Activation of the central pathway of the baroreceptor reflex,a possible mechanism of the hypotensive action of clonidine

Summary Arterial blood pressure, heart rate and discharges in the preganglionic splanchnic and a postganglionic renal sympathetic nerve were recorded in cats anaesthetized with urethane. Electrical stimulation of the posterior hypothalamus or the fastigial nucleus of the cerebellum elicited an immediate increase in sympathetic nerve activity and a rise in blood pressure and heart rate. The stimulation-induced discharge pattern in the sympathetic nerves was characterized by a strong initial burst followed by a phase of inhibition and a final stabilization of the discharges at a level definitely lower than the initial burst. This pattern was reversibly converted into a constant high amplitude firing during a lowering of the blood pressure by bleeding the cats and irreversibly so after cutting the buffer nerves. These findings indicate that the inhibitory phase of the sympathetic discharge pattern during central stimulation is due to the rise in blood pressure and the ensuing baroreceptor-reflex activation.Clonidine (0.03 and 0.1 mg/kg i.v.) reduced the spontaneous sympathetic nerve activity and lowered blood pressure and heart rate. The sympathetic discharges evoked by central stimulation were partially inhibited by clonidine, an effect which could be overcome by raising the voltage used for central stimulation. Independent of the strength of this stimulation an inhibitory phase in the evoked discharge pattern was not observed after clonidine, even when the low blood pressure due to the drug action was raised by a noradrenaline infusion. After clonidine, the evoked discharge pattern closely resembled that after simultaneous stimulation of both sinus nerves and the hypothalamus or the fastigial nucleus, and it was not altered by additional stimulation of the sinus nerves. These observations have led to the hypothesis that clonidine causes a long-lasting activation within an as yet unidentified part of the central pathway of the depressor baroreceptor reflex. In view of the well-known -adrenoceptor stimulating property of clonidine, and since the central effect of clonidine was antagonized by the -adrenoceptor blocking agent piperoxan, it is likely that the central part of the baroreceptor-reflex pathway is modified by or contains adrenergic neurones.     

Interaction of enantiomers of hydroxy tolazoline with adrenoceptors

Summary Adrenoceptor-mediated effects of the enantiomers of hydroxytolazoline and tolazoline (i. e., desoxy derivative) have been investigated in vitro. The enantiomers and tolazoline were partial agonists of postjunctional ₁-adrenoceptors in rat aorta. The rank order of potencies of the compounds in this system was as follows: tolazoline > R(–)-hydroxytolazoline > S(+)-hydroxytolazoline. The efficacy of R(–)-hydroxytolazoline was higher than that of tolazoline, though its affinity for the receptor was less. The K _B values for prazosin against these agonists were nearly equal, which indicated that these imidazolines activate the same type of receptor in rat aorta. The S(+)-isomer, however, produced both a prazosin sensitive and resistant component of the response. The interactions of the derivatives with presynaptic ₂-adrenoceptors were studied in field-stimulated myenteric plexus-longitudinal muscle of guinea-pig ileum. These substances were blockers at presynaptic ₂-adrenoceptors. Based on K _B values, the order of affinity in this system was as follows: tolazoline > S(+)isomer R(–)-isomer. -Adrenoceptor mediated activity was quantitated in guinea-pig and rat atria. R(–)-hydroxytolazoline lacked chronotropic effects either in guinea pig or rat atria. At 3 × 10^–4 M the isomer did not antagonize the effect of isoproterenol in the atria. On the other hand, S(+)-hydroxytolazoline produced a variable chronotropic effect in guinea-pig atria, but failled to show any significant activity in rat atria. Thus, the -adrenoceptor mediated action appears to be insignificant. Steric aspects of -adrenoceptor mediated events are discussed.This investigation was, in part, supported by a grant from the United States Public Service, National Institutes of Health, GM 29358 Send offprint requests to P. N. Patil at the above address     

