Alpha 1-adrenoceptor-mediated vasoconstriction in vivo to enantiomers of SK & F 89748-A

Central alpha1-adrenoceptors: their role in the modulation of attention and memory formation

Adrenoceptors presently are classified into three main subclasses: alpha1-, alpha2-, and beta-receptors, each with three (perhaps more) subtypes. All three alpha1-adrenoceptor subtypes are present in rat brain. The purpose of this review is to assess the role of alpha1-adrenoceptors in the modulation of synaptic transmission and plasticity, as well as their ability to modulate higher cerebral functions, such as attentional and memory processes. However, since there are no truly subtype-specific agonists or antagonists available at present, it is virtually impossible to allocate a particular central effect to one or other of the subtypes. The activation of alpha1-adrenoceptors reduces the firing probability and glutamate release in the cornu ammonis of the hippocampus. Alpha1-Adrenoceptors may flexibly modulate weak and strong activation of the pyramidal neurones in the neocortex. Alpha1-Adrenoceptors play only a minor role in the modulation of long-term potentiation in the hippocampus, and may influence many brain functions also via non-neuronal mechanisms. since glial cells can express alpha1-adrenoceptors. At the behavioural level, the activation of alpha1-adrenoceptors promotes vigilance and influences working memory and behavioural activation, while having only a minor role in the modulation of long-term memory.

Antagonism of the hypothermic effect of clozapine in mice by centrally-active alpha 2-adrenergic antagonists and alpha 1-adrenergic agonists

Hypothermia induced by either clozapine or clonidine in mice was blocked by the alpha 2-adrenergic antagonists yohimbine, idazoxan, CH-38083, SKF 86466, and L-657,743. These effects were dose related, and the ID50 values for inhibition of clozapine- or clonidine-induced hypothermia were fairly comparable. The order of potency for blocking clonidine-induced hypothermia was: L-657,743 greater than CH-38083 greater than yohimbine greater than idazoxan greater than SKF 86466. A very similar blockade hierarchy for clozapine-induced hypothermia was observed, with the order of the two most effective compounds being reversed. Hypothermia induced by either compound was not blocked by the peripherally-acting, selective alpha 2-adrenergic antagonist, L-659,066, indicating that blockade by the other compounds occurred centrally. The centrally-acting, alpha 1-adrenergic agonists St 587, cirazoline, and SKF 89748 were very effective in blocking the response to clozapine, but ineffective in antagonizing clonidine-induced hypothermia. The ED50 values for the blockade of this response to clozapine, however, did not correlate with their reported potencies in stimulating either peripheral or central alpha 1-adrenergic receptors. This indicates that clozapine-induced hypothermia in mice is not a suitable model for evaluating the properties of central alpha 1-adrenergic compounds. Moreover, since the clonidine-induced hypothermia is not influenced by alpha 1-adrenergic agonists, this paradigm is preferable to clozapine-induced hypothermia in the assessment of alpha 2-adrenergic antagonism The ability of alpha 2-adrenergic antagonists to block clozapine-induced hypothermia may result from the central overflow of norepinephrine, which is known to be brought about by this group of compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between clozapine and a lipophilic alpha 1-adrenergic agonist

Acute intraperitoneal injection of clozapine produced marked hypothermia and ataxia in Swiss-Webster mice. These two effects were almost completely blocked by the lipophilic alpha 1-adrenergic agonist, St 587, but not by the peripherally-acting alpha 1 agonist methoxamine. It was inferred that these effects of clozapine are central in origin and probably resulted from alpha 1 adrenergic blockade. However, since prazosin, a selective alpha 1-adrenergic antagonist did not elicit either hypothermia or ataxia in mice it became clear that the alpha 1 adrenergic blocking effect of clozapine is not entirely responsible for these effects, but has a major contributory role in their production. Both clozapine and prazosin inhibited the d-amphetamine-induced locomotor stimulation in mice. St 587 did not significantly reduce this amphetamine-blocking effect of clozapine. It was inferred that this response to d-amphetamine involving the release of mesolimbic dopamine is distinct from the other two St 587-sensitive responses. The hypothermic and ataxic effects of clozapine developed complete tolerance after just four days of treatment, but ten days of such treatment was required for the development of tolerance to the amphetamine-blocking effect of clozapine. The possible relationships between St 587-sensitive and insensitive effects of clozapine and its antipsychotic property are discussed.

