Central Pattern Generators

BOX 1, cont'd Central program for swimming in Tritonia. (A) Swimming movements as described previously. (B) Intracellular recordings from neurons in the Tritonia CNS. The top record represents a cell that drives the animal's downward (ventral) flexion; the lower record shows the cell that drives the upward (dorsal) flexion. The numbers between records correspond to the numbers in A and show the types of recordings obtained during the corresponding phases of the swimming movement. From Willows (24). Copyright by Scientific American.

BOX 1, cont'd Central program for swimming in Tritonia. (A) Swimming movements as described previously. (B) Intracellular recordings from neurons in the Tritonia CNS. The top record represents a cell that drives the animal's downward (ventral) flexion; the lower record shows the cell that drives the upward (dorsal) flexion. The numbers between records correspond to the numbers in A and show the types of recordings obtained during the corresponding phases of the swimming movement. From Willows (24). Copyright by Scientific American.

withdrawal reflex. The tail shock impinges upon this circuit by way of tail sensory neurons that make direct contacts (and interneurons that make indirect contacts) with the presynaptic terminals of the siphon sensory neurons.

It was soon realized that plasticity at the siphon sensory neuron/gill motor neuron synapse is one critical locus contributing to sensitization in the animal—one of the first demonstrations of the importance of synaptic plasticity in learning and memory. A predominant component of plasticity at this synapse is increased neurotransmitter release from the gill-and-siphon sensory neurons. Thus, tail shock and the attendant activity in tail sensory neurons and associated interneurons leads to the release of modulatory neurotransmitters onto the siphon sensory neuron presynaptic terminal, increasing the release of neurotrans-mitter from these cells and augmenting the defensive withdrawal reflex. These observations highlighted the role of presynaptic facilitation of neurotransmitter release as a mechanism for memory in this system.

Although all the modulatory neuro-transmitters involved in presynaptic facilitation in Aplysia sensory neurons are not yet identified, one important player is serotonin. Serotonin is released onto a subset of the siphon sensory neurons by a serotonergic tail sensory neuron stimulated by tail shock. In fact, serotonin application to siphon sensory neurons elicits the vast majority of the physiologic responses contributing to presynaptic facilitation of neurotransmitter release and sensitization in the animal.

Once it was realized that facilitation of neurotransmitter release from siphon sensory neurons (hereafter referred to simply as sensory neurons) was an important component of sensitization in the animal, and that serotonin could mimic the effects of sensitizing stimulation on sensory neuron physiology, it became clear that an effective model system for studying sensitization in

Aplysia was to study the cascade of events elicited by serotonin application to sensory neurons. This model system has been exploited to characterize the cellular, electro-physiologic, and biochemical mechanisms operating to achieve enduring presynaptic facilitation in these cells.

As mentioned earlier, sensitization in Aplysia exhibits both short-term and long-term forms. Similarly, in sensory neurons, serotonin application can lead to either short-term or long-term facilitation of neurotransmitter release. Single (5-minute) applications of serotonin give facilitation that lasts only a few minutes; repeated (5 x 5 minutes over the course of an hour) applications give facilitation lasting at least 24 hours. This is, of course, very reminiscent of the durations of behavioral sensitization in response to single or multiple presentations of tail shock stimuli. One very active area of Aplysia research over the last 20 years has been dissecting the biochemical cascades operating to cause these short- and long-term effects, in particular trying to understand how the different durations of effects are achieved. In the following sections, I will briefly describe the molecular mechanisms that have been discovered to play a role in short-term, intermediate-term, and long-term facilitation of neurotransmitter release in Aplysia sensory neurons.

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