alpha 2-Adrenoceptor agonists induced mydriasis in the rat by an action within the central nervous system

Alpha2-adrenoceptors do not mediate reflex mydriasis in rabbits

The aim of this study was to identify receptors that mediate reflex mydriasis in pentobarbital-anesthetized rabbits, in which the cervical sympathetic nerve was sectioned unilaterally. Voltage-response curves of pupillary dilation were generated bilaterally by stimulation of the sciatic nerve. Evoked mydriatic responses were mediated mainly by efferent parasympathetic innervation, and, to a lesser extent, by sympathetic innervation. The alpha-adrenergic antagonist, phenoxybenzamine (0.3 mg/kg, intravenously (i.v.)), antagonized mydriasis of the neurally intact eye, but not that on the sympathectomized side. The alpha2-adrenergic antagonist, RS 79948 (0.3 mg/kg, i.v.), potentiated mydriasis of the normal eye, but was without either a potentiating or inhibitory effect on the mydriasis of the sympathectomized eye. In addition, the dopamine-receptor antagonist, haloperidol (1 mg/kg, i.v.), inhibited evoked mydriasis of the sympathectomized eye. These results suggest that, unlike some other species (cats and rats), alpha2-adrenoceptors do not mediate reflex mydriasis elicited by sciatic-nerve stimulation in the rabbit, and support the previous finding in humans that dopamine receptors may mediate this response.

Rat clonidine mydriasis model: imidazoline receptors are not involved

The clonidine mydriasis model in rats has been widely applied in preclinical research to characterize alpha(2)-adrenoceptor antagonistic properties of drugs. The present study was undertaken to pharmacologically determine if imidazoline I(1) receptors are also involved in this model system. Sigmoid dose-response curves for pupillary dilation were produced in pentobarbital anesthetized rats by intravenous administration of increasing doses of agonists (guanabenz for alpha(2)-adrenoceptors, clonidine for both alpha(2)-adrenoceptors and imidazoline I(1) receptors, and rilmenidine for imidazoline I(1) receptors). Two antagonists (RS 79948 for alpha(2)-adrenoceptors and efaroxan for imidazoline I(1) receptors) were used to antagonize the mydriasis elicited by those three agonists, with antagonistic potencies calculated. In additional experiments, we examined the effect of the selective imidazoline I(1) receptor antagonist, AGN 192403, on clonidine-induced mydriasis. The results showed that pupillary response curves elicited by guanabenz, clonidine and rilmenidine were competitively antagonized by both RS 79948 (0.03-1 mg/kg) and efaroxan (0.03-1 mg/kg) in a dose-related fashion. The potencies of either antagonist against the three agonists were not significantly different. AGN 192403 (5 mg/kg) did not significantly shift the clonidine mydriasis curve. These results suggest that imidazoline I(1) receptors are not functionally involved in the rat clonidine mydriasis model and support this in vivo system as a useful model for studies of alpha(2)-adrenoceptors.

1-(2-pyrimidinyl)-piperazine, a buspirone metabolite, modulates bladder function in the anesthetized rat

AIMS

To examine the effects of 1-(2-pyrimidinyl)-piperazine (1-PP), a buspirone metabolite, on bladder function in vivo.

METHODS

Micturition reflexes in the rat were evaluated in two models of bladder function; a constant infusion model employing 0.5% acetic acid and an isovolumic model.

RESULTS

In the constant infusion model, 1-PP (0.14-1.32 mg/kg) dose-dependently and significantly decreased the number of bladder contractions measured during a 30 min recording period, with little effect on the pressure developed during each contraction. 1-PP is an alpha2-adrenergic receptor antagonist. The alpha2 antagonists BRL44408 (alpha2A vs. alpha2B selective; 0.3 and 1 mg/kg), imiloxan (alpha(2B) vs. alpha2A selective; 1 mg/kg), and yohimbine (non-subtype selective; 1 mg/kg; but not 0.3 mg/kg) also significantly reduced the number of contractions. Vehicle was without effect. In the isovolumic model, 1-PP (0.03-1.0 mg/kg) produced a dose-dependent and significant reduction in the number of bladder contractions recorded during a 15 min assessment period, with the maximum effect observed at 0.3 mg/kg. 1-PP had little effect on blood pressure; the only effect was observed at the highest dose (1 mg/kg) where it produced a transient 17% decrease in pressure. Cromakalim and tolterodine served as comparitors in all studies.

CONCLUSIONS

1-PP decreased the number of bladder contractions evoked by the micturition reflex at doses that had little effect on either the pressure developed during each bladd er contraction or on blood pressure. The effects of 1-PP are likely mediated primarily by alpha2 receptor antagonism.

Sympathetic mechanisms in cerebral blood flow alterations induced by spinal cord stimulation

OBJECT

Cervical spinal cord stimulation (SCS) has been found to augment cerebral blood flow (CBF) in a number of animal models, although the mechanisms underlying the cerebrovascular effects of SCS are poorly described. In this study, the authors examined the role of sympathetic tone in CBF alterations induced by SCS in rats.

METHODS

Spinal cord stimulation was performed at three intervals while CBF was monitored with laser Doppler flowmetry (LDF). Either hexamethonium (5, 10, or 20 mg/kg), prazosin (0.25, 0.5, or 1 mg/kg), idazoxan (0.5, 1, or 2 mg/kg), propranolol (1, 2, or 4 mg/kg), or vehicle was administered intravenously before the second stimulation. Changes in LDF values due to SCS were recorded as the percentage of change from baseline values and were analyzed. In vehicle-treated animals, SCS increased LDF values by 60.5 +/- 1.8% over baseline, whereas both high-dose hexamethonium and prazosin completely abolished the SCS-induced increases in LDF values. On the other hand, LDF values increased by 50.9 +/- 4% and 61.4 +/- 4% after SCS in the presence of idazoxan or propranolol, respectively. Administration of sympathetic nervous system blockers resulted in a variable degree of systemic hypotension as well. Nevertheless, induced hypotension without sympathetic blockade had only a minimal effect on SCS-induced augmentation of LDF values (48 +/- 1.4% over baseline).

CONCLUSIONS

Sympathetic tone plays a major role in SCS-induced increases in CBF. This effect seems to be mediated primarily by alpha1-adrenergic receptors. Systemic hypotension alone cannot explain the effects of sympathetic blockade on the SCS response. Clinical use of SCS in the treatment of cerebral ischemia should take alpha1-adrenergic receptor sympathetic tone into account.

Rilmenidine produces mydriasis in cats by stimulation of CNS alpha2-adrenoceptors

1. Experiments were undertaken to determine if the imidazoline/alpha2-adrenoceptor agonist, rilmenidine, would produce mydriasis in cats and, if so, to delineate its site of action and determine if this effect is mediated by imidazoline receptors or alpha2-adrenoceptors. 2. Rilmenidine produced dose-related pupillary dilator responses in pentobarbital anaesthetized cats that were independent of sympathetic innervation to the iris but were dependent upon intact parasympathetic neuronal tone. The ED50 for rilmenidine-induced pupillary dilation was approximately 200 microg kg(-1), i.v., and was sustained for at least 1 h. 3. The highly selective alpha2-adrenoceptor antagonist, RS-79948, administered either before or after rilmenidine, antagonized rilmenidine-induced mydriasis. Neuronally induced reflex inhibition of parasympathetic nerve activity was also inhibited by administration of RS-79948. 4. These results suggest that rilmenidine acts like clonidine to produce pupillary dilation by inhibition of parasympathetic tone to the iris sphincter and that this central nervous system parasympatho-inhibition is mediated by alpha2-adrenoceptors, rather than imidazoline receptors.

