Adenosine inhibits voltage‐dependent Ca²⁺ influx in cone photoreceptor terminals of the tiger salamander retina

Abstract

Endogenous adenosine has already been shown to inhibit transmitter release from the rod synapse by suppressing Ca 2+ influx through voltage-gated Ca 2+ channels. However, it is not clear how adenosine modulates the cone synapse. Cone photoreceptors, like rod photoreceptors, also possess L-type Ca 2+ channels that regulate the release of L-glutamate. To assess the impact of adenosine on Ca 2+ influx though voltage-gated Ca 2+ channels in cone terminals, whole-cell perforated-patch clamp recording and Ca 2+ imaging with fluo-4 were used on isolated cones and salamander retinal slices. Synaptic markers (VAMP and piccolo) and activity-dependent dye labeling revealed that tiger salamander cone terminals contain a broad, vesicle-filled cytoplasmic extension at the base of the somatic compartment, which is unlike rod terminals that contain one or more thin axons, each terminating in a large bulbous synaptic terminal. The spatiotemporal Ca 2+ responses of the cone terminals do not differ significantly from the Ca 2+ responses of the soma or inner segment like that observed in rods. Whole-cell recording of cone I Ca and Ca 2+ imaging of synaptic terminals in cones demonstrate that adenosine inhibited both I Ca and the depolarization-evoked Ca 2+ increase in cone terminals in a dose-dependent manner from 1 to 50 μM, with an EC 50 of 15.6 μM. These results indicate that, as in rods, adenosine's ability to suppress voltage-dependent Ca 2+ channels at the cone synapse will limit the amount of L-glutamate released. Therefore, adenosine has an inhibitory effect on L-glutamate release at the first synapse, which likely favors elevated adenosine levels in the dark or during dark-adapted conditions. Keywords FM 4-64; Synaptored-C2; fluo-4; VAMP; piccolo In the retina, adenosine has been shown to be under tight regulation by light and dark conditions (Ribelaya and Mangel, 2005), and it is the changing conditions of illumination that influence adenosine levels (Blazanski and Perez, 1991;Paes de Carvalho, 2002;Sun et al., 2002;Stella et al., 2003; Ribelaya and Mangel, 2005), making adenosine concentrations higher in the dark and lower in the light. Adenosine can also inhibit Ca 2+ channels and

Adenosine suppresses exocytosis from cone terminals of the salamander retina

“…In agreement with this observation, adenosine also plays an important role at the first synapse in visual processing by inhibiting voltage-dependent calcium channels that trigger l -glutamate release from both rod and cone photo-receptors [2-4]. Adenosine receptors are located on vertebrate photoreceptors [4,5], and adenosine has been shown to produce a dose-dependent inhibition of voltage-dependent Ca 2+ influx into cone terminals [2]. This reduction in Ca 2+ influx likely results in a decrease in fusion of synaptic vesicles at the ribbon synapse of cones, suggesting that adenosine suppresses exocytosis from cone photoreceptors.…”

“…The actions of adenosine have generally been well correlated with inhibitory modulation of excitatory synapses [1]. In agreement with this observation, adenosine also plays an important role at the first synapse in visual processing by inhibiting voltage-dependent calcium channels that trigger l -glutamate release from both rod and cone photo-receptors [2-4]. Adenosine receptors are located on vertebrate photoreceptors [4,5], and adenosine has been shown to produce a dose-dependent inhibition of voltage-dependent Ca 2+ influx into cone terminals [2].…”

“…Retinal slices from larval tiger salamanders were prepared as described earlier [2]. Slices were viewed by rotating 90° under a water-immersion objective (63 × 0.95 numerical aperture) and viewed on an upright, fixed-stage microscope (Zeiss Axioskop FS2; Zeiss, Thornwood, New York, USA).…”

Kinetics of Synaptic Transmission at Ribbon Synapses of Rods and Cones

“…Use-dependent feedback mechanisms include voltage-and calcium-dependent inhibition of I Ca [140][141][142][143], inhibition of cone I Ca by the pre-synaptic actions of glutamate on group III mGluRs [208], inhibition of cone I Ca by vesicular protons [208,209], inhibition of I Ca by vesicular zinc [116,210,211], inhibitory effects of adenosine which can be derived from vesicular ATP [212][213][214], inhibition of rod I Ca due to chloride efflux mediated by pre-synaptic glutamate transporters [215], inhibition of rod I Ca due to chloride efflux through calcium-activated chloride channels [138], and enhancement of rod I Ca due to K+ efflux through calcium-activated potassium channels [19]. Various neurotransmitters and neuromodulators also regulate rod and cone I Ca , including nitric oxide [216,217], dopamine [132], cannabinoids [218][219][220], somatostatin [221], insulin [222], retinoids, and polyunsaturated fats [223].…”

Comparison of the ontogeny of the vesicular glutamate transporter 3 (VGLUT3) with VGLUT1 and VGLUT2 in the rat retina

“…All tissue was labeled using the indirect fluorescence technique as described previously [23,44]. Retinal sections were briefly dipped in 0.1 M PB before being warmed for 10 minutes at 37°C.…”