Arginine vasopressin induces periaqueductal gray release of enkephalin and endorphin relating to pain modulation in the rat

Previous study has proven that microinjection of arginine vasopressin (AVP) into periaqueductal gray (PAG) raises the pain threshold, in which the antinociceptive effect of AVP can be reversed by PAG pretreatment with V2 rather than V1 or opiate receptor antagonist. The present work investigated the AVP effect on endogenous opiate peptides, oxytocin (OXT) and classical neurotransmitters in the rat PAG. The results showed that AVP elevated the concentrations of leucine–enkephalin (L–Ek), methionine–enkephalin (M–Ek) and β-endorphin (β-Ep), but did not change the concentrations of dynorphinA1–13 (DynA1–13), OXT, classical neurotransmitters including achetylcholine (Ach), choline (Ch), serotonin (5-HT), γ-aminobutyric acid (GABA), glutamate (Glu), dopamine (DA), norepinephrine (NE) and epinephrine (E), and their metabolic products in PAG perfusion liquid. Pain stimulation increased the concentrations of AVP, L–EK, M–Ek, β-Ep, 5-HT and 5-HIAA (5-HT metabolic product), but did not influence the concentrations of DynA1–13, OXT, the other classical neurotransmitters and their metabolic products. PAG pretreatment with naloxone — an opiate receptor antagonist completely attenuated the pain threshold increase induced by PAG administration of AVP, but local pretreatment of OXT or classical neurotransmitter receptor antagonist did not influence the pain threshold increase induced by PAG administration of AVP. The data suggested that AVP in PAG could induce the local release of enkephalin and endorphin rather than dynophin, OXT and classical neurotransmitters to participate in pain modulation.