Effect of medetomidine on electroencephalography and use of a quantitative electroencephalograph for evaluating sedation levels in dogs

This study was designed to characterize the effect of medetomidine (Med) on canine electroencephalography (EEG), to evaluate the use of quantitative EEG for assessing sedation levels and to explore the correlation between the serum concentration of Med and the quantitative EEG. Four groups of dogs were given Med at doses of 20, 40, 80 and 160 microg/kg (Med-20, Med-40, Med-80 and Med-160 groups). Following Med administration, there was synchrony between each unipolar EEG lead. On EEG power spectrum analysis of the bipolar leads, all groups showed a significant depression of the 14-30 Hz components. The power of the 1-3 Hz component in the Med-80 and Med-160 groups was significantly increased, although there were few changes in the other groups. Similar results were obtained from raw data analysis. As a result of quantitative EEG analysis, spectrum edge frequency 90 analysis (SEP90) showed that the frequency was significantly reduced in all groups after Med administration. A dose-response effect was observed in all groups except for the Med-160 group. Both of these EEG analyses were significantly correlated with the serum concentration of Med. However, the result of the SPF90 analysis sugested a stronger correlation than that for median edge frequency analysis. In conclusion, care must be taken in veterinary clinical diagnoses when Med is used during EEG recording, as Med may cause increased activity in the low frequency band and a decrease in high frequency band activity. In addition, quantitative EEG analysis may be useful in assessing the depth of sedation and in further studies on Med administration.

The effects of topical tropicamide and systemic medetomidine, followed by atipamezole reversal, on pupil size and intraocular pressure in normal dogs

Twenty normal Golden Retrievers being screeened for eye, hip and elbow diseases were given tropicamide topically and medetomidine systemically. Medetomidine effects were later reversed with systemic atipamezole. Pupil size and intraocular pressure changes were determined. Pupil size increased significantly following tropicamide administration and continued to increase slightly but significantly after medetomidine injection. It was unclear whether the slight increase in pupil size following medetomidine administration was due to continued effect of tropicamide or due to the medetomidine itself. Atipamezole did not influence pupil size. Intraocular pressure (IOP) was not affected by these drugs. Ophthalmic screening examination for inherited disease following tropicamide administration is equally feasible prior to sedation with medetomidine and after reversal with atipamezole, but not during the period of sedation.

Anti-obesity effect of MPV-1743 A III, a novel imidazoline derivative, in genetic obesity

MPV-1743 A III ((+/-)-4-(5-fluoro-2,3-dihydro-1H-inden-2-yl)-1H-imidazole) is a novel imidazoline derivative. In this study, it was shown to bind with high affinity to alpha2-adrenoceptor subtypes alpha2A (IC50) = 0.66 +/- 0.06 nM), alpha2B (IC50) = 3.8 +/- 0.53 nM), alpha2C (IC50) = 3.1 +/- 0.61 nM) in the recombinant S115 cells and to alpha2D (IC50 = 0.94 +/- 0.10 nM) in the rat submandibular gland. MPV-1743 A III also showed remarkably high affinity to alpha1-adrenoceptors (IC50 = 150 +/- 12 nM) in the rat cerebral cortex and to imidazoline I2b-binding sites (IC50) = 150 +/- 5.0 nM) in the rat liver. The functional alpha2-adrenoceptor antagonistic effect of MPV-1743 A III was demonstrated by studying the ability of orally administered MPV-1743 A III to reverse and prevent the alpha2-adrenoceptor agonist detomidine-induced mydriasis in rat. The anti-obesity effect of MPV-1743 A III was investigated in genetically obese (fa/fa) Zucker rats in two different phases of obesity. Chronic treatment with MPV-1743 A III (0.3 3 mg/kg per day p.o. for 3 weeks) dose dependently decreased weight gain in early-phase obesity. In fully established obesity, GDP binding to mitochondria and expression of uncoupling protein mRNA were increased in brown adipose tissue by MPV-1743 A III indicating an activation of non-shivering thermogenesis. The present study shows that MPV- 1743 A III has a modest anti-obesity effect in the genetic rodent model of obesity. The relative importance of alpha2- and alpha1-adrenoceptors and imidazoline I2b-binding sites in mediating the effects of MPV-1743 A III needs further evaluation.

A re-evaluation of the role of alpha 2-adrenoceptors in the anxiogenic effects of yohimbine, using the selective antagonist delequamine in the rat

1. The acute behavioural effects of the alpha2-adrenoceptor antagonists, yohimbine, idazoxan and delequamine (RS-15385-197) were compared in two tests of exploratory behaviour in the rat, operated in tandem. These were the elevated X-maze test (5 min) and a modified holeboard test (12 min), which comprised a holeboard arena with a small roof in one corner as a 'refuge'. Rats were first placed into this corner, thus enabling measurements of initial emergence latency and the number of forays. The experiments were always done with a concomitant vehicle control group, with 10-12 rats per group, and with the treatment blinded. 2. In order to validate the tests, the effects of representatives of four classes of psychoactive agents were examined, viz. picrotoxin (anxiogenic), chlordiazepoxide (anxiolytic), (+)-amphetamine (stimulant) and diphenhydramine (sedative). The modified holeboard tended to be more sensitive than the measurement of total arm entries in the elevated X-maze at detecting drug effects on exploratory behaviour, but unlike the X-maze it could not clearly identify each class of agent. Thus, picrotoxin (5 mg kg(-1), i.p.) reduced total arm entries and open arm exploration in the X-maze (P<0.02) and suppressed most measures of activity in the holeboard (P<0.05); chlordiazepoxide (7.5 mg kg(-1), i.p.) increased total arm entries and open arm exploration (P<0.02) in the X-maze, without clear-cut effects in the holeboard; (+)-amphetamine (1 mg kg(-1), i.p.) had no significant effects in the X-maze, but increased most holeboard activities (P<0.05), and diphenhydramine (30 mg kg(-1), i.p.) reduced total arm entries in the X-maze (P<0.002) and hole exploration in the holeboard (P<0.05).

Mydriasis elicited by imidazol(in)e alpha 2-adrenomimetics in comparison with other adrenoceptor-mediated effects and hydrophobicity

alpha 2-Adrenoceptor agonists cause both mydriasis and platelet aggregation. This work is aimed at identifying the factors accompanying and affecting mydriatic activity. For eight imidazol(in)e drugs mydriatic, hypotensive and bradycardic activities were determined in rats. The lipophilicity of the agents was determined chromatographically and calculated theoretically. A correlation was found between the hypotensive and the bradycardic potency and between the mydriatic activity and both the hypotensive and bradycardic activity. Mydriatic activity depended on the lipophilicity of the agents studied. The human platelet antiaggregatory activity of the drugs did not correlate with either the mydriatic or cardiovascular activity and it was independent of lipophilicity. The dependence of the centrally induced effects on lipophilicity and the lack of such a dependence in the case of the in vitro alpha 2-adrenoceptor-mediated platelet aggregation may be interpreted as resulting from heterogeneity of the rat cerebral and the human platelet alpha 2-adrenoceptors. The alpha 2-adrenergic activity of drugs in the model of mydriasis in rats cannot be predicted from their activity in causing human platelet aggregation in vitro.

