RESUMO
One of the few preclinical models used to identify mood stabilizers is an assay in which amphetamine-induced hyperactivity (AMPH) is potentiated by the benzodiazepine chlordiazepoxide (CDP), an effect purportedly blocked by mood stabilizers. Our data here challenge this standard interpretation of the AMPH-CDP model. We show that the potentiating effects of AMPH-CDP are not explained by a pharmacokinetic interaction as both drugs have similar brain and plasma exposures whether administered alone or in combination. Of concern, however, we find that combining CDP (1-12 mg/kg) with AMPH (3 mg/kg) results in an inverted-U dose response in outbred CD-1 as well as inbred C57Bl/6N and 129S6 mice (peak hyperactivity at 3 mg/kg CDP+3 mg/kg AMPH). Such an inverted-U dose response complicates interpreting whether a reduction in hyperactivity produced by a mood stabilizer reflects a "blockade" or a "potentiation" of the mixture. In fact, we show that the prototypical mood stabilizer valproic acid augments the effects of CDP on hypolocomotion and anxiolytic-like behavior (increases punished crossings by Swiss-Webster mice in the four-plate test). We argue that these data, in addition to other practical and theoretical concerns surrounding the model, limit the utility of the AMPH-CDP mixture model in drug discovery.
Assuntos
Afeto/efeitos dos fármacos , Anfetamina/administração & dosagem , Clordiazepóxido/administração & dosagem , Animais , Antimaníacos/administração & dosagem , Ansiedade/tratamento farmacológico , Transtorno Bipolar/tratamento farmacológico , Modelos Animais de Doenças , Relação Dose-Resposta a Droga , Avaliação Pré-Clínica de Medicamentos/métodos , Sinergismo Farmacológico , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Modelos Neurológicos , Atividade Motora/efeitos dos fármacos , Ácido Valproico/administração & dosagemRESUMO
Sensorimotor gating, the ability to automatically filter sensory information, is deficient in a number of psychiatric disorders, yet little is known of the biochemical mechanisms underlying this critical neural process. Previously, we reported that mice expressing a constitutively active isoform of the G-protein subunit Galphas (Galphas(*)) within forebrain neurons exhibit decreased gating, as measured by prepulse inhibition of acoustic startle (PPI). Here, to elucidate the biochemistry regulating sensorimotor gating and to identify novel therapeutic targets, we test the hypothesis that Galphas(*) causes PPI deficits via brain region-specific changes in cyclic AMP (cAMP) signaling. As predicted from its ability to stimulate adenylyl cyclase, we find here that Galphas(*) increases cAMP levels in the striatum. Suprisingly, however, Galphas(*) mice exhibit reduced cAMP levels in the cortex and hippocampus because of increased cAMP phosphodiesterase (cPDE) activity. It is this decrease in cAMP that appears to mediate the effect of Galphas(*) on PPI because Rp-cAMPS decreases PPI in C57BL/6J mice. Furthermore, the antipsychotic haloperidol increases both PPI and cAMP levels specifically in Galphas(*) mice and the cPDE inhibitor rolipram also rescues PPI deficits of Galphas(*) mice. Finally, to block potentially the pathway that leads to cPDE upregulation in Galphas(*) mice, we coexpressed the R(AB) transgene (a dominant-negative regulatory subunit of protein kinase A (PKA)), which fully rescues the reductions in PPI and cAMP caused by Galphas(*). We conclude that expression of Galphas(*) within forebrain neurons causes PPI deficits because of a PKA-dependent decrease in cAMP and suggest that cAMP PDE inhibitors may exhibit antipsychotic-like therapeutic effects.
Assuntos
Proteínas Quinases Dependentes de AMP Cíclico/fisiologia , AMP Cíclico/metabolismo , Subunidades alfa Gs de Proteínas de Ligação ao GTP/metabolismo , Transtornos Neurológicos da Marcha/metabolismo , Neurônios/metabolismo , Prosencéfalo/citologia , Estimulação Acústica/métodos , Anfetamina/farmacologia , Análise de Variância , Animais , Comportamento Animal/efeitos dos fármacos , Química Encefálica/efeitos dos fármacos , AMP Cíclico/análogos & derivados , AMP Cíclico/farmacologia , Antagonistas de Dopamina/farmacologia , Relação Dose-Resposta à Radiação , Subunidades alfa Gs de Proteínas de Ligação ao GTP/genética , Transtornos Neurológicos da Marcha/etiologia , Transtornos Neurológicos da Marcha/genética , Haloperidol/farmacologia , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Transgênicos , Neurônios/efeitos dos fármacos , Prosencéfalo/metabolismo , Inibidores de Proteínas Quinases/farmacologia , Reflexo de Sobressalto/efeitos dos fármacos , Reflexo de Sobressalto/fisiologia , Tionucleotídeos/farmacologiaRESUMO
Schizophrenia is a complex disorder characterized by wide-ranging cognitive impairments, including deficits in learning as well as sensory gating. The causes of schizophrenia are unknown, but alterations in intracellular G-protein signaling pathways are among the molecular changes documented in patients with schizophrenia. Using the CaMKIIalpha promoter to drive expression in neurons within the forebrain, we have developed transgenic mice that express a constitutively active form of G(s)alpha (G(s)alpha(*)), the G protein that couples receptors such as the D(1) and D(5) dopamine receptors to adenylyl cyclase. We have also generated mice in which the CaMKIIalpha promoter drives expression of a dominant-negative form of protein kinase A, R(AB). Here, we examine startle responses and prepulse inhibition of the startle reflex (PPI) in these G(s)alpha(*) and R(AB) transgenic mice. G(s)alpha(*) transgenic mice exhibited selective deficits in PPI, without exhibiting alterations in the startle response, whereas no deficit in startle or PPI was found in the R(AB) transgenic mice. Thus, overstimulation of the cAMP/PKA pathway disrupts PPI, but the cAMP/PKA pathway may not be essential for sensorimotor gating. G(s)alpha(*) transgenic mice may provide an animal model of certain endophenotypes of schizophrenia, because of the similarities between them and patients with schizophrenia in G-protein function, hippocampus-dependent learning, and sensorimotor gating.