Alterations in normal NMDA channel subunit composition and function are implicated in the pathophysiology of certain neurological and neuropsychiatric disorders such as Parkinson's Disease, Huntington's Chorea, schizophrenia, alcoholism and stroke. Binding of NMDA ligands to succinct modulatory sites located on the outside of or within the ion channel serves to modulate behavioral and neurochemical responses. For this proposal, the NMDA/PCP site has been specifically chosen as an imaging target since the involvement of this site in cognitive and neurodegenerative processes and psychotic behaviors is well documented. The development of effective PET tracers for the study of NMDA receptors is critical to providing much needed tools for understanding of the etiology and improving the clinical management of disorders thought to involve the associated ion channel. ? ? The PCP site is located inside the ion channel. Effective PCP site tracers will bind their target selectively during the active and """"""""open"""""""" state of the ion channel. Therefore, an effective PCP site tracer would serve as a non-invasive in vivo tool to report on the functional state of the channel. In addition to providing information regarding channel activation, these tracers would also be able to monitor the density of these sites. To identity effective PCP site tracers, novel substituted N, N'-alkyl-diphenyl guanidines will be synthesized and evaluated. Candidate ligands possessing high affinity, selectivity, and appropriate lipophilicity will be radiolabeled and further characterized in in vitro assays and in vivo models. ? ? In addition to aspects of radiotracer design synthesis, specific issues regarding appropriate methods to evaluate the new tracers for effective labeling of the ion channel in vivo will be addressed. To supplement the more classical methods of PET radiotracer evaluation, we will adopt an ex vivo method for determining the degree of in vivo saturable binding that occurs with these ligands. While well-established cell membrane assays will be used, an additional modified in vitro assay will allow us to probe the nature of binding of tracers to closed and open states of the channel. The use of these models will provide additional information to assist in the development of clinically useful PCP site radioligands. The data obtained will be pivotal for the design of studies to non-invasively assess NMDA receptor function and will identify suitable radiotracers and provide preliminary data to support future grant submissions.