A pharmacokinetic-pharmacodynamic linking model for the alpha 2-adrenergic antagonism of idazoxan on clonidine-induced mydriasis in the rat

The relationship between concentration and inhibitory effect of the alpha 2-adrenoceptor antagonist idazoxan on clonidine-induced mydriasis has been studied in the rat using pharmacokinetic-pharmacodynamic simultaneous modelling. Fifteen minutes after the anaesthesia of rats with sodium pentobarbitone (55 mg kg-1, i.p.), and 5 min after the administration of clonidine (0.3 mg kg-1, i.v.) to rats pretreated with idazoxan (3 mg kg-1, i.v., and 3 and 10 mg kg-1, orally) at different time intervals, pupil diameters were assessed. The pharmacokinetics of idazoxan were adequately described by a monoexponential equation. Using a pharmacokinetic-pharmaco-dynamic linking model, the concentration-effect relationships of idazoxan were derived, and were quantified by the inhibitory simple Emax model. At the effect compartment, the estimated apparent IC50 was 153.6 ng mL-1. Values of clearance, volume of distribution and elimination half-life were 71.2 mL kg-1 min-1, 3134 mL kg-1 and 30.5 min, respectively. These results could contribute to better characterization of the pharmacodynamic and toxicological profiles of idazoxan in experimental models in which a different pharmacokinetic behaviour of the drug is presumed.

Evidence for altered alpha 2-adrenoceptor function following isolation-rearing in the rat

This study investigated central alpha 2-adrenoceptor function in differentially reared rats. Rats reared from weaning were either housed singly or in groups of five. Measurements of spontaneous ambulatory activity at 4 weeks postweaning showed that isolates were more hyperactive on exposure to a novel environment than grouped rats. alpha 2-Adrenoceptors were investigated using alpha 2-adrenoceptor agonist-induced behaviours, [3H]-idazoxan binding and measurement of forskolin-stimulated cyclic AMP accumulation. Clonidine (0.001-1.0 mg/kg IP) induced mydriasis in both groups with no difference observed in the response between the isolation and group-reared animals. Clonidine (0.01-0.5 mg/kg IP) induced hypoactivity in both groups, with the effect significantly greater in the isolation-reared rats. Idazoxan markedly attenuated both responses, confirming their induction by alpha 2-adrenoceptor stimulation. Clonidine-induced hypoactivity and mydriasis are mediated by pre- and postsynaptic alpha 2-adrenoceptors, respectively; therefore the results suggest rats reared in isolation have enhanced presynaptic but unchanged postsynaptic alpha 2-adrenoceptor function. Saturation binding experiments using [3H]-idazoxan were undertaken to determine alpha 2-adrenoceptor number (Bmax) and affinity (Kd) in membranes prepared from the frontal cortex and hippocampus. Analysis of binding data revealed an increase in receptor number in the hippocampus of isolates. Cyclic AMP accumulation was measured in hippocampal slices from differentially reared rats. Isolation-rearing did not affect cyclic AMP accumulation in response to stimulation by forskolin (30 microM). However, the selective alpha 2-adrenoceptor agonist, UK14304, produced a significantly greater inhibition of cyclic AMP accumulation in slices from isolated rats, confirming changes in alpha 2-adrenoceptor function following isolation rearing.

Species-selective binding of [3H]-idazoxan to alpha 2-adrenoceptors and non-adrenoceptor, imidazoline binding sites in the central nervous system

1. We have used the imidazoline derivative [3H]-idazoxan to define alpha 2-adrenoceptors and non-adrenoceptor, imidazoline binding sites in cerebral cortex membranes of calf, mouse, rat, guinea-pig and man. 2. Competition experiments using the selective alpha-adrenoceptor drugs, rauwolscine and corynanthine, indicated that [3H]-idazoxan bound to a single population of sites in the calf and mouse membranes. However, [3H]-idazoxan also labelled non-adrenoceptor, imidazoline binding sites in the rat (15%), guinea-pig (30%) and human (40%) cerebral cortex membranes. 3. Competition experiments with adrenaline and cirazoline in the guinea-pig cortex, verified [3H]-idazoxan binding to both alpha 2-adrenoceptors and to non-adrenoceptor, imidazoline binding sites. 4. It has been postulated by several groups that [3H]-idazoxan may possess partial agonist activity. To investigate this further, saturation experiments were performed in the cerebral cortex membranes of all five species in the absence and presence of 300 microM guanosine triphosphate (GTP). GTP had no effect on [3H]-idazoxan binding in guinea-pig cerebral cortex; in both rat and mouse membranes 300 microM GTP increased the dissociation constant for [3H]-idazoxan by 2-3 fold without significantly affecting the Bmax. GTP reduced the Bmax by approximately 30% and 60% in calf and human cerebral cortex membranes, respectively, without significantly altering the Kd. 5. Saturation experiments were performed in the calf cerebral cortex membranes in the absence and presence of 300 microM GTP with the selective alpha 2-adrenoceptor agonist [3H]-clonidine and the selective muscarinic antagonist [3H]-quinuclidinyl benzilate (QNB). GTP reduced the Bmax for [3H]-clonidine without altering the Kd, but failed to affect either the Bmax or the Kd for [3H]-QNB.6. Saturation experiments were performed in human cerebral cortex membranes in the presence of alpha2-adrenoceptor blockade with and without GTP. GTP 300 microM reduced the Bmax for [3H]-idazoxan at the non-adrenoceptor, imidazoline binding sites, without affecting the Kd. GTP did not affect [3H]-QNB binding to muscarinic sites.7. Thus, there is a need to investigate further the pharmacological actions of [3H]-idazoxan in view of its ability to recognise both alpha2-adrenoceptors and non-adrenoceptor, imidazoline binding sites and because it might possess agonist activity at some of these sites.

