Identification of specific nerve growth factor (NGF) binding in superior cervical ganglia

The β subunit of NGF (β-NGF) has been isolated and labeled with I¹²⁵ to a specific activity of 110 mCi/mg. The I¹²⁵β-NGF retains biological activity as determined by its effects on chick embryo sympathetic ganglia in tissue culture. Specific binding, defined as the binding of I¹²⁵β-NGF minus its binding in the presence of excess nonradioactive β-NGF (10μg/ml) has been studied extensively with microsomal fractions from rabbit superior cervical ganglia. Specific binding reaches equilibrium at 25° in 5 to 10 min. Specific binding to ganglia is saturable and displacable by nonradioactive β-NGF. Binding is half saturated at 10^-10M β-NGF. Dissociation of bound I¹²⁵β-NGF at 25° is a first order process with a halflife of 15 min. In a survey of rabbit tissues, specific binding has been detected only in superior cervical ganglia. A variety of other peptide hormones and proteins do not compete with β-NGF for specific binding to ganglia. These findings indicate that a specific receptor for NGF exists in the superior cervical ganglion, a target organ for NGF, presumably on the surface membrane.