Delta-opioid agonist-stimulated inositol phosphate formation in isolated, rabbit iris-ciliary bodies: role of G i/o proteins and Gβγ-subunits

Abstract

Our laboratory has previously demonstrated the ability of kappa- and delta-opioid agonists to decrease aqueous flow rates and intraocular pressure of rabbits. The mechanisms by which these agents act in the ciliary body of the rabbit could involve inhibition of cAMP production, as well as increased generation of inositol phosphates (IPs). With regard to enhanced production of IPs, it has been suggested that their levels can be augmented by stimulation of phospholipase C via opioid receptors linked to either Gα-subunits derived from Gq proteins or Gβγ-subunits derived from G i/o-proteins. The aim of the current study is to investigate the role of pertussis toxin (PTX)-sensitive G-proteins and Gβγ-subunits in delta-opioid agonist-mediated changes in IP production in the isolated, rabbit iris-ciliary body (ICB). In one set of experiments, ICB segments were treated with the delta agonist, SNC80 (10 −12–10 −7 mol/l) alone. Other experiments were conducted utilizing SNC80 following pretreatment with either phosducin (Gβγ-subunit scavenger), PTX, or the delta antagonist, naltrindole. IP production was measured by ion exchange chromatography. Basal levels of IPs in the rabbit ICB were 58 287±2162, 15 218±969 and 2083±367 dpm/mg protein for IP 1, IP 2 and IP 3, respectively. The highly selective delta-opioid receptor agonist, SNC80, produced concentration-dependent increases in the levels of the IPs in the ICB, which were diminished in the presence of the delta antagonist, naltrindole, indicating the effect was mediated via activation of delta-opioid receptors. Pretreatment of tissues with PTX (75 ng/ml), completely abolished the concentration-dependent production of IPs generated by SNC80 (10 −11–10 −7 mol/l). In addition, pretreatment with phosducin (1 nmol/l) ablated the SNC80 (1 nmol/l)-induced increase in the formation of all three inositol phosphates. Results from this study indicate that the delta-opioid receptor-mediated increase in IP production is a PTX-sensitive G i/o response that involves participation of Gβγ-subunits. Thus, delta-opioid receptor activation by SNC80 in the ICB could be responsible, in part, for suppression of aqueous humor dynamics.