RESUMEN
Loss of nigrostriatal dopaminergic neurons in Parkinson's disease (PD) leads to increased activity of glutamatergic neurons in the subthalamic nucleus (STN). Recent studies reveal that the resultant increase in STN-induced excitation of basal ganglia output nuclei is responsible for the disabling motor impairment characteristic of PD. On the basis of this, it is possible that any manipulation that reduces activity at excitatory STN synapses onto basal ganglia output nuclei could be useful in the treatment of PD. We now report that group II metabotropic glutamate receptors (mGluRs) are presynaptically localized on STN terminals and that activation of these receptors inhibits excitatory transmission at STN synapses. In agreement with the hypothesis that this could provide a therapeutic benefit in PD, a selective agonist of group II mGluRs induces a dramatic reversal of catalepsy in a rat model of PD. These results raise the exciting possibility that selective agonists of group II mGluRs could provide an entirely new approach to the treatment of PD. These novel therapeutic agents would provide a noninvasive pharmacological treatment that does not involve the manipulation of dopaminergic systems, thus avoiding the problems associated with current therapies.
Asunto(s)
Compuestos Bicíclicos con Puentes/farmacología , Agonistas de Aminoácidos Excitadores/farmacología , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Receptores de Glutamato Metabotrópico/efectos de los fármacos , Receptores Presinapticos/efectos de los fármacos , Núcleo Subtalámico/efectos de los fármacos , Animales , Compuestos Bicíclicos con Puentes/uso terapéutico , Catalepsia/inducido químicamente , Catalepsia/tratamiento farmacológico , Catalepsia/fisiopatología , Antagonistas de Dopamina , Agonistas de Aminoácidos Excitadores/uso terapéutico , Potenciales Postsinápticos Excitadores/fisiología , Ácido Glutámico/farmacología , Ácido Glutámico/fisiología , Haloperidol , Masculino , Enfermedad de Parkinson/tratamiento farmacológico , Enfermedad de Parkinson/fisiopatología , Ratas , Ratas Sprague-Dawley , Receptores de Glutamato Metabotrópico/fisiología , Receptores Presinapticos/fisiología , Sustancia Negra/efectos de los fármacos , Sustancia Negra/fisiología , Núcleo Subtalámico/fisiologíaRESUMEN
The muscarinic receptor family (M(1)-M(4)) mediates cholinergic modulation of hippocampal transmission. Pharmacological and physiological studies have indicated that a presynaptic receptor on cholinergic terminals plays a key role in regulating ACh release, although the molecular identity of this subtype is uncertain. In this study, the localization of the M(2) receptor is described in detail for the pyramidal cell layer in the CAl region of the hippocampus. Electron microscopic analysis of M(2) immunoreactivity in this area revealed mainly presynaptic expression of this subtype. Double-labeling experiments using antibodies to M(2) and to the vesicular acetylcholine transporter, a novel, specific marker of cholinergic terminals, were used to investigate the nature of these presynaptic receptors. These studies have revealed that M(2) is located in cholinergic and non-cholinergic terminals. This is the first direct anatomical evidence that suggests that M(2) may indeed function as a cholinergic autoreceptor in the hippocampus. The distribution of the M(2) receptor in non-cholinergic terminals also suggests functional roles for M(2) as a presynaptic heteroreceptor.
