learning and reference memory in a rat water maze test (0.1-1.0 mg/kg). ABT-288 attenuated methamphetamine-induced hyperactivity in mice. In vivo rat brain H(3)R occupancy of ABT-288 was assessed in relation to rodent doses and exposure levels in behavioral tests. ABT-288 demonstrated a number of other favorable attributes, including good pharmacokinetics and oral bioavailability of 37 to 66%, with a wide central nervous system and cardiovascular safety margin. Thus, ABT-288 is a selective H(3)R antagonist with broad procognitive efficacy in rodents and excellent drug-like properties that support its advancement to the clinical area.

Structural abnormalities revealed by magnetic resonance imaging in rats prenatally exposed to methylazoxymethanol acetate parallel cerebral pathology in schizophrenia.

Schizophrenia is a highly familial, neurodevelopmental disorder that is associated with several neuropsychiatric, psychological, and neuropathological features. Although pharmacological animal models of dopaminergic and glutamatergic dysfunction have helped advance our understanding of the disease biology, there is a clear need for translational models that capture the neuropathological and functional manifestations associated with the intermediate phenotype and the clinical illness. Neuroimaging of preclinical neurodevelopmental approaches such as methylazoxymethanol acetate (MAM) exposure may afford a powerful translational tool to establish endpoints with greater congruency across animals and humans. Using in vivo volumetric magnetic resonance imaging (MRI), manganese-enhanced MRI, and diffusion tensor imaging (DTI), we investigated morphological and cytoarchitectural changes of brain structures in MAM-exposed rats, a neurodevelopmental model of schizophrenia. Compared to saline-exposed controls, MAM-exposed rats showed significant enlargement of lateral and third ventricles as well as reduced hippocampal volumes, which is consistent with findings observed in schizophrenia. In addition, DTI revealed that diffusion fractional anisotropy retrieved from corpus callosum and cingulum were significantly decreased in MAM-exposed rats, suggesting that demyelination occurred in these white-matter fiber tracts. Imaging findings were confirmed by conducting histological analysis using hematoxylin and eosin and Luxol fast blue stainings. In summary, structural abnormalities resulting from a MAM environmental challenge parallel cerebral pathology observed in schizophrenia. The MAM model incorporating noninvasive imaging techniques may therefore serve as an improved translational research tool for assessing new treatments for schizophrenia.

Profiling attention and cognition enhancing drugs in a rat touchscreen-based continuous performance test.

RATIONALE: A novel rodent continuous performance test (CPT) was developed as one of the goals of the NEWMEDS (Novel Methods leading to New Medications in Depression and Schizophrenia) consortium to improve its translatability to the CPT test used in human subjects. OBJECTIVES: The objective of the study is to investigate the effects of attention and cognition enhancing drugs in rodent CPT. METHODS: A single cohort of rats were trained to asymptotic performance in the test. Pharmacological test sessions were then performed twice per week in a full crossover design with the following drugs tested: methylphenidate (0.3, 1, and 3 mg/kg), the α4ß2 nicotinic agonist ABT-594 (0.0023, 0.007 and 0.023 mg/kg), modafinil (8, 16, and 32 mg/kg), atomoxetine (0.3, 1, and 3 mg/kg), donepezil (0.1, 0.3, and 1 mg/kg), and memantine (1.25, 2.5, and 5 mg/kg). RESULTS: The stimulant-like drugs methylphenidate, ABT-594, and modafinil were found to increase measures of impulsivity and overall responding with generally no positive effects on d', a putative measure of attention, with the exception of ABT-594 which improved d' at the highest dose tested. Atomoxetine and the memory-enhancing drugs donepezil and memantine, on the other hand, were found to reduce measures of impulsivity and responding and had either negligible or worsening effects on d'. CONCLUSIONS: Our results suggest rodent CPT can detect changes in impulsivity resulting from drugs known to improve attention in rodents and humans. However, additional work is needed to assess the sensitivity and validity of this assay for assessing effects on attention.

