Actions of 8-hydroxy-2-(N-dipropylamino) tetralin (8-OH-DPAT) at alpha 2-adrenoceptors

Actions of the prototypical 5-HT1A receptor agonist 8-OH-DPAT at human alpha2-adrenoceptors: (+)8-OH-DPAT, but not (-)8-OH-DPAT is an alpha2B subtype preferential agonist

8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin] is the prototypical agonist at serotonin 5-HT1A receptors; however, activity at other targets contributes to the functional effects of the compound as well. We examined the properties of 8-OH-DPAT and its enantiomers at recombinant human (h)alpha2-adrenoceptor subtypes, using a panel of radioligand binding and functional tests. In competition binding experiments using [3H]-RX821002, about 10-fold selectivity of (+)8-OH-DPAT for the halpha2B subtype (pKi about 7) over halpha2A- and halpha2C-adrenoceptors was observed. In contrast, the S(-) enantiomer of 8-OH-DPAT showed similar weak affinities for the three receptor subtypes (pKis<6). The binding affinity of (+)8-OH-DPAT at the halpha2B- and the halpha2A-adrenoceptor was found sensitive to GTPgammaS, a receptor/G protein-uncoupling agent, indicating agonist properties of the drug. Furthermore, using [35S]GTPgammaS binding determination at CHO-halpha2B or CHO-halpha2A cell membranes and G protein coupled inwardly rectifying potassium (GIRK) current recordings in Xenopus oocytes expressing halpha2B, partial agonist activity of (+)8-OH-DPAT at the respective receptors was confirmed in these two different functional assays. Potency of (+)8-OH-DPAT for stimulation of [35S]GTPgammaS incorporation was lower at the halpha2A- than at the halpha2B-adrenoceptor, consistent with binding affinities. Thus, (+)8-OH-DPAT and, as a consequence, racemic (+/-)8-OH-DPAT are partial agonists at halpha2-adrenoceptors with selectivity for the halpha2B subtype, a property that might contribute to the effects of the compound described in native systems.

The acute and chronic administration of (±)-8-hydroxy-2-(di-n-propylamino)tetralin significantly alters the activity of spontaneously active midbrain dopamine neurons in rats: An in vivo electrophysiological study

This study examined the effect of the acute and chronic systemic administration of (+/-)-8-Hydroxy-2-(Di-n-propylamino)Tetralin(8-OH-DPAT) on the number and firing pattern of spontaneously active dopamine (DA) neurons in the ventral tegmental area (VTA or A10) and substantia nigra pars compacta (SNC or A9) in anesthetized male rats. These parameters were measured using extracellular in vivo electrophysiology. A single s.c. injection of 0.01, 0.1, or 1 mg/kg of 8-OH-DPAT did not significantly alter the number of spontaneously active SNC DA neurons compared to vehicle-treated animals (controls). The acute administration of 0.01 or 0.1 mg/kg of 8-OH-DPAT did not significantly alter, whereas the 1 mg/kg dose significantly decreased the number of spontaneously active VTA DA neurons compared to controls. The acute administration of 8-OH-DPAT significantly increased the percentage of VTA DA neurons firing in a bursting pattern. In contrast, there was a significant decrease in the percentage of SNC DA neurons firing in a bursting pattern following the acute administration of 8-OH-DPAT. The number of spontaneously active SNC DA neurons was not significantly altered by the chronic s.c. administration of 8-OH-DPAT (0.01, 0.1, or 1 mg/kg s.c.) as compared to controls. However, the chronic s.c. administration of all doses of 8-OH-DPAT significantly decreased the number of spontaneously active VTA DA neurons compared to controls. The i.v. administration of (+)-apomorphine (50 microg/kg) did not reverse the 8-OH-DPAT-induced decrease in the number of spontaneously active VTA DA neurons, suggesting that this effect is unlikely due to depolarization blockade. The percentage of VTA DA neurons exhibiting burst firing was significantly increased by 0.01 and 0.1 mg/kg, but significantly decreased by 1 mg/kg of 8-OH-DPAT. Overall, the systemic administration of 8-OH-DPAT preferentially affects the activity of spontaneously active A10 DA neurons in rats.

