RESUMEN
Severe chronic stress can have a profoundly negative impact on the brain, affecting plasticity, neurogenesis, memory and mood. On the other hand, there are factors that upregulate neurogenesis, which include dietary antioxidants and physical activity. These factors are associated with biochemical processes that are also altered in age-related cognitive decline and dementia, such as neurotrophin expression, oxidative stress and inflammation. We exposed mice to an unpredictable series of stressors or left them undisturbed (controls). Subsets of stressed and control mice were concurrently given (1) no additional treatment, (2) a complex dietary supplement (CDS) designed to ameliorate inflammation, oxidative stress, mitochondrial dysfunction, insulin resistance and membrane integrity, (3) a running wheel in each of their home cages that permitted them to exercise, or (4) both the CDS and the running wheel for exercise. Four weeks of unpredictable stress reduced the animals' preference for saccharin, increased their adrenal weights and abolished the exercise-induced upregulation of neurogenesis that was observed in non-stressed animals. Unexpectedly, stress did not reduce hippocampal size, brain-derived neurotrophic factor (BDNF), or neurogenesis. The combination of dietary supplementation and exercise had multiple beneficial effects, as reflected in the number of doublecortin (DCX)-positive immature neurons in the dentate gyrus (DG), the sectional area of the DG and hippocampal CA1, as well as increased hippocampal BDNF messenger ribonucleic acid (mRNA) and serum vascular endothelial growth factor (VEGF) levels. In contrast, these benefits were not observed in chronically stressed animals exposed to either dietary supplementation or exercise alone. These findings could have important clinical implications for those suffering from chronic stress-related disorders such as major depression.
Asunto(s)
Suplementos Dietéticos , Hipocampo/fisiopatología , Carrera/fisiología , Estrés Psicológico/fisiopatología , Estrés Psicológico/terapia , Animales , Factor Neurotrófico Derivado del Encéfalo/metabolismo , Enfermedad Crónica , Trastorno Depresivo/patología , Trastorno Depresivo/fisiopatología , Trastorno Depresivo/terapia , Dieta , Modelos Animales de Enfermedad , Proteína Doblecortina , Hipocampo/patología , Factor I del Crecimiento Similar a la Insulina/metabolismo , Masculino , Ratones Endogámicos C57BL , Neurogénesis/fisiología , Tamaño de los Órganos , Condicionamiento Físico Animal/fisiología , Estrés Psicológico/patología , Resultado del Tratamiento , Incertidumbre , Factor A de Crecimiento Endotelial Vascular/sangreRESUMEN
Ionizing radiation continues to be a relevant tool in both imaging and the treatment of cancer. Experimental uses of focal irradiation have recently been expanded to studies of new neurons in the adult brain. Such studies have shown cognitive deficits following radiation treatment and raised caution as to possible unintentional effects that may occur in humans. Conflicting outcomes of the effects of irradiation on adult neurogenesis suggest that the effects are either transient or permanent. In this study, we used an irradiation apparatus employed in the treatment of human tumors to assess radiation effects on rat neurogenesis. For subjects we used adult male rats (Sprague-Dawley) under anesthesia. The irradiation beam was directed at the hippocampus, a center for learning and memory, and the site of neurogenic activity in adult brain. The irradiation was applied at a dose-rate 0.6 Gy/min for total single-fraction, doses ranging from 0.5 to 10.0 Gy. The animals were returned to home cages and recovered with no sign of any side effects. The neurogenesis was measured either 1 week or 6 weeks after the irradiation. At 1 week, the number of neuronal progenitors was reduced in a dose-dependent manner with the 50% reduction at 0.78 Gy. The dose-response curve was well fitted by a double exponential suggesting two processes. Examination of the tissue with quantitative immunohistochemistry revealed a dominant low-dose effect on neuronal progenitors resulting in 80% suppression of neurogenesis. This effect was partially reversible, possibly due to compensatory proliferation of the remaining precursors. At higher doses (>5 Gy) there was additional, nearly complete block of neurogenesis without compensatory proliferation. We conclude that notwithstanding the usefulness of irradiation for experimental purposes, the exposure of human subjects to doses often used in radiotherapy treatment could be damaging and cause cognitive impairments.
