Interneurons involved in mediation and modulation of gill-withdrawal reflex in Aplysia. III. Identified facilitating neurons increase Ca2+ current in sensory neurons.

Abstract

Stimulation of a single L29 neuron can produce facilitation at the synapses from sensory neurons to motor neurons in the circuit for the Aplysia gill-withdrawal reflex. It can also produce broadening of the spike in sensory neurons in the presence of 50 mM tetraethylammonium (TEA). Since inward current during the plateau of the spike in TEA is carried primarily by Ca2+ ions, an increase in spike duration is indicative of an increase in Ca2+ current. At least 2 different L29 neurons in the same ganglion produced broadening of the spike in sensory neurons. On average, duration of the sensory neuron spike increased by 91% compared to control and remained at that level for > 3 min following L29 stimulation. The full time course has not yet been investigated. These results were quantitatively very similar to those for facilitation of the complex PSP [post-synaptic potential] in a motor neuron by L29. Every L29 cell that produced facilitation of the complex PSP produced spike broadening when the 2 effects were tested separately. Stimulation of an L29 cell produced both broadening of the spike in a sensory neuron and facilitation of the unitary sensory-to-motor neuron PSP when the 2 effects were tested simultaneously, and again broadening and facilitation had roughly similar amplitudes and time courses. Results support the hypothesis that spike broadening in TEA solution reflects a mechanism of presynaptic facilitation: specifically, that stimulating an L29 neuron produces an increase in the Ca2+ current in the terminals and in the cell bodies of the sensory neurons, leading to greater transmitter release. The L29 neurons are compared to other identified modulatory neurons with which they may share certain biochemical properties. Neurons with a modulatory behavioral function may have the additional property of affecting voltage-dependent conductances in their target cells.