RESUMEN
Although gap junctions were first demonstrated in the mammalian brain about 30 years ago, the distribution and role of electrical synapses have remained elusive. A series of recent reports has demonstrated that inhibitory interneurons in the cerebral cortex, thalamus, striatum and cerebellum are extensively interconnected by electrical synapses. Investigators have used paired recordings to reveal directly the presence of electrical synapses among identified cell types. These studies indicate that electrical coupling is a fundamental feature of local inhibitory circuits and suggest that electrical synapses define functionally diverse networks of GABA-releasing interneurons. Here, we discuss these results, their possible functional significance and the insights into neuronal circuit organization that have emerged from them.
Asunto(s)
Encéfalo/fisiología , Uniones Comunicantes/fisiología , Interneuronas/fisiología , Sinapsis/fisiología , Ácido gamma-Aminobutírico/metabolismo , Potenciales de Acción/fisiología , Animales , Encéfalo/ultraestructura , Comunicación Celular/fisiología , Uniones Comunicantes/ultraestructura , Humanos , Interneuronas/ultraestructura , Inhibición Neural/fisiología , Sinapsis/ultraestructuraRESUMEN
The temporal pattern and relative timing of action potentials among neocortical neurons may carry important information. However, how cortical circuits detect or generate coherent activity remains unclear. Using paired recordings in rat neocortical slices, we found that the firing of fast-spiking cells can reflect the spiking pattern of single-axon pyramidal inputs. Moreover, this property allowed groups of fast-spiking cells interconnected by electrical and gamma-aminobutyric acid (GABA)-releasing (GABAergic) synapses to detect the relative timing of their excitatory inputs. These results indicate that networks of fast-spiking cells may play a role in the detection and promotion of synchronous activity within the neocortex.
Asunto(s)
Interneuronas/fisiología , Neocórtex/fisiología , Red Nerviosa/fisiología , Células Piramidales/fisiología , Transmisión Sináptica , Ácido gamma-Aminobutírico/metabolismo , Potenciales de Acción , Animales , Axones/fisiología , Potenciales Postsinápticos Excitadores , Femenino , Técnicas In Vitro , Cinética , Masculino , Neocórtex/citología , Ratas , Ratas Sprague-Dawley , Sinapsis/fisiología , Factores de TiempoRESUMEN
The increased release of oxytocin during lactation has been shown to be dependent upon glutamatergic transmission and is associated with an increased synaptic innervation of the supraoptic nucleus (SON). To determine whether the glutamatergic synaptic properties of oxytocin neurones are changed during lactation, we recorded excitatory postsynaptic currents (EPSCs) from identified oxytocin neurones in the SON of slices taken from adult virgin and lactating rats. The frequency of AMPA-mediated miniature EPSCs (mEPSCs) more than doubled during lactation. In addition, the decay time constant, but not the amplitude of the mEPSCs was significantly increased in both vasopressin and oxytocin neurones. Paired-pulse facilitation (PPF) was significantly reduced in oxytocin neurones during lactation, whereas no change was observed in vasopressin neurones. Elevating Ca(2+) reduced PPF in oxytocin neurones in virgin rats but did not alter PPF in oxytocin neurones from lactating rats. Collectively, our results suggest that excitatory glutamatergic transmission is strengthened in oxytocin neurones during lactation, probably by a combination of an increased number of terminals, slower decay kinetics, and an increase in the probability of release.