RESUMEN
The adaptive significance of adult hippocampal neurogenesis remains unknown. In the laboratory, it is influenced by a variety of environmental and physiological stimuli. In the wild, it may be influenced by the reliance on spatial memory and by environmental stressors. The one common denominator in both settings is that neurogenesis declines markedly with age. Red squirrels are long-lived rodents that store food (thousands of tree cones) to permit survival under harsh winter conditions. We compared a population from the eastern North America that stores its cones singly or in small clusters with one from the west that stores its cones in large stockpiles. The reliance on spatial memory should be much greater in the east than the west, and should not decline with age, as cone storage and recovery is a yearly necessity. We found no difference between the populations in the density of young neurons and both populations showed the same decline with age. Thus, we reject the spatial memory hypothesis for adult neurogenesis in mammals in its original form. Instead, our evidence is consistent with the neurogenic reserve hypothesis in which neuronal cell production early in life leads to enhanced hippocampal function later in life according to environmental demand but without requirement for ongoing cell production as a function of site- and species-specific needs. A more general interpretation of the data leads us to consider a possible role of neurogenesis in novel, flexible episodic memories.
Asunto(s)
Factores de Edad , Giro Dentado/fisiología , Conducta Alimentaria/fisiología , Memoria/fisiología , Neurogénesis/fisiología , Sciuridae/fisiología , Percepción Espacial/fisiología , Animales , Recuento de Células , Giro Dentado/citología , Proteínas de Dominio Doblecortina , Femenino , Antígeno Ki-67/metabolismo , Masculino , Proteínas Asociadas a Microtúbulos/metabolismo , Neuronas/citología , Neuropéptidos/metabolismo , Especificidad de la EspecieRESUMEN
OBJECTIVES: Adult neurogenesis in dentate gyrus (DG) is an evolutionarily preserved trait in most mammals examined thus far. Neuronal proliferation and subsequent integration of new neurons into the hippocampal circuit are regulated processes that can have profound effects on an animal's behaviour. A streptozotocin model of type I diabetes, characterized by low insulin and high plasma glucose levels, affects not only body's overall metabolism but also brain activity. MATERIALS AND METHODS: Neurogenesis was measured within the DG of the hippocampus using immunohistochemical markers Ki67, Doublecortin, Calbindin (CaBP) and bromodeoxyuridine (BrdU). RESULTS: Cell proliferation, measured with the endogenous marker Ki67, was reduced by 45%, and cell survival, measured with BrdU, was reduced by 64% of the control. Combined effects on proliferation and survival produced dramatically lower neuronal production. Among the surviving cells only 33% matured normally as judged by the co-labelling of BrdU and CaBP. CONCLUSION: Such a reduction lowered the number of surviving cells with neuronal phenotype by over 80% of the control values and this is expected to cause a significant functional impairment of learning and memory in diabetic animals. These results may shed light on causes of diabetic neuropathology and provide an explanation for the memory deficiencies seen in some diabetic patients.
Asunto(s)
Diabetes Mellitus Experimental/inducido químicamente , Diabetes Mellitus Experimental/patología , Hipocampo/metabolismo , Hipocampo/patología , Estreptozocina/farmacología , Animales , Antimetabolitos/metabolismo , Encéfalo/metabolismo , Encéfalo/patología , Bromodesoxiuridina/metabolismo , Calbindinas , Proliferación Celular/efectos de los fármacos , Diabetes Mellitus Experimental/metabolismo , Proteína Doblecortina , Inmunohistoquímica , Factor I del Crecimiento Similar a la Insulina/metabolismo , Antígeno Ki-67/metabolismo , Masculino , Ratas , Ratas Sprague-Dawley , Proteína G de Unión al Calcio S100/metabolismoRESUMEN
Postnatal hippocampal neurogenesis in wild mammals may play an essential role in spatial memory. We compared two species that differ in their reliance on memory to locate stored food. Yellow-pine chipmunks use a single cache to store winter food; eastern gray squirrels use multiple storage sites. Gray squirrels had three times the density of proliferating cells in the dentate gyrus (determined by Ki-67 immunostaining) than that found in chipmunks, but similar density of young neurons (determined by doublecortin immunostaining). Three explanations may account for these results. First, the larger population of young cells in squirrels may increase the flexibility of the spatial memory system by providing a larger pool of cells from which new neurons can be recruited. Second, squirrels may have a more rapid cell turnover rate. Third, many young cells in the squirrels may mature into glia rather than neurons. The densities of young neurons were higher in juveniles than in adults of both species. The relationship between adult age and cell density was more complex than that has been found in captive populations. In adult squirrels, the density of proliferating cells decreased exponentially with age, whereas in adult chipmunks the density of young neurons decreased exponentially with age.
