In addition to IPL, photoreceptors communicate with bipolar cells and horizontal cells at another synaptic layer in the retina, the outer plexiform layer (OPL). Therefore, we were interested in determining whether a similar α2 modulation of depolarization-induced Ca
2+ signals operates at OPL as well. Application of high K
+ Ringer solution elicited a robust Ca
2+ signal at OPL
(Fig. 4A)and IPL
(Fig. 4B)in the same retinal slice. As seen for IPL, the Ca
2+ signal at OPL could also be eliminated by 0 Ca
2+ Ringer and nimodipine
(Fig. 4C) , indicating that it is also produced predominantly by Ca
2+ influx through the L-type, voltage-gated Ca
2+ channels. However, the application of brimonidine had no effect on the high K
+-elicited Ca
2+ signal at OPL. In the same slice, a typical brimonidine-induced suppression of the Ca
2+ signal was observed at IPL (
Fig. 4C , third pair from left). These results demonstrate that brimonidine suppression of the depolarization-induced Ca
2+ signal is region specific: it potently suppresses the Ca
2+ signal at IPL but not at OPL.
High K
+ depolarization–induced Ca
2+ influx through L- and other types of Ca
2+ channels may trigger glutamate release from photoreceptor and bipolar cell synaptic terminals. Glutamate may activate NMDA receptors, which have a high Ca
2+ permeability and are expressed predominantly, if not exclusively, on the dendrites or processes of third-order retinal neurons, particularly ganglion cells. If the high K
+-elicited Ca
2+ signal is mediated mainly by NMDA receptors (secondary Ca
2+ signal) and if the α2 agonists also modulate NMDA receptor activity, this could explain the observed region-specific α2 modulation of high K
+-elicited Ca
2+ signal. We used a selective NMDA receptor antagonist, D-AP5, to test this possibility. At a concentration of 50 μM, D-AP5 completely blocked NMDA-induced whole-cell currents in retinal neurons (data not shown) and did have a small effect on the high K
+-elicited Ca
2+ signal at IPL but not at OPL. It reduced the signal by 17% ± 6% (
Fig. 4C , right pair). Thus, suppression of the high K
+-elicited Ca
2+ signal at IPL by α2 agonists appeared to have been caused mainly by the modulation of L-type Ca
2+ channels.