Structural and Functional Differences Distinguish Principal from Non- principal Cells in the Guinea Pig MSO Slice. Philip H. Smith. Dept. of Anatomy, University of Wisconsin Medical School, Madison, Wisconsin 53706.
APStracts 2:0028N, 1995.
SUMMARY AND CONCLUSIONS
1. Principal cells in the medial superior olive (MSO) receive low frequency information from both ears via left and right cochlear nuclei. In vivo extracellular records suggest that some MSO neurons respond optimally only when the binaural acoustic signal has a precise interaural delay. Thus, MSO cells, in particular principal cells, are thought to be the first stage in the processing of interaural time difference cues that provides information as to the location of a low frequency sound in space. 2. Despite this proposed fundamental role for the MSO, certain features of this nucleus make in vivo recordings from any cell type here very difficult to obtain. Only a small number of extracellular records and no intracellular recordings are reported in the literature. Using sharp, neurobiotin-filled glass electrodes to record intracellularly from cells in an ¯in vitro brain slice of the guinea pig superior olivary complex I have begun to assess the anatomical and physiological features of cells in the MSO that might be relevant to such a functional role in vivo. 3. Two basic MSO cell types, designated principal and non-principal, could be distinguished based on certain anatomical and physiological differences. 4. Labeled principal cell bodies were located at all dorsoventral locations within the MSO. Labeled non-principal cells were located in or around the dorsal aspect of the nucleus. Principal cells typically had thick bipolar dendrites, one directed medially - one laterally, that did not taper or branch significantly except at their terminations. Non- principal cells were multipolar with 3-9 thinner primary dendrites which did not branch preferentially in a mediolateral direction. Principal cell axons gave off collaterals terminating in and around the dorsal MSO. Non-principal cells also had axon collateral branches innervating dorsal MSO but these axons could branch more extensively and project further down the dorso-ventral aspect of the nucleus. 5. Principal cells typically responded to depolarizing current pulses with one or a few spikes at current onset. When bathed in saline containing 4-AP they fired repetitively to the same depolarizing current pulses. This would indicate a depolarization -induced nonlinearity similar to that seen in principal cell types of two other auditory brainstem nuclei, the anteroventral cochlear nucleus and medial nucleus of the trapezoid body. Non-principal cells normally fired repetitively to depolarizing current pulses even close to spike threshold. Both cell types could show a sag in the membrane potential to hyperpolarizing current pulses. 6. Both excitatory and inhibitory synaptic potentials could be elicited in both cell types by shocks applied to the trapezoid body (TB) at the midline and/or in ipsilateral TB just lateral to the lateral superior olive (LSO). 7. To determine the possible source of the inhibitory synaptic events I also injected a small population of cells in the medial and lateral nucleus of the trapezoid body (LNTB and MNTB). As shown in other species MNTB principal cells may project to the MSO while multipolar cells in the same nucleus do not. LNTB cells can also send collaterals into MSO. 

Received 22 September 1994; accepted in final form 8 January 1995.
APS Manuscript Number J594-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  3 April 1995.