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Quantal components of the excitatory postsynaptic currents at a rat central auditory synapse

Yoshinori Sahara *† and Tomoyuki Takahashi *

1. Paired whole-cell recordings were made from a glutamatergic giant nerve terminal, the calyx of Held, and its postsynaptic target cell in the medial nucleus of the trapezoid body (MNTB) in the brainstem slice of juvenile rat. Excitatory postsynaptic currents (EPSCs) were evoked by presynaptic action potentials triggered by brief (1 ms) depolarizing pulses.
2. In normal artificial cerebrospinal fluid (ACSF), EPSCs of several nanoamperes in amplitude were evoked at a relatively constant latency with no failure, whereas in low [Ca²⁺]_o-high [Mg²⁺]_o solutions, EPSCs fluctuated both in amplitude and latency, and stochastic failures of transmitter release were observed in response to presynaptic action potentials.
3. After blocking action potentials with tetrodotoxin (TTX), direct depolarization of the calyceal preterminal elicited asynchronous release of miniature EPSCs (mEPSCs). When the magnitude of depolarization was increased, mEPSCs increased in frequency. Being consistent with their quantal nature, their mean amplitude remained constant over a wide range of frequencies. The amplitude distribution of mEPSCs was slightly skewed (skewness = 1.06), with a mean conductance of 0.45 nS and a coefficient of variation (c.v.) of 0.43.
4. Single-channel conductance underlying mEPSCs was estimated using non-stationary fluctuation analysis. The weighted mean single channel conductance was 20.4 pS, suggesting that a single quantum opens 22 postsynaptic glutamate receptor channels on average.
5. After washing out TTX, EPSCs evoked by presynaptic action potentials were tested for quantal analysis based upon the mean amplitude of mEPSCs and their variance. In low [Ca²⁺]_o-high [Mg²⁺]_o solutions, quantal contents estimated from the EPSC/mEPSC ratio, rate of failures or c.v. assuming Poisson's statistics, coincided with each other. Evoked EPSCs could be fitted by integer multiples of mEPSCs with an assumption of incremental variance more adequately than the constant variance assumption.
6. It is concluded that the rat central auditory synaptic transmission is made in a quantal manner as at the frog neuromuscular junction.

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