Evidence for a central postsynaptic action of clonidine

Effect of clonidine in mice injected with Tityus discrepans scorpion venom

A study was conducted to assess the effect of clonidine (α(2)-adrenoceptor selective agonist) on glycemia, serum and urine α-amylase, blood urea nitrogen (BUN), serum creatinine, white blood cell count, kidney histology and zymogen granule content in pancreatic acini, in mice under the effect of Tityus discrepans (Td) scorpion venom. BALB/c male mice (20 ± 2 g, n = 7-11) were intraperitoneally (ip) injected with a sublethal dose (1 μg/g) of Td venom, and were treated (ip) with 0.1 μg/g of clonidine (Catapresan(®)) or 0.9% NaCl 30 min after the venom injection, and then every 2 h. Six hours later, mice were anesthetized with diethylether and urine and blood samples were withdrawn by cystocentesis and cardiocentesis, respectively. Tissue samples were obtained and fixed immediately in buffered formalin (2%, pH 7.4) and then processed for stain H&E. Td venom did not cause hyperglycemia by itself. However, clonidine induced hyperglycemia, which was synergized by Td venom. Although the venom did not produce hyperamylasemia, clonidine significantly diminished serum α-amylase activity in envenomed mice. Td venom did not significantly increase urinary α-amylase activity, which was unaffected by clonidine. Morphometric analysis using microphotographs of pancreata from mice injected with Td venom showed a reduced zymogen granule content as judged by the acidophilic bidimensional area of acini. This effect was significantly reduced by clonidine. Kidney samples showed histological changes which were partially affected by the drug. Clonidine reduced the increase in BUN and serum creatinine concentration in envenomed mice. Td venom produced neutrophilia and lymphopenia, which were clonidine-resistant at the assayed dose. These results suggest that α(2)-adrenoceptor selective agonists would be able to reduce some scorpion venom-induced renal and pancreatic disturbances, possibly through the inhibition of neurotransmitter release from presynaptic cholinergic and noradrenergic terminals, as well as from adrenal medulla.

In vivo evaluation of alpha(2)-adrenoceptors in cats with idiopathic cystitis

OBJECTIVE

To evaluate the in vivo response of alpha(2)-adrenoceptors to medetomidine administration in cats with feline idiopathic cystitis (FIC) during periods of stress and after environmental enrichment.

ANIMALS

13 cats with FIC and 12 healthy cats. Procedures-Cats were subjected to an acute-onset moderate stressor for 8 days. After stress, 20 microg of medetomidine/kg was administered IM on days 1, 3, and 8. Heart rate, blood pressure, pupil diameter, respiratory rate, and level of sedation were evaluated before and after administration of the drug. After day 8, cats were moved to an enriched environment, and tests were repeated on day 35.

RESULTS

Heart rate decreased and pupil diameter increased significantly after medetomidine administration in healthy cats, compared with cats with FIC. Cats with FIC had significantly lower respiratory rates. No significant differences in blood pressure or sedation level were found.

CONCLUSIONS AND CLINICAL RELEVANCE

Increased plasma catecholamine concentrations during the enrichment phase, which have been reported elsewhere, may have contributed to the differences in alpha(2)-adrenoceptor responses detected in cats with FIC.

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.

