Population Encoding In The Hippocampal CA1 During Rewarding Trace Associative Learning

Hippocampus at the temporal lobe is a part of the limbic system.One of its major functions is the online acquisition and short-term storage of declarative memory.Pavlovian conditioning is a classical paradigm for declarative memory,which associates events that are disparate in time.The time gap between the onset of conditioned stimulus(CS)and offset of unconditioned stimulus(US)is often referred as trace time,implying that a retention of CS after its offset is essential for CS-US association.Various studies have demonstrated that trace association is hippocampus-dependent.Majority of studies on hippocampus-dependent associative learning focus on comparison between pre-training and post-training of an associative learning paradigm,revealing a change in CS encoding or neural excitability.Only some have investigated the neural dynamic in hippocampus with days-lasting behavioral paradigms.The mechanism underlying the association of discontinuous events still eludes science.It is of great value to further explore the dynamic neural network of hippocampus during associative learning.A rewarding trace associative learning paradigm of moderate difficulty was established,in which mice learned the association between a tone,as CS,and water reward,as US,in the span of 200—300 pairings.Acute tetrode recording was performed during this training process in the hippocampal CA1.79 neurons were recorded in 10 mice,74 of which were identified as pyramidal cells.A prolonged excitatory response(0.34 s)of CA1 pyramidal cell population after tone stimulus was observed in the training group as compared to a short and fast firing(0.07 s)in the pseudo-training group.Water reward elicited inhibitory response of neuron population only in training group.We hence propose association learning produces excitatory encoding and inhibitory encoding for CS and US respectively.Next,obtained were response types and their according ratio in cell population after several behavioral events,including post CS onset(tone-trace period),post reward onset(reward)and post reward offset(post-reward period).36.5% and 2.7% population showed excitatory or inhibitory spike rate response in tone-trace period respectively.And 10.8%,14.9% respectively in reward period;28.4%,14.9% respectively in post-reward period,with 1.4% showing complex spike rate response.In general,excitatory,and inhibitory response significantly tended to exhibit in separate cell groups,implying these responses are conducted by separate pathway.The majority responses in the tone-trace period and post-reward period are excitatory,showing that forwarding information was coded after CS and reward obtaining.After examining the relationship between the responsive population in different periods,we found that responsive neurons in post-reward period significantly tended to be co-excited with those in the tone-trace period,while co-inhibited with those in the reward period,implying re-emerging of CS and US presentation after CSUS pairing,which makes association of discontinuous events potentially possible.Linear regression analysis was carried out between firing activity in respective periods and behavioral performance.24.3%,14.9%,23.0% of the neuron population exhibited linear relationship with significant accountability of behavioral performance in the tonetrace period,reward period,and post-reward period,respectively.Wherein,the linear relationship in the tone-trace period and post-reward period was majorly in the direction of stronger response specificity by increasing excitatory an inhibitory response.To examine the temporal relationship between firing activity in periods of interest and behavioral performance,cross-correlation analysis was utilized between activity of neurons mentioned above and behavioral performance.The change of firing activity in the tonetrace period usually was not early than behavioral performance change(88.9%),while that in the post-pairing period was not later than behavioral performance change(94.1%).But the change of firing activity in the reward period show no preference in temporal order.These results suggest neural activity after CS-US pairing precede and may contribute to the behavioral response,while neural activity after CS is a consequence of association learning,possibly a strengthening of cue memory,which further support the hypothesis that activity in hippocampus after CS-US pairing potentially associates discontinuous events.Next,firing activity of all the pyramidal neurons recorded was examined at 20pairing/block.Majority of neurons were found to be excitatory in at least one block in the tone trace period(93.2%)and post-reward period(96.0%).All the blocks were grouped by their according behavioral performance,and responsive block ratio in the tone-trace period was significantly linearly correlated with their mean behavioral performance in groups.Further analysis of the overlapping number of responsive cells between each two blocks showed a steadier responsive ensemble along learning.But the responsive block ratio in the post-pairing period showed no significant relationship with mean behavioral performance in groups,and responsive ensemble showed steadier tendency in the early and middle phase of learning but not in the late phase.We speculate that cue memory should be held over the span of learning process,while neural activity related to association is most essential in early and middle phase of learning.In summary,response specificity was increased in the tone-trace period with learning,and a steady coding population showed up.The change of neural activity was not early than behavioral performance change,and possibly a result of association of conditional stimulus and unconditional stimulus.Excitatory response in the post-reward period accompanies that in the tone-trace period,in which the coding population was steadied during the early and middle phase of learning.The change of neural activity in this period tended to precede behavioral performance change,suggested the important role of activity in post-reward period in the association between conditional stimulus and unconditional stimulus.Thus,we conclude that hippocampal CA1 network encodes important information during associative learning in the tone-trace period and post-reward period,the former of cue memory,while the latter potentially conduct the association of discontinuous events.