GRADED RESPONSES AND SPIKING PROPERTIES OF IDENTIFIED FIRST-ORDER VISUAL INTERNEURONS OF THE FLY COMPOUND EYE

1. We studied the graded and spiking properties of the ''non-spiking'' first-order visual interneurons of the fly compound eye in situ with the use of intracellular recordings. Iontophoretical QX-314 injections , Lucifer yellow marking, and (discontinuous) current-clamp method tog ether with transfer function analysis were used to characterize the ne ural signal processing mechanisms in these neurons. 2. A light-OFF Spi ke was seen in one identified anatomic subtype (L(3), n = 6) of the th ree first-order visual interneurons (L(1), L(2), and L(3), or LMCs) wh en recorded from synaptic region (i.e., in the 1st visual ganglion, la mina ganglionaris) in dark-adapted conditions. Hyperpolarization of th e membrane potential by current caused the identified L(1) (n = 4), as well as L(3) (n = 6), to produce an OFF spike, a number of action pot entials, and some subthreshold depolarizations after the light-ON resp onse. In L(2) the OFF spike or action potentials could not be elicited . 3. To produce action potentials in L(1) and L(3), it was found to be necessary to hyperpolarize the cells similar to 35-45 mV (n = 43) bel ow the resting potential (RP) in the synaptic zone. Recordings from th e axons of these cells revealed that near the second neuropil (chiasma ) the threshold of these spikes was near to (similar to 10 mV below, n = 16) or even at the RP when an ON spike was also produced (n = 4). 4 . The recorded spikes were up to 54 mV in amplitude, appeared with a m aximum frequency of up to 120 impulses/s, and had a duration of simila r to 8 ms. In L(1) and L(3) the spikes were elicited either after a li ght pulse (L(3)) Or after a negative current step that was superimpose d on a hyperpolarizing steady-state current (L(3) and L(1)). A positiv e current step (similarly superimposed on a hyperpolarizing steady-sta te current) also triggered the spikes during the step. 5. Iontophoreti c injection of a potent intracellularly effective blocker of voltage-g ated sodium channels, QX-314, irreversibly eradicated the spikes and s ubthreshold depolarizations (n = 5). In addition, further injections e longated the light-on responses and decreased or even abolished the Li ght-OFF response.6. Negative prepulses followed by positive current st eps were applied from the RP, to test the activation-inactivation prop erties of the channels responsible for the OFF spike. During this expe riment the increase of the negative prepulse (removal of the inactivat ion) increased the amplitude of the OFF spike from 7 to 21 mV, whereas the increase of the positive test pulse (activation test) led to the increase of the spike from 5 to 51 mV. 7. These results indicate that voltage-gated sodium channels that are normally highly inactivated are responsible for the OFF-spike generation in L(3) and L(1), and that p robably a component of the light-OFF response is mediated via the same conductance as well. The fact that L(2) did not show any spiking prop erties suggests that this subtype is responsible of feeding visual inf ormation to a different functional subsystem than L(1) or L(3) 8. Freq uency domain analysis suggested that the putative sodium channels in L MC axons considerably increase the gain of the signals at high frequen cies and produce a resonance. Thus in these neurons the OFF spike is p robably used to compensate the gain loss and to improve the signal-to- noise ratio (SNR) during the passive propagation through the long and thin axon. All three interneurons therefore regulate the gain in diffe rent manner and could be considered as parallel pathways with differen tly modulated responses.