A₂Adenosine Receptors Inhibit Calcium Influx Through L-Type Calcium Channels in Rod Photoreceptors of the Salamander Retina

Abstract

Presynaptic inhibition is a major mechanism for regulating synaptic transmission in the CNS and adenosine inhibits Ca(2+) currents (I(Ca)) to reduce transmitter release at several synapses. Rod photoreceptors possess L-type Ca(2+) channels that regulate the release of L-glutamate. In the retina, adenosine is released in the dark when L-glutamate release is maximal. We tested whether adenosine inhibits I(Ca) and intracellular Ca(2+) increases in rod photoreceptors in retinal slice and isolated cell preparations. Adenosine inhibited both I(Ca) and the [Ca(2+)]i increase evoked by depolarization in a dose-dependent manner with approximately 25% inhibition at 50 microM. An A2-selective agonist, (N(6)-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine) (DPMA), but not the A1- or A3-selective agonists, (R)-N(6)-(1-methyl-2-phenylethyl)adenosine and N(6)-2-(4-aminophenyl)ethyladenosine, also inhibited I(Ca) and depolarization-induced [Ca(2+)]i increases. An inhibitor of protein kinase A (PKA), Rp-cAMPS, blocked the effects of DPMA on both I(Ca) and the depolarization-evoked [Ca(2+)]i increase in rods. The results suggest that activation of A2 receptors stimulates PKA to inhibit L-type Ca(2+) channels in rods resulting in a decreased Ca(2+) influx that should suppress glutamate release.

Cannabinoid Receptor Activation Differentially Modulates Ion Channels in Photoreceptors of the Tiger Salamander

“…For example, the D2 and CB1 receptors, which activate G i/o -type G proteins, have been found to be negatively coupled to adenylyl cyclase (Howlett 1984;Howlett and Fleming 1984;Onali et al 1981)-reducing cAMP levels-whereas the G s -activating adenosine A2 receptor is positively coupled to adenylyl cyclase (van Calker et al 1979). Consistent with these findings, both CB1 and D2 effects on I Ca were mimicked by PKA inhibitor Rp-cAMPS, and the D2 effect occluded by PKA activator Sp-cAMPS (Stella and Thoreson 2000), whereas the A2 effect was occluded by RpcAMPS (Stella et al 2002). The cAMP/PKA-dependence of somatostatin action in salamander retina has not been investigated (Akopian et al 2000).…”

“…This stands in contrast to photoreceptors in the vertebrate retina, where neurotransmitter release relies on the action of L-type calcium channels. Here cannabinoids modulate L- (2000); † Akopian et al (2000); ‡ Stella et al (2002). type calcium currents in a PKA-dependent manner, an action that would be expected to occur via G-protein ␣ or ␤␥ subunit modulation of adenylyl cyclase (Simonds 1999). These results indicate a conservation of function-modulation of neurotransmitter release-by CB1 receptors across disparate neuronal settings, via distinct intracellular pathways and calcium channel effectors.…”

“…There is reason to think that GPCR modulation of L-type calcium currents in photoreceptors is mediated by cAMP via PKA. Specifically, three recent studies have described the G-protein-coupled receptor effects on ion channel function in photoreceptors of the tiger salamander: two examined the actions of dopamine D2 and adenosine A2 receptors, respectively, in modulating I Ca (Stella and Thoreson 2000;Stella et al 2002), whereas the other examined the effect of somatostatin on both calcium and potassium currents (Akopian et al 2000). These results have been summarized along with our own in Table 1.…”

“…Retinal cannabinoid receptor activation has been shown to inhibit dopamine release in the guinea pig (Schlicker et al 1996), to inhibit a delayed rectifier K ϩ current in ON cone bipolar cells of the goldfish (Yazulla et al 2000), and to inhibit L-type calcium currents in bipolar cells of the tiger salamander (Straiker et al 1999a). Three recent reports have described G-protein-coupled receptor-dependent modulation of ion channel function in photoreceptors of the tiger salamander by dopamine, adenosine, and somatostatin (Akopian et al 2000;Stella and Thoreson 2000;Stella et al 2002). These studies find that the calcium currents (I Ca ) of rods and large single cones respond differentially to activation of the same receptor type.…”

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].…”