A-366833: a novel nicotinonitrile-substituted 3,6-diazabicyclo[3.2.0]-heptane alpha4beta2 nicotinic acetylcholine receptor selective agonist: Synthesis, analgesic efficacy and tolerability profile in animal models.

5-[(1R,5S)-3,6-Diazabicyclo[3.2.0]heptan-6-yl]nicotinonitrile (A-366833) is a novel nicotinic acetylcholine receptor (nAChR) ligand that binds to the agonist-binding site ([3H]-cytisine) with Ki value of 3.1 nM and exhibits agonist selectivity at alpha4beta2 nAChR relative to the alpha3beta4 nAChR subtype. The analgesic effects of A-366833 were examined across a variety of animal models including the mouse model of writhing pain (abdominal constriction), the rat models of acute thermal (hot box), persistent chemical (formalin) and neuropathic (spinal nerve ligation, SNL) pain. In the abdominal constriction model, A-366833 was effective at doses ranging from 0.062 to 0.62 micromol/kg (i.p.). In addition, A-366833 demonstrated significant effects in acute thermal pain (6.2-19.0 micromol/kg, i.p.), formalin (1.9-19 micromol/kg i.p.) and SNL (1.9-19 micromol/kg i.p.) models. The systemic effects of A-366833 were attenuated by pretreatment with mecamylamine (5 micromol/kg i.p.) in both the formalin and SNL models, suggesting that the analgesic effects of A-366833 in models of persistent nociceptive and neuropathic pain are mediated by activation of nAChRs. Pharmacokinetic investigations of A-366833 in rat revealed moderate brain:plasma distribution, half-life of 1.5h and excellent oral bioavailability of 73%. Comparison of peak plasma levels at the minimal effective doses across rat models of acute thermal pain, formalin and SNL with the maximal exposure that does not evoke emesis in ferret revealed therapeutic margins ranging from 6- to 22-fold. These studies indicate that compounds like A-366833 with improved agonist selectivity at alpha4beta2 vs. alpha3beta4 nAChR can elicit a broad spectrum of analgesic efficacy without concurrent adverse effects.

The drug-induced helplessness test: an animal assay for assessing behavioral despair in response to neuroleptic treatment.

RATIONALE: Neuroleptic dysphoria encompasses a range of unpleasant subjective responses and, as a result, is difficult to study in preclinical animal models. OBJECTIVE: Based on the learned helplessness model of depression, increases in escape failures (EFs) in the drug-induced helplessness test (DH) are proposed to reflect drug-induced depressive-like state, a contributing factor to neuroleptic dysphoria in humans. MATERIALS AND METHODS: Effects of the typical antipsychotic haloperidol and the atypical antipsychotics risperidone, olanzapine, aripiprazole, quetiapine, and clozapine were investigated in the DH test. We further characterized this test by examining compounds affecting motor function, cognition, anxiety, and those with antidepressant activity. RESULTS: The antipsychotics haloperidol, risperidone, aripiprazole, and olanzapine, all increased EFs, while quetiapine had no effect, and clozapine reduced EFs. Amphetamine, diazepam, and ciproxifan, had no effect on EFs. Scopolamine significantly reduced EFs and MK-801 showed a trend toward reducing EFs at doses not significantly sti mulating locomotor activity. Subchronic, but not acute, imipramine and subchronic fluoxetine significantly reduced EFs at doses significantly suppressing locomotor activity. Dissociation appears to exist between performance in the DH test and compound effects on catalepsy or locomotor activity. CONCLUSIONS: After discussing potential alternative interpretations of the drug-induced changes of EFs, we propose the DH test as a useful test for assessing a drug-induced, depressive-like state that may contribute to neuroleptic dysphoria.

Effects of antipsychotics and selective D3 antagonists on PPI deficits induced by PD 128907 and apomorphine.

