Long-term increases in synaptic density in chick CNS after passive avoidance training are blocked by an inhibitor of protein synthesis.

Long-term increases in synaptic density (first recorded 24 h after training of chicks on a one-trial passive avoidance task, and still present 48 h post training), are found bilaterally in a part of the striatum, the lobus parolfactorius (LPO) [23,36], and are believed to reflect a trace of long-term memory formation. Such increases in synaptic density are most likely to occur by either de novo synthesis of new synaptic material, or via post-translational modification of pre-existing components. Several previous studies have shown that inhibitors of protein synthesis such as anisomycin injected just before, or after training, can prevent long-term memory formation in the chick. The present study therefore examined whether the long-term increases in synaptic density in the LPO that occur after passive avoidance training can be blocked by anisomycin. Our data show clearly that chicks injected with anisomycin 30 min pre-training were amnesic on testing 24 h later, and the bilateral increases in synaptic density (of spine and shaft synapses) seen in saline injected trained controls, were significantly reduced, demonstrating that protein synthesis de novo is involved in the post-training increase in synaptic density in the LPO.