Differentiation between rat brain and mouse vas deferens δ opioid receptors

Certain enkephalin analogues, including those which contain the conformationally restricted amino acid E-(2R,3S)-cyclopropylphenylalanine ((2R,3S)-{down triangle, open}^EPhe), have been shown to have high affinity for brain δ opioid receptors but are much less active in mouse vas deferens bioassays. To investigate whether there are differences between δ opioid receptors in brain and mouse was deferens, the ability of a selective δ opioid compound. [D-Pen²,pCl-Phe⁴,D-Pen⁵]enkephalin (pCl-DPDPE), and [D-Ala²,(2R,3S)-{down triangle, open}^EPhe⁴,Leu⁵]enkephalin methyl ester (CP-OMe), to inhibit [³H]pCl-DPDPE binding in both rat brain and mouse vas deferens were measured. pCl-DPDPE recognized brain and mouse vas deferences binding sites with equal affinity, however, CP-OMe showed 33 fold lower affinity in mouse vas deferens compared to brain. This suggests that mouse vas deferens δ opioid receptors may be distinct from brain δ opioid receptors.