An investigation into the selectivity of a novel series of benzoquinolizines for alpha 2-adrenoceptors in vivo.

The potencies and selectivities of a novel series of benzoquinolizines for the alpha 2-adrenoceptor have been investigated in the rat in comparison with yohimbine and indoramin. Peripheral postjunctional alpha 2- and alpha 1-adrenoceptor blockade was measured as the reversal of B-HT 933 and methoxamine-induced pressor responses, respectively, in the pithed rat. Peripheral prejunctional alpha 2-adrenoceptor blockade was measured as the reversal of B-HT 933-induced inhibition of an electrically evoked tachycardia in the pithed rat. Central alpha 2-adrenoceptor blockade was measured as a reversal of the hypotension induced in anaesthetized rats by central (i.c.v.) administration of clonidine. Wy 25309, Wy 26392, Wy 26703 and yohimbine (0.3-3 mg kg-1 i.v.) evoked dose-dependent shifts to the right of the dose-response curves to B-HT 933 whilst having minimal effects on the methoxamine dose-response curve. The selectivity for alpha 2-adrenoceptors increased with the dose of antagonist administered. In general, the order of selectivity was Wy 25309 greater than Wy 26392 greater than Wy 26703 greater than yohimbine. Indoramin (1 mg kg-1 i.v.) shifted the methoxamine pressor dose-response curve to the right without affecting the B-HT 933 dose-response curves, confirming its selective alpha 1-antagonist activity. Peripheral administration of all three benzoquinolizines (1-100 micrograms kg-1 i.v.) led to a dose-dependent reversal of the hypotension evoked by central administration of clonidine (500 ng i.c.v.). The reversal was incomplete, higher doses causing a further decrease in blood pressure. (ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of peripheral dopamine receptor and alpha-adrenoceptor blocking activity of sulpiride

The effect of sulpiride, a dopamine receptor antagonist, on peripheral alpha-adrenoceptors and dopamine receptors was determined. Positive chronotropic responses to cardioaccelerator nerve stimulation in pentobarbital-anesthetized dogs were potentiated by 1.0 and 2.0 mg/kg but not by 0.5 mg/kg intravenous sulpiride. This effect persisted after neuronal uptake blockade with desipramine, but was prevented by alpha 2-adrenoceptor blockade with yohimbine. Positive chronotropic effects of intravenous norepinephrine were unchanged by sulpiride, suggesting that sympathetic nerve function was facilitated via a presynaptic mechanism. Since yohimbine prevented the facilitatory action of sulpiride, an analysis of the ability of sulpiride to antagonize alpha-adrenoceptors was made. The reduction in stimulus-induced tachycardia caused by the alpha 2-adrenoceptor agonist tramazoline was significantly antagonized by sulpiride. Furthermore, while sulpiride did not antagonize the pressor effect of the alpha-adrenoceptor agonist phenylephrine, it significantly attenuated the increases in blood pressure produced by tramazoline. The alpha 2-adrenoceptor blocking action of sulpiride lasted for approximately 15 min at 1.0 mg/kg adn for 90 min at 2.0 mg/kg. In additional experiments, it was determined that the dopamine receptor-mediated bradycardic and hypotensive effects of N,N-di-n-propyldopamine were antagonized for at least 2 h by 0.1, 0.5, and 1.0 mg/kg sulpiride. These studies establish that peripheral neuronal and vascular dopamine receptors may be antagonized by sulpiride without affecting alpha-adrenergic mechanisms. However, at doses of 1.0 mg/kg and higher, sulpiride facilitates sympathetic nerve function via a preferential antagonism of alpha 2-adrenoceptors.     

The effect of yohimbine, WY 26392 and idazoxan on sympathetic nerve activity

Preganglionic sympathetic nerve activity, blood pressure, heart rate and femoral arterial conductance were recorded in anaesthetised, paralysed cats. Cumulative dose (0.01-1 mg kg-1) response curves were constructed for the alpha 2 adrenoceptor antagonists yohimbine, WY 26392 and idazoxan. Low doses of these drugs caused increases in preganglionic sympathetic nerve activity, heart rate and mean blood pressure, and decreases in femoral arterial conductance. In contrast, at the highest dose, all three drugs caused falls in mean blood pressure, decreases in heart rate and no further increases in preganglionic sympathetic nerve activity. The decreases in heart rate appeared to be due to increases in vagal tone since they were reversed by atropine methonitrate. These results demonstrate that low doses of alpha 2 adrenoceptors antagonists increase central sympathetic tone and blood pressure and are consistent with the hypothesis that central alpha 2 adrenoceptors are under a tonic input which is inhibitory to central sympathetic tone.     

