EFFECT OF THYROXINE ON THE DEVELOPMENT OF SOMATOSENSORY AND VISUAL-EVOKED POTENTIALS IN THE RAT

The thyroid hormone thyroxine (T4), administered post-natally to neona tal rats, has been shown to accelerate development of auditory functio n, as expressed by auditory nerve-brainstem evoked responses. This stu dy investigated whether this earlier development was also reflected in other sensory modalities. Rat pups were injected with T4 from the day of birth for 10 consecutive days. Somatosensory evoked potentials, bo th from the cortex and from sub-cortical structures, and flash-elicite d visual evoked potentials (VEP), were recorded at various ages up to 3 months. The recordings were compared with those from control rats fr om the same litters. Only a minimal difference was found between the e xperimental and control groups, the most significant being in the VEP at age 12 days, by which time the eyes of most of the experimental rat s had opened, which was not the case for the majority of control rats. This difference disappeared with eye-opening in the control rats. Alt hough T4 is known to affect myelinization and synaptic transmission in developing rat brain, this apparently only minimally affects the func tioning of the brain as expressed by evoked potentials, both in the sh ort and long term. The main effect of neonatal hyperthyroidism in thes e rats appeared to be accelerated development of the end organ (the ey e and the ear).