A comparison of the effects of the 5HT1A antagonists MM-77 and WAY-100635 on the mouse isolated vasa deferentia

1. Experiments were carried out to characterize the possible adrenergic properties of the 5-HT(1A) antagonists WAY 100635 and MM-77 using the mouse isolated vasa deferentia preparation. 2. When vasa deferentia were preincubated for 10 min in the presence of MM-77 (10(-8)-10(-6) m) or WAY100635 (10(-8)-7 x 10(-7) m), a concentration-dependent inhibition of the contractile response to submaximal electrical field stimulation (10 Hz, 50 V, 50 ms) was observed with pIC(50) values of 7.05 +/- 0.01 and 6.85 +/- 0.1 respectively. 3. MM-77 (10(-8)-10(-6) m) antagonized the contractile responses of the vasa deferentia to phenylephrine (PE) (10(-6)-10(-3) m) in a concentration-dependent manner. Schild plots of these data were linear and yielded a mean rhoA(2) value of 6.81 +/- 0.084. The mean slope was 1.42 +/- 0.22. 4. WAY100635 (10(-8)-10(-6) m) antagonized the contractile responses of the vasa deferentia to PE (10(-6)-10(-3) m) in a concentration-dependent manner. Schild plots of these data were linear and yielded a mean rhoA(2) value of 7.05 +/- 0.08. The mean slope was 0.97 +/- 0.1. 5. The results suggest that while WAY100635 acts as a competitive antagonist at alpha(1)-adrenoceptors, MM-77 displays non-competitive antagonist characteristics at this receptor subtype. 6. These results may have important implications for the use of these compounds as 5-HT(1A) receptor antagonists in in vivo studies.

The 5-HT1-like receptors mediating inhibition of sympathetic vasopressor outflow in the pithed rat: operational correlation with the 5-HT1A, 5-HT1B and 5-HT1D subtypes

1. It has been suggested that the inhibition of sympathetically-induced vasopressor responses produced by 5-hydroxytryptamine (5-HT) in pithed rats is mediated by 5-HT1-like receptors. The present study has re-analysed this suggestion with regard to the classification schemes recently proposed by the NC-IUPHAR subcommittee on 5-HT receptors. 2. Intravenous (i.v.) continuous infusions of 5-HT and the 5-HT1 receptor agonists, 8-OH-DPAT (5-HT1A), indorenate (5-HT1A), CP 93,129 (5-HT1B) and sumatriptan (5-HT(1B/1D)), resulted in a dose-dependent inhibition of sympathetically-induced vasopressor responses. 3. The sympatho-inhibitory responses induced by 5-HT, 8-OH-DPAT, indorenate, CP 93,129 or sumatriptan were analysed before and after i.v. treatment with blocking doses of the putative 5-HT receptor antagonists, WAY 100635 (5-HT1A), cyanopindolol (5-HT(1A/1B)) or GR 127935 (5-HT(1B/1D)). Thus, after WAY 100635, the responses to 5-HT and indorenate, but not to 8-OH-DPAT, CP 93,129 and sumatriptan, were blocked. After cyanopindolol, the responses to 5-HT, indorenate and CP 93,129 were abolished, whilst those to 8-OH-DPAT and sumatriptan (except at the lowest frequency of stimulation) remained unaltered. In contrast, after GR 127935, the responses to 5-HT, CP 93,129 and sumatriptan, but not to 8-OH-DPAT and indorenate, were abolished. 4. In additional experiments, the inhibition induced by 5-HT was not modified after 5-HT7 receptor blocking doses of mesulergine. 5. The above results suggest that the 5-HT1-like receptors, which inhibit the sympathetic vasopressor outflow in pithed rats, display the pharmacological profile of the 5-HT1A, 5-HT1B and 5-HT1D, but not that of 5-HT7, receptors.