The alpha2C-adrenoceptor modulates GABA release in mouse striatum

The alpha(2C)-adrenoceptor occurs in high density in the striatum relative to other brain regions, but its biological role in striatal physiology is perplexing because of the paucity of noradrenergic terminals in this region. In this study, mice with a targeted inactivation of the alpha(2C)-adrenoceptor gene (alpha(2C)-KO mice), and genetically related mice (WT mice), were used to study the potential role of the striatal alpha(2C)-adrenoceptor in modulating GABA release. Perfused brain slices were pre-loaded with [(3)H]GABA and were stimulated electrically. In WT mice, the alpha(2)-adrenoceptor agonist, UK14304 (brimonidine), significantly enhanced [(3)H]GABA release from striatal slices, while the alpha(2)-adrenoceptor antagonist, RX821002, alone evoked a significant decrease in [(3)H]GABA release. In alpha(2C)-KO mice, the effect of RX821002 was absent, while UK14304 retained its ability to enhance [(3)H]GABA release. Pharmacological depletion of monoamines in WT mice also abolished the effect of RX821002 on [(3)H]GABA release. In hippocampal slices, RX821002-induced reduction in [(3)H]GABA release was present in WT and alpha(2C)-KO mice. In the presence of tetrodotoxin, RX821002 increased [(3)H]GABA release in striatal slices from both WT and alpha(2C)-KO mice. Together, these data imply that alpha(2A)- and alpha(2C)-adrenoceptors are located on different neurons in the striatum, that alpha(2C)-adrenoceptor-mediated effects on striatal GABA release are mediated by an endogenous catecholamine that could be dopamine, and that the alpha(2C)-adrenoceptor effect of RX821002 does not occur at the GABAergic terminal.

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.

Clonidine-induced emesis: a multitransmitter pathway concept

The emetic effect of clonidine injected into the cerebral ventricles through chronically implanted cannulae was investigated in unanaesthetized cats. Clonidine (0.1-300 micrograms) induced dose-dependent and shortlasting emesis. The emesis induced by the supramaximal dose of clonidine (100 micrograms) was not abolished after the ablation of area postrema. Both the alpha 2 adrenoceptor blocking agent idazoxan and the mixed alpha 1 and alpha 2 adrenoceptor antagonist phenoxybenzamine, injected intracerebroventricularly, attenuated or abolished the emesis induced by clonidine (100 micrograms). On the other hand, the alpha 2 adrenoceptor blocking agent yohimbine, the alpha 1 adrenoceptor blocking drug prazosin and the non-selective beta-adrenoceptor antagonist propranolol, injected into the cerebral ventricles, had no significant effect on clonidine-induced emesis. The antimuscarinic drug atropine injected into the cerebral ventricles prevented the clonidine-induced emesis in a dose-dependent manner. The dopamine antagonist chlorpromazine, the 5-hydroxytryptamine blocking agent methysergide and the histamine H1 and H2 receptor antagonists, antazoline and cimetidine, injected intracerebroventricularly reduced or abolished the emesis produced by clonidine. The ganglionic blocking substance mecamylamine and the opioid antagonist naloxone, all injected into the cerebral ventricles, had no significant effect on clonidine-induced emesis. In cats pretreated with the intracerebroventricular competitive inhibitor of the synthesis of catecholamines, alpha-methyl-p-tyrosine, as well as with the inhibitor of acetylcholine synthesis hemicholinium-3, the emesis caused by clonidine was depressed or abolished. The clonidine-induced emesis was also abolished when catecholamine stores were depleted by intracerebroventricular reserpine. However, the clonidine-induced emesis was not significantly changed when 5-hydroxylryptaminergic nerve terminals were damaged by 5,6-dihydroxytryptamine. It follows, therefore, that cholinergic and noradrenergic mechanisms are of basic importance for the emetic action of clonidine. With regard to receptors, the emesis induced by clonidine injected into the cerebral ventricles, is mediated at least in part through alpha-adrenoceptors, muscarinic cholinoceptors, 5-hydroxytryptamine receptors and H1 and H2 histamine receptors. These receptors appear to be located mostly presynaptically and they transmit emetic impulses to neurones integrating them into emesis. However, the direct effect of clonidine on postsynaptic receptors cannot be excluded, particularly when muscarinic and 5-hydroxytryptamine receptors are implicated. Taken together, these results point to the existence of a multitransmitter pathway/s outside the area postrema, subserving the central regulation of emesis.

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.

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.

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.

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.

