3.2. Dopaminergic Degeneration Minds the Duration of Exposure to
6-OHDA
The first week of zebrafish development is rapid and vital that every
day counts for critical steps in forming a functional organism [46].
To assess the impact of age and treatment duration on the extent of
developmental changes caused by 6-OHDA treatment, we repeated the
experiments using 7dpf larvae. We exposed 2dpfpanx1a+/+ andpanx1a-/- larvae to 250µM 6-OHDA for either 72
or 120hrs and tested their performance at 7dpf (Fig. 3A and 3B). All
p-values were presented in Table 2. A 72hrs treatment significantly
reduced the RD and TBF of 7dpf panx1a+/+ larvae
by 63% and 40%, respectively. The results were consistent with the
data obtained for 5dpf larvae in section 3.1. The electric-induced RD
and TBF of 7dpf panx1a-/- larvae were unchanged
upon 72hrs treatment with 6-OHDA, like the behavioral response of 5dpf
mutants.
Extending the treatment from 72 to 120hrs had no effect on 7dpfpanx1a+/+ larvae, suggesting that both
exposure times led to the same and significant levels of motor
deficiency and cell death of dopaminergic cells. The results suggested
that panx1a+/+ larvae showed the greatest
sensitivity to the neurotoxin within the first 5 days of development. In
contrast to panx1a+/+ larvae, 7dpfpanx1a-/- larvae RD (24%) and TBF (52%)
declined when the 6-OHDA exposure time was extended from 72 to 120 hour.
This result showed that the duration of exposure of mutants to the
neurotoxin dictated the behavioral outcomes. We concluded that the
deregulation of the dopaminergic pathway inpanx1a-/- larvae shown previously[40] is
the probable cause of the 6-OHDA susceptibility at 7dpf. This idea was
tested using the rate-limiting enzyme TH as a surrogate for changes to
dopaminergic signaling. A significant reduction in TH expression levels
was observed after toxin treatments for both genotypes which was aligned
with the behavioral outcomes (Fig. 3C). We concluded that the absence of
functional Panx1a channels compromised dopaminergic signaling in 6-OHDA
treated zebrafish larvae following a reduction in the expression of the
critical rate-limiting enzyme TH.