The effects of 5-HT on sensory, central and motor neurons driving the abdominal superficial flexor muscles in the crayfish

Serotonin (5-HT) induces a variety of physiological and behavioral effects in crustaceans. However, the mechanisms employed by 5-HT to effect behavori al changes are not fully understood. Among the mechanisms by which these ch anges might occur are alterations in synaptic drive and efficacy of sensory , interneurons and motor neurons, as well as direct effects on muscles. We investigated these aspects with the use of a defined sensory-motor system, which is entirely contained within a single abdominal segment and consists of a 'cuticular sensory neurons-segmental ganglia-abdominal superficial fle xor motor neurons-muscles' circuit. Our studies address the role of 5-HT in altering (1) the activity of motor neurons induced by sensory stimulation; (2) the inherent excitability of superficial flexor motor neurons; (3) tra nsmitter release properties of the motor nerve terminal and (4) input resis tance of the muscle. Using en passant recordings from the motor nerve, with and without sensory stimulation, and intracellular recordings from the mus cle, we show that 5-HT enhances sensory drive and output from the ventral n erve cord resulting in an increase in the firing frequency of the motor neu rons. Also, 5-HT increases transmitter release at the neuromuscular junctio n, and alters input resistance of the muscle fibers (C) 2000 Elsevier Scien ce Inc. All rights reserved.