Initiating morphological changes associated with long‐term facilitation in Aplysia is independent of transcription or translation in the cell body

Abstract

In Aplysia, the growth of axonal arbor and the formation of new presynaptic varicosities are thought to contribute to long-term facilitation (LTF) produced by serotonin (5-HT). While it is known that there is a requirement for both transcription and translation in LTF and in the accompanying morphological changes, the mechanisms mediating the initiation and maintenance of these changes are poorly understood. We used long-term labeling of the presynaptic sensory neuron to carry out repeated imaging of axonal morphology, coupled with electrophysiology, to further elucidate the macromolecular requirements of this process. Robust synaptic facilitation, axonal growth, and the formation of axonal varicosities were elicited by 5-HT even when transcription was blocked with actinomycin. Increases in synaptic efficacy and varicosity number were detected 12 h after exposure to 5-HT but did not persist to 24 h. Even when sensory neuron cell bodies were removed, eliminating the contributions of both somal transcription and translation, 5-HT elicited these transient morphological and electrophysiological responses. New sensory varicosities contacting the postsynaptic neuron were filled with the neuropeptide sensorin. Under all conditions, global inhibition of protein synthesis completely blocked the formation of new axonal branches and varicosities. These results demonstrate that neither transcription nor somal translation is required to initiate the axonal growth that often accompanies long-term synaptic plasticity-protein synthesis in the axon is sufficient. Macromolecular synthesis in the cell body is, however, required to maintain the enlarged arbor.

Two Signaling Pathways Regulate the Expression and Secretion of a Neuropeptide Required for Long-Term Facilitation inAplysia

“…PI3K and rapamycin regulate sensorin levels at isolated sensory neuron processes After the cell body is removed, 5-HT produces a translation-dependent form of LTF at isolated sensory neuron synapses Grabham et al, 2005). We examined whether 5-HT also produces a rapid increase in sensorin in isolated axons and varicosities and whether the PI3K activity and rapamycin regulate that increase in expression.…”

“…Some kinases phosphorylate transcription factors and cytoplasmic substrates to regulate macromolecular synthesis (Bartsch et al, 1995(Bartsch et al, , 1998Chain et al, 1999;Yamamoto et al, 1999;Giustetto et al, 2003;Liu and Schwartz, 2003). Other kinases phosphorylate local substrates that may be critical for regulating synaptic transmission, local protein synthesis, and synaptic growth (Bailey et al, 1992(Bailey et al, , 1997Casadio et al, 1999;Angers et al, 2002;Si et al, 2003;Liu et al, 2004;Grabham et al, 2005;Udo et al, 2005). Type II PKA, which is concentrated at synaptic sites, is activated by 5-HT, and its increased expression and its interaction with anchoring proteins that maintain its proximity to substrates near the membrane are required for LTF (Liu et al, 2004).…”

Defining memories by their distinct molecular traces

“…This nascent growth is another example of a latent trace, because it is required for the formation of new synapses, but in the absence of additional events does not cause an increase in synaptic strength. Initial synaptic growth is independent of protein synthesis, whereas maintenance and elaboration of that growth requires local protein synthesis [18,19]. Moreover, maintenance and maturation of the synapse at 24 h requires gene expression [18].…”

“…Initial synaptic growth is independent of protein synthesis, whereas maintenance and elaboration of that growth requires local protein synthesis [18,19]. Moreover, maintenance and maturation of the synapse at 24 h requires gene expression [18]. Even after 24 h, the new synapses are unstable and require further events to become consolidated into more permanent structures [6].…”

Two mRNA-Binding Proteins Regulate the Distribution of Syntaxin mRNA inAplysiaSensory Neurons

“…Accumulation of the mRNA at the axon hillock leads to increases in the amounts of syntaxin protein at the axon hillock for transport into the axon (Hu et al, 2003), where it can contribute to axon growth and synaptic function (Tang et al, 2001;Horton and Ehlers, 2003). Because axon growth and the formation of new synapses occur both early during the initial interaction between the sensory neuron with a target motor neuron and later during the formation of long-term facilitation of stable sensory-motor neuron connections (Bailey and Chen, 1988;Glanzman et al, 1989Glanzman et al, , 1990Kim et al, 2003), increased syntaxin in the axon would be required for the formation, maintenance, and functioning of the new synapses (Hu et al, 2003;Kim et al, 2003;Grabham et al, 2005). Disrupting the accumulation of syntaxin mRNA at the axon hillock by interfering with specific interactions between cis-acting elements and trans-acting RNA-binding proteins blocked long-term facilitation (Figs.…”