Asunto(s)
Lactancia/metabolismo , Neuronas/metabolismo , Neurotransmisores/metabolismo , Oxitocina/metabolismo , Sinapsis/metabolismo , Animales , Calcio/metabolismo , Calcio/farmacología , Potenciales Evocados/efectos de los fármacos , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Femenino , Antagonistas de Receptores de GABA-A , Glutamina/metabolismo , Inmunohistoquímica , Técnicas In Vitro , Magnesio/metabolismo , Magnesio/farmacología , Plasticidad Neuronal/efectos de los fármacos , Neuronas/citología , Neuronas/efectos de los fármacos , Oxitocina/farmacología , Técnicas de Placa-Clamp , Ratas , Ratas Sprague-Dawley , Receptores AMPA/metabolismo , Receptores de N-Metil-D-Aspartato/antagonistas & inhibidores , Núcleo Supraóptico/citología , Núcleo Supraóptico/efectos de los fármacos , Núcleo Supraóptico/metabolismo , Vasopresinas/metabolismoRESUMEN
High-frequency activity produces transient depression at many synapses but also, as recently demonstrated, may accelerate the recovery from use-dependent depression. We have examined the possible consequences of this synaptic mechanism in neocortical excitatory synapses by recording simultaneously from presynaptic pyramidal neurons and their postsynaptic targets. Brief bursts of high-frequency spikes produced a strong depression of the amplitude of unitary excitatory postsynaptic currents (uEPSCs). However, when burst firing was combined with low-frequency ongoing activity, we found that the strong synaptic depression was followed by a transient rebound of synaptic strength. This rebound overshot the low-frequency baseline values and lasted 1-2 s. These results suggest that in the presence of ongoing activity, neocortical synapses may functionally facilitate following burst firing.
Asunto(s)
Potenciales Postsinápticos Excitadores/fisiología , Neocórtex/fisiología , Células Piramidales/fisiología , Sinapsis/fisiología , Animales , Estimulación Eléctrica , Técnicas In Vitro , Terminales Presinápticos/fisiología , Ratas , Ratas Wistar , Tiempo de Reacción , Factores de TiempoRESUMEN
Encoding of information in the cortex is thought to depend on synchronous firing of cortical neurons. Inhibitory neurons are known to be critical in the coordination of cortical activity, but how interaction among inhibitory cells promotes synchrony is not well understood. To address this issue directly, we have recorded simultaneously from pairs of fast-spiking (FS) cells, a type of gamma-aminobutyric acid (GABA)-containing neocortical interneuron. Here we report a high occurrence of electrical coupling among FS cells. Electrical synapses were not found among pyramidal neurons or between FS cells and other cortical cells. Some FS cells were interconnected by both electrical and GABAergic synapses. We show that communication through electrical synapses allows excitatory signalling among inhibitory cells and promotes their synchronous spiking. These results indicate that electrical synapses establish a network of fast-spiking cells in the neocortex which may play a key role in coordinating cortical activity.
Asunto(s)
Interneuronas/fisiología , Neocórtex/fisiología , Red Nerviosa/fisiología , Inhibición Neural , Sinapsis/fisiología , Potenciales de Acción , Animales , Electrofisiología , Técnicas In Vitro , Neocórtex/anatomía & histología , Ratas , Ratas Wistar , Transmisión Sináptica , Ácido gamma-Aminobutírico/metabolismoRESUMEN
Oxytocin (OT) and vasopressin (VP) hormone release from neurohypophysial terminals is controlled by the firing pattern of neurosecretory cells located in the hypothalamic supraoptic (SON) and paraventricular nuclei. Although glutamate is a key modulator of the electrical activity of both OT and VP neurons, a differential contribution of AMPA receptors (AMPARs) and NMDA receptors (NMDARs) has been proposed to mediate glutamatergic influences on these neurons. In the present study we examined the distribution and functional properties of synaptic currents mediated by AMPARs and NMDARs in immunoidentified SON neurons. Our results suggest that the properties of AMPA-mediated currents in SON neurons are controlled in a cell type-specific manner. OT neurons displayed AMPA-mediated miniature EPSCs (mEPSCs) with larger amplitude and faster decay kinetics than VP neurons. Furthermore, a peak-scaled nonstationary noise analysis of mEPSCs revealed a larger estimated single-channel conductance of AMPARs expressed in OT neurons. High-frequency summation of AMPA-mediated excitatory postsynaptic potentials was smaller in OT neurons. In both cell types, AMPA-mediated synaptic currents showed inward rectification, which was more pronounced in OT neurons, and displayed Ca2+ permeability. On the other hand, NMDA-mediated mEPSCs of both cell types had similar amplitude and kinetic properties. The cell type-specific expression of functionally different AMPARs can contribute to the adoption of different firing patterns by these neuroendocrine neurons in response to physiological stimuli.