Studies on alpha adrenoceptors in guinea-pig peripheral lung strips

Summary Contractile responses of guinea-pig peripheral lung strips to noradrenaline were determined in the presence of propranolol (2.5 × 10^–6 mol/l). All strips (n = 44) contracted to noradrenaline and a geometric mean EC₅₀ of 1.4 × 10^–6 mol/l (1.1 × 10^–6 mol/l, 1.8 x 10^–6 mol/l 95% confidence limits) was obtained. Contractions were antagonised by phentolamine (5 × 10^–7–10^–5 mol/l) and by prazosin (10–10^–7 mol/l). Dose-ratios (DR) were calculated and log (DR-1) was plotted against log concentration of antagonist to yield slopes (± SE) of 0.84 ± 0.14 and 0.73 ± 0.22 respectively which were not significantly different from unity. A pA₂ value (± SE) of 6.7 ± 0.2 was obtained for phentolamine and 7.5 ± 0.1 for prazosin. Yohimbine (10^–7–10^–5 mol/l) did not significantly affect the maximal tension generated or the EC₅₀ values for noradrenaline. These results suggest that ₁ adrenoceptors are mediating the contractile responses to noradrenaline. In the presence of cocaine (10^–5 mol/l, n=18), normetanephrine (2 × 10^–5 mol/l, n = 15), hydrocortisone (2.5 × 10^–5 mol/l, n = 15) and normetanephrine (2 × 10^–5-5 mol/l) plus cocaine (10^–6 5 mol/l, n=15) pA₂ values for phentolamine of 6.9, 6.7, 6.6, and 6.3 respectively were obtained. Since these pA₂ values are not significantly different from 6.7, it is unlikely that this original pA₂ value, which is lower than values obtained with phentolamine at -adrenoceptors in other tissues, may be explained by neuronal or extraneuronal uptake of noradrenaline. A possible explanation may be that more than one population of -adrenoceptors contribute to changes in tension in peripheral lung strips. Send offprint requests to J. P. Seale at the above address     

Depression of exploratory activity by clonidine in rats as a model for the detection of relative pre- and postsynaptic central noradrenergic receptor selectivity of α-adrenolytic drugs

Summary The effects of various -adrenoceptorblocking drugs on the depression of exploratory activity (ambulation and rearing) induced by 0.1 mg/kg i.p. clonidine were investigated in the rat. In parallel experiments, the effects of the same drugs on pre- and postsynaptic -receptors were determined in vitro (field-stimulated cortex slices and isolated vas deferens of the rat, respectively). Tolazoline, esproquine, yohimbine and piperoxan distinctly antagonized the inhibition of exploration produced by clonidine. All these drugs were found to possess relatively higher selectivity for the presynaptic -receptors, as judged by the ratios of the concentrations inducing a 50% increase in field-stimulated ³H-noradrenaline-overflow and the concentrations required to shift the EC₅₀ for the antagonism of noradrenaline-induced contractions of the vas deferens to the right by a factor of 2 (pA₂, ratio <1): In contrast, phentolamine and phenoxybenzamine which showed preferential postsynaptic -receptor blocking activity (ratio>1), potentiated rather than antagonized the effects of clonidine. Mianserin, although preferentially blocking the postsynaptic receptors, had no effect on clonidine-induced hypoactivity up to the high dose of 100 mg/kg i.p., probably because of its additional NA-uptake-inhibiting properties. The antagonism of clonidine by the selective presynaptic -receptor blockers was observed within a limited dose-range. Increasing the doses above an optimal level, which varied from one compound to another, resulted in a decrease in the effect. It is suggested that this phenomenon reflects the counter-balancing postsynaptic -adrenoceptor blockade occuring at higher concentrations of these drugs. In general, the results show a fairly good correlation between antagonism of clonidine in vivo and preferential blockade of presynaptic -receptors in vitro. Clonidine-induced suppression of exploration therefore seems to be a valuable model for the investigation of drug interactions with -adrenergic receptors in the central nervous system.These results were presented in part at the Spring Meeting of German Pharmacological Society, Mainz, 16–18 March, 1977     

Clonidine-induced inhibition of sympathetic nerve activity: No indication for a central presynaptic or an indirect sympathomimetic mode of action

