Acquisition-dependent modulation of hippocampal neural cell adhesion molecules by associative motor learning

Abstract

It is widely accepted that some types of learning involve structural and functional changes of hippocampal synapses. Cell adhesion molecules neural cell adhesion molecule (NCAM), its polysialylated form polysialic acid to NCAM (PSA-NCAM), and L1 are prominent modulators of those changes. On the other hand, trace eyeblink conditioning, an associative motor learning task, requires the active participation of hippocampal circuits. However, the involvement of NCAM, PSA-NCAM, and L1 in this type of learning is not fully known. Here, we aimed to investigate the possible time sequence modifications of such neural cell adhesion molecules in the hippocampus during the acquisition of a trace eyeblink conditioning. To do so, the hippocampal expression of NCAM, PSA-NCAM, and L1 was assessed at three different time points during conditioning: after one (initial acquisition), three (partial acquisition), and six (complete acquisition) sessions of the conditioning paradigm. The conditioned stimulus (CS) was a weak electrical pulse separated by a 250-ms time interval from the unconditioned stimuli (US, a strong electrical pulse). An acquisition-dependent regulation of these adhesion molecules was found in the hippocampus. During the initial acquisition of the conditioning eyeblink paradigm (12 h after 1 and 3 days of training), synaptic expression of L1 and PSA-NCAM was transiently increased in the contralateral hippocampus to the paired CS-US presentations, whereas, when the associative learning was completed, such increase disappeared, but a marked and bilateral upregulation of NCAM was found. In conclusion, our findings show a specific temporal pattern of hippocampal CAMs expression during the acquisition process, highlighting the relevance of NCAM, PSA-NCAM, and L1 as learning-modulated molecules critically involved in remodeling processes underlying associative motor-memories formation.

Keywords: L1; NCAM; PSA-NCAM; associative motor learning; classical eyeblink conditioning; hippocampus; synaptosomes.

FIGURE 1

Experimental design. (A) Bipolar hook electrodes were implanted on the right supraorbital nerve for electrical stimulation of this sensory nerve. The electromyographic activity of the ipsilateral orbicularis oculi muscle [orbicularis oculi (OO) electromyographic (EMG)] was also recorded. (B) Each conditioning session consisted of 6 blocks comprising 10 trails per block. The conditioned (CS) and unconditioned stimuli (US) consisted of square, cathodic pulses presented to the supraorbital nerve, lasting 0.05 ms (1.5X threshold) and 0.5 ms (3X threshold), respectively. The CS was followed by the US with an interval of 250 ms. The CS was presented alone the first trial of each block. (C) Representative examples of OO EMG profiles of eyelid responses (in mV) recorded during the 1st, 3rd, and 6th conditioning sessions (cond 1, 3, and 6). Conditioned responses (CRs) are indicated by arrows. (D) A typical eyelid response collected during pseudo-conditioning (i.e., unpaired presentation of the CS and the US). Calibration in panel (C) also applies to panel (D).

FIGURE 2

Eyeblink conditioning acquisition modulates synaptic expression of L1, PSA-NCAM, and NCAM in the hippocampus. (A) Percentage of conditioned eyelid responses [conditioned responses (CRs)] across six conditioning sessions. Data is expressed as mean ± SEM of animals sacrificed at the end of the sixth session. *p < 0.05, **p < 0.01, ***p < 0.001 vs. unpaired. (B–D) Optical density measurements of L1 (B), PSA-NCAM (C), and NCAM (D) expression, after the 1st, 3rd, and 6th training sessions, in hippocampal synaptosomes obtained from right (ipsilateral to CS and US presentation) and left (contralateral) hippocampi of conditioned and control (merged from unpaired and context groups) animals. Data is expressed as mean ± SEM for each session (1st: paired group n = 8 and control group n = 10; 3rd: paired group n = 9 and control group n = 11; 6th: paired group n = 9, and control group n = 10). a.u., arbitrary units. **p < 0.01, ***p < 0.001 vs. ipsi; ^#p < 0.05, ^##p < 0.01 vs. control; ⁺p < 0.05, ^{+ +}p < 0.01, ^{+ + +}p < 0.001 vs. session 6.