Effects of catecholamines on spinal motoneurones and spinal reflex discharges in the isolated spinal cord of the newborn rat.

The effects of catecholamines on spinal motoneurones and spinal reflex discharges were investigated in the isolated spinal cord of newborn rat. Noradrenaline (NA), adrenaline (Adr), dopamine (DA) and isoproterenol (Iso) caused depolarization of the motoneurones in a dose-dependent manner. The depolarizing action persisted in Ca2+-deficient Krebs solution. The order of potency was Adr greater than NA greater than DA much greater than Iso. The effects of NA and Adr on the monosynaptic reflex discharge varied; depression, potentiation or depression followed by potentiation. The polysynaptic reflex discharge was consistently depressed. DA depressed both the mono- and polysynaptic reflex discharges in all the preparations. Tyramine and adamantanamine induced a response similar to that to DA rather than to NA. Depolarization of the motoneurones and the effects on the spinal reflex discharges induced by all the catecholamines were decreased by phentolamine or phenoxybenzamine but not by propranolol or haloperidol. It is suggested that the endogenous catecholamines, mainly DA, depolarize the motoneurones and depress the mono- and polysynaptic reflex discharges through an alpha-adrenoceptor in the spinal cord of the newborn rat.