Summary The effects of clonidine on blood pressure, heart rate, contractile state of the nictitating membranes, spontaneous sympathetic nerve activity and response of sympathetic nerves to hypothalamic stimulation were compared in normal anaesthetized cats and in anaesthetized cats pretreated with reserpine and -methyl-p-tyrosine. The pretreatment lowered the noradrenaline content of various parts of the brain to less than 5 ng/g, i.e. to less than 1–3% of that of the controls. Under the conditions of this severe noradrenaline depletion, blood pressure and heart rate were low and spontaneous sympathetic nerve activity consisted of continous, high-amplitude discharges which contrasted with the low-amplitude bursts of activity—synchronous with the respiration—of the controls. In contrast to the controls, clonidine did not lower blood pressure and heart rate in the cats with noradrenaline depletion; however, the clonidine-induced contractions of the nictitating membranes were of similar magnitude and duration in both groups of animals. The efficacy of clonidine in reducing or abolishing spontaneous sympathetic nerve activity and in inhibiting the response of sympathetic nerves to hypothalamic stimulation was equal in controls and in cats with noradrenaline depletion, its potency being 3-fold higher in the former. The results indicate a direct stimulation of -adrenoceptors by clonidine both in the periphery and in the central nervous system and make it unlikely that the central effect of clonidine on blood pressure is due to a release of noradrenaline from central adrenergic neurones. It is further concluded that clonidine activates an adrenergic mechanism in the central nervous system by stimulation of postsynaptic -adrenoceptors. The inhibition of such a mechanism as a consequence of a diminished noradrenaline release due to stimulation of presynaptic -adrenoceptors—as proposed from in vitro experiments—seems to be of no importance for the central effect of clonidine on sympathetic nerve activity and blood pressure.Preliminary results have been presented at the 15th Spring Meeting of the German Pharmacological Society in Mainz (Haeusler, 1974a).     

Discontinuation of chronic clonidine treatment: Evidence for facilitated brain noradrenergic neurotransmission

Summary Clonidine, 0.1 mg/kg i.p., or saline was administered to mice twice daily for 12 days. After withdrawal of the drug (14–18.5 h after the last injection), the accumulation of Dopa during 30 min after inhibition of central aromatic amino acid decarboxylase by NSD 1015 (3-hydroxy-benzyl hydrazine, 150 mg/kg i.p.) was significantly increased in the noradrenaline (NA)-rich, but dopamine (DA)-poor, brain stem but not in the DA-rich, but NA-poor corpus striatum. An increased Dopa accumulation was also found in the limbic system and, probably, in the hemispheres. The central accumulation of 5-hydroxytryptophan (5-HTP) was significantly increased in the limbic system. Clonidine, 0.1 mg/kg i.p., administered to mice during the withdrawal phase caused reduction of the accumulation of Dopa and 5-HTP (during 30 min after NSD 1015, 150 mg/kg i.p.) in all brain regions studied to approximately the same levels, as when given to saline-pretreated controls. The disappearance rates of brain NA and DA after synthesis inhibition (-methyl-p-tyrosine methylester HCl 250 mg/kg i.p., 4 h) were probably not altered by clonidine withdrawal.The locomotor stimulation by the central catecholamine receptor agonists apomorphine plus clonidine (1.5 mg/kg i.p. both) after reserpine (10 mg/kg, 4 h) pretreatment was significantly enhanced in clonidine withdrawal mice. However, the motor stimulation by apomorphine (1.5 mg/kg i.p.) alone after reserpine pretreatment was not affected. The spontaneous motor activity was not significantly altered by the withdrawal of clonidine.The data show that brain NA and 5-HT systems are affected by clonidine withdrawal in the opposite direction to that seen after acute administration of low doses of the drug, when they are inhibited. Since postsynaptic central -adrenoceptors, which are stimulated by high but not by low doses of clonidine, showed an enhanced response to high doses of clonidine following discontinuation of a chronic low dose regimen, supersensitivity due to functional noradrenergic denervation might be implicated.     

