Clonidine-induced hypoactivity and mydriasis in mice are respectively mediated via pre- and postsynaptic alpha 2-adrenoceptors in the brain

Since brain alpha 2-adrenoceptors occur both pre- and postsynaptically, experiments were carried out to determine the synaptic locations of those receptors mediating clonidine-induced hypoactivity and mydriasis. Intraperitoneal (i.p.) injection of clonidine (1-3000 micrograms/kg) to mice dose dependently induced these two responses and also decreased brain concentrations of 3-methoxy-4-hydroxyphenylglycol (MHPG). The ED50 values were: 120 micrograms/kg for hypoactivity (95% confidence limits 103-140 micrograms/kg), 54 micrograms/kg for mydriasis (95% confidence limits 40-74 micrograms/kg) and 18 micrograms/kg for MHPG reduction (95% confidence limits 8-36 micrograms/kg) suggesting that these responses could all be presynaptically mediated. However, methamphetamine which increases noradrenaline turnover was found to dose dependently produce mydriasis, but not hypoactivity, after peripheral (0.1-5 mg/kg i.p.) or central (0.5-10 micrograms i.c.v.) injection. The mydriasis produced by methamphetamine (0.5 mg/kg i.p.) was abolished by i.c.v. injection of 1 micrograms idazoxan or yohimbine, but not 2.5 micrograms prazosin or pindolol, showing this effect was mediated by central alpha 2-adrenoceptors. Methamphetamine (1-10 micrograms i.c.v.) potentiated the mydriasis induced by clonidine (50 micrograms/kg i.p.) suggesting this was a postsynaptic alpha 2-adrenoceptor response. By contrast, methamphetamine (1-10 micrograms i.c.v.) dose dependently reversed clonidine (100 micrograms/kg i.p.) hypoactivity indicating this response was mediated by presynaptic alpha 2-adrenoceptors. These hypotheses were confirmed by destruction of noradrenergic neurones using DSP-4 (100 mg/kg i.p. x 2). This treatment prevented the mydriasis response to methamphetamine (0.5 mg/kg i.p.), but not clonidine (100 micrograms/kg i.p.) and markedly attenuated clonidine (100 micrograms/kg i.p.) hypoactivity.     

Opposing central autonomic actions of alpha 1- and alpha 2-adrenoceptor antagonists on oculomotor tone

Electrical stimulation of the afferent sciatic nerve produces reflex mydriasis in anesthetized rats. The alpha 2-antagonist idazoxan (10-100 micrograms/kg i.v.) inhibited this reflex in a dose-dependent fashion. In contrast, the alpha 1-antagonist prazosin (30-300 micrograms/kg i.v.), produced a dose-related enhancement of the reflex. Single dose administration of the alpha 2-antagonists yohimbine (3.0 mg/kg i.v.), rauwolscine (3.0 mg/kg i.v.) and idazoxan (1.0 mg/kg) also blocked the reflex, whereas the alpha 1-antagonists phenoxybenzamine (3.0 mg/kg i.v.), corynanthine (1.0 mg/kg i.v.) and prazosin (1.0 mg/kg i.v.) potentiated this response. These studies demonstrate that alpha 2-antagonists block and alpha 1-adrenoceptor antagonists potentiate alpha 2-adrenoceptor-mediated inhibition of oculomotor tone.     

Antagonism of clonidine-induced hypothermia by alpha adrenoceptor antagonists in electrically stimulated mice

The involvement of receptor subtypes in clonidine-induced hypothermia in electrically stimulated mice was studied using various alpha-adrenoceptor antagonists. Yohimbine, the selective alpha 2-adrenoceptor antagonist significantly blocked the action of low dose (50 micrograms/kg i.p.) of clonidine pretreated with Minimal Threshold Shock (12 mA, 0.2 Sec), while prazosin, selective alpha 1-adrenoceptor antagonist partially blocked the action, suggesting predominant involvement of alpha 2 and partial involvement of alpha 1-adrenoceptors. Under similar conditions when clonidine was used in higher dose (500 micrograms/kg i.p.), its action was blocked by prazosin, phenoxybenzamine and yohimbine suggesting involvement of both alpha 1 and alpha 2-adrenoceptors. In case of animals pretreated with Maximal Threshold Shock (36 mA, 0.2 Sec) the hypothermic action of clonidine in lower dose was blocked by all the three antagonists viz. prazosin, phenoxybenzamine and yohimbine, suggesting involvement of both alpha 1 and alpha 2-adrenoceptors, however, when clonidine was used in the higher dose, the action was significantly antagonised by prazosin and partially by yohimbine, suggesting predominant involvement of alpha 1 and partial involvement of alpha 2-adrenoceptors.     

