Actions of methoxamine and tryptamine and their interactions with cyproheptadine and phenoxybenzamine on cat spinal cord segmental reflexes.

The effects of norepinephrine, methoxamine and tryptamine were assessed on the monosynaptic and polysynaptic segmental reflexes in the unanesthetized, decerebrated acute spinal cat. Their selectivity of action was determined by studying the interactions of methoxamine and tryptamine with two antagonists, cyproheptadine and phenoxybenzamine. Potentials were evoked by stimulating either the L7 or S1 dorsal root and were recorded from the corresponding ipsilateral ventral root. Norepinephrine did not affect reflex activity, whereas methoxamine facilitated both the monosynaptic and polysynaptic potentials in a dose-related manner when infused over 20 minutes. Tryptamine facilitated both the monosynaptic and polysynaptic reflex potentials. This increase was dose related for the monosynaptic reflex but not for the polysynaptic reflex. Phenoxybenzamine blocked the facilitatory effects of methoxamine and did not antagonize the effects of tryptamine on the segmental reflex. The facilitatory effects of tryptamine were effectively blocked by cyproheptadine. Cyproheptadine failed to reduce the polysynaptic response to methoxamine, although it partially antagonized the monosynaptic facilitation. These findings demonstrate that methoxamine and tryptamine facilitate the segmental reflex by different modes of action and provide additional evidence for noradrenergic and tryptaminergic systems in the spinal cord.