CaMKII Requirement for in Vivo Insular Cortex LTP Maintenance and CTA Memory Persistence

Abstract

Calcium-calmodulin/dependent protein kinase II (CaMKII) plays an essential role in LTP induction, but since it has the capacity to remain persistently activated even after the decay of external stimuli it has been proposed that it can also be necessary for LTP maintenance and therefore for memory persistence. It has been shown that basolateral amygdaloid nucleus (Bla) stimulation induces long-term potentiation (LTP) in the insular cortex (IC), a neocortical region implicated in the acquisition and retention of conditioned taste aversion (CTA). Our previous studies have demonstrated that induction of LTP in the Bla-IC pathway before CTA training increased the retention of this task. Although it is known that IC-LTP induction and CTA consolidation share similar molecular mechanisms, little is known about the molecular actors that underlie their maintenance. The purpose of the present study was to evaluate the role of CaMKII in the maintenance of in vivo Bla-IC LTP as well as in the persistence of CTA long-term memory (LTM). Our results show that acute microinfusion of myr-CaMKIINtide, a selective inhibitor of CaMKII, in the IC of adult rats during the late-phase of in vivo Bla-IC LTP blocked its maintenance. Moreover, the intracortical inhibition of CaMKII 24 h after CTA acquisition impairs CTA-LTM persistence. Together these results indicate that CaMKII is a central key component for the maintenance of neocortical synaptic plasticity as well as for persistence of CTA-LTM.

Keywords: CTA; CaMKII; insular cortex; memory persistence; neocortical-LTP.

Figure 1

CaMKII inhibition reverts IC-LTP. (A) Schematic representation of the experimental procedure. (B) Schematic representation and representative micrographs of electrode placement showing the stimulated (S) and infused/recorded (I/R) sites in a coronal plane. The representative microphotography of IC shows the CaMKIINtide diffusion through the combination of the drug with Ponceau red. Bla, basolateral amygdaloid nucleus; IC, insular cortex. Scale bar: 1 mm. (C) Plot of IC evoked responses from HFS, HFS+CaMKIINtide, HFS+ACSF and CaMKIINtide groups. Note that the inhibition of CaMKII reverts Bla-IC potentiation. The top of each graph shows representative examples of IC field potentials obtained before (black line), 120 min after HFS (gray line) and 60 min after the infusion (dotted line). Scale bar: 5 ms, 0.5 mV. Bars indicate the infusion period (5 min). Arrows indicate HFS delivery. (D) Representative Western blots and densitometric analysis of phosphorylated and total CaMKII from HFS+CaMKIINtide and HFS+ACSF groups. Note that infusion of the inhibitor during IC-LTP maintenance phase decreased phosphorylation of CaMKII. AU, Arbitrary units. ^**p < 0.01.

Figure 2

CaMKII inhibition impairs the maintenance of CTA-LTM. (A) Schematic representation of the experimental procedure. (B) Schematic representation and representative micrographs showing cannulae placement. IC, insular cortex. Scale bar: 1 mm. (C) Saccharin consumption during acquisition, short-term memory aversion test (STM) and long-term memory aversion test (LTM) for CTA+STM+ACSF, CTA+STM+CaMKIINtide, CTA+CaMKIINtide and pCTA+STM+CaMKIINtide groups. Note that local infusion of myr-CaMKIINtide in the IC 24 h after acquisition, either in the presence or absence of STM-test produced a severe deficit during the LTM aversion test. ^**p < 0.001. Arrow indicates microinfusion.