Asunto(s)
Potenciales Evocados/fisiología , Hipotálamo/fisiología , Neuronas/fisiología , Oxitocina/fisiología , Receptores AMPA/fisiología , Receptores de N-Metil-D-Aspartato/fisiología , Núcleo Supraóptico/fisiología , Sinapsis/fisiología , Vasopresinas/fisiología , Animales , Potenciales Evocados/efectos de los fármacos , Antagonistas de Aminoácidos Excitadores/farmacología , Femenino , Técnicas In Vitro , Neuronas/citología , Neuronas/efectos de los fármacos , Oxitocina/análisis , Técnicas de Placa-Clamp , Quinoxalinas/farmacología , Ratas , Núcleo Supraóptico/citología , Vasopresinas/análisis , Ácido alfa-Amino-3-hidroxi-5-metil-4-isoxazol Propiónico/farmacologíaRESUMEN
The stability of cortical neuron activity in vivo suggests that the firing rates of both excitatory and inhibitory neurons are dynamically adjusted. Using dual recordings from excitatory pyramidal neurons and inhibitory fast-spiking neurons in neocortical slices, we report that sustained activation by trains of several hundred presynaptic spikes resulted in much stronger depression of synaptic currents at excitatory synapses than at inhibitory ones. The steady-state synaptic depression was frequency dependent and reflected presynaptic function. These results suggest that inhibitory terminals of fast-spiking cells are better equipped to support prolonged transmitter release at a high frequency compared with excitatory ones. This difference in frequency-dependent depression could produce a relative increase in the impact of inhibition during periods of high global activity and promote the stability of cortical circuits.
Asunto(s)
Neocórtex/fisiología , Inhibición Neural/fisiología , Sinapsis/fisiología , Potenciales de Acción/fisiología , Animales , Conductividad Eléctrica , Estimulación Eléctrica/métodos , Homeostasis/fisiología , Técnicas In Vitro , Terminales Presinápticos/fisiología , Ratas , Ratas Wistar , Factores de TiempoRESUMEN
We used paired recordings to study the development of synaptic transmission between inhibitory interneurons of the molecular layer and Purkinje cells in the cerebellar cortex of the rat. The electrophysiological data were combined with a morphological study of the recorded cells using biocytin or Lucifer yellow staining. Thirty-one interneuron-Purkinje cell pairs were obtained, and 11 of them were recovered morphologically. The age of the rats ranged from 11 to 31 d after birth. During this period synaptic maturation resulted in an 11-fold decrease in the average current evoked in a Purkinje cell by a spike in a presynaptic interneuron. Unitary IPSCs in younger animals exhibited paired-pulse depression, whereas paired-pulse facilitation was found in more mature animals. These data suggest that reduction in transmitter release probability contributed to the developmental decrease of unitary IPSCs. However, additional mechanisms at both presynaptic and postsynaptic loci should also be considered. The decrease of the average synaptic current evoked in a Purkinje cell by an action potential in a single interneuron suggests that as development proceeds interneuron activities must be coordinated to inhibit efficiently Purkinje cells.
