Characterisation of inhibitory and excitatory postsynaptic currents of therat medial superior olive

1. The medial superior olive (MSO) is part of the binaural auditory pathway , receiving excitatory projections from both cochlear nuclei and an inhibit ory input from the ipsilateral medial nucleus of the trapezoid body (MNTB). We characterised the excitatory and inhibitory synaptic currents of MSO ne urones in 3- to 14-day-old rats using whole-cell patch-clamp methods in a b rain 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-sen sitive response was evoked on stimulation of the MNTB, indicative of a glyc ine receptor-mediated IPSC. GABAA receptors contributed to IPSCs in rats un der 6 days old (bicuculline blocked 30% of the IPSC). In older rats little or no bicuculline-sensitive component was detectable, except in the presenc e of flunitrazepam. These glycinergic IPSCs showed a reversal potential tha t varied with changes in [Cl-](i), as predicted by the Nernst equation. 4. The IPSC exhibited two developmentally relevant changes. (i) At around p ostnatal day 6, the GABA, receptor-mediated component declined, leaving a p redominant glycine-mediated IPSC. The isolated glycinergic IPSC decayed wit h time constants of 7.8 +/- 0.3 and 38.3 +/- 1.7 ms, with the slower compon ent contributing 7.8 +/- 0.6% of the peak amplitude (n = 121, 3-11 days old , -70 mV, 25 degreesC). (ii) After day 11 the IPSC fast decay accelerated t o 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 amplit ude and were of large conductance (1.83 +/- 0.19 nS, n = 8). The amplitude was unchanged on lowering [Ca2+](i). 6. The time course of evoked and spontaneous miniature glycinergic IPSCs we re compared. The 10-90% rise times were 0.7 and 0.6 ms, respectively. The e voked IPSC decayed with a fast time constant of 7.2 +/- 0.7 ms, while the m IPSC 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 o ver 118 mV. The temperature dependence of the IPSC decay indicated a Q(10) value of 2.1. Picrotoxin and cyanotriphenylborate had little or no effect o n IPSCs from 6- to 14-day-old animals, implying homomeric channels are rare . 8. We conclude that the MSO receives excitatory inputs mediated by AMPA and NMDA receptors and a strong glycinergic IPSC which has a significant contr ibution from GAB(A) receptors in neonatal rats. Functionally, the IPSC coul d increase membrane conductance during the decay of binaural glutamatergic EPSCs, thus refining coincidence detection and interaural timing difference s.