A comparative study of the reversal by different alpha 2-adrenoceptor antagonists of the central sympatho-inhibitory effect of clonidine.

1. The recovery of the clonidine-induced hypotension, bradycardia and sympatho-inhibition produced by several putative alpha 2-adrenoceptor antagonists was investigated in pentobarbitone anaesthetized rats. The activity of four substances containing an imidazoline structure: idazoxan, methoxy-idazoxan, BRL44408 and atipamezole was compared with the effect of fluparoxan, yohimbine and L-657,743; in addition the effect of the alpha 1-adrenoceptor antagonist, prazosin, was also studied. 2. Prazosin (0.03-1 mg kg-1, i.v.) failed to alter the sympatho-inhibitory and hypotensive effects of clonidine (10 micrograms kg-1, i.v.). L-657,743 (0.01-1 mg kg-1, i.v.) induced a recovery of blood pressure, heart rate and renal sympathetic nerve activity. Yohimbine (0.03-3 mg kg-1, i.v.) completely reversed the sympatho-inhibitory effect of clonidine but did not alter its hypotensive effect. 3. The four imidazoline drugs: idazoxan (10-300 micrograms kg-1, i.v.), methoxy-idazoxan (1-100 micrograms kg-1, i.v.), BRL44408 (0.1-3 mg kg-1, i.v.) and atipamezole (0.03-1 mg kg-1, i.v.) and fluparoxan (10-300 micrograms kg-1, i.v.) reversed the clonidine-induced hypotension but produced only a partial recovery of the renal sympathetic nerve activity and of the heart rate. After pretreatment with prazosin (0.1 mg kg-1, i.v.), the recovery of the sympathetic nerve activity elicited by these compounds was significantly higher. In hexamethonium (10 mg kg-1, i.v.) pretreated rats, these five drugs induced dose-related hypertension which was reduced by pretreatment with prazosin (0.1 mg kg-1, i.v.). 4. Our results indicate that the putative alpha 2-adrenoceptor antagonists idazoxan, methoxy-idazoxan, BRL44408, atipamezole and fluparoxan also have a peripheral hypertensive effect which is mediated through activation of vascular alpha 1-adrenoceptors; this property of the compounds may be partly responsible for the reversal of the hypotensive action of clonidine. Considering the structure and the affinities of the drugs tested, our data indirectly suggest that alpha 2A-adrenoceptors may be implicated in the central sympatho-inhibitory effects of clonidine.     

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 agonists induced mydriasis in the rat by an action within the central nervous system.

1 The effects of intravenous administration of the selective alpha 2-adrenoceptor agonists clonidine, UK 14,304 and guanoxabenz on rat pupil diameter were investigated. 2 In rats anaesthetized with pentobarbitone, each agonist produced a marked dose-related increase in pupil diameter; the rank order of potency was: clonidine greater than UK 14,304 greater than guanoxabenz. 3 Pretreatment with the selective alpha 2-adrenoceptor antagonist, RX 781094 (0.5 mg/kg, i.v.), produced a parallel 30-40 fold shift to the right of the dose-pupil dilator response curves for the three agonists. Yohimbine (1.5 mg/kg, i.v.) produced about a 10 fold rightward shift of the dose-response curve for guanoxabenz. In contrast, the alpha 1-selective antagonist, prazosin (0.5 mg/kg, i.v.), failed to affect the dose-response relation for guanoxabenz. 4 Several antagonists of varying selectivities towards alpha 1- and alpha 2-adrenoceptors were tested for their ability to reverse the maximal mydriasis induced by guanoxabenz (0.3 mg/kg, i.v.). The rank order of potency of the antagonists producing a 50% reversal of this effect was: RX 781094 greater than yohimbine greater than piperoxan = rauwolscine greater than mianserin greater than RS 21361. Neither corynanthine nor prazosin reversed the guanoxabenz-induced mydriasis. 5 Topical application of RX 781094 (0.1 to 3% w/v solutions) onto one eye produced a slow reversal of guanoxabenz-induced mydriasis; the time course and degree of reversal were virtually the same in both eyes. 6 Intracerebroventricular administration of RX 781094 (1.25-15 micrograms total dose) caused a rapid dose-related reversal of the maximal mydriasis induced by guanoxabenz (0.3 mg/kg, i.v.). 7 Guanoxabenz (0.3 and 1.0 mg/kg, i.v.) did not produce any dilation of the physostigmine-constricted undamaged pupil of the pithed rat. Intravenous adrenaline was found to produce a small mydriatic effect, while atropine completely antagonized the effects of physostigmine in this preparation. 8 These results indicate that alpha 2-adrenoceptor agonists induce mydriasis in the rat through a central alpha 2-adrenoceptor mechanism. However, the site of action within the central nervous system remains to be determined.     

