Chronic alterations in serotonin function: dynamic neurochemical properties in agonistic behavior of the crayfish, Orconectes rusticus

Abstract

The biogenic amine serotonin [5-hydroxytryptamine (5-HT)] has received considerable attention for its role in behavioral phenomena throughout a broad range of invertebrate and vertebrate taxa. Acute 5-HT infusion decreases the likelihood of crayfish to retreat from dominant opponents. The present study reports the biochemical and behavioral effects resulting from chronic treatment with 5-HT-modifying compounds delivered for up to 5 weeks via silastic tube implants. High performance liquid chromatography with electrochemical detection (HPLC-ED) confirmed that 5,7-dihydroxytryptamine (5,7-DHT) effectively reduced 5-HT in all central nervous system (CNS) areas, except brain, while a concurrent accumulation of the compound was observed in all tissues analyzed. Unexpectedly, two different rates of chronic 5-HT treatment did not increase levels of the amine in the CNS. Behaviorally, 5,7-DHT treated crayfish exhibited no significant differences in measures of aggression. Although treatment with 5-HT did not elevate 5-HT content in the CNS, infusion at a slow rate caused animals to escalate more quickly while 5-HT treatment at a faster rate resulted in slower escalation. 5,7-DHT is commonly used in behavioral pharmacology and the present findings suggest its biochemical properties should be more thoroughly examined. Moreover, the apparent presence of powerful compensatory mechanisms indicates our need to adopt an increasingly dynamic view of the serotonergic bases of behavior like crayfish aggression.

Figure 1

Graph of 5-HT content (means ± standard error) for different experimental groups plotted on log-scale: (a) SUP, (b) SEG/CEG, (c) T1–T5, and (d) A1–A6. Significant Tukey-Kramer HSD tests are indicated with asterisks. In the 5,7-DHT treated group, all tissues except SUP were significantly lower than controls.

Figure 2

Scatter plots depict the amount of 5-HT (pg) as a function of treatment duration (days). Data points represent individual tissues with treatment groups arranged in columns and tissues arranged in rows.

Figure 3

Sample chromatograms for (a) an external standard, T1–T5 tissue from (b) a control animal, and individuals treated with (c) 5,7-DHT and (d) 5-HT “slow.” Substances are identified based on elution times across a range of mobile phase conditions: (DA) 116–120 s, (5,7-DHT) 198–202 s, and (5-HT) 290–294 s. Note the presence of a conspicuous peak (a likely 5-HT metabolite) that appears soon after the elution of DA.

Figure 4

Measures of aggression were largely unchanged by experimental treatments. (a) Total number of interactions observed is plotted on the ordinate: (5,7-DHT) n = 100 in eight trials, (control) n = 63 in five trials, (5-HT “slow”) n = 26 in four trials, and (5-HT “fast”) n = 65 in five trials. The number of fights reaching a particular intensity is represented with different shades. Intensities were pooled to reduce empty cells in statistical analyses: (No contest) intensity 0, (low intensities) intensity 1 or 2, and (high intensities) intensity 3 or 4. (b) Average duration of interactions for experimental groups.

Figure 5

The slope (i.e., regression coefficient) of each regression line represents the average rate of escalation and provides an estimate for the maximum intensity of an agonistic encounter given a certain duration. (a) The linear regression functions for each experimental group were: (5,7-DHT) y = 0.843 + 0.017 x, (control) y = 0.668 + 0.018x, (5-HT “slow”) y = 0.334 + 0.031x, and (5-HT “fast”) y = 0.224 + 0.008x. (b) Graph of regression coefficients of the linear function for experimental groups and their respective 95% confidence intervals.