Down-regulation of thyrotropin-releasing hormone (TRH) receptors in spinal cord after transection as revealed by quantitative autoradiography.

The autoradiographic localization of thyrotropin-releasing hormone (TRH) receptors was investigated in the rat spinal cord after transection at the level of T8-T9. The discrete distribution of [3H]-MeTRH binding was measured with a computerized image analyzer at the cervical (C6-C7) and lumbar (L2-L3) level, one week and three weeks after injury. The TRH receptor density was expressed in fmol/mg protein. There was no significant change in the density of TRH receptors below the injury site. In the cervical region, TRH receptor concentration in the dorsal gray matter did not differ from normal controls; in contrast we found a time dependent change in lamina 10 and in the ventral gray, with a significant decrease (25% and 19%, respectively) of TRH receptor binding sites one week after transection and a return to control levels by three weeks. From these data and the known increase of TRH immunoreactivity above a spinal injury, a down-regulation of spinal cord TRH receptors in response to elevated levels of TRH is suggested.