Evidence for imidazoline receptors involvement in the agmatine antidepressant-like effect in the forced swimming test

This study investigated the involvement of the imidazoline receptors in the antidepressant-like effect of agmatine in the forced swimming test. The antidepressant-like effects of agmatine (10 mg/kg, i.p.) in the forced swimming test was blocked by pretreatment of mice with efaroxan (1 mg/kg, i.p., an imidazoline I1/alpha2-adrenoceptor antagonist), idazoxan (0.06 mg/kg, i.p., an imidazoline I2/alpha2-adrenoceptor antagonist) and antazoline (5 mg/kg, i.p., a ligand with high affinity for the I2 receptor). A subeffective dose of agmatine (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with clonidine (0.06 mg/kg, i.p, an imidazoline I1/alpha2-adrenoceptor agonist), moxonidine (0.5 mg/kg, i.p., an imidazoline I1/alpha2-adrenoceptor agonist), antazoline (1 mg/kg, i.p.) and MK-801 (0.001 mg/kg, i.p., a non-competitive NMDA receptor antagonist), but not with efaroxan (1 mg/kg, i.p.) and idazoxan (0.06 mg/kg, i.p.). Pretreatment of mice with yohimbine (1 mg/kg, i.p., an alpha2-adrenoceptor antagonist) blocked the synergistic antidepressant-like effect of agmatine (0.001 mg/kg, i.p.) with clonidine (0.06 mg/kg, i.p). A subeffective dose of MK-801 (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with antazoline (5 mg/kg, i.p.), but not with efaroxan (1 mg/kg, i.p.) or idazoxan (0.06 mg/kg, i.p.). In conclusion, this study suggests that the anti-immobility effect of agmatine in the forced swimming test is dependent on its interaction with imidazoline I1 and I2 receptors.     

THE EFFECT OF SOME α2-ADRENOCEPTOR AGONISTS AND ANTAGONISTS ON GASTROINTESTINAL TRANSIT IN MICE: INFLUENCE OF MORPHINE, CASTOR OIL AND GLUCOSE

1. The effects of graded doses of the α₂-adrenoceptor agonists clonidine, tizanidine and BHT-920, and the α₂-adrenoceptor antagonists yohimbine and idazoxan, on gastrointestinal transit were investigated in mice using the charcoal meal test. 2. The agonists produced significant and dose-dependent decreases in gastrointestinal transit, and the antagonists produced the opposite effect. In affecting the gastrointestinal transit, clonidine (1 mg/kg) was as effective as tizanidine (12 mg/kg) and BHT-920 (40 mg/kg), while yohimbine (2 mg/kg) was as effective as idazoxan (1 mg/kg). 3. Morphine (2, 4 and 8 mg/kg) significantly inhibited gastrointestinal transit. This effect was significantly reversed by the co-administration of yohimbine (2 mg/kg) and idazoxan (1 mg/kg). 4. The acute administration of glucose (5.04 g/kg, i.p.) potentiated the inhibition of gastrointestinal transit produced by clonidine (1 mg/kg) and BHT-920 (40 mg/kg). Glucose treatment, however, had no significant effect on the increase in gastrointestinal transit induced by yohimbine (2 mg/kg) or idazoxan (1 mg/kg). 5. Castor oil (0.25 mL/mouse, orally) induced diarrhoea in saline-treated animals within about 45 min. Clonidine (1 mg/kg), tizanidine (12 mg/kg) and BHT-920 (40 mg/kg) delayed the occurrence of diarrhoea to 2.1, 1.2 and 1.4 h, respectively.     