Rats treated with apomorphine and amphetamine display sensorimotor gating impairments, as measured by prepulse inhibition (PPI), and these impairments can be reversed by antipsychotic treatment. However, it remains unknown whether the dopamine (DA) D(3) receptor plays a role in mediating these effects on PPI, as none of these DA agonists or antipsychotics are exclusively selective at either D(2) or D(3) receptors. To address this question, the current study was designed to investigate whether antipsychotic drugs and selective D(3) antagonists could block the PPI-disruptive effects of PD 128907 (a preferential D(3) agonist) and apomorphine. We found that the effect of PD 128907 on PPI in rats could be antagonized by risperidone, clozapine, and the selective D(3) antagonists SB 277011 and A-691990, but not by raclopride or haloperidol, while the apomorphine-induced PPI deficit could be reversed by risperidone, clozapine and haloperidol, but not by SB 2770111 and A-691990. These results suggest that the D(3) receptor does not mediate apomorphine-induced disruption of PPI in rats, however, given the findings that PD 128907 elicited a PPI-disruptive effect that was blocked by selective D(3) antagonists, a role of D(3) receptor in mediating PPI in rats cannot be ruled out. The possible mechanisms of D(3) receptor involvement in PPI are discussed.

Effect of dopamine D3 antagonists on PPI in DBA/2J mice or PPI deficit induced by neonatal ventral hippocampal lesions in rats.

Schizophrenic patients typically exhibit impairment of sensorimotor gating, which can be modeled in animal models such as the test of prepulse inhibition of startle response (PPI) in rodents. It has been found that antipsychotics enhanced PPI in DBA mice and reversed the PPI deficit induced by neonatal ventral hippocampal (NVH) lesions in rats. However, the relative involvement of D(3) and D(2) receptors in these effects is unknown since all antipsychotics are D(2)/D(3) antagonists with limited binding preference at D(2) receptors. Therefore, in the current study, we investigated the influence of several dopamine antagonists with higher selectivity at D(3) vs D(2) receptors on PPI in DBA/2J mice and in NVH-lesioned rats. The PPI in DBA/2J mice was enhanced by the nonselective D(2)/D(3) antagonists, haloperidol at 0.3-3 mg/kg, or risperidone at 0.3-1 mg/kg, while PPI-enhancing effects were observed after the administration of higher doses of the preferential D(3)/D(2) antagonist, BP 897 at 8 mg/kg, and the selective D(3) antagonists, SB 277011 at 30 mg/kg and A-437203 at 30 mg/kg. No effect was observed following the treatment with the selective D(3) antagonist, AVE 5997 up to 30 mg/kg. The PPI deficits induced by NVH lesions were reversed by haloperidol but not by the more selective D(3) antagonists, A-437203 and AVE 5997. BP 897 enhanced PPI nonselectivity, that is, in both lesioned and nonlesioned rats. In summary, the present study indicates that PPI-enhancing effects induced by antipsychotics in DBA/2J mice and in NVH-lesioned rats are unlikely to be mediated by D(3) receptors.

Synthesis and structure-activity relationships of 3,8-diazabicyclo[4.2.0]octane ligands, potent nicotinic acetylcholine receptor agonists.

A series of potent neuronal nicotinic acetylcholine receptor (nAChR) ligands based on a 3,8-diazabicyclo[4.2.0]octane core have been synthesized and evaluated for affinity and agonist efficacy at the human high affinity nicotine recognition site (halpha4beta2) and in a rat model of persistent nociceptive pain (formalin model). Numerous analogs in this series exhibit picomolar affinity in radioligand binding assays and nanomolar agonist potency in functional assays, placing them among the most potent nAChR ligands known for the halpha4beta2 receptor. Several of the compounds reported in this study (i.e., 24, 25, 28, 30, 32, and 47) exhibit equivalent or greater affinity for the halpha4beta2 receptor relative to epibatidine, and like epibatidine, many exhibit robust analgesic efficacy in the rat formalin model of persistent pain.

Lack of efficacy of melanin-concentrating hormone-1 receptor antagonists in models of depression and anxiety.