Effects of alpha 1- and alpha 2-adrenoceptor blocking drugs on noradrenaline release rate in anesthetized rabbits

[3H]noradrenaline was infused intravenously into pentobarbitone-anesthetized rabbits to reach a steady-state plasma (3H]noradrenaline level, from which the noradrenaline plasma clearance was calculated. The plasma level of endogenous noradrenaline was determined simultaneously, and the rate of noradrenaline release was then derived. The effects of a series of selective alpha 1- and alpha 2-adrenoceptor blocking drugs on the noradrenaline release rate and noradrenaline clearance were investigated. Yohimbine (1 mg/kg i.v.), rauwolscine (1 mg/kg i.v.), corynanthine (1 mg/kg i.v.), prazosin (0.3 and 1 mg/kg i.v.), phenoxybenzamine (4 mg/kg i.v.), and sodium nitroprusside (10 micrograms/kg/min i.v.) decreased the plasma noradrenaline clearance. The selective alpha 2-adrenoceptor blocking drugs yohimbine and rauwolscine, as well as phenoxybenzamine, increased the noradrenaline release rate more than equihypotensive doses of the directly acting vasodilators hydralazine and sodium nitroprusside. The selective alpha 1-adrenoceptor blocking drugs prazosin and corynanthine increased the noradrenaline release rate less than equihypotensive doses of the vasodilators. These results suggest that, in vivo, blockade of alpha 2-adrenoceptors results in increased noradrenaline release, probably due to blockade of inhibitory presynaptic alpha 2-adrenoceptors at sympathetic nerve endings. Blockade of alpha 1-adrenoceptors, on the other hand, appears to depress baroreceptor-mediated increases in noradrenaline release in response to a fall in blood pressure.     

Alpha 2-adrenoceptor agonists potentiate responses mediated by alpha 1-adrenoceptors in the cat nictitating membrane.

Alpha 1 but not alpha 2-adrenoceptors mediate contractions of the cat nictitating membrane. The contractions of this tissue evoked by alpha 1-adrenoceptor agonists, but not those evoked by angiotensin II, are potentiated by pre-dosing with alpha 2-adrenoceptor agonists. This potentiation is reversed by the alpha 2-adrenoceptor antagonist, WY 26392. Pressor responses evoked by alpha 1-adrenoceptor agonists or angiotensin II were not affected by alpha 2-adrenoceptor agonists. Contractions of the nictitating membrane evoked by noradrenaline were reduced by pretreatment with WY 26392. These results suggest that in some tissues the role of alpha 2-adrenoceptors may be to modulate responses to alpha 1-adrenoceptors, rather than to evoke a discrete response themselves.     

Mechanism of ketanserin-induced sympatho-inhibition.

Experiments were undertaken to determine if sympatho-inhibition produced by ketanserin is due to antagonism of central nervous system alpha 1-adrenoceptors rather than central 5-HT2 receptors and if (like prazosin) it produces sympatho-inhibition indirectly via a central (presynaptic) alpha 2-adrenoceptor mechanism. Administration of ketanserin (0.03-3.0 mg/kg i.v.) caused a dose-related depression of sympathetic-cholinergic electrodermal responses evoked by electrical stimulation of the hypothalamus in pentobarbital anesthetized cats. No effect of ketanserin was observed on electrodermal responses evoked by preganglionic sympathetic nerve stimulation nor did the more specific 5-HT2 receptor antagonist, cinanserin, produce a central sympatholytic effect at dosages up to 3 mg/kg i.v. Pretreatment with alpha 2-adrenoceptor blockers yohimbine, idazoxan, or rauwolscine significantly antagonized ketanserin-induced sympatho-inhibition. Depletion of central nervous system (CNS) monoamines totally prevented ketanserin-induced sympatho-inhibition although clonidine (30 micrograms/kg i.v.) continued to be effective. These results suggest that ketanserin acts in the CNS to reduce sympathetic reactivity by blocking alpha 1-adrenoceptors and not 5-HT2 receptors. In this regard, ketanserin appears to act in a manner similar to other alpha 1-adrenoceptor antagonists (e.g. prazosin and indoramin) by an apparent presynaptic facilitation of alpha 2-adrenoceptor mediated tonic inhibition descending from the lower brainstem.     

