ethadone, Yes, 2 _ 10-8 M Levorphanol, Yes, 2 _ 10-9 M Phenobarbital, No, No effect at 10-4 M Atropine, No, No effect at 10-4 M Serotonin, No, No effect at 10-4 M A team of researchers was looking for opiate receptors in the mammalian brain. Knowing that the drug naloxone blocks the analgesic effect of opiates, they hypothesized that naloxone acts by binding tightly to brain opiate receptors without activating them. In this exercise, you will interpret the results of an experiment that the researchers conducted to test their hypothesis. The researchers added radioactive naloxone to a protein mixture prepared from rodent brains. If the mixture contained opiate receptors or other proteins that could bind naloxone, the radioactivity would stably associate with the mixture. To determine whether the binding was due to specific opiate receptors, they tested other drugs, opiate and non-opiate, for their ability to block naloxone binding. Diagram showing the steps of the experiment. Step 1: Radioactive naloxone and a test drug are incubated with a protein mixture. Step 2: Proteins are trapped on a filter. Bound naloxone is detected by measuring radioactivity. What is the lowest concentration of morphine that blocked naloxone binding, in standard notation?