Characterization of antinociception to opioid receptor selective agonists after antisense oligodeoxynucleotide-mediated "knock-down" of opioid receptors in vivo

Pharmacological studies in vivo and in vitro have suggested the existence of subtypes of the delta opioid receptor termed delta₁ and delta₂ (δ₁ and δ₂). The hypothesis of subtypes of ä receptors was further explored by assessing the effects of administration of antisense or mismatch oligodeoxynucleotides (ODN) in vivo to the cloned DOR, or to a conserved region of the cloned opioid receptors, on the antinociceptive responses elicited by selective μ, κ and opioid receptor agonists in mice. Additionally, the density of opioid δ receptors in brain after δ opioid receptor (DOR) ODN treatment was investigated. Repeated twice daily intracerebroventricular (i.c.v.) administration of DOR antisense, but not mismatch, ODN, produced a dose-and time-related blockade of i.c.v. [D-Ala², Glu⁴]deltorphin (δ₂ agonist), but not [D-Pen², D-Pen⁵]enkephalin (δ₁ agonist), antinociception. The antinociceptive responses to selective μ and κ opioid agonists were unaffected by DOR antisense or mismatch ODN treatments. The antinociceptive effect of an A₉₀ dose of [D-Ala², Glu⁴]deltorphin was significantly reduced by the third day of DOR antisense ODN administration and persisted over a treatment period of 6 days with recovery by the third posttreatment day. Saturation studies in mouse whole brain preparations with the selective δ-radioligand [³H]naltrindole showed that DOR antisense, but not mismatch, ODN treatment produced a significant time-related reduction in B_max values of approximately 30 to 40% by day 6, without changing the K_d value. The reduction in DOR density was reversible and returned to control levels within 3 days after cessation of antisense ODN treatment. The i.c.v. administration of an antisense, but not mismatch, ODN directed to a conserved region of the cloned opioid receptors, termed common opioid receptor antisense ODN, inhibited the antinociceptive effects of i.c.v. μ, κ and δ agonists, including [D-Pen², D-Pen⁵]enkephalin. These data further support the hypothesis of subtypes of opioid δ receptors.