Phylogenetic Analysis of Learning‐related Neuromodulation in Molluscan Mechanosensory Neurons

Abstract

In spite of significant advances in our understanding of mechanisms of learning and memory in a variety of organisms, little is known about how such mechanisms evolve. Even mechanisms of simple forms of learning, such as habituation and sensitization, have not been studied phylogenetically. Here we begin an evolutionary analysis of learning-related neuromodulation in species related to the well-studied opisthobranch gastropod, Aplysia californica. In Aplysia, increased spike duration and excitability in mechanosensory neurons contribute to several forms of learning-related changes to defensive withdrawal reflexes. The modulatory transmitter serotonin (5-hydroxytryptamine, or 5-HT), is thought to play a critical role in producing these firing property changes. In the present study, we tested mechanosensory homologs of the tail-withdrawal reflex in species related to Aplysia for 5-HT-mediated increases in spike duration and excitability. Criteria used to identify homologous tail-sensory neurons included position, relative size, resting electrical properties, expression of a sensory neuron-specific protein, neuroanatomy, and receptive field. The four ingroup species studied (Aplysia californica, Dolabella auricularia, Bursatella leachii, and Dolabrifera dolabrifera) belong to two clades (two species each) within the family Aplysiidae. In the first clade (Aplysia/Dolabella), we found that the tail-sensory neurons of A. californica and tail-sensory homologs of a closely related species, D. auricularia, responded to bath-applied serotonin in essentially similar fashion: significant increases in spike duration as well as excitability. In the other clade (Dolabrifera/Bursatella), more distantly related to Aplysia, one species (B. leachii) showed spike broadening and increased excitability. However, the other species (D. dolabrifera) showed neither spike broadening nor increased excitability. The firing properties of tail-sensory homologs of D. dolabrifera were insensitive to 5-HT over a wide range of concentrations. We also performed experiments on two outgroup species (Akera bullata and Bulla gouldiana) and found that spike duration was unaffected by 5-HT, whereas excitability was increased. This study suggests that 5-HT-induced spike broadening arose more recently in opisthobranch evolution, whereas 5-HT-induced excitability increase is a more ancestral trait that may have been expressed in the earliest opisthobranchs. Both traits are absent in the aplysiid species D. dolabrifera, demonstrating that a lineage can lose learning-related mechanisms. The phylogenetic variation observed in the present study presents the opportunity to test general models about learning mechanisms and their evolution in unique ways.

Interspecific variation in palatability suggests cospecialization of antipredator defenses in sea hares

“…Finally, the original motivation for executing these experiments was physiological research in our laboratory, which found significant phylogenetic variation in mechanisms of sensitization across 7 aplysiomorph species (Wright et al 1996, Erixon et al 1999, as well as behavioral experiments verifying correlated variation in behavioral phenotypes (Wright 1998, Erixon et al 1999, Marinesco et al 2003. Sensitization is a simple form of learning, which refers to the strengthening of defensive reflexes following a noxious stimulus.…”

“…Although chemical defenses have been well studied in Aplysia and its relatives (Carefoot 1987, Pennings 1994, Avila 1995, Nolen & Johnson 2001, Kicklighter et al 2005, Nusnbaum & Derby 2010, chemical defenses in Phyllaplysia and Dolabrifera have been scarcely studied (Ciavatta et al 1996). We chose these 3 species (Phyllaplysia and Dolabrifera are sister genera in the subfamily Dolabriferinae, and Aplysia is a more distant relative in the subfamily Aplysiinae; Klussmann-Kolb 2004) because we have previously discovered significant variation among them in a simple form of learning called sensitization (Wright et al 1996, Wright 1998, Erixon et al 1999, Marinesco et al 2003. Sensitization refers to a generalized increase in reflex response after a novel or noxious stimulus (Kandel 2004, Barco et al 2006.…”

Associative memory in three aplysiids: Correlation with heterosynaptic modulation

“…In Phyllaplysia taylori, the modulatory effects of noxious stimulation of sensory neurons although significant, are weaker than those observed in Aplysia. This is likely due to inhibition by unknown mechanisms, because serotonin release ) and the response of sensory neurons to serotonin (Wright et al 1996), are indistinguishable from that of Aplysia. At the behavioral level, Phyllaplysia shows only very limited sensitization (Erixon et al 1999).…”

“…Both of these nonassociative manipulations cause neuromodulatory changes in sensory-neuron firing properties of Aplysia, but their effects on two related species are much weaker. Specifically, in Dolabrifera dolabrifera, no serotonin-induced changes are observed, and a cladistic analysis (Wright et al 1996) strongly suggests that this absence is due to a relatively recent evolutionary loss of response by sensory neurons to applied serotonin. Furthermore, stimulation of peripheral nerves has no effect on sensory neurons, and behavioral experiments (Wright 1998) confirm the complete lack of sensitization (short-or long-term) or dishabituation.…”

“…Modulation is produced by application of the modulatory transmitter, serotonin (Wright et al 1996;Erixon et al 1999), or by strong stimulation of peripheral nerves . Both of these nonassociative manipulations cause neuromodulatory changes in sensory-neuron firing properties of Aplysia, but their effects on two related species are much weaker.…”