We examined the physiological properties of neurons in the dorsal nucleus of the lateral lemniscus (DNLL) of the rat in a 400-microns tissue slice taken in the frontal plane through the auditory midbrain. The brain slice was placed in a small chamber and was perfused fully submerged in a warm, continuously circulating oxygenated saline solution. We made intracellular recordings with glass pipettes filled with 4 M potassium acetate. Synaptic potentials were evoked by electrical stimulation of either the lateral lemniscus or the commissure of Probst. 2. We tested the membrane characteristics of DNLL neurons by recording the electrical potentials produced by intracellular injection of positive or negative current. Typically, DNLL neurons had nearly linear current-voltage curves and responded to depolarizing currents with a sustained train of action potentials. Injection of intense or prolonged depolarizing currents frequently resulted in a pronounced afterhyperpolarization of the cell membrane. Intense hyperpolarizing currents were often followed by a large rebound depolarization. 3. The action potentials of most DNLL neurons were characterized by a double undershoot, i.e., the initial hyperpolarization after a spike was followed by a second, longer-latency hyperpolarization. Seventy-nine percent of the cells recorded had this type of double undershoot. The remaining cells had a single undershoot in which the postspike hyperpolarization was followed by a steady return to resting potential without any indication of a second phase of hyperpolarization. 4. Electrical stimulation of the lateral lemniscus evoked both excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) in DNLL. The EPSPs were evoked alone without any evidence of an IPSP in 67% of neurons and IPSPs were evoked alone in 6% of the neurons from which recordings were made. In 27% of the recordings both EPSPs and IPSPs were elicited in the same neuron by stimulation of a single location on the lateral lemniscus. 5. The combined EPSPs and IPSPs produced by lemniscal stimulation could often be dissociated by their different thresholds and/or different response latencies. For 35% of the neurons in which both an EPSP and IPSP were present, the IPSP had a lower threshold; for 23% of the cells, the EPSP had a lower threshold. For the remaining cells the thresholds for producing an EPSP and IPSP were the same. 6. DNLL neurons were capable of responding with great fidelity to a single pulse of stimulation delivered to the lateral lemniscus, i.e., an action potential was evoked after every stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)
