Overview
Colonies of the red harvester ant, Pogonomyrmex barbatus, regulate foraging activity based on food availability and local conditions. Colony variation in foraging behavior is thought to be linked to biogenic amine signaling and metabolism. Measurements of differences in neurotransmitters have not been made among ant colonies in a natural environment. Here, for the first time, we quantified tissue content of 4 biogenic amines (dopamine, serotonin, octopamine, and tyramine) in single forager brains from 9 red harvester ant colonies collected in the field. Capillary electrophoresis coupled with fast-scan cyclic voltammetry (CE-FSCV) was used to separate and detect the amines in individual ant brains. Low levels of biogenic amines were detected using field-amplified sample stacking by preparing a single brain tissue sample in acetonitrile and perchloric acid. The method provides low detection limits: 1 nM for dopamine, 2 nM for serotonin, 5 nM for octopamine, and 4 nM for tyramine. Overall, the content of dopamine (47 ± 9 pg/brain) was highest, followed by octopamine (36 ± 10 pg/brain), serotonin (20 ± 4 pg/brain), and tyramine (14 ± 3 pg/brain). Relative standard deviations were high, but there was less variation within a colony than among colonies, so the neurotransmitter content of each colony might change with environmental conditions. This study demonstrates that CE-FSCV is a useful method for investigating natural variation in neurotransmitter content in single ant brains and could be useful for future studies correlating tissue content with colony behavior such as foraging.
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Methods and contributions
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Findings and contributions
- The method achieved detection limits of 1 nM dopamine, 2 nM serotonin, 5 nM octopamine, and 4 nM tyramine.
- Dopamine was the most abundant amine per forager brain, followed by octopamine, serotonin, and tyramine.
- Variation in neurotransmitter content was larger among colonies than within colonies.
- Colony dopamine did not correlate with the other amines, while octopamine correlated strongly with serotonin and tyramine.
- Colonies with higher brain dopamine showed less foraging increase after exogenous dopamine, though the effect was largely driven by one colony (D24).
Methods
- Field-collected P. barbatus foragers from 9 colonies near Rodeo, NM — Foragers were collected into liquid nitrogen at a long-term study site and single brains dissected for analysis (n = 53 brains).
- Capillary electrophoresis with fast-scan cyclic voltammetry (CE-FSCV) — An in-house CE-FSCV system with a carbon-fiber microelectrode separated and detected four biogenic amines in individual ant brains.
- Field-amplified sample stacking of single-brain homogenates — Each brain was homogenized in 5 μl acetonitrile/perchloric acid so low-conductivity stacking could preconcentrate trace amines.
- ANOVA, Pearson correlations, and RSD comparisons in GraphPad Prism — Tested colony effects on amine content, inter-amine correlations, and within- vs. among-colony relative standard deviations.
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