Characterization of a high-affinity galanin receptor in the rat anterior pituitary: absence of biological effect and reduced membrane binding of the antagonist M15 differentiate it from the brain/gut receptor.
Structure-activity studies demonstrate that galanin fragments 1-15 and 2-29 are fully active, whereas fragment 3-29 has been reported to be inactive, in a number of different in vivo models. M15, a chimeric peptide comprising galanin 1-13 and substance P5-11, has recently been found to be a potent galanin antagonist. Direct effects of galanin at the level of the pituitary have been defined, yet, paradoxically, a number of studies have been unable to demonstrate galanin binding to an anterior pituitary receptor. Porcine galanin stimulated prolactin release from dispersed rat anterior pituitary cells up to 180% +/- 12% (mean +/- SEM) of control secretion. The addition of a specific galanin antiserum caused a profound inhibition of basal prolactin release, maximal inhibition being 12% +/- 0.5% of control secretion. Addition of M15 produced no effect on basal or galanin-stimulated prolactin release. Galanin fragment 3-29 was fully active when compared to galanin 1-29. Fragments 5-29 and 8-29 stimulated prolactin release to a lesser extent and galanin 1-15, 10-29, and 20-29 had no significant prolactin-releasing activity. Using [mono(125I)iodo-Tyr26]galanin or porcine 125I-labeled Bolton-Hunter [mono(125I)iodo-Lys25]galanin, no anterior pituitary membrane binding was observed. In contrast, 125I-labeled Bolton-Hunter N-terminally labeled galanin allowed characterization of a single high-affinity anterior pituitary galanin receptor with a Kd of 4.4 +/- 0.34 nM and a Bmax of 79 +/- 8.3 fmol/mg of protein. The IC50 for porcine galanin was 0.51 +/- 0.04 nM but for M15 was in excess of 10 microM. Galanin 3-29 fully displaced the label with an IC50 of 0.96 +/- 0.7 nM. The IC50 for galanin 5-29 was 200 nM, whereas 8-29 and 1-15 were > 1 microM. Galanin 10-29 and 20-29 failed to displace the label. These data suggest the presence of a high-affinity pituitary galanin receptor, designated GAL-R2, in which region 3-10 and amino acid 25 are crucial for membrane binding and biological activity, in contrast to the known gut/brain galanin receptor (designated GAL-R1). A number of tissues known to bind or respond to galanin were screened. GAL-R2 would appear to be expressed only in the anterior pituitary and hypothalamus.
Wynick
D
D
Department of Medicine, Hammersmith Hospital, London, United Kingdom.
Smith
D M
DM
Ghatei
M
M
Akinsanya
K
K
Bhogal
R
R
Purkiss
P
P
Byfield
P
P
Yanaihara
N
N
Bloom
S R
SR
D000818:Animals; D001667:Binding, Competitive; D002462:Cell Membrane / Q000378:metabolism; D005260:Female; D019004:Galanin / Q000031:analogs & derivatives; D066298:In Vitro Techniques; D007700:Kinetics; D009479:Neuropeptides / Q000378:metabolism / Q000494:pharmacology; D010446:Peptide Fragments / Q000494:pharmacology; D010455:Peptides / Q000378:metabolism* / Q000494:pharmacology*; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism*; D011388:Prolactin / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D044088:Receptors, Galanin; D011964:Receptors, Gastrointestinal Hormone / Q000378:metabolism*; D013373:Substance P / Q000031:analogs & derivatives*; D013552:Swine; D013788:Thalamus / Q000378:metabolism*
D009479:Neuropeptides; D010446:Peptide Fragments; D010455:Peptides; D044088:Receptors, Galanin; D011964:Receptors, Gastrointestinal Hormone; C074975:galantide; D013373:Substance P; D019004:Galanin; D011388:Prolactin