Receptor‐mediated retrograde transport in CNS neurons after intraventricular administration of NGF and growth factors

Abstract

Radiolabel tracer techniques were used to follow the distribution of nerve growth factor (NGF) and other neuromodulatory factors after intraventricular injection. Autoradiography showed that shortly after intraventricular injection of radio‐iodinated NGF (¹²⁵I‐NGF), substantial amounts of radioactivity had penetrated the ventricular wall surfaces; this binding was transient and nonspecific. The ¹²⁵I‐NGF was progressively cleared from the central nervous system (CNS), presumably via the flow of cerebrospinal fluid (CSF) into the blood. A relatively small proportion of the injected ¹²⁵I‐NGF was taken up by NGF receptor‐positive neurons in the CNS. Retrograde accumulation of radiolabel was observed within the basal forebrain cholinergic neurons at 5 hours after intraventricular injection. Labeling intensity was maximal at 18 hours and much reduced by 30 hours. This labeling was blocked by co‐injection of an excess of unlabeled NGF. Specific and saturable retrograde labeling was also observed within other NGF receptor‐bearing neurons, including the prepositus hypoglossal nucleus and the raphe obscurus nucleus. When epidermal growth factor (EGF), transforming growth factor‐β1 (TGF‐β1), platelet‐derived growth factor‐AA (PDGF‐AA), PDGF‐BB, leukemia inhibitory factor (LIF), insulin‐like growth factor‐I (IGF‐I), or IGF‐II was radiolabeled and injected intraventricularly, specific labeling of neurons was observed for ¹²⁵I‐IGF‐II and ¹²⁵I‐LIF within separate subpopulations of the dorsal and medial raphe. No retrograde accumulation within neurons was observed for EGF, TGF‐β1, PDGF‐AA, PDGF‐BB, or IGF‐I. This study describes an in vivo method for identifying putative neuromodulatory factors and their responsive neurons.