Lack of apparent receptor reserve at postsynaptic 5-hydroxytryptamine1A receptors negatively coupled to adenylyl cyclase activity in rat hippocampal membranes.
Previous studies have demonstrated the existence of a large receptor reserve for agonists at somatodendritic 5-hydroxytryptamine1A (5-HT1A) serotonin receptors in the raphe nuclei of the rat. 5-HT1A agonists with anxiolytic properties (e.g., buspirone, gepirone, and ipsapirone) display full intrinsic activity at these receptors but are partial agonists at postsynaptic 5-HT1A receptors, which suggests that the latter sites may be devoid of a receptor reserve. In the present studies, this was directly determined by examining the relationship between receptor occupancy and response at postsynaptic 5-HT1A receptors, in rat hippocampus, mediating the inhibition of forskolin-stimulated adenylyl cyclase activity, using the method of partial irreversible receptor inactivation. Rats were treated with vehicle or the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), and 24 hr later hippocampi were removed for saturation analysis of [3H]8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) binding to 5-HT1A receptors or for adenylyl cyclase assays. EEDQ (1 and 6 mg/kg) dose-dependently reduced the maximal number of [3H]8-OH-DPAT binding sites by 68.5 and 80%, respectively, without altering the Kd. Concentration-response curves were generated for inhibition of forskolin-stimulated adenylyl cyclase activity by 5-HT and the selective 5-HT1A agonist N,N-dipropyl-5-carboxamidotryptamine (DP-5-CT). EEDQ treatment dose-dependently reduced the maximal inhibitory effect of 5-HT [percentage of inhibition: control, 23.6; EEDQ (1 mg/kg), 13.4; EEDQ (6 mg/kg), 8.9], without altering either the slope factor (1.01) or the EC50 (96.4 nM). Analogous results were obtained with DP-5-CT [percentage of maximal inhibition: control, 24.1; EEDQ (1 mg/kg), 15.2; EEDQ (6 mg/kg), 10.7), again without changes in slope factor (0.89) or EC50 (9.9 nM). Analysis of double-reciprocal plots of equieffective concentrations of agonist, followed by calculation of fractional receptor occupancy, revealed a linear relationship between receptor occupancy and response for both 5-HT and DP-5-CT (i.e., an absence of receptor reserve). The receptor specificity of the effect of EEDQ was demonstrated in two ways. First, it was shown that pretreatment of rats with the selective 5-HT1A partial agonist BMY 7378 (10 mg/kg) before EEDQ afforded substantial protection (about 75%) against loss of the inhibitory effect of DP-5-CT on forskolin-stimulated adenylyl cyclase activity. Second, EEDQ did not alter the inhibition of forskolin-stimulated adenylyl cyclase activity induced by the adenosine A1 receptor agonist phenylisopropyladenosine (PIA).(ABSTRACT TRUNCATED AT 400 WORDS)
CNS Neuropharmacology, Bristol-Myers Squibb Co., Wallingford, Connecticut 06492.
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D017371:8-Hydroxy-2-(di-n-propylamino)tetralin; D000262:Adenylyl Cyclases / Q000187:drug effects / Q000378:metabolism*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D005576:Colforsin / Q000037:antagonists & inhibitors / Q000494:pharmacology; D006624:Hippocampus / Q000201:enzymology / Q000502:physiology / Q000648:ultrastructure*; D008297:Male; D008566:Membranes / Q000201:enzymology / Q000378:metabolism; D011804:Quinolines / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011985:Receptors, Serotonin / Q000378:metabolism / Q000502:physiology*; D012701:Serotonin / Q000494:pharmacology; D013268:Stimulation, Chemical; D013379:Substrate Specificity; D013569:Synapses / Q000502:physiology / Q000648:ultrastructure*; D013764:Tetrahydronaphthalenes / Q000378:metabolism; D014363:Tryptamines / Q000494:pharmacology
D000317:Adrenergic alpha-Antagonists; D011804:Quinolines; D011985:Receptors, Serotonin; D013764:Tetrahydronaphthalenes; D014363:Tryptamines; D005576:Colforsin; D012701:Serotonin; C000470:EEDQ; C055895:N,N-dipropylcarboxamidotryptamine; D017371:8-Hydroxy-2-(di-n-propylamino)tetralin; D000262:Adenylyl Cyclases