Asunto(s)
Giro Dentado/fisiología , Conducta Alimentaria/fisiología , Memoria/fisiología , Sciuridae/anatomía & histología , Factores de Edad , Animales , Recuento de Células , Giro Dentado/citología , Proteínas de Dominio Doblecortina , Antígeno Ki-67/metabolismo , Proteínas Asociadas a Microtúbulos/metabolismo , Neuroglía/citología , Neuroglía/fisiología , Neuronas/citología , Neuronas/metabolismo , Neuronas/fisiología , Neuropéptidos/metabolismo , Sciuridae/fisiología , Percepción Espacial/fisiología , Especificidad de la EspecieRESUMEN
Adult hippocampal neurogenesis has been linked to learning but details of the relationship between neuronal production and memory formation remain unknown. Using low dose irradiation to inhibit adult hippocampal neurogenesis we show that new neurons aged 4-28 days old at the time of training are required for long-term memory in a spatial version of the water maze. This effect of irradiation was specific since long-term memory for a visibly cued platform remained intact. Furthermore, irradiation just before or after water maze training had no effect on learning or long-term memory. Relationships between learning and new neuron survival, as well as proliferation, were investigated but found non-significant. These results suggest a new role for adult neurogenesis in the formation and/or consolidation of long-term, hippocampus-dependent, spatial memories.
Asunto(s)
Proliferación Celular/efectos de la radiación , Hipocampo/fisiopatología , Aprendizaje por Laberinto/fisiología , Trastornos de la Memoria/fisiopatología , Memoria/fisiología , Plasticidad Neuronal/fisiología , Animales , Bromodesoxiuridina , Diferenciación Celular/fisiología , Diferenciación Celular/efectos de la radiación , División Celular/fisiología , División Celular/efectos de la radiación , Movimiento Celular/efectos de los fármacos , Movimiento Celular/fisiología , Señales (Psicología) , Rayos gamma/efectos adversos , Hipocampo/efectos de la radiación , Masculino , Aprendizaje por Laberinto/efectos de la radiación , Memoria/efectos de la radiación , Trastornos de la Memoria/etiología , Trastornos de la Memoria/patología , Plasticidad Neuronal/efectos de la radiación , Neuronas/fisiología , Neuronas/efectos de la radiación , Ratas , Ratas Long-Evans , Células Madre/fisiología , Células Madre/efectos de la radiación , Factores de TiempoRESUMEN
Adult animals continue to produce new neurons in the dentate gyrus of hippocampus. Until now, the principal method of studying neurogenesis has been to inject either tritiated thymidine or 5'-Bromo-2-deoxyuridine (BrdU) intraperitoneally followed by autoradiographic or immunohistochemical detection methods respectively. However, such exogenous markers may produce toxic effects. Our objective was to determine whether Ki-67, a nuclear protein expressed in all phases of the cell cycle except the resting phase, can be used as an alternative, endogenous marker. Using immunohistochemistry, we examined Ki-67 and BrdU expression pattern in rats. Ki-67 was expressed within the proliferative zone of the dentate gyrus and its expression pattern mimicked that of BrdU when examined soon after exogenous BrdU administration. Quantitative comparison of BrdU and Ki-67-positive cells showed 50% higher numbers of the latter when examined 24 h after the BrdU injection. This was expected, since BrdU can be incorporated into DNA only during the S-phase of the mitotic process, whereas Ki-67 is expressed for its whole duration. Experimental increases (by ischemia) or reductions (by radiation) in the number of mitotic cells produced parallel changes in BrdU and Ki-67 signals. Thus, Ki-67 is an effective mitotic marker and has most of the benefits of BrdU and none of the costs. This study provides evidence for Ki-67 to be used as a marker of proliferation in the initial phase of adult neurogenesis.
