Bioamine disruption leads to memory deficits in the whip spider, Phrynus marginemaculatus

Amblypygi (whip spiders) are an order of arthropods (Subphylum: Chelicerata, Class: Arachnida) that navigate primarily by their keen olfactory abilities. Experiments in the laboratory reveal that the species Phrynus marginemaculatus forms both short and long-term memories for odors associated with access to a shelter. While bioamine function and location in the whip spider nervous system remain completely unexplored, comparative anatomy indicates that their olfactory memory may involve serotonin and dopamine signaling in brain regions involved in processing olfactory input. Preliminary immunohistochemistry detected serotonin and tyrosine hydroxylase immunoreactivity in mushroom body-associated calycal regions and antenniform primary olfactory glomeruli, supporting future work on monoaminergic modulation of olfactory circuits. Therefore, if serotonin or dopamine signaling involved in olfactory processing is pharmacologically disrupted, their performance in an associative olfactory learning task should be significantly impaired. Subjects were trained on an olfactory memory paradigm, subsequently injected with physiological saline, the serotonin receptor antagonist methiothepin mesylate (MET), or the dopamine receptor antagonist SCH-23390 (SCH), and tested for memory retention 24 h afterwards. Controls injected with saline demonstrated robust associative memory (serotonin antagonism control: n = 10, p = 0.006 against chance performance on the task, dopamine antagonism control: n = 10, p = 0.039), while treated groups displayed no such memory (MET: n = 10, p = 0.375, SCH: n = 10, p = 0.892). There was also a significant difference in performance between the treated and control groups on test day (serotonin antagonism groups: p = 0.048, dopamine antagonism groups: p = 0.016). Additionally, there were no significant differences in locomotor activity between treatment and control groups on test day (p > 0.05). Together, these results are consistent with serotonergic and dopaminergic signaling contributing to olfactory memory consolidation without grossly impairing locomotion in Phrynus marginemaculatus.