Asunto(s)
Corteza Cerebelosa/citología , Interneuronas/citología , Células de Purkinje/citología , Sinapsis/fisiología , Transmisión Sináptica/fisiología , Potenciales de Acción/efectos de los fármacos , Factores de Edad , Animales , Corteza Cerebelosa/crecimiento & desarrollo , Interneuronas/fisiología , Células de Purkinje/fisiología , Ratas , Ratas Wistar , Relación Estructura-ActividadRESUMEN
Rapid applications of GABA (from 10 microM to 10 mM) to outside-out patches were used to study the role that the kinetic properties of GABAA receptors play in determining the time course of IPSCs in neocortical pyramidal neurons. Currents induced by rapid applications of brief (1 msec) pulses of GABA (1 mM) showed a biexponential decay phase that seems to involve the entry of GABAA receptors into desensitized states. This conclusion is based on the similar fast decay kinetics of the response to brief and prolonged pulses of GABA and on the correlation between the degree of paired-pulse depression and the decay rate of the currents induced by brief pulses. Under nonequilibrium conditions we found that the concentration-response curve of pyramidal GABAA receptors has an EC50 of 185 microM (GABA pulse of 1 msec). The decay time course of the patch currents in response to brief applications of GABA was insensitive to agonist concentrations at the range from 50 microM to 10 mM. Faster decay rates were observed only in response to pulses of 10 microM GABA. These data are compatible with the suggestion that briefer openings derive from a monoliganded state and that these are negligible when receptor activation is >2%. Assuming that GABA transients at neocortical synapses are fast, a several millimolar GABA concentration would be needed to saturate the postsynaptic GABAA receptors.
Asunto(s)
Corteza Cerebral/fisiología , Inhibición Neural/fisiología , Células Piramidales/fisiología , Receptores de GABA-A/fisiología , Sinapsis/fisiología , Animales , Corteza Cerebral/citología , Relación Dosis-Respuesta a Droga , Conductividad Eléctrica , Electrofisiología , Masculino , Concentración Osmolar , Ratas , Ratas Wistar , Sinapsis/efectos de los fármacos , Factores de Tiempo , Ácido gamma-Aminobutírico/farmacologíaRESUMEN
Native AMPA receptors (AMPARs) were investigated in neocortical fast-spiking (FS) and regular-spiking nonpyramidal (RSNP) cells. The onset of and recovery from desensitization as well as current rectification and single-channel conductance were studied by using fast glutamate application to outside-out patches. The GluR1-4 subunit, flip/flop splicing, and R/G editing expression patterns of functionally characterized cells were determined by single-cell reverse transcription-PCR to correlate the subunit composition of native AMPARs with their functional properties. Our sample, mostly constituted by RSNP neurons, predominantly expressed GluR3 flip and GluR2 flop. In individual cells, flip/flop splicing of each subunit appeared to be regulated independently, whereas for R/G editing all subunits were either almost fully edited or unedited. We confirmed that the relative GluR2 expression controls the permeation properties of native AMPARs, whereas none of the single molecular parameters considered appeared to be a key determinant of the kinetics. FS neurons displayed AMPARs with relatively homogeneous functional properties characterized by fast desensitization, slow recovery from desensitization, marked inward rectification, and large single-channel conductance. In contrast, these parameters varied over a wide range in RSNP neurons, and their combination resulted in various AMPAR functional patterns. Indeed, in different cells, fast or slow desensitization was found to be associated with either slow or fast recovery from desensitization. Similarly, fast or slow kinetics was associated with either strong or weak rectification. Our results suggest that kinetic and permeation properties of native AMPARs can be regulated independently in cortical neurons and probably do not have the same molecular determinants.