The pharmacology of RS-15385-197, a potent and selective alpha 2-adrenoceptor antagonist

1. RS-15385-197 ((8aR, 12aS, 13aS)-5,8,8a,9,10,11,12,12a,13,13a-decahydro- 3-methoxy-12-(methylsulphonyl)-6H-isoquino [2,1-g][1,6]-naphthyridine) was evaluated in a series of in vitro and in vivo tests as an antagonist at alpha 2-adrenoceptors. 2. RS-15385-197 had a pKi of 9.45 for alpha 2-adrenoceptors in the rat cortex (pA2 in the guinea-pig ileum of 9.72), whereas the 8aS, 12aR, 13aR enantiomer, RS-15385-198, had a pKi of only 6.32 (pA2 6.47) indicating a high degree of stereoselectivity. The racemate RS-15385-196 had a pKi of 9.18. 3. RS-15385-197 showed unprecedented alpha 2 vs. alpha 1 adrenoceptor selectivity in vitro. In the rat cortex, RS-15385-197 had a pKi of 9.45 in displacing [3H]-yohimbine and 5.29 in displacing [3H]-prazosin (alpha 2/alpha 1 selectivity ratio in binding experiments > 14000). The compound had a pA2 of 9.72 as a competitive antagonist of the inhibitory effects of UK-14,304 in transmurally-stimulated guinea-pig ileum and 10.0 against BHT-920-induced contractions in dog saphenous vein (DSV); this latter value was unaltered by phenoxybenzamine. An apparent pKB of 5.9 was obtained against cirazoline-induced contractions in DSV, whilst a pA2 of 6.05 was obtained against phenylephrine-induced contractions in the rabbit aorta (alpha 2/alpha 1 selectivity ratio in functional experiments > 4000). 4. RS-15385-197 was highly selective for alpha 2-adrenoceptors over other receptors: the compound showed low affinity for 5-HT1A (pKi 6.50) and 5-HT1D (pKi 7.00) receptor subtypes, and even lower affinity (pKi < or = 5) for other 5-HT receptor subtypes, dopamine receptors, muscarinic cholinoceptors, beta-adrenoceptors and dihydropyridine binding sites. RS-15385-197 was devoid of affinity for the non-adrenoceptor imidazoline binding site, labelled by [3H]-idazoxan, which provides further evidence that these sites are not related to alpha 2-adrenoceptors. In the DSV, contractile responses to 5-hydroxytryptamine (5-HT) were unaffected by a concentration of 1 microM RS-15385-197. 5. RS-15385-197 was non-selective for the alpha 2A- and alpha 2B-adrenoceptor subtypes in that the pKi for the alpha 2A-adrenoceptor in human platelets was 9.90 and the pKi for the alpha 2B-adrenoceptor in rat neonate lung was 9.70. However, RS-15385-197 showed lower affinity for the alpha 2-adrenoceptor subtype in hamster adipocytes (pKi 8.38). 6. In anaesthetized rats, RS-15385-197 was a potent antagonist of the mydriasis response induced by UK-14,304 or clonidine (AD50 5 and 7 microg kg-1, i.v., respectively; 96 microg kg-1, p.o.) and of UK-14,304-induced pressor responses in pithed rats (AD50 7 microg kg-1, i.v.); the compound therefore is both centrally and orally active. Even at a high dose (10 mg kg-1, i.v.), RS-15385-197 did not antagonize pressor responses to cirazoline in pithed rats, indicating that the selectivity for alpha2 vs. alpha1-adrenoceptors was maintained in vivo.8 RS-15385-197 is therefore a very potent, selective, competitive alpha2-adrenoceptor antagonist, both in vitro and in vivo, is orally active and readily penetrates the brain. It will thus be a powerful pharmacological tool for exploring the various physiological roles of alpha2-adrenoceptors.

Continuous α2-adrenoceptor blockade by atipamezole decreases neocortical high-voltage spindle activity in rats

The present study investigates the effects of a subchronic continuous infusion of atipamezole, a potent and selective alpha 2-adrenoceptor antagonist, on neocortical high-voltage spindle (HVS) activity in rats. Six days' subcutaneous infusion of atipamezole (0.125 mg/kg per h) with osmotic minipumps decreased HVS activity significantly. The HVS activity-decreasing effect of atipamezole persisted at the same level throughout the infusion. A single subthreshold dose of an alpha 2-adrenoceptor agonist, guanfacine (0.001 mg/kg i.p.), did not affect HVS activity either before or after the continuous atipamezole treatment. The central alpha 2-adrenoceptor blocking effect of atipamezole (0.1 mg/kg per h s.c.) was confirmed to be at the same level after one, three or seven days' infusion, as assessed by measuring the antagonism of detomidine-induced mydriasis in the rat. The serum concentration of atipamezole (0.1 mg/kg per h s.c.) increased slightly from day 3 (37 +/- 11 ng/ml) to day 7 (45 +/- 4 ng/ml). In conclusion, the results of the study suggest that the suppressant effects of atipamezole on neocortical high-voltage spindle activity are preserved during subchronic continuous treatment. In addition, alpha 2-adrenoceptor blockade, as measured in the rat mydriasis model, persists at the same level during subchronic infusion.

The effects of the α2-adrenoceptor antagonist idazoxan on sleep in normal volunteers

The effects of idazoxan, a novel antidepressant that is a potent and selective α(2)-adrenoceptor antagonist, were studied on sleep. Twelve normal male volunteers had sleep recordings made using the Oxford ambulatory system (Medilog 9000) with subsequent analysis by automatic sleep staging and visual inspection. Idazoxan was given in a dose of 40 mg three times a day for 21 days. Sleep recording on day 3 demonstrated markedly reduced indices of rapid eye movement (REM) sleep, although other sleep parameters were only minimally affected. The effect on REM persisted unchanged at day 18, however on the second day after withdrawal a rebound in REM measures was observed. These data show that idazoxan has an effect on sleep which is very similar to that of other antidepressants, and emphasize a role for noradrenaline in the regulation of REM sleep.

Effects of antidepressant drugs and electroconvulsive shock on pre- and postsynaptic alpha 2-adrenoceptor function in the brain: rapid down-regulation by sibutramine hydrochloride

Clonidine (0.1 mg/kg IP)-induced hypoactivity and mydriasis responses were respectively used as functional indices of pre- and postsynaptic alpha 2-adrenoceptors in mouse brain. A single injection of various antidepressant drugs had no effect on either response when measured 24 h later. However, 14 days' treatment with sibutramine HCl (3 mg/kg IP), dothiepin (50 mg/kg IP), amitriptyline (10 mg/kg IP), desipramine (10 mg/kg IP) or tranylcypromine (10 mg/kg IP) markedly attenuated both clonidine-induced hypoactivity and mydriasis. Repeated administration of zimeldine (10 mg/kg IP), mianserin (10 mg/kg IP) or clenbuterol (5 mg/kg IP) had no effect on either response. Subchronic treatment with sibutramine HCl (3 mg/kg IP; 3 days) also attenuated pre- and postsynaptic alpha 2-adrenoceptor function. Five ECS (200 V, 2 s) spread over 10 days, but not a single shock, reduced the hypoactivity and mydriasis responses to clonidine. Together, the results indicate that pre- and postsynaptic alpha 2-adrenoceptor function is attenuated by repeated treatment with those antidepressants which acutely increase synaptic levels of noradrenaline. These adrenergic receptor populations are also desensitized by ECS, although this effect is probably mediated via a different mechanism. Finally, the rapid down-regulation observed with sibutramine HCl is not confined to beta-adrenoceptors alone, because pre- and postsynaptic alpha 2-adrenoceptor function is also attenuated by 3 days of treatment with this novel antidepressant drug.

