Effect of metrazol on the discharge characteristics of muscle spindle afferents from the hind limb of the rat.

The nature of altered fusimotor discharge induced by subconvulsive doses of Metrazol has been investigated in deeply anaesthetized rats. Large amplitude stretching (ramp and hold or sinusoidal) of the appropriate muscle during the course of drug action shows Metrazol to have a predominant gamma d effect on primaries, and an exclusive gamma s effect on secondaries in both plantar and dorsiflexors of ankle and toes. Small amplitude stretches revealed gamma s activity which large amplitude stretching failed to show. Experiments with convulsive doses of Metrazol show that this elevated spindle discharge precedes development of electromyographic activity. It is suggested that fusimotor activity must be considered when analysing the mechanisms of Metrazol-induced seizures in non-paralysed animals.