Electrochemical sensing of hormonal serotonin levels in Crayfish

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) has been implicated as a key biomarker that regulates a range of neurobehavioral functions. 5-HT also plays a prominent role in modulating social behavior across species. Here, we report the successful differentiation of hormonal (circulatory) 5-HT concentrations in socially isolated and communal crayfish samples by a surface-modified carbon fiber microelectrode electrochemical sensor. Monitoring 5-HT concentrations across social conditions provided important insights into mechanisms linking isolation to neurohormones that guide behavior. Utilizing surface coatings and electrochemical etching, our sensor can detect endogenous, nanomolar concentrations of 5-HT in vitro from crayfish hemolymph (i.e., blood) collected from communally housed animals and those isolated for one or seven days. Our measurements revealed that crayfish responses to isolation varied by species: Procambarus crayfish isolated for seven days exhibited a major increase in 5-HT compared to communally housed or short-term isolated conspecifics, whereas for Faxonius crayfish, the hormonal concentrations of 5-HT peaked after one day of isolation but paralleled the levels of communally housed crayfish after one week of isolation. These findings underscore the complex nature of neurochemical pathways in response to social conditions, demonstrate the sensor's utility for studying 5-HT dynamics, and provide a method for monitoring temporal changes in hormonal 5-HT following social isolation. 

Methods
All statistical analyses were performed using IBM SPSS (IBM Corp.). Ipa data is represented as the mean of measurements from a sample (n=3). Samples where no 5-HT was detected (Ipa=0 µA) were excluded from statistical analysis. Statistical comparisons among social groups and species were completed using a one-wayanalysis of variance (ANOVA). Differences between species were analyzed using non-parametric Kruskal-Wallis tests, followed by Mann-Whitney U tests for pair-wise comparison. Single-variable cross-species groups were evaluated using Mann-Whitney U tests. Statistical significance was defined at p<0.05.