Effects of idazoxan on 5-hydroxytryptamine-mediated behaviour in the mouse and rat

The α(2)-adrenoceptor antagonists idazoxan and RX811059 induced reciprocal forepaw treading, a component of the 5-HT-behavioural syndrome in rats. This response is independent of 'non-α(2)-adrenoceptor idazoxan binding sites' (NAIBS) at which RX811059 is inactive. Idazoxan pre-treatment, in rats, enhanced forepaw treading, head weaving and tremor induced by the 5-HT agonist 5-methoxy-N,N dimethyltryptamine (5-MeODMT), increased head twitches (but decreased hindlimb abduction) induced by the 5-HT releaser p- chloroamphetamine (pCA), but did not clearly alter head twitches induced by the 5-HT precursor L-5-hydroxytryptophan in mice. The α(1)-antagonist prazosin did not alter behaviour induced by either 5-MeODMT or pCA in rats. The α( 2)-agonist, guanoxabenz, did not alter 5-MeODMT-induced behaviour in rats. St587, an α(1)-agonist, selectively potentiated tremor induced by 5-MeODMT, but no other behaviour. A possible mechanism for these interactions could be through enhanced, α(2)-adrenoceptor-mediated, 5-HT release in specific brain areas. Other possibilities, e.g. direct action at subtypes of 5-HT receptors and the importance of these NA-5-HT interactions in the treatment of resistant depression, are discussed.

Specific α2-adrenoreceptor antagonists induce behavioural activation in the rat

The behavioural effects of the specific and selective α(2)-adrenoreceptor antagonists, idazoxan, efaroxan and RX811059, have been investigated in the rat. All three drugs induced periods of behavioural activation characterized by increased locomotion and exploration (rearing and hole dipping). However, these effects were only apparent in animals which were fully habituated to their environments and thus displayed low baseline activity. The behaviour observed lay within the normal range of activity and was not apparent under conditions when exploration was stimulated such as in a novel environment. α( 2)-Adrenoreceptor antagonist- induced activation was a weak response when compared with the intense and prolonged hyperactivity, in both novel and non-novel environments, induced by the amine releaser D- amphetamine. Possible mechanisms involving a direct action of noradrenaline at postsynaptic α( 1)-adrenoreceptors (subsequent to enhanced presynaptic α(2)-receptor feedback blockade) or an indirect action of α(2)-antagonists on dopamine function in mesolimbic pathways are discussed.

Modulation by antidepressant drugs of CNS postsynaptic alpha 2-adrenoceptors mediating mydriasis in the rat

The modulation of central postsynaptic alpha 2-adrenoceptors mediating mydriasis in the pentobarbitone-anaesthetized rat was studied after the acute and short/long-term administration of antidepressant treatments (drugs, electroshock). The acute administration of cyclic antidepressant drugs (2.5 mg/kg, i.v.) resulted in different mydriatic effects (amitriptyline greater than protriptyline approximately imipramine greater than clomipramine greater than nortriptyline greater than desipramine approximately maprotiline) which were attenuated (17-55%) by idazoxan (1 mg/kg, i.v., 5 min) and reserpine (5 mg/kg, s.c., 18 h) indicating that, besides the well-known anticholinergic properties of some of these drugs, their mydriatic effects are due in part to activation of alpha 2-adrenoceptors (through endogenous noradrenaline). In contrast, the long-term (7-21 days) but not the short-term (1-4 days) administration of cyclic antidepressant drugs (2.5-10 mg/kg, i.p.), MAO inhibitors (1 mg/kg, i.p.), lithium (20 mg/kg, i.p.) and electroshock (150 mA, 63 Hz, 8 ms during 300 ms) resulted in dose- and time-dependent reductions of the dose-pupillary response curve for clonidine (ED50 increased 1.2-2.0-fold; Emax decreased by 9-29%) which indicated desensitization of postsynaptic alpha 2-adrenoceptors. In line with these findings, treatment for 7 days with clonidine (0.1-1 mg/kg, i.p.) or idazoxan (3-10 mg/kg, i.p.) led to an opposite modulation (down- and up-regulation) of the dose-pupillary response curve for clonidine. The main results demonstrate that cyclic antidepressant drugs, through indirect mechanisms which involve endogenous noradrenaline, can modulate the sensitivity of brain postsynaptic alpha 2-adrenoceptors mediating mydriasis in the rat.

Clonidine produces mydriasis in conscious mice by activating central alpha 2-adrenoceptors

Intraperitoneal (i.p.) injection of the alpha 2-adrenoceptor agonist clonidine (1-3000 micrograms/kg) produced dose-dependent pupil dilatation in conscious C57/Bl/6 mice with an ED50 of 54 micrograms/kg (95% confidence limits 40-74 micrograms/kg). This response was rapid in onset and of approximately 30 min duration. The alpha 2-adrenoceptor antagonists idazoxan (1 or 3 mg/kg i.p.) and yohimbine (1 or 3 mg/kg i.p.) both produced dose-related miosis, but the alpha 1- and beta-adrenoceptor antagonists prazosin (1 or 3 mg/kg i.p.) and pindolol (1 or 3 mg/kg i.p.) were without effect. These doses of idazoxan and yohimbine potently reversed the mydriasis induced by clonidine (100 micrograms/kg i.p.), while prazosin and pindolol were again ineffective. Clonidine-induced mydriasis was also unaltered by the 5-HT antagonists, methysergide (2.5 mg/kg i.p.) and ketanserin (0.1 mg/kg i.p.) or 0.1 mg/kg i.p. of the dopamine antagonists, haloperidol, SCH 23390 and BRL 34778. A dose of 0.25 microgram clonidine, which was ineffective when administered i.p., produced marked mydriasis after intracerebroventricular (i.c.v.) injection. In addition, the mydriasis produced by i.p. injection of clonidine (100 micrograms/kg) was abolished by i.c.v. dosing of 2.5 micrograms idazoxan or yohimbine, but again not by prazosin or pindolol. Together, these data provide strong evidence to indicate that clonidine-induced mydriasis is exclusively mediated via central alpha 2-adrenoceptors and that this response provides a useful model for studying the function of these receptors.

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.

Alpha 2-adrenoceptor agonists enhance responses to certain other vasoconstrictor agonists in the rat tail artery

1. The effects of the alpha2-adrenoceptor agonists clonidine, rilmenidine, TL99 and UK14304 on the vasoconstrictor response to sympathetic nerve stimulation and on the concentration-response curves to noradrenaline and phenylephrine were compared in two isolated, perfused vascular tissues: the rat tail artery (which has both postjunctional alpha 1- and alpha 2-adrenoceptors), and the rabbit ear artery (in which only alpha 1-adrenoceptors are present postjunctionally). 2. In the rabbit ear artery, the first observable effect of alpha 2-adrenoceptor agonists was inhibition of vasoconstrictor responses to sympathetic nerve stimulation. This occurred with concentrations of the alpha 2-adrenoceptor agonists which were far below those producing vasoconstriction. Responses to noradrenaline were not affected. 3. In contrast, in the rat isolated perfused tail artery, alpha 2-adrenoceptor agonists, in concentrations that produced no other observable effects, enhanced the vasoconstrictor responses to sympathetic nerve stimulation and to noradrenaline. Much higher concentrations of alpha 2-adrenoceptor agonists produced vasoconstriction in most preparations and only then reduced the response to sympathetic nerve stimulation. The enhancing effect of alpha 2-adrenoceptor agonists was blocked by idazoxan, but not by prazosin. 4. Vasoconstrictor responses in the rat tail artery to the relatively selective alpha 1-adrenoceptor agonist phenylephrine were enhanced by alpha 2-adrenoceptor agonists. The enhancement of the response to phenylephrine was greater than that to the mixed alpha 1- and alpha 2-adrenoceptor agonist noradrenaline. 5. Vasoconstrictor responses in the rat tail artery to vasopressin, ATP and KCl, like those to alpha 1-adrenoceptor agonists, were enhanced by alpha 2-adrenoceptor agonists.2+owever, vasoconstrictor responses to

