Influence of Thyroid Hormones on Enzyme Activities of Myelinating Rat Central‐Nervous Tissues

Abstract

The effects of triiodothyronine administration and of hypothyroidism on the rapidly developing enzymes UDP galactose: sphingosine galactosyltransferase and 2′:3′‐nucleotide 3′‐phosphohydrolase associated with central nervous system myelination were investigated. The activity of these enzymes in the spinal cords of young rats injected daily with triiodothyronine up to sacrifice on day 5 was significantly increased over control animals. In normal animals, circulating plasma thyroxine increased gradually from the second postnatal day to a maximum value at days 15–17. Rats, born of mothers treated with n ‐propylthiouracil from the thirteenth day of pregnancy, did not exhibit the increase in plasma thyroxine. Determination of the specific activity of these enzymes in central nervous tissue of such hypothyroid rats at day 12 showed a significant reduction compared with normal animals. Intraperitoneal injection of the hypothyroid rats with triiodothyronine on day 8 resulted in a partial restoration of the activity of the enzymes in brain and of 2′:3′‐nucleotide 3′‐phosphohydrolase in spinal cord when assayed on day 12. Restoration of UDPgalactose: sphingosine galactosyltransferase activity did not occur in the spinal cord of such animals. However, when hypothyroid rats were injected with triiodothyronine on days 1, 4, and 7, the psychosine‐synthesising activity in their spinal cords on day 8 was restored to that of normal animals. This suggests that there is a critical period during the first 8 days of postnatal life when thyroid hormone levels must be adequate in order that spinal cord activity of this enzyme develops normally. It is concluded that the flux in circulating thyroid hormone is a factor in the normal development of these enzymes.