Asunto(s)
Corteza Cerebral/metabolismo , Neuronas/metabolismo , Receptores AMPA/metabolismo , Animales , Corteza Cerebral/citología , Electrofisiología , Isomerismo , Cinética , Permeabilidad , Reacción en Cadena de la Polimerasa , Ratas , Ratas Wistar , Receptores AMPA/química , Receptores AMPA/fisiología , Transcripción GenéticaRESUMEN
The physiological and molecular features of nonpyramidal cells were investigated in acute slices of sensory-motor cortex using whole-cell recordings combined with single-cell RT-PCR to detect simultaneously the mRNAs of three calcium binding proteins (calbindin D28k, parvalbumin, and calretinin) and four neuropeptides (neuropeptide Y, vasoactive intestinal polypeptide, somatostatin, and cholecystokinin). In the 97 neurons analyzed, all expressed mRNAs of at least one calcium binding protein, and the majority (n = 73) contained mRNAs of at least one neuropeptide. Three groups of nonpyramidal cells were defined according to their firing pattern. (1) Fast spiking cells (n = 34) displayed tonic discharges of fast action potentials with no accommodation. They expressed parvalbumin (n = 30) and/or calbindin (n = 19) mRNAs, and half of them also contained transcripts of at least one of the four neuropeptides. (2) Regular spiking nonpyramidal cells (n = 48) displayed a firing behavior characterized by a marked accommodation and presented a large diversity of expression patterns of the seven biochemical markers. (3) Finally, a small population of vertically oriented bipolar cells, termed irregular spiking cells (n = 15), fired bursts of action potentials at an irregular frequency. They consistently co-expressed calretinin and vasoactive intestinal polypeptide. Additional investigations of these cells showed that they also co-expressed glutamic acid decarboxylase and choline acetyl transferase. Our results indicate that neocortical nonpyramidal neurons display a large diversity in their firing properties and biochemical patterns of co-expression and that both characteristics could be correlated to define discrete subpopulations.
Asunto(s)
Corteza Cerebral/citología , Interneuronas/química , Interneuronas/citología , Potenciales de Acción/fisiología , Animales , Biomarcadores , Calbindina 1 , Calbindina 2 , Calbindinas , Corteza Cerebral/química , Corteza Cerebral/enzimología , Colecistoquinina/genética , Colina O-Acetiltransferasa/genética , Glutamato Descarboxilasa/genética , Interneuronas/enzimología , Proteínas del Tejido Nervioso/genética , Neuropéptido Y/genética , Parvalbúminas/genética , Técnicas de Placa-Clamp , Reacción en Cadena de la Polimerasa/métodos , ARN Mensajero/análisis , Ratas , Ratas Wistar , Proteína G de Unión al Calcio S100/genética , Sensibilidad y Especificidad , Somatostatina/genética , Péptido Intestinal Vasoactivo/genéticaRESUMEN
The electrophysiological and morphological properties of layer I neurons were studied in visual cortex slices from 7- to 19-d-old rats using whole-cell recording and biocytin labeling. A heterogeneous population of small, nonpyramidal neurons was found. Approximately one third of the cells we recorded were neurogliaform cells; another third were multipolar neurons with axons descending out of layer I. The remaining cells were heterogeneous and were not classified. In slices from 7- to 10-d-old animals only, we identified Cajal-Retzius cells. Neurogliaform neurons had a very dense local axonal field, which was largely contained within layer I. Cells with descending axons had a relatively sparse local axonal arbor and projected at least to layer II and sometimes deeper. Spiking in neurogliaform neurons was followed by an afterdepolarizing potential, whereas spiking in cells with descending axons was followed by a slow after-hyperpolarizing potential (AHP). In addition, neurogliaform cells exhibited less spike broadening and a larger fast AHP after single spikes than did cells with descending axons. Generally, cells in layer I received synaptic inputs characterized as either GABA- or glutamate-mediated, suggesting the presence of excitatory and inhibitory inputs. With their output largely limited to layer I, neurogliaform cells could synapse with other layer I neurons, the most distal dendritic branches of pyramidal cells, or the dendrites of layer II/III interneurons, which invade layer I. Cells with descending axons could contact a wide variety of cortical cells throughout their vertical projection.
