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Effects of intramuscular injection of botulinum toxin and doxorubicin on the survival of abducens motoneurons.

PURPOSE
To investigate in vivo the survival of abducens motoneurons (AMNs) at different periods of time after a single intramuscular injection of the neurotoxin botulinum toxin A (BTxA) or doxorubicin (DXR).


METHODS
In Sprague-Dawley rats, the AMNs were labeled with fluorogold (FG), which was applied intramuscularly in the lateral rectus muscle. The numbers of labeled neurons were determined in adult control animals; in young animals that had received intramuscular injections of 0.125, 0.250, 1, or 2 U BTxA; and in adult rats that had received 100 microg, 200 microg, or 300 microg DXR, at various survival times.


RESULTS
In control animals, the numbers of FG-labeled motoneurons were similar to the numbers found by other investigators with the use of other retrogradely transported tracers; motoneuron numbers diminished with time after FG application. The numbers of FG-labeled neurons in the animals that had been injected with BTxA were similar to those found in control animals. However, there were fewer FG-labeled neurons in the animals injected with DXR.


CONCLUSIONS
Fluorogold injected into the lateral rectus muscle can be used to label the AMNs. However, this tracer does not persist within the cytoplasm of the labeled neurons for more than 37 days. The intramuscular injection of 0.125, 0.250, 1, or 2 U BTxA does not induce significant motoneuron death in young rats 30, 60, or 90 days after the injection. Doxorubicin injected intramuscularly causes variable amounts of motoneuron death that is related both to the survival period and to the amount of DXR injected.