Localisation of the GABA(C) receptors at the axon terminal of the rod bipolar cells of the mouse retina

In the vertebrate retina, the rod bipolar cells make reciprocal synapses wi th amacrine cells at the axon terminal. Amacrine cells may perform a fine c ontrol of the transmitter release from rod bipolar cells by means of GABAer gic synapses acting on different types of GABA receptors. To clarify this p ossibility GABA-induced currents were recorded by the patch-clamp whole cel l method in rod bipolar cells enzymatically dissociated from the mouse reti na. All cells tested showed a desensitising chloride-sensitive GABA-induced current. When GABA 30 mu M was applied in presence of 100 mu M biccuculine , a blocker of the GABA(A) receptors, a slow-desensitising component of the current still remains. This current was blocked when GABA 30 mu M was appl ied in presence of 100 mu M 3-aminopropylphosphonic acid, an antagonist of the GABA(C) receptors. The current mediated by GABA(C) receptors showed an EC50 of less that 5 mu M; the ionic current through the GABA(A) receptor sh owed an EC50 of ca. 30 mu M. Two pieces of evidence demonstrated that the G ABA(C)-mediated current was localised at the axon terminal of rod bipolar c ells: (1) cells lacking the axon terminal only showed the biccuculine-sensi tive GABA-induced current; and (2) after mechanical section of the axon ter minal, bipolar cells lost the slow-desensitising component of the GABA-indu ced current. We conclude that the rod bipolar cells express two types of io notropic GABA receptors, and that the high sensitive GABA(C) receptors are mainly localised at the level of the axon terminal and therefore may contri bute to the modulation of the transmitter release from the rod bipolar cell . (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.