Role of α-adrenoceptors in the effects of buspirone and 5-carboxamidotryptamine in rabbit isolated thoracic aorta

Differential Distribution of Functional α1-Adrenergic Receptor Subtypes along the Rat Tail Artery

The rat tail artery has been used for the study of vasoconstriction mediated by alpha(1A)-adrenoceptors (ARs). However, rings from proximal segments of the tail artery (within the initial 4 cm, PRTA) were at least 3-fold more sensitive to methoxamine and phenylephrine (n = 6-12; p < 0.05) than rings from distal parts (between the sixth and 10th cm, DRTA). Interestingly, the imidazolines N-[5-(4,5-dihydro-1H-imidazol-2-yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl]methanesulfonamide hydrobromide (A-61603) and oxymetazoline, which activate selectively alpha(1A)-ARs, were equipotent in PRTA and DRTA (n = 4-12), whereas buspirone, which activates selectively alpha(1D)-AR, was approximately 70-fold more potent in PRTA than in DRTA (n = 8; p < 0.05). The selective alpha(1D)-AR antagonist 8-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione dihydrochloride (BMY-7378) was approximately 70-fold more potent against the contractions induced by phenylephrine in PRTA (pK(B) of approximately 8.45; n = 6) than in DRTA (pK(B) of approximately 6.58; n = 6), although the antagonism was complex in PRTA. 5-Methylurapidil, a selective alpha(1A)-antagonist, was equipotent in PRTA and DRTA (pK(B) of approximately 8.4), but the Schild slope in DRTA was 0.73 +/- 0.05 (n = 5). The noncompetitive alpha(1B)-antagonist conotoxin rho-TIA reduced the maximal contraction induced by phenylephrine in DRTA, but not in PRTA. These results indicate a predominant role for alpha(1A)-ARs in the contractions of both PRTA and DRTA but with significant coparticipations of alpha(1D)-ARs in PRTA and alpha(1B)-ARs in DRTA. Semiquantitative reverse transcription-polymerase chain reaction revealed that mRNA encoding alpha(1A)- and alpha(1B)-ARs are similarly distributed in PRTA and DRTA, whereas mRNA for alpha(1D)-ARs is twice more abundant in PRTA. Therefore, alpha(1)-ARs subtypes are differentially distributed along the tail artery. It is important to consider the segment from which the tissue preparation is taken to avoid misinterpretations on receptor mechanisms and drug selectivities.

Buspirone, a 5-HT1A receptor agonist, dilates the perfused rat uterine vascular bed through α1-adrenoceptor blockade

In the perfused rat uterine vascular bed, 5-hydroxytryptamine (5-HT) produced dose-dependent vasoconstrictor responses. Buspirone, a selective 5-HT(1A) receptor agonist, was not effective at low doses but produced a response at high doses. When perfusion pressure was raised with phenylephrine, responses to 5-HT were enhanced while buspirone produced dose-dependent vasodilator responses. Buspirone did not produce vasodilation when perfusion pressure was raised with vasopressin or U46619. Buspirone-induced vasodilator responses were not affected by selective 5-HT(1A) receptor antagonists, 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]-decane-7,9-dione (BMY 7378) and N-tert-butyl-3-(4-[2-methoxyphenyl]piperazin-1-yl)-2-phenylpropanamide (WAY 100478), indicating that specific 5-HT(1A) receptors might not be involved in buspirone-induced vasodilation. Buspirone (3 x 10 (-5) M) and prazosin (3 x 10(-9) M) antagonized noradrenaline-induced constriction with dose ratios of 19.1+/-2.9 and 11.7+/-2.1, respectively. The dose ratio of these antagonists in combination was 46.6+/-8.1. Since the combination ratio is closer to the sum of their individual dose ratios less 2 (i.e. DR(p)+DR(b)-2) than it is to the product of their individual dose ratios, our data suggest an interaction of buspirone with alpha(1)-adrenoceptors. Buspirone also protected adrenoceptors against inactivation by phenoxybenzamine confirming that buspirone interacted with alpha(1)-adrenoceptors. We concluded that buspirone-induced vasodilation of the perfused rat uterine vascular bed is mediated through blockade of alpha(1)-adrenoceptors rather than through 5-HT(1A) receptors.

Buspirone functionally discriminates tissues endowed with alpha1-adrenoceptor subtypes A, B, D and L

