Characterization of serotonin receptors Aj5-HTR2 and Aj5-HTR7 reveals potential roles in circadian regulation of Apostichopus japonicus

Abstract

Serotonin (5-hydroxytryptamine, 5-HT) is an evolutionarily conserved neurotransmitter that mediates neuroendocrine signaling through 5-hydroxytryptamine receptors (5-HTRs). In this study, two 5-HT receptor subtypes, Aj5-HTR₂ and Aj5-HTR₇, were cloned and functionally characterized from the sea cucumber Apostichopus japonicus. Functional assays demonstrated that both receptors are signaling-competent molecules, exhibiting rapid receptor internalization upon 5-HT stimulation, along with dose-dependent increases in intracellular Ca²⁺ or cyclic adenosine monophosphate (cAMP) levels and ERK1/2 phosphorylation. Quantitative real-time PCR showed that Aj5-HTR₂ was highly expressed in the nerve ring and tentacles, whereas Aj5-HTR₇ was highly expressed in the Polian vesicle and muscle. Moreover, both receptors exhibited distinct day-night expression rhythms, suggesting subtype-specific involvement in circadian neuroendocrine regulation. In summary, this work expands the repertoire of 5-HTRs in sea cucumbers and provides new insights into their potential roles in circadian neuroendocrine regulation.