Abstract

The constrained opioid peptide (2S,3R)β-methyl-2′,6′-dimethyltyrosine-l-tetrahydroisoquinoline-3-carboxylic acid [(2S,3R)TMT-l-Tic-OH] exhibits high affinity and selectivity for the δ-opioid receptors (Liao et al., 1997). In the present study, we examined the pharmacological properties of (2S,3R)TMT-l-Tic-OH in mouse brain. A 5′-O-(3-[³⁵S]thiotriphosphate) ([³⁵S]GTPγS) binding assay was used to determine the effect of (2S,3R)TMT-l-Tic-OH on G protein activity in vitro, in mouse brain membranes. δ- (SNC80; (+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxy-benzyl]-N,N-diethyl-benzamide) or μ- (DAMGO; [d-Ala², Me-Phe⁴,Gly(ol)⁵]enkephalin) selective opioid full agonists stimulated [³⁵S]GTPγS binding in mouse brain membranes 150 ± 4.5% and 152 ± 5.7% over the basal level, respectively. (2S,3R)TMT-l-Tic-OH did not influence basal [³⁵S]GTPγS binding in mouse brain membranes but dose dependently shifted the dose-response curve of SNC80 to the right, with a K_e value of 3.6 ± 0.7 nM. In contrast, (2S,3R)TMT-l-Tic-OH had no effect on the dose-response curve of the μ-selective opioid agonist, DAMGO. Warm water (55°C) tail-flick and radiant heat paw-withdrawal tests were used to determine the in vivo nociceptive properties of (2S,3R)TMT-l-Tic-OH in the mouse. Intracerebroventricular injection of (2S,3R)TMT-l-Tic-OH had no significant effect on withdrawal latencies in either nociceptive tests. (2S,3R)TMT-l-Tic-OH (30 nmol/mouse) attenuated deltorphin II- but not DAMGO-mediated antinociception (40 ± 13 and 100% of maximal possible effect, respectively) when administered intracerebroventricularly 10 min before the agonist. Taken together these results suggest that (2S,3R)TMT-l-Tic-OH is a potent highly selective neutral δ-opioid antagonist in mouse brain.