Chronic cocaine reduces alpha 2-adrenoceptor elicited mydriasis and inhibition of locus coeruleus neurons

The effects of chronic cocaine (50 mg/kg per day for two weeks) administration on two alpha 2-adrenoceptor-mediated responses were studied in rats. Chronic administration of cocaine significantly (compared to sham controls) attenuated the alpha 2-adrenoceptor-mediated inhibition of noradrenergic locus coeruleus (LC) neurons as well as alpha 2-adrenoceptor elicited mydriasis. Noradrenergic LC neurons from the cocaine treated and sham sham groups differed significantly in their responsiveness to the inhibitory effects of clonidine (ED50 values micrograms/kg: sham 7.35 +/- 1.13 and cocaine-treated 17.17 +/- 4.40, P less than 0.05). The ED50 values for the mydriatic response were sham 5.71 +/- 0.49 and cocaine-treated 16.42 +/- 0.69 micrograms/kg, respectively, P less than 0.001. No differences in cardiovascular responses to systemically injected clonidine between the chronic cocaine- and sham-treated groups were observed. Chronic cocaine treatment attenuates the two alpha 2-adrenoceptor-mediated responses most likely via an interaction with central catecholaminergic neurotransmission.

Mechanism of dexamphetamine-induced mydriasis in the anaesthetized rat

1. The effect of intravenous administration of dexamphetamine [+)-Amp) on rat pupil diameter was investigated. In all experiments, the vagosympathetic trunks were sectioned bilaterally at the cervical level. 2. In rats anaesthetized with urethane, (+)-Amp (0.1-0.3 mg kg-1, i.v.) produced a dose-related increase in pupil size. The mydriatic effects of (+)-Amp were evident immediately after administration. 3. Pretreatment with the 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 (+)-Amp. Yohimbine caused about a 30 fold shift to the right in the dose-response curve whereas idazoxan almost completely abolished the mydriatic response to (+)-Amp. 4. In contrast, pretreatment with the alpha 1-adrenoceptor antagonist phenoxybenzamine (2 mg kg-1, i.v.), failed to alter significantly the pupillary response to (+)-Amp. 5. Depletion of central nervous system (CNS) monoamines with reserpine (5 mg kg-1, i.p.) and alpha-methyl-p-tyrosine (2 x 300 mg kg-1, i.p.) prevented the pupillary response to (+)-Amp. 6. The mydriatic effect of (+)-Amp was present only in preparations that had intact parasympathetic neural tone to the iris. Central preganglionic denervation of the oculomotor nerve abolished the mydriatic response of (+)-Amp. 7. These results indicate the (+)-Amp acts in the CNS to produce mydriasis in the anaesthetized rat by stimulating CNS postsynaptic alpha 2-adrenoceptors, findings that are consistent with the hypothesis that (+)-Amp acts predominantly as an indirect sympathomimetic agent to release endogenous stores of a monoaminergic neurotransmitter (perhaps noradrenaline).

Medetomidine--a novel alpha 2-adrenoceptor agonist: a review of its pharmacodynamic effects

1. The pharmacodynamic effects of medetomidine, a novel alpha 2-adrenoceptor agonist, are reviewed. 2. In receptor binding experiments, and in isolated organ preparations medetomidine shows high specificity and selectivity to alpha 2-adrenoceptors. Its alpha 2/alpha 1 selectivity ratio is 1620 compared to 220 of clonidine. It is a highly potent full agonist at alpha 2-adrenoceptors, a fact that also distinguishes it from clonidine. 3. Medetomidine induces a dose-dependent decrease in the central release and turnover of norepinephrine (NE) measured as changes in metabolite concentrations or using pharmacological intervention techniques. 4. The selectivity, specificity and potency of medetomidine is further supported by various in vivo experiments showing dose-dependent hypotensive, bradycardic, sedative, anxiolytic mydriatic, hypothermic and analgesic effects. 5. The pharmacological, neurochemical and behavioral effects of medetomidine can be inhibited by prior, simultaneous or subsequent administration of selective and specific alpha 2-antagonists. 6. In humans medetomidine is well-tolerated and pharmacodynamic effects including e.g. dose-dependent decrease of vigilance, blood pressure, heart rate, salivary secretion and plasma NE are compatible with an agonistic action at alpha 2-adrenoceptors.

The thermogenic actions of alpha 2-adrenoceptor agonists in reserpinized mice are mediated via a central postsynaptic alpha 2-adrenoceptor mechanism

1. The dose-related effects of the selective alpha 2-adrenoceptor agonists clonidine, UK-14,304 and B-HT 933 on the body temperature of untreated and reserpine-treated mice were investigated. 2. In untreated mice all three agonists induced a dose-related hypothermia. The highest doses of UK-14,304 and B-HT 933, 3 and 100 mg kg-1 respectively, elicited a marked (10 degrees C) hypothermia, whereas the maximal hypothermic effect of clonidine (5.5 degrees C) was less pronounced and reached a plateau at a dose of 0.5 mg kg-1 i.p. 3. Reserpine (2.5 mg kg-1, s.c.) induced a marked hypothermia in the mouse; 18 h after injection body temperature had decreased to only slightly (0.5-1.5 degrees C) above ambient (19 degrees C). 4. All three alpha 2-agonists produced a partial dose-related reversal of reserpine-induced hypothermia; maximal thermogenic responses (9-10 degrees C increases in body temperature) were elicited by doses of 0.2, 0.5 and 16 mg kg-1 i.p. of clonidine, UK-14,304 and B-HT 933 respectively, and the log dose-response curves for all 3 agonists were bell-shaped. 5. Following intracerebroventricular administration to reserpine-treated mice, the thermogenic response to clonidine was more rapid in onset, and the agonist was 20 fold more potent than when injected i.p. 6. The selective alpha 2-adrenoceptor antagonists, idazoxan (0.05-0.5 mg kg-1), Wy 26392 (0.3-5.0 mg kg-1) and yohimbine (0.1-1.6 mg kg-1) given orally attenuated the thermogenic responses to all 3 agonists in reserpinized mice in a dose-related manner. Pretreatment with a single dose of idazoxan (0.3 mg kg-1, orally) elicited a 6 fold parallel shift to the right in the dose-response curve to clonidine. 7. The selective alpha 1-adrenoceptor antagonists, prazosin (10 mg kg-1) and indoramin (3-10 mg kg-1), and the beta-adrenoceptor antagonist, propranolol (10 mg kg-1), only partially attenuated the thermogenic responses to the alpha 2-agonists in reserpinized mice. These effects were variable and not clearly dose-related. 8. Pretreatment of reserpinized mice with the catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine, markedly attenuated (60-95%) the thermogenic response to the noradrenaline uptake inhibitor, desipramine (0.13-12.5 mg kg-1, i.p.), but only slightly reduced (10-35%) that to clonidine (0.032-0.5 mg kg-1, i.p.). 9. These results suggest that alpha2-adrenoceptor agonists reverse reserpine-induced hypothermia via a central mechanism involving activation of postsynaptic alpha 2-adrenoceptors.

