Gamma-aminobutyric acid (GABA), released by GABAergic neurons, is an abundant inhibitory neurotransmitter in the central nervous system. GABAergic neuronal activity is strongly associated with reproductive cyclicity in mammals. However, the consequences of GABA action on GnRH neurons, the key reproductive regulator in hypothalamus and pituitary, remain controversial.

A recent study carried out by Prof. HU Wei’s group at Institute of Hydrobiology (IHB) of Chinese Academy of Sciences explored the organization of GABAergic neurons in zebrafish, and found that GABA executed ambiequal modulatory effects on Gonadotropin-releasing hormone 3 (GnRH3) neurons. This study was published in the journal of Endocrinology.

In order to reveal the relationship between the GABAergic neurons and GnRH3 neurons in zebrafish, a transgenic fish line was established in which GABAergic neurons were labeled with red fluorescence (mCherry) and GnRH3 neurons were labeled with green fluorescence (EGFP).

It showed that GABAergic neurons were widely distributed in forebrain, midbrain, hindbrain, retina and spinal cord, while GnRH3 neurons originated in the olfactory bulb and migrated along the ventral telencephalon into the hypothalamus. GnRH3 neurons were contacted by GABAergic neurons directly in the telencephalon and hypothalamus. A subpopulation of GnRH3 neurons in telencephalon was also GABAergic.

Pituitary is the regulative centre connecting brain and gonad, in which gonadotropin is synthesized and released into the circulation system. In pituitary, a mass of GABAergic cell bodies and GnRH3 nerve fibers are identified. Both in the dorsal and medial parts, GnRH3 fibers are in contact with GABAergic cells. Moreover, contacts between GnRH3 fibers and GABAergic cells are also evident in the proximal pars distalis (PPD) and pars intermedia (PI) where gonadotrophs (LH/FSH cells) are located.

To elucidate the role of GABA in regulating GnRH and gonadotropin, GABA receptor agonist (baclofen) and antagonist (CGP55845) were administrated to larval and adult fish. GnRH3 neuron migration and gnrh3 expression was inhibited in fish treated with baclofen while enhanced with CGP55845.

It indicated that GABA receptor signaling pathway inhibits GnRH3 neurons activity in larval fish. However, gnrh3 expression was stimulated by baclofen while inhibited by CGP55845 in three months adult fish. It indicated GABA receptor signaling positively regulated GnRH3 neurons in adult fish. Gonadotropin (LH/FSH) expression was also increased when treated with baclofen, but the effect was delayed several hours after gnrh3 increased. It was suggested GABA regulate gonadotropin via GnRH3 neurons indirectly.

This study revealed the distribution patterns of GABAerigc neurons in zebrafish, focusing on the relationship between GABAergic neurons and GnRH3 neurons. It demonstrated that GABAergic neurons inhibited GnRH3 neurons in larval stage while stimulated GnRH3 neruons in adult stage. It is the first experimental evidence that GABA shows both inhibitory and stimulating effects on GnRH neurons in teleost fish.

This work was supported by National Natural Science Foundation and Chinese Academy of Sciences. It was also supported by the Natural Sciences and Engineering Research Council of Canada and the University of Ottawa International Research Acceleration Program.