A correlative physiological and morphological analysis of monosynapticallyconnected propriospinal axon-motoneuron pairs in the lumbar spinal cord offrogs

Intracellular stimulation of single propriospinal axons evoked excitatory p ostsynaptic. potentials (EPSPs) in lumbar motoneurons. Mean EPSP amplitudes differed by two orders of magnitude when measured in different connections . After analyzing the distribution of mean amplitudes of 47 single-fiber EP SPs, two populations of responses could be defined: (1) those with mean amp litudes between 0.1 and 1.2 mV (mean +/- S.D.: 0.48 +/- 0.30 mV, 34 pairs), which is in the range of values typical for single-fiber EPSPs evoked by s timulation of supraspinal fibers and primary muscle afferents, (2) those wi th mean amplitudes between 1.6 and 8 mV (4.2 +/- 2.0 mV, 13 pairs). Both po pulations of responses had similarly short latencies and rise times and res ponded similarly to paired-pulse stimulation, consistent with monosynaptic transmission. However, the high-efficacy connections had significantly smal ler coefficients of variation of EPSPs, as well as increased quantal conten t and quantal size. Tetanic stimulation gradually depressed the amplitude o f large EPSPs by 81-86%, but did not affect small EPSPs. Recovery of large EPSPs was exponential with a time constant of 3-5.6 min. During post-tetani c depression the amplitude ratio between the test and conditioned EPSPs evo ked by paired-pulse stimulation was not changed but the coefficient of vari ation was increased, suggesting that the depression was due to depletion of synaptic vesicles available for release. Intracellular labeling of seven electrophysiologically studied propriospina l axon-motoneuron pairs revealed that the number of axon varicosities estab lishing close appositions with dendrites of the labeled motoneuron was hi.- her for connections where large-amplitude EPSPs were recorded. These varico sities were more often located on proximal dendrites of motoneurons than th ose of low-efficacy connections. In addition, the number of boutons in high ly effective connections was several times lower than the maximal number of available quanta estimated from physiological data, implying that the larg e EPSPs: may be generated by multivesicular release from presynaptic bouton s. We conclude that the efficacy and related mode of use-dependent modulation of propriospinal connections is determined by a number of factors, includin g the number and position of synaptic contacts and the number of active zon es or vesicles available for release. (C) 2001 Published by Elsevier Scienc e Ltd on behalf of IBRO.