Functional dissociation within the entorhinal cortex for memory retrieval of an association between temporally discontiguous stimuli

Abstract

Anatomical connectivity and single neuron coding suggest a segregation of information representation within lateral (LEC) and medial (MEC) portions of the entorhinal cortex, a brain region serving as the primary input/output of the hippocampus and maintaining widespread connections to many association cortices. The present study aimed to expand this idea by examining whether these two subregions differentially contribute to memory retrieval for an association between temporally discontiguous stimuli. We found that reversible inactivation of the LEC, but not the MEC, severely impaired the retrieval of the recently and remotely acquired memory in rat trace eyeblink conditioning, in which a stimulus-free interval was interposed between the conditioned and unconditioned stimulus. Conversely, inactivation of the LEC had no effect on retrieval in delay eyeblink conditioning, where two stimuli were presented without an interval. Therefore, the LEC, but not the MEC, plays a long-lasting role in the retrieval of a memory for an association between temporally discontiguous stimuli.

Figure 1.

Cannula locations and spread of muscimol. A, A fluorescent tagged muscimol conjugate infused into the lateral (LEC, left) and medial (MEC, right) entorhinal cortex. Spread was confined to a maximal 1 mm range. A magnified view of a LEC infusion site is also provided (center). Scale bars, 600 μm. B, Histological reconstructions of cannula locations in the LEC (left) and MEC (right) of the animals included in the analyses. In 18 animals (1 d retention, Recent, n = 8, red circles; 1 month retention, Remote, n = 10, black circles), the cannulae were bilaterally positioned in the LEC, and of those 18 animals, 12 also had bilateral cannulae in the MEC (Recent, n = 6; Remote, n = 6). Images were adapted from Paxinos and Watson, Copyright Elsevier (1998) with permission.

Figure 2.

Design, acquisition, and retention in trace conditioning. A, Rats were trained in the trace paradigm and tested 1 d (Recent group) or 1 month (Remote group) after training. B, Conditioning sessions consisted of a conditioned stimulus (CS, 100 ms tone) paired with an unconditioned stimulus (US, 100 ms periorbital shock) separated by a 500 ms stimulus-free interval. Animals' eyeblink responses were monitored by recording electromyogram (EMG) in obicularis occuli (top). Approximately 15 of 100 trials were discarded because of hyperactivity before the CS presentation (bottom). Before conditioning, two adaptation sessions (Adt.) recorded spontaneous eyeblinks in the absence of CS or US. The Recent and Remote groups acquired the conditioned response (CR), but were not different from one another. C, Muscimol infusion into the lateral entorhinal cortex (LEC, black bar) severely impaired CR expression compared with aCSF infusion (white bar) at recent (n = 8) and remote (n = 10) time-points (left). No effect was observed after muscimol infusion into the MEC (center) at either time-point (Recent, n = 6; Remote, n = 6). Combined muscimol infusion into the LEC and MEC impaired CR expression compared with aCSF infusion at recent (n = 6) and remote (n = 6) test times (right). **p < 0.01, ***p < 0.001. D, The increase in averaged amplitude of EMG and the integrated area of the CR following CS onset during sessions with muscimol infusion (black line) into the LEC (left) was smaller than that during sessions with aCSF infusion into either region (gray line) or muscimol infusion into the MEC (right) at both recent (top) and remote (bottom) time points.

Figure 3.

Design, acquisition, and retention in delay conditioning. A, Following retention sessions in trace conditioning, the rats received one acquisition session (Acq.) and two retention sessions in a delay paradigm where the conditioned stimulus (CS, 350 ms tone) coterminated with the unconditioned stimulus (US, 100 ms periorbital shock). B, In the session with muscimol (Mus.) infusions into the lateral entorhinal cortex (LEC), rats showed a comparable percentage of the conditioned response (CR) to the session with aCSF infusion (n = 14). C, The temporal pattern of averaged EMG amplitude or integrated area of the CR following CS onset did not differ between sessions with Muscimol infusion (black line) and sessions with aCSF infusion (gray line).