Displacement of [3H] GABA against GABA in brain stem of control and experimental neonatal rats. Competition studies were carried out with 30 nM [3H] baclofen in each tube with the unlabelled baclofen concentrations varying from 10-12 to10-4 M. Values are representation of 4-6 separate experiments. Data from the curves as determined from nonlinear regression analysis using computer program PRISM fitted to a two-site model. The affinity for the first and second site for the competing drug is designated as Ki-1 (for high affinity) and Ki-2 (for low affinity). EC50 is the concentration of competitor that competes for half the specific binding. The equation built-in to the program is defined in terms of the log (EC50). If the concentrations of unlabelled compound are equally spaced on a log scale, the uncertainty of the log (EC50) will be symmetrical, but uncertainty of the EC50 will not be symmetrical.