In vivo studies on alpha-adrenergic receptor subtypes in human veins

Summary We studied in vivo responsiveness of venous ₁ and ₂-adrenoceptors, measuring the diameter changes in superficial veins in response to -adrenergic agonists and antagonists in healthy human volunteers. The dorsal hand vein technique was used because it permits complete dose-response studies of venous constriction without confounding reflex alterations.Local infusions of all agonists studied induced dose-dependent contraction of the hand vein; the maximal effects (Emax) were: norepinephrine (88% ± 10%), methox amine (97% ± 5%), phenylephrine (95% ± 6%), clonidine (54% ± 12%), and azepexole (68% ± 26%). Clonidine reduced the norepinephrine-induced venoconstriction by 11% ± 10%. Oral doses of 1 mg prazosin antagonized the venoconstriction induced by norepinephrine, methoxamine, and clonidine, but not by azepexole. Yohimbineantagonism was observed against all agonists studied. Inhibition by yohimbine of clonidine-induced venoconstriction was irreversible over 60–180 min.Results show that the in vivo effects on veins of -adrenergic agonists are in good agreement with results from in vitro experiments. Agonists with ₁- and ₂-adrenoceptor subtype selectivity cause venoconstriction in vivo, but ₂-receptor mediated constriction is intrinsically weaker. Clonidine acts as a partial antagonist against norepinephrine, presumably on postsynaptic ₂-receptors. At high doses, ₂-adrenoceptor subtype selectivity of clonidine and yohimbine appear to be partially lost in vivo. Send offprint requests to H. G. Eichler at the above address     

Binding of 3H-clonidine to an α-adrenoceptor in membranes of guinea-pig ileum

Summary The binding of ³H-clonidine to membrane particles from guinea-pig ileum was investigated. The specific binding, i.e. the binding that could be inhibited by high concentrations of unlabeled clonidine or noradrenaline, was of high affinity, K _D3 nM. The number of sites was approximately 25 fmol/mg protein. Rate constants of association and dissociation were 5.3×10⁷ M^–1 min^–1 and 0.18 min^–1, respectively. Affinites of various drugs to the binding site were determined by measuring their effect on the binding of ³H-clonidine. The affinity of adrenergic agonists decreased in the order clonidine = tramazoline > (–)-erythro--methylnoradrenaline > (–)-noradrenaline (–)-phenylephrine. (–)-Noradrenaline had about 20 times more affinity than the (+)-isomer. The affinity of -adrenoceptor antagonists decreased in the order phentolamine > rauwolscine = yohimbine > WB 4101 > pseudoyohimbine > prazosin = corynanthine. Yohimbine and rauwolscine had about 100 times more affinity than their stereoisomer corynanthine. Serotonin 10 M and metiamide 10 M did not affect the binding, and propranolol inhibited it only at high concentrations. — The results indicate that ³H-clonidine labels an ₂-adrenoceptor in guinea-pig ileum. The orders of affinity of -adrenoceptor agonists and antagonists agree well with their orders of potency in functional tests, namely as modulators of cholinergic transmission in the guinea-pig ileum and as modulators of noradrenaline release in the rabbit pulmonary artery. An -adrenoceptor should be classified as ₂ when the affinities of clonidine, tramazoline and -methylnoradrenaline greatly exceed the affinity of phenylephrine, and when the affinities of rauwolscine and yohimbine exceed those of prazosin and corynanthine.     

Transforming Growth Factor-α (TGF-α) in a Semisolid Dosage Form: Preservative and Vehicle Selection

The selection of an ideal semisolid vehicle for growth factors presents a challenge. Some antimicrobial agents are known to delay wound healing. The objective of this investigation was to identify appropriate preservatives and vehicles for TGF-. Criteria for acceptance are noninterference with the mitogenic activity of TGF- as well as adequate product preservation. Vehicles considered were o/w creams, ointments, and a gel. Combinations of six preservatives were tested. Selection was determined using both microbial preservative challenge and TGF- mitogenic assay. In the former, 10 species of microorganisms were inoculated into formulation samples. At selected time intervals, it was determined whether colonies decreased, increased, or remained constant. In the mitogenic assay, samples of either preservatives or formulation prototypes were introduced to TGF--stimulated fibroblast cell cultures. Mitogenesis was determined by measuring ³H-dThd uptake into newly synthesized DNA. As preservatives, sorbic acid and quaternium-15 appear to satisfy both selection criteria. A thermosetting gel appears most promising as vehicle.     

