Nonassociative Learning in Invertebrates

Abstract

The simplicity and tractability of the neural circuits mediating behaviors in invertebrates have facilitated the cellular/molecular dissection of neural mechanisms underlying learning. The review has a particular focus on the general principles that have emerged from analyses of an example of nonassociative learning, sensitization in the marine mollusk Aplysia. Learning and memory rely on multiple mechanisms of plasticity at multiple sites of the neuronal circuits, with the relative contribution to memory of the different sites varying as a function of the extent of training and time after training. The same intracellular signaling cascades that induce short-term modifications in synaptic transmission can also be used to induce longterm changes. Although short-term memory relies on covalent modifications of preexisting proteins, long-term memory also requires regulated gene transcription and translation. Maintenance of long-term cellular memory involves both intracellular and extracellular feedback loops, which sustain the regulation of gene expression and the modification of targeted molecules.

Associative Learning in Invertebrates

“…The mechanisms of several simple forms of learning have been studied extensively in the marine mollusk Aplysia, which has a number of advantages for a reductionist approach (see Byrne and Hawkins 2015). Although conditioning has been shown for some more complex behaviors (e.g., Walters et al 1981;Cook and Carew 1986), many of those studies have examined the gill-and siphon-withdrawal reflex, in which a light touch to the siphon (an exhalant funnel for the gill) produces contraction of the gill and siphon.…”

“…Although conditioning has been shown for some more complex behaviors (e.g., Walters et al 1981;Cook and Carew 1986), many of those studies have examined the gill-and siphon-withdrawal reflex, in which a light touch to the siphon (an exhalant funnel for the gill) produces contraction of the gill and siphon. As described in Byrne and Hawkins (2015), a noxious stimulus, such as a shock to the tail, produces an enhancement of subsequent responses to siphon stimulation or sensitization. In addition to this nonassociative form of learning, the reflex undergoes two associative forms of learning, operant conditioning (Hawkins et al 2006) and classical conditioning, which has been studied much more extensively for this behavior.…”

“…Thus, it is reasonable to suppose that at least some of the underlying mechanisms may be similar as well. That idea has encouraged the study of neural mechanisms of learning in relatively simple invertebrate species, which have a number of experimental advantages for such studies (Byrne and Hawkins 2015). In particular, many invertebrate nervous systems have comparatively few neurons, making it easier to elucidate the neural circuits for specific behaviors.…”

“…earning can be divided into two general categories: nonassociative learning, in which an animal learns about the properties or occurrence of a single stimulus (see Byrne and Hawkins 2015), and associative learning, in which an animal learns about the relationship between two stimuli or events. Associative learning includes classical and operant conditioning.…”

Acute Sleep Deprivation Blocks Short- and Long-Term Operant Memory inAplysia

“…27 Aplysia has been an excellent model for dissecting the molecular mechanisms involved in long-term and short-term associative and nonassociative memory formation. [28][29][30][31][32][33] The underlying mechanisms of memory formation identified in Aplysia extrapolate to more complex systems as they are highly conserved between vertebrates and invertebrates species. [34][35][36] Thus, the marine mollusk A. californica, with its relatively simple neural circuitry, provides an ideal model for studying the interactions between sleep and memory formation.…”