Expression of functional kainate and AMPA receptors in developing lateral superior olive neurons of the rat.

A functional analysis of AMPA and kainate receptors (AMPARs and KARs) in the lateral superior olive (LSO), a major nucleus in the auditory brainstem, has not been performed so far, to our knowledge. Here we investigated the presence and characteristics of such receptors in the rat LSO by means of whole-cell patch-clamp recordings in combination with pharmacology. Current responses evoked by 200 microM AMPA were completely blocked by the specific AMPAR antagonist GYKI 52466 (100 microM). Properties of the AMPAR-mediated currents (latency, activation time constant, and peak amplitude) remained constant between postnatal day 3 (P3) and P10. Current responses evoked by 100 microM KA were not completely blocked by 100 microM GYKI 52466, indicating that the residual component was mediated by KARs. Throughout development, two groups of KAR-mediated currents (fast I(KA) and slow I(KA)) were distinguished because they had significantly different mean activation time constants. Moreover, the mean peak amplitude of fast I(KA) was significantly higher than that of slow I(KA). The differentiation into fast I(KA) and slow I(KA) can be explained by the existence of two groups of LSO neurons displaying different KAR densities, distributions, and/or diverse types with differences in conductance. Application of the specific KAR subunit agonists SYM 2081 (10 microM), ATPA (10 microM), or iodowillardiine (1 microM) evoked currents in almost all cells tested, showing that GluR5 subunits are a component of functional KARs in LSO neurons. Electrical stimulation of ipsilateral input fibers in the presence of KAR antagonists (NS-102 and GAMS), modulators (WGA), or GYKI 52466 revealed the presence of synaptic KARs in LSO neurons.