SYNAPTIC EXCITATION IN THE DORSAL NUCLEUS OF THE LATERAL LEMNISCUS - WHOLE-CELL PATCH-CLAMP RECORDINGS FROM RAT-BRAIN SLICE

The synaptic events underlying the excitation of neurons in the rat's dorsal nucleus of the lateral lemniscus were studied by whole-cell pat ch-clamp recordings in a brain slice preparation of the auditory midbr ain. Both current-clamp and voltage-clamp data were obtained with the brain slice submerged in artificial cerebrospinal fluid. The rats were between 21 and 35 dais of age at the time the recordings were made. S ynaptic responses were evoked by a bipolar stimulating electrode place d on the lateral lemniscus just ventral to the dorsal nucleus. To elim inate glycinergic inhibitory responses, all physiological data were ga thered with 0.5 mu M strychnine added to the saline bath. Under curren t-clamp conditions, excitatory postsynaptic potentials could be subdiv ided into early and late components. The early component produced a si ngle, highly reliable, short-latency spike and the later component pro duced a more variable, long-latency spike or train of spikes. The non- N-methyl-D-aspartate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, completely blocked the early excitatory postsynaptic potential and it s associated action potential. The Ai-methyl-D-aspartate antagonist, D ,L-2-amino-5-phosphonovaleric acid, blocked the later excitatory posts ynaptic potential and its action potentials. Typically, both early and late excitatory postsynaptic potentials could be recorded from the sa me cell, but the early excitatory postsynaptic potential was evoked at lower stimulus levels and had a larger amplitude than the later excit atory postsynaptic potential. Under voltage-clamp conditions, dorsal n ucleus of the lateral lemniscus neurons responded to stimulation of th e lateral lemniscus with excitatory postsynaptic currents. Outward exc itatory postsynaptic currents were recorded with holding potentials th at depolarized the cell membrane and inward currents were seen when th e cell was hyperpolarized. The current-voltage (I-V) relation of the e arly peak portion of the excitatory postsynaptic current was nearly li near, whereas the I-V relation of the later excitatory postsynaptic cu rrent (12 ms after the peak) was non-linear over the range between -50 and -100 mV. The outward excitatory postsynaptic current consisted of an early current that was selectively blocked by 6-cyano-7-nitroquino xaline-2,3-dione and a later current that was blocked by D,L-2-amino-5 -phosphonovaleric acid. In artificial cerebrospinal fluid with normal concentrations of Mg2+, the inward excitatory postsynaptic current was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione, bur was not affected by D,L-2-amino-5-phosphonovaleric acid. In Mg2+-free artificial cereb rospinal fluid, however, the early component of the inward excitatory postsynaptic current was selectively blocked by 6-cyano-7-nitroquinoxa line-2,3-dione and a later component was blocked by D,L-2-amino-5-phos phonovaleric acid. The results indicate that both N-methyl-D-aspartate and non-iv-methyl-D-aspartate receptor-mediated synaptic responses ar e present in dorsal nucleus of the lateral lemniscus neurons of rats a t 21-35 days of age. The N-methyl-D-aspartate component had a longer t ime-course and a higher threshold than the non-N-methyl-D-aspartate co mponent: and was subject to a voltage-dependent Mg2+ block when the ce ll's membrane was hyperpolarized. The long-duration N-methyl-D-asparta te component is probably responsible for the prolonged inhibitory effe ct of dorsal nucleus of the lateral lemniscus; neurons on physiologica l responses in the rat's inferior colliculus. (C) 1997 IBRO.