Alpha 2-adrenoceptor antagonist activity may account for the effects of buspirone in an anticonflict test in the rat

The anxiolytic effects of buspirone, its metabolite, 1-(2-pyrimidyl)piperazine (1-PP) and several alpha 2-adrenoceptor antagonists have been compared in an anticonflict (shock-induced suppression of drinking) paradigm in rats. Idazoxan, WY 26392 and yohimbine had anticonflict effects comparable to those of buspirone and 1-PP, and enhanced the release of suppressed responding induced by buspirone. The response to buspirone was antagonised by the alpha 2-adrenoceptor agonist clonidine. In tests of clonidine-induced mydriasis, the antagonist potencies of buspirone, 1-PP, idazoxan, WY 26392 and yohimbine corresponded closely to the doses of the compounds active in the anticonflict test. Clonidine-induced hypolocomotion was also antagonised by 1-PP, although this response was potentiated by buspirone. The results suggest that the anticonflict effects of buspirone involve an alpha 2-adrenoceptor mechanism.     

Adrenergic and purinergic neurotransmission in arterial resistance vessels of the cat intestinal circulation

The contribution of adrenoceptors and purine receptors in mediating neurogenic vasoconstriction was investigated in the autoperfused intestinal circulation of anaesthetised cats treated with atropine and propranolol. Prazosin (0.5 mg/kg) and yohimbine (1.5 mg/kg) reduced but did not abolish the vasoconstrictor responses to stimulation of the efferent sympathetic nerves. The inhibitory actions of the two antagonists were additive but even after alpha 1- and alpha 2-adrenoceptor blockade nerve stimulation still elicited a residual, frequency-related vasoconstriction. The initial, rapid, phase of this response was completely abolished after desensitisation of P2x-purinoceptors with a high dose (1.5 mg i.a.) of alpha,beta-methylene ATP. In the absence of alpha-adrenoceptor antagonists, alpha,beta-methylene ATP reduced neurogenic vasoconstriction particularly at low frequency (1 Hz) nerve stimulation, but also caused a short-lasting decrease in noradrenaline and methoxamine responses which indicates that the drug may have some non-specific effects on arterial smooth muscle. The results suggest that neurotransmission in arterial resistance vessels of the cat intestinal circulation is predominantly under adrenergic control mediated by postsynaptic alpha 1- and alpha 2-adrenoceptors, with a possible purine involvement in the initial rapid response of the blood vessels, particularly to low frequency nerve stimulation.     

Cardiovascular effects of irindalone, a new S2-serotonergic antagonist, in the rat

Irindalone is a new antihypertensive agent with affinity to serotonin (5-HT2) receptors and at higher concentrations also to alpha 1-adrenoceptors. The present study was designed to evaluate the relative importance of the antagonism of central and peripheral alpha 1- and 5-HT2-receptors in the blood pressure lowering properties or irindalone after acute administration. In conscious Sprague-Dawley rats intravenous irindalone (0.05-1.5 mg/kg) dose-dependently reduced the blood pressure. In the same dose-range irindalone antagonized pressor responses to phenylephrine and electrical stimulation of the spinal sympathetic outflow (SNS) in the pithed rats, indicating that the acute blood pressure lowering effect is primarily related to the blockade of alpha 1-adrenoceptors. However, the concomitant 5-HT2-receptor blockade may contribute since irindalone in a dose (0.15 mg/kg) where it had no alpha-adrenoceptor blocking properties enhanced the hypotensive response to selective alpha 1-adrenoceptor blockade by prazosin (1 micrograms/kg). We found no evidence that central mechanisms contributed to the blood pressure lowering effect of irindalone. In anaesthetized rats irindalone (1 mg/kg) did not reduce the directly recorded sympathetic nerve activity. Intracerebroventricular administration of irindalone in conscious rats (10-100 micrograms) had no consistent effects on the blood pressure and did not enhance the hypotensive response to intracerebroventricularly administered prazosin (10 micrograms). Finally, the hypotensive response to irindalone was not influenced by depletion of central serotonin stores (by PCPA). It is concluded that the blood pressure lowering effect of irindalone following acute administration is related primarily to blockade of peripheral alpha-adrenoceptors but that the concomitant blockade of 5-HT2-receptors may contribute.     