Effect of chronic lithium administration on adrenoceptor binding and adrenoceptor regulation in rat cerebral cortex

Summary Lithium (Li) at a concentration, which exerts prophylactic effects in affective disorders is known to alter noradrenaline turnover and the -adrenoceptor-dependent cAMP accumulation. In the present study the action of chronic Li administration (at least 5 weeks) on agonist and antagonist binding to adrenoceptors and on the regulation of adrenoceptors was investigated in rat cerebral cortex. Li treatment caused a small but significant decrease in the number of -adrenoceptor binding sites by 10% (³H-dihydroalprenolol binding) leaving the number of ₁- and ₂-adrenoceptor binding sites (³H-prazosin and ³H-rauwolscine, respectively) unchanged. The affinity of the radioligands as well as the affinity of agonists to these binding sites were not altered. The up-regulation of -adrenoceptor binding sites produced by repeated reserpine injections was inhibited by 32% in rats treated concomitantly with Li, although the noradrenaline depleting effect of reserpine was not impaired. In contrast, Li treatment had no effect on the up-regulation of -adrenoceptor binding induced by 6-OH-dopamine, nor did it alter the -adrenoceptor down-regulation following chronic administration of desipramine. The up-regulation of ₁-adrenoceptor binding sites caused by reserpine or 6-OH-dopamine also remained unaffected by Li. It is concluded that chronic Li has limited effects on cortical adrenoceptors and their regulation. The inhibition of -adrenoceptor up-regulation caused by reserpine may reflect an action of Li on non-adrenergic systems rather than a general stabilizing effect on adrenoceptors proposed previously.Send offprint requests to: G. Gross     

Role of adrenoceptors in the potentiation of opioid antinociception by ephedrine and phenylpropanolamine in mice

Ephedrine and phenylpropanolamine (PPA) (10 mg/kg) pretreatment intraperitoneally (IP) potentiated the antinociceptive effects of subcutaneous (SC) morphine (5 mg/kg) and codeine (60 mg/kg) in mice using the tail-flick method. Prior administration of propranolol (6 mg/kg, SC) 10 min before the sympathomimetics had no effect on this action. Phentolamine (2 mg/kg, SC), on the other hand, abolished the enhancing effects of ephedrine and PPA on opioid antinociception. Prazosin (2 mg/kg, SC) pretreatment did not significantly affect the potentiation of opioid antinociception by ephedrine and PPA, while yohimbine (4 mg/kg, SC) effectively antagonised this enhancing effect. None of the adrenoceptor antagonists had any effect on the tail-flick reaction time on their own in the doses used, and neither did they affect opioid antinociceptive responses. It is concluded that ephedrine and PPA potentiate the antinociceptive effects of morphine and codeine, most probably through an action on ₂-adrenoceptors.     

Evidence against a functional link between noradrenaline uptake mechanisms and presynaptic alpha-2 adrenoceptors

Summary Slices prepared from rat cerebral cortex were labelled with ³H-noradrenaline and superfused. Electrical field stimulation was carried out 15 min (S₁) and 45 min (S₂) after the start of collection of 5-min samples using 4 pulses delivered at 100 Hz. Drugs acting at ₂-adrenoceptors were added 20 min before S₂, and their effects were evaluated using the S₂/S₁-ratio. The ₂-adrenoceptor antagonists idazoxan (1 mol/l) and rauwolscine (1 mol/l) failed to increase stimulation-evoked overflow of radioactivity in the absence or presence of the noradrenaline reuptake inhibitor desipramine (1 gmol/l). This indicates that the duration of electrical stimulation was too short to allow development of ₂-adrenoceptor-mediated auto-inhibition by released noradrenaline. The effect of clonidine (3–1000 nmol/l) on stimulation-evoked overflow of radioactivity was tested in the absence and presence of three different reuptake inhibitors (desipramine, 1 ol/l; maprotiline, 1 ol/l; cocaine, 10 mol/l). The analysis yielded identical concentration-response curves under all conditions. These results argue against an action of inhibitors of neuronal reuptake of noradrenaline at the presynaptic ₂-adrenoceptor and against the concept of a functional link between uptake site and receptor. Send offprint requests to E. A. Singer     

Alpha2-adrenoceptor-mediated responses to so-called selective alpha1-adrenoceptor agonists after partial blockade of alpha1-adrenoceptors

