DRUG RECEPTORS IN VIVO--ANIMAL MODELS AND IMAGING

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Drug receptors are readily studied in vitro. However, there may be differences in vivo as the environment of the receptor may be quite different. Accordingly, one of our projects is to study receptors in vivo. This involves developing methods for the study of receptors for drugs of abuse. RTI-121 is a potent and selective analog for the dopamine transporter. In another project report, we note that the radiolabeled form of this compound is an excellent in vitro binding ligand which is potent and selective for the dopamine transporter. We have also shown that carbon-11 labeled RTI-121 is useful as a PET scanning ligand for the dopamine transporter. Because of its greater selectivity than many other ligands, there is significant interest in RTI-121 as a binding ligand. It can be used to study changes in transporters during the addictive process or in various neurodegenerative diseases such as Parkinson's disease. A reasonable criteria for medications for drugs of abuse is that they be long acting. This feature allows convenience in terms of scheduling doses and in overall treatment efficiency. We previously showed that in vivo receptor binding competition with radioactive ligands could be used to establish the duration of occupancy of a drug binding site by a relevant drug. In the past year, we utilized this approach of PET scanning in baboons with radiolabeled cocaine to show that RTI-55 was very long acting. It bound to the transporter for about a day. This supports the notion that RTI-55, or a substance like it, might be a useful medication for drug abusers. PET scanning and SPECT scanning is a very useful procedure, primarily because of its noninvasive nature. A serious deficiency in the approach is the lack of cellular resolution. Accordingly, we have utilized antibodies for the dopamine transporter to study the localization of the dopamine transporter by immunohistochemical methods in, at first, rats. We find that the transporter is localized to dopaminergic neurons as expected. However, all of the neuronal groups do not possess the same amount of transporter. In the dopaminergic cell bodies in the midbrain, for example, the dopamine transporter is highly concentrated. By contrast, in the arcuate nucleus, the cell bodies contain surprisingly little immunoreactive material while the nerve terminal regions contain high quantities. This suggests that the transporter is made in the cell body at a low rate but transported to the nerve terminal region where it is retained and stabilized. These studies are ongoing, and particularly at the electron microscopic level, we hope to find additional information which will clarify the cellular localization of the transporter protein.