Inhibitory synaptic transmission via GABA and glycine
receptors plays a crucial role in shaping the excitatory response of
neurons in the retina. Whole-cell recordings were obtained from
ganglion cells in the intact rabbit eyecup preparation to correlate
GABA- and glycine-activated currents with the presence of their
specific receptors on morphologically identified α ganglion cells.
Alpha ganglion cells were chosen based upon their large somata when
viewing the retinal surface, and responses to light and dark spots were
used to identify OFF-alpha ganglion cells. Light responses were
abolished by superfusion of Ringer's containing cobalt to
synaptically isolate the cell by blocking all Ca2+-mediated
transmitter release. Pressure pulses of GABA and glycine were delivered to
an area that encompassed the dendritic field while receptor antagonists
were applied through superfusion to characterize the direct inhibition onto
the ganglion cell. Physiological results indicated that OFF-α cells did
not have any GABAC receptor-activated currents, but did express
currents mediated by ionotropic GABAA receptors and
metabotropic GABAB receptors that were blocked by their
specific antagonists bicuculline and CGP55845, respectively. The
amplitudes of strychnine-sensitive glycine-activated currents were
always larger than the currents elicited by GABA. Confocal optical
sections of physiologically identified, sulforhodamine B-stained cells
displayed the localization of glycine and GABAA receptor
subunit labeling dispersed over the stained dendrites.
GABAB
receptors couple to Gαq
to mediate increases in voltage-dependent calcium current during development