Spike rates of a hippocampal place cell are not constant and vary even when an animal visits an identical place field with nearly identical behavior. As one potential neurophysiological source underlying place cell spiking variability, we focused on the temporally fluctuating activity states of neuronal ensembles. Spike patterns of hippocampal neurons were recorded from rats performing a linear track task. Within a single consummatory period, similar sets of neurons were more frequently recruited in synchronous firing events, whereas different synchronized firing patterns of neuronal populations tended to be identified in different consummatory periods. A linear regression analysis indicated that the time-varying activation patterns of neuronal populations during consummatory periods are correlated with the spike rates of a place cell within its place field during running. These findings suggest that place cell in-field spiking is not only triggered by static inputs that represent external environments but also strongly depends on the time-varying internal states of neuronal populations.

Acute alcohol exposure impairs hippocampus-dependent spatial memory. However, there is little evidence for the effects of ethanol on the spike patterns of hippocampal cell populations. Here, we examined how the spatial firing patterns of place cells, neurons that encode specific locations, were altered in rats that were intraperitoneally injected with 1.5 g/kg ethanol. Ethanol administration partly reduced or abolished place-selective spiking of a subset of place cells during running periods in a spatial task, whereas a subset of place fields newly emerged, suggesting a partial reorganization of hippocampal spatial maps by ethanol. On the other hand, ethanol administration did not significantly alter the frequency of hippocampal sharp-wave ripple (SWRs) and synchronous spike patterns during resting periods, suggesting that offline memory consolidation and retrieval mechanisms underpinned by hippocampal neuronal synchronization are not strongly affected by ethanol. These results indicate that acute ethanol intake mainly affects the encoding of external information but has little impact on internal memory processing.

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Alcohol exposure impairs the retention of spatial memory. Consistently, previous reports have demonstrated widespread changes in a variety of receptor functions and gene expressions in the hippocampus, a brain region involved in spatial memory. However, it remains unknown how these molecular mechanisms are integrated to alter neuronal spike patterns. Hippocampal neurons consist of place cells as they fire preferentially when an animal visits a specific area (a place field), which are considered to play a crucial role in spatial memory. Here, we recorded spatial spike patterns of hippocampal neuronal ensembles from freely moving rats running on familiar linearized tracks. The rats were tested in two 20-min sessions of running and during a 10-min inter-session interval, they were injected intraperitoneally with 1.5 g/kg ethanol, a dose comparable to those generally consumed by humans. The alcohol administration triggered the emergence of a subset of place cell populations, while abolishing place-selective firing of the other place cell populations. Moreover, a subset of place cells altered their place fields. These results demonstrate that hippocampal spatial maps are dynamically reorganized by ethanol administration. The neuronal mechanism may underlie alcohol-induced impairments in hippocampus-dependent memory. Poster Sessions