The effect of microelectrophoretically applied clonidine on single cerebral cortical neurones in the rat. Evidence for interaction with alpha 1-adrenoceptors

Pharmacological screening of a new alpha-2 adrenergic receptor agonist, mafedine, in zebrafish

Pharmacological agents acting at alpha-2 adrenergic receptors are widely used in physiology and neuroscience research. Mounting evidence of their potential utility in clinical and experimental psychopharmacology, necessitates new models and novel model organisms for their screening. Here, we characterize behavioral effects of mafedine (6-oxo-1-phenyl-2- (phenylamino)-1,6-dihydropyrimidine-4-sodium olate), a novel drug with alpha-2 adrenergic receptor agonistic effects, in adult zebrafish (Danio rerio) in the novel tank test of anxiety and activity. Following an acute 20-min exposure, mafedine at 60 mg/L produced a mild psychostimulant action with some anxiogenic-like effects. Repeated acute 20-min/day administration of mafedine for 7 consecutive days at 1, 5 and 10 mg/L had a similar action on fish behavior as an acute exposure to 60 mg/L. Since mafedine demonstrated robust behavioral effects in zebrafish - a sensitive vertebrate aquatic model, it is likely that it may modulate rodent and human behavior as well. Thus, further studies are needed to explore this possibility in detail, and whether it may foster clinical application of mafedine and related alpha-2 adrenergic agents.

Interaction of clonidine and labetalol in hypertension

Concomitant administration of the alpha 2-adrenoceptor agonist, clonidine, and the alpha 1- and non-selective beta-adrenoceptor antagonist, labetalol, was studied in 12 hypertensive outpatients treated with thiazide diuretics. Clonidine, 0.15 mg b.i.d., effectively lowered supine, standing and isometric blood pressure values when administered to patients on cyclothiazide, 2.5 mg q.d. Addition of labetalol, 200 mg b.i.d., to the diuretic-clonidine treatment resulted in further reduction of blood pressure at rest both in the supine and standing position. In contrast, during isometric work, addition of labetalol to the diuretic-clonidine regimen did not exhibit the same additive antihypertensive effect. When clonidine was given in a dose of 0.3 mg once daily in the evening and the other two drugs as previously, the same antihypertensive effect was observed in the afternoon. Labetalol did not provoke new side-effects of its own.

The alpha-2 antagonists idazoxan and rauwolscine but not yohimbine or piperoxan are anxiolytic in the Vogel lick-shock conflict paradigm following intravenous administration

The alpha 2 agonist clonidine has been shown to be anxiolytic in a number of preclinical anxiety models. Interestingly, intravenous infusion of the alpha 2 antagonists idazoxan at 10 mg/kg and rauwolscine at 2.24 mg/kg significantly disinhibited lick-shock conflict responding in rats similar to the alpha 2 agonist clonidine (0.022 mg/kg) and the benzodiazepine diazepam (0.5 mg/kg). However, the alpha 2 antagonists yohimbine and piperoxan, the alpha 2 agonists medetomidine, guanfacine, and guanabenz, the non-specific alpha antagonist phentolamine, and the alpha 1 antagonist prazosin did not disinhibit conflict responding in the Vogel lick-shock paradigm. In fact, yohimbine has been shown to be anxiogenic in both animals and man. This may be due to yohimbine's lack of specificity and its ability to inhibit GABAergic release. In addition, all of these agents, except idazoxan, did not increase water consumption in water deprived rats. Idazoxan (10 mg/kg) significantly decreased water consumption by 45%. Therefore, idazoxan increased conflict responding for water reward at a dose (10 mg/kg) which also decreased water consumption in a non-conflict paradigm. These data suggest that agents with selective antagonism at the alpha 2 receptor site may be anxiolytic while agents with less specificity at this site such as yohimbine, piperoxan, and phentolamine are not anxiolytic.

Modification of receptive fields of posteriomedial barrel subfield neocortical single units by known concentrations of iontophoresed noradrenaline in the rat

Using quantitative electromechanical stimulation of vibrissae poststimulus time histograms were used to map the receptive fields of single units from laminae II/III (12 units), IV (24 units) and V (12 units) of the posteriomedial barrel subfield in urethane anaesthetized rats prior to, and during the iontophoresis of known concentrations of Noradrenaline. All units had multivibrissae receptive fields prior to noradrenaline iontophoresis with laminae II/III units having the smallest size of center and surround receptive field and laminae V the largest. Lamina IV showed the strongest center receptive field response probability magnitude (spikes/stimulus) and the shortest modal latency of evoked responses. Noradrenaline iontophoresis diminished the size of surround receptive fields in each lamina and additionally decreased the size of center receptive fields in laminae IV and V. In Lamina IV, in contrast to laminae II/III and V, response probability magnitude from center receptive field vibrissae was not significantly depressed whilst modal latency of the evoked response was significantly reduced.

