Thyrotropin-releasing hormone labeled with 99mTc: a potential agent for neuroendocrinological studies.

Abstract

Abstract To determine the ability of the cerebrospinal fluid (CSF) and the median eminence to transport thyrotropin-releasing hormone (TRH) to the anterior pituitary and to systemic blood, synthetic TRH labeled with technetium-99m was introduced in the cisterna magna of 8 male adult beagle dogs. Following these injections, sequential blood samples were collected and examined for the presence of radioactivity. The time course of concentration of radioactivity in the systemic blood appeared to be a double stage kinetic process: the first step reached a plateau as soon as 20 min after the injection and lasted about 30 min: then, a second increase of the activity appeared very markedly and the ratio of the radioactivity at 4 h versus that at the plateau was found to be around 3.56. A similar pattern was found in all the dogs and can be explained as follows: the first stage might represent the “active” transport of TRH via the median eminence to the portal system. and the second and later step might reflect the “passive” transport of CSF molecules through the arachnoidal granules of Paccioni.