The aim of this study was to validate melanin-concentrating hormone (MCH)-1 receptor antagonism as a potential treatment of mood disorders. We attempted to replicate the effects previously reported with SNAP-7941 and expanded the investigation to three other orally bioavailable MCH-1 receptor antagonists with good brain penetration. SNAP-7941 (3-30 mg/kg, i.p.) and T-226296 (5-60 mg/kg, p.o.) (+/- racemate), were evaluated in the rat forced swim and mouse tail suspension tests. (+)SNAP-7941 (3-10 mg/kg, p.o.) was also tested in a modified 5-min rat forced swim protocol as previously reported. A-665798 (3-30 mg/kg, p.o.) and A-777903 (3-30 mg/kg, p.o.) were tested in mouse tail suspension and rat Vogel tests. None of the compounds showed meaningful efficacy in the paradigms tested. The lack of efficacy with four structurally different MCH-1 receptor antagonists does not support a role for therapeutic treatment of depression/anxiety via this mechanism of action.

Anticonvulsant effects of structurally diverse GABA(B) positive allosteric modulators in the DBA/2J audiogenic seizure test: Comparison to baclofen and utility as a pharmacodynamic screening model.

The GABA(B) receptor has been indicated as a promising target for multiple CNS-related disorders. Baclofen, a prototypical orthosteric agonist, is used clinically for the treatment of spastic movement disorders, but is associated with unwanted side-effects, such as sedation and motor impairment. Positive allosteric modulators (PAM), which bind to a topographically-distinct site apart from the orthosteric binding pocket, may provide an improved side-effect profile while maintaining baclofen-like efficacy. GABA, the major inhibitory neurotransmitter in the CNS, plays an important role in the etiology and treatment of seizure disorders. Baclofen is known to produce anticonvulsant effects in the DBA/2J mouse audiogenic seizure test (AGS), suggesting it may be a suitable assay for assessing pharmacodynamic effects. Little is known about the effects of GABA(B) PAMs, however. The studies presented here sought to investigate the AGS test as a pharmacodynamic (PD) screening model for GABA(B) PAMs by comparing the profile of structurally diverse PAMs to baclofen. GS39783, rac-BHFF, CMPPE, A-1295120 (N-(3-(4-(4-chloro-3-fluorobenzyl)-6-methoxy-3,5-dioxo-4,5-dihydro-1,2,4-triazin-2(3H)-yl)phenyl)acetamide), and A-1474713 (N-(3-(4-(4-chlorobenzyl)-3,5-dioxo-4,5-dihydro-1,2,4-triazin-2(3H)-yl)phenyl)acetamide) all produced robust, dose-dependent anticonvulsant effects; a similar profile was observed with baclofen. Pre-treatment with the GABA(B) antagonist SCH50911 completely blocked the anticonvulsant effects of baclofen and CMPPE in the AGS test, indicating such effects are likely mediated by the GABA(B) receptor. In addition to the standard anticonvulsant endpoint of the AGS test, video tracking software was employed to assess potential drug-induced motor side-effects during the acclimation period of the test. This analysis was sensitive to detecting drug-induced changes in total distance traveled, which was used to establish a therapeutic index (TI = hypoactivity/anticonvulsant effects). Calculated TIs for A-1295120, CMPPE, rac-BHFF, GS39783, and A-1474713 were 5.31x, 5.00x, 4.74x, 3.41x, and 1.83x, respectively, whereas baclofen was <1. The results presented here suggest the DBA/2J mouse AGS test is a potentially useful screening model for detecting PD effects of GABA(B) PAMs and can provide an initial read-out on target-related motor side-effects. Furthermore, an improved TI was observed for PAMs compared to baclofen, indicating the PAM approach may be a viable therapeutic alternative to baclofen.

Antidepressant-like effect of D(2/3) receptor-, but not D(4) receptor-activation in the rat forced swim test.