Effect of catecholaminergic drugs on quinolinate- and kynurenine-induced seizures in mice

Administration of reserpine, trifluperidol, chlorpromazine, haloperidol, spiroperidol, and thioproperazine to adult mice shortened the latency and increased the number of animals with clonic seizures induced by 1-kynurenine sulfate or its metabolite quinolinic acid. Haloperidol dose-dependently intensified kynurenine-induced seizures and did not alter pentylenetetrazole seizures. Dopamine abolished the effect of haloperidol while serotonin was ineffective. Pretreatment with 6-hydroxydopamine potentiated kynurenine-induced seizures, but not quinolinic acid-induced seizures. The seizure thresholds of kynurenine and quinolinic acid were not affected by pretreatments with yohimbine, clonidine, piperoxan, phentolamine and tricyclic antidepressants. Apomorphine and amphetamine (i.p.), noradrenaline and adrenaline (i.c.v.) possess anticonvulsant action against kynurenine and not against quinolinic acid. The data obtained suggest a similarity of kynurenine and known convulsants in the involvement of the catecholaminergic processes in their convulsant action. Quinolinic acid markedly differs from kynurenine in its mechanism of action as indicated by their interactions with numerous endogenous substances.

Biphasic effects of clonidine on conflict behavior: involvement of different alpha-adrenoceptors

The effect of the alpha-2-adrenoceptor agonist clonidine on anxiety-related behavior was investigated using two different rat anxiety models: a modified Vogel's drinking conflict model and Montgomery's elevated plus-maze. In both models biphasic dose-response curves were obtained; in a narrow low-dose range (6.25-10.0 micrograms/kg) the drug produced anxiolytic-like effects, while anxiogenic-like properties were found after higher doses (12.5-80.0 micrograms/kg). Attempts to block the effects obtained were made in Montgomery's elevated plus-maze. The specific alpha-2-adrenoceptor antagonist idazoxan blocked the anxiolytic-like effect but did not influence the anxiogenic-like activity. Conversely, the specific alpha-1-adrenoceptor antagonist prazosin blocked the anxiogenic-like effect but did not alter the anxiolytic-like activity. These findings may suggest that alpha-1- and alpha-2-adrenergic receptor mechanisms are reciprocally involved in anxiety-related behavior.

Advantages and disadvantages of alpha 2-adrenoceptor agonists for systemic hypertension

Alpha 2-receptor agonists are effective antihypertensive drugs that reduce sympathetic activity by both central and peripheral mechanisms. Plasma noradrenaline and renin concentrations are reduced. Hemodynamic disturbances are minor with a near-normal cardiovascular response to standing, exercise and volume depletion. The main problems are due to central adverse effects, particularly sedation, and a syndrome of sympathetic overactivity on sudden withdrawal. These problems can be minimized by using lower doses, while therapy with a useful antihypertensive drug is maintained. Clonidine and its analogs have a low toxicity and very few contraindications. They remain a useful part of the therapeutic armamentarium in hypertension.

Effects of continuous infusions (10 days) and cessation of infusions of clonidine and rilmenidine (S 3341) on cardiovascular and behavioral parameters of spontaneously hypertensive rats

Clonidine is a centrally acting antihypertensive drug that acts in vivo at both alpha 1- and alpha 2-adrenoceptor sites, whereas rilmenidine (S 3341) is more selective for alpha 2 adrenoceptors. The present study compared the effects of continuous 10-day infusions of clonidine (5 micrograms/kg/hour) with those of rilmenidine (100 micrograms/kg/hour) on various cardiovascular and behavioral parameters in the spontaneously hypertensive rat. The changes in these parameters after cessation of the infusions were also compared. At these rates of infusion, clonidine and rilmenidine produced quantitatively similar reductions in mean arterial pressure (MAP), lability of MAP and cardiovascular responsiveness during normal behaviors such as eating and grooming. Neither drug infusion affected heart rate. The cessation of the clonidine infusion resulted in a "withdrawal" syndrome characterized by prominent rapid eye movement-sleep rebound, and cardiovascular and behavioral disturbances including an increased lability of MAP, exaggerated cardiovascular responses during normal behaviors, tachycardia, and an "opiate abstinence-like" syndrome including head and body shakes. Cessation of rilmenidine infusion resulted in somewhat similar cardiovascular and behavioral disturbances, but unlike clonidine there was a return to normal rapid eye movement sleep without rebound.

