Abstract
Abstract: :
Purpose: Pharmacological evidence suggests that activation of α7 (Strang et al., 2004) and ß2–containing nicotinic acetylcholine receptors (nAChRs; Strang et al., 2003) contributes to retinal ganglion cell (GC) responses. Acetylcholine (ACh) also enhances the preferred direction responses of directionally selective (DS) GCs (Ariel & Daw, 1982). Additional physiological data suggest that DS GCs express at least two types of nicotinic acetylcholine receptors, those that are sensitive to the partially subtype–specific antagonist methyllycaconitine (MLA), and those that are MLA–insensitive (Reed et al., 2002). Our purpose was to confirm the expression of α7 nAChRs by DS GCs and to assess the contributions of additional nAChR subtypes to DS GC responses. Methods: In isolated retina/choroid preparations, DS GCs were identified using standard extracellular recording techniques. After physiological identification, DS GCs were synaptically isolated with 2 mM cobalt in Ames medium and the partially subtype–specific α7 agonist choline (500 µm) was applied (Alkondon et al., 1997). MLA blockade of choline responses was used to confirm α7 nAChR identity. After physiological characterization, subsets of DS GCs cells were either injected with fluorescent dye and processed for nAChR immunoreactivity, or harvested for single cell RT–PCR analysis of nAChR subunit expression. Results: Synaptically isolated DS GCs responded to application of choline. Bath application of 30–60 nM MLA decreased or blocked choline responses, confirming α7 nAChR activation. Some DS cells, including those that were choline–responsive, were also immunoreactive for ß2–containing nAChRs. Transcripts for α3 and/or α7 nAChR subunits were amplified from other DS GCs. Conclusions: DS GCs demonstrated direct evidence for the expression of functional α7 nAChRs. Indirect evidence for the expression of additional nAChR subunits included cells with mRNA transcripts for α3 and α7 nAChRs, immunolabeling consistent with the expression of ß2–containing nAChRs, and residual choline responses after blockade of α7 nAChRs by 30–60 nM MLA. Identification of nAChR subtypes expressed by GC types may help clarify retinal GC response modulation.
Keywords: ganglion cells • acetylcholine • receptors: pharmacology/physiology