NMDA-evoked [Ca²⁺]_i increase in salamander retinal ganglion cells: Modulation by PKA and adrenergic receptors

Abstract

Application of NMDA induces a depolarization and increase of intracellular calcium concentration ([Ca²⁺]_i) in retinal ganglion cells, which cause ganglion cell death in models of glaucoma. In the present study, we investigated the pharmacological mechanism of how NMDA-evoked increase in calcium could be modulated in dissociated retinal ganglion cells from tiger salamander. In these neurons, protein kinase A (PKA) up-regulated the NMDA-evoked [Ca²⁺]_i increase. In the presence of 8-bromo-cAMP or forskolin to stimulate PKA, the elevation level of [Ca²⁺]_i induced by NMDA became even higher; In the presence of H-89, a PKA inhibitor, the NMDA-evoked [Ca²⁺]_i increase was attenuated. In addition, applications of adrenergic compounds were also found to influence the NMDA-evoked [Ca²⁺]_i increase. UK-14,304, a selective α₂ agonist, reduced the elevation level of [Ca²⁺]_i caused by NMDA. In contrast, isoproterenol, a β agonist, augmented the NMDA-evoked [Ca²⁺]_i increase. These adrenergic regulations were due to direct activation of adrenoceptors, since modulations of both UK-14,304 and isoproterenol on the NMDA-evoked [Ca²⁺]_i increase were abolished by their respective antagonists. Furthermore, adrenergic regulations were mediated through a PKA-related pathway since PKA inhibitor blocked adrenergic regulations. The possible modulatory site(s) by PKA was also discussed.