Anatomical distribution of mu, delta, and kappa opioid‐ and nociceptin/orphanin FQ‐stimulated [ 35 S]Guanylyl‐5′‐ O ‐(γ‐Thio)‐triphosphate binding in guinea pig brain

An in vitro autoradiographic technique has recently been developed to visualize receptor‐activated G‐proteins by using agonist‐stimulated [ 35 S]guanylyl‐5′‐ O ‐(γ‐thio)‐triphosphate ([ 35 S]GTPγS) binding in the presence of excess guanosine 5′‐diphosphate. This technique was used to localize opioid‐activated G‐proteins in guinea pig brain, a species that contains the three major types of opioid receptors. This study used selective μ, δ, and κ opioid agonists as well as nociceptin or orphanin FQ (N/OFQ) peptide, an endogenous ligand for an orphan opioid receptor‐like (ORL1) receptor, to stimulate [ 35 S]GTPγS binding in guinea pig brain sections. Opioid receptor specificity was confirmed by blocking agonist‐stimulated [ 35 S]GTPγS binding with the appropriate antagonists. In general, the distribution of agonist‐stimulated [ 35 S]GTPγS binding correlated with previous reports of receptor binding autoradiography, although quantitative differences suggest regional variations in receptor coupling efficiency. Mu, δ, and κ opioid‐stimulated [ 35 S]GTPγS binding was found in the caudate‐putamen, nucleus accumbens, amygdala, and hypothalamus. Mu‐stimulated [ 35 S]GTPγS binding predominated in the hypothalamus, amygdala, and brainstem, whereas κ‐stimulated [ 35 S]GTPγS binding was particularly high in the substantia nigra and cortex and was moderate in the cerebellum. N/OFQ‐stimulated [ 35 S]GTPγS binding was highest in the cortex, hippocampus, and hypothalamus and exhibited a unique anatomical distribution compared with opioid‐stimulated [ 35 S]GTPγS binding. The present study extends previous reports on opioid and ORL1 receptor localization by anatomically demonstrating functional activity produced by μ, δ, and κ opioid and ORL1 receptor activation of G‐proteins. J. Comp. Neurol. 386:562–572, 1997. © 1997 Wiley‐Liss, Inc.