Effect of mild and chronic neonatal hypothyroidism on sensory information processing in a rodent model: A behavioral and electrophysiological study

Thyroid hormones are essential for neonatal brain development. It is well established that absence of thyroid hormones during critical periods of development can alter sensory functions such as visual and auditory processing, but there are few studies on rat somatosensory system development at mild, long-term, and irreversible neonatal hypothyroidism. Thus, the current study was conducted to investigate whether chronic thyroid hormone insufficiency would alter different cognitive aspects of tactile information processing and sensory motor filtering at behavioral or neuronal levels. Neonatal Wistar male rats were exposed to 0 and 6 ppm of propylthiouracil for 150 days. Behavioral tests including tactile discrimination tests and acoustic startle reflex test were performed. Using extracellular single unit recording technique, barrel cortex neurons' excitatory and inhibitory responses to controlled displacement of whiskers were evaluated. Results indicated that percentage of correct choice in tactile learning and discrimination of a new texture decreased in hypothyroid group compared to the control group (P < 0.05). In addition, acoustic startle reflex of hypothyroid group significantly decreased compared to the control rats when the prepulse intensity was 71 dB (P < 0.05). Data obtained from electrophysiological tests showed that spontaneous activity and response magnitude of barrel cortex neurons decreased in hypothyroid group compared to the control group (P < 0.05). It is concluded that, thyroid hormones can regulate tactile and auditory sensory processing in male rats, and mild and long-term absence of these hormones can result in deficiency in natural functions of these sensory systems.