The impact of day, night and other external synchronizers on brain serotonin levels in the American lobster Homarus americanus

Persistent proliferation occurs among the local (cluster 9) and projection (cluster 10) neurons in the lobster brain. Serotonin influences the rate of neurogenesis in these cell clusters. Neurogenesis among the cluster 10 projection neurons is also under circadian control. Because serotonin and the light/dark cycle influence the rate of neurogenesis, experiments conducted for this thesis have tested whether serotonin may be part of the circadian pathway regulating neurogenesis. High performance liquid chromatography (HPLC) measurements of serotonin levels in the brains of juvenile lobsters exposed to a 12:12 light: dark cycle, followed by 3 days in constant darkness, demonstrate that serotonin levels in the brain follow an endogenous rhythm that can be entrained by light; that separate rhythms can be found in functionally distinct lobes; that phase shifted entrained feeding alters the endogenous rhythm of both neurogenesis and serotonin; that feeding frozen brine prior to sampling decreases the characteristic peak and troughs of the circadian rhythm of both neurogenesis and serotonin; that physical activity shortly increases the rate of neurogenesis but overall decreases neurogenesis; that physical activity phase shifts the circadian rhythm of serotonin; and that feeding functions as a stronger synchronizer on both neurogenesis and serotonin than light. Immunocytochemical staining methods examining the intensity of serotonin labelling in the brain over 24-h support the HPLC findings; show that melatonin, a biosynthetic product of serotonin which has been implicated to modify the release and synthesis of serotonin in other species, can be found throughout various sites within the lobster brain; and that serotonin transporter (SERT) expressing cells can be found within the region of life-long neurogenesis and adjacent to newly generated cells. Taken together, these results provide evidence that serotonin may function as one component of a light-/feeding-regulated molecular pathway influencing neurogenesis of the cluster 10 projection neurons. Supported by LGF Baden-Württemberg, DAAD, NSF, Staley Fellowship and Klothilde Eberhardt Stiftung