Effects of the selective 5-HT1A receptor antagonist WAY100135 and its enantiomers on 8-OH-DPAT-induced hyperglycaemia in conscious rats

8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) increases plasma glucose levels in conscious rats probably by stimulation of central 5-HT1A receptors. We have examined the effects of WAY100135 (N-tert-butyl-3-[4-(2-methoxyphenyl)piperazin-1-yl]-2-phenylpropan amide), a selective 5-HT1A receptor antagonist and its enantiomers on plasma glucose levels and on the hyperglycaemia induced by 8-OH-DPAT. (R,S)-WAY100135 (minimum effective dose (MED) 3 mg/kg i.v.) and (S)-WAY100135 (MED 1 mg/kg i.v.) dose-dependently attenuated 8-OH-DPAT-induced hyperglycaemia. In contrast, (R)-WAY100135 at doses up to 3 mg/kg i.v. was unable to block hyperglycaemia induced by 8-OH-DPAT. When the antagonists were examined for intrinsic effects on plasma glucose levels only (S)-WAY100135 (3 mg/kg i.v.) caused a significant but transient hyperglycaemia (20% increase). These results are consistent with previous suggestions that (R,S)-WAY100135 and (S)-WAY100135 are selective 5-HT1A receptor antagonists and that 8-OH-DPAT-induced hyperglycaemia is mediated by 5-HT1A receptors. The antagonist action of WAY100135 is stereoselective, and more potent activity being observed with the (S) enantiomer.

Lack of synergistic feeding enhancement by systemic clonidine and 8-OH-DPAT

To expand on recent suggestions that brain alpha 2-adrenergic and serotonergic systems may interact in their controls over feeding, three experiments were conducted to determine if combining clonidine and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) injections subcutaneously might produce exponential feeding enhancements greater than responses elicited by either agent alone. Ad lib fed adult male rats were tested in 2 x 2 drug designs in all studies. In Experiment 1, 30 micrograms/kg clonidine reliably enhanced feeding over a 6-hr test period. However, 250 micrograms/kg 8-OH-DPAT showed no signs of enhancing this response, but instead seemed to impede it as determined by analyses of cumulative intake curves. In Experiment 2, either 10 micrograms/kg clonidine or 250 micrograms/kg 8-OH-DPAT reliably enhanced 6-h interval intakes, but their combination again failed to synergize other than to interact with time in suppressing intake. To determine if feeding synergy might occur if subthreshold doses of each agent were combined, Experiment 3 tested 1 microgram/kg clonidine and 15 micrograms/kg 8-OH-DPAT in 4-h feeding tests. Neither agent alone elicited reliable feeding as planned, but their combination also failed to stimulate feeding. These findings, combined with other recent work, do not support the possibility that previously demonstrated interactions between brain alpha 2-adrenergic and serotonergic systems extend to circumstances wherein enhancements of the former combined with suppressions of the latter might underly robust overeating responses.

Investigations of the subtype of alpha 2-adrenoceptor mediating contractions of the human saphenous vein

1. We have examined the effects of a series of alpha 2-adrenoceptor antagonists against isometric contractions to noradrenaline in human saphenous vein, and correlated these potencies with affinities for the alpha 2A-ligand binding site of human platelet and the alpha 2B-ligand binding site of rat kidney. 2. The alpha 2B-selective adrenoceptor antagonists, prazosin, ARC 239 and HV 723 showed high, and the alpha 2A-selective antagonist BRL 44408 showed low, potency in human saphenous vein. 3. Potency in human saphenous vein correlated better with affinity for the alpha 2B-ligand binding site (r = 0.71, n = 12, P less than 0.01) than with affinity for the alpha 2A-ligand binding site (r = 0.56, n = 12, non significant). 4. It is concluded that the postjunctional alpha 2-adrenoceptor of human saphenous vein resembles an alpha 2B-ligand binding site more than an alpha 2A-ligand binding site.