Dynamics of memory state of the brain

本研究通过诱导大鼠海马突触的长时程增强（LTP）并观察海马CA3和CA1区对周期性刺激反应的动力学特征，(1)CA3传入突触的LTP对CA3自发活动的产生没有贡献，而CA3的周期性兴奋突触的LTP对CA3自发活动的产生有贡献。(2)周期性苔藓纤维刺激诱发LTP的CA3区引起锁相和混沌响应。(3) LTP可在CA3区的局部区域诱导。即使LTP增强并导致自发性癫痫样发作，局部区域仍能维持。本地区域可能对应一个内存状态。(4)强直刺激对侧CA3可增强对侧CA3经同侧CA3至CA1的兴奋通路，而对侧CA3至CA1的直接通路则无增强作用。(5) CA1对同侧CA3周期性刺激的场电位响应随刺激频率的变化而变化；20 ~ 40 Hz刺激的反应特征随时间变化显著。初始瞬态后的负场电位特别引起锁相和混沌响应。这是由CA3的动力学特性决定的。综上所述，具有丰富循环兴奋连接的CA3区，当LTP在CA3中被诱导时，对周期性刺激产生锁相和混沌反应，这些活动通过Schaffer侧边被投射到CA1兴奋，通过中间神经元被抑制。

In this research project, long-term potentiation (LTP) at synapses in the rat hippocampus followed by spontaneous bursting discharges was induced and dynamical features of the responses of the hippocampal CA3 and CA1 regions to periodic stimulation were investigated, (l) LTP at afferent synapses to CA3 did not contribute to generate the spontaneous activity of CA3 but LTP at recurrent excitatory synapses in CA3 did. (2) The CA3 region where LTP was induced in recurrent excitatory circuits caused phase-locked and chaotic responses to periodic mossy fiber stimulation. (3) LTP can be induced in a local area of the CA3 region. The local area was maintained even if LTP was enhanced and resulted in spontaneous epileptiform bursts. The local area might correspond to a memory state. (4) The excitatory pathway from contralateral CA3 to CA1 via ipsilateral CA3 was augmented by tetanic stimulation of the contralateral CA3, while the direct pathway from the contralateral CA3 to CA1 was not. (5) Field potential responses of CA1 to periodic stimulation of ipsilateral CA3 depend on the stimulus frequency; the feature of the responses to 20 40 Hz stimulation changes dramatically with time. The negative field potential after an initial transient especially caused phase-locked and chaotic responses. This results from the dynamical features of CA3.The above results conclude that the CA3 region, which has rich recurrent excitatory connections, causes phase-locked and chaotic responses to periodic stimulation when LTP is induced in CA3, and those activities are projected to CA1 excitatory through Schaffer collaterals and inhibitory through interneurons.