Effect of prazosin on spontaneous sympathetic-cholinergic activity

Tonic sympathetic-cholinergic electrodermal (sudomotor) activity was measured in intact anesthetized and unanesthetized decerebrate and decerebrate-spinalized cats. Prazosin (3-100 micrograms/kg i.v.) depressed spontaneous electrodermal activity in intact anesthetized cats in a dose-dependent fashion (ED50 4.8 micrograms/kg). Prazosin's action was almost totally abolished by monoamine depletion with reserpine and a synthesis inhibitor. The alpha 2-adrenoceptor blocker, yohimbine (0.5 mg/kg i.v.) also antagonized prazosin-induced sympatho-inhibition. Prazosin reduced tonic sudomotor activity in unanesthetized decerebrate cats in a dose-dependent fashion (ED50 5.5 micrograms/kg i.v.) but was without effect in spinalized preparations. These results support the hypothesis that, in this system, prazosin produces sympatho-inhibition indirectly by means of an alpha 2-adrenoceptor-mediated mechanism, an effect which appears to be prejunctional. It is proposed that prazosin acts at the level of the spinal cord to facilitate ongoing alpha 2-adrenergic inhibition arising from supra-spinal loci.

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.

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).

Role of central biogenic amines on the protection afforded by clonidine against the toxicity of soman, an irreversible cholinesterase inhibitor

Studies in our laboratory have demonstrated that alpha 2-adrenergic agonists such as clonidine offer protection against the toxicity caused by cholinesterase inhibitors such as soman. Experiments were designed to determine whether central catecholaminergic systems are implicated in the toxic and lethal manifestations of soman toxicity and whether the protection afforded by clonidine involves such pathways. Clonidine pretreatment resulted in a significant degree of protection from the lethal effects of soman in the mouse. Depletion of brain catecholamines did not alter soman-induced lethality or behavioral toxicity. Furthermore, catecholamine depletion was not effective in blocking clonidine-induced protection against soman toxicity. In contrast, elevation of brain catecholamines using the monoamine oxidase inhibitor, pargyline, resulted in significant protection against soman toxicity which was additive with that of clonidine.

Role of stress in the exacerbation of chronic illness: effects of clonidine administration on blood pressure and plasma norepinephrine, cortisol, growth hormone and prolactin concentrations

Systolic blood pressure (SBP), diastolic blood pressure (DBP), and plasma norepinephrine (NE), cortisol (CRT), growth hormone (GH) and prolactin (PRL) were studied before and after clonidine (2.5 micrograms/kg i.m.) administration in 193 chronic severely ill patients and 193 normal subjects matched by age and sex. During exacerbation periods (positive manifestations of impairment and progressive disease), the patients showed higher NE, CRT and DBP than the normals or when they were investigated during non-exacerbation periods (92 of the 193). Clonidine induced sharp, marked reductions of NE, CRT and DBP, plus a sudden increase of GH, in all the patients during exacerbation periods. Non-significant reductions of NE, CRT and DBP were observed in normals and in patients during non-exacerbation periods. On the other hand, the GH increase registered during exacerbation periods was of an order of magnitude higher than that registered in normals and in patients during non-exacerbation periods. Significant reduction of SBP was registered both in normals and patients (exacerbation and non-exacerbation periods). Some tendency to PRL lowering was observed during exacerbation periods only. A high positive correlation between NE and DBP (pre- and post-clonidine values) was obtained during exacerbation periods in patients, but not in normals or during non-exacerbation periods in the patients. Similarly, a close negative correlation was obtained between CRT and GH (postclonidine values) during exacerbation periods, but not in normals or during non-exacerbation periods. No significant correlation was found between NE and SBP in any group of subjects. The clonidine-induced changes in GH and CRT observed in the patients during exacerbation periods were in striking contrast to the absence of these changes in depressed patients. This finding is consistent with the low rate of depression (6.7%) registered among our patients during exacerbation periods. The high plasma NE and CRT levels registered in chronic severely ill patients during exacerbation periods reflect a central and peripheral sympathetic hyperactivity, accompanied by an overactivity of the pituitary--adrenocortical axis. The strong reduction of DBP, NE and CRT, along with the sharp and great increase of GH, might be useful as indicators in assessing the exacerbation and progression of severe chronic illnesses.

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.

