Characterization of Voltage–Dependent Ca2+ Channels and Inhibitory Neurotransmitter–Gated Receptors of a Cone Bipolar Cell in the Mouse Retina

Abstract

Abstract: : Purpose: Mammalian cone bipolar cells consist of multiple subtypes, which show heterogeneous expression of voltage–dependent membrane channels and neurotransmitter–gated receptors. Recently, a transgenic mouse strain has been generated in which green fluorescence protein (GFP) is strongly expressed in a single subtype of ON cone bipolar cells (Huang et al., 2003). The purpose of this study is to examine the propreties of voltage–activated Ca²⁺ channels and inhibitory transmitter–gated receptors in this subtype of cone bipolar cells. Methods: Whole–cell patch–clamp recordings were performed on acutely isolated GFP–labeled bipolar cells. Results: Two types of voltage–activated Ca²⁺ currents were observed in the GFP–labeled cone bipolar cells: a small low voltage–activated (LVA) Ca²⁺ current and a large high voltage–activated (HVA) Ca²⁺ current. The LVA Ca²⁺ current was suppressed by mibefradil (5 µM). The HVA Ca²⁺ current was blocked by nimodipine (5–10 µM) and markedly enhanced by BayK–8644 (1 µM), indicating the expression of both T– and L–type Ca²⁺ currents in this subtype of cone bipolar cells. Similar properties of Ca²⁺ currents were also observed in GFP–labeled cone bipolar cells that did not retain axon terminals, suggesting that both types of Ca²⁺ currents were located at least in the somatic/dendritic regions. Application of GABA always elicited large chloride–mediated currents. In contrast, application of glycine evoked no or small current. The majority of the GABA–evoked currents were blocked by bicuculline. The remaining current was blocked by 3–APMPA or TPMPA. Conclusions:We characterized the properties of voltage–activated Ca²⁺ channels and inhibitory neurotransmitter–gated receptors in a single subtype of cone bipolar cells in the mouse retina. These cone bipolar cells express both L–type and T–type Ca²⁺ channels. The L–type Ca²⁺ current is larger than the T–type. These cone bipolar cells also show large GABA–evoked currents but no or small glycine–evoked current. The majority of the GABA–evoked currents are mediated by GABA_A receptors.

Keywords: bipolar cells • ion channels • inhibitory receptors