Protein synthesis inhibition blocks the induction of mossy fiber long-termpotentiation in vivo

Protein synthesis inhibitors block the maintenance of NMDA receptor-depende nt long-term potentiation (LTP) both in vivo and in vitro. Protein synthesi s inhibitors block mossy fiber( MF) LTP maintenance in vitro, but little is known about the effect of protein synthesis inhibitors on either induction or maintenance in MF-LTP in vivo. Here we study the role of protein synthe sis in the induction of long-term potentiation at the mossy fiber-CA3 hippo campal synapse in vivo in anesthetized rats. The protein synthesis inhibito r anisomycin was administered at different doses (0.04, 10, or 40 nmol) int o area CA3 15 min before delivering high-frequency stimulation (two times a t 100 Hz, 1 sec). Anisomycin blocked MF-LTP induction in a dose-dependent m anner; both 40 and 10 nmol blocked MF-LTP induction, but a lower dose of 0. 04 nmol was without effect. The inhibitory effect of anisomycin on protein synthesis was determined by measuring the incorporation of [S-35] methionin e into the newly synthesized proteins. Percentages of protein synthesis inh ibition were determined by comparing [S-35] incorporation of anisomycin-tre ated samples with vehicle controls. Doses of 0.04, 10, or 40 nmol of anisom ycin produced 21, 82, or 83% inhibition of [S-35] methionine incorporation, respectively. The effect of anisomycin was verified using a single dose of the protein synthesis inhibitor cycloheximide (40 nmol). Cycloheximide als o blocked MF-LTP induction. These results suggest that protein synthesis pl ays an important role in the induction of mossy fiber long-term potentiatio n in vivo.