To provide a quantitative description of the postnatal development of dendritic trees in alpha ganglion cells of the rabbit retina, these cells were stained either by intracellular injection of Lucifer yellow or by application of the lipophilic dye Oil. This was done at three developmental stages: postnatal day (P) 8/9, P 16/17, and in adults. For different retinal locations we quantified the alpha cell dendritic field area, the number of dendritic branch points, and the average dendritic length between branch points. According to the alpha cell location, the data were collected in three groups representing the retinal center, midperiphery, and far periphery, respectively. The data were then correlated with the postnatal retinal expansion which is known to differ among the above topographic regions of the retinae (Reichenbach et al., 1993). Our results show that the growth of alpha ganglion cell dendrites is not proportional to, but significantly exceeds, that of the local retinal tissue. Between P 8/9 and adulthood, the area of central alpha cells increases almost six-fold from 26,000 to 144,000 μm2 (retinal expansion: 2.2-fold), and that of peripheral cells more than 15-fold from 35,000 to 556,000 μm2 (retinal expansion: four-fold). During this period, the coverage factor of alpha cell dendritic fields increases about three-fold, and reaches adult levels of about 3 (retinal center) and 2.2 (periphery), respectively. The number of dendritic branch points remains nearly constant, and the distance between them increases by a factor close to the square root of the factor by which the dendritic field area grows. Thus, it appears that, from the second postnatal week on, dendritic trees of rabbit alpha ganglion cells increase by intense “interstitial growth,” rather than by outgrowth of (new) dendritic branches. This growth pattern is different from that of some other rabbit retinal ganglion cell types, and of alpha ganglion cells of the cat retina, whose dendritic trees expand at a rate equal to or less than that of the surrounding retinal tissue. The consequences for synaptic contacts with bipolar and amacrine cells are discussed; they suggest a high degree of synaptic plasticity during normal postnatal retinal growth.

The correlation between cholinergic sensitivity and the level of stratification for ganglion cells was examined in the rabbit retina. As examples, we have used ON or OFF α ganglion cells and ON/OFF directionally selective (DS) ganglion cells. Nicotine, a cholinergic agonist, depolarized ON/OFF DS ganglion cells and greatly enhanced their firing rates but it had modest excitatory effects on ON or OFF α ganglion cells. As previously reported, we conclude that DS ganglion cells are the most sensitive to cholinergic drugs. Confocal imaging showed that ON/OFF DS ganglion cells ramify precisely at the level of the cholinergic amacrine cell dendrites, and co-fasciculate with the cholinergic matrix of starburst amacrine cells. However, neither ON or OFF α ganglion cells have more than a chance association with the cholinergic matrix. Z-axis reconstruction showed that OFF α ganglion cells stratify just below the cholinergic band in sublamina a while ON α ganglion cells stratify just below cholinergic b. The latter is at the same level as the terminals of calbindin bipolar cells. Thus, the calbindin bipolar cell appears to be a prime candidate to provide the bipolar cell input to ON α ganglion cells in the rabbit retina. We conclude that the precise level of stratification is correlated with the strength of cholinergic input. Alpha ganglion cells receive a weak cholinergic input and they are narrowly stratified just below the cholinergic bands.

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.