Apparent Receptor Kinetics Analyses Of Synaptic Responses And Its Application In LTP Of The Spinal Cord Motoneurons

To explore an analysis method of apparent receptor kinetics of synaptic response andits application in the long-term potentiation (LTP) in motoneurons (MNs), theintracellular recordings were performed in MNs of spinal cord slices isolated fromneonatal rats (8-14days old), and excitatory postsynaptic potentials (EPSPs) were evokedby ipsilateral ventrolateral funiculus (iVLF) electrical stimulation (iVLF-EPSP). Threetypes of iVLF-EPSPs with different kinetic process evoked by threshold stimulusintensity (T) were selected to conduct apparent receptor kinetics analysis. The linearizedformulas of receptor kinetics of association and dissociation kinetic curves were appliedto estimate the apparent association rate constant (K1), apparent dissociation rate constant(K2), apparent equilibrium dissociation constant (KT) and apparent maximum response(Vmax) by linear regression analysis of the rising phase and declining phase ofiVLF-EPSPs. Morever, the effects of iVLF tetanic electrical stimulation on iVLF-EPSP、K1、K2and KTwere observed. The results were reported as follows：1. Parametric analyses of iVLF-EPSPs in6MNs indicated that the amplitude andarea under curve were both positively correlated with the intensity of stimulation (P <0.01and P <0.05) while the maximum left slope of iVLF-EPSP was negativelycorrelated with the intensity of stimulation (P <0.05), all of which were stimulusintensity-dependent.2. By linear regression analyses of3types of iVLF-EPSPs selected from the6MNsmentioned above with different rising phase and declining phase evoked by thresholdstimulation, the values of correlation coefficient (r) were0.9397-0.9621(all P <0.01) and-0.8836--0.9690(all P <0.01) for linearization of association and dissociationkinetic curves, respectively. And the values of K1, K2, KTand Vmaxwere estimated to be0.364-2.330T-1·ms-1,0.055-0.189ms-1,0.025-0.284T and1.968-4.367mV.3. Apparent receptor kinetics analyses of iVLF-EPSPs in6MNs indicated that K2was negatively correlated with the intensity of stimulation (P <0.05), while the linearcorrelation between K1, KTand stimulus intensity was not detected.4. Among6MNs which were applied the tetanic electrical stimulation (100Hz,50pulses/train, duration0.4-1.0ms,6trains, main interval10seconds,10-100V) on iVLF,the amplitude of iVLF-EPSPs was enhanced to or above120%of the baseline and theenlargement lasted longer than30min in two MNs, which could be referred to as LTP(iVLF-LTP).5. During iVLF-LTP elicited in one MN, the area under curve of iVLF-EPSPs wasincreased significantly to300%of the baseline which was consistent with the increasingtrend of EPSP amplitude. The maximum left slope was persistently increased and thelargest enhancement could reach180%of the baseline. In addition, the duration andmaximum right slope of postsynaptic potentials were increased in tendency. But thelatency was unchanged. In the other MN, the area under curve of iVLF-EPSPs wassustained increased after transient decrease and could reach140%of the baseline whilethe duration maintained at baseline after transient decrease, and the latency was decreasedsignificantly within20min.6. During iVLF-LTP, receptor kinetics analyses of iVLF-EPSPs indicated that K2and KTwere reduced significantly to60%and50%of the baseline within10minrespectively, while K1was increased to140%of the baseline. However, there were nosignificant changes of K1, K2and KTin the other MN before and after tetanic stimulation.7. Analyses of membrane electrical properties in MNs with LTP indicated that therewere no obvious changes in the membrane resistance, threshold voltage and overshoot.But threshold current was reduced obviously after tetanic stimulation. In the other MN, the membrane resistance and overshoot were increased while the threshold voltage wasdecreased.These results suggest that the analysis method of apparent receptor kinetics issuitable to analyses of the transmitter-receptor binding kinetics in synaptic response.Morever, the obvious changes in K1, K2and KTwithin10min during LTP indicated anenhancement of postsynaptic receptor affinity. While in the other MN, there were nochanges in K1, K2, KTand postsynaptic receptor affinity which was possibly due toregulation of presynaptic transmitter release. The different postsynaptic receptormechanisms may be involved during early LTP in two MNs. Therefore, analysis methodof apparent receptor kinetics may be a simple and feasible method to estimate the receptorkinetics property in synaptic plasticity research.