GABA action and sex steroid feedback in the goldfish (Carassius auratus) neuroendocrine brain

The inhibitory amino acid neurotransmitter gamma-aminobutyric acid (GABA) has a significant role in vertebrate reproduction. In teleost fish, GABA will stimulate the release of LH from the pituitary during certain periods of gonadal development. This thesis characterizes the mRNA transcript distribution of the enzymes that are responsible for the synthesis (glutamic acid decarboxylase) and degradation (GABA-transaminase) of GABA in the brain of the goldfish and shows that regions containing estrogen receptors and reproductive neuropeptides also contain high amounts of GABA-related transcripts. I show that GABAergic synaptic transmission may be modulated by GABA via genomic effects on GABA receptor subunits. In order to better understand GABA stimulated LH release in fish, I studied GABA agonist action on gene expression. This experiment showed that GABA agonists modulate GABA synthesis and degradation differentially. I also show that a rapid and pronounced increase in activin betaalpha transcription in neuroendocrine tissues via GABAB receptors and GABAA mediated changes in tyrosine hydroxylase mRNA are putative mechanisms by which GABA mediates pituitary LH release in fish. Lastly, I outline the development of a goldfish brain enriched cDNA microarray to study estrogen feedback in the neuroendocrine brain. Male goldfish exposed to the xenoestrogen ethinylestradiol (EE2) had significantly reduced gonads and circulating sex steroids. Microarray and real-time RT-PCR showed that brain aromatase, secretogranin-III, and interferon-related developmental regulator 1 (IFDR-1) were altered after EE2 treatment. This thesis (1) further characterizes the GABAergic system in goldfish using different approaches to gain a better understanding of the role of GABA in neuroendocrine function in fish; (2) identifies evolutionary conserved patterns of gene expression changes after GABA and estrogenic treatment in fish; and (3) demonstrates that the goldfish microarray is a useful tool to study the effects of endocrine disrupting chemicals on neuroendocrine function