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

Abstract

Staiker, Alex and Jane M. Sullivan. Cannabinoid receptor activation differentially modulates ion channels in photoreceptors of the tiger salamander. J Neurophysiol 89: 2647-2654, 2003; 10.1152/jn.00268.2002. Cannabinoid CB1 receptors have been detected in retinas of numerous species, with prominent labeling in photoreceptor terminals of the chick and monkey. CB1 labeling is well-conserved across species, suggesting that CB1 receptors might also be present in photoreceptors of the tiger salamander. Synaptic transmission in vertebrate photoreceptors is mediated by L-type calcium currents-currents that are modulated by CB1 receptors in bipolar cells of the tiger salamander. Presence of CB1 receptors in photoreceptor terminals would therefore be consistent with presynaptic modulation of synaptic transmission, a role seen for cannabinoids in other parts of the brain. Here we report immunohistochemical and electrophysiological evidence for the presence of functional CB1 receptors in rod and cone photoreceptors of the tiger salamander. The cannabinoid receptor agonist WIN 55212-2 enhances calcium currents of rod photoreceptors by 39% but decreases calcium currents of large single cones by 50%. In addition, WIN 55212-2 suppresses potassium currents of rods and large single cones by 44 and 48%, respectively. Thus functional CB1 receptors, present in the terminals of rod and cone photoreceptors, differentially modulate calcium and potassium currents in rods and large single cones. CB1 receptors are therefore well positioned to modulate neurotransmitter release at the first synapse of the visual system. I N T R O D U C T I O NCannabinoid CB1 receptors have been detected in the retinas of numerous species, including salamander, goldfish, chick, rat, mouse, monkey, and human (Straiker et al. 1999a,b;Yazulla et al. 1999) with a well-conserved pattern of staining that is particularly intense in both synaptic layers. In species for which double labeling has been performed-chick and monkey-outer plexiform layer staining is found to localize to photoreceptor terminals. CB1 is a member of the Gi/o-type G-protein-coupled receptor family and mediates a wide range of effects on ion channels, including voltage-dependent calcium and potassium channels (Deadwyler et al. 1995;Mackie and Hille 1992;Mackie et al. 1995;Mu et al. 1999). 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 ...

Synaptic transmission at retinal ribbon synapses

“…In zebrafish retina, WIN55212-2 inhibited I Ca at concentrations above 1 μM but enhanced I Ca at concentrations below 1 μM (Fan and Yazulla, 2003), a biphasic effect not seen in salamander rods (Straiker and Sullivan, 2003). Cannabinoid receptors also act on potassium and chloride channels in photoreceptors that influence synaptic transmission through their effects on membrane potential and excitability (Straiker and Sullivan, 2003;Fan and Yazulla, 2003). Activation of CB1 receptors by higher concentrations of WIN55212-2 stimulates pertussis toxin-sensitive G proteins (G i/o ) that in turn inhibit adenylyl cyclase, similar to the signaling cascade engaged by D 2 -like dopamine receptors (Straiker and Sullivan, 2003;Fan and Yazulla, 2003).…”

“…The CB1 agonist WIN 55212-2 (1 μM) enhances I Ca in rods but inhibits I Ca in red-sensitive large single cones of the salamander retina (Straiker and Sullivan, 2003). Another red-sensitive cone subtype in salamander retina, the accessory member of double cones, was not affected by WIN55212-2.…”

“…These effects were blocked by a cannabinoid antagonist (SR141716A). In zebrafish retina, WIN55212-2 inhibited I Ca at concentrations above 1 μM but enhanced I Ca at concentrations below 1 μM (Fan and Yazulla, 2003), a biphasic effect not seen in salamander rods (Straiker and Sullivan, 2003). Cannabinoid receptors also act on potassium and chloride channels in photoreceptors that influence synaptic transmission through their effects on membrane potential and excitability (Straiker and Sullivan, 2003;Fan and Yazulla, 2003).…”

“…The amplitude and voltage dependence of photoreceptor I Ca also are affected by a large number of extrinsic neuromodulators, including dopamine , somatostatin (Akopian et al, 2000), cannabinoids (Straiker and Sullivan, 2003;Fan and Yazulla, 2003), adenosine , nitric oxide (Kurenny et al, 1994), insulin (Stella et al, 2001), polyunsaturated fats (Vellani et al, 2000) and various ions. The list of substances that can modulate photoreceptor I Ca is probably not yet complete.…”

“…RT-PCR and in situ hybridization indicate the presence of both CB1 and CB2 receptors throughout the retina (Lu et al, 2000;Buckley et al, 1998;Porcella et al, 2000). CB1 receptor antibodies label many neurons in the retina, including rod and cone terminals (Straiker et al, 1999;Yazulla et al, 1999;Straiker and Sullivan, 2003). Although the endogenous cannabinoid, anandamide, has not been detected, the CB1 agonist 2-arachidonylglycerol and CB2 agonist palmitoylethanolamide were found to be present in high levels in retina (Straiker et al, 1999).…”