The serotonin receptor Am5HTR2a links neuromodulation to MRJP1 expression and temporal polyethism in honey bees

Division of labor in honey bees is accompanied by coordinated changes in neural and glandular physiology, yet the molecular links between neuromodulatory signaling and Major Royal Jelly Protein 1 (MRJP1) expression remain poorly understood. Here we show that the serotonin receptor Am5HTR2a is dynamically associated with nurse-bee physiology and MRJP1 expression in Apis mellifera. Among four serotonin receptor subtypes examined, Am5HTR2a showed the strongest age-dependent expression pattern. Expression peaked in the brain during the nurse stage (day 11) and slightly later in the hypopharyngeal glands (HG; day 16), where it rose from undetectable levels to a peak relative expression level of approximately 98, normalized to rp49, whereas the other receptor transcripts remained comparatively stable. Pharmacological manipulation showed that serotonin increased Am5HTR2a and MRJP1 expression in the brain and HG in a dose-dependent manner, whereas melatonin produced the opposite effect. A multifactorial general linear model identified pharmacological treatment as the dominant predictor of Am5HTR2a expression (F ₂,₂₂ = 65.31, p < 0.001, partial η² = 0.856; n = 3 biological replicates per group), with a significant tissue × treatment interaction (F ₂,₂₂ = 7.58, p = 0.003, partial η² = 0.408), indicating that serotonergic and melatonergic effects differed between neural and exocrine tissues. Consistently, endogenous serotonin depletion with 5,7-dihydroxytryptamine reduced MRJP1 expression. RNA interference-mediated knockdown of Am5HTR2a efficiently suppressed receptor transcript abundance and was accompanied by a progressive decline in MRJP1 mRNA. Notably, serotonin remained able to elevate MRJP1 expression in forager bees in a dose-dependent manner, indicating that serotonergic responsiveness persists beyond the nurse stage and that aspects of temporal polyethism remain physiologically reversible. Together, these results identify Am5HTR2a as a component of a serotonergic signaling pathway associated with worker bee physiology and HG-associated MRJP1 expression. The findings support a model in which serotonergic and melatonergic signaling are potentially linked to arylalkylamine N-acetyltransferase (AANAT)-dependent indoleamine metabolism, although AANAT activity was not directly measured here. In this framework, these signals help shape temporal polyethism by modulating molecular programs underlying worker allocation to behavioral and metabolic tasks.