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Quantitative aspects of ionic conductance mechanisms contributing to firing pattern of motor cells mediating inking behavior in Aplysia californica.

1. The release of ink occurs selectively to long-lasting stimuli (18). There is a good correspondence between the features of the behavior and the firing pattern of the ink motor cells since the initial synaptic input is ineffective in firing the ink gland motor neurons. The features of the firing pattern of these cells have in turn been attributed to a fast K+ current that shunts the initial excitatory input and a late buildup of synaptic input. The purpose of the present paper is to examine these features quantitatively using a kinetic model of the ink gland motor neurons. 2. A modified Hodgkin-Huxley (13) model was developed, which includes seven membrane currents: a ) a fast inward Na+ current, 6) a slow inward Ca2+ current, c) a fast outward K+ current, d) a slow (delayed) outward K+ current, e) a synaptic current, f) a leakage current, and g) a capacitative current. The three ink gland motor neurons are electrically coupled to each other. The model also includes the features of this electronic coupling. 3. The Na+ current underlies the spike initiation, while the delayed K+ current accounts for spike repolarization. The fast transient K+ current also contributes to the repolarization process and slows the rate of rise of the membrane potential during the interspike interval. The slow Ca2+ current makes only a small contribution to the generation of the action potential, but contributes to the rise of the membrane potential during the interspike interval. 4. A depolarizing current pulse to one ink gland motor neuron yields an initial rapid depolarization followed by a slower component, which after a several-second period results in a burst of action potentials. The activation and inactivation sequence of a fast transient K+ current contributes to the cell’s selective response to long-lasting depolarizing current pulses. If the suprathreshold current pulse is preceded by a small subthreshold pulse, spikes are elicited immediately with the application of the suprathreshold pulse, since the subthreshold pulse inactivates the transient outward current. 5. The ink gland motor neurons also show a selective response to long-lasting trains of synaptic input. A train of synaptic input causes an initial large depolarization, but the initial synaptic input is ineffective in firing the cell. As a result, there is a severalsecond silent period or pause before an accelerating burst of spikes is produced. The fast transient K+ current shunts the initial synaptic input, preventing the cell from reaching spike threshold. Over a several-second period this current inactivates and the synaptic current is more effective in firing the cell. 6. While the fast outward current is inactivating, the synaptic current builds up and contributes to the accelerating burst of spikes.