A 2 adenosine receptors inhibit calcium influx through L-type calcium channels in rod photoreceptors of the salamander retina. J Neurophysiol 87: 351-360, 2002; 10.1152/jn.00010.2001. Presynaptic inhibition is a major mechanism for regulating synaptic transmission in the CNS and adenosine inhibits Ca 2ϩ currents (I Ca ) to reduce transmitter release at several synapses. Rod photoreceptors possess L-type Ca 2ϩ channels that regulate the release of L-glutamate. In the retina, adenosine is

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... in the dark when L-glutamate release is maximal. We tested whether adenosine inhibits I Ca and intracellular Ca 2ϩ increases in rod photoreceptors in retinal slice and isolated cell preparations. Adenosine inhibited both I Ca and the [Ca 2ϩ ] i increase evoked by depolarization in a dose-dependent manner with ϳ25% inhibition at 50 M. An A 2 -selective agonist, (N 6 -[2-(3,5-dimethoxyphenyl)-2-(2methylphenyl)-ethyl]adenosine) (DPMA), but not the A 1 -or A 3 -selective agonists, (R)-N 6 -(1-methyl-2-phenylethyl)adenosine and N 6 -2-(4aminophenyl)ethyladenosine, also inhibited I Ca and depolarizationinduced [Ca 2ϩ ] 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 2ϩ ] i increase in rods. The results suggest that activation of A 2 receptors stimulates PKA to inhibit L-type Ca 2ϩ channels in rods resulting in a decreased Ca 2ϩ influx that should suppress glutamate release.