Possible role of alpha 1- and alpha 2-adrenoceptors in the modulation of the sympathetic component of the baroreflex

In anaesthetized and bilaterally vagotomized dogs, reflex bradycardia elicited by intravenous injection of noradrenaline was facilitated by AR-C 239, a new alpha 1-adrenoceptor blocking drug and inhibited by the alpha 2-adrenoceptor antagonist, yohimbine. Both alpha-blocking drugs were administered into the vertebral artery. In another group of bilaterally vagotomized dogs, unilateral electrical stimulation of the carotid sinus nerve induced a frequency-dependent decrease in mean blood pressure solely mediated through the sympatho-inhibitory component of the baroreflex. Administration of the alpha 1-adrenoceptor blocking drugs, AR-C 239 and prazosin (5 micrograms/kg) into the vertebral artery decreased basal mean blood pressure and increased depressor responses to the carotid sinus nerve stimulation, whereas the intracisternal injection of phenylephrine (30 micrograms/kg), a preferential alpha 1-agonist, increased mean blood pressure but inhibited the hypotension resulting from electrical stimulation. In addition, the injection into the vertebral artery of yohimbine (100 micrograms/kg), an alpha 2-adrenoceptor blocking agent which caused no change in mean arterial pressure, inhibited the decrease in the sympathetic component. In conclusion, these results suggest the possible participation of the two types of alpha-adrenoceptors in the modulation of the sympathetic component of the baroreflex: alpha 1-adrenoceptor stimulation could inhibit, whereas alpha 2-adrenoceptor activation facilitates the reflex activity in the sympathetic fibres.     

Involvement of alpha 2-adrenoceptors in the activation of kininogenesis by clonidine

The effects of clonidine and alpha 2-adrenoceptor blocking agents, yohimbine and tolazoline on kininogen and prekallikrein levels in plasma and on blood pressure were investigated in male Wistar rats. Clonidine (0.5 mg/kg) decreased concentration of kininogen. Yohimbine (3 mg/kg) and tolazoline (5 mg/kg) counteracted this effect of clonidine and increased the kininogen level in plasma. Clonidine did not change the level of prekallikrein, however in rats receiving alpha 2-adrenoceptor blockers alone or together with clonidine kallikrein utilization was reduced, which indicated indirectly that the level of prekallikrein in plasma augmented. Yohimbine and tolazoline in the same doses (3 mg/kg and 5 mg/kg resp.) increased the blood pressure and counteracted the hypotensive action of clonidine, which indicated the presynaptic alpha 2-adrenoceptor blocking effect of the doses used. The data indicate that clonidine-induced activation of kininogenesis as well as its hypotensive effect are counteracted by yohimbine and tolazoline in doses blocking specifically presynaptic alpha 2-adrenoceptors. It can be suggested that alpha 2-adrenoceptors are involved in the kininogenesis-activating effect of clonidine.     

