Protein synthesis at synapse versus cell body: Enhanced but transient expression of long‐term facilitation at isolated synapses

Abstract

Protein synthesis at synaptic terminals contributes to LTP in hippocampus and to the formation of new synaptic connections by sensory neurons (SNs) of Aplysia. Here we report that after removal of the SN cell body, isolated SN synapses of Aplysia in culture express protein-synthesis dependent long-term facilitation (LTF) produced by 5-HT that decays rapidly. Changes in expression of a SN-specific neuropeptide sensorin in isolated SN varicosities parallel the changes in synaptic efficacy. At 24 h after 5-HT the magnitude of LTF produced at isolated SN synapses was significantly greater than that produced when SN cell bodies were present. LTF was maintained at 48 h at connections with SN cell bodies, but not at isolated SN synapses. The increase in synaptic efficacy at isolated SN synapses at 24 h was blocked by the protein synthesis inhibitor anisomycin. LTF was accompanied by changes in expression of sensorin. The increase in sensorin level at isolated SN varicosities with 5-HT was blocked by anisomycin or was reversed 48 h after 5-HT treatment alone. The results suggest that, as is the case for initial synapse formation between SNs and L7, changes in protein synthesis at synaptic terminals may contribute directly to LTF of stable synapses. Changes in expression within the cell body provide additional contributions for long-term maintenance of the new level of synaptic efficacy that was initiated directly by local changes in protein synthesis at or near synaptic terminals.

Protein Kinase C Regulates Local Synthesis and Secretion of a Neuropeptide Required for Activity-Dependent Long-Term Synaptic Plasticity

“…Immunocytochemistry was used to monitor the expression and distribution of sensorin throughout the sensory neurons and total p42/44 MAPK or phosphorylated p42/44 MAPK in the cell bodies of sensory neurons ( Hu et al, 2004aHu et al, , 2006. Cultures at various times after stimuli or after the application of control solutions were rinsed briefly in artificial seawater, fixed in 4% paraformaldehyde, and processed as described previously (Liu et al, 2003;Hu et al, 2004a). Cultures were exposed to rabbit polyclonal antibody specific for sensorin (1:1000) or total p42/44 MAPK and phospho-p42/44 MAPK (1:200; Cell Signaling Technology, Beverly, MA) diluted in 2% normal goat serum in 0.01 M PBS with 0.3% Triton X-100 at 4°C for 24 h. The incubated cultures were washed in 0.01 M PBS and incubated in FITC-conjugated goat anti-rabbit IgG (1:200; Sigma) at 4°C for 4 h. After washing in 0.01 M PBS, cultures were imaged directly with the appropriate filter set for detecting the fluorescent signal.…”

Multifunctional Role of Protein Kinase C in Regulating the Formation and Maturation of Specific Synapses

“…Protein synthesis, including synthesis at distal sites of sensory neurons, is required for synapse maturation and growth (Schacher and Wu, 2002). Protein synthesis inhibitors blocked synapseassociated growth of sensory neurons (Schacher and Wu, 2002;Hu et al, 2004a) and interfered with the local synthesis of sensorin (Hu et al, 2002(Hu et al, , 2004a(Hu et al, , 2007Liu et al, 2003). Inhibition of PKC also blocked the rapid increase in local synthesis of sensorin after paired stimuli that produce an associative form of LTF (Hu et al, 2007).…”

“…This was surprising because at mature synapses after stimuli that produce LTF, PKA and PKC are both required for the rapid release of newly synthesized sensorin, and PKA activity is required for sensorininduced activation and translocation of MAPK (Hu et al, 2004a. During the initial phases of synapse formation, PKC may specifically regulate sensorin release from varicosities, structures containing both high levels of sensorin and transmitter release sites (Glanzman et al, 1989;Hatada et al, 1999;Liu et al, 2003;Hu et al, 2004b). Because varicosities contacting L11 contain low levels of sensorin and no active zones (Glanzman et al, 1989), these results might explain the low levels of sensorin release when sensory neurons contact L11 (Hu et al, 2004b).…”

“…Immunocytochemistry was used to monitor expression of sensorin, total p42/44 MAPK, or phosphorylated p42/44 MAPK ( Martin et al, 1997;Liu et al, 2003;Hu et al, 2004a,b). Cultures were rinsed briefly in artificial seawater and fixed in 4% paraformaldehyde and processed as described previously Hu et al, 2004a,b).…”

“…To test the specificity of the primary antibody, controls were performed, including the substitution of normal rabbit serum for the primary antibody and omission of the primary antibody. To test specificity of the sensorin antibody, sensorin and its antibody at equimolar concentrations were preincubated for 30 min before their application to fixed cultures for immunostaining (Liu et al, 2003;Hu et al, 2004a,b). All controls showed little immunocytochemical reaction.…”