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The protein kinases that are activated by Ca2+ /calmodulin, Ca2+ /phos pholipid, and cAMP are now thought to govern many types of slow (or modulatory) synaptic mechanisms and to mediate many forms of short. term synaptic plasticity (Reichardt & Kelly 1983, Nairn et a1 1985, Nish izuka 1986). These processes, which do not depend on the synthesis of new proteins and can endure from minutes to a substantial part of an hour, are the neurophysiological correlates of short-term memory (see Kandel & Schwartz 1982, Schwartz et a1 1983, Goelet et aI1986). Unlike the rapid conformational changes in the polypeptides that constitute chemically gated ion channels and that last only fractions of seconds after the stimu lating ligand is removed, the slow onset of short-term synaptic mechan isms on the one hand and their persistence for periods of time exceeding even the longest allosteric rearrangements of protein molecules on the other (Frieden 1970) reflect the receptor stimulated appearance, accumu lation, and relatively slow dissipation of free Ca2+, diacylglycerol, or cAMP within the stimulated nerve cell. As second messengers, these mol-
Schwartz et al. (Sun,) studied this question.