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Proceedings of the Seventy‐ninth Meeting of The British Neuro‐pathological Society held at the London Hospital 11–12 January 1990

Experimental cytotoxic models of motor neuron degeneration have exploited a primary lethal action on either DNA (e.g. with doxorubicin) or RNA (with ricin or diphtheria toxin). Using an experimental diphtheritic neuronopathy in cat spinal cord, the possibility of a trans-synaptic passage of diphtheria toxin (DTX) from a group of spinal motor neurons retrogradely intoxicated by intraneural injection of DTX has been examined using a combination of immunocytochemical and colloidal gold-DTX conjugate labelling procedures. Forty-eight hours after unilateral injection of DTX into intercostal nerves T7 or T9 two groups of immunostained ipsilateral neurons were identified. One group comprised large diameter (35 pm) neurons localized to the ventral horn (i.e. motor neurons); the other consisted of variously sized neurons (20-40pm) which were sited more dorsally between the motor nuclei and the central canal. These results strongly suggested a trans-neuronal passage of toxin since there was no evidence of either systemic intoxication or non-specific staining. To provide independent confirmation of this conclusion, evidence was sought for the presence of colloidal gold in neurons 24-48 h following unilateral injection of a 6 nm colloidal gold-DTX conjugate. Light microscopical examinations of silver-enhanced sections of cord revealed an identical labelling