Measurement of 3-methoxy-4-hydroxyphenylglycol (MHPG) in mouse brain by h.p.l.c. with electrochemical detection, as an index of noradrenaline utilisation and presynaptic alpha 2-adrenoceptor function.

1. A novel method for measurement of 3-methoxy-4-hydroxyphenylglycol (MHPG) in mouse brain by use of high performance liquid chromatography (h.p.l.c.) with electrochemical detection is described. This technique incorporates an ethyl acetate purification procedure and uses 3-hydroxy-4-methoxyphenylglycol (iso-MHPG) as the internal standard. 2. Inhibition of monoamine oxidase by injection of tranylcypromine (5 and 10 mg kg-1) or pargyline (50 and 100 mg kg-1) markedly decreased brain MHPG concentrations. After injection of the tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine (200 mg kg-1), there were time-dependent linear decreases in the concentrations of noradrenaline and MHPG in mouse brain. In addition, a very good correlation (r = 0.95, n = 30; P less than 0.001) was found between the concentrations of noradrenaline and MHPG present in the brains of the same mice after alpha-methyl-p-tyrosine treatment. 3. Mouse brain MHPG concentrations were dose-dependently reduced after administration of the alpha 2-adrenoceptor agonist, clonidine (1-3000 micrograms kg-1), and elevated by the antagonists, idazoxan (1 and 5 mg kg-1), and yohimbine (1 and 5 mg kg-1). Intracerebroventricular injection of the alpha 1-adrenoceptor agonist, phenylephrine (5-50 micrograms) dose-dependently increased MHPG levels. The alpha 1-adrenoceptor antagonist, prazosin, had no effect at the moderate dose of 1 mg kg-1, but increased MHPG concentrations at 5 mg kg-1. The beta-adrenoceptor agonist, clenbuterol (10-1000 micrograms kg-1) and the antagonist, pindolol (1 and 5 mg kg-1), were both without effect. 4. The decrease in brain MHPG concentrations induced by clonidine (100 micrograms kg-1) was prevented by prior injection of 1 mg kg-1 of idazoxan or yohimbine, but not by prazosin or pindolol. 5. MHPG levels were decreased after administration of the noradrenaline reuptake inhibitor desipramine (5 and 10 mg kg-1) and the non-selective monoamine reuptake inhibitors, sibutramine HCl (BTS 54 524; 1 and 3 mg kg-1) and amitryptyline (5 mg kg-1). However, the selective 5-hydroxytryptamine reuptake inhibitor, zimeldine (5 and 10 mg kg-1), was without effect. Dexamphetamine (1 and 5 mg kg-1) and methamphetamine (1 and 5 mg kg-1) both decreased brain MHPG concentrations in a dose-related fashion. 6. Overall the data show that MHPG can be used as a functional index of both presynaptic alpha 2-adrenoceptor activity and noradrenaline turnover and utilisation.     

Evidence for a noradrenergic component in the antinociceptive effect of the analgesic agent tramadol in an animal model of clinical pain, the arthritic rat.

The analgesic agent tramadol has a potent antinociceptive effect in arthritic rats. In the present study, the actions of the selective alpha 2-adrenoceptor antagonists yohimbine and idazoxan on this antinociceptive effect were tested in arthritic rats, using vocalization thresholds to paw pressure as a nociceptive test. The antagonists were administered 30 min before tramadol, at doses (0.5 and 1 mg/kg i.v.) without action per se, but which prevented the antinociceptive action of the prototypic alpha 2-adrenoceptor agonist clonidine (0.1 mg/kg i.v.) in these animals. The potent antinociceptive effect of tramadol (1 mg/kg i.v.) was significantly decreased (mean total effect reduced about 2-fold) by yohimbine and idazoxan. In alpha 2-adrenoceptor antagonists-pretreated arthritic rats, the effect of tramadol was almost abolished when tramadol was coinjected with the opioid antagonist naloxone. In addition to the involvement of opioid receptors, these results provide evidence for a noradrenergic component to the antinociceptive action of tramadol in this model of clinical pain.     

