PHYSIOLOGICAL-PROPERTIES OF NEURONS IN THE MOUSE SUPERIOR OLIVE - MEMBRANE-CHARACTERISTICS AND POSTSYNAPTIC RESPONSES STUDIED INVITRO

1. The physiological properties of cells in the superior olivary complex (SOC) were studied in 400-mu-m brain slices taken through the mouse auditory brain stem. Coronal sections were prepared from fresh brain tissue and were placed fully submerged in an oxygenated saline solution. The boundaries of the medial nucleus of the trapezoid body (MNTB), the lateral superior olive (LSO), and the fibers of the trapezoid body were visualized through a dissecting microscope, and micropipettes filled with 4 M potassium acetate were inserted into the LSO or MNTB. 2. Bipolar stimulating electrodes were placed along the trapezoid body usually at the midline decussation and at a location just lateral to the LSO. This arrangement allowed for stimulation of the trapezoid body both contralateral and ipsilateral to the SOC. Synaptic potentials were elicited by delivering brief (0.1 ms) current pulses to the fibers of the trapezoid body. In some cases the integrity of the fibers was confirmed by transport of horseradish peroxidase (HRP) after extracellular microinjections at various locations along the pathway. The HRP reaction product revealed active transport within the trapezoid body and characteristic synaptic and terminal morphology in the MNTB and LSO. The MNTB contained primarily large-diameter fibers terminating in specialized endings (the calyces of Held), whereas the LSO contained mainly small-diameter fibers and punctate terminal boutons. 3. Membrane characteristics of cells in MNTB and LSO were determined by injecting current into the cell and measuring the corresponding voltage change. Neurons in LSO exhibited a roughly linear relation between voltage and intracellularly injected current. Negative current resulted in a graded hyperpolarization of the cell membrane, and positive current resulted in a graded depolarization that led to the production of action potentials. The number of action potentials was directly related to the strength of the current injected. In contrast, the neurons in MNTB had current-voltage relations that were strongly nonlinear around resting potential. The injection of negative current led to graded hyperpolarization, but injection of positive current produced a limited depolarization that resulted in either a single large action potential or an action potential followed by several spikes with greatly reduced amplitude. 4. Excitatory postsynaptic potentials (EPSPs) could be elicited in LSO by ipsilateral stimulation of the trapezoid body and in MNTB by contralateral stimulation. In response to repeated stimulation, some cells in LSO exhibited temporal summation, that is, a series of slightly subthreshold current pulses produced postsynaptic potentials that combined to elicit action potentials. None of the cells tested in MNTB showed temporal summation even at very rapid rates of stimulation. 5. MNTB cells were excited by contralateral stimulation and were unaffected by ipsilateral stimulation of the trapezoid body. Current pulses delivered to the midline trapezoid body resulted in EPSPs with latencies between 0.4 and 0.6 ms. In contrast, the cells in LSO were most often excited by ipsilateral stimulation and inhibited by contralateral stimulation of the trapezoid body. The mean latency for contralateral stimulation of the trapezoid body. The mean latency for contralateral inhibitory postsynaptic potentials (IPSPs) was 1.3 ms and for ipsilateral EPSPs 1.1 ms. These data are consistent with in vivo studies that show comparable latencies for ipsilateral and contralateral effects produced by acoustic stimulation. 6. Introduction of a 10(-6)-M solution of strychnine to the recording chamber abolished the contralaterally elicited IPSP in LSO cells. The reintroduction of normal saline solution brought about the restoration of the IPSP. This result supports the idea that glycine is the neurotransmitter mediating contralateral inhibition in the LSO.