Intracellular postsynaptic potentials of cat lateral geniculate cells and the effects of degeneration of the optic terminals.

(1) The synaptic signal transmission was studied in the lateral geniculate nucleus (LGN) during the late phase of degeneration, 50-80 h after light coagulation of the optic disc and the retinal vessels of one eye. Postsynaptic potentials and action potentials were intracellularly recorded from normal LGN principal cells (N-cells) and cells with degenerating optic terminals (D-cells). EPSPs and postsynaptic action potentials were elicited by electrical stimulation of the optic tract. Two neuronal classes (I and II) were characterized according to their response latency to optic tract stimuli. (2) Spontaneous small EPSPs and slow changes of the membrane potential were found in D-cells and the EPSP-IPSP pattern normally present in N-cells was absent. (3) The responses to optic tract stimuli applied at low stimulus frequencies (less than 5 stimuli/sec) were identical in D-cells and N-cells. (4) At higher stimulus frequencies (greater than 50 stimuli/sec), the EPSP amplitude was, on average, significantly smaller in D-cells than in N-cells. (5) At frequencies above 50 stimuli/sec, the fluctuation of the EPSP amplitudes and the latency scattering of postsynaptic action potentials were larger in D-cells than in N-cells. (6) The slope of the rising phase of the EPSPs increased slightly in N-cells and decreased in D-cells when the stimulus frequency was raised from 5/sec to 100/sec (class I cells). (7) The decay time constant of the EPSP, the latency of the onset and the peak amplitude of EPSPs did not change during degeneration. (8) The hypothesis put forward to account for the results is that the amount of transmitter released by each action potential in degenerating terminals is reduced, but its variability is increased. This is probably caused by a reduction in both the transmitter reserve and the transmitter resynthesis, and also by a larger fluctuation of the size of transmitter quanta.