High concentration of vitamin E decreases thermosensation and thermotaxis learning and the underlying mechanisms in the nematode Caenorhabditis elegans

Abstract

α-tocopherol is a powerful liposoluble antioxidant and the most abundant isoform of vitamin E in the body. Under normal physiological conditions, adverse effects of relatively high concentration of vitamin E on organisms and the underlying mechanisms are still largely unclear. In the present study, we used the nematode Caenorhabditis elegans as an in vivo assay system to investigate the possible adverse effects of high concentration of vitamin E on thermosensation and thermotaxis learning and the underlying mechanisms. Our data show that treatment with 100-200 µg/mL of vitamin E did not noticeably influence both thermosensation and thermotaxis learning; however, treatment with 400 µg/mL of vitamin E altered both thermosensation and thermotaxis learning. The observed decrease in thermotaxis learning in 400 µg/mL of vitamin E treated nematodes might be partially due to the moderate but significant deficits in thermosensation, but not due to deficits in locomotion behavior or perception to food and starvation. Treatment with 400 µg/mL of vitamin E did not noticeably influence the morphology of GABAergic neurons, but significantly decreased fluorescent intensities of the cell bodies in AFD sensory neurons and AIY interneurons, required for thermosensation and thermotaxis learning control. Treatment with 400 µg/mL of vitamin E affected presynaptic function of neurons, but had no remarkable effects on postsynaptic function. Moreover, promotion of synaptic transmission by activating PKC-1 effectively retrieved deficits in both thermosensation and thermotaxis learning induced by 400 µg/mL of vitamin E. Therefore, relatively high concentrations of vitamin E administration may cause adverse effects on thermosensation and thermotaxis learning by inducing damage on the development of specific neurons and presynaptic function under normal physiological conditions in C. elegans.

Figure 1. Effects of vitamin E treatment at different concentrations on thermotaxis learning behavior as monitored by 25/20°C or 17/20°C thermotaxis assays in C. elegans.

IT, isothermotracking. Data are expressed as mean ± SEM. **p<0.01 vs. N2.

Figure 2. Effects of vitamin E treatment at different concentrations on thermotaxis and locomotion behaviors in C. elegans.

(A) Effects of vitamin E treatment at different concentrations on thermotaxis behavior. In the thermotaxis assay system, movement to 25°C was scored as thermophilic (T); movement to 17°C was scored as cryophilic (C); movement across the thermal gradient (17°C/25°C) was scored as athermotactic (A); movement at 20°C was scored as isothermal tracking behavior (IT). (B) Effects of vitamin E treatment at different concentrations on body bends of nematodes inside and outside learning assay model. (C) Effects of vitamin E treatment at different concentrations on basic movements of the examined nematodes. (D) Effects of vitamin E treatment at different concentrations on basal and enhanced slowing responses of the examined nematodes. Data are expressed as mean ± SEM. **p<0.01 vs. N2.

Figure 3. Effects of vitamin E treatment at different concentrations on development of GABAergic motor neurons in C. elegans.

(A) Effects of vitamin E treatment on morphology of GABAergic motor neurons. The heads of nematodes were at the left of the images. (B) Effects of vitamin E treatment on neuronal loss of GABAergic motor neurons. (C) Effects of vitamin E treatment on dorsal/ventral cord gaps of GABAergic motor neurons. Data are expressed as mean ± SEM.

Figure 4. Effects of vitamin E treatment at different concentrations on development of AFD sensory neurons and AIY interneurons in C. elegans.

(A) Effects of vitamin E treatment on morphology of AFD sensory neurons. (B) Effects of vitamin E treatment on fluorescent intensities of cell bodies in AFD sensory neurons. (C) Effects of vitamin E treatment on morphology of AIY interneurons. (D) Effects of vitamin E treatment on fluorescent intensities of cell bodies in AIY interneurons. L, left, R, right. Data are expressed as mean ± SEM. **p<0.01 vs. N2.

Figure 5. Effects of vitamin E treatment at different concentrations on synaptic neurotransmission in C. elegans.

(A) Effects of vitamin E treatment on presynaptic function as evaluated by aldicarb resistance. (B) Effects of vitamin E treatment on postsynaptic function as evaluated by levamisole resistance. Data are expressed as mean ± SEM. **p<0.01 vs. control.

Figure 6. Promotion of synaptic transmission effectively retrieved deficits in learning and thermotaxis behaviors in nematodes treated with 400 µg/mL of vitamin E.

(A) Promotion of synaptic transmission effectively retrieved deficits in thermotaxis learning behavior in nematodes treated with 400 µg/mL of vitamin E as monitored by 25/20°C thermotaxis assay. (B) Promotion of synaptic transmission effectively retrieved deficits in thermotaxis learning behavior in nematodes treated with 400 µg/mL of vitamin E as monitored by 17/20°C thermotaxis assay. (C) Promotion of synaptic transmission effectively retrieved deficits in thermotaxis in nematodes treated with 400 µg/mL of vitamin E. In the thermotaxis assay system, movement to 25°C was scored as thermophilic (T); movement to 17°C was scored as cryophilic (C); movement across the thermal gradient (17°C/25°C) was scored as athermotactic (A); movement at 20°C was scored as isothermal tracking behavior (IT). Data are expressed as mean ± SEM. **p<0.01 vs. N2.