MODULATION OF MONOSYNAPTIC EXCITATION IN THE NEONATAL RAT SPINAL-CORD

1. The effects of high-frequency (5-50 Hz) stimulation of dorsal root afferents on monosynaptic excitation of a motoneurons was studied in t he in vitro spinal cord preparation of the neonatal rat, using sharp-e lectrode intracellular recordings. 2. Double pulse stimulation of dors al root afferents induced severe depression of testing excitatory post synaptic potentials (EPSPs) at each of the tested interstimulus interv als (15 ms-5 s). After perfusion of the preparation with low-calcium, high-magnesium Krebs saline, the amplitude of the conditioning EPSPs w as markedly decreased and the testing EPSPs exhibited substantial faci litation that was maximal at the 20-ms interval and that was accompani ed by depression at intervals greater-than-or-equal-to 60-100 ms. 3. S hort-duration stimulus trains applied to dorsal root afferents normall y induced tetanic depression of the intracellularly recorded monosynap tic EPSPs. Switching the bathing solution to low-calcium, high-magnesi um saline decreased the control EPSP and induced facilitation and then tetanic potentiation (TP) of the EPSPs within the applied train. The magnitude of potentiation (% potentiation) of these EPSPs depended on the interpulse interval of the short stimulus train and on the degree of attenuation of the unpotentiated control EPSP after the solution wa s changed from normal- to low-calcium Krebs solution. 4. Long-duration stimulus trains applied to dorsal root afferents at 5-10 Hz induced m arked depression of monosynaptic EPSPs during the train. The depressio n was alleviated after cessation of the tetanic stimulation and was fo llowed in some cases by slight posttetanic potentiation. Perfusion of the preparation with low-calcium, high-magnesium solution reduced the amplitude of the pretetanic EPSPs and induced moderate potentiation or slight depression during the tetanic train. After the tetanic train, EPSPs exhibited posttetanic potentiation (PTP) that lasted less-than-o r-equal-to 10 min. 5. On the basis of these results, it is suggested t hat monosynaptic EPSPs produced by dorsal root afferents in the neonat al rat spinal cord are modulated simultaneously (as in the adult spina l cord) by processes acting to increase the EPSP amplitude (facilitati on, TP, and PTP) and to decrease it (depression). Unlike the adult spi nal cord, the immature transmitter release machinery of the developing spinal cord imposes, under normal conditions, a prolonged synaptic de pression that masks facilitation and TP but allows occasionally the ex pression of moderate PTP. Substantial expression of facilitation TP an d PTP is revealed only by conditions that decrease calcium influx to p resynaptic terminals, and, concomitantly, transmitter release. The lon g duration of PTP in low-calcium solution in the neonatal rat may be a ttributed to slower kinetics of processes involved in extrusion and in traterminal buffering of calcium in the developing mammalian spinal co rd.