Asunto(s)
Hipocampo/química , Proteínas de Transporte de Membrana , Terminales Presinápticos/química , Receptores Muscarínicos/análisis , Proteínas de Transporte Vesicular , Animales , Anticuerpos Monoclonales , Biomarcadores/análisis , Proteínas Portadoras/análisis , Hipocampo/citología , Inmunohistoquímica , Masculino , Microscopía Electrónica , Terminales Presinápticos/ultraestructura , Células Piramidales/química , Células Piramidales/ultraestructura , Ratas , Ratas Sprague-Dawley , Receptor Muscarínico M2 , Receptores Muscarínicos/fisiología , Proteínas de Transporte Vesicular de AcetilcolinaRESUMEN
Activation of muscarinic acetylcholine (ACh) receptors (mAChRs) increases excitability of pyramidal cells by inhibiting several K+ conductances, including the after-hyperpolarization current (Iahp), the M-current (Im), and a leak K+ conductance (Ileak). Based on pharmacological evidence and the abundant localization of M1 receptors in pyramidal cells, it has been assumed that the M1 receptor is responsible for mediating these effects. However, given the poor selectivity of the pharmacological agents used to characterize these mAChR responses, rigorous characterization of the receptor subtypes that mediate these actions has not been possible. Surprisingly, patch clamp recording from CA1 pyramidal cells in M1 knockout mice revealed no significant difference in the degree of inhibition of Iahp, Im, or Ileak by the mAChR agonist, carbachol (CCh), as compared with wildtype controls. In addition, the M1-toxin was not able to block CCh's inhibition of the Iahp, Im, or Ileak These data demonstrate that the M1 receptor is not involved in increasing CA1 pyramidal cell excitability by mediating ACh effects on these K+ conductances.
Asunto(s)
Hipocampo/citología , Activación del Canal Iónico/efectos de los fármacos , Transporte Iónico/efectos de los fármacos , Canales de Potasio/efectos de los fármacos , Potasio/metabolismo , Células Piramidales/efectos de los fármacos , Receptores Muscarínicos/fisiología , Potenciales de Acción/efectos de los fármacos , Animales , Atropina/farmacología , Carbacol/farmacología , Venenos Elapídicos/farmacología , Hipocampo/efectos de los fármacos , Ratones , Ratones Noqueados , Agonistas Muscarínicos/farmacología , Antagonistas Muscarínicos/farmacología , Técnicas de Placa-Clamp , Pirenzepina/farmacología , Receptor Muscarínico M1 , Receptores Muscarínicos/deficiencia , Receptores Muscarínicos/genéticaRESUMEN
The basal ganglia (BG) are a set of interconnected subcortical structures that play a critical role in motor control. The BG are thought to control movements by a delicate balance of transmission through two BG circuits that connect the input and output nuclei: the direct and the indirect pathways. The BG are also involved in a number of movement disorders. Most notably, the primary pathophysiological change that gives rise to the motor symptoms of Parkinson's Disease (PD) is the loss of dopaminergic neurons of the substantia nigra pars compacta (SNc) that are involved in modulating function of the striatum and other BG structures. This ultimately results in an increase in activity of the indirect pathway relative to the direct pathway and the hallmark PD symptoms of rigidity, bradykinesia, and akinesia. A great deal of effort has been dedicated to finding treatments for this disease. The current pharmacotherapies are aimed at replacing the missing dopamine, while the current surgical treatments are aimed at reducing transmission through the indirect pathway. Dopamine replacement therapy has proven to be helpful, but is associated with severe side effects that limit treatment and a loss of efficacy with progression of the disease. Recently developed surgical therapies have been highly effective, but are highly invasive, expensive, and assessable to a small minority of patients. For these reasons, new effort has been dedicated to finding pharmacological treatment options that will be effective in reducing transmission through the indirect pathway. Members of the metabotropic glutamate receptor (mGluR) family have emerged as interesting and promising targets for such a treatment. This review will explore the most recent advances in the understanding of mGluR localization and function in the BG motor circuit and the implications of those findings for the potential therapeutic role of mGluR-targeted compounds for PD.
