Characteristics of inhibitory post‐synaptic currents in brain‐stem neurones of the lamprey.

Abstract

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.

Amplitude fluctuations in synaptic potentials evoked in cat spinal motoneurones at identified group Ia synapses.

“…The probability of release would be different at different boutons in the termination. This idea is supported by recent results obtained from other synaptic connexions: at an inhibitory synapse on the Mauthner cell in the goldfish (Korn et al 1981), at an inhibitory synapse on the Muller cell in lamprey (Gold & Martin, 1982) and at excitatory synapses on mouse spinal cord neurones in culture (Nelson, Macdonald, Neale, Pun, Marshall & Sheriff, 1982). However, this release process alone is unlikely to account for all of the observations on the incremental e.p.s.p.…”

Patch Clamp Techniques Used for Studying Synaptic Transmission in Slices of Mammalian Brain

“…These estimates were based solely on the analysis of fluctuations of evoked unitary IPSCs. In lamprey inhibitory synapses the size of spontaneous IPSCs also indicated quantal conductance changes in the range of tens of nS (Gold & Martin, 1983), but evoked unitary (Osborn, 1981 Finkel & Redman, 1983;Gold & Martin, 1983). If the size of the quantal components of unitary IPSCs were limited by the number of available GABA receptors opposite a bouton then the weight of a particular synaptic knob could be drastically modified by inserting only a few additional receptors.…”

Voltage dependence of Ia reciprocal inhibitory currents in cat spinal motoneurones.

“…6B , the lamprey (Gold & Martin, 1983a) or in mammalian brain and spinal cord slices in vitro (Konnerth et at. 1988).…”