Differential effects of norepinephrine on hippocampal complex-spike and theta-neurons

The central noradrenergic system has long been postulated to modulate learning and memory. A brain structure known to be important in these functions is the hippocampus. Since the hippocampus receives a noradrenergic projection from the locus coeruleus, knowledge of norepinephrine's actions in the hippocampus may help determine its role in learning and memory. In the present study, the effects of norepinephrine were examined on two hippocampal cell types: complex-spike and theta-neurons. In the hippocampus, there is good evidence that complex-spike cells are pyramidal neurons, while theta-neurons are interneurons. Extracellular action potentials from hippocampal neurons were recorded using multibarrel glass micropipettes. Drugs were locally applied using pressure micro-ejection. Norepinephrine inhibited the spontaneous firing of complex-spike cells, while theta-neurons were excited. The inhibitory response of complex-spike neurons was mediated by an alpha 1-receptor. However, selective agonists for the alpha 2- and beta-noradrenergic receptors excited the complex-spike cells. The noradrenergic-induced excitatory response of theta-neurons was also mediated by alpha 2- and beta-receptors. This study provides evidence that locally applied norepinephrine produces different responses on two types of hippocampal neurons. Furthermore, these differential responses arise primarily from the activation of distinct populations of noradrenergic receptors.

Pharmacological characterization of the receptor mediating the adrenergic inhibition of responses to substance P in the cingulate cortex

The excitatory responses of neurones in the anterior cingulate cortex of the rat to iontophoretically applied substance P (SP) are reduced by noradrenaline (NA) applied iontophoretically or released from noradrenergic pathways. In order to determine the receptor involved in this inhibitory effect we have studied the effects of a number of receptor-specific adrenergic agonists and antagonists on responses of cingulate neurones to SP in rats anaesthetized with chloral hydrate. Low iontophoretic currents (0-15 nA) of NA, adrenaline and the beta-agonist, clenbuterol, all strongly reduced responses to SP. Isoprenaline was also effective but less consistently so, although problems were experienced with its iontophoretic release from micropipettes. The alpha 1-agonists, phenylephrine and methoxamine were also able to reduce responses to SP. However, this reduction required higher iontophoretic currents (15-60 nA) and was associated with depressant effects on baseline firing rate. The alpha 2-agonist clonidine was only weakly active at high currents and this too was associated with depression of baseline firing. Similar weak effects were noted with dopamine. The inhibitory effects of NA on SP responses were convincingly blocked or reversed by the beta-antagonist, practolol, but not by the alpha 1-antagonist, prazosin. The reduction of SP responses by phenylephrine was also blocked by practolol but unaffected by prazosin. Finally, reduction of SP excitations by activation of the coeruleocortical pathway was also blocked by practolol applied iontophoretically to the cortical cells. These results are consistent with the hypothesis that the effect of NA on SP responsiveness in the cingulate cortex is mediated by beta-adrenoreceptors.

Dual effects of the snake venom polypeptide vipoxin on receptors for acetylcholine and biogenic amines in Aplysia neurons

Vipoxin, a 13,000-dalton polypeptide component of Russell's viper venom, has a dual pattern of effects on the responses of voltage-clamped Aplysia neurons to acetylcholine and biogenic amines. Application of low doses of vipoxin by pressure ejection reversibly antagonized all three types of ionic response to acetylcholine and carbachol. The blockade by vipoxin of acetylcholine responses was not prevented by eserine. The order of susceptibility of acetylcholine responses to blockade by vipoxin was Na+ greater than K+ greater than Cl-. Low doses of vipoxin also produced a reversible potentiation of excitatory responses to dopamine with a slower time course of onset and recovery. Inhibitory responses to dopamine (Cl-, K+) and both excitatory and inhibitory responses to histamine and 5-hydroxytryptamine were little affected by vipoxin. Higher doses of vipoxin directly evoked current responses which were always of the same ionic type as that evoked by acetylcholine or carbachol. Responses to cholinergic agonists and vipoxin were both blocked by cholinergic antagonists but not by antagonists of biogenic amine receptors, which reversibly antagonized the responses to amines on the same cell. These results suggest that vipoxin, which has no demonstrated actions on vertebrate acetylcholine receptors, acts as a partial agonist at all three types of acetylcholine receptor in Aplysia neurons. Our observations thus provide evidence for some degree of phylogenetic difference between vertebrate and molluscan acetylcholine receptors.

