EFFECTS OF THYROID HORMONE ON THE LATENCY OF EVOKED POTENTIAL AND ELECTRORETINOGRAM IN ADULT RATS

Abstract

To analyze effects of thyroid hormone on the central nervous system, we recorded visual evoked potentials (VEPs) by photic and electrical stimulations in normal, thyroidectomized (Tx) and T3 or T4-supplemented (Tx-T3, T4) adult rats under pentobarbital anesthesia. The latencies of VEPs recorded in the visual cortex (VC) and lateral geniculate nucleus (LGN) and b-wave of electroretinogram (ERG) were compared in these three groups. A significant prolongation of latency in the visual system of Tx rats 14 to 28 days after operation was observed compared with that in intact control rats. The prolonged VEP latencies in the LGN and the VC showed a positive correlation with the prolonged b-wave latency of ERG between the three groups. These prolonged latencies observed in the VC, the LGN and ERG were partially normalized 24 hours after T3 supplement (50μg/kg, sc) and completely restored to normal levels by 48 hours after T4 injection (100μg/kg, sc). There was a positive correlation between recovered latencies to observed in the LGN and VC and those in b-wave of ERG. When electrical stimulation was delivered to the optic chiasma or the LGN, and evoked potential were recorded in the VC and LGN, neither latencies recorded in the VC nor those in the LGN were significantly different in intact and Tx rats. Moreover, there was no significant difference between optic nerve conduction velocities in these two groups.
These results suggest that the prolongation of latency of VEPs observed in hypothyroid rats are mainly attributed to that occurring in the retina, indicating the essential role of thyroid hormone in maintaining retinal function in adult animals.