Influence of afferent synaptic innervation on the discharge variability of cat abducens motoneurones

Abstract

The discharge variability of abducens motoneurones was studied after blocking inhibitory synaptic inputs or both excitatory and inhibitory inputs by means of an intramuscular (lateral rectus) injection of either a low (0.5 ng kg _1 ) or a high dose (5 ng kg _1) of tetanus neurotoxin (TeNT), respectively. Motoneuronal firing increased after low-dose TeNT. High-dose treatment, however, produced a firing depression, and in some cells, a total lack of modulation in relation to eye movements. Firing became increasingly more regular with larger TeNT doses as shown by significant reductions in the coefficient of variation after low-and high-dose treatments. Similarly, autocorrelation histograms of interspike intervals increased the number of resolvable peaks twofold in low-dose-treated motoneurones and sevenfold in high-dose-treated motoneurones. The plots of standard deviation versus the mean instantaneous firing frequency showed an upward deflexion with low firing frequencies. The upward deflexion occurred in controls at 39.9 ± 4.9 ms, an interval similar to the mean afterhyperpolarisation (AHP) duration (48.4 ± 8.8 ms). Low-dose TeNT treatment shifted the deflexion point to 20.9 ± 3.9 ms, whereas the high dose increased it to 60.7 ± 6.1 ms, in spite of the fact that no differences in AHP parameters between groups were found. The density of synaptophysin-immunoreactive boutons decreased by 14 % after the low-dose treatment and 40.5 % after the high-dose treatment, indicating that protracted synaptic blockade produces elimination of synaptic boutons. It is concluded that abducens motoneurone spike variability during spontaneous ocular fixations depends largely on the balance between inhibitory and excitatory synaptic innervation.

Respiratory-related Discharge of Genioglossus Muscle Motor Units

“…This is consistent with recent data in feline geniohyoid muscle motor units, which showed increased motor unit discharge variability on removal of inhibitory afferent inputs from pulmonary stretch receptors (29). Similarly, pharmacologic elimination of excitatory synaptic inputs to abducens motoneurons reduced the CV of the ISI (30). Together, these observations support the view (14) that excitatory synaptic input increases the variability of neural spike train discharge.…”

Nerve Growth Factor Regulates the Firing Patterns and Synaptic Composition of Motoneurons

“…The dependence of the SD of the mean firing rate on the mean firing frequency has been previously described in spinal and extraocular motoneurons (Powers and Binder, 2000;González-Forero et al, 2002). We selected the coefficient of variation (i.e., the percentage ratio of the SD to the mean), because it is insensitive to scale changes and thus adequate to establish comparisons between groups.…”

Synaptic structural modification following changes in activity induced by tetanus neurotoxin in cat abducens neurons

“…Experimental procedures were in accordance with the directive of the European Union (86/609/EEC) and the current Spanish legislation for the use and care of laboratory animals (BOE 67/8509-12, 1988). A single low (0.5 ng/kg) or high dose (5 ng/kg) of TeNT was injected in the left lateral rectus muscle following procedures described extensively in detail previously (Gonzá lez Forero et al, 2002a,b, 2003Pastor and Gonzá lez-Forero, 2003). Two animals received low-dose injections, and six animals were injected with high doses.…”

“…Both somatic and dendritic membrane compartments were analyzed. In these animals inhibitory synapses were effectively blocked, and the ensuing disinhibition induced increased firing activity in both abducens neuronal types (Gonzá lez-Forero et al, 2001, 2002a,b, 2003. In contrast, 7 days following a high-dose injection of TeNT, the density of gephyrin-IR clusters over the somata and primary dendrites of CGRP-IR or calretinin-IR abducens neurons was significantly reduced in the experimental side (ipsilateral to the injection site) compared with control (contralateral to the injection; Figs.…”

“…A low-dose (0.5 ng/kg) TeNT injection in the lateral rectus muscle induces specific blockade of afferent inhibitory signals and increases firing in abducens motoneurons. In contrast, a high-dose injection (5 ng/kg) reduces simultaneously excitatory and inhibitory synaptic activity, resulting in decreased firing (Gonzá lez Forero et al, 2002a,b, 2003. The degree of inhibitory synaptic blockade was, however, the same after a low or a high dose of TeNT (Gonzá lez Forero et al, 2002a,b, 2003.…”