Alkylation of opioid receptors by 5'-naltrindole-isothiocyanate injected into the nucleus accumbens of rats : Receptor selectivity and anatomical diffusion

Subtypes of the δ opioid receptor (Oprd1) have been suggested based on pharmacology studies. However, these subtypes have not been confirmed biochemically using either receptor binding assays or molecular cloning. Naltrindole-5′-isothiocyanate (5′-NTII) is an irreversible opioid antagonist that appears to selectively inhibit the actions of a subset of δ opioid agonists in vivo, referred to as putative δ-2 agonists. The biochemical and anatomical selectivity of wash-resistant inhibition of binding of [3H]DAMGO (Oprm1), [3H]DPDPE (Oprd1, putative subtype 1 agonist), or [3H]deltorphin II (Oprd1, putative subytpe 2 agonist) in coronal sections was assessed using quantitative in vitro autoradiography following injection of 5′-NTII into the nucleus accumbens in rats. 5′-NTII decreased [3H]deltorphin II to a greater extent than the binding of the other two radioligands following administration of 0.05–2.5 nmol. The effects of 5′-NTII were largely confined to the nucleus accumbens; however, some loss in the ventral caudate was also noted. In contrast, administration of the nonselective opioid receptor alkylating antagonist β-chlornaltexamine (β-CNA) over a similar range of doses was found to be nonselective for either δ radioligand, and produced greater inhibition of Oprm1 relative to Oprd1 binding, consistent with the nonselective pharmacological activity of this antagonist. Although 5′-NTII inhibited [3H]deltorphin II binding to a greater extent, the binding of the other two radioligands was decreased over a similar range of doses. Absolute conclusions regarding the involvement of δ-2 opioid receptors in pharmacological or physiological effects based on studies with 5′-NTII should therefore be tempered, and for site-directed studies it would be best to employ doses of 0.5 nmol or lower. Synapse 60:384–391, 2006. © 2006 Wiley-Liss, Inc.