Asunto(s)
Ganglios Basales/fisiología , Enfermedad de Parkinson/fisiopatología , Receptores de Glutamato Metabotrópico/fisiología , Ganglios Basales/patología , Cuerpo Estriado/fisiología , Globo Pálido/fisiología , Humanos , Enfermedad de Parkinson/tratamiento farmacológico , Receptores de Glutamato Metabotrópico/efectos de los fármacos , Transmisión Sináptica/fisiología , Tálamo/fisiologíaAsunto(s)
Receptores de Ácido Kaínico/metabolismo , Receptores de Glutamato Metabotrópico/metabolismo , Receptores de N-Metil-D-Aspartato/metabolismo , Animales , Encéfalo/metabolismo , Cicloleucina/análogos & derivados , Cicloleucina/farmacología , Electrofisiología , Metoxihidroxifenilglicol/análogos & derivados , Metoxihidroxifenilglicol/farmacología , N-Metilaspartato/metabolismo , Fármacos Neuroprotectores/farmacología , Oocitos/metabolismo , Fosfoproteínas Fosfatasas/metabolismo , Fosforilación , Ratas , Proteínas Recombinantes/genética , XenopusRESUMEN
The metabotropic glutamate receptor, mGluR5, has a critical role in induction of NMDA-receptor-dependent forms of synaptic plasticity and excitotoxicity. This is likely mediated by a reciprocal positive-feedback interaction between these two glutamate receptor subtypes in which activation of mGluR5 potentiates NMDA receptor currents and NMDA receptor activation potentiates mGluR5-mediated responses. We have investigated the mechanism by which NMDA receptor activation modulates mGluR5 function and find evidence that this response is mediated by activation of a protein phosphatase and a resultant dephosphorylation of protein kinase C phosphorylation sites on mGluR5. This form of neuromodulation may be important in a number of normal and pathological processes that involve activation of the NMDA receptor.
Asunto(s)
Receptores de Glutamato Metabotrópico/metabolismo , Receptores de N-Metil-D-Aspartato/metabolismo , Animales , Hipocampo/metabolismo , Hipocampo/fisiología , Técnicas In Vitro , Masculino , Oocitos , Técnicas de Placa-Clamp , Monoéster Fosfórico Hidrolasas/metabolismo , Fosforilación , Proteína Quinasa C/fisiología , Ratas , Ratas Sprague-Dawley , Receptor del Glutamato Metabotropico 5 , Proteínas Recombinantes , XenopusRESUMEN
Muscarinic receptors modulate hippocampal activity in two main ways: inhibition of synaptic activity and enhancement of excitability of hippocampal cells. Due to the lack of pharmacological tools, it has not been possible to identify the individual receptor subtypes that mediate the specific physiological actions that underlie these forms of modulation. Light and electron microscopic immunocytochemistry using subtype-specific antibodies was combined with lesioning techniques to examine the pre- and postsynaptic location of m1-m4 mAChR at identified hippocampus synapses. The results revealed striking differences among the subtypes, and suggested different ways that the receptors modulate excitatory and inhibitory transmission in distinct circuits. Complementary physiological studies using m1-toxin investigated the modulatory effects of this subtype on excitatory transmission in more detail. The implications of these data for understanding the functional roles of these subtypes are discussed.
Asunto(s)
Hipocampo/química , Receptores Muscarínicos/análisis , Animales , Colinérgicos/farmacología , Hipocampo/efectos de los fármacos , Hipocampo/fisiología , Hipocampo/ultraestructura , Terminaciones Nerviosas/química , Terminaciones Nerviosas/efectos de los fármacos , Terminaciones Nerviosas/fisiología , Terminaciones Nerviosas/ultraestructura , Vías Nerviosas/química , Vías Nerviosas/efectos de los fármacos , Vías Nerviosas/fisiología , Terminales Presinápticos/química , Terminales Presinápticos/efectos de los fármacos , Terminales Presinápticos/fisiología , Terminales Presinápticos/ultraestructura , Receptores Muscarínicos/fisiología , Receptores de N-Metil-D-Aspartato/fisiología , Transmisión Sináptica/efectos de los fármacosRESUMEN
Evidence suggests that cholinergic input to the hippocampus plays an important role in learning and memory and that degeneration of cholinergic terminals in the hippocampus may contribute to the memory loss associated with Alzheimer's disease. One of the more prominent effects of cholinergic agonists on hippocampal physiology is the potentiation of N-methyl-D-aspartate (NMDA)-receptor currents by muscarinic agonists. Here, we employ traditional pharmacological reagents as well as m1-toxin, an m1 antagonist with unprecedented selectivity, to demonstrate that this potentiation of NMDA-receptor currents in hippocampal CA1 pyramidal cells is mediated by the genetically defined m1 muscarinic receptor. Furthermore, we demonstrate the colocalization of the m1 muscarinic receptor and the NR1a NMDA receptor subunit at the electron microscopic level, indicating a spatial relationship that would allow for physiological interactions between these two receptors. This work demonstrates that the m1-muscarinic receptor gene product modulates excitatory synaptic transmission, and it has important implications in the study of learning and memory as well as the design of drugs to treat neurodegenerative diseases such as Alzheimer's.
