Suppressive actions of betaxolol on ionic currents in retinal ganglion cells may explain its neuroprotective effects

Betaxolol, a β₁-selective adrenoceptor antagonist, is widely used in the treatment of glaucoma. In addition to its ocular hypotensive effects, betaxolol has been suggested to act as a retinal neuroprotective agent (Osborne et al., 1997). To investigate possible mechanisms underlying the neuroprotective effects, we tested the actions of betaxolol on ion channels and calcium signaling in isolated retinal ganglion cells. Betaxolol (50 μM) reduced by about 20% the high-voltage-activated (HVA) Ca channel currents in ganglion cells isolated from tiger salamander retina. In contrast, the β₁- adrenoceptor antagonists propranolol (10 μM) and timolol (50 μM) had no inhibitory actions on HVA Ca channel currents. The L-type Ca channel antagonist, nisoldipine, blocked the HVA Ca channel current partially and the remaining current was not inhibited by betaxolol. Outward current was inhibited in the presence of betaxolol. Both iberiotoxin (IBTX; 10 nM), a selective inhibitor of large-conductance Ca-activated K channels, and Cd²⁺ (100 μM), which suppresses Ca-activated K channels subsequent to its block of Ca channels, reduced outward current and the remaining current was not blocked significantly with betaxolol. In the presence of betaxolol. Na channel currents were reduced by about 20%, as were currents evoked by glutamate (10 mM) and GABA (1 mM). Current clamp recordings from isolated ganglion cells showed that betaxolol had several effects on excitability: Spike height decreased, repetitive spike activity was suppressed, spike width increased and hyperpolarization following spikes was reduced. Calcium imaging in isolated rat retinal ganglion ceils revealed that betaxolol inhibited glutamate-induced increases in [Ca²⁺](i). These results suggest that betaxolol has a diversity of suppressive actions on ganglion cell ion channels and that, as a consequence, one of the net actions of the drug is to reduce Ca²⁺ influx. The subsequent reduction in [Ca²⁺](i) may contribute to the apparent neuroprotective actions of betaxolol in promoting ganglion cell survival following ischemic insult, as may occur in glaucoma and retinal disease. (C) 2000 Academic Press.