Dopamine plays a role in the pathophysiology of depression and therapeutic effects of antidepressants but the contribution of individual D(2)-like receptor subtypes (D(2), D(3), D(4)) to depression is not known. We present evidence that activation of D(2)/D(3), but not D(4) receptors, can affect the outcome in the rat forced swim test (FST). Nomifensine, a dopamine uptake inhibitor (7, 14, and 28 micromol/kg); quinpirole, a D(2)-like receptor and agonist (0.4, 1.0, and 2.0 micromol/kg); PD 12,8907, a preferential D(3) receptor agonist (0.17, 0.35, and 0.7 micromol/kg); PD 168077 (0.1, 0.3, and 1.0 micromol/kg) and CP 226269 (0.3, 1.0, and 3.0 micromol/kg), both selective D(4) receptor agonists, were administered s.c. 24, 5, and 0.5/1 h before testing. Nomifensine, quinpirole at all doses and PD 128907 at the highest dose decreased immobility time in FST. PD 168077 and CP 226269 had no effect on the model. To further clarify what type of dopamine receptors were involved in the anti-immobility effect of quinpirole, we tested different antagonists. Haloperidol, a D(2)-like receptor antagonist (0.27 micromol/kg), completely blocked the effect of quinpirole; A-437203 (LU-201640), a selective D(3) receptor antagonist (17.46 micromol/kg), showed a nonsignificant trend to attenuate the effect of the low dose of quinpirole, and L-745,870, a selective D(4) receptor antagonist (1.15 micromol/kg), had no effect. The pharmacological selectivity of the compounds tested suggests that the antidepressant-like effects of quinpirole are most likely mediated mainly by D(2) and to a lesser extent by D(3) but not D(4) receptors.

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.

Loss of functional neuronal nicotinic receptors in dorsal root ganglion neurons in a rat model of neuropathic pain.

Recent evidence has suggested that the anti-allodynic effect of neuronal acetylcholine receptor (nAChR) agonists may have a peripheral component [L.E. Rueter, K.L. Kohlhaas, P. Curzon, C.S. Surowy, M.D. Meyer, Peripheral and central sites of action for A-85380 in the spinal nerve ligation model of neuropathic pain, Pain 103 (2003) 269-276]. In further studies of the peripheral anti-allodynic mechanisms of nAChR agonists, we investigated the function of nAChRs in acutely isolated dorsal root ganglion (DRG) neurons from allodynic [L5-L6 spinal nerve ligation (SNL)] and naive adult rats. Following determination of cell diameter and membrane capacitance, responses to rapid applications of nAChR agonists were recorded under whole cell patch clamp. nAChR inward currents were observed in approximately 60% of naive neurons, across small, medium, and large diameter cells. Evoked nAChR currents could be clustered into three broad classes: fast transient, biphasic, and slow desensitizing currents, consistent with multiple subtypes of nAChR expressed in DRG [J.R. Genzen, W. Van Cleve, D.S. McGehee, Dorsal root ganglion neurons express multiple nicotinic acetylcholine receptor subtypes, J. Neurophysiol. 86 (2001) 1773-1782]. In contrast, in neurons from allodynic animals, the occurrence and amplitude of responses to nAChR agonists were significantly reduced. Reduced responsiveness to nAChR agonists covered the range of DRG neuron sizes. The decrease in the responsiveness to nAChR agonists was not seen in neighboring uninjured L4 neurons. The significant decrease in the number of cells with nAChR agonist responses, compounded with the significant decrease in response amplitude, indicates that there is a marked down regulation of functional nAChRs in DRG somata associated with SNL.

Cross-site strain comparison of pharmacological deficits in the touchscreen visual discrimination test.