Asunto(s)
Bromodesoxiuridina , Diferenciación Celular/fisiología , División Celular/fisiología , Giro Dentado/crecimiento & desarrollo , Antígeno Ki-67 , Neuronas/metabolismo , Células Madre/metabolismo , Animales , Biomarcadores/análisis , Isquemia Encefálica/metabolismo , Isquemia Encefálica/patología , Isquemia Encefálica/fisiopatología , Bromodesoxiuridina/farmacología , Recuento de Células , Giro Dentado/citología , Giro Dentado/metabolismo , Rayos gamma/efectos adversos , Inmunohistoquímica/métodos , Inyecciones Intraperitoneales , Masculino , Proteínas del Tejido Nervioso/metabolismo , Neuronas/citología , Ratas , Ratas Wistar , Células Madre/citologíaRESUMEN
Ongoing neurogenesis in the adult hippocampal dentate gyrus (DG) generates a substantial population of young neurons. This phenomenon is present in all species examined thus far, including humans. Although the regulation of adult neurogenesis by various physiologically relevant factors such as learning and stress has been documented, the functional contributions of the newly born neurons to hippocampal functions are not known. We investigated possible contributions of the newly born granule neurons to synaptic plasticity in the hippocampal DG. In the standard hippocampal slice preparation perfused with artificial cerebrospinal fluid (ACSF), a small (10%) long-term potentiation (LTP) of the evoked field potentials is seen after tetanic stimulation of the afferent medial perforant pathway (MPP). The induction of this ACSF-LTP is resistant to a N-methyl-D-aspartate (NMDA) receptor blocker, D,L-2-amino-5-phosphonovaleric acid (APV), but is completely prevented by ifenprodil, a blocker of NR2B subtype of NMDA receptors. In contrast, slices perfused with picrotoxin (PICRO), a GABA-receptor blocker, revealed a larger (40--50%), APV-sensitive but ifenprodil-insensitive LTP. The ACSF-LTP required lower frequency of stimulation and fewer stimuli for its induction than the PICRO-LTP. All these characteristics of ACSF-LTP are in agreement with the properties of the putative individual new granule neurons examined previously with the use of the whole cell recording technique in a similar preparation. A causal relationship between neurogenesis and ACSF-LTP was confirmed in experiments using low dose of gamma radiation applied to the brain 3 wk prior to the electrophysiological experiments. In these experiments, the new cell proliferation was drastically reduced and ACSF-LTP was selectively blocked. We conclude that the young, adult-generated granule neurons play a significant role in synaptic plasticity in the DG. Since DG is the major source of the afferent inputs into the hippocampus, the production and the plasticity of new neurons may have an important role in the hippocampal functions such as learning and memory.
Asunto(s)
Giro Dentado/citología , Giro Dentado/fisiología , Plasticidad Neuronal/fisiología , Neuronas/citología , Neuronas/fisiología , 2-Amino-5-fosfonovalerato/farmacología , Factores de Edad , Animales , División Celular/fisiología , División Celular/efectos de la radiación , Giro Dentado/efectos de la radiación , Antagonistas de Aminoácidos Excitadores/farmacología , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Potenciales Postsinápticos Excitadores/fisiología , Antagonistas del GABA/farmacología , Potenciación a Largo Plazo/efectos de los fármacos , Potenciación a Largo Plazo/fisiología , Masculino , Técnicas de Cultivo de Órganos , Picrotoxina/farmacología , Piperidinas/farmacología , Ratas , Ratas WistarRESUMEN
The dogma that the adult brain produces no new neurons has been overturned, but the critics are still asking, so what? Is adult neurogenesis a biologically relevant phenomenon, or is it perhaps harmful because it disrupts the existing neuronal circuitry? Considering that the phenomenon is evolutionarily conserved in all mammalian species examined to date and that its relevance has been well documented in non-mammalian species, it seems self-evident that neurogenesis in adult mammals must have a role. In birds, it has been established that neurogenesis varies dramatically with seasonal changes in song production. In chickadees, the learning behaviour related to finding stored food is also correlated with seasonal adult neurogenesis. Such studies are still nonexistent in mammals, but the related evidence suggests that neurogenesis does vary seasonally in hamsters and shows sexual differences in meadow voles. To promote studies on natural populations asking fundamental questions of the purpose and function of neurogenesis, we organized a Workshop on "Hippocampal Neurogenesis in Natural Populations" in Toronto in May 2000. The Workshop highlighted recent discoveries in neurogenesis from the lab, and focused on its functional consequences. The consensus at the Workshop was that demonstration of a role for neurogenesis in natural behaviours will ultimately be essential if we are to understand the purpose and function of neurogenesis in humans.
