hydroxylase activity or the uptake in tyrosine, suggesting a potential increase in the dopaminergic efflux due to diabetes [33]. In a more recent study, butyrylcholinesterase activity was reduced 30% in retinas of STZ-diabetic rats, implying a potential loss of subtypes of amacrine cells [34]. A similar study confirmed the depletion of the butyrylcholinesterase enzyme activity in STZ rats [35]. Similarly, NADPH diaphorase immunoreactivity was reduced in the processes of amacrine cells [36].

Retinal Ganglion Cell Loss

Evidence for the loss of RGCs in animal models of diabetes was recently reviewed [7]. Apoptosis had been noted in RGCs using TUNEL in retinal cross sections [11]. An approximation of ganglion cell loss was also made by counting large nuclei in H&E sections of STZ-diabetic rat retinas. There was a 10% reduction in these cells after 7 months of diabetes [16]. A similar approach identified a 20–25% loss of ganglion cells from retinas of STZ-diabetic mice after 14 weeks of hyperglycemia [17]. The number of ganglion cells in the retinas of Brown-Norway STZ-diabetic rats was also found to be reduced by about 16% within 4–5 weeks, using a fluorescent retrograde labeling technique [37]. Another study on STZ-diabetic mice, however, reported no change in H&E sections [21]. Ganglion cell bodies can be easily confused with astrocytes and amacrine cells in this type of preparation, which may account for the variation in results using this method. To overcome this potential confounding variable, RGCs were quantified in whole retinas of mice-expressing endogenous ganglion cell markers. Ins2Akita mice were crossed with Thy1-CFP transgenic mice, which express an endogenous fluorescent protein in the cell body of the majority of RGCs. There was a 16% reduction in RGCs in the peripheral retina within 3 months of the onset of diabetes in these mice [38].

Abnormalities in Ganglion Cell Morphology

Accelerated loss of RGCs due to diabetes has been suggested in a number of studies, but it is likely that the apoptosis is accompanied by other pathological features in these important neurons. A study of flat-mounted retinas from Ins2Akita diabetic mice with endogenously fluorescent markers under the Thy1 promoter revealed several abnormal features in subsets of RGCs [38]. Cell bodies were enlarged, and there were numerous axonal swellings, often associated with a constriction between the cell body and the swelling. The morphology of dendrites was also altered, most dramatically in the large On-ganglion cells. The dendritic fields of these neurons tended to be more complex, possessing more branches and terminals. It was suggested that the reason for this apparent increase in plasticity among certain ganglion cells was in compensation for loss of input from bipolar or amacrine cells or an attempt to compensate for the loss of neighboring ganglion cells (Fig. 4).

A morphological study of RGCs in rats used DiI (1,1¢-dioctadecyl-3,3,3¢,3¢- tetramethylindocarbocyanine perchlorate) applied to whole retinas of STZ-diabetic rats with a gene gun [39]. The largest subtype of cell defined in this study appeared to have enlarged dendritic field areas in the diabetic animals compared to controls. A similar study in a small sample of human postmortem samples with advanced stages of diabetic retinopathy also suggested abnormal morphology of some parasol and midget ganglion cells, including axon swelling and beading, while the dendritic field areas of