This project focuses on the molecular mechanisms that modulate the functions of the N-Methyl-D-aspartate preferring subtype of glutamate receptor (NMDA-R). NMDA-R is important because phencyclidine interacts with sites located inside the ion channel formed by this receptor, and neurotransmission mediated by this receptor has been implicated in opioid tolerance and dependence (see project report on the physiological effects of opioids). Discovery of the molecular mechanisms modulating NMDA-R function may lead to a better understanding of substance abuse and to the discovery of new treatments for substance abuse. Studies on the modulation of this receptor by polyamines reveal that the glutamate recognition site exists in high- and low-affinity states. The relative proportions of sites in these affinity states, assayed in vitro under conditions of low molarity buffer, in unperturbed tissue are 20% and 80%, respectively. Polyamines convert NMDA-Rs from the low affinity state to the high affinity state in a dose-dependent manner. This conversion can also be accomplished by incubating membranes in the presence of mono- and divalant cations. However, the potency of these cations is lower than that of polyamines. These studies suggest that polyamines may modulate the NMDA-R by more than one mechanism. At low temperature, radiolabeled ifenprodil (in the presence of GBR 12909 to mask sigma receptors; see project report on imaging sigma receptors) binds to polyamine sites on NMDA-Rs. Endogenous mechanisms regulating the function of the nicotinic acetylcholine receptor (nAChR) also are studied. Radiolabeled mecamylamine is used as a probe for the NACHR channel. At concentrations near those at which they occur in brain, polyamines exert an uncompetitive inhibition of binding of mecamylamine, suggesting that polyamines are endogenous modulators of NACHR.
