Characterization of Thyroid Hormone Effect on the Visual System of the Adult Rat

Abstract

To characterize the effect of thyroid hormone on the central nervous system in adult rats, we recorded evoked potentials by photic (VEP) and electrical stimulation in normal, thyroidectomized (Tx) and T₃- or T₄-supplemented (Tx-T₃, Tx-T₄) male adult rats under pentobarbital anesthesia. The latencies of VEPs recorded in the visual cortex (VC) and lateral geniculate nucleus (LGN), and the b-waves of the electroretinograms (ERG) of these three groups were compared. A significant prolongation of the latency of VEP by photic stimulation (flash light intensity 2 J, 0.2 Hz) in the VC was observed 14 to 28 days after thyroidectomy (27.9 ± 0.4 vs. 33.4 ± 0.43 msec,p < 0.01). Similar prolongations were also observed when those of the VEP in the LGN and the b-wave of the ERG were compared to those of normal rats (22.4 ± 0.37 vs. 27.3 ± 0.41 msec,p < 0.01; 21.8 ± 0.21 vs. 25.3 ± 0.41 msec,p < 0.01, respectively). The prolonged latencies observed in the VC, LGN, and ERG-b-wave were partially normalized 24 hr after T3 supplement (50 μg/kg, sc) and fairly restored to normal levels by 48 hr after T4 injection (100 μg/kg sc). The VEP latencies in the LGN and VC showed a positive correlation with the b-wave latency of the ERG in these three groups. When field potentials to electrical shocks (0.3 Hz, 4-fold threshold intensity, 100 μsec duration) to the optic chiasm (OC) or the LGN were recorded in the VC and LGN, or in the VC, neither the latencies recorded in the VC nor those in the LGN were significantly different in normal and Tx rats. Moreover, there was no significant difference between the conduction velocities of the optic nerves in these two groups. The autoradiographic 2-[¹⁴C]deoxyglucose method was employed to determine changes in the rate of local cerebral glucose utilization due to thyroid states. Thyroidectomy and T₄ supplement had no effect on the rates of glucose utilization in the brain structures including the visual cortex, the LGN, and other areas. These results indicate that the component in the visual system most sensitive to thyroid hormone is localized in front of the optic chiasm, probably in the retina.