Binding of Muscimol–Conjugated Quantum Dots to GABAC Receptors

Abstract

Abstract: : Purpose: GABA_C receptors mediate inhibitory synaptic signaling at multiple locations within the retina. Recent data indicate that chain–derivatized muscimol can activate GABA_C receptors, a finding potentially relevant to the design of receptor–modulating molecular structures containing a tethered GABA_C agonist (1). Highly fluorescent quantum dots (qdots) functionalized with biomolecules have been used to visualize cell surface receptors (e.g., 2). We have investigated whether muscimol tethered to this bulky fluorescent tag exhibits GABA_C receptor binding activity. Methods: The test conjugate consisted of aminohexanoyl muscimol joined to AMP^®–coated CdSe/ZnS core–shell qdots (9–12 nm diameter) (Quantum Dot Corp.) via a PEG–3400 linker. A given qdot contained ∼100–150 tethered muscimols. The binding of muscimol–qdots (m–q) was investigated in Xenopus oocytes expressing human ρ1 or perch ρ1B GABA_C receptors (1). Oocytes were incubated with m–q for 5 min and then analyzed for fluorescence by confocal microscopy (excitation at 476 nm; emission at 580–620 nm). In competition experiments, oocytes were incubated with test agents prior to the incubation with m–q. Results: Upon incubation with 34 nM m–q, oocytes expressing ρ1 or ρ1B GABA_C receptors exhibited a fluorescent halo at their surface. This halo was absent following m–q treatment of non–transfected oocytes. Preincubation (15–20 min) of oocytes with 500 µM free muscimol, PEG–muscimol (500 µM), or GABA (500 µM) prior to incubation with 34 nM m–q reduced or eliminated the fluorescence halo. AMP coated qdots (34 nM) that lacked conjugated muscimol showed no receptor–binding activity, and did not diminish the subsequent binding of m–q. Conclusions: Muscimol tethered to qdots via a PEG linker exhibits specific binding to expressed GABA_C receptors, despite the presence of both the long–chain PEG linker and the bulky nanocrystal platform. The data encourage further testing of tethered muscimol as a candidate receptor effector for a GABA_C–modulating molecular device (1). (1) Vu et al., Biomaterials, in press. (2) Rosenthal et al., 2002, J Am Chem Soc 124: 4586–4594

Keywords: neurotransmitters/neurotransmitter systems • retina: neurochemistry