Sexually dimorphic transcriptomic responses in the teleostean hypothalamus: a case study with the organochlorine pesticide dieldrin

Abstract

Organochlorine pesticides (OCPs) such as dieldrin are a persistent class of aquatic pollutants that cause adverse neurological and reproductive effects in vertebrates. In this study, female and male largemouth bass (Micropterus salmoides) (LMB) were exposed to 3mg dieldrin/kg feed in a 2 month feeding exposure (August-October) to (1) determine if the hypothalamic transcript responses to dieldrin were conserved between the sexes; (2) characterize cell signaling cascades underlying dieldrin neurotoxicity; and (3) determine whether or not co-feeding with 17β-estradiol (E(2)), a hormone with neuroprotective roles, mitigates responses in males to dieldrin. Despite also being a weak estrogen, dieldrin treatments did not elicit changes in reproductive endpoints (e.g. gonadosomatic index, vitellogenin, or plasma E(2)). Sub-network (SNEA) and gene set enrichment analysis (GSEA) revealed that neuro-hormone networks, neurotransmitter and nuclear receptor signaling, and the activin signaling network were altered by dieldrin exposure. Most striking was that the majority of cell pathways identified by the gene set enrichment were significantly increased in females while the majority of cell pathways were significantly decreased in males fed dieldrin. These data suggest that (1) there are sexually dimorphic responses in the teleost hypothalamus; (2) neurotransmitter systems are a target of dieldrin at the transcriptomics level; and (3) males co-fed dieldrin and E(2) had the fewest numbers of genes and cell pathways altered in the hypothalamus, suggesting that E(2) may mitigate the effects of dieldrin in the central nervous system.

Fig. 1

Dieldrin in control and experimental feeds (dieldrin alone and dieldrin + E₂) (upper graph) and concentration of dieldrin in muscle samples after 2 month feeding regime (lower graph). Dieldrin was undetectable in muscle tissue from control animals.

Fig. 2

(A) Gonadosomatic index of control and treated female and male LMB (control females, n = 10; dieldrin-treated females, n = 7; control males, n = 4; dieldrin-treated males, n = 10; dieldrin + E₂ treated males, n = 4). There were animals in each group in which GSI could not be calculated because the gonad was too small to quantify accurately. (B) Plasma vitellogenin in female and male LMB (control females, n = 8; dieldrin-treated females, n = 6; control males, n = 9; dieldrin-treated males, n = 10; dieldrin + E₂ treated males, n = 4). (C) Plasma levels of 17β-estradiol (E₂) (log 10 scale) in female and male LMB (control females, n = 5; dieldrin-treated females, n = 6; control males, n = 8; dieldrin-treated males, n = 7; dieldrin + E₂ treated males, n = 4). Horizontal line in box plots is the median, the boundaries of the box represent the 25th and 75th percentiles (boundaries of the box) and the minimum and maximum data points are represented by the whiskers. Different letters denote significant differences among groups (p < 0.05).

Fig. 3

Heat map for control and dieldrin-treated LMB using hierarchical clustering. Each row (i.e. gene probe) was first centered to a mean of zero (0) across microarrays prior to analysis, followed by further scaling of the rows to a variance of one (1) prior to clustering. Red intensity means an increase in relative expression while blue means a decrease in relative expression.

Fig. 4

Pathway analysis of significantly changing expression sub-networks in LMB hypothalamus. The networks shown in the pathway are dopamine receptors DRD1 and DRD2, GABA_A receptor, AR, and retinoic acid receptor expression targets. These signaling networks will have a significant impact on neurotransmitter systems that include GABA, catecholamines, and glutamate. Abbreviations are provided in the abbreviation list.

Fig. 5

Pathway analysis for the expression network of nuclear receptor subfamily 5, group A, member 1 and 2 in males fed dieldrin and E₂. This network was unique to this group. The majority of the genes found within the cell are increased in expression. Abbreviations are provided in the abbreviation list. Legend follows that provided in Fig. 4.

Fig. 6

Sub-network for activin signaling in the hypothalamus after males were fed dieldrin. However, this network was significantly reduced in all three treatment groups (~20–30%), suggesting it is affected by dieldrin. This network has the potential to also affect steroid production in the neuroendocrine brain and is involved in calcium signaling and response to reactive oxygen species. Abbreviations are provided in the abbreviation list. Legend follows that provided in Fig. 4.