Heterogeneity between soluble human and rabbit splenic alpha 2-adrenoceptors

The pharmacological and biochemical characteristics of soluble alpha 2-adrenoceptors were investigated to determine whether differences observed in membranes were maintained in solution and to probe the nature of any such differences. alpha 2-Adrenoceptors were solubilized from purified plasma membrane preparations of human and rabbit spleen using digitonin. [3H]yohimbine bound to one population of alpha 2-adrenoceptors in the preparations with dissociation constants of 2.4 nM and 7.8 nM respectively. The pharmacological profile of the alpha 2-adrenoceptors has been examined. Upon solubilization the affinity of the alpha 2-adrenoceptors for yohimbine was unchanged. In contrast, the potency of idazoxan and RX 811066 were increased, whereas the potency for prazosin (human only), phentolamine and WY 26392 was decreased 2-3-fold. The potency of the agonists oxymetazoline, UK 14304 and adrenaline were all reduced upon solubilization of alpha 2-adrenoceptors. The selectivity of yohimbine, idazoxan, RX 811066 and WY 26392 for human rather than rabbit alpha 2-adrenoceptors was maintained in solution. Possible sources of heterogeneity between human and rabbit alpha 2-adrenoceptors were investigated. The protein structure was probed by comparing the susceptibility of the receptors to inactivation by sulphydryl modifying agents. No differences were observed in the potency of N-ethylmaleimide or p-chloromercuribenzoate to inactivate the receptor. The carbohydrate component of the receptors was investigated using agarose-linked lectins. Rabbit splenic alpha 2-adrenoceptors had a lower affinity for the lectins wheatgerm agglutinin (Triticum vulgaris) and soybean (Glycine max) which bind the sugars N-acetyl d-glucosamine and N-acetyl d-galactosamine respectively. These findings suggest that heterogeneity of the alpha 2-adrenoceptor derives from its structural characteristics rather than its environment in the membrane.     

Studies on RX 781094: a selective, potent and specific antagonist of alpha 2-adrenoceptors.

1 The selectivity and specificity of RX 781094 [2-(2-(1,4 benzodioxanyl))2-imidazoline HCl] for alpha-adrenoceptors have been examined in peripheral tissues. 2 In isolated tissue experiments RX 781094 was a competitive antagonist at prejunctional alpha 2-adrenoceptors situated on the sympathetic nerve terminals of the rat (pA2 = 8.56) and mouse (pA2 = 7.93) vas deferens and on the parasympathetic nerve terminals of the guinea-pig ileum (pA2 = 8.55). 3 Although RX 781094 was also a competitive antagonist at the postjunctional alpha 1-adrenoceptors of the rat anococcygeus muscle (pA2 = 6.10) its affinity for these receptors was markedly less than that displayed for prejunctional sites. From pA2 values obtained in the rat vas deferens and anococcygeus muscle the calculated alpha 2/alpha 1-adrenoceptor selectivity ratio for RX 781094 was 288. 4 The rank order of alpha 2/alpha 1-adrenoceptor selectivities for the antagonists studied was RX 781094 greater than RS 21361 greater than yohimbine greater than piperoxan greater than phentolamine greater than WB 4101 greater than prazosin. 5 RX 781094 had extremely low affinity for beta-adrenoceptors, histamine receptors, cholinoceptors, 5-hydroxytryptamine and opiate receptors in vitro. 6 In pithed rats, intravenous administration of RX 781094 antagonized the prejunctional alpha 2-adrenoceptor agonist effects of clonidine and guanabenz on electrically-induced contractions of the vas deferens and anococcygeus muscle respectively. 7 In the vas deferens the rank order of alpha 2-adrenoceptor antagonist potencies was RX 781094 greater than phentolamine greater than piperoxan greater than yohimbine greater than RS 21361 greater than WB 4101. Only RX 781094, yohimbine and RS 21361 were active against guanabenz in the anococcygeus muscle. 8 In the pithed rat, RX 781094 preferentially antagonized the pressor responses evoked by postjunctional alpha 2-adrenoceptor activation by UK 14,304 although higher doses also inhibited the effects of phenylephrine and cirazoline at postjunctional alpha 1-adrenoceptors. 9 RX 781094 had little effect on the cardiovascular responses to 5-hydroxytryptramine, angiotensin II, histamine, acetylcholine and isoprenaline in pithed rats and rats anaesthetized with pentobarbitone. 10 These results demonstrate that RX 781094 is a potent and selective alpha 2-adrenoceptor antagonist with a high degree of specificity for these receptors.     

