Modulation of ischaemic retinal glutamate transport by protein kinase A

Purpose: To investigate the effect of protein kinase A (PKA) modulation on glutamate transporter expression and activity under normal and simulated ischaemic conditions. Glutamate is the major excitatory neurotransmitter in the retina. The glial and neuronal glutamate transporters, GLAST, GLT–1, EAAC1 and EAAT5, normally maintain low extracellular glutamate concentrations. However, during an ischaemic insult, glutamate transport can fail, thus precipitating a dangerous elevation of extracellular glutamate to neurotoxic levels. Methods: Adult rats were euthanased with sodium pentobarbitone (200mg/kg i.p.) and their retinas incubated in vitro with PKA modulators in either normal (oxygenated and glucose) or ischaemic (oxygen/glucose free) medium. The medium contained the non–metabolisable glutamate analogue, D–aspartate (50µM). These retinas were then labelled immunohistochemically to determine the expression of glutamate transporters and the distribution D–aspartate uptake. The effect of PKA modulation upon ischaemic retinal function was assessed in vivo using electroretinography (ERG). Results: Under normal conditions, PKA activation with forskolin (10µM) or PACAP–38 (pituitary adenylate cyclase–activating polypeptide, 0.1µM) reduced the uptake of D–aspartate by Müller cells. Under ischaemic conditions, PKA inhibition with KT5720 (1µM) increased the uptake of D–aspartate by Müller cells. The expression of the four glutamate transporters was not affected by the modulation of PKA activity under normal or simulated ischaemic conditions. These results were confirmed by Western blotting. Neither PKA activation nor inhibition affected the amplitude of the ERG a– or b–waves when recorded 1 or 7 days after an acute ischaemic insult induced by elevated intraocular pressure. Conclusions: These data suggest that retinal glutamate transport can be modulated by protein kinase A.