Table 1.

Opioid Receptor Subtypes and Naltrexone (NTX)–Binding Affinity^45,197–203

Opioid Receptor	Endogenous Ligand(s)	Primary Effects	NTX Binding Affinity	Relative 6βN Binding Affinity
μ	β-endorphin	Euphoria	Referent (0.0825–1 nM)	35%–50% of NTX (0.74–2.1 nM)
κ	Dysnorphin A and B Neoendorphine	Dysphoria Stress Negative affect	15%–25% of μ (0.509–3.9 nM)	~50% of NTX (2.0–7.4 nM)
δ	Met-enkephalin Leu-enkephalin	Anxiolysis Positive affect	≤1% of μ (8.02–149 nM)	~25% of NTX (29–213 nM)

6βN, 6-β-naltrexol