Functional modifications in rod bipolar cells in a mouse model of retinitis pigmentosa

The rd mouse has been widely used as an animal model of retinitis pigmentosa. In this model, a mutation of rod-specific phosphodiesterase leads to a loss of rods during the early period of postnatal life. Morphological modifications at the level of the outer plexiform layer have been shown (Proc. Nat. Acad. Sci. USA 97 (2000) 11020) in bipolar and horizontal cells. However, very little is known about the functional changes suffered by these cells postsynaptic to the degenerated rods. In the present work we have studied the neurotransmitter-induced currents in rod bipolar cells from the rd mouse retina. Currents induced by glutamate and GABA were studied by the patch clamp-whole cell technique, on rod bipolar cells enzymatically dissociated from the rd mouse retina. Data from rd animals were compared with non-dystrophic NMRI mice. GABA (30-100 micro M) and glutamate (100 micro M) were applied from a puff pipette in the near proximity of rod bipolar cell dendrites, clamped at physiological membrane potentials, and their evoked currents were studied. In rod bipolar cells from non-dystrophic mouse, puff application of glutamate induced an outward current. This current was increased twofold in absence of extracellular calcium (nominally 0 calcium). In rod bipolar cells from adult rd mouse, currents induced by glutamate were absent. Two types of GABA mediated currents were isolated in rod bipolar cells both in control and rd mouse retinas. The currents mediated by GABA(C) receptors were observed exclusively at the axon terminal, while the currents mediated by the GABA(A) receptors were observed upon GABA application to the bipolar cell dendrites. The currents mediated by GABA(A) receptors in rod bipolar cells from rd mouse were larger than those from control animals. We conclude that after the degeneration of rod photoreceptors in rd mouse, rod bipolar cells lost their glutamate (rod-neurotransmitter) input while they increase their response to GABA (horizontal cell-neurotransmitter). In our opinion, this work describes for the first time the changes in neurotransmitter sensitivity that affect rod bipolar cells after photoreceptor degeneration of the mouse retina.