The relative potencies of cholinomimetics and muscarinic antagonists on the rat iris in vivo: effects of pH on potency of pirenzepine and telenzepine

The effects of cholinomimetics and muscarinic antagonists were compared following topical administration to the eyes of anaesthetized rats. For tests with cholinomimetics, clonidine (0.3 mg/kg) was used to induce mydriasis via central inhibition of parasympathetic tone. Full, dose-dependent miosis was induced by acetylcholinesterase inhibitors [physostigmine greater than neostigmine greater than tetrahydroaminoacridine (THA)] and by membrane channel blockers (4-aminopyridine greater than 3,4-diaminopyridine). Oxotremorine was the most potent direct agonist tested [oxotremorine greater than arecaidine propargylester (APE) greater than arecoline greater than carbachol greater than ethoxyethyltrimethyl-ammonium iodide (EOE) greater than RS 86]. Some putative M1 selective agonists were weakly active or behaved as partial agonists (pilocarpine greater than AH6405 greater than Mc-A-343 greater than isoarecoline). Of the antagonists, compared in non-clonidine treated rats, scopolamine hydrochloride was the most potent. Of the receptor selective antagonists the M2 (ileal) selective compounds hexahydrosiladifenidol and 4-DAMP were more potent than either M1 selective (pirenzepine, telenzepine) or M2 (atrial) selective (AF DX 116) drugs. These data tentatively suggest the involvement of an M2 (ileal) type muscarinic receptor. Potency was lower for quaternary structures, probably due to impaired corneal penetration. The potency of pirenzepine and telenzepine was increased 60-fold at low pH following topical administration. Acid induced corneal damage does not appear to account for this potency shift as the effects of scopolamine and several agonists (oxotremorine, pilocarpine and McNA-343) were not substantially altered by acid media. For pirenzepine the potency shift appears to be related to protonation of the second amino group (N1) in the piperazine tail (pKa = 2.05).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Dose-related effects of detomidine on autonomic responses in the horse

1. Detomidine is a novel veterinary sedative analgesic which is thought to act by stimulation of alpha 2 adrenoreceptors. The present study was undertaken to determine the direction, time course, and dose-response relationship of detomidine on specific autonomic responses in the unanaesthetized horse. 2. Detomidine was administered intravenously to eight adult thoroughbred racehorses at doses of 0.010-0.040 mg kg-1, according to a double-blind Latin square crossover design. Cardiac and respiratory rates, pupil diameter and rectal temperature were monitored for 180 min postinjection. 3. Detomidine produced prolonged dose-related bradycardia and bradypnoea. This was accompanied by a briefer period of dose-related mydriasis. Response duration, rather than peak was consistently increased as a function of dose. 4. Rectal temperature was not altered in a dose-dependent fashion. Low doses of detomidine produced late onset hypothermia, while high doses produced early and late onset hyperthermia. 5. These data indicate the uniqueness in autonomic response by the horse to alpha 2 adrenoreceptor stimulation.

Alpha 1- and alpha 2-adrenoreceptor antagonists produce opposing mydriatic effects by a central action

1. In anaesthetized rats, intravenous administration of alpha 2-adrenoreceptor antagonists yohimbine (0.3-3.0 mg kg-1), idazoxan (0.03-0.3 mg kg-1) and raulwolscine (0.3-3.0 mg kg-1) produced a dose-related inhibition of sciatic nerve-(ScN) mediated reflex pupillary dilation (parasympatho-inhibition). The rank order of potency was idazoxan greater than yohimbine greater than rauwolscine. 2. Under similar experimental conditions, intravenous administration of alpha 1-adrenoreceptor antagonists prazosin (0.03-1.0 mg kg-1), phenoxybenzamine (0.3-3.0 mg kg-1) and corynanthine (0.03-1.0 mg kg-1) produced a dose-dependent potentiation of the reflex mydriasis with the potency order being prazosin greater than corynanthine greater than phenoxybenzamine. Intravenous yohimbine (1.5 mg kg-1) reversed the potentiation caused by the alpha 1-adrenoreceptor antagonists and blocked the reflex mydriasis. 3. Parasympatho-inhibition and mydriasis elicited by hypothalamic stimulation was not affected by the alpha 2-adrenoreceptor antagonists yohimbine (0.3-3.0 mg kg-1), idazoxan (0.03-0.3 mg kg-1) or the alpha 1-adrenoreceptor antagonist prazosin (0.3-1.0 mg kg-1). 4. Microinjection of prazosin (3-30 ng) into the oculomotor nuclear complex (IIIn), produced a dose-related potentiation, whereas microinjection of yohimbine (0.3-3.0 micrograms) produced a dose-related blockade of reflex mydriasis. 5. The above findings support the hypothesis that ascending mechanisms (e.g. afferent ScN) produce inhibition of parasympathetic oculomotor tone to the iris by activation of central postsynaptic alpha 2-adrenoreceptors. Furthermore, these studies demonstrate that alpha 2-adrenoreceptor antagonists block and alpha 1-adrenoreceptor antagonists potentiate the reflex mydriasis. These actions appear to be localized within the pupilloconstrictor regions of the brain (oculomotor nuclear complex).

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.

Characterization of the selectivity, specificity and potency of medetomidine as an alpha 2-adrenoceptor agonist

Medetomidine (4-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole) was tested for alpha 2-adrenoceptor agonist activity and compared to several reference agents. In binding studies carried out with rat brain membrane preparations, medetomidine showed high affinity for alpha 2-adrenoceptors, as measured by the displacement of [3H]clonidine (Ki 1.08 nM compared to 1.62, 3.20, 6.22 and 194 nM for detomidine, clonidine, UK 14,304 and xylazine, respectively). The affinity of medetomidine for alpha 1-adrenoceptors, as measured by [3H]prazosin displacement, was much weaker, yielding a relative alpha 2/alpha 1 selectivity ratio of 1620 which is 5-10 times higher than that of the reference compounds. Medetomidine caused a concentration-dependent inhibition of the twitch response in electrically stimulated mouse vas deferens with a pD2 value of 9.0 compared to that of 8.6, 8.5, 8.2 and 7.1 for detomidine, clonidine, UK 14,304 and xylazine, respectively. The effect of medetomidine was antagonized by idazoxan. In anaesthetized rats, medetomidine caused a dose-dependent mydriasis which could be reversed by alpha 2-adrenoceptor blockade. In receptor binding experiments and isolated organs medetomidine had no affinity or effects on beta 1-, beta 2-, H1, H2, 5-HT1, 5-HT2, muscarine, dopamine, tryptamine, GABA, opiate and benzodiazepine receptors. Based on these results, medetomidine can be classified as a potent, selective and specific alpha 2-adrenoceptor agonist.

