Developmental changes of impulse transmission in rat gastrocnemius muscles revealed by neostigmine

Developing rats between 5 and 44 days of age as well as rats about 2 months old were anesthetized with urethane. Spontaneous activities and muscle potentials to sciatic stimuli were recorded from their exposed medial gastrocnemius muscles using concentric electrodes. Neostigmine of 0.12-0.40 mg/kg was injected into the contralateral muscles. Regardless of age, spontaneous activities were not observed and muscle potentials were evoked by single stimuli primarily in a biphasic wave (main component) before the drug treatment. Developmental differences were revealed in the presence of the drug. 1) Spontaneous activities were detected only in single spikes around postnatal 10 days. Single or double spikes were often found in rats of about two weeks. Burst discharges such as seen in adult rats were observed immediately before weaning. 2) In rats of 2 weeks or so, one or more components were observed obscurely following the main component of muscle potentials, which appeared definitely at the preweaning period. 3) When double shocks with the interval of 2 sec were delivered in the presence of the drug, the second potential was greatly depressed in rats older than two weeks as well as in adult rats. The potential was only slightly reduced in rats around 10 days. Thus, an adult pattern as to impulse transmission was observed immediately before weaning. These alterations would, at least in part, reflect the maturation of acetylcholine receptors at neuromuscular junctions.