Asunto(s)
Hipocampo/crecimiento & desarrollo , Molécula L1 de Adhesión de Célula Nerviosa , Neuronas/fisiología , Adulto , Animales , Apoptosis/fisiología , Hipocampo/citología , Hormonas/metabolismo , Humanos , Mamíferos/crecimiento & desarrollo , Memoria/fisiología , Moléculas de Adhesión de Célula Nerviosa/metabolismo , Neuronas/citología , Ácidos Siálicos/metabolismoRESUMEN
The dentate gyrus is one of the few areas of the mammalian brain where new neurons are continuously produced in adulthood. Certain insults such as epileptic seizures and ischemia are known to enhance the rate of neuronal production. We analyzed this phenomenon using the temporary occlusion of the two carotid arteries combined with arterial hypotension as a method to induce ischemia in rats. We measured the rate of cell production and their state of differentiation with a mitotic indicator, bromodeoxyuridine (BrdU), in combination with the immunohistochemical detection of neuronal markers. One week after the ischemic episode, the cell production in dentate gyrus was increased two- to threefold more than the basal level seen in control animals. Two weeks after ischemia, over 60% of these cells became young neurons as determined by colabeling with BrdU and a cytoplasmic protein (CRMP-4) involved in axonal guidance during development. Five weeks after the ischemia, over 60% of new neurons expressed calbindin, a calcium-binding protein normally expressed in mature granule neurons. In addition to more cells being generated, a greater proportion of all new cells remained in the differentiated but not fully mature state during the 2- to 5-week period after ischemia. The maturation rate of neurons as determined by the calbindin labeling and by the rate of migration from a proliferative zone into the granule cell layer was not changed when examined 5 weeks after ischemia. The results support the hypothesis that survival of dentate gyrus after ischemia is linked with enhanced neurogenesis. Additional physiological stimulation after ischemia may be exploited to stimulate maturation of new neurons and to offer new therapeutic strategies for promoting recovery of neuronal circuitry in the injured brain.
Asunto(s)
Giro Dentado/irrigación sanguínea , Giro Dentado/citología , Ataque Isquémico Transitorio/patología , Neuronas/citología , Animales , Antimetabolitos , Bromodesoxiuridina , Calbindinas , Diferenciación Celular/fisiología , División Celular/fisiología , Supervivencia Celular/fisiología , Citoplasma/química , Neuronas/química , Ratas , Ratas Sprague-Dawley , Proteína G de Unión al Calcio S100/análisis , Accidente Cerebrovascular/patologíaRESUMEN
Electroconvulsive shock (ECS) seizures provide an animal model of electroconvulsive therapy (ECT) in humans. Recent evidence indicates that repeated ECS seizures can induce long-term structural and functional changes in the brain, similar to those found in other seizure models. We have examined the effects of ECS on neurogenesis in the dentate gyrus of the adult rat using bromodeoxyuridine (BrdU) immunohistochemistry, which identifies newly generated cells. Cells have also been labeled for neuronal nuclear protein (NeuN) to identify neurons. One month following eight ECS seizures, ECS-treated rats had approximately twice as many BrdU-positive cells as sham-treated controls. Eighty-eight percent of newly generated cells colabeled with NeuN in ECS-treated subjects, compared to 83% in sham-treated controls. These data suggest that there is a net increase in neurogenesis within the hippocampal dentate gyrus following ECS treatment. Similar increases have been reported following kindling and kainic acid- or pilocarpine-induced status epilepticus. Increased neurogenesis appears to be a general response to seizure activity and may play a role in the therapeutic effects of ECT.
Asunto(s)
Giro Dentado/citología , Electrochoque , Plasticidad Neuronal/fisiología , Neuronas/citología , Animales , Bromodesoxiuridina/metabolismo , División Celular/fisiología , Giro Dentado/metabolismo , Terapia Electroconvulsiva/efectos adversos , Electrochoque/efectos adversos , Masculino , Neuronas/metabolismo , Ratas , Ratas WistarRESUMEN
Adenosine, an endogenous modulator of synaptic transmission, has been implicated in regulation of sleep and arousal. The effect of adenosine on neuronal excitability depends on its concentration in the extracellular space. The present study shows that the state of activity of laboratory rats determines the level of tonic inhibition by adenosine in hippocampal slices prepared from these animals. Thus, slices taken at the end of the active period showed significantly more inhibition by adenosine, as determined by the effects of the A1 receptor blocker 8-CPT, in comparison to slices taken in the inactive state. The results support the proposed role of adenosine in regulation of sleep and arousal and point to the importance of the time of day at which slices for electrophysiological experiments are prepared.
