Function of insulin in snail brain in associative learning

Abstract

Insulin is well known as a hormone regulating glucose homeostasis across phyla. Although there are insulin-independent mechanisms for glucose uptake in the mammalian brain, which had contributed to a perception of the brain as an insulin-insensitive organ for decades, the finding of insulin and its receptors in the brain revolutionized the concept of insulin signaling in the brain. However, insulin's role in brain functions, such as cognition, attention, and memory, remains unknown. Studies using invertebrates with their open blood-vascular system have the promise of promoting a better understanding of the role played by insulin in mediating/modulating cognitive functions. In this review, the relationship between insulin and its impact on long-term memory (LTM) is discussed particularly in snails. The pond snail Lymnaea stagnalis has the ability to undergo conditioned taste aversion (CTA), that is, it associatively learns and forms LTM not to respond with a feeding response to a food that normally elicits a robust feeding response. We show that molluscan insulin-related peptides are up-regulated in snails exhibiting CTA-LTM and play a key role in the causal neural basis of CTA-LTM. We also survey the relevant literature of the roles played by insulin in learning and memory in other phyla.

A flavanoid component of chocolate quickly reverses an imposed memory deficit

“…It should be noted that the Yerkes and Dodson (1908) paper did not present such an inverted-U curve. The inverted-U function is actually a figure adapted from Donald Hebb's 1955 presidential address to the American Psychological Association (Hebb, 1955; see also Diamond et al, 2007;Ito et al, 2015a;Kojima et al, 2015). It appears that Hebb was unaware of the earlier Yerkes and Dodson paper (Diamond et al, 2007).…”

Relationship between the grades of a learned aversive-feeding response and the dopamine contents inLymnaea

“…In snails, CTA persists for at least one month (Kojima et al, 2015). Previously, we demonstrated that the CGCs play a key role in CTA (Yamanaka et al, 1999; Kojima et al, 2001; Otsuka et al, 2013).…”

Linking the why and how of aging; evidence for somatotropic control of long-term memory function in the pond snail Lymnaea stagnalis

“…The idea that ILPs play important and diverse roles in the fate of the nervous system is supported by various lines of evidence implicating ILPs in aspects of neuronal plasticity and cognitive function in a wide range of invertebrate and vertebrate species, including L. stagnalis (Creyghton et al, 2004;Zhao et al, 2004;Azami et al, 2006;Isgaard et al, 2007;Fernandez and TorresAlemán, 2012;Murakami et al, 2013;Mita et al, 2014;Pirger et al, 2014;Chambers et al, 2015;Kojima et al, 2015;Studzinksi et al, 2015;Akinola, 2016;Tanabe et al, 2017). Moreover, ILPs are also known to be important regulators of adult neurogenesis, angiogenesis and synaptogenesis, and insulin has been shown to affect synapse remodelling through downstream neurotrophic signalling cascades (Granata et al, 2007;Nelson et al, 2008;Åberg, 2010).…”

“…For example, in the mammalian brain, ILPs exert beneficial effects on processes such as growth and development, cell survival, adult neurogenesis, synaptic plasticity and cognition (Isgaard et al, 2007;Åberg, 2010;Fernandez and Torres-Alemán, 2012;Chambers et al, 2015). Lymnaea stagnalis appears to form no exception to this rule; several studies report that MIP genes are upregulated during LTM consolidation and that insulin/MIP signalling is necessary for LTM formation (Azami et al, 2006;Murakami, et al, 2013;Mita et al, 2014;Pirger et al, 2014;Kojima et al, 2015). ILPs could, however, also have a 'dark' side.…”