The low rate of success for identifying effective treatments for cognitive dysfunction has prompted recent efforts to improve pharmaceutical discovery and development. In particular, investigators have emphasized improving translation from pre-clinical to clinical research. A specific area of focus has been touchscreen technology; this computer-automated behavioral testing method provides an objective assessment of performance that can be used across species. As part of a larger multi-site study with partners from the Innovative Medicines Initiative (IMI), two US sites, AbbVie and Pfizer, conducted a cross-site experiment with a common protocol for the visual discrimination (VD) task using identical testing equipment, stimuli, and rats of the same strains, sex, and age from the same supplier. As most touchscreen-based rodent experiments have used Lister-Hooded rats that are not readily available outside of Europe, a strain comparison with male Long-Evans rats was conducted as part of the study. Rats were trained for asymptotic performance, and test sessions were performed once per week in a full crossover design with cognition-impairing drugs. Drugs tested were phencyclidine and S-ketamine (N-methyl-D-aspartate (NMDA) antagonists), D-amphetamine (indirect dopamine agonist), and scopolamine (muscarinic antagonist). Satellite brain and plasma samples were taken to confirm appropriate exposures. Results indicate that both rat strains show similar patterns of impairment, although Lister-Hooded rats were more sensitive than Long-Evans rats to three out of four drugs tested. This suggests that researchers should fully explore dose-response relationships in their strain of choice and use care in the interpretation of reversal of cognitive impairment.

Nicotinic modulation of auditory evoked potential electroencephalography in a rodent neurodevelopmental model of schizophrenia.

Schizophrenia is a chronic disease that has been hypothesized to be linked to neurodevelopmental abnormalities. Schizophrenia patients exhibit impairments in basic sensory processing including sensory gating deficits in P50 and mismatch negativity (MMN). Neuronal nicotinic acetylcholine receptor (nAChR) agonists have been reported to attenuate these deficits. Gestational exposure of rats to methylazoxymethanol acetate (MAM) at embryonic day 17 leads to developmental disruption of the limbic-cortical system. MAM exposed offspring show neuropathological and behavioral changes that have similarities with those seen in schizophrenia. In this study, we aimed to assess whether N40 auditory sensory gating (the rodent form of P50 gating) and MMN deficits as measures of auditory evoked potential (AEP) electroencephalography (EEG) are present in MAM rats and whether nAChR agonists could attend the deficit. E17 male MAM and sham rats were implanted with cortical electrodes at 2 months of age. EEG recordings evaluating N40 gating and MMN paradigms were done comparing effects of vehicle (saline), nicotine and the α7 agonist ABT-107. Deficits were seen for MAM rats compared to sham animals in both N40 auditory sensory gating and MMN AEP recordings. There was a strong trend for N40 deficits to be attenuated by both nicotine (0.16mg/kg i.p. base) and ABT-107 (1.0mg/kg i.p. base). MMN deficits were significantly attenuated by ABT-107 but not by nicotine. These data support the MAM model as a useful tool for translating pharmacodynamic effects in clinical medicine studies of novel therapeutic treatments for schizophrenia.

Peripheral and central sites of action for A-85380 in the spinal nerve ligation model of neuropathic pain.

Neuronal nicotinic receptor (NNR) agonists such as ABT 594 have been shown to be effective in a wide range of preclinical models of acute and neuropathic pain. The present study, using the NNR agonist A-85380, sought to determine if NNR agonists are acting via similar or differing mechanisms to induce anti-nociception and anti-allodynia. A systemic administration of the quaternary NNR antagonist chlorisondamine (0.4 micromol/kg, intraperitoneal (i.p.)) did not alter A-85380-induced (0.75 micromol/kg, i.p.) anti-nociception in the rat paw withdrawal model of acute thermal pain. In contrast, previous studies have demonstrated that blockade of central NNRs by prior administration of chlorisondamine (10 microg i.c.v.) prevents A-85380 induced anti-nociception indicating a predominantly central site of action of NNR agonists in relieving acute pain. In the rat spinal nerve ligation model of neuropathic pain, A-85380 induced a dose-dependent anti-allodynia (0.5-1.0 micromol/kg) that was blocked by pretreatment with mecamylamine (1 micromol/kg). Interestingly, unlike acute pain, both systemic and central administration of chlorisondamine blocked A-85380-induced anti-allodynia, an effect that was determined not to be due to a non-specific effect of chlorisondamine or to chlorisondamine crossing the blood-brain barrier. The peripheral site of action was shown not to be the primary receptive field, since A-85380 had equally potent anti-allodynic effects when it was injected into either the affected or unaffected paw. In contrast, infusion of A-85380 directly onto the L5 dorsal root ganglion on the affected side resulted in a dose-dependent and marked anti-allodynia (10-20 microg) at doses that had no effect when injected systemically. This effect was blocked by pretreatment with chlorisondamine. Together these data further support the idea that different mechanisms underlie different pain states and suggest that the effects of NNR agonists in neuropathic pain may be due in part to peripheral actions of the compounds.

