Time constants of facilitation and depression in Renshaw cell responses to random stimulation of motor axons

Abstract

Summary. In 9 adult anaesthetized cats, 22 lumbosacral Renshaw cells recorded with NaCl-filled micropipettes were activated by random stimulation of ventral roots or peripheral nerves. The stimulus patterns had mean rates of 9.5-13 or 20-23 or 45 pulses per second and were pseudo-Poisson; short intervals below ca. 5 ms (except in two cases) were excluded. The Renshaw cell responses were evaluated by two kinds of peristimulus-time histograms (PSTHs). "Conventional" PSTHs were calculated by averaging the Renshaw cell discharge with respect to all the stimuli in a train. These PSTHs showed an early excitatory response which was often followed by a longer-lasting slight reduction of the discharge probability. These two response components were positively correlated. "Conditional" PSTHs were determined by averaging the Renshaw cell discharge with respect to the second ("test") stimulus in pairs of stimuli which were separated by varied intervals, 6. The direct effect of the first "conditional" response was subtracted from the excitation following the second ("test") stimulus so as to isolate the effect caused by the second stimulus per se. After such a correction, the effect of the first "conditioning" stimulus showed pure depression, pure facilitation or mixed facilitation/depression. Analysis of such conditioning curves yielded two time constants of facilitation (ranges: ca. 435 ms and 93-102 ms) and two of depression (ranges: ca. 7-25 ms and 50-161 ms). It is concluded that these time constants are compatible with processes of short-term synaptic plasticity known from other synapses. Other processes such as afterhyperpolarization and mutual inhibition probably are of less importance.

Expression of vesicular glutamate transporters, VGLUT1 and VGLUT2, in cholinergic spinal motoneurons

“…Long‐term potentiation of recurrent inhibition has not yet been explored (Windhorst, 1996), but high‐frequency motoneuron excitation may induce post‐tetanic potentiation (Eccles et al ., 1954) or depression (Hultborn & Pierrot‐Deseilligny, 1979) of RC responses. Random stimulation of motoneurons induces facilitation or depression of RC excitability depending on the strength of their activation (Laouris et al ., 1988). Glutamate released by the motoneurons is a likely candidate for participating in the plasticity of RC excitability.…”

Recurrent Inhibition of Wrist Extensor Motoneurones: A Single Unit Study on a Deafferented Patient

“…This might be in‐keeping with the long‐lasting (up to several hundred milliseconds) muscarinic activation of RCs by the motor axon collaterals (Curtis & Ryall, 1966). Another possibility might be that of a presynaptic facilitation of the recurrent inhibition occurring either at the cholinergic recurrent collateral synapse and/or at the inhibitory synapse between the RC and the α‐MN, as suggested by Laouris et al . (1988).…”

A method to estimate the effects of parallel inputs on neuronal discharge probability

“…Auto-und cross-conditioning curves are of interest in their own right (see, for instance, Windhorst et al 1987Windhorst et al , 1988Laouris et al 1988).…”

“…The experimental methods have been described in detail elsewhere Christakos et al 1987;Laouris et al 1988). Briefly, adult cats were anesthetized initially by i.p.…”

“…But after this (at greater 3's), the curve fluctuates around zero, implying that the activation of the parallel (LGS) input per se did not cause postsynaptic or other changes which might account for the depression in curve a. Therefore, this depression likely is homosynaptic and due to presynaptic depression of transmitter release (see also Laouris et al 1988). The depression evident in the (thick) curves cl and c2 is stronger than that apparent in curve a.…”