Effects of alpha-adrenoceptor agonists and antagonists in a maze-exploration model of 'fear'-motivated behaviour

An elevated X-maze with alternating open and enclosed arms was investigated as a model for the study of fear-induced behaviour. As predicted, the anxiolytics diazepam and amylobarbitone increased, and the putative anxiogenics ACTH and picrotoxin decreased the proportion of open arm entries. The alpha 1-adrenoceptor agonists phenylephrine and ST587, and the alpha 2-adrenoceptor antagonists idazoxan, piperoxane, RS-21361 and yohimbine decreased relative open-arm entries, thus resembling the putative anxiogenics. On the other hand, azepexole, clonidine and guanabenz, agonists at alpha 2-adrenoceptors, and the alpha 1-adrenoceptor antagonists prazosin and thymoxamine, enhanced the proportion of open arm entries at low doses, suggesting anxiolytic-like properties. A paradoxical fall in open arm entries occurred with these agents at higher doses. These results provide further evidence for the involvement of noradrenergic systems in 'fear'-motivated behaviour.

Neuronal responses to noradrenaline in the cerebral cortex: evidence against the involvement of alpha 2-adrenoceptors

The technique of microelectrophoresis was used to test the hypothesis that alpha 2-adrenoceptors are involved in mediating the excitatory responses of single neurones to noradrenaline in the somatosensory cerebral cortex of the rat. In the first series of experiments the effects of two alpha 2-adrenoceptor antagonists, yohimbine and idazoxan (RX-781094), were compared on excitatory responses to noradrenaline, phenylephrine and acetylcholine. The response to noradrenaline was not more susceptible to antagonism by these drugs than the response to the alpha 1-adrenoceptor stimulant, phenylephrine. Yohimbine antagonized responses to all three agonists equally, while idazoxan antagonized responses to noradrenaline and phenylephrine equally with relative preservation of responses to acetylcholine. In the second series of experiments the effects of the selective alpha 2-adrenoceptor stimulant, UK-14304, were examined. UK-14304 produced weak and inconsistent excitations on a small number of cells; however, most of the cells did not respond to this drug. When applied continuously using low ejection currents, UK-14304 selectively and reversibly antagonized responses to noradrenaline and phenylephrine without affecting responses to acetylcholine. These results suggest that, in the somatosensory cortex of the rat, neuronal excitation to noradrenaline is unlikely to be mediated either wholly or partly by alpha 2-adrenoceptors. The antagonism of neuronal responses to noradrenaline and phenylephrine by idazoxan probably reflects the alpha 1-adrenoceptor antagonistic properties of the drug which is known to occur at higher concentrations. The low agonistic potency of UK-14304 and the antagonism of responses to noradrenaline and phenylephrine by UK-14304 suggest that this drug, like clonidine, may act as a partial agonist at alpha-adrenoceptors.

A regional difference in the distribution of postsynaptic alpha-adrenoceptor subtypes in canine veins

The responsiveness of helical venous strips isolated from fifteen different sites in the body of dogs to relatively selective alpha 1- and alpha 2-adrenoceptor agonists was studied, as well as to a non-selective alpha-adrenoceptor agonist. Longitudinal strips of the portal and mesenteric veins and the inferior vena cava between the liver and the renal vein (segment C) were also investigated. All veins contracted to noradrenaline or phenylephrine whereas only seven veins responded significantly to clonidine: the saphenous, cephalic, jugular and femoral veins and longitudinal strips of the portal and mesenteric veins and the segment C of the inferior vena cava. The brachiocephalic, azygos, pulmonary and splenic veins and the superior vena cava and the supradiaphragmatic portion (segment A) and the infrarenal portion (segment D) of the inferior vena cava responded little to clonidine. Unlike the longitudinal strips, the helical strips of the portal and mesenteric veins and the segment C of the inferior vena cava did not respond to clonidine. According to the relative sensitivities to phenylephrine and clonidine, those veins which responded to clonidine could be divided into three groups. (1) The veins in which the sensitivity to phenylephrine was higher than to clonidine: longitudinal strips of the portal vein and segment C of the inferior vena cava, (2) the veins whose sensitivity to phenylephrine was lower than to clonidine: the saphenous, cephalic, femoral and external jugular veins, (3) the vein whose sensitivity to the two agonists was comparable: longitudinal strips of the mesenteric vein. Subtype characteristics were further analyzed in the saphenous vein and in the portal vein using prazosin, phentolamine and yohimbine as antagonists. Analysis of Schild plots to noradrenaline suggested that a mixed population of alpha-adrenoceptor subtypes might be present in the saphenous vein, whereas a rather homogeneous population of a single subtype might occur in the portal vein. The results of the antagonism experiment against phenylephrine and clonidine suggested that contractions of the saphenous vein are mediated by both alpha 1- and alpha 2-adrenoceptors whereas contractions of the portal vein are exerted mainly through alpha 1-adrenoceptors. The results suggest that there may be a distinct regional difference with respect to postsynaptic alpha- adrenoceptor subtypes in the canine venous system.