Pupillary dilation as an index of central nervous system alpha 2-adrenoceptor activation

In recent years there has been increasing evidence that some antihypertensive drugs like clonidine and alpha-methyldopa (after conversion in the brain to alpha-methylnorepinephrine) may decrease sympathetic tone by stimulating central nervous system (CNS) alpha 2-adrenoceptors. These same drugs also produce pupillary dilation in cats and rats. In this review, evidence is presented supporting the hypothesis that clonidinelike drugs act either directly or indirectly on CNS postsynaptic alpha 2-adrenoceptors to cause pupillary dilation by reduction of parasympathetic neural tone to the iris. It is further suggested that the underlying physiologic mechanism for this mydriatic action is activation of an ascending pathway that provides tonic inhibitory input by releasing norepinephrine on neurons in the Edinger-Westphal complex. Yohimbine-sensitive pupillary dilation in these species may provide a simple and effective model for quantitatively accessing CNS alpha 2-adrenoceptor activity.

Tachycardic and hypertensive effects of centrally administered clonidine in conscious rats

The centrally mediated cardiovascular changes induced by clonidine were studied in conscious rats. Clonidine administered intracerebroventricularly (i.c.v.), and intravenously (i.v.) caused hypotension following an initial pressor response. I.v. clonidine caused significant greater hypotension than i.c.v. clonidine (30 micrograms/kg; p less than 0.05). With the 30 micrograms/kg i.c.v. dose, a tachycardia was observed in all rats following initial transient bradycardia. No tachycardia was observed when clonidine was administered i.v. Propranolol (3 mg/kg i.v.) did not modify the cardiovascular actions of i.c.v. clonidine except initial pressure response. While combined treatment with propranolol (3 mg/kg i.v.) and atropine (1 mg/kg i.v.) abolished both the bradycardic and tachycardic actions of i.c.v. clonidine (30 micrograms/kg), but did not modulate the hypotensive action. Yohimbine (30 micrograms/kg i.c.v.) converted the hypotension induced by i.c.v. clonidine (30 micrograms/kg) to hypertension, attenuated the bradycardia but did not modulate the tachycardia. The same dose of i.c.v. yohimbine attenuated the hypotensive effect of i.v. clonidine (30 micrograms/kg) but did not affect the initial pressor response. Prazosin (30 micrograms/kg i.c.v.) did not modulate either phase of the heart rate response to i.c.v. clonidine. These results provide evidence of centrally mediated pressor and tachycardic actions of clonidine in conscious rats. The tachycardia appears to be mediated through the inhibition of parasympathetic tone and is not dependent on alpha-adrenoceptor mechanism. In conscious rats the opposing influence of centrally mediated pressor and depressor actions may result in the apparently low hypotensive potency of i.c.v. clonidine.

Comparison of the effects of detomidine and xylazine on some alpha 2-adrenoceptor-mediated responses in the central and peripheral nervous systems

The effects of detomidine, a novel veterinary sedative analgesic, on some alpha 2-adrenoceptor-mediated responses in the central and peripheral nervous systems were studied. In pithed rats, detomidine was a very potent agonist at both pre- and postsynaptic alpha 2-adrenoceptors. Doses of 1.9 micrograms/kg and 6.5 micrograms/kg inhibited electrically induced tachycardia by 50% and increased mean blood pressure by 50 mmHg, respectively. In comparison, xylazine, though similar in specificity, was 40 times less potent than detomidine in this preparation. In unanaesthetized rats, detomidine both caused sedation and induced complex changes in body temperature. Low doses caused decreases in rectal temperature but these were reversed as the dose was increased. The decrease in rectal temperature could be blocked by yohimbine. Prazosin somewhat inhibited but did not eliminate the hyperthermia seen with the very high doses of detomidine. Xylazine caused much more severe falls in rectal temperature which could not be completely antagonized by alpha 2-adrenoceptor blockade. Both detomidine and xylazine caused dose-dependent mydriasis in anaesthetized rats, detomidine being about 10 times more potent than xylazine. The mydriatic effects of detomidine could be prevented by alpha 2- but not by alpha 1-adrenoceptor blockade. It is concluded that detomidine is a potent and rather specific alpha 2-adrenoceptor agonist in the central and peripheral nervous systems. In comparison with xylazine, detomidine has higher potency and greater specificity, especially at central alpha 2-adrenoceptors.