Modification of certain pharmacological effects of ethanol by lipophilic alpha-1 adrenergic agonists

The influence of four centrally-acting alpha-1 adrenoceptor agonists, namely, 2(2-chloro-5-trifluoromethylphenylimino) imidazolidine (St 587), cirazoline, (-) 1,2,3,4-tetrahydro-8-methoxy-5-methylthio-2-naphthalenamine ((-)SKF 89748A) and 2-(2-methylindazol-4-imino)imidazolidine (Sgd 101/75) on the pharmacological effects of ethanol was investigated. All four drugs reduced the duration of ethanol-induced hypnosis in C57B1/6 mice, this effect being proportional to their relative potencies to exert central alpha-1 agonism. In prazosin-pretreated mice, St 587 failed to reduce the hypnotic effect of ethanol, which provided strong evidence for the role of alpha-1 agonism for the hypnosis reducing effect of St 587. Hyperactivity induced in C57B1/6 mice by a subhypnotic dose of ethanol and St 587 was reported earlier. In the present study, St 587, cirazoline and (-)SKF 89748A produced similar response, but no correlation between this effect and ethanol hypnosis blockade could be established. Interestingly, this hyperactivity response was not exhibited by Swiss-Webster, BALB/c or DBA-2 mice--strains in which St 587 exerted little or no antagonism to ethanol-induced hypnosis. Of the alpha-1 agonists, only St 587 reduced the ethanol-induced hypothermia in C57B1/6 mice. St 587 also blocked this effect of ethanol in BALB/c mice in which this drug failed to reduce the ethanol-induced hypnosis. It was concluded that ethanol-induced hypothermia and hypnosis are not interrelated. None of the alpha-1 agonists modified the pentobarbitone-induced hypnosis or the rate of elimination of ethanol in C57B1/6 mice. In this strain, the reduction of the duration of ethanol-induced hypnosis by the alpha-1 agonists is a selective and centrally-mediated response.

Equal potency of nifedipine to inhibit alpha 1-(dobutamine and BDF 6143) and alpha 2-adrenoceptor (B-HT 920) induced pressor responses in pithed rats; lack of effect of phenoxybenzamine

Intravenous (i.v.) dobutamine and BDF 6143 were partial agonists in increasing diastolic pressure in beta-adrenoceptor-blocked pithed rats. The log dose-pressor effect curves were not influenced by yohimbine (1 mg/kg i.v., -15 min) but were markedly shifted to the right by prazosin (0.1 mg/kg i.v., -15 min) indicating the exclusive involvement of alpha 1-adrenoceptors. Nifedipine (0.1-1 mg/kg i.a., -15 min) non-competitively inhibited the pressor effects of dobutamine and BDF 6143 as well as of the alpha 2-adrenoceptor agent B-HT 920 with equal potency. The -log ED50 values calculated for nifedipine amounted to 6.25 +/- 0.12, 6.16 +/- 0.14 and 6.20 +/- 0.10, respectively. Phenoxybenzamine (3 or 10 micrograms/kg i.v., -60 min) did not affect the effectiveness of nifedipine (0.1 mg/kg) to inhibit the pressor effects of dobutamine and BDF 6143. Following treatment with Bay k 8644 (1 mg/kg i.a., -15 min), the log dose-pressor effect curves for dobutamine and BDF 6143 were shifted to the left and the maximum responses were elevated. Our findings suggest that the alpha 1-adrenoceptor-induced pressor effects of dobutamine and BDF 6143 rely heavily on the influx of Ca2+, and are indistinguishable in this respect from the effects initiated by alpha 2-adrenoceptor stimulation. The data further support the view that the sensitivity of alpha-adrenoceptor-mediated pressor effects to inhibition by Ca2+ entry blockers depends on the extent to which Ca2+ influx contributes to the overall response and is not determined by the intrinsic activity or by the receptor reserve of the alpha-adrenoceptor agonist.

