Effects of neurotrophins on synaptic protein expression in the visual cortex of dark-reared rats

Abstract

Total lack of visual experience [dark rearing (DR)] is known to prolong the critical period and delay development of sensory functions in mammalian visual cortex. Recent results show that neurotrophins (NTs) counteract the effects of DR on functional properties of visual cortical cells and exert a strong control on critical period duration. NTs are known to modulate the development and synaptic efficacy of neurotransmitter systems that are affected by DR. However, it is still unknown whether the actions of NTs in dark-reared animals involve interaction with neurotransmitter systems. We have studied the effects of DR on the expression of key molecules in the glutamatergic and GABAergic systems in control and NT-treated animals. We have found that DR reduced the expression of the NMDA receptor 2A subunit and its associated protein PSD-95 (postsynaptic density-95), of GRIP (AMPA glutamate receptor interacting protein), and of the biosynthetic enzyme GAD (glutamic acid decarboxylase). Returning dark-reared animals to light for 2 hr restored normal expression of the above-mentioned proteins almost completely. NT treatment specifically counteracts DR effects; NGF acts primarily on the NMDA system, whereas BDNF acts primarily on the GABAergic system. Finally, the action of NT4 seems to involve both excitatory and inhibitory systems. These data demonstrate that different NTs counteract DR effects by modulating the expression of key molecules of the excitatory and inhibitory neurotransmitter systems.

Fig. 1.

DR reduces the expression of synaptic proteins in the glutamatergic and GABAergic system, and visual experience rapidly restores the glutamatergic system. a–d, Quantitative immunoblotting was performed on protein extracts prepared from visual cortices of LR rats (n = 12), DR rats (n = 11), or DR rats exposed to light for 2 hr (n = 3) for NR2A (a), PSD-95 (b), GRIP (c), and GAD (d) (asterisks indicate significant difference vs LR; t test; p < 0.05). Data are presented as percentage of OD values measured in LR rats (mean ± SEM). Arrows indicate bands relative to the proteins of interest; arrowheads indicate G-6-PDH.

Fig. 2.

NGF, BDNF, and NT4 differentially rescue DR effects on the glutamatergic and GABAergic system. Protein extracts prepared from visual cortices of DR rats and infused with NGF or with BDNF, NT4, or CYT from P23 to P30 were immunoblotted for NR2A (a), PSD-95 (b), GRIP (c), and GAD (d). Data concerning synaptic protein expression levels in LR and DR animals were the same as shown in Figure 1 (one-way ANOVA; p < 0.001; asterisks indicate significant difference vs LR rats in Tukey test; p < 0.05). Data are presented as percentage of OD values measured in LR rats (mean ± SEM). Arrows indicate bands relative to the proteins of interest; arrowheads indicate G-6-PDH.