Neurochemical aspects of depression: the past and the future?

The role of aberrant neurochemical substrates in the etiology of depression and the neurochemical mechanisms of antidepressant therapies have been the subjects of many hypotheses in the last 30 years. Pharmacological studies of early antidepressant drugs indicated that brain monoamines were significantly affected by these drugs and these led to the formulation of the biogenic amine hypothesis of depression. Although this hypothesis has been of heuristic value in the study of drug mechanisms and has provided a basis for screening drugs for antidepressant potential, deficiencies in it have become apparent. Neuroanatomical and neurochemical considerations favour the view that brain noradrenaline and serotonin systems may serve as bias adjusting systems for each other and numerous other neural systems. As a consequence of such a relationship, a primary defect in some other neural system would appear amplified in measurements of serotonin or noradrenaline. A possible site for this primary defect may be in membrane composition and function. Recent studies have found that typical and other antidepressant therapies have a pronounced effect on membrane lipids. Thus, in view of the important functions of membrane lipids and the fact that they have been linked to the initiation and development of a number of other disease processes, it is now suggested that consideration be given to them as playing primary causal roles in the etiology of depression and as a site of action for antidepressant drugs.

Studies on the mechanism of clonidine-induced mydriasis in the rat

Intravenous administration of clonidine hydrochloride (3-100 micrograms/kg) produced a dose-dependent pupillary dilation in anaesthetized rats. All experiments were carried out in rats in which vagosympathetic nerve trunks were sectioned bilaterally at the cervical level. Clonidine-induced mydriasis was present only in those preparations having intact parasympathetic neural tone to the iris. Depletion of CNS monoamines by more than 95% with reserpine (5 mg/kg) and alpha-methyl-para-tyrosine (2 X 300 mg/kg) failed to alter the dose-response relation to clonidine. Pretreatment with the alpha-2-adrenoceptor antagonist, yohimbine hydrochloride (1.5 mg/kg), produced about a 10-fold shift to the right in the pupillary dose-response curve to clonidine. Yohimbine administered after the highest dose of clonidine also antagonized the mydriatic response. The above results suggest that clonidine acts on CNS post-synaptic alpha-2-adrenoceptors to produce mydriasis by withdrawal of parasympathetic neural tone to the iris. In an attempt to assess the physiological substrate(s) involved, mydriatic responses, due to parasympatho-inhibition, were evoked by electrical stimulation of ascending (sciatic nerve and medullary) and descending (hypothalamic) pathways. Yohimbine (0.3 and 1.0 mg/kg) produced a dose-dependent inhibition of the pupillary dilation evoked by stimulation of the sciatic nerve and medullary loci, whereas these doses of yohimbine failed to alter the dilation in response to hypothalamic stimulation. Similarly, monoamine depletion greatly antagonized the pupillary dilation elicited by sciatic nerve and medullary stimulation without significantly affecting mydriasis due to hypothalamic stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

CNS adrenergic inhibition of parasympathetic oculomotor tone

Inhibition of parasympathetic neural tone to the iris was produced by electrical stimulation of the afferent sciatic nerve, medullary reticular formation, and posterior hypothalamus in anesthetized cats in which only the parasympathetic nerves to the eye were intact. Stimulation of all 3 sites of activation produced a graded pupillary dilation and reduction of tonic nerve activity in the short ciliary nerves. Intravenous administration of the alpha-2-adrenoceptor antagonist, yohimbine hydrochloride, (0.03-1.0 mg/kg) produced a dose-dependent antagonism of the mydriasis elicited by activation of the ascending (sciatic nerve and medullary) mechanisms but did not block the pupillary dilation evoked by stimulation of the system descending from the hypothalamus. This differential action of yohimbine was confirmed directly by means of nerve recordings taken from the parasympathetic nerve to the eye. Depletion of CNS monoamines with reserpine and alpha-m-p-tyrosine reduced the norepinephrine concentration of the medulla and midbrain by 95% and 97%, respectively. In these depleted preparations, stimulation of the hypothalamus still produced the characteristic mydriasis and inhibition of parasympathetic tonic activity whereas activation of ascending mechanisms (sciatic or medullary) were no longer effective in producing these effects. Taken together, these results suggest that ascending parasympatho -inhibition is mediated by a monoamine (probably norepinephrine) and that inhibition descending from the hypothalamus is mediated by a non-monoaminergic mechanism.