Asunto(s)
Adenosina/fisiología , Ritmo Circadiano , Hipocampo/fisiología , Transmisión Sináptica/fisiología , Animales , Potenciales Postsinápticos Excitadores , Técnicas In Vitro , Masculino , Ratas , Ratas WistarRESUMEN
Postnatal neurogenesis contributes substantially to the neuronal population of the adult dentate gyrus. We report here that the neurons located in the deep aspects of the granule cell layer, near the proliferative zone, have different properties from those located in the superficial layers. The former group of neurons, tentatively designated as young, can be readily identified in a standard hippocampal slice preparation by morphological, immunohistochemical, and electrophysiological criteria. Electrophysiological recordings and imaging with Lucifer yellow from these neurons in the standard hippocampal slice preparation showed one or two main dendrites and conically shaped branches possessing varicose protrusions. These features are in agreement with the appearance of the same population of young neurons immunopositive for TOAD-64, a marker for immature neurons. In disinhibited slices, these putative young neurons are distinguished from the mature neurons, located in the superficial granule cell layer, by showing paired pulse facilitation and having a lower threshold for induction of long-term potentiation. The putative young neurons are completely unaffected by GABA(A) inhibition and always display robust long-term potentiation. In contrast, the mature neurons never produce long-term potentiation when the GABA(A) inhibition is intact. We propose that the heterogeneity of the functional properties of the granule neurons is related to the ongoing neurogenesis in the adult animals.
Asunto(s)
Giro Dentado/citología , Neuronas/citología , Neuronas/fisiología , Animales , Bicuculina/farmacología , Biomarcadores , Senescencia Celular/fisiología , Dendritas/metabolismo , Giro Dentado/fisiología , Estimulación Eléctrica , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Potenciales Postsinápticos Excitadores/fisiología , Colorantes Fluorescentes , Antagonistas del GABA/farmacología , Antagonistas de Receptores de GABA-A , Inmunohistoquímica , Técnicas In Vitro , Isoquinolinas , Potenciación a Largo Plazo/fisiología , Masculino , Neuronas/efectos de los fármacos , Técnicas de Placa-Clamp , Ratas , Ratas WistarRESUMEN
To incorporate variation of neuron shape in neural models, we developed a method of generating a population of realistically shaped neurons. Parameters that characterize a neuron include soma diameters, distances to branch points, fiber diameters, and overall dendritic tree shape and size. Experimentally measured distributions provide a means of treating these morphological parameters as stochastic variables in an algorithm for production of neurons. Stochastically generated neurons shapes were used in a model of hippocampal dentate gyrus granule cells. A large part of the variation of whole neuron input resistance R(N) is due to variation in shape. Membrane resistivity Rm computed from R(N) varies accordingly. Statistics of responses to synaptic activation were computed for different dendritic shapes. Magnitude of response variation depended on synapse location, measurement site, and attribute of response.
Asunto(s)
Simulación por Computador , Potenciales Postsinápticos Excitadores , Neuronas/fisiología , Transducción de Señal/fisiología , Procesos Estocásticos , Algoritmos , Tamaño de la Célula , Dendritas/ultraestructura , Giro Dentado/citología , Giro Dentado/fisiología , Humanos , Técnicas In Vitro , Modelos NeurológicosRESUMEN
Silent synapses, defined as structural specializations for neurotransmission that do not produce a physiological response in the receiving cell, may occur frequently in neural circuits. Their recruitment to physiological effectiveness may be an important component of circuit modification. In several nervous systems, evidence from electrophysiological and optophysiological measurements has established a strong case for the existence of silent synapses and for their emergence as active synapses with appropriate stimulation. During normal development and aging, synapses of individual neurons change in number, and many of these may be functionally silent at certain stages of their developmental trajectory. Changes in their status may contribute to shaping the properties of neural pathways during development, often in response to neural activity. In general, it is often difficult to distinguish physiological emergence of pre-established silent synapses from developmental maturation or de novo formation of new synapses. Several possible mechanisms for silent synapses and their recruitment are reviewed. These include incompletely assembled synapses that lack structural components, insufficient availability of key presynaptic proteins, and nonfunctional postsynaptic receptors, or presence of receptors that do not mediate a postsynaptic response except under specific conditions (conditionally silent synapses). The available silent synapses can often be rapidly activated, and conversely, active synapses appear to be rapidly silenced in many instances. These properties enable silent synapses to participate in short-term facilitation and depression. In addition, they may contribute to long-term facilitation and potentiation, especially during development.
