Table 2. Effect of pretreatment with naltrindole on acute antinociceptive potency of selected μ-δ bivalent ligands administered i.c.v.

	ED50, nmol (95% C.I.)
Ligand	No pretreatment	NTI (50 pmol) pretreatment
3 (MDAN-16)	1.70 (1.50–2.00)	0.30 (0.19–0.43)
6 (MDAN-19)	0.43 (0.36–0.50)	0.05 (0.05–0.07)
8 (MDAN-21)	0.08 (0.06–0.10)	0.06 (0.05–0.07)
11 (MA-19)	0.04 (0.023–0.045)	0.08 (0.065–0.085)

NTI was administered i.c.v. 10 min before each compound, and tail-flick response was measured at the time of peak compound antinociceptive activity.