Neuro-glial cannabinoid receptors modulate signaling in the embryonic avian retina

Endocannabinoids are endogenous lipids that activate selective G protein coupled receptors (CB₁ and CB₂), mostly found at neuronal presynaptic sites in the nervous system. One of the main consequences of the activation of CB receptors is a decrease in GABA or glutamate release, controlling cell excitability. Here we studied the expression of CB₁ and CB₂ receptors in E8C8 cultured retina cells (embryonic day 8 and 8 days in vitro) using immunocytochemistry and western blot analysis. We also evaluated their functions in terms of cyclic AMP (cAMP) production, single cell calcium imaging (SCCI) and GABA release induced in basal conditions or activated by l-Aspartate (L-ASP) in cell cultures or under ischemia in young chick retina. We show that both cannabinoid receptors are expressed in retinal neurons and glial cells. WIN 55,212-2 (WIN, a CB₁/CB₂ agonist) decreased cAMP production in cultured avian embryonic retinal cells in basal conditions. WIN also led to a decrease in the number of glial cells that increased Ca²⁺ levels evoked by ATP, but had no effect in Ca²⁺ shifts in neuronal cells activated by KCl. Finally, WIN inhibited [³H]-GABA release induced by KCl or L-ASP, accumulated in amacrine cells, but had no effect in the amount of GABA released in an oxygen glucose deprivation (OGD) condition. Altogether, our data indicate that cannabinoid receptors function as regulators of avian retina signaling at critical embryonic stages during synapse formation.