Effects of tetramethylpyrazine on heart rate and pupil diameter in rats

The alpha-adrenergic nature of tetramethylpyrazine (TMPZ), a commonly used cardiovascular drug in China, was studied with a combined bradycardia and mydriasis model in the pentobarbital anesthesized rat. Intravenous injections of TMPZ at 1-30 mg/kg failed to change heart rate or pupil size, whereas iv injections of both clonidine (alpha 2-adrenoceptor agonist, 1-30 micrograms/kg) and methoxamine (alpha 1-adrenoceptor agonist, 10-300 micrograms/kg) caused dose-dependent bradycardia and mydriasis. Pretreatment of rats with TMPZ (30 mg/kg) did not significantly change clonidine- or methoxamine-induced bradycardia or mydriasis. However, when TMPZ was injected 2 min after clonidine administration (30 micrograms/kg), while the mydriasis reached a maximum, it slightly but significantly reversed the clonidine-induced mydriasis for 15-20 s. In contrast, the alpha 2-adrenoceptor antagonist idazoxan (6 micrograms/kg) caused the same degree of reduction of the clonidine-induced mydriasis for greater than 2 min. Our results suggested that TMPZ at the doses studied did not have alpha-adrenoceptor agonistic activities, but may have slight alpha 2-adrenoceptor antagonistic activities.     

Permissive role of spinal alpha 1-adrenoceptors in sudomotor efferents

The electrodermal potential (EDP) recorded in the forepaws of anaesthetized cats in response to stimulation of the cholinergic-sympathetic nervous system at different levels was taken as a measure for sudomotor activity. Electrical stimulation of the hypothalamus with square wave pulses (1 ms duration, 0.5-64 Hz, 2 s train length) at intervals of 1 min induced rate-dependent EDPs which were inhibited at all rates of stimulation by intravenous (i.v.) injection of doses less than 40 micrograms/kg of the alpha 1-adrenoceptor blocking drug prazosin. However, prazosin (50-500 micrograms/kg i.v.) did not impair EDPs induced by preganglionic stimulation (0.5-64 Hz) or by injection of the nicotinic ganglion stimulant DMPP (50 micrograms/kg i.v.). Prazosin (50 micrograms/kg i.v.) inhibited EDPs induced by unilateral electrical stimulation (square wave pulses, 1 ms duration, 16 Hz, 2 s train length, 1 min intervals) of the spinal cord at C 1 in cats with an axotomy at the level of the medulla oblongata, thus indicating a spinal site of prazosin action and suggesting a permissive role of spinal catecholamines by activation of alpha 1-adrenoceptors. In spinal preparations pretreated with 250 micrograms/kg yohimbine i.v. to block inhibition by alpha 2-adrenoceptors of catecholamine reuptake, cocaine 2.5 mg/kg i.v. potentiated EDPs induced by spinal stimulation with 8 Hz. This effect could be antagonized by 50 micrograms/kg prazosin or 1000 micrograms/kg corynanthine i.v. In spinal preparations pretreatment with 5 mg/kg reserpine i.p. for depletion of catecholamines and 250 micrograms/kg yohimbine i.v., EDPs (16 Hz) were smaller than in undepleted preparations. Under these conditions injection of 100 micrograms/kg clonidine i.v. caused amplification of EDPs. This effect was antagonized by 50 micrograms/kg prazosin i.v. After i.v. pretreatment with 250 micrograms/kg yohimbine the i.v. injection of 2.5 mg/kg cocaine also potentiated EDPs which were induced by hypothalamic stimulation in intact cats. The results indicate that catecholaminergic neurons influence sudomotor activity by interaction with efferents of the cholinergic-sympathetic nervous system at the level of the spinal cord. Catecholamines seem to facilitate impulse transmission in non-catecholaminergic synapses by activation of alpha 1-adrenoceptors.     

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.     

Effect of amitraz and chlordimeform on heart rate and pupil diameter in rats: mediated by alpha 2-adrenoreceptors

