The Neuroprotective and Anxiolytic Effects of Magnesium Sulfate on Retinal Dopaminergic Neurons in 6-OHDA-Induced Parkinsonian Rats: A Pilot Study

Abstract

This study investigates the protective effects of magnesium sulfate on dopamine neurons in the retinas of rats with 6-hydroxydopamine (6-OHDA)-induced Parkinson's disease (PD). Rapidly progressing cognitive decline often precedes or coincides with the motor symptoms associated with PD. PD patients also frequently exhibit visual function abnormalities. However, the specific mechanisms underlying visual dysfunction in PD patients are not yet fully understood. Therefore, this study aims to investigate whether magnesium homeostasis affects dopaminergic neurons in the retina of PD rats. Thirty-six rats were divided into four groups: (1) control, (2) control with magnesium sulfate (control/MgSO₄), (3) Parkinson's disease (PD), and (4) Parkinson's disease with magnesium sulfate (PD/MgSO₄). The apomorphine-induced (APO) rotation test assessed the success of the PD models. The open-field experiment measured the rats' anxiety levels. Tyrosine hydroxylase (TH) and glutamate levels, indicators of dopamine neuron survival, were detected using immunofluorescence staining. Protein levels of solute carrier family 41 A1 (SCL41A1), magnesium transporter 1 (MagT1), and cyclin M2 (CNNM2) in the retina were analyzed using Western blot. Results showed that, compared to the PD group, rats in the PD/MgSO₄ group had improved psychological states and motor performance at two and four weeks post-surgery. The PD/MgSO₄ group also exhibited significantly higher TH fluorescence intensity in the left retinas and lower glutamate fluorescence intensity than the PD group. Additional experiments indicated that the protein levels of SLC41A1, MagT1, and CNNM2 were generally higher in the retinas of the PD/MgSO₄ group, along with an increase in retinal magnesium ion content. This suggests that magnesium sulfate may reduce glutamate levels and protect dopamine neurons in the retina. Thus, magnesium sulfate might have therapeutic potential for visual functional impairments in PD patients.

Keywords: Parkinson’s disease; magnesium ion; magnesium ion transporter; retina; tyrosine hydroxylase.

Figure 1

The APO-induced rotation test was performed 14 days and 28 days after surgery. # indicates the PD group vs. the control group, p < 0.01; ** indicates the PD/MgSO₄ group vs. the PD group at 28 days after surgery and the PD/MgSO₄ group at 28 days vs. the PD/MgSO₄ group at 14 days, p < 0.01; N = 6.

Figure 2

(A) Total distance traveled. (B) Distance traveled in the central zone. (C) Activity time in the central zone. (D) Activity traces. # indicates the Control/MgSO₄ group vs. the control group at 28 days after surgery, p < 0.05; ** indicates the PD group vs. the control group at 28 days after surgery, p < 0.01; * indicates the PD group vs. the control group and the PD/MgSO₄ group vs. the PD group at 28 days after surgery, p < 0.05; N = 6.

Figure 3

(A) TH fluorescence was observed under a laser confocal microscope (bar = 25 μm) (red: CY3, blue: DAPI). IPL: inner plexiform layer; INL: inner nuclear layer. (B) TH immunofluorescence staining intensity in rat retinal tissue. * indicates the PD group vs. the control group (left side) and the PD/MgSO₄ group vs. the PD group (right side) at 14 days after surgery and the PD group vs. the control group (left side) and the PD/MgSO₄ group vs. the PD group (left side) at 28 days after surgery, p < 0.05, N = 6.

Figure 4

(A) Glutamate fluorescence was observed under a laser confocal microscope (bar = 25 μm) (red: CY3, blue: DAPI). IPL: inner plexiform layer; INL: inner nuclear layer. (B) Glutamate immunofluorescence staining intensity in rat retinal tissue. * indicates the PD group vs. the control group (left side) and the PD/MgSO₄ group vs. the PD group (left side) at 14 days after surgery and the PD group vs. the control group (left side) at 28 days after surgery, p < 0.05; N = 6.

Figure 5

Correlation analysis of the TH fluorescence intensity with the glutamate fluorescence intensity in the rat retina at 14 days ((A), p > 0.05, N = 8) and 28 days ((B), p < 0.05, N = 8) after surgery.

Figure 6

Magnesium ion content in rat retinal tissue at 14 and 28 days after surgery was determined with a magnesium ion kit. ** indicates the left side in the PD group vs. the left side in the control group at 14 days after surgery, p < 0.01; * indicates the right side in the PD/MgSO₄ group vs. the right side in the PD group at 28 days after surgery, p < 0.05; N = 6.

Figure 7

(A) Protein expression of the magnesium ion transporters CNNM2, SLC41A1, and MagT1 in rat retina at 14 days after surgery. (B) Representative Western blot. The relative expression in the right retina of the control group was set as 100%, and the relative expression for each group was calculated as (the experimental group gray value/the internal reference gray value)/(the control group gray value/the internal reference gray value). * indicates p < 0.05; ** indicates p < 0.01; N = 6.

Figure 8

(A) Protein expression of the magnesium ion transporters CNNM2, SLC41A1, and MagT1 in rat retina at 28 days after surgery. (B) Representative Western blot. The relative expression in the right retina of the control group was set as 100%, and the relative expression for each group was calculated as (the experimental group gray value/the internal reference gray value)/(the control group gray value/the internal reference gray value). ** indicates p < 0.01; N = 6.