Journal of neurophysiology
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1. The amplitudes of end-plate currents (EPCs) evoked by stimulating the nerve with frequencies ranging from 1 to 5 Hz and the amplitudes of miniature end-plate currents (MEPCs) gradually diminish if choline uptake is blocked by hemicholinium-3 (HC-3, 20 microM). This reduction of EPC amplitudes is predominantly of presynaptic origin, although an observed decrease in MEPC amplitudes suggests that some postsynaptic changes [due to direct action of HC-3 on acetylcholine (ACh) receptors or on open ACh channels] also occurs. 2. ⋯ Even with the choline uptake system intact, prolonged high-frequency stimulation leads to a gradual shortening of EPCs and, to a small extent, MEPCs. Shortening of EPCs appears to be mainly a result of a reduction of their quantal size. 5. It is estimated from the shortening of EPCs and the known EPC versus MEPC relationship that the reduction of the quantal sizes of nerve-evoked quanta probably contributes very significantly to synaptic depression that occurs during prolonged high-frequency nerve stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)
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1. The contribution of midline medullary bulbospinal neurons to descending inhibition from the locus coeruleus (LC) and the funicular trajectories of coeruleo- and raphe-spinal fibers mediating inhibition of spinal nociceptive transmission were examined in different experiments. Extracellular recordings of lumbar dorsal horn neurons were made in deeply pentobarbital-anesthetized, paralyzed rats. ⋯ The effects of ipsilateral and bilateral ventrolateral funiculus (VLF) lidocaine microinjections on spontaneous and heat-evoked unit activity were examined in other experiments. Spontaneous activity increased following ipsilateral VLF lidocaine microinjections for 13/18 units; decreases and no change in spontaneous activity were observed for three and two units, respectively. Heat-evoked unit activity was increased significantly following ipsilateral VLF lidocaine microinjections.(ABSTRACT TRUNCATED AT 400 WORDS)