EFFECTS OF BOTULINUM NEUROTOXIN TYPE-A ON ABDUCENS MOTONEURONS IN THECAT - ULTRASTRUCTURAL AND SYNAPTIC ALTERATIONS

The synaptic alterations induced in abducens motoneurons by the inject ion of 3 ng/kg of botulinum neurotoxin type A into the lateral rectus muscle were studied using ultrastructural and electrophysiological tec hniques. Motoneurons identified by the retrograde transport of horsera dish peroxidase showed a progressive synaptic stripping already notice able by four days post-injection which increased over the study period . By 35 days post-injection, the normal coverage of motoneurons by syn aptic boutons (66.4 +/- 4.0%) significantly decreased to 27.2 +/- 4.0% . Synaptic boutons detached by a widening of the subsynaptic space but remained apposed by synaptic contacts and desmosomes to the motoneuro n. Detachment did not affect equally flat and round vesicle-containing boutons. The control motoneuron had almost equal numbers of both type s of boutons, but after 35 days post-injection the ratio of round to h at vesicle-containing boutons was 1.20 +/- 0.01. Synaptic boutons impi nging on motoneurons showed signs of alterations in membrane turnover, as indicated by an increase in the number of synaptic Vesicles and a decrease in the number of coated vesicles and synaptic vesicles near t he active zone. Abducens motoneurons had a transient increase in soma size by 15 days that returned to normal at 35 days, but no signs of ch romatolysis or organelle degeneration were seen. Accompanying the swel ling of motoneurons, a 15-fold increase in the number of spines, very infrequent in controls, was observed. Spines located in the soma and p roximal dendritic trunk received synaptic contacts from both flat and round vesicle-containing boutons that could be either partly detached or completely attached to the motoneuron. An increased turnover of the plasmatic membrane of the motoneuron was observed, as indicated by a four-fold increase in the number of somatic coated vesicles. Animals w ere implanted with bipolar electrodes in the ampulla of both horizonta l semicircular canals for evoking contralateral excitatory and ipsilat eral inhibitory postsynaptic potentials. Motoneurons were antidromical ly identified from the lateral rectus muscle. Synaptic potentials of v estibular origin were recorded in abducens motoneurons. In the period between two and six days post-injection, a complete abolition of inhib itory synaptic potentials was observed. By contrast, excitatory synapt ic potentials remained, but were reduced by 82%. The imbalance between excitatory and inhibitory inputs to motoneurons induced a progressive increase of firing frequency within a few stimuli applied to the cont ralateral canal. Between 7 and 15 days post-injection, both excitatory and inhibitory postsynaptic potentials were virtually abolished and r emained so up Co the longest time checked (105 days). Some motoneurons recorded beyond 60 days post-injection showed signs of recovery of ex citatory postsynaptic potentials. During the whole time-span studied, presynaptic wavelets were present, indicating no affecting of the cond uction of afferent volleys to the abducens nucleus. Taken together, th ese data indicate that botulinum neurotoxin at high doses causes profo und synaptic alterations in motoneurons responsible for the effects se en in the behavior of motoneurons recorded in alert animals. (C) 1997 IBRO. Published by Elsevier Science Ltd.