Pupillary and cardiac responses to the insecticide/acaricide amitraz (0.03 to 1.0 mg/kg, iv) and chlordimeform (0.03 to 10.0 mg/kg, iv), as well as the alpha 2-adrenergic agonists clonidine (1 to 30 micrograms/kg, iv) and xylazine (10 to 300 micrograms/kg, iv), were investigated in rats anesthetized with an ether and pentobarbital combination. Amitraz, clonidine, and xylazine caused a dose-dependent mydriasis and bradycardia. The order of potency of the mydriatic and bradycardic effects was: clonidine greater than xylazine greater than amitraz. Chlordimeform did not cause mydriasis or bradycardia at the dosages studied. Amitraz-induced mydriasis and bradycardia were blocked by antagonists with alpha 2-adrenoreceptor blocking activity: yohimbine and phentolamine (2.5 mg/kg each, iv). In contrast, these effects of amitraz were not affected by prazosin (2.5 mg/kg, iv), an alpha 1-adrenoreceptor antagonist. In rats pretreated with reserpine (7.5 mg/kg, sc, 20 hr) and alpha-methyl-p-tyrosine (250 mg/kg, ip, 5 hr) to deplete catecholamine, amitraz (0.03-1.0 mg/kg, iv) produced mydriasis of similar magnitude as in the control animals. However, amitraz did not lower the heart rate in the pretreated animals as it did in the control animals. The results demonstrated that amitraz, a formamidine, induced mydriasis and bradycardia which were not observed with administration of another formamidine, chlordimeform. The data also suggest that amitraz-induced mydriasis is mediated by postsynaptic alpha 2-adrenoreceptors while amitraz-induced bradycardia is mediated by presynaptic alpha 2-adrenoreceptors.     

alpha-Methyldopa produces mydriasis in the rat by stimulation of CNS alpha 2-adrenoceptors.

1. The effects of i.v. administration of alpha-methyldopa (MD) on rat pupil diameter were investigated. All experiments were carried out in rats in which vagosympathetic nerve trunks were sectioned bilaterally at the cervical level. 2. In anaesthetized rats MD produced a marked dose-related increase in pupil diameter. The onset of pupillary response to MD was gradual and reached maximal levels 2-3 h after administration. 3. Pretreatment with alpha 2-adrenoceptor antagonists yohimbine (1.5 mg kg-1, i.v.) or idazoxan (0.5 mg kg-1, i.v.) blocked the pupillary response to MD. In contrast, the alpha 1-antagonists prazosin (1.0 mg kg-1, i.v.) and phenoxybenzamine (1.5 mg kg-1, i.v.) did not significantly alter the pupillary effects of MD. 4. Selective enzymatic blockade with 3-hydroxy-benzyl-hydrazine (NSD-1015; 25 mg kg-1, i.p.), a dopa-decarboxylase inhibitor, as well as bis (4-methyl-homopiperazinyl-thiocarbonyl) disulphide (FLA-63, 5.0 mg kg-1, i.p.), a dopamine-beta-hydroxylase inhibitor, prevented the mydriatic effect of MD. 5. The above findings support the hypothesis that MD produces a clonidine-like CNS mydriasis in the rat. This effect appears to be mediated primarily by the MD metabolite, alpha-methylnoradrenaline. 6. These results indicate that MD produces mydriasis in the rat by a CNS action. The mydriatic action of MD appears to be produced by its metabolite alpha-methylnoradrenaline which in turn stimulates CNS postsynaptic alpha 2-adrenoceptors.     

Influence of alpha-adrenoceptor agonists and antagonists on imipramine-induced hypothermia in mice

Effects of adrenoceptor agonists and antagonists on imipramine-induced hypothermia in mice were studied. Intraperitoneal injection of imipramine (10-40 mg x kg(-1)), alpha2-adrenoceptor agonist clonidine (0.05-0.1 mg x kg(-1)), alpha1-adrenoceptor agonist phenylephrine (6 mg x kg(-1)) and alpha1-adrenoceptor antagonist prazosin (1-4 mg x kg(-1)) but not alpha2-adrenoceptor antagonist yohimbine (1-4 mg x kg(-1)) induced significant hypothermia. The hypothermic response induced by imipramine (10-30 mg x kg(-1)) was not altered by clonidine (0.05-0.1 mg x kg(-1)) or phenylephrine (2-6 mg x kg(-1)). The response of imipramine (10-30 mg x kg(-1)) was reduced significantly by yohimbine (2 mg x kg(-1)) and was potentiated by prazosin (1 mg x kg(-1)). The hypothermic effect of clonidine (0.1 mg x kg(-1)) and imipramine (20 mg x kg(-1)) were also decreased significantly by different doses of yohimbine (1-4 mg x kg(-1)). The hypothermia induced by different doses of prazosin (1-4 mg x kg(-1)) was not altered by yohimbine (2 mg x kg(-1)) or by low dose of imipramine (10 mg x kg(-1)). It is concluded that alpha2-adrenoceptor mechanism may be involved in the hypothermic effect of imipramine.     