An in vivo pharmacological method for the quantitative evaluation of the central effects of alpha 1 adrenoceptor agonists and antagonists

A new in vivo pharmacological method for the quantitative evaluation of alpha 1-adrenoceptor agonists and antagonists has been developed. It consists of recording the myoclonic twitch activity (MTA) of the suprahyoideal muscle of rats anesthetized with urethane. In these animals, the isomers of amphetamine elicited myoclonic twitch activity; their effects were dose-related and the d-isomer was approximately 3.5 times more effective than the l-isomer. While pimozide did not block this response, the postsynaptic alpha 1-antagonist prazosin fully blocked the myoclonic twitch activity induced by d-amphetamine. Other postsynaptic alpha 1-antagonists, such as haloperidol, phenoxybenzamine and clozapine, were also effective in blocking this response to d-amphetamine. Since d-amphetamine elicited myoclonic twitch activity in rats pretreated with reserpine and alpha-methyl-p-tyrosine, it was concluded that d-amphetamine exerted a direct alpha 1-adrenoceptor stimulation. In rats pretreated with nialamide and pimozide, l-DOPA elicited myoclonic twitch activity which was dose-related. This effect of l-DOPA was promptly and fully blocked by prazosin. It was concluded that this response to l-DOPA resulted from stimulation of alpha 1-adrenoceptors. The relative potencies of four alpha 1-adrenoceptor stimulants, namely, cirazoline, St-587, (-)SKF 89748A and Sgd 101/75 were determined using this method. The results correlated very well with their relative potencies to increase the diastolic blood pressure of pithed rats. Evidence that myoclonic twitch activity is a centrally-mediated response has also been presented. It appears that the method is a simple, sensitive, versatile and easily quantifiable procedure for the evaluation of the central effects of alpha 1-adrenoceptor agonists and antagonists.

Further studies on the ethanol antagonism exhibited by 2(2-chloro-5-trifluoromethyl phenylimino) imidazolidine (St 587)

A lipid soluble alpha 1-adrenoceptor agonist 2-(2-chloro-5-trifluoromethyl phenylimino) imidazolidine (St 587) dose-dependently antagonized the hypnotic, hypothermic and respiratory depressant effects of ethanol in C57B1/6 mice. This effect was present whether St 587 was given before or after ethanol. St 587 did not block the pentobarbitone-induced hypnosis. It also did not influence the elimination of ethanol. Combined treatment with a subhypnotic dose of ethanol and St 587 resulted in marked hyperactivity in mice. This effect was completely abolished by pimozide pretreatment. It was inferred that the dopamine released from brain areas by this dose of ethanol together with the norepinephrine receptor activation offered by St 587 resulted in this hyperactivity. Cirazoline, a more potent alpha 1-adrenoceptor agonist than St 587 was relatively more effective than the latter in blocking the ethanol-induced hypnosis in mice. It seems that alpha 1-adrenoceptor stimulation is a major contributing factor to the ethanol antagonism exerted by St 587. This drug might prove to be useful in the treatment of acute ethanol intoxication and in understanding the mode of action of ethanol.

Effects of the irreversible alpha-adrenoceptor antagonists phenoxybenzamine and benextramine on the effectiveness of nifedipine in inhibiting alpha 1- and alpha 2-adrenoceptor mediated vasoconstriction in pithed rats

