RECOMBINANT HUMAN NERVE GROWTH-FACTOR IS BIOLOGICALLY-ACTIVE AND LABELS NOVEL HIGH-AFFINITY BINDING-SITES IN RAT-BRAIN

Iodinated recombinant human nerve growth factors (I-125-rhNGF) stimulated neurite formation in PC12 cell cultures with a half-maximal potency of 35-49 pg/ml, compared with 39-52 pg/ml for rhNGF. In quantitative ligand autoradiography, the in vitro equilibrium binding of I-125-rhNGF to brain sections showed a 10-fold regional variation in density and was saturable, reversible, and specifically displaced by up to 74% with rhNGF or murine NGF (muNGF). At equilibrium, I-125-rhNGF bound to these sites with high affinity (K(d) 52-85 pM) and low capacity (B(max) less-than-or-equal-to 13.2 fmol/mg of protein). Calculation of I-125-rhNGF binding affinity by kinetic methods gave average K(d) values of 24 and 31 pM. Computer-generated maps revealed binding in brain regions not identified previously with I-125-rhNGF, including hippocampus; dentate gyrus; amygdala; paraventricular thalamus; frontal, parietal, occipital, and cingulate cortices; nucleus accumbens; olfactory tubercle; subiculum, pineal gland; and medial geniculate nucleus. NGF binding sites were distributed ina 2-fold increasing medial-lateral gradient in the caudate-putamen and a 2-fold lateral-medial gradient in the nucleus accumbens. I-125-rhNGF binding sites were also found in most areas labeled by I-125-muNGF, including the interpeduncular nucleus, cerebellum, forebrain cholinergic nuclei, caudoventral caudate-putamen, and trigeminal nerve nucleus. I-125-rhNGF binding sites were absent from areas replete with low-affinity NGF binding sites, including circumventricular organs, myelinated fiber bundles, and choroid plexus. The present analysis provides an anatomical differentiation of high-affinity I-125-rhNGF binding sites and greatly expands the number of brain structures that may respond to endogenous NGF or exogenously administered rhNGF.