Precipitation by yohimbine of the withdrawal syndromes of clonidine, guanfacine, and methyldopa in the spontaneously hypertensive rat

In conscious spontaneously hypertensive (SH) rats, continuously infused with clonidine (500 micrograms/kg/day s.c.) for 12 days, yohimbine (3 or 10 mg/kg i.p.) dose dependently precipitated an overshoot of heart rate and other withdrawal symptoms, such as blood pressure upswings, diarrhea, ptosis, body shivering, jumping, and wet-dog shakes. Naloxone (3 or 30 mg/kg i.p.) or prazosin (1 mg/kg i.p.) did not evoke withdrawal signs in clonidine-treated SH rats. In SH rats treated for 12 days with guanfacine (10 mg/kg/day s.c.), the administration of yohimbine elicited a withdrawal pattern similar in severity and appearance to that observed in clonidine-treated rats. In methyldopa-treated (200 mg/kg/day) SH rats, the yohimbine-precipitated withdrawal symptoms were much less pronounced than those observed in the clonidine- and guanfacine-treated animals, despite the equipotency of the infusions in reducing mean arterial pressure. These results indicate that the clonidine-withdrawal syndrome can be precipitated by acute alpha 2-adrenoceptor blockade. The previously observed minor severity of the guanfacine-discontinuation syndrome in the rat should be attributed to the longer half-life of this drug as compared to clonidine. On the other hand, alpha-methylnoradrenaline much less intensively triggers the mechanisms underlying withdrawal symptoms than clonidine or guanfacine.     

Clonidine mydriasis in the rat.

Pupillary responses to clonidine (3--100 micrograms/kg, i.v.) and epinephrine (1--30 micrograms/kg, i.v.) were observed in anesthetized rats. Clonidine caused a dose-dependent mydriasis which was effectively antagonized by pretreatment with yohimbine (1.5 mg/kg, i.v.). Pretreatment with phentolamine (5 mg/kg, i.v.) was less effective in antagonizing this clonidine-induced mydriasis. Phenoxybenzamine (2 mg/kg, i.v.) was almost without effect. In contrast, both phentolamine and phenoxybenzamine blocked the pupillary dilation produced by epinephrine while yohimbine pretreatment resulted in no antagonism of epinephrine-induced mydriasis. These results suggest that clonidine-induced mydriasis in the rat is mediated by a central adrenergic inhibitory mechanism.     

Antagonism of apomorphine-enhanced startle by alpha 1-adrenergic antagonists

The present study investigated the possible involvement of central noradrenergic neurons in mediating the excitatory effect of the dopamine agonist apomorphine on the acoustic startle response in rats. Experiment 1 assessed the effects of intraperitoneal (i.p.) administration of adrenergic antagonists on apomorphine-enhanced startle. The excitation of startle produced by apomorphine (1.0-3.0 mg/kg i.p.) was blocked by the alpha 1-adrenergic antagonists prazosin (0.03-1.0 mg/kg) and WB-4101 (1.0 mg/kg). Prazosin was very potent in this regard, having an ED50 of 0.03 mg/kg. Blockade of beta-adrenergic receptors with propranolol (20 mg/kg) or blockade of peripheral alpha-adrenergic receptors with phentolamine (10 mg/kg) failed to alter the effect of apomorphine. Prazosin did not block the enhancement of startle produced by other drugs (5-methoxy-N,N-dimethyltryptamine, strychnine), nor did it alter the entry of apomorphine into the brain. The alpha 1-adrenergic antagonists piperoxane (0.03 mg/kg), yohimbine (0.03 mg/kg) or RX781094 (0.07 mg/kg) markedly potentiated apomorphine excitation. These data indicated that specific blockade of central alpha 1-adrenergic receptors prevents apomorphine-enhanced startle. In contrast to the effects of alpha 1-adrenergic antagonists, Experiment 2 found that other drugs that produce an acute (clonidine, 0.040 mg/kg) or chronic (intraventricular 6-hydroxydopamine, 2 X 200 micrograms; DSP4, 50 mg/kg i.p.) disruption of noradrenergic transmission failed to affect apomorphine excitation. Thus, the ability of alpha 1-adrenergic antagonists to block apomorphine's excitation of startle cannot be explained by a simple dopamine-norepinephrine interaction. Alternative hypothesis are discussed.