Motoneurone excitability during antidromically evoked inhibition after administration of a thyrotrophin releasing hormone (TRH) analogue.

Acute experiments have been performed in urethane anaesthetised rats to determine the effect of a TRH analogue (RX77368) on the excitability of motoneurone pools in the lumbar region of the spinal cord to paired antidromic stimuli ( interval 10ms) delivered to the ventral root, by measuring the field potentials produced. Following the response (CR) to the first stimulus (conditioning stimulus), the response (TR) to the second stimulus (test stimulus) is reduced in size. In control experiments, the greater the amplitude of CR, the smaller TR becomes. After RX77368 injection, CR progressively increases over the initial post injection period. It would be expected therefore that during this time there would be a progressive decline in TR relative to CR. It has been shown however, that over this period there is no decline of TR relative to CR. There is speculation on a possible role of putative neuromodulators such as TRH in the maintenance of control parameters during motoneurone recruitment and/or increases in discharge frequency.