Nicotinic acetylcholine receptor agonists: a potential new class of analgesics.

Current analgesics, such as opioids and nonsteroidal anti-inflammatory drugs (NSAIDs), are largely refinements of approaches available for more than 100 years and have critical liabilities and limitations. A number of new molecular targets for analgesia have been proposed in recent years, including the neuronal nicotinic acetylcholine receptor (nAChR). Agonists at neuronal nAChRs have antinociceptive effects in a variety of preclinical pain models. Moreover, nicotine can decrease experimentally-induced pain in humans without disrupting normal tactile sensation. These data from both experimental animals and humans suggest that compounds targeting neuronal nAChRs may represent a new class of analgesic agents. In this paper, we provide brief overviews of the physiology of pain, the animal models used to assess potential analgesics preclinically, and the biology of nAChRs. We then provide a review of preclinical data on the antinociceptive effects of a variety of neuronal nAChR agonists and a discussion of potential mechanisms, including evidence that antinociception is mediated by activation of brainstem nuclei with descending inhibitory inputs to the spinal cord. An evaluation of the clinical potential of this approach must also consider potential side effects. Undesirable side effects of nicotine are well known, but as we will discuss in detail, these effects are not produced by all neuronal nAChR agonists and the existence of neuronal nAChR subtypes may provide a basis for separating therapeutic effects from toxicities.

Chronic low dose risperidone and clozapine alleviate positive but not negative symptoms in the rat neonatal ventral hippocampal lesion model of schizophrenia.

RATIONALE: The rat neonatal ventral hippocampal (VH) ibotenic lesion model has been proposed as a developmental model of schizophrenia, based on evidence that it encompasses aspects of the disorder including psychomotor agitation (hyperactivity), deficits in prepulse inhibition (PPI), and deficits in social interaction (SI), measures presumed to reflect positive symptoms, sensory gating deficits and negative symptoms, respectively. However, validation of the model as a predictive pharmacological screening tool has been minimal. OBJECTIVE: Determine the effects of a chronic 3-week low dose treatment of clozapine or risperidone on locomotor hyperactivity, PPI and SI in lesioned and control rats. RESULTS: Both clozapine, 2.5 mg/kg per day IP and risperidone, 0.1 mg/kg per day IP, reversed lesion-induced locomotor hyperactivity; however, the compounds also decreased locomotor activity in the non-lesioned controls. Clozapine 2.5 mg/kg per day and risperidone 0.1 mg/kg per day significantly attenuated lesion-induced PPI deficits. Neither compound induced a significant attenuation of lesion-induced SI deficits. In order to see if SI deficits required a higher dose of an antipsychotic, the dose of clozapine was increased to 4 mg/kg per day; however this dose induced such marked decreases in the activity and startle responses in the control rats, i.e. up to 74% decrease, that the effects on the lesioned rats could not be adequately interpreted. CONCLUSIONS: These data add further support to the neonatal VH lesion model as a predictive pharmacological screening assay for identifying compounds effective in the treatment of positive symptoms of schizophrenia. However, the usefulness of the model in detecting compounds effective in treating negative symptoms of schizophrenia is still in question.