Targeted inhibition of electrical activity alters axonal pathfinding of gonadotropin-releasing hormone-3 neurons located in the terminal nerve of embryonic transgenic zebrafish (Danio rerio)
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Society of Neuroscience
There are multiple populations of gonadotropin releasing hormone (GnRH) neurons in all vertebrates studied to date. Based on their neuroanatomy, physiology, and expression of different GnRH genes, it appears that these different populations perform different functions. In this study, we examined the neurophysiological development of a population of GnRH neurons located near the terminal nerve (TN), and associated with the olfactory bulb. We generated stable transgenic zebrafish lines with the GnRH3 promoter driving expression of fluorescent proteins to examine the relationship between morphological and electrophysiological changes that occur in these neurons during an early period of embryonic development (24 h - 80 h post fertilization).
Time-lapse confocal microscopy of live zebrafish embryos revealed dynamic changes in axonal projections of TN-GnRH3 neurons starting at 24 hpf. Between 24-34 hpf axonal projections extended caudally away from the soma located in the olfactory region, following which they began looping towards the dorsal and rostral surfaces along with increased branching of processes by 48 hpf. The number of neurons within the population more than doubled between 24 hpf and 72 hpf (3.25± 0.7 neurons vs. 8.75 ± 0.8 neurons). TN-GnRH3 neurons are located close to the anterior surface of the zebrafish embryo allowing access for electrophysiological analysis in the living intact embryo. We examined the physiology of the TN-GnRH3 neurons using single-cell loose-patch electrophysiology on 2 day and 3 day postfertilization (dpf) embryos. There were significantly more electrically active TN-GnRH neurons on 3 dpf (81% of 26 neurons) than on 2 dpf (52% of 29 neurons; P < 0.05). The frequency of tonic firing of neurons was significantly increased from 2 dpf to 3 dpf (0.2± 0.04 Hz vs. 0.68 ± 0.12 Hz; P < 0.001). Targeted misexpression of an inward rectifying K+ channel (hKir2.1) in GnRH3 neurons suppressed electrical activity and led to dramatic changes in axonal pathfinding of TN-GnRH3 neurons during embryonic development. We show that the acquisition of a stable adult-like electrical activity in TN-GnRH3 neurons is important in regulating their development and appropriate projection to the olfactory bulb.
S. Ramakrishnan, N.L. Wayne, S. Navarre, D.J. Kozlowksi. Targeted inhibition of electrical activity alters axonal pathfinding of gonadotropin-releasing hormone-3 neurons located in the terminal nerve of embryonic transgenic zebrafish (Danio rerio). Program No. 186.19. 2009 Neuroscience Meeting Planner. Chicago, IL: Society for Neuroscience, 2009. Online.