Roles of Calcium/Calmodulin-Dependent Kinase II in Long-Term Memory Formation in Crickets

Abstract

Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a key molecule in many systems of learning and memory in vertebrates, but roles of CaMKII in invertebrates have not been characterized in detail. We have suggested that serial activation of NO/cGMP signaling, cyclic nucleotide-gated channel, Ca2+/CaM and cAMP signaling participates in long-term memory (LTM) formation in olfactory conditioning in crickets, and here we show participation of CaMKII in LTM formation and propose its site of action in the biochemical cascades. Crickets subjected to 3-trial conditioning to associate an odor with reward exhibited memory that lasts for a few days, which is characterized as protein synthesis-dependent LTM. In contrast, animals subjected to 1-trial conditioning exhibited memory that lasts for only several hours (mid-term memory, MTM). Injection of a CaMKII inhibitor prior to 3-trial conditioning impaired 1-day memory retention but not 1-hour memory retention, suggesting that CaMKII participates in LTM formation but not in MTM formation. Animals injected with a cGMP analogue, calcium ionophore or cAMP analogue prior to 1-trial conditioning exhibited 1-day retention, and co-injection of a CaMKII inhibitor impaired induction of LTM by the cGMP analogue or that by the calcium ionophore but not that by the cAMP analogue, suggesting that CaMKII is downstream of cGMP production and Ca2+ influx and upstream of cAMP production in biochemical cascades for LTM formation. Animals injected with an adenylyl cyclase (AC) activator prior to 1-trial conditioning exhibited 1-day retention. Interestingly, a CaMKII inhibitor impaired LTM induction by the AC activator, although AC is expected to be a downstream target of CaMKII. The results suggest that CaMKII interacts with AC to facilitate cAMP production for LTM formation. We propose that CaMKII serves as a key molecule for interplay between Ca2+ signaling and cAMP signaling for LTM formation, a new role of CaMKII in learning and memory.

Effects of Caffeine on Olfactory Learning in Crickets

“…Indeed, in the present study, an intracellular Ca 2+ chelator, BAPTA-AM, impaired Multiple-trial conditioning (MTC) activates a series of molecular pathways including NO-cGMP and cAMP systems, which ultimately leads to de novo protein synthesis, eliciting long-term synaptic plasticity (modified from Matsumoto et al, 2009;Mizunami et al, 2014;. Single-trial conditioning (STC) is usually insufficient to activate the LTM formation pathway, but treatment with caffeine makes it possible to activate this pathway even by STC.…”

“…An increase in intracellular Ca 2+ concentration may play a significant role in LTM formation, as the Ca 2+ -calmodulin signaling pathway is implicated in our model Mizunami et al, 2014). Indeed, in the present study, an intracellular Ca 2+ chelator, BAPTA-AM, impaired Multiple-trial conditioning (MTC) activates a series of molecular pathways including NO-cGMP and cAMP systems, which ultimately leads to de novo protein synthesis, eliciting long-term synaptic plasticity (modified from Matsumoto et al, 2009;Mizunami et al, 2014;.…”

“…Based on our previous studies, we have proposed a molecular model of LTM formation in cricket olfactory learning Mizunami et al, 2014Mizunami et al, , 2015 (Fig. 7).…”

“…In cricket olfactory learning, an increase in intracellular Ca 2+ concentration is also considered to be a critical process related to Ca 2+ -calmodulin pathways for LTM formation (Matsumoto et al, 2006;Mizunami et al, 2014;Fig. 7).…”

“…Matsumoto and Mizunami (2002) developed an olfactory learning paradigm in the field cricket, wherein preference for a specific odor is evaluated before and after a training session during which the crickets associate the CS (odor) with the US (reward/punishment). Using this paradigm, a series of pharmacological studies proposed a biochemical model explaining the formation of long-term memory (LTM), involving NO-cGMP and cAMP pathways (Matsumoto et al, 2006Mizunami et al, 2014).…”