TIME-DEPENDENT CHANGES IN EXCITABILITY AFTER ONE-TRIAL CONDITIONING OF HERMISSENDA

The visual system of Hermissenda has been studied extensively as a sit e of cellular plasticity produced by classical conditioning. A one-tri al conditioning procedure consisting of light paired with the applicat ion of serotonin (5-HT) to the exposed, but otherwise intact, nervous system produces suppression of phototactic behavior tested 24 h after conditioning. Short-and long-term enhancement (STE and LTE) of excitab ility in identified type B photoreceptors is a cellular correlate of o ne-trial conditioning. LTE can be expressed in the absence of STE sugg esting that STE and LTE may be parallel processes. To examine the deve lopment of enhancement, we studied its time-dependent alterations afte r one-trial conditioning. Intracellular recordings from identified typ e B photoreceptors of independent groups collected at different times after conditioning revealed that enhanced excitability follows a bipha sic pattern in its development. The analysis of spikes elicited by 2 a nd 30 s extrinsic current pulses at different levels of depolarization showed that enhancement reached a peak 3 h after conditioning. From i ts peak, excitability decreased toward baseline control levels 5-6 h a fter conditioning followed by an increase to a stable plateau at 16 to 24 h postconditioning. Excitability changes measured in cells from un paired control groups showed maximal changes 1 h posttreatment that ra pidly decremented within 2 h. The conditioned stimulus (CS) elicited s ignificantly more spikes 24 h postconditioning for the conditioned gro up as compared with the unpaired control group. The analysis of the ti me-dependent development of enhancement may reveal the processes under lying different stages of memory for this associative experience.