Desensitization and resensitization rates of glutamate-activated channels may regulate motoneuron excitability

Abstract

1. Single-channel properties of desensitizing glutamate-activated channels were analyzed in outside-out patch-clamp recordings from a motoneuron-enriched cell fraction from embryonic chick. A piezo-driven device was used to achieve fast solution exchange at the electrode tip, resulting in maximum activation within 2 ms. 2. Quisqualate/AMPA receptors, with a 13-pS conductance, desensitized rapidly; the desensitization rate depended on agonist concentration but not on membrane potential. When quisqualate was applied slowly, the quisqualate-activated channels desensitized without prior channel opening, indicating desensitization from the closed state. After a 10-ms refractory period, resensitization of all channels required up to 300 ms; resensitization rate did not depend on the duration of the preceding quisqualate application. 3. At agonist concentrations less than or equal to 1 mM, kainate receptors, with a 20-pS conductance, did not desensitize. At kainate concentrations greater than or equal to 1 mM, though, kainate receptors desensitized to a low steady-state conductance within approximately 200 ms. Resensitization of all channels required as long as 3 s, which could render kainate receptors inexcitable during high-frequency activation. 4. Desensitization rates of whole-cell currents were similar to those observed in outside-out mode. Glutamate- and quisqualate-activated responses were similar, suggesting that the rapidly desensitizing quisqualate-sensitive receptor type may dominate the kinetics of whole-cell excitatory postsynaptic currents (EPSCs) in this preparation. 5. It may be concluded that the efficacy of glutamate-mediated synaptic transmission is modulated by differences in the rates of desensitization and resensitization.

AMPA Receptor Current Density, Not Desensitization, Predicts Selective Motoneuron Vulnerability

“…AMPA receptors in motoneurons consistently exhibited fast deactivation and rapid, extensive desensitization. AMPA receptor desensitization kinetics in rat motoneurons were similar to those previously measured in chick spinal motoneurons (Smith et al, 1991). Deactivation and desensitization kinetics of AMPA receptors in motoneurons also resembled AMPA receptor kinetics in cerebellar Purkinje neurons (Raman et al, 1994;Häusser and Roth, 1997), were slower than AMPA Figure 5.…”

Synaptic Control of Motoneuronal Excitability

“…Thus high levels of GluR4 in spinal and cranial motoneurons in conjunction with reduced levels of GluR2 in spinal motoneurons may predict AMPA receptor complexes with rapid deactivation and desensitization kinetics (416,868) as well as slow recovery rates from desensitization (752). For example, lumbar motoneurons in chicks have rapidly gated receptors with rapid desensitization kinetics (1166) and glutamatergic inspiratory drive to hypoglossal motoneurons is potentiated by cyclothiazide (which blocks AMPA receptor desensitization) (406). Not only are the rapid desensitization kinetics of AMPA receptors likely to play a role in shaping synaptic currents (50, 406,1166), but rates of desensitization threefold faster than rates of resensitization can produce rapid frequency-dependent modulation of excitability, since synapses rich in this type of receptor should become much less excitable during high-frequency activation.…”

“…For example, lumbar motoneurons in chicks have rapidly gated receptors with rapid desensitization kinetics (1166) and glutamatergic inspiratory drive to hypoglossal motoneurons is potentiated by cyclothiazide (which blocks AMPA receptor desensitization) (406). Not only are the rapid desensitization kinetics of AMPA receptors likely to play a role in shaping synaptic currents (50, 406,1166), but rates of desensitization threefold faster than rates of resensitization can produce rapid frequency-dependent modulation of excitability, since synapses rich in this type of receptor should become much less excitable during high-frequency activation. Furthermore, if desensitization kinetics are subject to endogenous modulation similar to that produced by exogenous drugs (50, 406,1126), allosteric modulation of desensitization kinetics can provide a powerful mechanism for modulating motoneuron excitability to glutamatergic inputs.…”

Proton-activated currents in chick spinal motoneurons

“…The effectiveness of this procedure for enrichment of motoneurons was verified occassionally during the course of these and related experiments (e.g. Smith et al 1991) by labeling the motoneurons via cut ventral roots with the fluorescent dye rhodamine prior to dissociation. The percentage of recovered cells containing the fluorescent label and were thus identified specifically as motoneurons ranged from 88 to 97%; the average (n = 12) was 92%.…”

“…Membrane currents were recorded at room temperature (22 to 24~ in whole-cell (n = 46) or outside-out (n = 14) configuration (Smith et al 1991). Prior to recording, the DME medium was replaced by HEPES-buffered bath saline containing (mM) NaCI (140), KC1 (5.3), CaC12 (2), Na/HPO 4 (0.67), KH2PO 4 (0.22), HEPES (15), and glucose (5.6).…”