Molluscan memory of injury: evolutionary insights into chronic pain and neurological disorders

Abstract

Molluscan preparations have yielded seminal discoveries in neuroscience, but the experimental advantages of this group have not, until now, been complemented by adequate molecular or genomic information for comparisons to genetically defined model organisms in other phyla. The recent sequencing of the transcriptome and genome of Aplysia californica, however, will enable extensive comparative studies at the molecular level. Among other benefits, this will bring the power of individually identifiable and manipulable neurons to bear upon questions of cellular function for evolutionarily conserved genes associated with clinically important neural dysfunction. Because of the slower rate of gene evolution in this molluscan lineage, more homologs of genes associated with human disease are present in Aplysia than in leading model organisms from Arthropoda (Drosophila) or Nematoda (Caenorhabditis elegans). Research has hardly begun in molluscs on the cellular functions of gene products that in humans are associated with neurological diseases. On the other hand, much is known about molecular and cellular mechanisms of long-term neuronal plasticity. Persistent nociceptive sensitization of nociceptors in Aplysia displays many functional similarities to alterations in mammalian nociceptors associated with the clinical problem of chronic pain. Moreover, in Aplysia and mammals the same cell signaling pathways trigger persistent enhancement of excitability and synaptic transmission following noxious stimulation, and these highly conserved pathways are also used to induce memory traces in neural circuits of diverse species. This functional and molecular overlap in distantly related lineages and neuronal types supports the proposal that fundamental plasticity mechanisms important for memory, chronic pain, and other lasting alterations evolved from adaptive responses to peripheral injury in the earliest neurons. Molluscan preparations should become increasingly useful for comparative studies across phyla that can provide insight into cellular functions of clinically important genes.

Fig. 1

Features of a molluscan model organism, Aplysia californica. A Phylogenetic relationships of gastropods (Aplysia) to other prominent model organisms: insects (Drosophila), nematodes (Caenorhabditis elegans), sea urchins (Strongylocentrotus), ascidians (Ciona), vertebrates (represented by dog, Canis, and zebrafish, Danio), and cnidarians (hydrozoan Hydra). The evolutionary distance (indicated as relative branch length) from Canis or Danio to Aplysia is shorter than the distance from the vertebrates to Drosophila or C. elegans, suggesting that the amino acid replacement rate has been lower in the lineage leading to the gastropod Aplysia than in the lineages leading to Drosophila and C. elegans. B Nociceptive sensory neurons in the right ventrocaudal (VC) cluster revealed by in situ hybridization staining for a sensory neuron-specific neuropeptide, sensorin. A somatotopic map (‘Aplunculus’) of the ipsilateral body surface is represented in the cluster. Note that sensorin mRNA is found in neurites and axons throughout the pedal ganglion. C Expression of huntingtin occurs in the giant mucus release motor neuron, LPl1, and other neurons in the left pleural ganglion (in situ hybridization). Scale: 450 μm. D In situ hybridization shows choline acetyltransferase mRNA (a marker for cholinergic neurons) in diverse neurons, including LPl1, in the left pleural and pedal ganglia. Panels A, C and D are modified from Moroz et al. [2006]; panel B is modified from Walters et al. [2004].

Fig. 2

Sensorin-expressing neurons in Aplysia are ‘true’ nociceptors and exhibit long-term nociceptive sensitization. A Body regions innervated by different sensory clusters. Sensory neurons innervating the siphon (LE cluster) and tail (VC cluster) have received the most study. B Evidence that mechanosensory neurons are nociceptors. Responses of an LE siphon sensory neuron (SN) to mechanical stimulation (B1) exhibit a relatively high threshold, are maximal with damaging stimuli (pinch), and increase (sensitize) following the pinch (B1 and B2) [modified from Illich and Walters, 1997]. C Long-term, sensitizing effects in a nociceptive tail sensory neuron. One day after noxious tail shock a SN innervating the shocked site, but not a contralateral control SN, exhibited afterdischarge following a brief (10-msec) test pulse to the soma, and pronounced facilitation of its synapse onto a tail motor neuron (MN) [modified from Walters, 1987b]. D Intrinsic injury signals are sufficient to produce SN hyperexcitability. Example showing how the spike accommodation normally seen in control SNs is greatly reduced in a dissociated SN one day after transection of its neurites [modified from Ambron et al., 1996].

Fig. 3

Similarity of initial signals for induction of long-term neuronal plasticity in molluscan and mammalian nociceptors (left side) and mammalian neurons that form memories within the CNS (right side). In each case local signals include intense depolarization of an injured segment or synaptic region (darkened areas), Ca²⁺ influx, and modulation by various neuroactive substances released from nearby cells. In addition, global signals released during injury or memorable events can influence the soma and other parts of the neuron, at least in part by regulating gene transcription. These initiating events for nociceptive sensitization and memory are mediated by common sets of highly conserved intracellular signals (see text).