Synaptic responses in cochlear nucleus neurons evoked by activation of the olivocochlear system

The action of olivocochlear collaterals to the cochlear nucleus is not fully established. Synaptic ultrastructure suggests an excitatory role. Extracellular recordings show spikes evoked by electrical stimulation of olivocochlear axons, but these spikes in the cochlear nucleus may be antidromic (activation of output axons) or orthodromic (synaptic input). We therefore recorded intracellular responses to shocks to olivocochlear axons in anaesthetized guinea pigs. In chopper and primary-like neurons shocks caused either no response or an inhibitory synaptic response (IPSP), but never an excitatory one (EPSP). In contrast, onset neurons never showed IPSPs but showed a variety of other responses; antidromic spikes, EPSPs, orthodromic spikes or no effect. The results agree with earlier extracellular observations in that olivocochlear collaterals provide excitatory input to onset neurons. Because some onset neurons are inhibitory they may be the source of the IPSPs observed in other cochlear nucleus neurons. The data also show that electrical stimulation at the floor of the IVth ventricle results in antidromic spikes as well. However, intracellular recording enabled the orthodromic action to be verified and the presumed olivocochlear action to be better understood. Our data support the hypothesis that olivocochlear collaterals initiate excitatory input onto onset-chopper neurons.