Hippocampal State-Dependent Behavioral Reflex to an Identical Sensory Input in Rats

We examined the local field potential of the hippocampus to monitor brain states during a conditional discrimination task, in order to elucidate the relationship between ongoing brain states and a conditioned motor reflex. Five 10-week-old Wistar/ST male rats underwent a serial feature positive conditional discrimination task in eyeblink conditioning using a preceding light stimulus as a conditional cue for reinforced trials. In this task, a 2-s light stimulus signaled that the following 350-ms tone (conditioned stimulus) was reinforced with a co-terminating 100-ms periorbital electrical shock. The interval between the end of conditional cue and the onset of the conditioned stimulus was 4±1 s. The conditioned stimulus was not reinforced when the light was not presented. Animals successfully utilized the light stimulus as a conditional cue to drive differential responses to the identical conditioned stimulus. We found that presentation of the conditional cue elicited hippocampal theta oscillations, which persisted during the interval of conditional cue and the conditioned stimulus. Moreover, expression of the conditioned response to the tone (conditioned stimulus) was correlated with the appearance of theta oscillations immediately before the conditioned stimulus. These data support hippocampal involvement in the network underlying a conditional discrimination task in eyeblink conditioning. They also suggest that the preceding hippocampal activity can determine information processing of the tone stimulus in the cerebellum and its associated circuits.

本研究通过检测海马体（hippocampus）的局部场电位（local field potential），以监测条件辨别任务（conditional discrimination task）过程中的脑状态，旨在阐明持续脑状态与条件运动反射（conditioned motor reflex）之间的关联。实验选取5只10周龄的雄性Wistar/ST品系大鼠，开展基于眨眼条件反射（eyeblink conditioning）的序列特征阳性条件辨别任务，以先行光刺激作为强化试次（reinforced trials）的条件线索（conditional cue）。在该任务中，2秒时长的光刺激预示后续350毫秒的音调刺激（即条件刺激（conditioned stimulus））将与同步终止的100毫秒眶周电刺激（periorbital electrical shock）配对强化。条件线索结束至条件刺激起始的间隔为4±1秒；未呈现光刺激时，条件刺激不会获得强化。受试动物可有效利用光刺激作为条件线索，对相同的条件刺激产生差异化反应。本研究发现，条件线索的呈现可诱发海马θ振荡（theta oscillations），且该振荡会在条件线索与条件刺激的间隔期间持续存在。此外，针对音调刺激（条件刺激）的条件反应（conditioned response）的表达，与条件刺激出现前即刻出现的θ振荡存在相关性。上述数据表明，海马体参与了眨眼条件反射中条件辨别任务的神经环路基础。同时本研究还提示，先行的海马活动可决定小脑（cerebellum）及其相关环路对音调刺激的信息处理过程。