Glutamate infused posttraining into the hippocampus or caudate-putamen differentially strengthens place and response learning

TLDR

The study used a cross‑maze task that can be learned by place or response strategies to test whether post‑training glutamate injections into the hippocampus or caudate‑putamen bias rats toward a particular memory system. Male Long‑Evans rats underwent 15 days of cross‑maze training with probe trials on days 8 and 16, and received post‑training intrahippocampal (1 µg/0.5 µl) or intracaudate (2 µg/0.5 µl) glutamate or saline injections on days 4–6. Saline‑treated rats shifted from place to response learning over time, whereas hippocampal glutamate blocked this shift and caudate glutamate accelerated it, demonstrating that post‑training glutamate infusions can bias the use of specific memory systems and alter the timing of the transition from cognitive to habitual behavior.

Abstract

A cross-maze task that can be acquired through either place or response learning was used to examine the hypothesis that posttraining neurochemical manipulation of the hippocampus or caudate-putamen can bias an animal toward the use of a specific memory system. Male Long-Evans rats received four trials per day for 7 days, a probe trial on day 8, further training on days 9–15, and an additional probe trial on day 16. Training occurred in a cross-maze task in which rats started from a consistent start-box (south), and obtained food from a consistent goal-arm (west). On days 4–6 of training, rats received posttraining intrahippocampal (1 μg/0.5 μl) or intracaudate (2 μg/0.5 μl) injections of either glutamate or saline (0.5 μl). On days 8 and 16, a probe trial was given in which rats were placed in a novel start-box (north). Rats selecting the west goal-arm were designated “place” learners, and those selecting the east goal-arm were designated “response” learners. Saline-treated rats predominantly displayed place learning on day 8 and response learning on day 16, indicating a shift in control of learned behavior with extended training. Rats receiving intrahippocampal injections of glutamate predominantly displayed place learning on days 8 and 16, indicating that manipulation of the hippocampus produced a blockade of the shift to response learning. Rats receiving intracaudate injections of glutamate displayed response learning on days 8 and 16, indicating an accelerated shift to response learning. The findings suggest that posttraining intracerebral glutamate infusions can ( i ) modulate the distinct memory processes mediated by the hippocampus and caudate-putamen and ( ii ) bias the brain toward the use of a specific memory system to control learned behavior and thereby influence the timing of the switch from the use of cognitive memory to habit learning to guide behavior.