Neuromodulation is a form of signal transduction that results in the biochemical control of neuronal excitability. Many neurotransmitters act through second messengers, and the examination of biochemical cascades initiated by neurotransmitter-receptor interaction has advanced the understanding of how information is acquired and stored in the nervous system (Kaczmarek and Levitan 1987). For example, as described by Sweatt et al. (this volume), 5-HT and other facilitory transmitters increase cAMP in sensory neurons of Aplysia, which enhances excitability and facilitates transmitter output. cAMP promotes protein phosphorylation, which closes Ks, a specific K+ channel, thereby delaying membrane repolarization (Klein and Kandel 1980; Siegelbaum et al. 1982). Within sensory neurons, a novel second-messenger cascade using metabolites of arachidonic acid and activated by the peptide transmitter FMRF-amide has two actions on the same K+ channel that is modulated by 5-HT to produce facilitation: (1) FMRF-amide through the arachidonic acid cascade increases the probability of channel...
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Shapiro et al. (1988) studied this question.