sensitivity to the excitatory transmitter NMDA as compared to cells without a GABA response (Sun et al., 2003). This argues against a simple protective eVect of GABA receptors alone. Expression of NMDA receptors did not correlate with susceptibility of neurons to glaucomatous cell death (Hof et al., 1998). This suggests that additional neurochemicals may alter the balance and impact survival. The following sections briefly review evidence for the involvement three neurotransmitters with the potential to damage ganglion cells in glaucoma. Although a contribution from other molecules is likely, the questions addressed are of general relevance. As discussed in the final section on purines, the balance of inhibitory and excitatory responses may be modulated by both receptor expression and the availability of specific transmitters.

1. Nitric Oxide

Nitric oxide (NO) can kill motor neurons (Estevez et al., 1998) and it may also contribute to the loss of ganglion cells in glaucoma (Neufeld, 2004). The enzyme responsible for the production of NO, nitric oxide synthase (NOS), is altered in both animal and human forms of the disease. Increased levels of the NOS 2 isoform were detected in reactive astrocytes from the optic nerve head of humans with glaucoma as compared to controls (Liu and Neufeld, 2000). In vitro experiments with astrocytes obtained from the optic nerve head of humans found that an increase in hydrostatic pressure led to an elevation of protein and mRNA for NOS 2 (Liu and Neufeld, 2001). Rats with experimental glaucoma treated with the NOS 2 inhibitor aminoguanidine had reduced rates of ganglion cell death (Neufeld et al., 1999). Activation of epidermal growth factor receptor (EGFR) in astrocytes of the optic nerve head may be an early step in the astrocyte response to stress. Attenuating this activation with a tyrosine kinase inhibitor reduced ganglion cell death (Liu et al., 2006). Ganglion cell loss accompanying retinal ischemia was also decreased by the NOS inhibitors aminoguanidine and methyl ester No nitro L arginine methyl ester (L NAME), suggesting that NOS acted in a pathway common to both stresses (Geyer et al., 1995; Adachi et al., 1998).

As with many areas of glaucoma research, there is some inconsistency surrounding the role of NOS in glaucoma. A study on a rat model of chronic glaucoma found no evidence for an increase in either NOS 2 protein or mRNA in the ganglion cell layer or optic nerve head (Pang et al., 2005). This study also failed to find an increase in immunoreactivity for NOS 2 in humans with POAG. The discovery that L NAME can actually lower IOP in rabbits independent of any protective eVects (GiuVrida et al., 2003) urges caution when interpreting evidence for involvement of NOS, and for neurochemical changes in general, in glaucoma. It is of course possible that

diVerences found between model systems actually reflect the variety of pathological disorders clustered under the heading ‘‘glaucoma,’’ stressing another parameter to be considered.

2. Glutamate

Perhaps no topic concerning the fate of ganglion cells in glaucoma has aroused as much debate as the role of the excitatory amino acid glutamate. Evidence exists both for and against it having a major role. Finding a way to explain these discrepancies will ultimately benefit the field.

Exogenously added glutamate agonists can kill retinal ganglion cells (Lei et al., 1992; Manabe and Lipton, 2003). Over stimulation of the ionotropic NMDA glutamate receptor can lead to an excess elevation of intracellular calcium (Sucher et al., 1990; Lei et al., 1992) and apoptotic cell death (Lam et al., 1999), consistent with a downstream activation of endonucleases and proteases typically observed in calcium mediated apotosis (Choi, 1988). The central role of calcium elevation in ganglion cell death triggered by NMDA is supported by the observation that inhibition of L type calcium channels with dihydropyridine reduced cell loss (Sucher et al., 1991). The ability of the NMDA receptor antagonist memantine to prevent pressure triggered ganglion cell death in rats is consistent with the hypothesis that glutamate might indeed play a role in the endogenous pathophysiology of glaucoma (WoldeMussie et al., 2002), although the eVectiveness of extending this protection to patients remains to be determined.

An excess of glutamate has been associated with a secondary susceptibility of ganglion cells under conditions of ischemic challenge (Osborne et al., 1999b) and optic nerve crush (Yoles and Schwartz, 1998). However, a direct link between elevated IOP, increased glutamate and stimulation of the NMDA receptor remains elusive, and numerous inconsistencies complicate the relationship. For example, NMDA preferentially kills small and medium diameter ganglion cells (Vorwerk et al., 1999) while large diameter cells are more susceptible in glaucoma (Glovinsky et al., 1991). The distribution of NMDA receptors was unrelated to the patterns of ganglion cell loss in primates with experimental glaucoma (Hof et al., 1998). The relationship between vitreal glutamate levels and elevated IOP is at best inconsistent (Dreyer et al., 1996; Levkovitch Verbin et al., 2001; Carter Dawson et al., 2002; Honkanen et al., 2003). While diVerences in the interval between pressure increase and vitreal sampling may explain some of the discrepancies, even the ability of NMDA to kill ganglion cells is debated, with some studies suggesting they are relatively insensitive (Ullian et al., 2004). It is likely that multiple factors, particularly the membrane potential and the voltage sensitive block of the NMDA channel by Mg2þ, can influence the eVect of NMDA.