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Characterisation of inhibitory and excitatory postsynaptic currents of the rat medial superior olive

Amanda J. Smith, Steven Owens and Ian D. Forsythe

1. The medial superior olive (MSO) is part of the binaural auditory pathway, receiving excitatory projections from both cochlear nuclei and an inhibitory input from the ipsilateral medial nucleus of the trapezoid body (MNTB). We characterised the excitatory and inhibitory synaptic currents of MSO neurones in 3- to 14-day-old rats using whole-cell patch-clamp methods in a brain slice preparation.

2. A dual component EPSC was mediated by AMPA and NMDA receptors. The AMPA receptor-mediated EPSC decayed with a time constant of 1.99 ± 0.16 ms (n = 8).

3. Following blockade of glutamate receptors, a monosynaptic strychnine-sensitive response was evoked on stimulation of the MNTB, indicative of a glycine receptor-mediated IPSC. GABA_A receptors contributed to IPSCs in rats under 6 days old (bicuculline blocked 30% of the IPSC). In older rats little or no bicuculline-sensitive component was detectable, except in the presence of flunitrazepam. These glycinergic IPSCs showed a reversal potential that varied with changes in [Cl^-]_i , as predicted by the Nernst equation.

4. The IPSC exhibited two developmentally relevant changes. (i) At around postnatal day 6, the GABA_A receptor-mediated component declined, leaving a predominant glycine-mediated IPSC. The isolated glycinergic IPSC decayed with time constants of 7.8 ± 0.3 and 38.3 ± 1.7 ms, with the slower component contributing 7.8 ± 0.6% of the peak amplitude (n = 121, 3-11 days old, -70 mV, 25°C). (ii) After day 11 the IPSC fast decay accelerated to 3.9 ± 0.3 ms (n = 12) and the magnitude of the slow component declined to less than 1%.

5. Spontaneous miniature glycinergic IPSCs (mIPSCs) were variable in amplitude and were of large conductance (1.83 ± 0.19 nS, n = 8). The amplitude was unchanged on lowering [Ca²⁺]_o.

6. The time course of evoked and spontaneous miniature glycinergic IPSCs were compared. The 10-90% rise times were 0.7 and 0.6 ms, respectively. The evoked IPSC decayed with a fast time constant of 7.2 ± 0.7 ms, while the mIPSC decayed with a fast time constant of 5.3 ± 0.4 ms in the same seven cells.

7. The glycinergic IPSC decay was voltage dependent with an e-fold change over 118 mV. The temperature dependence of the IPSC decay indicated a Q₁₀ value of 2.
8. Picrotoxin and cyanotriphenylborate had little or no effect on IPSCs from 6- to 14-day-old animals, implying homomeric channels are rare.

9. We conclude that the MSO receives excitatory inputs mediated by AMPA and NMDA receptors and a strong glycinergic IPSC which has a significant contribution from GABA_A receptors in neonatal rats. Functionally, the IPSC could increase membrane conductance during the decay of binaural glutamatergic EPSCs, thus refining coincidence detection and interaural timing differences.

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