Department of Psychology, University of Otago, Dunedin, New Zealand.
Hippocampus. 2012 Jun;22(6):1325-37. doi: 10.1002/hipo.20969. Epub 2011 Aug 9.
The accurate recall of an event is usually dependent on a memory trace that encodes three pieces of information; what happened, when the event happened, and where. The established phenomenology of hippocampal CA1 pyramidal neurons could reflect mechanisms via which some of this information (where and what) is encoded; but so far there has been little evidence for a mechanism by which these cells might represent "when." It was therefore of interest to examine the activity of CA1 neurons over a substantial temporal duration. Forty-eight CA1 neurons were recorded once an hour during long (24-48 h) exposures to a single, stable environment where minimal time-of-day cues were available. Only data from the first 25 h of recording was analyzed quantitatively. We found that the mean ensemble firing rate of these cells changed predictably such that it was closely correlated (r = 0.707) to a reference sine wave with a 25-h period and a positive peak at recording start. This relationship was not explained by changes in the animal's running speed or amount of the recording environment covered in each recording session. When data were referenced to the onset or offset of the normal light-on period, the correlation with the sinusoid was abolished. At an individual cell level, the majority of neurons (n = 31) had significant correlations (P < 0.05) with the reference sine. We conclude that the firing rate of a large proportion of cells in area CA1 of the hippocampus are modulated over a circadian period but that this modulation is not entrained to light. Rather, entry into the environment and the associated food availability appear to be the entraining factors. We hypothesize that these neurons may be part of the putative food-entrainable oscillator. Such a system could enable an animal to discriminate between spatial representations on a temporal dimension with reference to the time of food availability.
事件的准确回忆通常依赖于一个记忆痕迹，该痕迹编码了三个信息：发生了什么，事件发生的时间，以及地点。已建立的海马 CA1 锥体神经元现象学可以反映出某些信息（地点和内容）的编码机制；但是到目前为止，几乎没有证据表明这些细胞可以代表“时间”的机制。因此，研究 CA1 神经元在相当长的时间内的活动情况是很有意义的。在长时间（24-48 小时）暴露于单个稳定环境中时，每个小时对 48 个 CA1 神经元进行一次记录，在该环境中几乎没有时间线索。仅对记录的前 25 小时的数据进行定量分析。我们发现，这些细胞的平均集合放电率发生了可预测的变化，以至于与具有 25 小时周期和记录开始时的正峰值的参考正弦波密切相关（r = 0.707）。这种关系不能用动物的奔跑速度或每次记录会话中记录环境的覆盖量的变化来解释。当将数据参考正常光照期的开始或结束时，与正弦波的相关性就消失了。在单个细胞水平上，大多数神经元（n = 31）与参考正弦波有显著的相关性（P <0.05）。我们得出结论，海马体 CA1 区的很大一部分细胞的放电率在一个昼夜周期内受到调制，但这种调制不受光的影响。相反，进入环境以及相关的食物供应似乎是调节因素。我们假设这些神经元可能是所谓的食物可调节振荡器的一部分。这样的系统可以使动物能够根据食物可获得的时间，在时间维度上区分空间表示。
