Identification and functional evidence of GABAergic neurons in parts of the brain of adult zebrafish (Danio rerio).

The distribution of GABA-containing neurons was studied in the brain of the adult zebrafish by Nissl staining and immunohistochemistry. GABA immunoreactivity (GABA-IR) was demonstrated in parts of the brain such as olfactory bulb (OB), telencephalon, tectum stratum, and in the hypothalamus. GABA-IR appeared in the area where Nissl-stained cell bodies were abundant. The internal cellular layer of the OB was most densely stained by Nissl staining, and also showed a high level of GABA-IR. The telencephalon and the hypothalamus revealed a similar pattern to the OB in terms of Nissl staining and GABA-IR. However, the distribution and shape of stained cells of the tectum stratum were distinct from those in other regions: Nissl-stained neurons were ubiquitously present throughout all cellular layers including the stratum griseum centrale, the stratum album centrale (SAC), and the stratum periventriculare (SP). However, GABA-IR was weakly expressed in a limited number of neurons only in the SAC and SP. Whether GABA serves as an inhibitory neurotransmitter was also tested in the isolated telencephalon preparation by using extracellular field potential recordings. The synaptic activity recorded in the posterior dorsal telencephalon in response to the electrical stimulation of the anterior dorsal telencephalon was increased in the presence of the GABAA receptor antagonist, BMI, suggesting an inhibitory role for GABA-immunoreactive neurons in the adult brain of the zebrafish.