Monosynaptic excitatory amino acid transmission from the posterior rhombencephalic reticular nucleus to spinal neurons involved in the control of locomotion in lamprey.

1. The reticulospinal neurons in the lamprey posterior rhombencephalic reticular nucleus (PRRN) and their projections to different types of spinal neurons have been investigated by the use of simultaneous paired intracellular recordings from one pre- and one postsynaptic cell. PRRN is of particular importance for the initiation of locomotion. 2. Intracellular stimulation of single PRRN neurons produced monosynaptic excitatory postsynaptic potentials (EPSPs) in simultaneously recorded motoneurons and spinal premotor interneurons of both the excitatory and inhibitory type. Individual PRRN neurons produced EPSPs in several different types of target cells, as revealed by signal averaging. Each single PRRN neuron had extensive monosynaptic connections to approximately 73% of the motoneuronal population. Conversely, several PRRN neurons converge on individual spinal neurons. The average amplitude of the EPSPs was 0.43 +/- 0.40 (SD) mV. The EPSPs varied in time course (time to peak = 7.5 +/- 2.8 ms; duration at one-half peak amplitude = 21.9 +/- 18.1 ms). 3. The EPSPs produced by reticulospinal cells were composed of either exclusively chemical, exclusively electrical, or mixed chemical and electrical components. The electrical EPSPs remained when the ordinary physiological solution was substituted for one without Ca2+ but with Mn2+. The chemical component of the EPSPs was always depressed when a broad-spectrum excitatory amino acid (EAA) antagonist, such as kynurenic acid, was applied, suggesting that the chemical component was because of EAA transmission. The chemical EPSP could have two components, one late, suppressed by N-methyl-D-aspartate (NMDA) antagonists, and one early because of activation of kainate/quisqualate receptors. 4. Three-dimensional reconstructions of Lucifer yellow-filled PRRN neurons were performed with a confocal laser scanning microscope. PRRN neurons producing monosynaptic excitatory amino acid EPSPs were found to have a fusiform cell body located near the surface of the fourth ventricle and an extensive fanlike dendritic tree extending to the ventral and lateral margin of the brain stem within the basal plate. The axons descend in the lateral funiculi of the spinal cord. 5. PRRN neurons utilizing EAA transmission are active during fictive locomotion. They presumably initiate and reinforce ongoing spinal locomotor activity by monosynaptically increasing the general excitability of the spinal premotor interneurons of the spinal locomotor networks by means of their extensive divergent and convergent monosynaptic connections.