Regulation of serotonin levels by multiple light-entrainable endogenous rhythms

Abstract

This study examined whether serotonin levels in the brain of the American lobster, Homarus americanus, are under circadian control. Using high-performance liquid chromatography and semi-quantitative immunocytochemical methods, we measured serotonin levels in the brains of lobsters at six time points during a 24-h period. Lobsters were maintained for 2·weeks on a 12·h:12·h light:dark cycle followed by 3·days of constant darkness. Under these conditions, brain serotonin levels varied rhythmically, with a peak before subjective dusk and a trough before subjective dawn. This persistent circadian rhythm in constant darkness indicates that serotonin levels are controlled by an endogenous clock. Animals exposed to a shifted light cycle for >10·days, followed by 3·days in constant darkness, demonstrate that this rhythm is light entrainable. Separate analyses of two pairs of large deutocerebral neuropils, the accessory and olfactory lobes, show that serotonin levels in these functionally distinct areas also exhibit circadian rhythms but that these rhythms are out of phase with one another. The olfactory and accessory lobe rhythms are also endogenous and light entrainable, suggesting the presence of multiple clock mechanisms regulating serotonin levels in different brain regions.

Putative pacemakers of crayfish show clock proteins interlocked with circadian oscillations

“…Both 5-hydroxytryptamine (5-HT)-ir and 5-HT receptors have been reported in these locations (Sandeman et al, 1988;Spitzer et al, 2005). 5-HT has been proposed to be a modulator of circadian rhythms in crayfish (Castañón-Cervantes et al, 1999;Wildt et al, 2004) …”

Circadian clocks in crustaceans: identified neuronal and cellular systems

“…5-HT is already detectable during post-embryonic development in the crayfish retina, eyestalk and brain, and displays circadian fluctuations in 5-HT-concentrations in each tissue, linking the 5-HT system to a pacemaker system (103,155,248). In the lobster H. americanus, light-entrainable circadian changes in 5-HT concentrations have been observed in both the central brain and the eyestalk (249). Changes in the circadian rhythm of 5-HT (and likely its ratelimiting enzyme tryptophan-hydroxylase; 250) during crayfish development according to a 9-12 hr rhythm have been interpreted as (i) being superimposed upon the basic circadian rhythm (248) or (ii) representing an example for an initially ultradian-like rhythmicity of, e.g., the ERG and retinal shielding pigment movements (15min to 4hrs), that about 30 days later is being replaced by circadian rhythmicity as has been shown for first instar crayfish P. clarkii (22).…”

Adult neurogenesis and cell cycle regulation in the crustacean olfactory pathway: from glial precursors to differentiated neurons

“…Various in vitro and in vivo studies have shown that increases in serotonin levels result in an upregulation of neurogenesis in both the MPZ and LPZ of juvenile clawed lobsters (Benton and Beltz 2001a;Beltz et al 2001) and that decreases inhibit the branching of newborn projection neurons (Sullivan et al 2000). Furthermore, brain serotonin levels are under circadian control (Wildt et al 2004). Current studies are aimed at determining whether serotonergic pathways form an integral part of the mechanism underlying circadian regulation of neurogenesis, or whether this pathway functions in parallel with photoperiodic controls.…”