Abstract
1. Twitches produced by stimulation of the nerve leading to the pharyngeal retractor muscle (PRM) relax more rapidly after a tetanic stimulation. 2. Perfusion of the PRM with 5-hydroxytryptamine (5-HT) mimics this relaxing action. 5-HT is the only factor present in the PRM that causes an increase in twitch relaxation rate. When the 5-HT content of the PRM is reduced by pre-treatment with reserpine, the magnitude of the post-tetanic increase in relaxation rate is also reduced. 3. The post-tetanic increase in relaxation rate appears to require the activation of a specific serotonergic axon(s) since there is a discrete threshold for the effect. 4. Peptides derived from nervous and other tissues of the snail also modulate PRM activity. 5. One peptide (SCP) mimics the action of 5-HT, but is not present in the PRM. 6. A second peptide class (LCP), at concentrations similar to those found in the hemolymph, potentiates the amplitude of sub-maximal contractions of the PRM. 7. It was concluded that 5-HT and LCP are physiological modulators of PRM activity, while the significance of SCP's action is unknown.
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