Potency of 5-hydroxytryptamine1a agonists to inhibit adenylyl cyclase activity is a function of affinity for the "low-affinity" state of [3H]8-hydroxy-N,N-dipropylaminotetralin ([3H]8-OH-DPAT) binding.
In this study, the radiolabeled 5-hydroxytryptamine1A agonist, [3H]8-hydroxy-N,N-dipropylamino tetralin ([3H]8-OH-DPAT), was shown to have both a high (Kd, 0.7 +/- 0.2 nM) and a low (Kd, 17 +/- 4 nM) affinity binding component in rat hippocampal homogenate preparations in the absence of guanine nucleotides. The high-affinity binding component was markedly reduced by the elimination of Mg++ from the incubation medium and the addition of both the nonhydrolyzable guanine nucleotide guanylylimidodiphosphate (Gpp(NH)p) (100 microM) and 1.0 mM EDTA to the incubation medium. Under these latter conditions, a single binding affinity component was observed with a Kd of 11 +/- 1 nM, a value in good agreement with the value for the low-affinity component measured in the absence of Gpp(NH)p. Further, the Bmax value for the single low-affinity binding component measured in the presence of Gpp(NH)p was essentially equivalent to the total of the two Bmax values found in the absence of Gpp(NH)p. A binding assay was developed using 15 nM [3H]8-OH-DPAT to determine the affinities of serotonergic drugs for the low-affinity component of [3H]8-OH-DPAT binding and these values were compared with their affinities for the high-affinity binding component as well as their potencies in a hippocampal adenylyl cyclase assay. For agonists, the Ki value for the high-affinity binding component was always less than the low-affinity Ki value, whereas the antagonist spiperone had similar values for both the high-affinity and low-affinity binding components.(ABSTRACT TRUNCATED AT 250 WORDS)
Chamberlain
J
J
Neuropsychopharmacology Unit, Department of Veterans Affairs Medical Center, Philadelphia, Pennsylvania.
Offord
S J
SJ
Wolfe
B B
BB
Tyau
L S
LS
Wang
H L
HL
Frazer
A
A
D017371:8-Hydroxy-2-(di-n-propylamino)tetralin / Q000378:metabolism*; D000067956:Adenylyl Cyclase Inhibitors; D000818:Animals; D019204:GTP-Binding Proteins / Q000502:physiology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D011985:Receptors, Serotonin / Q000378:metabolism*; D017366:Serotonin Receptor Agonists / Q000494:pharmacology*
D000067956:Adenylyl Cyclase Inhibitors; D011985:Receptors, Serotonin; D017366:Serotonin Receptor Agonists; D006165:Guanylyl Imidodiphosphate; D017371:8-Hydroxy-2-(di-n-propylamino)tetralin; D019204:GTP-Binding Proteins