Asunto(s)
Hipocampo/fisiología , Células Piramidales/metabolismo , Receptores Muscarínicos/metabolismo , Receptores de N-Metil-D-Aspartato/metabolismo , Animales , Carbacol/farmacología , Venenos Elapídicos/farmacología , Electrofisiología , Hipocampo/efectos de los fármacos , Masculino , Microscopía Inmunoelectrónica , Antagonistas Muscarínicos/farmacología , N-Metilaspartato/farmacología , Enfermedades Neurodegenerativas/fisiopatología , Ratas , Ratas Sprague-Dawley , Receptor Muscarínico M1 , Receptores Muscarínicos/genética , Transmisión Sináptica/efectos de los fármacos , Transmisión Sináptica/fisiologíaRESUMEN
A family of muscarinic acetylcholine receptor proteins mediates diverse pre- and postsynaptic functions in the hippocampus. However the roles of individual receptors are not understood. The present study identified the pre- and postsynaptic muscarinic acetylcholine receptors at the perforant pathway synapses in rat brain using a combination of lesioning, immunocytochemistry and electron microscopic techniques. Entorhinal cortex lesions resulted in lamina-specific reductions of m2, m3, and m4 immunoreactivity in parallel with the degeneration of the medial and lateral perforant pathway terminals in the middle and outer thirds of the molecular layer, respectively. In contrast, granule cell lesions selectively reduced m1 and m3 receptors consistent with degeneration of postsynaptic dendrites. Direct visualization of m1-m4 by electron microscopic immunocytochemistry confirmed their differential pre- and postsynaptic localizations. Together, these findings provide strong evidence for both redundancy and spatial selectivity of presynaptic (m2, m3 and m4) and postsynaptic (m1 and m3) muscarinic acetylcholine receptors at the perforant pathway synapse.
Asunto(s)
Corteza Entorrinal/fisiología , Vía Perforante/metabolismo , Terminales Presinápticos/metabolismo , Receptores Muscarínicos/biosíntesis , Sinapsis/metabolismo , Animales , Encéfalo/citología , Encéfalo/metabolismo , Dendritas/metabolismo , Dendritas/ultraestructura , Inmunohistoquímica , Masculino , Microscopía Inmunoelectrónica , Terminaciones Nerviosas/metabolismo , Terminaciones Nerviosas/ultraestructura , Neuronas/metabolismo , Neuronas/ultraestructura , Vía Perforante/citología , Terminales Presinápticos/ultraestructura , Ratas , Ratas Sprague-Dawley , Receptor Muscarínico M1 , Receptor Muscarínico M2 , Receptor Muscarínico M3 , Receptor Muscarínico M4 , Receptores Muscarínicos/análisis , Sinapsis/ultraestructuraRESUMEN
The present study addressed the hypothesis that differential localization of ml-m4 subtypes in the inner one-third of the dentate molecular layer is due to selective presynaptic expression of the receptor proteins on the hippocampal commissural/associational pathways. Physical and chemical lesions of the commissural and associational pathways were used to denervate afferent terminals in the inner one-third of the molecular layer, and fluid injections were used to lesion granule cells, their postsynaptic target. Immunocytochemistry utilizing muscarine acetylcholine receptor (mAChR) subtype-specific antibodies was used to identify changes in expression patterns in the molecular layer postlesion. m1 immunoreactivity in the molecular layer did not change after commissural/associational pathway lesions. m2 immunoreactivity in the inner one-third of the molecular layer was attenuated only after lesions involving the associational pathway. In contrast, m3 and m4 immunoreactivity in the inner one-third of the molecular layer was almost completely abolished by lesions of both pathways simultaneously. Granule cell lesions greatly attenuated m1 and m3 immunoreactivity in the molecular layer, with little to no diminution of m2 and m4 immunoreactivity. The results indicate that, in the inner one-third of the molecular layer, m1 and m3 are mainly postsynaptic on granule cells, whereas m2 and m4 are presynaptic on the commissural/associational pathways. This study provides direct anatomical evidence for the diversity of molecular subtypes presynaptically on the commissural/associational pathways.
