EXCITATION OF LUMBAR MOTONEURONS BY THE MEDIAL LONGITUDINAL FASCICULUS IN THE IN-VITRO BRAIN-STEM SPINAL-CORD PREPARATION OF THE NEONATAL RAT

1. The excitation of lumbar motoneurons by reticulospinal axons travel ing in the medial longitudinal fasciculus (MLF) was investigated in th e newborn rat using intracellular recordings from lumbar motoneurons i n an in vitro preparation of the brain stem and spinal cord. The trace r Dil dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine) was introduce d into the MLF of 6-day-old littermate rats that had been fixed with p araformaldehyde to evaluate the anatomic extent of this developing pat hway. 2. Fibers labeled from the MLF by Dil were present in the cervic al ventral and lateral white matter and a smaller number of labeled fi bers extended to the lumbar enlargement. Patches of sparse terminal la beling were seen in the lumbar ventral gray. 3. In the in vitro prepar ation of the brain stem and spinal cord, MLF stimulation excited moton eurons through long-latency pathways in most motoneurons and through b oth short- (<40 ms) and long-latency connections in 16 of 40 motoneuro ns studied. Short- and longer-latency components of the excitatory res ponse were evaluated using mephenesin to reduce activity in polysynapt ic pathways. 4. Paired-pulse stimulation of the MLF revealed a modest temporal facilitation of the short-latency excitatory postsynaptic pot ential (EPSP) at short interstimulus intervals (20-200 ms). Trains of stimulation at longer interstimulus intervals (1-30 s) resulted in a d epression of EPSP amplitude. The time course of the synaptic depressio n was compared with that found in EPSPs resulting from paired-pulse st imulation of the dorsal root and found to be comparable. 5. The short- latency MLF EPSP was reversibly blocked by 6-cyano-7-nitroquinoxaline (CNQX), an antagonist of non-N-methyl-D-aspartate glutamate receptors, with a small CNQX-resistant component. Longer-latency components of t he MLF EPSP were also blocked by CNQX, and some late components of the PSP were sensitive to strychnine. MLF activation of multiple polysyna ptic pathways in the spinal cord is discussed.