Adenosine Suppresses Transmitter Release by Inhibiting Calcium Channels in Cone Photoreceptors of the Tiger Salamander Retina

Abstract

Abstract: : Purpose:ATP and adenosine are released from the retina and endogenous adenosine has been shown to inhibit excitatory transmission in the outer retina. It has been proposed that adenosine release occurs in the dark, when photoreceptors are depolarized and L–glutamate release from photoreceptors is at its greatest. The tonic release of L–glutamate is regulated by the activity of L–type calcium channels present on photoreceptors. L–type Ca²⁺ channels regulate the release of L–glutamate from cone photoreceptors. Activation of adenosine receptors in the CNS has been shown to inhibit transmitter release through inhibition of voltage–dependent Ca²⁺ influx through Ca²⁺ channels in rod photoreceptors. We therefore tested whether adenosine could inhibit Ca²⁺ influx through L–type Ca²⁺ channels and alter exocytosis from isolated cone photoreceptors. Methods:[Ca²⁺]_i changes in cones were assessed using the dye, fluo–4, and exocytosis was monitored with the activity dependent dye, Synaptored–C2. Immunocytochemistry was performed on isolated cones and retinal sections, to determine the localization of adenosine receptors (A₁, A_2A, A_2B, A₃). Results:Adenosine inhibited the K⁺–evoked Ca²⁺ increases in cones in a concentration–dependent manner (1–50 µM). K⁺–evoked depolarizations in cones resulted in destaining of Synaptored–C2 in cone terminals, which was inhibited by adenosine. A₂ receptor immunoreactivity was localized to photoreceptors. The pharmacological subtype of adenosine receptor mediating the inhibition of voltage–dependent calcium channels was investigated using selective adenosine receptor agonists [A₁ (N6–Cyclopentyladenosine), A_2A (CGS 21680), A₃ (HEMADO)]. Conclusions:These results suggest that adenosine inhibits exocytosis from cones by inhibiting voltage–dependent Ca²⁺ channels. Therefore, adenosine regulates cone transmitter release in the outer retina by suppressing L–type Ca²⁺ channels.

Keywords: retina: distal (photoreceptors, horizontal cells, bipolar cells) • neurotransmitters/neurotransmitter systems • calcium