The bradycardic and mydriatic effects of chlordimeform and its demethylated analogs in the rat: antagonism by idazoxan but not by prazosin

Pupillary and cardiac responses to i.v. injections of chlordimeform (CDM, 0.3-10 mg/kg), a formamidine insecticide, and its metabolites demethylchlordimeform (DCDM, 0.03-1 mg/kg) and didemethylchlordimeform (DDCDM, 0.1-3 mg/kg) were studied in rats anesthetized with pentobarbital. Both CDM and DCDM induced a dose-dependent mydriasis and bradycardia and DCDM was 10 times more potent than CDM in causing these effects. In contrast, DDCDM did not induce a mydriasis or bradycardia. The alpha 2-adrenoreceptor antagonist, idazoxan (0.2 mg/kg, i.v.) abolished or reduced CDM- and DCDM-induced mydriasis and bradycardia, whereas the alpha 1-adrenoreceptor antagonist, prazosin (1.5 mg/kg, i.v.) did not change these effects of CDM and DCDM. SKF 525-A (50 mg/kg, i.p.), an inhibitor of enzymatic demethylation, administered 10 min before the first dose of CDM, failed to reduce the effects of CDM. The results suggested: 1) the mydriatic and bradycardic effects of CDM and DCDM are mediated by alpha 2-adrenoreceptors, 2) the monodemethylation of CDM increases its alpha 2-adrenoreceptor agonistic activities, but the didemethylation of CDM abolishes these activities, and 3) CDM can exert alpha 2-adrenoreceptor agonistic activities without undergoing a demethylation process.

Pharmacological profile of a new potent and specific alpha 2-adrenoceptor antagonist, L-657,743

L-657,743,(2S,12bS)1',3'-dimethylspiro(1,3,4,5',6,6',7,12 b-octahydro-2H- benzo[b]furo[2,3-a]quinolizine)-2,4'-pyrimidin-2'-one, was tested in several in vitro and in vivo models for alpha 2-adrenoceptor antagonism. L-657,743 exhibited a high affinity (less than or equal to 1 nM) for alpha 2-adrenoceptors labelled by [3H] rauwolscine or [3H]clonidine with a 240-fold selectivity versus alpha 1-adrenoceptors labelled by [3H]prazosin. L-657,743 was a potent, selective, and competitive alpha 2-adrenoceptor antagonist in the rat isolated vas deferens (pA2 = 9.3 vs. clonidine; pA2 = 7.1 vs methoxamine). In vivo, L-657,743 potently blocked clonidine-induced mydriasis in the rat and stimulated cerebrocortical norepinephrine synthesis, two indices of central alpha 2-adrenoceptor antagonism. L-657,743 exhibited a comparatively low affinity for several monoamine receptor subtypes (D1, D2, 5-HT1, 5-HT2) in radioligand binding assays in vitro and a comparatively low potency to alter the synthesis of brain DA and 5-HT in vivo indicating a marked alpha 2-specificity versus other monoamine receptor mechanisms. Compared to yohimbine, L-657,743 had considerably higher alpha 2-antagonist potency and alpha 2/alpha 1 selectivity and was significantly more alpha 2-specific (i.e., vs. DA, 5-HT receptors).

L-654,284 a new potent and selective alpha 2-adrenoceptor antagonist

L-654,284 [(2R, 12bS)-N-(1,3,4,6,7,12b-hexahydro-2H-benzo[b]-furo[2,3-a] quinolizine-2-yl)-N-methyl-2-hydroxyethanesulfonamide) was tested in several in vitro and in vivo models for alpha 2-adrenoceptor antagonist activity and compared to several reference agents. In vitro L-654,284 completed for the binding of 3H-clonidine or 3H-rauwolscine (Ki's 0.8 nM, 1.1 nM) and blocked the presynaptic effects of clonidine in the rat isolated vas deferens (pA2, 9.1). L-654,284 exhibited marked alpha 2-vs. alpha 1-adrenoceptor selectivity in vitro, inhibiting 3H-prazosin binding with a Ki of 110 nM and blocking the effects of methoxamine on the vas deferens with a pA2 of 7.5. In vivo L-654,284 at 22 nmoles/kg i.v. doubled the ED50 of clonidine to produce mydriasis in rats. Given orally, the potency of L-654,284 in this test was reduced by a factor of 5.5. L-654,284 also potently increased cerebrocortical NE turnover in the rat, another in vivo index of alpha 2-adrenoceptor blockade in the central nervous system. In the periphery, L-654,284 demonstrated alpha 2-adrenoceptor selectivity by preferentially blocking the pressor effects of UK 14304 versus those of methoxamine in the pithed rat. Overall, L-654,284 was generally a more potent alpha 2-adrenoceptor antagonist than RX 781094 with comparable alpha 2/alpha 1 selectivity and was several times more potent and alpha 2-selective than WY 26703 or yohimbine. In addition, L-654,284 had better (5-6 times) oral bioavailability than RX 781094 or WY 26703.

Biochemical and pharmacological effects of fluperlapine on noradrenaline and acetylcholine systems in some rodent, bovine and crustacean preparations

Fluperlapine was compared with clozapine, chlorpromazine, haloperidol and imipramine regarding its effects on some cholinergic and noradrenergic animal systems. Fluperlapine and clozapine showed the most pronounced anticholinergic effects. Fluperlapine was equipotent with clozapine in displacing [3H]-QNB from muscarinic receptors of the calf cerebral cortex (IC50 about 15 nM). In the mydriasis test in the mouse and in the crayfish hindgut bioassay the differences between fluperlapine and clozapine were small. Like the other antischizophrenic drugs tested, fluperlapine displayed a marked affinity for alpha 1-adrenoceptors (calf cerebral cortex: IC50 about 10 nM) but a negligible affinity for alpha 2-adrenoceptors in the same tissue. Only clozapine showed a weak affinity for the latter receptor type. Fluperlapine was as effective as imipramine in antagonizing tetrabenazine-induced ptosis in the rat, the anti-ptotic effect remaining constant after up to ten daily drug administrations. Still, imipramine was stronger than fluperlapine as an inhibitor of the accumulation of [3H]-noradrenaline ([3H]-NA) in rat cerebral cortex slices. Fluperlapine's effects on the spontaneous and the electrically-induced release of [3H]-NA from rat cerebral cortex slices, with and without protriptyline, showed it to be an inhibitor of the reuptake of NA. The results indicate that the pharmacological profile of fluperlapine is similar to that of clozapine, with additional antidepressant properties.

Acute reversible cataract induced by xylazine and by ketamine-xylazine anesthesia in rats and mice

Combined administration of ketamine and xylazine is used increasingly for safe, effective anesthesia of small laboratory animals. We found that rats injected systemically with ketamine and xylazine at doses recommended for effective anesthesia developed acute reversible lens opacities. Mice given the same drug doses were similarly affected. Testing of each drug alone demonstrated that xylazine was the causative agent. The appearance of cataract was associated to varying degree with proptosis, suppression of the blink reflex, corneal surface drying, and mydriasis. All of these ocular effects, including cataract also could be induced locally by topical application of xylazine to one eye, with untreated contralateral eyes showing no drug effects. A possible cause of xylazine-induced transient lens opacification is trans-corneal water loss and alteration of aqueous humor composition due to corneal exposure. Additional action on aqueous humor formation and the lens itself may be due to the alpha-2-adrenoceptor nature of xylazine. Whatever the cause of cataract induction, the occurrence of this phenomenon during ketamine-xylazine anesthesia appears to be associated with marked changes in the physiological state of the eye. Therefore, the side-effects of anesthetic drug combination should be considered prior to its use on animals for studies of ocular physiology.