Influence of afferent synaptic innervation on the discharge variability of cat abducens motoneurones.
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.
González-Forero
David
D
Departamento de Fisiología y Zoología, Universidad de Sevilla, 41012-Sevilla, Spain.
Alvarez
Francisco J
FJ
de la Cruz
Rosa R
RR
Delgado-García
José María
JM
Pastor
Angel M
AM
D000010:Abducens Nerve / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D002415:Cats; D019706:Excitatory Postsynaptic Potentials / Q000502:physiology; D005110:Extracellular Space / Q000187:drug effects / Q000378:metabolism; D007091:Image Processing, Computer-Assisted; D007150:Immunohistochemistry; D008564:Membrane Potentials / Q000502:physiology; D009046:Motor Neurons / Q000187:drug effects / Q000502:physiology*; D009475:Neurons, Afferent / Q000187:drug effects / Q000502:physiology*; D017729:Presynaptic Terminals / Q000187:drug effects; D013569:Synapses / Q000187:drug effects / Q000502:physiology*; D016708:Synaptophysin / Q000494:pharmacology; D013744:Tetanus Toxin / Q000494:pharmacology; D014722:Vestibule, Labyrinth / Q000187:drug effects / Q000502:physiology
D016708:Synaptophysin; D013744:Tetanus Toxin