This project focuses on N-methyl-D-aspartate (NMDA) and nicotinic cholinergic receptors (NMDA-R, nAChR), ligand-gated ion channels. NMDA-R is involved in the psychotropic actions of phencyclidine, and in opioid tolerance and dependence; nAChR is the primary site of action of nicotine.
This research aims to clarify mechanisms by which polyamines (PAs) modulate function of NMDA-R and other neurotransmitter receptors or transporters. NMDA recognition sites in brain occur in two affinity states, and PAs (as well as mono- and divalent cations) convert these sites from the low- to the high-affinity conformation. PAs also enhance binding of [H-3]dizocilpine (DZ), even at saturating glutamate and glycine concentrations. Furthermore, arcaine, a PA antagonist, which inhibits the facilitation by PAs of DZ binding at saturating concentrations of glutamate, does not affect conversion between affinity states. Arcaine-sensitive modulation may involve enhancement in the efficiency of transduction between NMDA-R activation and channel opening, or it may involve an action of PAs to increase affinity of NMDA-R for DZ. Studies of interactions of spermine (SP) with cocaine binding to dopamine and serotonin transporters (DAT, ST, respectively) showed that SP inhibited binding of [H-3]CFT and [H-3]mazindol to DAT, but had no effect on binding of [H-3]paroxetine to ST. The effect of SP on binding to DAT involved a decrease in the density of binding sites. The results suggest that PAs can alter effects of cocaine, and they have implications for treatment of cocaine abuse. As an extension of our previously demonstration that ascorbic acid (AA) protects neurons in culture from NMDA toxicity, we examined potential neuroprotective effects of AA against toxicity produced by nitric oxide (NO), generated from the breakdown of sodium nitroprusside (SNP). Although AA protected against NMDA toxicity, it enhanced toxicity produced by SNP. The data support a model in which the redox state of NO determines whether the free radical produces neuroprotection or neurotoxicity. [H-3]Cytisine was evaluated as an in vivo ligand for nAChR. Distribution of the radiotracer in brain and competition assays indicated that cytisine, appropriately labeled with a positron-emitting radionuclide, may be useful for study of nAChR by emission computed tomography.
