Characteristics of Inhibitory Post-synaptic Currents in Brain-stem Neurones of the Lamprey

Voltage-clamp techniques were used to record spontaneously occurring inhibitory post-synaptic currents (i.p.s.c.s) from Müller cells in the brain stem of the lamprey. In normal bathing solution, the i.p.s.c.s in most cells had unimodally distributed amplitudes with a mean corresponding to a peak conductance of 107 nS and a coefficient of variation of about 15%. About 20% of the cells displayed, in addition, events of approximately twice the modal amplitude. The falling phase of the i.p.s.c.s was exponential with a mean time constant of about 32 msec. This is the same as the relaxation time constant of glycine-activated channels in these cells under comparable conditions (Gold & Martin, 1983b). When tetrodotoxin (TTX) was added to the bathing solution the spontaneous i.p.s.c.s disappeared, suggesting that they were due to release of transmitter by action potentials in presynaptic terminals. Spontaneous activity was also abolished by removing Ca2+ from the bathing solution. When extracellular Ca2+ was increased, or 4-aminopyridine (4-AP) was added to the bathing solution, the mean amplitude of the i.p.s.c.s increased and the amplitude distribution showed two or more distinct peaks. Analysis of the amplitude distributions suggested that the peaks represented single and multiple quantal events and that the release process obeyed binomial statistics. In TTX-blocked preparations, spontaneous i.p.s.c.s could be induced by raising extracellular K+. These had the same time constant of decay as in normal solution and a unimodal amplitude distribution, with the mean corresponding to a peak conductance of 45 nS. In solutions with raised extracellular K+ and reduced extracellular Cl-, the mean i.p.s.c. amplitude corresponded to a peak conductance of 67 nS. These variations in conductance corresponded closely to variations in conductance with extracellular K+ and Cl- of single glycine-activated channels. It is concluded that the i.p.s.c.s are produced by activation of conductance channels identical to those activated by glycine, and that in normal bathing solution i.p.s.c.s produced by individual presynaptic action potentials are the result of the release of one or, at most, two quanta of transmitter. Each quantum activates approximately 1500 elementary channels.