The role of the interstimulus interval in heterosynaptic facilitation in Aplysia californica

Abstract

1. (1) From a great number of recordings, 16 cases were selected in which heterosynaptic facilitation (HSF) or inhibition (HSI) could be repeated without fatigue of the test or priming responses for a period up to 2 h.2. (2) Eight of these recordings represented unitary, the rest compound EPSPs. The unitary responses could be divided into those which changed their amplitude during HSF or HSI and those which were facilitated or inhibited in an all or nothing fashion by heterosynaptic interference.3. (3) Various interstimulus intervals (ISIs) for the test and the priming stimulation ranging from 250 to 2,000 msec were tested. The interval which produced the highest amplitude of HSF was between 250 and 450 msec (approximately 350 msec) in 15 out of 16 cases. Shorter or longer intervals showed less heterosynaptic facilitation. No differences of the optimal ISI were found in unitary as compared to compound synaptic potentials.4. (4) The interval which was correlated to the longest total duration of HSF was also 350 msec.5. (5) In 2 cells, HSI was found to be strongest and longest when ISIs of 350 msec were used.6. (6) Since in psychological conditioning experiments ISIs of the same magnitude have been found to be optimal, this paper further allows indicating another similarity between HSF and behavioral conditioning.