Vasopressor nerve responses in the pithed rat, previously identified as α2-adrenoceptor mediated, may be α1D-adrenoceptor mediated

Responses to pressor nerve stimulation in the pithed rat have been variously described as mediated, at least in part, by α(2)-adrenoceptors and by α(1A) and α(1D)-adrenoceptors. We have examined the subtypes of α-adrenoceptor involved in rises in diastolic blood pressure in the pithed rat preparation produced by vasopressor nerve stimulation with 10 pulses at 1 Hz or 20 pulses at 5 Hz. Vasopressor nerve responses to 1 Hz stimulation were markedly inhibited by the α(1A)-adrenoceptor antagonist RS 100329 (0.1mg/kg) and by the α(1D-)adrenoceptor antagonist BMY 7378 (0.1mg/kg). The α(2)-adrenoceptor antagonist yohimbine (0.1mg/kg) significantly increased pressor nerve responses to 1 Hz stimulation, but yohimbine (1mg/kg) significantly reduced pressor nerve responses. However, following BMY 7378 (0.1mg/kg), yohimbine (1mg/kg) did not produce any further inhibition of pressor nerve responses to 1 Hz stimulation. The α(2A)-adrenoceptor antagonist BRL 44408 (1mg/kg) did not reduce pressor responses to 1 Hz stimulation. BMY 7378 produced much less inhibition of pressor nerve responses to 5 Hz stimulation, whereas RS 100329 produced similar inhibition of 1 Hz and 5 Hz responses. Yohimbine (0.1 and 1mg/kg) did not significantly affect pressor nerve responses to 5 Hz stimulation. In conclusion, pressor nerve responses in the pithed rat involve both α(1A) and α(1D)-adrenoceptor, but there is no clear evidence for the involvement of α(2)-adrenoceptors.     

Sympathetic control of nasal blood flow in the rat mediated by alpha(1)-adrenoceptors

Experiments were undertaken, using laser-Doppler flowmetry, to determine the nature of adrenoceptors mediating sympathetic nerve evoked nasal vasoconstrictor responses in anesthetized rats. Presence of sympathetic tone was confirmed by a large (330%) increase of nasal blood flow following section of the ipsilateral preganglionic cervical sympathetic nerve. Electrical nerve stimulation produced reproducible, frequency-related nasal vasoconstrictor responses with near maximal response, observed at less than 10 Hz. Evoked nasal vasoconstrictor responses were largely blocked with intravenous treatment with the non-selective alpha-adrenoceptor antagonists, phentolamine (5 mg kg(-1)) and phenoxybenzamine (2 mg kg(-1)), as well as with the selective alpha(1)-adrenoceptor antagonist, prazosin (300 microg kg(-1)). alpha(2)-Adrenoceptor antagonism with rauwolscine (500 microg kg(-1)) potentiated neurally evoked nasal vasoconstriction. Neither atropine (1 mg kg(-1)) nor propranolol (1 mg kg(-1)) altered the evoked responses. Rats with intact cervical sympathetic nerves responded to rauwolscine with a modest constriction. Subsequent prazosin administration produced an increase of nasal blood flow of approximately 275%. These results suggest that the nasal vasculature of the rat is under intense sympathetic tone and that the resulting neurogenic vasoconstriction is mediated exclusively by activation of alpha(1)-adrenoceptors.     

Functional characterization of alpha-adrenoceptors mediating pupillary dilation in rats

