LONG-TERM HABITUATION (LTH) IN THE CRAB CHASMAGNATHUS - A MODEL FOR BEHAVIORAL AND MECHANISTIC STUDIES OF MEMORY

A decade of studies on long-term habituation (LTH) in the crab Chasmag nathus is reviewed. Upon sudden presentation of a passing object overh ead, the crab reacts with an escape response that habituates promptly and for at least five days. LTH proved to be an instance of associativ e memory and showed context, stimulus frequency and circadian phase sp ecificity. A strong training protocol (STP) (greater than or equal to 15 trials, intertrial interval (ITI) of 171 s) invariably yielded LTH, while a weak training protocol (WTP) (less than or equal to 10 trials , ITI = 171 s) invariably failed. STP was used with a presumably amnes tic agent and WTP with a presumably hypermnestic agent. Remarkably, sy stemic administration of low doses was effective, which is likely to b e due to the lack of an endothelial blood-brain barrier. LTH was block ed by inhibitors of protein and RNA synthesis, enhanced by protein kin ase A (PKA) activators and reduced by PKA inhibitors, facilitated by a ngiotensin II and IV and disrupted by saralasin. The presence of angio tensins and related compounds in the crab brain was demonstrated. Dive rse results suggest that LTH includes two components: an initial memor y produced by spaced training and mainly expressed at an initial phase of testing, and a retraining memory produced by massed training and e xpressed at a later phase of testing (retraining). The initial memory would be associative, context specific and sensitive to cycloheximide, while the retraining memory would be nonassociative, context independ ent and insensitive to cycloheximide.