Summary In dog saphenous vein — a tissue possessing both postsynaptic ₁- and ₂-adrenoceptors — the effects of two selective ₁-adrenoceptor agonists (phenylephrine and methoxamine) were compared with that of the selective ₂-adrenoceptor agonist, UK-14,304, before and after phenoxybenzamine. Furthermore, the influence exerted by prazosin, yohimbine and verapamil on the effects of these agonists was also studied before and after phenoxybenzamine. In the absence of phenoxybenzamine, prazosin (56 nmol/l) caused a parallel shift of the concentration-response curves of both phenylephrine and methoxamine to the right (by 0.94 and 1.1 log units, respectively) and had no effect on the concentration-response curve of UK-14,304, while 20 nmol/l yohimbine caused a marked parallel shift of the concentration-response curve of UK-14,304 to the right (by 1.18 log units) and caused only minor displacements of those of phenylephrine and methoxamine (by 0.2 and 0.33 log units, respectively). After exposure of the strips to 30 nmol/l phenoxybenzamine, prazosin (56 nmol/l) caused small shifts of the concentration-response curves of both phenylephrine (by 0.36 log units) and methoxamine (by 0.31 log units) and did not change that of UK-14,304, while yohimbine (20 nmol/l) caused pronounced parallel shifts of the concentration-response curves (to the right) of all the agonists: phenylephrine (by 1.0 log units), methoxamine (by 0.93 log units) and UK-14,304 (by 1.28 tog units). When UK-14,304 was added to the bath during a sub-maximal contraction to phenylephrine it caused a further contraction almost up to the maximum; if this procedure was repeated after phenoxybenzamine (30 nmol/1), there was no further contraction to UK-14,304.In the absence of phenoxybenzamine, verapamil (5 mol/l) caused a parallel shift of the concentration-response curve of phenylephrine (or methoxamine) to the right and a non-parallel shift (with marked depression of the maximal effect) of that of UK-14,304. However, after phenoxybenzamine (30 nmol/l), the same concentration of verapamil caused non-parallel shifts of the concentration-response curves of the three agonists to the right with about equal depression of the maximal effects. We conclude that, after removal of ₁-adrenoceptor reserve by phenoxybenzamine, the responses to selective ₁-adrenoceptor agonists are predominantly ₂-adrenoceptor-mediated. This may explain why under these conditions, the selective ₁-and ₂-adrenoceptor agonists are equally antagonized by calcium entry blockers.This work was supported by a grant from the University of Porto (Subsidio para acção de investigação no. 36/85) Send offprint requests to S. Guimarães at the above address     

Smooth muscle of rabbit aorta contains α1- but not α2-Adrenoceptors

Summary We have investigated the residual contractile response to noradrenaline remaining after phenoxybenzamine (3×10^–7 mol/l) in rabbit aorta, since it has been reported that phenoxybenzamine at low doses completely and irreversibly blocks ₁- but not ₂-adrenoceptors. The contraction elicited by noradrenaline slowly recovered with time after it had been almost abolished by phenoxybenzamine. This residual response was blocked by the ₁-selective antagonist prazosin (3×10^–8 mol/l) but not by the ₂-selective antagonist rauwolscine (3×10^–7 mol/l). The results confirm that the smooth muscle of rabbit aorta contains ₁- not ₂-adrenoceptors.     

Pharmacological characterization of the imidazoline receptor which mediates inhibition of noradrenaline release in the rabbit pulmonary artery

