Immunocytochemical mapping and quantification of expression of a putative type 1 serotonin receptor in the crayfish nervous system

Serotonin is an important neurotransmitter that is involved in modulation of sensory, motor, and higher functions in many species. In the crayfish, which has been developed as a model for nervous system function for over a century, serotonin modulates several identified circuits. Although the cellular and circuit effects of serotonin have been extensively studied, little is known about the receptors that mediate these signals. Physiological data indicate that identified crustacean cells and circuits are modulated via several different serotonin receptors. We describe the detailed immunocytochemical localization of the crustacean type 1 serotonin receptor, 5‐HT1crust, throughout the crayfish nerve cord and on abdominal superficial flexor muscles. 5‐HT1crust is widely distributed in somata, including those of several identified neurons, and neuropil, suggesting both synaptic and neurohormonal roles. Individual animals show very different levels of 5‐HT1crust immunoreactivity (5‐HT1crustir) ranging from preparations with hundreds of labeled cells per ganglion to some containing only a handful of 5‐HT1crustir cells in the entire nerve cord. The interanimal variability in 5‐HT1crustir is great, but individual nerve cords show a consistent level of labeling between ganglia. Quantitative RT‐PCR shows that 5‐HT1crust mRNA levels between animals are also variable but do not directly correlate with 5‐HT1crustir levels. Although there is no correlation of 5‐HT1crust expression with gender, social status, molting or feeding, dominant animals show significantly greater variability than subordinates. Functional analysis of 5‐HT1crust in combination with this immunocytochemical map will aid further understanding of this receptor's role in the actions of serotonin on identified circuits and cells. J. Comp. Neurol. 484:261–282, 2005. © 2005 Wiley‐Liss, Inc.