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The role of MLCK in synaptic plasticity and memory formation

Final Activity Report Summary - MLCK AND MEMORY (The role of MLCK in synaptic plasticity and memory formation)

Long-term memory (LTM) formation is believed to involve alterations of synaptic efficacy produced by changes in neural transmission and/or structural modifications within neuronal networks. The proposed project investigated the roles of cytoskeleton regulatory molecules in long-term memory formation. The actin-cytoskeleton and regulatory proteins have been shown to be intimately involved in structural plasticity and neuronal transmission during brain development cellular processes believed to be central for memory formation.

Toward understanding more about the role of neuronal cytoskeleton in memory formation we investigated the involvement of the myosin light chain kinase (MLCK) and Rho-associated kinase (ROCK), two cytoskeletal regulatory proteins, in taste memory formation. Toward that end we studied whether MLCK or ROCK are involved in conditioned taste aversion (CTA) memory in gustatory cortex (GC), a brain area mediating CTA long term memory formation.

The ROCK inhibitor Y-27632 was microinjected into the GC 30 minutes before CTA (0.1% saccharin (CS; conditioned stimulus) followed 40 min later by injection of 0.15 M LiCl i.p. 2% of body weight, (US; unconditioned stimulus)). Three days later the rats were tested for long-term CTA memory for 3 consecutive days. Animals that were microinjected with the ROCK inhibitor into the GC were significantly impaired in the first test day for CTA memory when compared to animals injected with vehicle (p<0.03). ROCK inhibitor had no effect on memory retrieval when injected 30 minutes before LTM test. These results show that ROCK is essential for the formation of CTA LTM in gustatory cortex.

As shown above the ROCK is involved in CTA paradigm. One of the ROCK substrates is the MLC. To study another pathway where MLC might be involved in taste memory formation we inhibited the activity its regulator the MLCK using the ML-7 inhibitor. We microinjection the ML-7 (220 µM) 1µl bilaterally immediately before the taste conditioned stimulus (CS; the unfamiliar taste NaCl (0.3% w/v)) in CTA and tested taste memory 3 days later. For the control group we infused 1 µl 1% DMSO bilaterally immediately before the CS. We found no significant changes in when comparing the ML-7 group to the control group.

In conclusion, we revealed that inhibition of ROCK in GC before CTA impairs LTM memory formation whereas MLCK has no effect. Although both kinases affect cytoskeleton activity in neurons they are differentially involved in cortical-dependent manner. Further studies will be needed to determine the molecular mechanisms whereby ROCK mediates consolidation of taste memory and to reveal which substrates are essential for these processes.