Evidence for two distinct sympathoinhibitory bulbo-spinal systems

Yohimbine hydrochloride (0.5 mg/kg, i.v.) caused a long-lasting potentiation of electrodermal (sympathetic-cholinergic) reflexes in intact anaesthetized and decerebrate unanaesthetized cats. Transection of the cervical spinal cord also resulted in an increased amplitude of the sudomotor reflex in unanaesthetized decerebrate preparations. Depletion of monoamines in the CNS by pretreatment with reserpine (5 mg/kg, i.p.) and alpha-methyl-p-tyrosine (2 X 300 mg/kg, i.p.) reduced the concentrations of norepinephrine, dopamine and serotonin to less than 93% of control levels in the thoracic spinal cord. In monoamine-depleted preparations, yohimbine no longer facilitated the reflex amplitude whereas the effect of spinal transection was not altered. These results suggest that there are two distinct sympathoinhibitory systems in the lower brain stem that converge on spinal sympathetic neurons, one of which is monoaminergic and one of which is not. Evidence for the baroreceptor-independent nature of these descending inhibitory systems is discussed.

Alpha-methyl-DOPA induced mydriasis in the cat. Relationship between pupillary response and the oculomotor perfusate concentration of methyldopa and its metabolites

In pentobarbital anaesthetized cats, intravenous administration of 30 mg/kg, alpha-methyl-DOPA produced mydriasis that reached a maximum plateau in 2--2.5 h. The oculomotor nucleus was perfused with saline using a push-pull cannula system chronically implanted over the nucleus. Perfusate samples were collected and subsequently analyzed by liquid chromatography with electrochemical detection (LC-EC). Alpha-methyl-DOPA administration resulted in a gradual build up of alpha-methyl-noradrenaline to approximately 32 mumoles over the 3 h sampling period. In contrast, the concentration of alpha-methyl-dopamine was below the detection level for the first 90 min with peak levels of less than 6 mumoles after 3 h. A linear regression analysis demonstrated a negative correlation (R = 0.90) between the pupil size and the perfusate concentration of alpha-methyl-DOPA and a positive correlation for both alpha-methyl-dopamine and alpha-methyl-noradrenaline (R = 0.88 and 0.94 respectively). Pretreatment with the DOPA-decarboxylate inhibitor, 3-hydroxy-benzyl-hydrazine (NSD-1015; 25 mg/kg, i.p.) completely blocked the mydriatic response to alpha-methyl-DOPA, with neither alpha-methyl-dopamine nor alpha-methyl-noradrenaline reaching detectable levels in the oculomotor perfusate. After treatment with the dopamine-beta-hydroxylase inhibitor, bis (4-methyl-homopiperazinyl thiocarbonyl) disulfide (FLA-63; 2.5 mg/kg, i.p.) there was a significant accumulation of alpha-methyl-dopamine when compared to that obtained in the alpha-methyl-DOPA controls but with no apparent alpha-methyl-dopamine related pupillary dilation. However, the correlation between alpha-methyl-noradrenaline concentration and the increase in the diameter of the pupil was maintained.(ABSTRACT TRUNCATED AT 250 WORDS)

Clonidine-induced growth hormone secretion in chronic schizophrenia

Clonidine (0.15 mg intravenously), an alpha-adrenergic receptor agonist, was administered to 13 male chronic schizophrenics, who had been withdrawn from chronic neuroleptic therapy, and to 18 normal male controls. There was no significant difference in growth hormone (GH) response between the two groups. There was no significant correlation between duration of psychosis, duration of neuroleptic therapy or length of neuroleptic withdrawal and GH response. These results suggest that postsynaptic alpha-adrenergic receptor function in the hypothalamic-pituitary axis is unaltered in chronic schizophrenia or by prior chronic neuroleptic therapy.