Asunto(s)
Plasticidad Neuronal/fisiología , Sinapsis/fisiología , Animales , MamíferosRESUMEN
Kindling, a form of neuronal plasticity produced by repeated low intensity electrical brain stimulation, leads to epileptic seizures. To address possible causes of this phenomenon, we have prepared amygdala-kindled animals and measured neurogenesis, by bromodeoxyuridine incorporation. Early, when focal seizures were present, there was no evidence of a change in the rate of hippocampal neurogenesis. In contrast, during the later phases of kindling, when secondary generalization was well established and motor seizures were present, neurogenesis was enhanced by 75-140%, depending on the hippocampal region. Double labelling with the neuron-specific marker TOAD-64 demonstrated the presence of numerous new-born granule neurons in the kindled animals. We propose that the newly-born neurons contribute to the cellular changes and behavioral symptoms associated with this type of epileptiform brain plasticity.
Asunto(s)
Giro Dentado/patología , Excitación Neurológica , Neuronas/patología , Animales , Bromodesoxiuridina , División Celular , Núcleo Celular/ultraestructura , Giro Dentado/ultraestructura , Masculino , Proteínas del Tejido Nervioso/análisis , Neuronas/ultraestructura , Ratas , Ratas Wistar , Convulsiones/patologíaRESUMEN
The mechanisms underlying the differential expression of long-term potentiation (LTP) by AMPA and NMDA receptors, are unknown, but could involve G-protein-linked metabotropic glutamate receptors. To investigate this hypothesis we created mutant mice that expressed no metabotropic glutamate receptor 5 (mGluR5), but showed normal development. In an earlier study of these mice we analyzed field-excitatory postsynaptic potential (fEPSPs) in CA1 region of the hippocampus and found a small decrease; possibly arising from changes in the NMDAR-mediated component of synaptic transmission. In the present study we used whole-cell patch clamp recordings of evoked excitatory postsynaptic currents (EPSCs) in CA1 pyramidal neurons to identify the AMPAR- and NMDAR-mediated components of LTP. Recordings from control mice following tetanus, or agonist application (IS, 3R-1-amino-cyclopentane 1,3-dicarboxylic acid) (ACPD), revealed equal enhancement of the AMPA and NMDA receptor-mediated components. In contrast, CA1 neurons from mGluR5-deficient mice showed a complete loss of the NMDA-receptor-mediated component of LTP (LTP(NMDA)), but normal LTP of the AMPA-receptor-mediated component (LTP(AMPA)). This selective loss of LTP(NMDA) was seen in three different genotypic backgrounds and was apparent at all holding potentials (-70 mV to +20 mV). Furthermore, the LTP(NMDA) deficit in mGluR5 mutant mice could be rescued by stimulating protein kinase C (PKC) with 4beta-phorbol-12,13-dibutyrate (PDBu). These results suggest that PKC may couple the postsynaptic mGluR5 to the NMDA-receptor potentiation during LTP, and that this signaling mechanism is distinct from LTP(AMPA). Differential enhancement of AMPAR and NMDA receptors by mGluR5 also supports a postsynaptic locus for LTP.
Asunto(s)
Potenciales Postsinápticos Excitadores/fisiología , Hipocampo/fisiología , Potenciación a Largo Plazo/fisiología , N-Metilaspartato/fisiología , Neuronas/fisiología , Receptores de Glutamato Metabotrópico/genética , Receptores de Glutamato Metabotrópico/fisiología , Animales , Estimulación Eléctrica , Exones , Ratones , Ratones Noqueados , Células Piramidales/fisiología , Receptor del Glutamato Metabotropico 5 , Receptores de Glutamato Metabotrópico/deficiencia , Mapeo Restrictivo , Especificidad de la EspecieRESUMEN
Metabotropic glutamate receptors are thought to be important regulators of synaptic transmission and plasticity in the hippocampus. The metabotropic glutamate receptor subtype mGluR5 is expressed in hippocampal pyramidal neurons but its function remains unknown due to the lack of selective pharmacological blockers. We inhibited the synthesis of mGluR5 with antisense oligonucleotides injected into the hippocampus in vivo. The functional effects of altered mGluR5 expression were measured electrophysiologically in the CA1 region of the hippocampus during applications of the metabotropic agonist 1S,3R-ACPD (50 microM) to hippocampal slices from injected animals. The results show a concomitant reduction of the mGluR5 receptor protein and physiological effects in the hippocampus. The major effect found in the antisensetreated animals was the lack of an excitatory action normally produced by 1S,3R-ACPD. Another effect attributed to metabotropic glutamate receptors, depression of synaptic transmission, had a more rapid onset, but unchanged magnitude, while long-term potentiation remained unchanged. The specificity and effectiveness of the antisense treatment were confirmed using mismatched oligonucleotides and immunoblotting. We conclude that the metabotropic glutamate receptor subtype mGluR5 plays a major role in the regulation of cell excitability in the hippocampus without directly affecting synaptic transmission or long-term potentiation. Moreover, in vivo applications of antisense deoxynucleotides are a useful approach in studies of neurotransmitter receptor subtypes.
