In the retinas of Royal College of Surgeons (RCS) rats light induces an increase in distal extracellular potassium irrespective of the age, between days 19-24 and days 29-35 postpartum, but by days 29-35 the ERG b-wave has become reduced. The synaptic blocker 2-amino-4-phosphonobutyric acid (APB) causes the abolition of both the b-wave and the potassium increase at any age. MgCl2 greatly reduces the b-wave at all ages and abolishes the potassium increase in older rats, but in younger rats the potassium increase is enlarged. Since this increase occurs in the absence of the b-wave it is unlikely that the on-bipolar cells are the only sources of the b-wave. Because the NMDA receptor blocker ketamine reduces the b-wave, third order neurons, which possess NMDA receptors, could contribute to the b-wave.

The ERG and the extracellular potassium concentration, [K+]o, of the isolated superfused rat retina were measured in a physiological solution and in solutions containing 10 mM MgCl2 or 100 mu M APB. MgCl2 nearly abolished the b-wave, but the light-induced distal [K+]o increase was enlarged from 0.13 +/- 0.05 to 0.28 +/- 0.08 mM. There was also an increase in the light-induced [K+]o in the proximal retina. APB abolished the b-wave completely, and the distal light-induced [K+]o increase was then replaced by a [K+]o decrease. Upon return to the control solution, there was a larger transitory [K+]o increase than under control conditions, and this occurred before the b-wave had returned. Under these experimental conditions, the distal [K+]o increase could not be correlated with the b-wave, and so the Muller cells are unlikely to be the main source of the rising phase of the b-wave. More probable sources of the b-wave are the on-bipolar cells with their metabotropic and ionotropic receptors, with only the latter apparently being blocked by MgCl2. The extracellular [K+]o changes, however, had an influence upon the slow potentials of the ERG.

No abstract

An increase in the extracellular GABA concentration decreases the b-wave; an impairment of the function of ACs increases the b-wave. These conditions are discussed in the context of the lack of consistent changes to the b-wave during therapy with vigabatrin.

No abstract