Analysis of CNS sympatho-inhibition produced by guanabenz

The effects of guanabenz on several autonomic systems were observed in vagotomized, anesthetized cats with the aim of determining, in a quantitative sense, the degree to which guanabenz produces a clonidine-like central nervous system action. Guanabenz given as a single dose (50 micrograms/kg i.v.) produced a transient hypertension associated with a more sustained bradycardia and depression of centrally (hypothalamic) evoked electrodermal responses (EDR). Increasing cumulative doses of guanabenz (3-1000 micrograms/kg i.v.) also resulted in a dose-related depression of EDR amplitude, transient hypertension followed by hypotension, sustained bradycardia, and mydriasis. All responses were antagonized by pretreatment with yohimbine (0.5 mg/kg i.v.). The ED50 for depression of the centrally evoked EDR was in the range of 50-100 micrograms/kg i.v. in the non-pretreated preparations. Guanabenz (100 micrograms/kg i.v.) was shown to be devoid of significant ganglionic blocking properties. These experiments suggest that guanabenz acts like clonidine in the CNS and that an alpha 2-adrenergic inhibitory mechanism is involved in its myriad of central autonomic effects.

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.

Effects of apomorphine, clonidine or 5-methoxy-NN-dimethyltryptamine on approach and escape components of lateral hypothalamic and mesencephalic central gray stimulation in two inbred strains of mice

The effects of intraperitoneal injections of increasing doses of apomorphine, clonidine or 5-methoxy-NN-dimethyltryptamine (5-m-DMT) on approach and escape reactions induced by lateral hypothalamic (LH) or mesencephalic central gray (CG) stimulation were compared in BALB/c and DBA/2 mice. Apomorphine increased both the approach latency for LH stimulation and the escape latency for CG stimulation; the BALB/c strain was more reactive than DBA/2 animals. Clonidine reduced the approach latency for LH stimulation only in the BALB/c strain. 5-m-DMT increased escape latency both for LH and CG stimulation only in the DBA/2 strain. These results suggest that the neurochemical regulation of escape reactions respectively generated by LH or CG activation is partially different: dopamine seems to be involved only in CG aversion, whereas serotonin (5-HT) modulates both LH and CG escape reactions. Moreover, our results demonstrate a noradrenergic influence on the appetitive component of LH stimulation. Finally, they confirm that approach and escape reactions, particularly when induced from lateral hypothalamus, depend on distinct neuronal populations.

Studies on the mechanism of methyl-dopa-induced mydriasis in the cat

Alpha-methyl-Dopa (10-100 mg/kg, i.v.) produced a dose-dependent mydriasis in cats anaesthetized with pentobarbital (30 mg/kg, i.p.). The onset was gradual, reaching a maximum plateau in 2-2.5 h. Intracerebroventricular administration of 1 or 3 mg of alpha-methyl Dopa (MD) also produced pupillary dilation with a similar time course. These dosages were without effect when given intravenously. Pretreatment with the alpha 2-adrenoceptor antagonist, yohimbine (0.5 mg/kg i.v.), blocked the pupillary response to MD. The alpha 1-adrenoceptor antagonist, prazosin (1.0 mg/kg. i.p.), was ineffective. Selective enzymatic blockade with 3-hydroxy-benzyl-hydrazine (NSD-1015; 25 mg/kg, i.p.), a Dopa-decarboxylase enzyme inhibitor, as well as with bis (4-methyl-homopiperazinyl-thiocarbonyl) disulfide (FLA-63; 2.5 mg/kg, i.p.), a dopamine-beta-hydroxylase blocker, prevented the mydriatic effect of MD. These results support the hypothesis that MD produces a clonidine-like, CNS mediated mydriasis in the cat, primarily by action of its metabolite alpha-methyl-noradrenaline acting on alpha 2-adrenoceptors.