Asunto(s)
Neuronas/fisiología , Corteza Visual/citología , Animales , Axones/ultraestructura , Inhibición Neural , Neuroglía/citología , Neuronas/citología , Técnicas de Placa-Clamp , Ratas , Ratas Wistar , Sinapsis/fisiologíaRESUMEN
In the cortex fast excitatory synaptic currents onto excitatory pyramidal neurons and inhibitory nonpyramidal neurons are mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors exhibiting cell-type-specific differences in their kinetic properties. AMPA receptors consist of four subunits (GluR1-4), each existing as two splice variants, flip and flop, which critically affect the desensitization properties of receptors expressed in heterologous systems. Using single cell reverse transcription PCR to analyze the mRNA of AMPA receptor subunits expressed in layers I-III neocortical neurons, we find that 90% of the GluR1-4 in nonpyramidal neurons are flop variants, whereas 92% of the GluR1-4 in pyramidal neurons are flip variants. We also find that nonpyramidal neurons predominantly express GluR1 mRNA (GluR1/GluR1-4 = 59%), whereas pyramidal neurons contain mainly GluR2 mRNA (GluR2/GluR1-4 = 59%). However, the neuron-type-specific splicing is exhibited by all four AMPA receptor subunits. We suggest that the predominance of the flop variants contributes to the faster and more extensive desensitization in nonpyramidal neurons, compared to pyramidal cells where flip variants are dominant. Alternative splicing of AMPA receptors may play an important role in regulating synaptic function in a cell-type-specific manner, without changing permeation properties.
Asunto(s)
Corteza Cerebral/metabolismo , Receptores AMPA/metabolismo , Empalme Alternativo , Animales , Secuencia de Bases , Corteza Cerebral/citología , Cartilla de ADN/química , Datos de Secuencia Molecular , ARN Mensajero/genética , Ratas , Ratas WistarRESUMEN
The biochemical and functional characteristics of the AMPA subtype of the glutamate receptors expressed by pyramidal and non-pyramidal neurons of the neocortex have been studied in acute slices by means of single-cell RT-PCR and fast applications of glutamate on outside-out patches. Our results suggest that the predominant expression of the flop splice variants of the GluR1-4 AMPA subunits contributes to the faster desensitization of these receptors in non-pyramidal neurons compared to pyramidal cells where flip variants of GluR1-4 are dominant. Alternative splicing of AMPA receptors may therefore play an important role in regulating synaptic function in a cell-type specific manner.
Asunto(s)
Corteza Cerebral/fisiología , Plasticidad Neuronal , Neuronas/fisiología , Receptores AMPA/biosíntesis , Sinapsis/fisiología , Empalme Alternativo , Animales , Variación Genética , Técnicas In Vitro , Sustancias Macromoleculares , Reacción en Cadena de la Polimerasa , Receptores AMPA/fisiología , Corteza Somatosensorial/fisiología , Corteza Visual/fisiologíaRESUMEN
Spontaneous excitatory postsynaptic currents (sEPSCs) and responses to rapid application of glutamate were recorded in excitatory spiny, pyramidal neurons and compared with those recorded in inhibitory aspiny interneurons. The sEPSC decay time constant was faster in aspiny interneurons (2.5 ms) compared with pyramidal neurons (4.6 ms). The decay time constant in response to a brief application (1 ms) of glutamate (10 mM) in patches excised from pyramidal and aspiny interneurons were similar (1.9 and 2.7 ms, respectively). However, the rate of desensitization was faster in patches from interneurons compared with pyramidal neurons (3.4 and 12.0 ms, respectively). In addition, single-channel conductance was larger in aspiny interneurons (27 pS) compared with pyramidal neurons (9 pS). These results indicate that pyramidal neurons and aspiny interneurons express different non-N-methyl-D-aspartate receptors and that selective desensitization of interneuron receptors may contribute to depression of inhibition.
Asunto(s)
Potenciales Evocados , Interneuronas/fisiología , Canales Iónicos/fisiología , Neuronas/fisiología , Receptores de Glutamato/fisiología , Sinapsis/fisiología , Corteza Visual/fisiología , Análisis de Varianza , Animales , Conductividad Eléctrica , Técnicas In Vitro , Tractos Piramidales/fisiología , RatasRESUMEN
Brief glutamate applications to membrane patches, excised from neurons in the rat visual cortex, were used to assess the role of desensitization in determining the AMPA/kainate receptor-mediated excitatory postsynaptic current (EPSC) time course. A brief (1 ms) application of glutamate (1-10 mM) produced a response that mimicked the time course of miniature EPSCs (mEPSCs). Direct evidence is presented that the rate of onset of desensitization is much slower than the decay rate of the response to a brief application of glutamate, implying that the decay of mEPSCs reflects channel closure into a state readily available for reactivation. Rapid application of glutamate combined with nonstationary variance analysis provided an estimate of the single-channel conductance and open probability, allowing an approximation of the number of available channels at a single synaptic site.
