The effect of thyroliberin and some of its analogues on the hind limb flexor reflex in the spinal rat.

Thyroliberin (TRH) (0.5-4 mg/kg) enhanced the flexor reflex (increase in the reflex amplitude) in a dose-dependent way. A similar though weaker effect was exerted by one of its analogues examined in our experiment (Pyr-His-Pro-NH-NH2 . 2HCI, 2--8 mg/kg). The other two analogues tested (Pyr-Tyr-Pro-NH2 and Pic-His-Pro-NH2) were inactive in this respect. TRH (0.5-8 mg/kg) produced no effect on the neuro-muscular transmission. Serotoninolytics (metergoline, pizotifen) and noradrenolytics (phenoxybenzamine, haloperidol) did not counteract the TRH-induced stimulation of the reflex. On the other hand, ti was found that TRH enhanced the stimulating effect of LSD and, especially, of clonidine on the flexor reflex. The results obtained suggest that the stimulatory action of TRH on the flexor reflex is not connected with its direct effect either on the serotoninergic or nonradrenergic transmission, and that TRH increases the reactivity of central noradrenaline receptors and, to a smaller extent, of serotonin ones.