1. The calcium influx pathway in large synaptic terminals of acutely isolated bipolar neurons from goldfish retina was characterized using Fura‐2 measurements of intracellular calcium and patch‐clamp recordings of whole‐cell calcium current. 2. Depolarization of bipolar cells with high [K+]o resulted in a sustained, reversible increase in [Ca2+]i in both synaptic terminals and somata. Removal of external calcium abolished the response, as did the addition of 200 microM‐cadmium to the bathing solution, indicating that the rise in [Ca2+]i was due to entry of external calcium. Dihydropyridine blockers of voltage‐gated Ca2+ channels also blocked the influx, and the Ca2+ channel agonist Bay K 8644 potentiated influx, implicating voltage‐activated, dihydropyridine‐sensitive channels in the influx pathway. 3. Under voltage clamp, depolarization from a holding potential of ‐60 mV evoked a slowly inactivating inward current that began to activate at ‐50 to ‐40 mV and reached a maximal amplitude between ‐20 and ‐15 mV. This current was identified as a calcium current because it decreased when the extracellular calcium concentration was lowered, increased when barium was the charge carrier, and was blocked by 200 microM‐external cadmium. The current was substantially blocked by 1 microM‐nitrendipine and potentiated by 0.1 microM‐Bay K 8644, as expected for L‐type Ca2+ channels; it was unaffected by omega‐conotoxin. No evidence for transient or rapidly inactivating Ca2+ current was found. 4. At a given level of potassium depolarization, both the amplitude and the speed of increase in [Ca2+]i were greater in synaptic terminals than in somata. For instance, depolarization by 32.6 mM‐potassium caused an increase in intracellular calcium of 400 +/‐ 23 nM in terminals and 180 +/‐ 20 nM in somata (mean +/‐ S.E.M., n = 73 terminals, n = 30 somata), with maximal rates of change of 40 +/‐ 3 and 12 +/‐ 2 nM/s, respectively. 5. The contribution of terminal and somatic currents to the total whole‐cell Ca2+ current was determined under voltage clamp by local application of calcium or of blocking agents. While there was no qualitative difference between currents in terminals and somata, synaptic terminals accounted for 64 +/‐ 3% (mean +/‐ S.E.M., n = 12) of the total whole‐cell calcium current, and somata accounted for 39 +/‐ 2%. Thus, the density of Ca2+ current was higher in the terminal, accounting for the greater magnitude and speed of Ca2+ influx observed in terminals in Fura‐2 experiments.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanisms of presynaptic inhibition were examined in giant presynaptic terminals of retinal bipolar neurons, which receive GABAergic feedback synapses from amacrine cells. Two distinct inhibitory actions of GABA are present in the terminals: a GABAA-like Cl conductance and a GABAB- like inhibition of voltage-dependent Ca current. Both of the receptors underlying these actions have unusual pharmacology that fits neither GABAA nor GABAB classifications. The GABA-activated Cl conductance was not blocked by the classical GABAA antagonist bicuculline, while the inhibition of Ca current was neither mimicked by the GABAB agonist baclofen nor blocked by the GABAB antagonist 2-hydroxysaclofen. The “GABAC” agonist cis-4-aminocrotonic acid (CACA) both activated the Cl conductance and inhibited Ca current, but the inhibition of Ca current was observed at much lower concentrations of CACA (< 1 microM) than was the activation of the Cl conductance (K1/2 = 50 microM). Thus, by the criterion of being insensitive to both bicuculline and baclofen, both GABA receptors qualify as potential GABAC receptors. However, it is argued on functional grounds that the two GABA receptors coupled to Cl channels and to Ca channels are best regarded as members of the GABAA and GABAB families, respectively.