Previously, we reported that the alpha(1A)-adrenoceptor, but not the alpha(1D)-adrenoceptor, mediates pupillary dilation elicited by sympathetic nerve stimulation in rats. This study was undertaken to further characterize the alpha-adrenoceptor subtypes mediating pupillary dilation in response to both neural and agonist activation. Pupillary dilator response curves were generated by intravenous injection of norepinephrine in pentobarbital-anesthetized rats. Involvement of alpha(1)-adrenoceptors was established as mydriatic responses were inhibited by systemic administration of nonselective alpha-adrenoceptor antagonists, phentolamine (0.3-3 mg/kg) and phenoxybenzamine (0.03-0.3 mg/kg), as well as by the selective alpha(1)-adrenoceptor antagonist, prazosin (0.3 mg/kg). The alpha(2)-adrenoceptor antagonist, rauwolscine (0.5 mg/kg), was without antagonistic effects. alpha(1A)-Adrenoceptor selective antagonists, 2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane (WB-4101; 0.1-1 mg/kg) and 5-methylurapidil (0.1-1 mg/kg), the alpha(1B)-adrenoceptor selective antagonist, 4-amino-2-[4-[1-(benzyloxycarbonyl)-2(S)- [[(1,1-dimethylethyl)amino]carbonyl]-piperazinyl]-6,7-dimethoxyquinazoline (L-765314; 0.3-1 mg/kg), as well as the alpha(1D)-adrenoceptor selective antagonist, 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY-7378; 1 mg/kg), were used to delineate the adrenoceptor subtypes involved. Mydriatic responses to norepinephrine were significantly antagonized by intravenous administration of both WB-4101 and 5-methylurapidil, but neither by L-765314 nor by BMY-7378. L-765314 (0.3-3 mg/kg, i.v.) was also ineffective in inhibiting the mydriasis evoked by cervical sympathetic nerve stimulation. These results suggest that alpha(1B)-adrenoceptors do not mediate sympathetic mydriasis in rats, and that the alpha(1A)-adrenoceptor is the exclusive subtype mediating mydriatic responses in this species.     

Alpha 2-adrenoceptor antagonists: effects on ejaculation, penile erection and pelvic thrusting behavior in dogs

We previously reported that systemic administration of yohimbine, an alpha2-adrenoceptor antagonist, exerts a biphasic effect (stimulating and suppressing) on ejaculation in dogs, when this function is analyzed using the amount of ejaculated semen in response to genital stimulation. To clarify the effect of alpha2-adrenoceptor blockade on male sexual function, we investigated the effects of four selective alpha2-adrenoceptor antagonists, rauwolscine, idazoxan, RX821002 and mydaglizole, on sexual responses (ejaculation, penile erection and pelvic thrusting behavior) elicited by manual penile stimulation in dogs. Rauwolscine (intraperitoneal, 30 min before the testing) caused a biphasic effect on ejaculation; the amount of ejaculated semen produced by the stimulation was significantly increased by the lower doses (0.1 and 0.3 mg/kg), whereas it was decreased by the higher doses (1.0 and 2.0 mg/kg). The higher doses of rauwolscine also markedly inhibited both penile erection and pelvic thrusting behavior. Idazoxan and RX821002, at doses of 0.1 and 0.3 mg/kg, caused a significant increase in the amount of ejaculated semen without affecting other sexual functions. RX821002 (2.0 mg/kg), but not idazoxan (2.0 mg/kg), moderately inhibited both penile erection and pelvic thrusting behavior. Mydaglizole, a peripherally acting alpha2-adrenoceptor antagonist, did not affect the sexual responses at any doses (0.1-4.0 mg/kg). In the ejaculatory declining test, all alpha2-adrenoceptor antagonists (0.1 mg/kg), except for mydaglizole, completely prevented the decrease in ejaculatory capacity produced by antecedent ejaculation. These results indicate that, though the range of the effective dose is narrow, the alpha2-adrenoceptor antagonists that can block the central alpha2-adrenoceptors have the stimulatory effects on ejaculatory function. The difference of the sexual effects may be based on the action except for the alpha2-adrenoceptor blockade.     

Comparison of effects of urapidil and prazosin on vascular influence of circulating and neuronally released catecholamines in canine muscle vascular bed

Two series of experiments were conducted in anesthetized dogs to compare the vascular effects of the alpha 1-adrenoceptor antagonists urapidil and prazosin. In the first series of experiments vasoconstrictor responses to sympathetic nerve stimulation and phenylephrine injected intraarterially were elicited in the pump-perfused hindlimb. Urapidil and prazosin administered intravenously in the doses of 2 and 0.25 mg/kg, respectively, were equivalent in their ability to block these responses. Yohimbine, an alpha 2-adrenoceptor antagonist, in the dose of 0.05 mg/kg, had no additional effect on the responses, but did decrease the vascular resistance after prazosin infusion. This suggested that urapidil, but not prazosin, exerted an alpha 2-blocking effect which decreased vascular resistance. In the second series of experiments the effect of urapidil administered intravenously was determined on the innervated and denervated gracilis muscle, after maximal alpha 1-adrenoceptor blockade with prazosin. The purpose of these experiments was to confirm whether or not urapidil had an additional effect on vascular resistance attributable to alpha 2-antagonism. Prazosin, 0.25 mg/kg followed by 0.50 mg/kg i.v., decreased vascular resistance in both the innervated and denervated gracilis muscles, and maximal alpha 1-blockade was achieved based on the blockade of the pressor responses to phenylephrine. A further reduction in vascular resistance was found after urapidil infusion, 2 mg/kg, which appears to be due to blockade of the effect of circulating catecholamines on alpha 2-adrenoceptors. The difference between the vascular resistance seen immediately following acute sympathetic denervation and the level achieved 15 min afterward appears to be due to the influence of circulating catecholamines.     

