Intracellular injection of t he catalytic subunit of cyclic AMP-dependent protein kinase simulates facilitation of transmitter release underlying behavioral sensitization in Aplysia.

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Abstract
It has been difficult to establish whether cAMP-mediated protein phosphorylation in nerve cells plays a specific role in synaptic transmission. This difficulty can be overcome in higher invertebrates because their large neurons allow the injection of protein molecules into the cell. Intracellular injection was used to study whether protein phosphorylation is involved in the mechanism of sensitization, a simple form of learning. Sensitization of the gill-withdrawal reflex in Aplysia involves enhancement of transmitter release by presynaptic facilitation at a particular set of synaptic connections between identified sensory neurons and their follower cells. Injection of the catalytic subunit of cAMP-dependent protein kinase purified from bovine heart mimics the action of the natural transmitter and of serotonin, the putative transmitter, by simulating the physiological changes that accompany presynaptic facilitation. Intracellular injection of the kinase into a sensory cell broadens the action potential in the presence of tetraethylammonium, indicating an increase in Ca2+ current, decreases the input conductance of the cell, presumably as a result of a decrease in the K+ current, and increases the amount of transmitter released by terminals of the sensory cell onto follower neurons.