SPINAL INPUTS FROM LATERAL COLUMNS TO RETICULOSPINAL NEURONS IN LAMPREYS

This study characterizes the inputs from the lateral columns of the sp inal cord to reticulospinal neurons in the lampreys, using the in vitr o isolated brainstem and spinal cord preparation. Synaptic responses t o the electrical stimulation of the lateral columns were recorded in r eticulospinal neurons of the posterior and middle rhombencephalic reti cular nuclei. The responses consisted of a mixture of excitation and i nhibition. They were markedly potentiated when using trains of two to five pulses, suggesting that the larger part of these synaptic respons es was mediated via an oligosynaptic pathway. An early component, howe ver, persisted when using twin pulses at 10-20 Hz on the ipsilateral s ide, suggesting the presence of an early mono- or disynaptic component . When increasing the stimulation strength, an early fast rising excit atory component appeared. It most likely resulted from an antidromic a ctivation of vestibulospinal axons in the lateral tracts, which make e n passant synaptic contacts with reticulospinal neurons. Responses wer e practically abolished by adding CNQX and AP5 to the Ringer's solutio n. The late component of excitatory responses was decreased by AP5, su ggesting that NMDA receptors were activated. The NMDA receptor-mediate d component was larger when using trains of stimuli or in Mg2+-free Ri nger's. The application of NMDA depolarized reticulospinal neurons. Th e glycinergic inhibitory component was markedly increased in Mg2+-free Ringer's. Moreover, GABA(B)-receptor activation with(-)-baclofen abol ished both excitatory and inhibitory responses. Taken together, the pr esent results indicate that ascending lateral column axons generate la rge excitatory and inhibitory synaptic potentials in reticulospinal ne urons. The possible role of these inputs in modulating the activity of reticulospinal neurons during locomotion is discussed. (C) 1998 Elsev ier Science B.V. All rights reserved.