检索历史 清除

摘要In order to explore the evidence of transient neuronal plasticity observed inhuman sensorimotor cortex during development of learning, the mechanisms of activity-dependent synaptic modification in cultured cortical sensorimotor neurons in rats were studied. Long-term potentiation (LTP) is an activity dependent strengthening of synaptic efficacy that is considered to be one of indexes of learning and memory in the present study. Voltage pulse, as a model of repetitive electrical stimulation, was applied to test the relationship between repetitive electrical activity and a persistent change of synaptic efficacy. NMDA (N-methyl-D-aspartate) was administrated to cells to examine its dose dependent effects on the agonist-induced synaptic transmission in cultured cortical sensorimotor neurons. Patch clamp techniques with whole cell recording mode was performed to assess the effects of depolarizing pulses and changing concentrations of NMDA on spontaneous neuronal activity. In the present study, the voltage-gated potassium and sodium channels of thecultured sensorimotor neurons were activated by step depolarization anddemonstrated their current and voltage relations. The slow bursting rhythmdischarged from cultured sensorimotor neurons showed spontaneous neuronalactivity alternating with quiet interval. At very low density cultured condition, the cultured cortical sensorimotorneurons have their basic properties of amplitude, rise time, decay time andfrequency of sEPSCs. Similar as the findings of cultured hippocampal neuronsstudies, the plot of amplitude histogram of the sEPSCs obtained from corticalsensorimotor neurons during the control period showed that the distribution wasskewed toward larger events that indicates a typical glutamatergic distribution.In the repetitive stimulation experiments, voltage pulse successfully inducedsynaptic response of sEPSCs and the effect was highly reproducible (95％,n=12). Predominantly, the rapid response increased significantly in the averagedfrequency (typically around 2-4 folds) accompanying a change in averagedamplitude, of sEPSCs. The enhanced EPSCs frequency response generallymaintained within 30-40 min occurring in most cells. In addition, voltage pulseinduced potentiation also involved increase in spike frequency in some neurons(around 8-10 folds). To examine the mechanism of induction and the expressionof EPSCs frequency response, it appeared, under conditions of our experiments,to imply an increase of presynaptic sensitivity that may be due in part, to theturning on of silent synapses. However, the change of EPSCs amplitudeindicating this switch might reflect an increase in the postsynaptic sensitivity ofnon-NMDA or NMDA receptors at previously active synapses, or alternatively,it could reflect the turning on of release sites that were functionally silent priorto the voltage pulsing. Nevertheless, we cannot exclude the possibility thatsome fraction of potentiation of EPSCs is postsynaptic which requires anincrease in postsynaptic Ca2+. With respect to spike frequency response, thepossible mechanism might be involved an alteration of the voltage-gated Na+channels that may account for the change in synaptic strength. In agreement of recent studies of cultured hippocampal neurons, voltagepulse-induced potentiation in cultured cortical sensorimotor neurons is transientsuggesting that sustained potentiation may require the combined some additionalcomponents, such as activation of NMDA and metabotropic receptors. Thus, thepresent findings are perhaps more pertinent to the early short-term potentiationthat occurs following repetitive stimulation. Consistent with the results of the study in hippocampus, the NMDA application to bath solution did potentiate synaptic transmission in cultured sensorimotor neurons in vitro. The present results demonstrated that NMDA application to bath at both concentration of 500 μM and 1 mM was sufficient to cause NMDA receptor activation. This agonist-induced potentiation shared many features with LTP that exhibited a large increase in spike frequency, EPSCs amplitude, as well as in the number of events ofsEPSCs. The analysis oftime, amplitude and frequency domain data indicated that there was a significantdifference between two concentrations. The evidence suggests that the mannerof the synaptic transmission via NMDA receptor is concentration-dependent. Itis likely that the concentration is an important factor during the critical period ofplasticity that is relevant to the interpretation of the phenomena observed inhuman motor cortex during learning. The present results provided informationthat the enhancement of synaptic transmission might be influenced by the degreeof NMDA receptor activation, and may be dependent on the complex temporaland spatial relationships of afferent synaptic activity onto a given postsynapticcell as well. Furthermore, NMDA receptor function may have profound effectson the synaptic modification elicited by a fixed pattern of synaptic activity. Itmay be important when modulatory neurotransmitter systems are active duringdifferent behavioral states. Although the present experimental designs were not very precise and thepresent results are preliminary, the findings, at least to some extent, provideinsight into understanding the cellular mechanism of plasticity observed inhuman motor cortex during different learning stages.