The affinity for functional alpha1-adrenoceptor subtypes of buspirone in comparison with its close structural analogs and selective alpha1D-adrenoceptor antagonists, BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]dec ane-7,9-dione) and MDL 73005EF (8-[2-(1,4-benzodioxan-2-ylmethylamino)ethyl]-8-azaspiro+ ++[4.5]decane-7,9-dione), was determined, namely at subtype A in rat vas deferens and perfused kidney, at subtype B in guinea-pig and mouse spleen, at subtype L in rabbit spleen, and at subtype D in rat aorta and pulmonary artery against noradrenaline-evoked contractions. BMY 7378 and MDL 73005EF were confirmed as 30- and 20-fold selective antagonists, respectively, for alpha1D- over both alpha1A- and alpha1B-adrenoceptors. Buspirone was a weak antagonist without intrinsic activity at alpha1A-adrenoceptors in rat vas deferens (pA2 = 6.12), at alpha1B-adrenoceptors in guinea-pig and mouse spleen (pA2 = 5.54 and 5.59) and at alpha1L-adrenoceptors in rabbit spleen (pA2 = 4.99), but caused partial vasoconstriction in rat kidney that was attenuable by the subtype D-selective adrenoceptor antagonist BMY 7378, but hardly by the subtype A-selective adrenoceptor antagonist B8805-033 ((+/-)-1,3,5-trimethyl-6-[[3-[4-((2,3-dihydro-2-hydroxymethyl)-1,4-be nzodioxin-5-yl)-1-piperazinyl]propyl]amino]-2,4(1H,3H)-pyrimidinedion e), confirming the additional presence of alpha1D-adrenoceptors mediating rat renal vasoconstriction. Buspirone behaved as a partial agonist at alpha1D-adrenoceptors in rat aorta (pD2 = 6.77, intrinsic activity (i.a.)= 0.40) and pulmonary artery (pD2 = 7.16, i.a. = 0.59). With buspirone as agonist in these tissues, the pA2 values of subtype-discriminating antagonists were consistent with their alpha1D-adrenoceptor affinity determined in rat aorta against noradrenaline and with published binding data on cloned alpha1d-adrenoceptors. The results provide pharmacological evidence that (1) in functional preparations for the A subtype, like rat vas deferens and perfused kidney, for the B subtype, like guinea-pig and mouse spleen, and for the L subtype, like rabbit spleen, buspirone is a weak antagonist without intrinsic activity, but (2) behaves as a partial agonist in rat aorta and pulmonary artery as models for the D subtype and (3) detects an additional vasoconstrictor alpha1D-adrenoceptor in rat kidney. Buspirone, like its close analogs BMY 7378 and MDL 73005EF, thus might also be a useful tool for functionally discriminating alpha1D- from alpha1A-, alpha1B- and alpha1L-adrenoceptors in various tissues.

8-OH-DPAT and male rat sexual behavior: partial blockade by noradrenergic lesion and sexual exhaustion

As previously shown, the 5-HT1A agonist 8-OH-DPAT is a potent facilitator of male rat copulatory behavior in both sexually experienced and sexually exhausted male rats. The basis of this facilitation is still not clear. Therefore, the purpose of the present study was to determine whether 8-OH-DPAT-induced sexual-behavior facilitation could be counteracted by lesioning the NA system with the noradrenergic neurotoxin DSP4. In NA-lesioned, sexually experienced, non-exhausted rats, the facilitatory effects of 8-OH-DPAT on the number of mounts and the postejaculatory interval were reduced, the effect on the intromission latency disappeared, while the percentage of copulating rats was not significantly altered. In sexually exhausted rats bearing a lesion of the NA system, the facilitatory effects of 8-OH-DPAT on the percentage of copulating rats was blocked. Data are discussed on the basis of the interactions between the noradrenergic and serotonergic systems in the mediation of the facilitatory effect of 8-OH-DPAT in sexually exhausted and non-exhausted rats.

Role of alpha1-adrenoceptors in the reduction of external carotid blood flow induced by buspirone and ipsapirone in the dog

The effects of the 5-HT1A receptor agonist with anxiolytic properties, buspirone and ipsapirone, in the external carotid bed of anaesthetized dogs were analyzed. Since these agonists produce several vascular effects via activation of both 5-HT receptors and alpha1-adrenoceptors, their effects were compared with those elicited by the 5-HT agonist, quipazine, and the alpha1-adrenoceptor agonist, methoxamine. 1-Min intracarotid (i.c.) infusions of buspirone (300 microgram/min), ipsapirone (40 microgram/min), quipazine (300 microgram/min) and methoxamine (15 microgram/min) produced consistent decreases in external carotid blood flow (ECBF); since these changes in blood flow were not accompanied by modifications in systemic blood pressure, the agonists produced parallel increases in external carotid resistance. After interruption of the sympathetic tone by bilateral cervical vagosympathectomy, the vasoconstrictor responses to all the agonists remained unaffected. The intravenous (i.v.) administration of the nonselective 5-HT1-like receptor antagonist, methiothepin (1-100 microgram/kg), potently and dose-dependently antagonized buspirone-, ipsapirone- and quipazine-induced vasoconstriction; methiothepin similarly antagonized the vasoconstrictor responses to methoxamine. Interestingly, the alpha1-adrenoceptor antagonist, prazosin (1-100 microgram/kg, i.v.), also antagonized the vasoconstrictor responses to buspirone, ipsapirone and methoxamine in a dose-dependent manner. Finally, buspirone (300 microgram/min, i.c.) and ipsapirone (40 microgram/min, i.c.) did not modify the responses to noradrenaline (10 microgram/min, i.c.) or tyramine (100 microgram/min, i.c.). It is concluded that canine external carotid vasoconstriction induced by buspirone and ipsapirone is mainly mediated by activation of alpha1-adrenoceptors located in vascular smooth muscle. These data further highlight the ability of the above anxiolytics to produce significant vascular effects under in vivo conditions.

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.