Thyroid hormones are essential for the development and function of the brain and 
also for the maturation and repair of the peripheral nervous system. In the 
brain, most of the 3,5,3'-triiodothyronine is locally produced by 
5'-deiodination of thyroxine catalyzed by the type 2 deiodinase. The absence of 
any information about thyroid hormone metabolism in the peripheral nervous 
system prompted us to study the expression of type 2 deiodinase (mRNA and 
activity) in the peripheral nervous system. Expression of type 2 deiodinase mRNA 
was very low in the sciatic nerve of rats until day 5 after birth, then 
increased from day 10 to 35-45 and gradually decreased afterwards, down to the 
low basal levels observed in the adult. A lesion of the sciatic nerve in the 
adult induced an increase in type 2 deiodinase mRNA and activity. After a 
cryolesion, the stimulation was observed as early as 4 h and mRNA levels 
increased until 24-48 h, then gradually declined down to basal levels around 28 
days, when regeneration and functional recovery were completed. After a 
permanent transection, up-regulation of type 2 deiodinase persisted in both 
proximal and distal segments until the end of the experiment (28 days). 
Transection and cryolesion were also followed by increased type 2 deiodinase 
mRNA expression in the ipsilateral L4/L6 dorsal root ganglia within 24 h. Both 
mRNA and activity were found in the peripheral nerve sheaths but not in the 
internal compartment of the intact or injured nerve. Cultured fibroblasts from 
the sciatic nerve expressed type 2 deiodinase 4 h after stimulation by 10 microM 
forskolin, whereas purified Schwann cells did not. The present study provides 
evidence that the peripheral nervous system has its own system responsible for 
the local production of 3,5,3'-triiodothyronine, which may play a key role 
during the regeneration process.