subset of the starburst cells [47]. Acetylcholine (Ach) excites RGCs, particularly Y cells [18, 44, 46, 53, 63, 91], and a role for Ach has been implicated in directionsensitivity in the retina.

Dopamine released by amacrine cells regulates the spread of activity through gap junctions in the retina. Dopamine D1 receptors decrease the conductance of gap junctions between amacrine cells and bipolar cells [35, 107]. Therefore connections are dynamically regulated in photopic and scotopic light conditions by dopamine [4, 5].

9.2.2.5  Neuropeptides

Amacrine cells also produce a number of neuropeptides, including substance P, somatostatin, vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), corticotropin releasing factor (CRF) and opiates. The roles of peptides in retinal processing are less well understood, and due to the long-term instability of proteins, and complications in exogenous application of peptides, they are not likely to be useful in a neurotransmitter-based prosthesis.

9.2.2.6  Putative neurotransmitters for retinal prosthesis

The neurotransmitter and gap–junction interactions in the IPL and ganglion cell layer (GCL) provide a variety of means to stimulate the retina, possibly in a more naturalistic way. Glutamate application to the retina can directly excite RGCs, and indirectly activate RGCs through the amacrine-bipolar-RGC pathway. In addition, glutamate stimulation may activate amacrine pathways which are used for feature detection. Activation of amacrine cells can modulate many retinal processing pathways. Acetylcholine may also be effective in selectively activating large ganglion cells such as the Y or type A RGC. In addition, GABA or glycine application could reduce activity and may also evoke rebound activity at the offset of application [30, 96].

9.3  Neurophysiological Changes in Retinal Degeneration

An important consideration for any retinal prosthesis is how retinal function is affected beyond photoreceptor loss due to neurodegeneration and reorganization. Degenerative changes in biophysical and morphological cell properties, reorganization of connections, endogenous transmitter release, and transmitter receptor alterations have been observed in animal models of retinitis pigmentosa [60–62]. Such changes may affect the excitability of RGCs to exogenously applied neurotransmitters. Late stages of retinal degeneration have been shown to severely limit RGC stimulation via electrical charge, as thresholds for eliciting electrically evoked cortical potentials increase and will likely impact the efficacy of neurotransmitter stimulation [45, 76].

Various animal models of RP express many similarities, but differ in time course of degenerative and physiological changes. Photoreceptor loss in the pink-eyed RCS

rat (rdy+/rdy+) is apparent by postnatal day 20 (P20), progresses rapidly to only a few nuclear layers by P40, and is nearly complete by P100 [6, 50, 69]. The S344-ter rat has a true rhodopsin gene mutation and therefore is an important model for studying human RP. Different lines of S344-ter rats exhibit different rates of progressive photoreceptor loss. In the rd1 mouse model, which has a mutation in phosphodiesterase [7], PRs exhibit a rapid loss of in the first 2–3 postnatal weeks [22]. This early loss of PR is associated with abnormal development of bipolar mGLUR6 receptors, and an early remodeling both in horizontal cells, which exhibit atrophy of terminal dendrites, and in rod bipolar cells, where photoreceptor directed dendrites do not develop [99]. However, amacrine cells do not appear to be affected [100], and in recent work from the same group the many types of RGCs also exhibit normal morphology in rd10 mice [99]. Bipolar and other cell remodeling occurs in stage 3, with onset varying with molecular deficit. In the RCS, s334ter, and P23H rat models, remodeling is relatively late in the disease with the onset on or after P270 [60, 62].

Visual function in RCS rats based on electroretinogram (ERG) recordings [8, 28, 75, 79, 90] shows a progressive loss of rod function to near total loss by P100. Cone function, although declining, can be measured up to P200 [85]. Visual receptive fields in pigmented RCS rats are recorded in the superior colliculus up to P180, albeit with expected increases in threshold [90]. Thus, even after substantial loss of visual function due to photoreceptor loss, RGCs are relaying information to the central nervous system.

Studies on degenerated retinas have in part focused on the changes in neurotransmitter levels and glutamate receptors. Glutamate and aspartate are reduced by approximately 50% in RCS rats at 23 weeks of age [77], and this is likely to be a consequence of photoreceptor loss. GABA is reduced to a lower extent, while glycine levels increase in 23 week RCS rats [77]. However, other studies found that both GABA and glycine levels increase in degenerating retinas [23, 78, 92]. In addition, of the transmitters used by amacrine cells, dopamine is reduced by approximately 50%, but acetylcholine levels are not affected [77]. The reduced dopamine levels correspond with a loss of dopaminergic amacrine cells associated with retinal degeneration [16, 23]. A reduction or loss of many subunits of NMDA receptors (NR1, NR2A-D) has been found in RCS rat by P120 [29]. However, decreased expression of NMDA NR1 subunits in IPL was also observed in congenic non-pigmented rats compared to brown Norway [29]. Kainate binding sites also decrease by P180 in the IPL and OPL of RCS rats [98]. Excitation of RGCs can be shown in response to activation of AMPA, kainate and NMDA receptors [10, 58, 59, 109]. AMPA receptors subunit mRNA for GluR2, GluR3 and GluR4 increase in degenerating retinas of rd1 mice by P40, but the flop:flip ratio (the ratio of the two AMPA receptor splice variants which affect binding and currents evoked by glutamate) is unchanged [71]. The levels of GluR1 mRNA do not change, but the flop:flip ratio of Glur1 (flip responses have slower desensitization and a greater steady-state component) does not exhibit the normal increase between p10 and p40 [71].

The activation of RGCs by exogenous glutamate may also be affected by excitation of bipolar and amacrine cells. Bipolar cells express either mGluR or kainite glutamate receptors. Kainate receptor expression in the IPL and OPL is high at early stages of development (P17) and decreases by postnatal 180 days in pink-eyed