Asunto(s)
Glutamatos/farmacología , Canales Iónicos/fisiología , Sinapsis/fisiología , Corteza Visual/fisiología , Animales , Membrana Celular/efectos de los fármacos , Membrana Celular/fisiología , Conductividad Eléctrica , Ácido Glutámico , Canales Iónicos/efectos de los fármacos , Cinética , Neuronas/efectos de los fármacos , Neuronas/fisiología , Pirrolidinonas/farmacología , Ratas , Receptores AMPA , Receptores de Ácido Kaínico , Receptores de Neurotransmisores/fisiologíaRESUMEN
The central nervous system has extraordinary plasticity in early life. This is thought to involve N-methyl-D-aspartate (NMDA) receptors which, along with the non-NMDA receptors, mediate fast excitatory synaptic transmission. Although NMDA receptors may be transiently enhanced early in life, it has not been possible to demonstrate directly a functional change in the NMDA receptor-mediated synaptic response because of the voltage-dependence of the NMDA conductance and the overlapping inhibitory synaptic conductances. Here I report that the duration of evoked NMDA-receptor-mediated excitatory postsynaptic currents (e.p.s.cs) in the superior colliculus is several times longer at early developmental stages compared to that measured in older animals. In contrast, the amplitude of NMDA-receptor-mediated miniature e.p.s.cs does not change during development. The kinetic response of excised membrane patches to a brief activation of NMDA receptors is similar to that of the NMDA e.p.s.c, which suggests that the time course of the NMDA e.p.s.c. in the superior colliculus reflects slow NMDA channel properties as in the hippocampus. Therefore, these data indicate that the molecular properties of NMDA receptors are developmentally regulated and thus may be controlling the ability of synapses to change in early life.
Asunto(s)
Neuronas/fisiología , Receptores de N-Metil-D-Aspartato/fisiología , Colículos Superiores/fisiología , Sinapsis/fisiología , 2-Amino-5-fosfonovalerato/farmacología , 6-Ciano 7-nitroquinoxalina 2,3-diona , Envejecimiento , Animales , Estimulación Eléctrica , Potenciales Evocados/efectos de los fármacos , Glutamatos/farmacología , Ácido Glutámico , Técnicas In Vitro , Canales Iónicos/efectos de los fármacos , Canales Iónicos/fisiología , Magnesio/farmacología , Potenciales de la Membrana , Neuronas/efectos de los fármacos , Picrotoxina/farmacología , Quinoxalinas/farmacología , Receptores de N-Metil-D-Aspartato/efectos de los fármacos , Colículos Superiores/crecimiento & desarrollo , Sinapsis/efectos de los fármacos , Factores de TiempoRESUMEN
1. We studied excitatory synaptic currents activated by stimulation of Schaffer collateral-commissural fibres and recorded from interneurones in the CA1 region of hippocampal slices using whole-cell techniques. 2. Interneurones were identified by their location outside the cell layer and their morphology as seen with differential interference contrast (DIC) microscopy and by filling with Lucifer Yellow (LY). 3. The excitatory postsynaptic current (EPSC) had a fast, voltage-insensitive component and a slow component which had a region of negative slope resistance between -70 and -40 mV. The slow voltage-dependent component was abolished by the N-methyl-D-aspartate (NMDA) receptor antagonist (DL-2-amino-5-phosphonovalerate (APV) 50 microM) which had little effect on the fast component. Conversely, the fast component was abolished by the non-NMDA receptor antagonist 6-cyano-7-nitoquinoxaline-2,3-dione (CNQX; 10 microM), which had no effect on the slow component. 4. The rise time of the fast component ranged from 1 to 3 ms and the decay time constant ranged from 3 to 15 ms. The rise time of the slow component ranged from 5 to 11 ms and the decay time constant ranged from 50 to 100 ms. 5. It is concluded that although the morphology of the excitatory synapses onto interneurones differs considerably from those onto pyramidal cells, their electrophysiological and pharmacological properties are very similar.