In pithed normotensive rats, i.v. injection of the selective alpha 1-adrenoceptor agonist cirazoline produced vasoconstriction which was largely resistant to inhibition by nifedipine. On the other hand, the pressor effects of the selective alpha 1-adrenoceptor agonists St 587 and Sgd 101/75 were much more effectively blocked by nifedipine, although not as effectively as the pressor effects to the selective alpha 2-adrenoceptor agonist B-HT 920. The sensitivity to inhibition of vasoconstriction in pithed rats to the different agonists increased in the order cirazoline much less than St 587 less than Sgd 101/75 less than B-HT 920. Phenoxybenzamine (3-300 micrograms/kg, i.v., -60 min) irreversibly antagonized the vasoconstriction to cirazoline, St 587, Sgd 101/75 and B-HT 920. After treatment of the rats with phenoxybenzamine the potency and efficacy of nifedipine in antagonizing vasoconstriction to alpha 1-, but not to alpha 2-adrenoceptor activation was dose-dependently enhanced. The potency of nifedipine to inhibit alpha 1-adrenoceptor-mediated vasoconstriction by cirazoline, St 587 and Sgd 101/75 was increased maximally to the level of efficacy at which nifedipine antagonized B-HT 920-induced vasoconstriction. The dose of phenoxybenzamine required to maximally increase the potency and efficacy of nifedipine to antagonize vasoconstriction of the alpha 1-adrenoceptor agonists was inversely related to the level of sensitivity to blockade by nifedipine of the vasoconstriction they produced. In contrast, pretreatment of rats with the irreversible antagonist, benextramine (10 mg/kg, i.v., -100 to -60 min) did not increase the potency or efficacy of nifedipine to antagonize vasoconstriction to cirazoline, St 587, Sgd 101/75 or B-HT 920, despite irreversible blockade of alpha 1- and alpha 2-adrenoceptors. These data suggest that phenoxybenzamine, but not benextramine, selectively inhibits the alpha 1-adrenoceptor mediated vasoconstrictor mechanism that is independent of influx of extracellular calcium. Moreover, the results show that the existence of receptor reserve or the number of alpha 1-adrenoceptors activated does not determine the relative contribution of calcium influx-independent mechanisms in alpha 1-adrenoceptor-mediated vasoconstriction.

Lack of effect of D600 on alpha 1-adrenoceptor-mediated contractions of rat isolated anococcygeus muscle

The experiments concerned the contractile responses of rat anococcygeus muscle to the selective alpha 1-adrenoceptor agonists cirazoline, phenylephrine, methoxamine, St 587, Sgd 101/75, amidephrine and SK&F 89748-A and the effect of the calcium entry blocker D600 on the responses. The effects of noradrenaline, adrenaline, K+, tyramine and electrical field stimulation were studied as well. The potency series of the various agonists on rat anococcygeus muscle differed from the series for rat and guinea-pig aorta, indicating differences in the structure of the alpha 1-adrenoceptors on these tissues. D600 was ineffective in inhibiting contractions of rat anococcygeus muscle to the various agonists, but effectively attenuated the responses to K+ in anococcygeus muscle from rats pretreated with reserpine or in prazosin-induced preparations. These data indicate that alpha 1-adrenoceptor activation in rat anococcygeus muscle triggers contractions which do not primarily require the influx of extracellular calcium. In this respect, the smooth muscle from the rat anococcygeus muscle differs from vascular smooth muscle from this species.

The contractions induced in rat and guinea-pig aortic strips by the alpha 2-adrenoceptor selective agonists B-HT 920 and UK 14,304 are mediated by alpha 1-adrenoceptors

The alpha-adrenergic action of the selective alpha 2-adrenoceptor agonists B-HT 920 and UK 14,304 was investigated on helically cut preparations of rat and guinea-pig isolated aorta. The alpha 1-adrenoceptor selective stimulant (-)-phenylephrine was included for comparison. All agonists induced concentration-dependent contractions in both preparations. Calcium entry blockade by D 600 almost abolished the contractions evoked by B-HT 920 and UK 14,304 in rat aorta while those evoked in guinea-pig aorta were less strongly affected. Contractions elicited by (-)-phenylephrine were moderately impaired by D 600 in rat aorta whereas there was only a limited effect in guinea-pig aorta. Analysis of the prazosin and yohimbine antagonism of B-HT 920- and UK 14,304-evoked contractions showed the involvement of alpha 1-like adrenoceptors in rat and guinea-pig aorta, prazosin being approximately 1000 times more potent that yohimbine. The results show that B-HT 920 and UK 14,304 contract rat and guinea-pig aorta via alpha 1-like adrenoceptors which are not identical. It is submitted that rat and guinea-pig alpha 1-adrenoceptors activate different contractile processes.