Distinct inhibitory effects of spinal endomorphin‐1 and endomorphin‐2 on evoked dorsal horn neuronal responses in the rat

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Abstract
Intrathecal endomorphin‐1 and endomorphin‐2 (0.25–50 μg) dose‐relatedly reduced all components of electrical evoked C‐fibre responses of spinal neurones. These effects were partially reversed by naloxone. Endomorphin‐1, but not endomorphin‐2, dose‐relatedly reduced the Aβ‐fibre evoked responses. Peak inhibitory effects of endomorphin‐1 and −2 were at 15–20 min post‐administration. Thus spinal endomorphin‐2 had selective effects on noxious responses, whereas endomorphin‐1 was non‐selective.