Asunto(s)
Cicloleucina/análogos & derivados , ADN sin Sentido/farmacología , Regulación hacia Abajo , Hipocampo/efectos de los fármacos , Hipocampo/metabolismo , Fármacos Neuroprotectores/farmacología , Receptores de Glutamato Metabotrópico/metabolismo , Animales , Cicloleucina/farmacología , Electrofisiología , Hipocampo/fisiología , Immunoblotting , Masculino , Ratas , Ratas Wistar , Receptores de Glutamato Metabotrópico/efectos de los fármacosRESUMEN
Class I metabotropic glutamate receptors (mGluRs) have been postulated to play a role in synaptic plasticity. To test the involvement of one member of this class, we have recently generated mutant mice that express no mGluR5 but normal levels of other glutamate receptors. The CNS revealed normal development of gross anatomical features. To examine synaptic functions we measured evoked field EPSPs in the hippocampal slice. Measures of presynaptic function, such as paired pulse facilitation in mutant CA1 neurons, were normal. The response of mutant CA1 neurons to low concentrations of (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) was missing, which suggests that mGluR5 may be the primary high affinity ACPD receptor in these neurons. Long-term potentiation (LTP) in mGluR5 mutants was significantly reduced in the NMDA receptor (NMDAR)-dependent pathways such as the CA1 region and dentate gyrus of the hippocampus, whereas LTP remained intact in the mossy fiber synapses on the CA3 region, an NMDAR-independent pathway. Some of the difference in CA1 LTP could lie at the level of expression, because the reduction of LTP in the mutants was no longer observed 20 min after tetanus in the presence of 2-amino-5-phosphonopentanoate. We propose that mGluR5 plays a key regulatory role in NMDAR-dependent LTP. These mutant mice were also impaired in the acquisition and use of spatial information in both the Morris water maze and contextual information in the fear-conditioning test. This is consistent with the hypothesis that LTP in the CA1 region may underlie spatial learning and memory.
Asunto(s)
Hipocampo/fisiología , Discapacidades para el Aprendizaje/metabolismo , Potenciación a Largo Plazo , Receptores de Glutamato Metabotrópico/deficiencia , Animales , Conducta Animal , Unión Competitiva , Sistema Nervioso Central/crecimiento & desarrollo , Condicionamiento Psicológico , Cicloleucina/análogos & derivados , Cicloleucina/metabolismo , Miedo , Aprendizaje por Laberinto , Ratones , Ratones Endogámicos , Mutación , Receptores de Glutamato Metabotrópico/agonistas , Receptores de Glutamato Metabotrópico/genética , Valores de Referencia , Sinapsis/fisiologíaRESUMEN
Dendritic arborization permits convergence of synaptic inputs and their integration in single neurons. The granule neuron in the dentate gyrus represents a relatively simple example where anatomically and functionally distinct medial and lateral perforant pathways terminate on different regions of the dendritic tree. High-frequency stimulation of either pathway alone results in the induction of long-term potentiation. However, whether the potentiated synapses in different parts of the dendrites interact is not known. In this study we have compared long-term potentiation and synaptic interactions in the lateral and medial perforant pathways in the "disinhibited" hippocampal slice preparation in the presence of the GABA(A) receptor blocker bicuculline. The data show that the magnitude of long-term potentiation induced by tetanic stimulation was similar in both pathways, but differences between the two pathways were revealed after two or more tetanizations. A significantly smaller capacity for further long-term potentiation in the lateral, as compared to the medial, perforant pathway was found and can be attributed to stronger postsynaptic GABA(B) inhibition in distal dendrites of granule neurons. Blockade of GABA(B) inhibition with CGP36742 (100 microM) unmasked additional long-term potentiation in the lateral pathway. Presynaptically, GABA(B) receptors produced a short-lasting heterosynaptic depression in the medial pathway, which was reduced by CGP36742. Coincident activation of the two pathways boosted long-term potentiation only in the medial pathway. We propose that the interactions between the two pathways are orchestrated to maximize associative long-term potentiation in the medial pathway; this may be important for types of learning attributed to the hippocampus.