Thyrotropin releasing hormone-induced potentiation of spinal monosynaptic reflex in rats in vitro

Superfusion of thyrotropin-releasing hormone (TRH) in neonatal rat spinal cord in vitro produced dose (0.01-1.00 microM) dependent potentiation of monosynaptic reflex (MSR) which was maximum (44% of control) at 1 microM of TRH. But no ventral root depolarization was observed with TRH (1 microM) although potassium concentration out side ([K+]0) when increased produced a depolarization at the magnitude of 0.2 mV/mM of [K+]0. TRH-induced potentiation of MSR was not altered in spinal cords, obtained from the animals pretreated with 5,7-dihydroxytryptamine or 6-hydroxydopamine. Neither serotonin antagonists (spiperone, ketanserin, cyproheptadine or 3-troponyl-indole-3-carboxylate) nor adrenergic antagonists (phentolamine or haloperidol) could attenuate TRH-induced potentiation. Inhibition of MSR observed in the spinal cord elicited by stimulating the adjacent dorsal root was unaffected by TRH. The results suggest that, TRH potentiates MSR by directly acting on the motoneurons, without involving presynaptic serotonergic or catecholaminergic neuronal systems or the disinhibition of pre- or post-synaptic inhibition in the spinal cord.