【摘要】：In order to explore the evidence of transient neuronal plasticity observed in human 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 the cultured sensorimotor neurons were activated by step depolarization and demonstrated their current and voltage relations. The slow bursting rhythm discharged from cultured sensorimotor neurons showed spontaneous neuronal activity alternating with quiet interval.At very low density cultured condition, the cultured cortical sensorimotor neurons have their basic properties of amplitude, rise time, decay time and frequency of sEPSCs. Similar as the findings of cultured hippocampal neurons studies, the plot of amplitude histogram of the sEPSCs obtained from cortical
sensorimotor neurons during the control period showed that the distribution was skewed toward larger events that indicates a typical glutamatergic distribution. In the repetitive stimulation experiments, voltage pulse successfully induced synaptic response of sEPSCs and the effect was highly reproducible (95%, n=12). Predominantly, the rapid response increased significantly in the averaged frequency (typically around 2-4 folds) accompanying a change in averaged amplitude, of sEPSCs. The enhanced EPSCs frequency response generally maintained within 30-40 min occurring in most cells. In addition, voltage pulse induced potentiation also involved increase in spike frequency in some neurons (around 8-10 folds). To examine the mechanism of induction and the expression of EPSCs frequency response, it appeared, under conditions of our experiments, to imply an increase of presynaptic sensitivity that may be due in part, to the turning on of silent synapses. However, the change of EPSCs amplitude indicating this switch might reflect an increase in the postsynaptic sensitivity of non-NMDA or NMDA receptors at previously active synapses, or alternatively, it could reflect the turning on of release sites that were functionally silent prior to the voltage pulsing. Nevertheless, we cannot exclude the possibility that some fraction of potentiation of EPSCs is postsynaptic which requires an increase in postsynaptic Ca2+. With respect to spike frequency response, the possible 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, voltage pulse-induced potentiation in cultured cortical sensorimotor neurons is transient suggesting that sustained potentiation may require the combined some additional components, such as activation of NMDA and metabotropic receptors. Thus, the present findings are perhaps more pertinent to the early short-term potentiation that 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 uM 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 of sEPSCs. The analysis of time, amplitude and frequency domain data indicated that there was a significant difference between two concentrations. The evidence suggests that the manner of the synaptic transmission via NMDA receptor is concentration-dependent. It is likely that the concentration is an important factor during the critical period of plasticity that is relevant to the interpretation of the phenomena observed in human motor cortex during learning. The present results provided information that the enhancement of synaptic transmission might be influenced by the degree of NMDA receptor activation, and may be dependent on the complex temporal and spatial relationships of afferent synaptic activity onto a given postsynaptic cell as well. Furthermore, NMDA receptor function may have profound effects on the synaptic modification elicited by a fixed pattern of synaptic activity. It may be important when modulatory neurotransmitter systems are active during different behavioral states.Although the present experimental designs were not very precise and the present results are preliminary, the findings, at least to some extent, provide insight into understanding the cellular mechanism of plasticity observed in human motor cortex during different learning stages.