State-dependent recall can be induced by protein synthesis inhibition: behavioural and morphological observations

The possibility that the amnesia induced by protein synthesis inhibition is state dependent was investigated. Chicks injected with the protein synthesis inhibitor anisomycin and then trained in a single-trial passive avoidance learning task showed no recall for the task when tested 6 h later in the absence of the drug. If, however, the same chicks were subsequently retested 30 min after a second administration of the drug they demonstrated clear recall for the task. Control groups showed that this effect was not the result of the administration of anisomycin per se but was due to state-dependent recall. Quantitative morphological characterisation of synapses in a region of the chick forebrain (the intermediate part of the medial hyperstriatum ventrale (IMHV) previously shown to be involved in passive avoidance learning was performed. The characteristic increase in the length of the postsynaptic density in the left IMHV was only evident in chicks killed after the behavioural test in which they had demonstrated recall. No synaptic changes were observed in chicks in which state-dependent recall had been demonstrated in a previous test but which were killed after a test in which they appeared amnestic. These results suggest that a memory trace may be established even in the absence of protein synthesis but that this trace may not normally be accessible. It is also suggested that the synaptic changes observed following learning may be dependent on some aspect of the recall phenomenon.