Serotonin, a biogenic amine known to be a neuromodulator of insect behavior, has recently been associated with age-related patterns of task performance in the ant Pheidole dentata. We identified worker age- and subcaste-related patterns of serotonergic activity within the optic lobes of the P. dentata brain to further examine its relationship to polyethism. We found strong immunoreactivity in the optic lobes of the brains of both minor and major workers. Serotonergic cell bodies in the optic lobes increased significantly in number as major and minor workers matured. Old major workers had greater numbers of serotonergic cell bodies than minors of a similar age. This age-related increase in serotonergic immunoreactivity, as well as the presence of diffuse serotonin networks in the mushroom bodies, antennal lobes, and central complex, occurs concomitantly with an increase in the size of worker task repertoires. Our results suggest that serotonin is associated with the development of the visual system, enabling the detection of task-related stimuli outside the nest, thus playing a significant role in worker behavioral development and colony-wide division of labor.
To gain basic understanding of the reproductive and developmental effects of endocrine disrupting chemicals in invertebrates, we have used C. elegans as an animal model. The completion of the C. elegans genome sequence brings to bear microarray analysis as a tool for these studies. We previously showed that the C. elegans genome was responsive to vertebrate steroid hormones, and changes in gene expression of traditional biomarkers used in environmental studies were detected; i.e., vitellogenin (vtg), cytochrome P450 (cyp450), glutathione-S-transferase (gst) and heat shock proteins (hsp). The data were interpreted to suggest that exogenous lipophilic compounds can be metabolized via cytochrome P450 proteins, and that the resulting metabolites can bind to members of the Nuclear Receptor (NR) class of proteins and regulate gene expression. In the present study, using DNA microarrays, we examined the pattern of gene expression after progesterone (10(-5), 10(-7) M), estradiol (10(-5) M), cholesterol (10(-9) M) and cadmium (0.1, 1 and 10 μM) exposure, with special attention to the members of NRs. Of approximately 284 NRs in C. elegans, expression of 25 NR genes (representing 9% of the total NRs in C. elegans) was altered after exposure to steroids. Of note, each steroid activated or inhibited different subsets of NR genes, and only estradiol regulated NR genes implicated in neurogenesis. These results suggest that NRs respond to a variety of exogenous steroids, which regulate important metabolic and developmental pathways. The response of the C. elegans genome to cholesterol and cadmium was analyzed in more detail. Cholesterol is a probable precursor to signaling molecules that may interact with NRs and we focused on expression of genes related to lipid metabolism (cyp450), transport and storage (i.e., vitellogenin). Worms exposed to cadmium respond principally by activating the expression of genes encoding stress-responsive proteins, such as mtl-2 and cdr-1, and no significant changes in expression of NRs or vtg genes were observed. The possible implications of these results with regard to the evolution of steroid receptors, endocrine disruption and the role of vitellogenin as a lipid transporter are discussed.
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