Asunto(s)
Hipocampo/fisiología , Terminales Presinápticos/metabolismo , Receptores Muscarínicos/metabolismo , Animales , Hipocampo/efectos de los fármacos , Inmunohistoquímica , Ácido Kaínico/farmacología , Masculino , Ratas , Ratas Sprague-DawleyRESUMEN
The muscarinic acetylcholine receptor (mAChR) molecular subtype, m2, has been postulated to be the presynaptic cholinergic autoreceptor in many brain regions. However, due to a lack of subtype-specific pharmacological agents, conclusive evidence for m2 as an autoreceptor remains elusive. The development of subtype-specific antibodies has enabled extensive characterization of the synaptic localization of the m2 subtype. Specifically, double-labeling immunocytochemistry with m2 antibodies and antibodies to the vesicular acetylcholine transporter (VAChT), a novel specific marker of cholinergic terminals, in the striatum has allowed the first direct anatomical evidence of m2 localization in cholinergic terminals. Additionally, other anatomical studies in striatum and the septohippocampal pathway have revealed that this subtype is also expressed presynaptically in non-cholinergic terminals, and is postsynaptically expressed in both cholinergic and non-cholinergic neurons. The implications of these data for understanding the functional roles of this subtype are discussed.
Asunto(s)
Receptores Muscarínicos/análisis , Sinapsis/química , Animales , Autorreceptores/análisis , Cuerpo Estriado/química , Hipocampo/química , Inmunohistoquímica , Ratas , Ratas Sprague-Dawley , Receptor Muscarínico M2 , Receptores Muscarínicos/fisiología , Receptores Presinapticos/análisisRESUMEN
Muscarinic cholinergic transmission plays an important role in modulating hippocampal activity and many higher brain functions. Many of the modulatory effects of acetylcholine on hippocampal function result from direct effects in the hippocampus or from actions on the hippocampal afferent neurons. At each site, the differential expression of a family of five distinct but related receptor subtypes governs the nature of the response. The aim of the present study was to identify the subtypes expressed in the hippocampal afferent neurons by combining retrograde tracing with immunocytochemistry. The retrograde tracer, wheat germ agglutinin conjugated to horseradish peroxidase, was injected into the hippocampus unilaterally to label afferent neurons, and was combined with muscarinic (m) acetylcholine (ACh) receptors (mAChRs) with immunocytochemistry to identify the m1-m4 subtypes expressed. The retrogradely labeled cells in the basal forebrain that contribute to the septohippocampal pathway were found to express m2, m3, and, to a lesser extent, m1. Commissural/associational pathway neurons, which were identified by retrogradely labeled cells in the ipsi- and contralateral dentate gyrus, expressed m1, m3, and m4. The retrogradely labeled cells in the entorhinal cortex of the perforant pathway expressed predominantly m1 and m3, with fewer neurons expressing m2 and m4. Raphe-hippocampal cells were found to express m1. Thus, this study provides evidence for the diversity of mAChR subtypes expressed in neurons that project to the hippocampus. The complex modulation by acetylcholine of hippocampal function, therefore, is governed not only by the variety of mAChRs expressed in the hippocampus but also by their differential expression in extrinsic hippocampal afferents.