Caruana Douglas A, Chapman C Andrew
Center for Studies in Behavioral Neurobiology, Department of Psychology, Concordia University, 7141 Sherbrooke St. W., Room SP-244, Montréal, Québec H4B 1R6, Canada.
J Neurophysiol. 2004 Aug;92(2):1226-35. doi: 10.1152/jn.00038.2004. Epub 2004 Mar 24.
Although a major output of the hippocampal formation is from the subiculum to the deep layers of the entorhinal cortex, the parasubiculum projects to the superficial layers of the entorhinal cortex and may therefore modulate how the entorhinal cortex responds to sensory inputs from other cortical regions. Recordings at multiple depths in the entorhinal cortex were first used to characterize field potentials evoked by stimulation of the parasubiculum in urethan-anesthetized rats. Current source density analysis showed that a prominent surface-negative field potential component is generated by synaptic activation in layer II. The surface-negative field potential was also observed in rats with chronically implanted electrodes. The response was maintained during short stimulation trains of < or =125 Hz, suggesting that it is generated by activation of monosynaptic inputs to the entorhinal cortex. The piriform cortex also projects to layer II of the entorhinal cortex, and interactions between parasubicular and piriform cortex inputs were investigated using double-site stimulation tests. Simultaneous activation of parasubicular and piriform cortex inputs with high-intensity pulses resulted in smaller synaptic potentials than were expected on the basis of summing the individual responses, consistent with the termination of both pathways onto a common population of neurons. Paired-pulse tests were then used to assess the effect of parasubicular stimulation on responses to piriform cortex stimulation. Responses of the entorhinal cortex to piriform cortex inputs were inhibited when the parasubiculum was stimulated 5 ms earlier and were enhanced when the parasubiculum was stimulated 20-150 ms earlier. These results indicate that excitatory inputs to the entorhinal cortex from the parasubiculum may enhance the propagation of neuronal activation patterns into the hippocampal circuit by increasing the responsiveness of the entorhinal cortex to appropriately timed inputs.
虽然海马结构的主要输出是从下托投射到内嗅皮层的深层，但旁下托投射到内嗅皮层的浅层，因此可能会调节内嗅皮层对来自其他皮层区域的感觉输入的反应方式。首先在戊巴比妥麻醉的大鼠中，通过在内嗅皮层的多个深度进行记录，来表征旁下托刺激所诱发的场电位。电流源密度分析表明，II层中的突触激活产生了一个显著的表面负性场电位成分。在长期植入电极的大鼠中也观察到了表面负性场电位。在频率≤125Hz的短刺激串期间，该反应得以维持，这表明它是由内嗅皮层单突触输入的激活所产生的。梨状皮层也投射到内嗅皮层的II层，并且使用双位点刺激试验研究了旁下托和梨状皮层输入之间的相互作用。用高强度脉冲同时激活旁下托和梨状皮层输入所产生的突触电位，比根据单个反应的总和所预期的要小，这与两条通路都终止于共同的神经元群体一致。然后使用配对脉冲试验来评估旁下托刺激对梨状皮层刺激反应的影响。当旁下托提前5毫秒受到刺激时，内嗅皮层对梨状皮层输入的反应受到抑制；而当旁下托提前20 - 150毫秒受到刺激时，反应则增强。这些结果表明，旁下托向内嗅皮层的兴奋性输入可能通过增加内嗅皮层对适时输入的反应性，来增强神经元激活模式向海马回路的传播。