Summary Imidazoline receptors involved in modulation of noradrenaline release were characterized in the rabbit pulmonary artery preincubated with [³H]noradrenaline and superfused with physiological salt solution containing cocaine, corticosterone and propranolol. Tritium overflow was evoked by transmural electrical stimulation.The ₂-adrenoceptor blocking imidazolines tolazoline, BDF 6100 [2-(2-imidazoline-2-ylamino)-isoindoline] and BDF 7572 (4,7-dichloro-derivative of BDF 6100) increased the electrically evoked ³H overflow; the concentration-response curves were bell-shaped. In contrast, two other imidazolines, i. e. moxonidine and clonidine, two guanidine derivatives structurally related to BDF 6100, i. e. aganodine and BDF 7579 [4-chloro(2-isoindolinel)-guanidine], as well as the catecholamine noradrenaline concentration-dependently inhibited the evoked ³H overflow. The concentration-response curves for moxonidine, clonidine, aganodine, BDF 7579 and noradrenaline were shifted to the right by rauwolscine. The apparent pA₂ value of rauwolscine against moxonidine was 8.22, whereas those against clonidine, aganodine, BDF 7579 and noradrenaline were in the range of 6.37–6.77 and, hence, considerably lower than reported for ₂-adrenoceptors. In the presence of rauwolscine an inhibitory effect was also observed with the ₂-adrenoceptor blocking imidazolines tolazoline, BDF 6100, BDF 7572, and the imidazolineST 587 [2-(2-chloro-5-trifluoromethylphenylimino)-imidazoline]; the rank order of potency of all guanidines and imidazolines investigated was: aganodine > BDF 7579 > BDF 7572 > BDF 6100 > clonidine > ST 587 > moxonidine > tolazoline. Amiloride, 1-benzylimidazole and histamine were ineffective. After irreversible blockade of -adrenoceptors by preexposure to phenoxybenzamine, evoked ³H overflow was still inhibited by aganodine, BDF 7579 and noradrenaline. Under this condition the maximal effects obtainable with the guanidines and in particular with noradrenaline were lower than in the presence of rauwolscine.These findings are compatible with our previous suggestion that imidazoline receptors mediating inhibition of noradrenaline release exist on the sympathetic nerve terminals of the rabbit pulmonary artery. Comparison of the present data with those obtained in other preparations containing imidazoline recognition sites revealed that those sites are different from the present ones. It is conceivable that the receptor characterized here represents an allosteric site of the ₂-adrenoceptor or a so far undescribed -adrenoceptor subtype since it can be activated not only by imidazolines and guanidines but also by noradrenaline and can be blocked by rauwolscine. Comparison of the properties of isoindolines substituted with either aminoimidazoline or guanidine reveals that the imidazolines (e.g. BDF 7572) possess both ₂-adrenoceptor antagonistic and imidazoline receptor agonistic properties, whereas the analogous guanidines (e. g. aganodine) are imidazoline receptor agonists as well as ₂-adrenoceptor agonists. Send offprint requests to M. Göthert at the above address     

Pharmacological characterization of central α-adrenoceptors which mediate clonidine-induced locomotor hypoactivity in the developing rat

Summary The present experiment was designed to pharmacologically characterize receptors which mediate the clonidine-induced locomotor change in the developing rat. A subcutaneous injection of clonidine (0.78 mol/kg) produced locomotor hyperactivity in 7-day-old rats but hypoactivity in 20-day-old rats. Phenoxybenzamine (1.5 mol/kg, 5.9 mol/kg and 15 mol/kg) decreased spontaneous activity in a dosedependent manner but did not antagonize clonidineinduced hypoactivity in 20-day-old rats. By contrast, the significant reversal of the clonidine-induced hypoactivity by pretreatment with phentolamine (1.6 mol/kg and 6.3 mol/kg), yohimbine (1.3 mol/kg and 5.1 mol/kg) and piperoxan (7.4 mol/kg) was observed at such doses when the blockers did not cause and hypoactivity by themselves. It is suggested that clonidine could induce locomotor hypoactivity by activating presynaptic (₁-type) -adrenoceptors in the CNS of 20-day-old rat.     

The effect of microelectrophoretically applied clonidine on single cerebral cortical neurones in the rat

Summary The technique of microelectrophoresis was used in order to examine the effects of clonidine on single neurones in the somatosensory cortex of the rat, and to compare its actions with those of noradrenaline and phenylephrine. Clonidine evoked only excitatory responses on cortical neurones. The clonidine-sensitive neurones were also excited by noradrenaline and phenylephrine. Clonidine had a consistently lower apparent potency than either noradrenaline or phenylcphrine. Responses to clonidine had a slower time-course than responses to the other two adrenoceptor agonists, both the latencies to onset and the recovery times being longer for responses to clonidine than for responses to noradrenaline and phenylephrine. When the mobilities of clonidine and phenylephrine were compared using an in vitro method, no significant difference was found between the mobilities of the two ionic species, suggesting that they have similar transport numbers. Thus the difference between the potencies and time-courses of responses to clonidine and phenylephrine are presumably of biological origin. Responses to clonidine were antagonised by microelectrophoretically applied prazosin; responses to phenylephrine were equally antagonised, while responses to acetylcholine were not affected. Clonidine could reversibly antagonise excitatory responses to both noradrenaline and phenylephrine, whithout affecting responses to acetylcholine. The results suggest that clonidine may act as a partial agonist at excitatory ₁-adrenoceptors on cortical neurones.     

