Serotonin-stimulated biochemical events in the procerebrum of Limax.

1. The procerebrum (PC) of the terrestrial slug Limax maximus is of interest as a potential site of olfactory information processing (Gelperin et al., 1989). The neuromodulator serotonin is present in the procerebrum and can elicit action potentials from cultured procerebral neurons. We have investigated the effects of serotonin on second-messenger signaling systems and protein phosphorylation as a prelude to studies on long-term synaptic plasticity in the Limax procerebral lobe. 2. We found that several biochemical changes are triggered within 20 min of adding serotonin to the isolated procerebral lobe: adenylate cyclase is activated, protein phosphorylation and synthesis are modulated, and phosphatidylinositol-metabolism is stimulated. 3. Serotonin causes a rapid synthesis of cAMP, reaching a 20- to 30-fold increase within 1 min. Serotonin affects the rate of phosphorylation of several proteins, detected after a brief (20-min) incubation of the procerebral lobe in [32P]phosphate-containing medium. The level of synthesis of several proteins is altered by serotonin, as determined by alterations in [35S]methionine incorporation during a 20-min incubation. Serotonin also causes a slow accumulation of inositoltrisphosphate. 4. Our study shows that within a short time (less than 20 min) serotonin can influence several second-messenger signaling systems and the functional state and abundance of proteins in the procerebral lobe. These serotonin-stimulated events should have direct consequences for intercellular communication in the odor-processing network of the procerebral lobe.