Interaction of Cannabinoid CB1 Receptor Agonist WIN 55212–2 and Dopamine on Voltage–gated Potassium Currents of Goldfish Retina ON–Bipolar Cells

Abstract

Abstract: : Purpose: Cannabinoid CB1 receptor (via Gs) and dopamine D2 receptor (via Gi/o) modulate goldfish cone membrane currents in an antagonistic manner (ARVO Abs. 4167, 2003). As ON–bipolar cells have CB1 and D1 receptors, but not D2 receptors, this work focused on whether CB1 receptor agonist and dopamine interact to modulate voltage–dependent outward membrane K⁺ currents of the large ON mixed rod/cone (Mb) bipolar cells. Methods: The stock solution of cannabinoid CB1 receptor agonist WIN 55212–2 (WIN) was prepared with DMSO. As ascorbate itself suppresses voltage–dependent K⁺ currents, dopamine was dissolved in saline without the addition of ascorbate and was used within 20 min after dissolving. Conventional patch–clamp method was used to record whole–cell currents from Mb bipolar cells in goldfish retinal slices. All Mb bipolar cells were identified by intracellular filling of Lucifer yellow. The bath solution was calcium–free and contained 1 mM cobalt to block indirect calcium–dependent transmitter effects. Results: : Dopamine (10 µM) consistently increased the depolarization–elicited outward K⁺ currents by 48 ± 14% (sem, n = 10), while the effects of WIN (0.25 – 1 µM) were variable and only mildly affected the currents. However, if the K⁺ currents were first increased by 10 µM dopamine, application of WIN (0.25 – 1 µM) not only blocked the effect of dopamine but the K⁺ currents were decreased below control levels. This effect was reversible. 0.1 µM WIN did not block the dopamine effect. If WIN was applied first and dopamine (5 or 10 µM) was then added to the WIN–containing solution, the K⁺ currents were not affected. Thus, WIN potently blocked the effects of dopamine but dopamine had no effect in the presence of WIN. The effects of WIN were blocked by the CB1 antagonist SR141716A, indicating specific involvement of CB1 receptors. Conclusions: A CB1 agonist suppresses the effect of dopamine on bipolar cell K⁺ currents but not the reverse, suggesting that activation of an endocannabinoid system supercedes the action of dopamine on bipolar cell K⁺ currents. As cannabinoids and dopamine also have antagonistic actions on cone–membrane currents, the present data further support our notion that endocannabinoids function as a dark signal to oppose the action of dopamine that serves as a light signal.

Keywords: bipolar cells • dopamine • electrophysiology: non–clinical