Studies on drug-induced neuropathies. III. Motor nerve deficit in cats with experimental acrylamide neuropathy

To assess motor nerve and motor nerve terminal function in acrylamide neuropathy, cats were given i.m. injections of acrylamide (15 mg/kg) daily for 10 days to induce a peripheral neuropathy. Tests of function were performed on the day of the 10th injection (day 0) and 7, 21 and 35 days thereafter. In untreated animals tetanic conditioning evoked stimulus-bound repetition (SBR) in 85% of soleus alpha-motoneurones. Following administration of acrylamide, the percent of axons elaborating SBR were: day 0 -- 79%, day 7 -- 71%, day 21 -- 31%, day 35 -- 22%. The response of soleus muscle to SBR is normally a post-tetanic potentiation (PTP) of contractile tension which is proportional to the tetanic conditioning frequency; during the development of the neuropathy, PTP in response to all tetanic frequencies progressively declined, concomitant with and as a result of the declining incidence of SBR. These data indicate that initial functional alterations in motor nerves during acrylamide neuropathy occurs at the level of the nerve terminal, preceding alterations in conduction velocities in the axons. However, the motor nerve deficit is not adequate, in either time to onset or severity, to account for the clinical manifestations of the neuropathy. The possible contribution to clinical signs of the neuropathy made by lesions to other peripheral nerves is discussed.

Glucocorticoids and mammalian motor nerve excitability

Short-term treatment of cats with high doses of triamcinolone and related steroids strikingly increased the capacity of soleus motoneurons to generate posttetanic stimulus-bound repetition (SBR) and the obligatory postetanic potentiation (PTP) of muscle. The edrophonium chloride (Tensillon)-induced SBR and twitch potentiation were likewise augmented. These effects reflect an increase in the excitability of the motoneuron. This glucocorticoid effect suggests that the motoneuron is the site of the antimyasthenic action of the hormone. Certainly, the enhanced SBR is a neuronal representation of the adverse epileptogenic action of the glucocorticoids. The glucocorticoid effect on motoneuron outlasts the dosing period, suggesting an underlying alteration in the neuron. Other glucocorticoids caused the same effects, but varied in their potencies. Mineralocorticoids were less effective. The single androgen that was tested proved to be minimally effective.

Motor nerve terminal response to edrophonium in delayed DFP neuropathy

A localized peripheral neuropathy was induced in cats with di-isopropyl fluorophosphate (DFP). Soleus nerve-muscle preparations, and the motor nerve terminals in particular, were evaluated for responsiveness to edrophonium (200 mug/kg i.v.). Potentiation of contractile strength was absent 24 hr after DFP, and showed a trend toward recovery 7-14 days post-DFP; it then fell to about 25% of normal 3 weeks following DFP administration. During the ensuing 5 weeks this aspect of edrophonium responsiveness was largely regained. The underlying post-drug repetition which gives rise to the potentiated responses was not demonstrable in either the nerve or muscle 3 weeks after DFP, but was again observed 8 weeks after poisoning. These findings suggest a delayed peripheral neuropathy indicative of a trophic deprivation and further illustrate a motor nerve terminal deficit as the initial function alteration in DFP neuropathy.