KRASNE, F.B. ; EDWARDS, D.H.: Modulation of crayfish escape reflexes I. Neural circuitry of escape and modulatory systems

The neural circuitry that mediates escape behavior in crayfish constitutes one of the best understood behavioral circuits in the animal kingdom. Escape is composed of two major types, stereotyped escape reflexes mediated by neurons with giant axons (giant fiber or GF escape) and more flexible non giant-mediated (NG) escape; each type is controlled by different mediational circuitry joining partially common afferent and efferent circuits. The portion of the circuitry that mediates GF escape is ideal for analyzing mechanisms responsible for one of the most fascinating aspects of animal (including human) behavior, variation in the likelihood of behavioral actions depending on the state of the animal. The circuitry for GF escape displays intrinsic plasticity, including synaptic depression and long-term potentiation, and is played upon by two classes of modulatory neurons, ones using GABA as a transmitter and ones using serotonin. The modulatory effects of serotonin differ according to the pattern of application of the drug, whether fast or slow, high or low concentration, and brief or long exposures. This sensitivity suggests that the GF neurons have multiple serotonin receptors that are sensitive to the source of the modulator, whether from neurotransmission, paracrine transmission, or hormonal release. The modulatory systems thus formed play roles in control of GF escape during other activities such as feeding and agonistic behavior as well as during simple forms of learning such as habituation. Serotonergic modulation, which is believed to play important roles during agonistic behavior, is a focus of this symposium. Of particular interest is the observation that the neuromodulatory effects of serotonin are qualitatively altered by social experience.