Clonidine inhibition of pancreatic secretion in rats: a possible central site of action

The effects of clonidine on pancreatic secretion were studied in rats fitted with chronic or acute fistulas. Subcutaneous and intracerebroventricular injections of clonidine in conscious rats induced a dose-dependent inhibition of basal pancreatic secretion involving volume, bicarbonate output and protein output with an ED50 of about 10 micrograms/kg. Clonidine inhibition of pancreatic secretion was not dependent on the associated inhibition of gastric acid output. In conscious rats, the pancreatic inhibitory effect of clonidine was completely antagonized by yohimbine and slightly by piperoxane and prazosin. Propranolol, mianserin, naloxone and cimetidine did not antagonize the clonidine effect. Clonidine decreased the basal pancreatic secretion in anaesthetized rats and this action was completely reversed by yohimbine. Clonidine inhibited the pancreatic secretion stimulated by 2-deoxyglucose. This effect was reversed by yohimbine, while prazosin had no effect. Clonidine did not inhibit the pancreatic secretion induced by electrical stimulation of the vagus nerves. These results suggest that clonidine inhibition of pancreatic secretion is mediated through alpha 2-adrenergic receptors, and at least in part by a central nervous system mechanism. Yohimbine alone increased basal pancreatic secretion in conscious rats. This suggests that alpha 2-adrenergic receptors might be involved in the physiological nerve tone to the pancreas.

Analysis of pupillary dilation produced by analogs of clonidine

Pupillary responses to intravenous administration of clonidine and five congeneric derivatives of clonidine were observed in anesthetized cats. All of the agents tested produced a dose-dependent mydriasis of long duration. The order of potency for these compounds was clonidine (St-155) greater than St.375 greater than St-606 greater than St-608 greater than St-91. Pretreatment with yohimbine hydrochloride (0.5 mg/kg i.v.) shifted the dose-response curve for all of the compounds tested to the right. A CNS inhibition of parasympathetic nerve activity was demonstrated for clonidine and St-375 by means of direct recordings from the postganglionic ciliary nerves. An effect of high doses of St-91 on nerve activity was also observed. These results suggest that all the analogs of clonidine tested (with the possible exception of St-91) act like clonidine to produce mydriasis by a central alpha-adrenergic inhibition of parasympathetic outflow to the eye.

Methyldopa produces central inhibition of parasympathetic activity in the cat

alpha-Methyldopa (10-100 mg/kg i.v.) produced a dose-dependent pupillary dilation in anaesthetized cats which was antagonized by subsequent administration of yohimbine hydrochloride (0.5 mg/kg i.v.). The peak effects were observed approximately 2-3 h after injection. This alpha-methyldopa-induced mydriasis was present only when the parasympathetic innervation to the iris was intact. Prior treatment with yohimbine (0.5 mg/kg i.v.) 30 min before alpha-methyldopa also antagonized the mydriatic effect, whereas pretreatment with phenoxybenzamine (2.5 mg/kg i.v.) did not. In contrast, phenoxybenzamine, but not yohimbine, effectively antagonized the pupillary dilation produced by adrenaline (0.3-10.0 microgram/kg i.v.). These results suggest that alpha-methyldopa produces mydriasis in the cat by means of CNS inhibition of tonic outflow from the oculomotor nucleus and that an alpha-adrenergic inhibitory mechanism may be involved. This conclusion is supported further by experiments in which direct measurements of ciliary nerve activity were made.

Effects of clonidine and chlorpromazine on a sympathetic-cholinergic reflex

Intravenous administration of clonidine and chlorpromazine resulted in a dose-dependent inhibition of the amplitude of reflexly evoked electrodermal responses in intact and spinal cats. Yohimbine pretreatment (0.5 mg/kg, i.v.) antagonized the effects of clonidine but not chlorpromazine in both preparations. These findings confirm and expand previous observations that both clonidine and chlorpromazine inhibit the amplitude of centrally evoked responses in this sympathetic-cholinergic system. In addition, both drugs appeared to have a spinal site of action. The antagonism of the effects of clonidine by yohimbine suggests that the mechanism of the action of clonidine may be a result of activation of central inhibitory alpha-adrenergic receptors. The failure of yohimbine to antagonize the effects of chlorpromazine suggests that clonidine and chlorpromazine may depress these sympathetic reflexes by different mechanisms.

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.