Comparative behavioral characterization of the neuroactive steroids 3α-OH,5α-pregnan-20-one and 3α-OH,5β-pregnan-20-one in rodents

Pregnan steroids have been shown to possess anesthetic, hypnotic, anticonvulsant and anxiolytic properties. In this study, two endogenous neuroactive steroid isomers, 3-hydroxy-5-pregnan-20-one (3,5-P) and 3-hydroxy-5-pregnan-20-one 3,5-P), were studied for differences in their pharmacological properties using behavioral assays. 3,5-P and 3,5-P were similar in their potencies and efficacies in blocking pentylenetetrazol-induced seizures in mice (ED₅₀: 3,5-P=2.8 mg/kg and 3,5-P=3.0 mg/kg). Similarly, both neuroactive steroids produced roto-rod deficits within the same range of potency (TD₅₀:3,5-P=18.8 mg/kg and 3,5-P=21.2 mg/kg). However, in animal models of anxiety, subtle differences were observed between the two isomers. In both the light/dark transition test and elevated plus-maze, 3,5-P was more efficacious than 3,5-P, though both compounds had similar potencies. In the Geller-Seifter test, 3,5-P was more potent and efficacious than 3,5-P. Neither compound had significant effects on unpunished responding within the dose range tested. Both compounds produced similar biphasic curves in the locomotor test. All together, the data indicate that 3,5-P and 3,5-P have similar anticonvulsant activity, but the 5-isomer possesses more potent and efficacious anxiolytic properties than the 5-isomer.     

Hypotensive and bradycardic effects of talipexole (B-HT 920) in anaesthetized rabbits are antagonized by metoclopramide but not by yohimbine

Summary The interactions of talipexole (B-HT 920) and clonidine with selective -adrenoceptor antagonists, yohimbine (a ₂) and prazosin (a ₁), as well as with dopamine receptor antagonists, metoclopramide (D₂), domperidone (D₂) and SCH23 390 (D₁) were investigated in anaesthetized rabbits after i. v. administration.Both talipexole (0.03–0.1 mg/kg) and clonidine (0.01–0.03 mg/kg) dose-dependently induced hypotension and bradycardia. Talipexole had a shorter duration of action.The hypotensive effect of the ₂-adrenoceptor and D₂ agonist talipexole (0.03 mg/kg) was antagonized by pretreatment with metoclopramide (3 mg/kg) or domperidone (0.3–3 mg/kg),but not with yohimbine (3 mg/kg),prazosin (0.1 mg/kg) orSCH 23 390 (1 mg/kg). Its bradycardic effect was antagonized only by metoclopramide (3 mg/kg). The hypotensive and bradycardic effects of clonidine (0.03 mg/kg) were most effectively antagonized by yohimbine (0.3–3 mg/kg).These findings indicate that in anaesthetized rabbits after i. v. administration, talipexole may lower blood pressure by peripheral, and heart rate by central, dopamine D₂ agonism.Parts of the results were presented at the 33rd Spring Meeting of the German Pharmacological and Toxicological Society in Mainz, March 10–12, 1992 (Palluk R, Schilling JC, Stockhaus K, Peil H (1992) Naunyn-Schmiedeberg's Arch Pharmacol 345:R82)Correspondence to R. Palluk at the above address     

Chronic effects of miaserin on noradrenaline metabolism in the rat brain: Evidence for a pre-synaptic α-adrenolytic action in vivo

Chronic administration of the pre-synaptic -adrenoreceptor agonist clonidine decreases the concentration of the extra-neuronal metabolite of noradrenaline normetanephrine in the amygdaloid cortex and increases it in the mid-brain. Conversely, blockade of these pre-synaptic receptors by yohimbine increases the normetanephrine concentration in the amygdaloid cortex and decreases it in the mid-brain. Mianserin had a qualitatively similar action to that of yohimbine. When given clinically to rats in combination with clonidine, mianserin antagonizes both the depression of behaviour of the rats in the open field apparatus and also the effects of the -agonist in reducing the concentration of normetanephrine in the amygdaloid cortex. It thus appears that the chronic effects of mianserin are due to an increase in noradrenaline release as a consequence of the inhibition of pre-synaptic -adrenoreceptors.