We are actively pursuing several projects in the area of metalloneurochemistry centered on understanding the role of mobile zinc in synaptic transmission, particularly during sensory perception. We have synthesized dozens of fluorescent probes for detecting both intracellular and extracellular zinc. The absorption and emission wavelengths of our sensors span the entire visible spectrum and the apparent dissociation constants for the corresponding zinc-sensor complexes range from picomolar to millimolar, allowing us to image mobile zinc in a wide variety of biological contexts. In particular, we have applied these tools to studies of mobile zinc in the olfactory bulb, dorsal cochlear nucleus (DCN), optic nerve, and hippocampus. With our collaborator Thanos Tzounopoulos (hyperlink to his website: http://www.audres.pitt.edu/people/tzounopoulos.php) and his group at the University of Pittsburgh, we found that extrasynaptic NMDA receptors in the DCN are inhibited by synaptic and tonic zinc. In further studies, we discovered that synaptically released zinc blocks AMPA receptors in the DCN and the hippocampus. Because these ionotropic glutamate receptors mediate excitatory neurotransmission throughout the central nervous system, zinc can modulate excitatory postsynaptic currents and neuronal plasticity in many different types of glutamatergic synapses. For example, working with James McNamara at Duke University Medical School, we learned that vesicular zinc in the hippocampus promotes presynaptic long-term potentiation (LTP) but inhibits postsynaptic LTP in mossy fiber-CA3 synapse. Work in progress includes the development of new strategies to precisely control the activity and localization of zinc sensors in acute brain slices and in live animals, as well as the preparation of highly selective zinc chelators with extremely rapid zinc binding kinetics. Our ultimate goal is to understand the exact molecular mechanisms by which zinc regulates neurotransmission and the attendant physiological and pathological consequences for memory, learning, and sensory perception.
We also develop of metal-based fluorescent sensors for detecting biological nitric oxide and nitroxyl. Nitric oxide (NO) is the endothelium-derived relaxing factor. A diatomic radical, NO functions as a biological signaling agent for a variety of processes related to the cardiovascular and immune systems, neuroprotection, protein regulation and chemotherapeutic resistance. Nitroxyl (HNO) is the protonated, one-electron reduced form of nitric oxide. Even though endogenous generation of HNO has not been yet demonstrated in vivo, several of its biosynthetic pathways were recently elucidated in vitro. Exogenously applied HNO induces vasoprotection, heart muscle contractility, and neurotoxicity. HNO also confers unique therapeutic effects in the treatment of heart failure and alcoholism. Since NO and HNO display contrasting biochemistry, molecular reporters are needed to discriminate between the two species in cells, and at the same time maintain selectivity in the presence of other biological reactants such as hydrogen sulfide, superoxide, peroxynitrite, and thiols. To study the occurrence and biological actions of NO and HNO, we employ fluorescence microscopy with small-molecule metal-based probes. In 2006, we synthesized CuFL1, the first sensor for direct detection of biological NO, which has since been improved to afford greater dynamic range and cellular trappability. Recently, we developed and implemented CuDHX1, the first near-infrared Cu(II)-based sensor for nitroxyl.