enkephalin--ala(2)-mephe(4)-gly(5)- and mastoparan

Mastoparan activated in a concentration-dependent manner the low Km GTPase activity in P2 fractions from mouse periaquedultal grey matter (PAG). This peptide at 1-10 mM produced increases of 30-70% over the basal value of 90-120 pmol Pi/mg/min. A series of substances displaying antagonist activity at cellular receptors and not modifying the GTPase function, when used at nanomolar and micromolar concentrations enhanced the effect of mastoparan upon this enzyme. These included antagonists of receptors coupling G proteins: naloxone (non selective opioid antagonist), CTOP (m opioid receptors), ICI 174,864 (d opioid receptors), nor-BNI (k opioid receptors), sulpiride (D2 dopaminergic antagonist), idazoxan (a2 adrenergic antagonist). Bicuculline, antagonist of a receptor not linked to G proteins, GABAA, did not alter the effect of mastoparan on the GTPase. The m opioid agonist, DAMGO, prevented naloxone from increasing the function of the mastoparan-activated enzyme. Thus, mastoparan appears to act on Gi/Go proteins at a site not directly related to the receptor binding domain.

Topics: Animals; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; Enzyme Activation; GTP Phosphohydrolases; GTP-Binding Proteins; In Vitro Techniques; Intercellular Signaling Peptides and Proteins; Kinetics; Male; Mice; Naloxone; Peptides; Periaqueductal Gray; Receptors, Cell Surface; Somatostatin; Wasp Venoms

Intracerebroventricular (i.c.v.) administration of the venom peptide, mastoparan, to mice decreased to a limited extent opioid-induced supraspinal analgesia in a non-competitive fashion. The mu-opioid receptor agonists, [D-Ala2,N-MePhe4,Gly-ol5]-enkephalin (DAMGO) and morphine, the mu/delta-opioid receptor ligands, human beta-endorphin-(1-31) and [D-Ala2,D-Leu5]-enkephalin (DADLE), and the selective ligands of delta-opioid receptors, [D-Pen2,5]enkephalin (DPDPE) and [D-Ala2]deltorphin II, showed an impaired analgesic effect in mice given mastoparan. Mastoparan diminished the analgesic activity of DPDPE and [D-Ala2]deltorphin II to the same extent as observed after giving the delta-opioid receptor-selective antagonist, ICI 174864. The mu-opioid receptor-mediated analgesia that remained after mastoparan was abolished in the presence of the opioid antagonist, naloxone. Mastoparan after binding to Gi alpha/Go alpha subunits could block opioid antinociception. The existence of a class of G protein functionally coupled to mu-opioid receptors, but resistant to the effect of mastoparan is suggested.

Topics: Amino Acid Sequence; Analgesics; Animals; Dose-Response Relationship, Drug; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, D-Penicillamine (2,5)-; Enkephalin, Leucine-2-Alanine; Enkephalins; GTP-Binding Proteins; Injections, Intraventricular; Intercellular Signaling Peptides and Proteins; Male; Mice; Molecular Sequence Data; Peptides; Receptors, Opioid; Wasp Venoms