Motoneurone excitability after administration of a thyrotrophin releasing hormone analogue

Recording have been made of motoneurone field potentials at vertebral level L1 to stimulation of the ventral root, in rats anaesthetized with urethane. Injection of thyrotrophin releasing hormone (TRH) analogue RX77368 (2 mg kg-1) produced an increase in the amplitude of the field potential within 1 min of injection, reaching peak amplitude of 51-69% above control values 6-12 min after injection; the duration of the elevation was 30-65 min. Arguments are presented which show that this effect is compatible with an RX77368-induced depolarization of the motoneurone, allowing the antidromic volley to invade more cell bodies.