Tissue slices in radioligand binding assays: studies in brain, pineal and muscle

The use of tissue homogenates in receptor binding assays raises serious questions as to the physiological value of a preparation which examines receptors (binding sites) in disrupted tissue. In order to usefully study the regulatory properties of neurotransmitter receptors under physiological conditions, the necessity for tissue preparations which retain some degree of cellular integrity is clear. We review here the experiments which have utilized intact tissue - largely in the form of thick slices - to perform radioligand binding assays. There are many reports which note marked differences between studies in intact versus broken cell preparations. For example, significant discrepancies in KD and Bmax values are apparent for [3H] quinuclidinyl benzilate (muscarinic) and [3H] ouabain (Na+/K+-ATP ase, sodium pump) sites in brain and muscle respectively. A further example is the well-described stimulatory effect of GABA on benzodiazepine binding sites which is not seen in tissue slices. Other examples are highlighted. For all ligands so far examined, binding to slices is reversible, stereospecific, saturable, displaceable by appropriate drugs and of high affinity (nM). The method developed in our own laboratory is inexpensive, rapid and involves a minimum of tissue preparation. The technique is so simple as to allow many workers to enter this field who would not otherwise have done so. We suggest that metabolically active tissue slices offer the simplest approach to the study of cell-surface receptor regulation in living tissue.