Fig. 13. A model of the peptide actions on the excitatory transmission in the vasoconstrictor muscle. AMRP acts mainly on the presynaptic receptors to inhibit the excitatory transmitter release. Part of this inhibition is assumed to be due to the activation of the 4-AP sensitive K⁺ channels. Because 4-AP does not completely block the inhibitory action, other route to inhibit the transmitter release seems to exist. Enterin activates both pre and postsynaptic receptors. Activation of the postsynaptic receptors induces the opening of the 4-AP sensitive K⁺ channels, and inhibits the contraction. Because 4-AP does not completely inhibit the action of enterin, the presynaptic inhibitory receptors are also assumed. NdWFamide also activates both pre and postsynaptic receptors. Activation of the postsynaptic receptors evokes the muscle contraction, and that of the presynaptic receptors enhances the transmitter release.