Studies on the mechanism of prazosin induced sympatho-inhibition

Intravenous administration of the alpha 1-adrenoceptor antagonist, prazosin (3-300 micrograms/kg), produced a depression of sympathetic-cholinergic electrodermal responses evoked by electrical stimulation of the posterior hypothalamus in pentobarbital anesthetized cats. Pretreatment with the alpha 2-adrenoceptor antagonists yohimbine (0.5 mg/kg) or idazoxan (0.1 mg/kg) significantly blocked the depressant effects of prazosin but had no effect on hypothalamic evoked electrodermal responses when given alone. Electrodermal responses were readily elicited in animals depleted of CNS monoamines. Monoamine depletion, however, totally abolished prazosin's depression of centrally evoked electrodermal responses. Prazosin also depressed the amplitude of electrodermal responses evoked by electrical stimulation of the cervical spinal cord in spinalized cats. In contrast to hypothalamic stimulation, yohimbine when given alone potentiated spinally evoked electrodermal responses which suggests that both excitatory and inhibitory mechanisms were being activated. Taken together these results suggest that prazosin produces its CNS sympatholytic effect by enhancing inhibition mediated by alpha 2-adrenoceptor mechanisms and not directly by blockade of excitatory alpha 1-adrenergic receptors in the central nervous system. A spinal cord site of action for prazosin is also implicated.     

Behavioural and biochemical evidence for the release of noradrenaline in mouse brain by TRH and some of its biologically stable analogues

Small doses of clonidine probably induce hypoactivity (a distinct form of sedation) by stimulating presynaptic alpha 2-adrenoceptors. This was attenuated by injection of 0.1-10 mg/kg of thyrotropin releasing hormone (TRH) or its biologically stable analogues, CG3509, CG3703 and RX77368, when these were given 10 min before clonidine. This effect was dose-dependent in all cases, but the analogues were more potent than TRH. The TRH metabolites, TRH acid and histidyl-proline diketopiperazine (10 mg/kg) were without effect. This response was still attenuated by the analogues, but not TRH, when these were given 1 hr before clonidine. The results, therefore, suggested that it was the basic tripeptide structure which was active and TRH was less potent than its analogues because of rapid metabolism. Attenuation of hypoactivity by TRH and analogues was not due to increased dopaminergic function because apomorphine (5 mg/kg) was ineffective. Thyrotropin releasing hormone (20 mg/kg), CG3509 (10 mg/kg) and CG3703 (1 mg/kg) also induced locomotor activity and produced various other behavioural changes. This was inhibited by prazosin (3 mg/kg) and haloperidol (0.5 mg/kg) but not by yohimbine (1 mg/kg). Apomorphine (5 mg/kg)-induced activity was inhibited by haloperidol and yohimbine but not by prazosin. This indicated that the activity produced by the TRH compounds, but not apomorphine, was partly mediated by alpha 1-adrenoceptors. Both CG3509 (10(-5) and 10(-4) M) and RX77368 (10(-4) M) evoked the release of endogenous noradrenaline from slices of hypothalamus in vitro. The TRH analogues, however, had no affinity for alpha 1- or alpha 2-adrenoceptors in ligand-receptor binding experiments. Viewed overall, the data showed that TRH and its analogues induced the release of noradrenaline in the brain. In addition, a comparison of the behavioural effects of TRH compounds with dopamine and alpha 1-adrenoceptor agonists suggested that in mice these behavioural responses resulted from stimulation of both noradrenergic and dopaminergic function.