Asunto(s)
Hipocampo/fisiología , Interneuronas/fisiología , Sinapsis/fisiología , 2-Amino-5-fosfonovalerato/farmacología , 6-Ciano 7-nitroquinoxalina 2,3-diona , Potenciales de Acción/efectos de los fármacos , Animales , Hipocampo/citología , Técnicas In Vitro , Fibras Nerviosas/fisiología , Neuronas Aferentes/fisiología , Tractos Piramidales/fisiología , Quinoxalinas/farmacología , Ratas , Factores de TiempoRESUMEN
We studied with the whole-cell recording techniques, the mechanisms underlying the time course of the slow N-methyl-D-aspartate (NMDA), and fast non-NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) in hippocampal slices. The rising phase of the NMDA receptor-mediated component of the EPSC as well as the decaying phase of the NMDA and non-NMDA component were highly temperature-sensitive, suggesting that neither of these processes is determined by free diffusion of transmitter. Moreover, glutamate uptake blockers enhanced the responses to exogenously applied glutamate, but had no effect on the decay of either the NMDA or non-NMDA components of the EPSCs. On the other hand, open channel blockers known to modify NMDA channel kinetics reduced the EPSC decay time. Thus, the present results support a model in which the rise time and decay of the NMDA component are determined primarily by slow channel kinetics and the decay of the non-NMDA component is due either to channel kinetics or to desensitization.
Asunto(s)
Hipocampo/fisiología , Sinapsis/fisiología , Animales , Ácido Aspártico/análogos & derivados , Ácido Aspártico/antagonistas & inhibidores , Dibenzocicloheptenos/farmacología , Maleato de Dizocilpina , Electrofisiología , Glutamatos/metabolismo , Glutamatos/farmacología , Ácido Glutámico , Técnicas In Vitro , Ácido Kaínico/análogos & derivados , Ácido Kaínico/farmacología , Ketamina/farmacología , N-Metilaspartato , Ratas , Receptores de N-Metil-D-Aspartato , Receptores de Neurotransmisores/fisiología , Temperatura , Factores de TiempoRESUMEN
The N-methyl-D-aspartate (NMDA) and non-NMDA classes of glutamate receptor combine in many regions of the central nervous system to form a dual-component excitatory postsynaptic current. Non-NMDA receptors mediate synaptic transmission at the resting potential, whereas NMDA receptors contribute during periods of postsynaptic depolarization and play a role in the generation of long-term synaptic potentiation. To investigate the receptor types underlying excitatory synaptic transmission in the cerebellum, we have recorded excitatory postsynaptic currents (EPSCS), by using whole-cell techniques, from Purkinje cells in adult rat cerebellar slices. Stimulation in the white matter or granule-cell layer resulted in an all-or-none synaptic current as a result of climbing-fibre activation. Stimulation in the molecular layer caused a graded synaptic current, as expected for activation of parallel fibres. When the parallel fibres were stimulated twice at an interval of 40 ms, the second EPSC was facilitated; similar paired-pulse stimulation of the climbing fibre resulted in a depression of the second EPSC. Both parallel-fibre and climbing-fibre responses exhibited linear current-voltage relations. At a holding potential of -40 mV or in the nominal absence of Mg2+ these synaptic responses were unaffected by the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV), but were blocked by the non-NMDA receptor antagonist 6-cyano-2,3-dihydro-7-nitroquinoxalinedione (CNQX). NMDA applied to the bath failed to evoke an inward current, whereas aspartate or glutamate induced a substantial current; this current was, however, largely reduced by CNQX, indicating that non-NMDA receptors mediate this response. These results indicate that both types of excitatory input to adult Purkinje cells are mediated exclusively by glutamate receptors of the non-NMDA type, and that these cells entirely lack NMDA receptors.