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David Glanzman"Postsynaptic Mechanisms of Sensitization and Habituation in Aplysia"

Abstract :


U
ntil recently, nonassociative forms of learning in the marine snail Aplysia californica have been thought to be due to simple, presynaptic cellular mechanisms. In particular, sensitization has been ascribed to presynaptic facilitation and habituation to (presynaptic) homosynaptic depression. However, it is now apparent that learning in Aplysia depends critically upon postsynaptic, in addition to presynaptic, mechanisms.
We have found that serotonin (5-HT), the endogenous monoamine that mediates dishabituation and sensitization, causes upregulation of AMPA receptor function in Aplysia motor neurons. This functional upregulation of AMPA receptors depends upon release of calcium from postsynaptic intracellular stores and postsynaptic exocytosis. We hypothesize that stimuli that induce dishabituation and sensitization in Aplysia modulate AMPA receptor trafficking in motor neurons that mediate that withdrawal reflex. Support for this hypothesis comes from experiments in which prior injection of botulinum toxin, an inhibitor of exocytosis, into identified siphon motor neurons blocks behavioral dishabituation of the siphon withdrawal reflex.
In other experiments we have investigated the mechanisms that underlie long-term habituation (LTH) of the gill withdrawal reflex. We have found that LTH depends upon both protein and RNA synthesis. Moreover, LTH requires activation of both AMPA and NMDA receptors. Finally, LTH involves protein phosphatase and calcineurin activity. Our results are consistent with a role for an LTD-related mechanism in LTH in Aplysia.
We propose that presynaptic facilitation and homosynaptic depression are short-lived forms of plasticity that mediate early stages of learning in Aplysia. Persistent learning is mediated by postsynaptic mechanisms, and by presynaptic alterations induced by retrograde signals.

Selected publications

Roberts AC, Glanzman DL.
Learning in Aplysia: looking at synaptic plasticity from both sides.
Trends Neurosci. 2003 Dec;26(12):662-70.
Ezzeddine Y, Glanzman DL.
Free Full Text Prolonged habituation of the gill-withdrawal reflex in Aplysia depends on protein synthesis, protein phosphatase activity, and postsynaptic glutamate receptors.
J Neurosci. 2003 Oct

Romuald Nargeot