The lateral superior olive (LSO) is a binaural brainstem nucleus, involved in sound source localisation. Principal LSO neurones are excited by tones applied to the ipsilateral ear, inhibited by contralateral tones and show a mediolateral tonotopic gradient (high to low frequencies). Their action potential (AP) firing response to tone bursts is described as a ‘chopper’ pattern in post-stimulus time histograms. Dendrotoxin (DTX)-sensitive low-threshold K⁺ currents (I_LT) mediated by Kv1 channels are important in regulating AP firing in other auditory neurones (Dodson et al. 2002). We used whole-cell voltage and current clamp recording in rat brainstem slices to study low-threshold currents underlying LSO principal neurone firing.

Lister Hooded rats (8-14 days old) were killed by decapitation and 150 µm transverse brainstem slices prepared. Slices were superfused with normal ACSF, bubbled with 5 % CO₂ and 95 % O₂ at 25 °C and LSO neurones whole-cell patch clamped with pipettes containing a 130 mM KCl-based patch solution and 1 mg ml^-1 Lucifer Yellow. Data are means ± S.E.M.

Principal neurones had sustained outward currents in response to depolarising commands and a slowly developing inward current in response to hyperpolarizing commands. The inward current was blocked by 10 µM ZD7288, confirming that it was the non-selective cation current I_h. Principal neurones had a spectrum of AP firing patterns in response to depolarising current injection. Some cells fired a single ‘onset’ AP whilst others had an accommodating train following the precisely timed initial AP, described as a ‘chopper’ response (Adam et al. 1999). I-V relationships revealed that ‘onset’ cells had more outward current at -40 mV, suggesting I_LT may regulate their firing. Current density at -40 mV was 0.039 ± 0.003 nA pF^-1; n = 10 compared with 0.016 ± 0.002 nA pF^-1; n = 11 in ‘chopper’ cells (P < 0.001; unpaired t test). Application of 100 nM DTX-I reduced the outward current at -40 mV by 94 ± 3 % (n = 5) in ‘onset’ neurones and converted the single ‘onset’ AP into a ‘chopper’-like train, confirming the presence of Kv1 conductances.

Plotting the location of each principal neurone on a stereotypical LSO profile showed that ‘onset’ cells dominated in the low frequency lateral limb and ‘chopper’ cells in the high frequency medial limb. Immunohistochemical localisation of Kv1.1 subunits in three rats showed a mediolateral intensity gradient with highest levels in the lateral LSO. We conclude that LSO principal cells display a range of AP firing patterns which are tonotopically distributed and Kv1 regulated.

This work was supported by The Wellcome Trust and The Physiological Society.