Effects of short- and long-term rat hind limb immobilization on spinal cord insulin-like growth factor-I and its receptor

In this study we investigated changes in the spinal cord insulin-like growt h factor-I peptide (IGF-I) and its receptors (IGF-IR) after hind limb immob ilization for 5 days, 2, 4, and 8 weeks. Moreover, effects on IGF-I and nic otinic cholinergic receptors (nAChRs) in two types of skeletal muscle were also investigated. IGF-I levels were measured by radioimmunoassay (RIA) whe reas IGF-IR and nAChRs were measured by quantitative receptor autoradiograp hy. Spinal cord IGF-I levels decreased significantly after 5 days, 2 and 4 weeks of immobilization, whereas IGF-IR increased significantly after 4 and 8 weeks compared to controls. In skeletal muscles, nAChRs increased signif icantly after 5 days and 2 weeks in the soleus (SOL) and tibialis anterior (TIB) muscles, respectively, and continued up to 8 weeks in both muscles. I GF-I concentration decrease significantly after 4 and 8 weeks in the SOL an d TIB muscles, respectively. Despite the normal levels of IGF-I in both mus cles at the early time points (5 days and 2 weeks), low levels of IGF-I wer e observed concurrently in the spinal cord ipsilateral to the immobilized l imb. Our findings suggest that the early decrease in the IGF-I level and th e late uppregulation in the IGF-IR in the spinal cord might represent a ner vous system response to disuse. (C) 2001 Elsevier Science B.V. All rights r eserved.