Neural circuits and temporal plasticity in hindlimb representation of rat primary somatosensory cortex: revisited by multi-electrode array on brain slices

Abstract

Objective

The well-established planar multi-electrode array recording technique was used to investigate neural circuits and temporal plasticity in the hindlimb representation of the rat primary somatosensory cortex (S1 area).

Methods

Freshly dissociated acute brain slices of rats were subject to constant perfusion with oxygenated artificial cerebrospinal fluid (95% O₂ and 5% CO₂), and were mounted on a Med64 probe (64 electrodes, 8×8 array) for simultaneous multi-site electrophysiological recordings. Current sources and sinks across all the 64 electrodes were transformed into two-dimensional current source density images by bilinear interpolation at each point of the 64 electrodes.

Results

The local intracortical connection, which is involved in mediation of downward information flow across layers II–VI, was identified by electrical stimulation (ES) at layers II–III. The thalamocortical connection, which is mainly involved in mediation of upward information flow across layers II–IV, was also characterized by ES at layer IV. The thalamocortical afferent projections were likely to make more synaptic contacts with S1 neurons than the intracortical connections did. Moreover, the S1 area was shown to be more easily activated and more intensively innervated by the thalamocortical afferent projections than by the intracortical connections. Finally, bursting conditioning stimulus (CS) applied within layer IV of the S1 area could successfully induce long-term potentiation (LTP) in 5 of the 6 slices (83.3%), while the same CS application at layers II–III induced no LTP in any of the 6 tested slices.

Conclusion

The rat hindlimb representation of S1 area is likely to have at least 2 patterns of neural circuits on brain slices: one is the intracortical circuit (ICC) formed by interlaminar connections from layers II–III, and the other is the thalamocortical circuit (TCC) mediated by afferent connections from layer IV. Besides, ICC of the S1 area is spatially limited, with less plasticity, while TCC is spatially extensive and exhibits a better plasticity in response to somatosensory afferent stimulation. The present data provide a useful experimental model for further studying microcircuit properties in S1 cortex at the network level in vitro.

摘要

目的

运用平面微电极阵列记录技术探讨大鼠初级躯体感觉皮层(primary somatosensory cortex, S1 area)后肢代表区的神经回路和时间可塑性。

方法

将急性分离的大鼠脑片置于MED-64 系统的电极平皿中, 持续灌流通入 95% O₂ 和5% CO₂ 混合气的人工脑脊液, 随后进行多电极同时同步记录。 利用双线性内插法计算出64个点的电流源与电流井, 并将其转换为二维电流源密度分布图。

结果

刺激S1 后肢代表区脑片的II–III层, 可鉴定出局部皮层内的连接回路, 而刺激IV 层(丘脑传入的主要终末端)则鉴定出丘脑皮层间的连接回路。 首先, 丘脑皮层投射的激活可以诱发更多的有效场电位, 并且与S1 区更多的神经元建立突触联系。 与之相比, 皮层内回路的激活引起的电反应活动范围则较小。 其次, 刺激IV 层在每一刺激强度下诱发的场电位幅度远远大于刺激II–III的场电位幅度, 即S1 区更容易被丘脑皮层连接回路的投射纤维所激活。 最后, 在S1 区IV 层给予强直刺激可以记录到长达两个小时的局部场电位幅度的增加, 长时程增强(long-term potentiation, LTP)的诱出率约为83.3% (5/6 例)。 相反, 在II–III层, 同样的强直刺激并未能明显诱导LTP的发生。

结论

大鼠S1区的后肢代表区似乎存在两种不同类型的神经回路, 一种是由层间联系介导的皮层内回路, 另一种则是由丘脑皮层传入介导的丘脑皮层连接回路。 皮层内回路在空间上联系较局限且可塑性不强, 而丘脑皮层回路的空间连接范围则相对较广, 对躯体感觉信息的传入也显示出很强的可塑性。 本结果为进一步在体外神经网络水平研究S1 区的神经回路特性提供了很好的实验模型。