Selective blockade of brain alpha 2-autoreceptors by yohimbine: effects on motor activity and on turnover of noradrenaline and dopamine

The anxiogenic drug yohimbine is a reinforcer in male and female rats

The indole alkaloid yohimbine is an anxiogenic drug that activates stress-responsive systems in the brain. However, because yohimbine also elicits approach behaviors, this study employed male and female Sprague-Dawley rats to explore its potential reinforcing effects. Thus, it was first determined if intravenous (IV) infusions of yohimbine (0.25 mg/kg/infusion) could maintain lever pressing, whether intake could be modulated by dose/infusion, and if lever pressing would persist in the absence of yohimbine or yohimbine-paired cues. Next, to assess yohimbine's effect on memory consolidation, 0.3, 1.25 or 3 mg/kg yohimbine was administered post-training using an object recognition memory task. Finally, place conditioning assessed whether doses of yohimbine that elevate blood serum corticosterone levels (1.25 or 3 mg/kg) could elicit a conditioned place preference. It was found that both sexes acquired yohimbine IV self-administration, that intake was modulated by dose/infusion, and that lever pressing persisted during extinction and in the absence of the yohimbine-paired cue. As well, post-training injections of 1.25 mg/kg yohimbine enhanced consolidation of object memory, and 1.25 and 3 mg/kg elevated corticosterone levels and elicited a place preference in both sexes. Finally, in behavioral tests of psychomotor functions, acute yohimbine increased lever pressing for a visual cue and elevated locomotor activity. These findings reveal a profile of yohimbine's behavioral effects that is consistent with that of psychostimulant reinforcing drugs.

Modulation of Spatial Memory Deficit and Hyperactivity in Dopamine Transporter Knockout Rats via α2A-Adrenoceptors

Attention deficit hyperactivity disorder (ADHD) is manifested by a specific set of behavioral deficits such as hyperactivity, impulsivity, and inattention. The dopamine neurotransmitter system is postulated to be involved in the pathogenesis of ADHD. Guanfacine, a selective α2A-adrenoceptor agonist, is prescribed for ADHD treatment. ADHD also is known to be associated with impairment of multiple aspects of cognition, including spatial memory, however, it remains unclear how modulation of the norepinephrine system can affect these deficits. Hyperdopaminergic dopamine transporter knockout (DAT-KO) rats are a valuable model for investigating ADHD. The DAT-KO rats are hyperactive and deficient in spatial working memory. This work aimed to evaluate the effects of noradrenergic drugs on the fulfillment of spatial cognitive tasks by DAT-KO rats. The rats were tested in the Hebb - Williams maze during training and following noradrenergic drugs administration. The efficiency of spatial orientation was assessed as to how fast the animal finds an optimal way to the goal box. Testing in a new maze configuration allowed us to evaluate the effects of drug administration after the acquisition of the task rules. The behavioral variables such as the distance traveled, the time to reach the goal box, and the time spent in the error zones were analyzed. It has been observed that α2A-adrenoceptor agonist Guanfacine (0.25 mg/kg) had only a minimal inhibitory effect on hyperactivity of DAT-KO rats in the maze but significantly ameliorated their perseverative pattern of activity and reduced the time spent in the error zones. In contrast, α2A-adrenoceptor antagonist Yohimbine, at the dose of 1 mg/kg, increased the distance traveled by DAT-KO rats and elevated the number of perseverative reactions and the time spent in the error zones. Guanfacine caused minimal effects in wild-type rats, while Yohimbine altered several parameters reflecting a detrimental effect on the performance in the maze. These data indicate that modulation of α2A-adrenoceptor activity potently affects both dopamine-dependent hyperactivity and cognitive dysfunctions. Similar mechanisms may be involved in the beneficial effects of Guanfacine on cognitive deficits in ADHD patients. This study further supports the translational potential of DAT-KO rats for testing new pharmacological drugs.

Bidirectional pharmacological perturbations of the noradrenergic system differentially affect tactile detection

The brain neuromodulatory systems heavily influence behavioral and cognitive processes. Previous work has shown that norepinephrine (NE), a classic neuromodulator mainly derived from the locus coeruleus (LC), enhances neuronal responses to sensory stimuli. However, the role of the LC-NE system in modulating perceptual task performance is not well understood. In addition, systemic perturbation of NE signaling has often been proposed to specifically target the LC in functional studies, yet the assumption that localized (specific) and systemic (nonspecific) perturbations of LC-NE have the same behavioral impact remains largely untested. In this study, we trained mice to perform a head-fixed, quantitative tactile detection task, and administered an α2 adrenergic receptor agonist or antagonist to pharmacologically down- or up-regulate LC-NE activity, respectively. We addressed the outstanding question of how bidirectional perturbations of LC-NE activity affect tactile detection, and tested whether localized and systemic drug treatments exert the same behavioral effects. We found that both localized and systemic suppression of LC-NE impaired tactile detection by reducing motivation. Surprisingly, while locally activating LC-NE enabled mice to perform in a near-optimal regime, systemic activation impaired behavior by promoting impulsivity. Our results demonstrate that localized silencing and activation of LC-NE differentially affect tactile detection, and that localized and systemic NE activation induce distinct behavioral changes.

Noradrenergic versus dopaminergic modulation of impulsivity, attention and monitoring behaviour in rats performing the stop-signal task: possible relevance to ADHD

RATIONALE

Deficient response inhibition is a prominent feature of many pathological conditions characterised by impulsive and compulsive behaviour. Clinically effective doses of catecholamine reuptake inhibitors are able to improve such inhibitory deficits as measured by the stop-signal task (SST) in humans and other animals. However, the precise therapeutic mode of action of these compounds in terms of their relative effects on dopamine (DA) and noradrenaline (NA) systems in prefrontal cortical and striatal regions mediating attention and cognitive control remains unclear.

OBJECTIVES

We sought to fractionate the effects of global catecholaminergic manipulations on SST performance by using receptor-specific compounds for NA or DA. The results are described in terms of the effects of modulating specific receptor subtypes on various behavioural measures such as response inhibition, perseveration, sustained attention, error monitoring and motivation.

RESULTS

Blockade of α2-adrenoceptors improved sustained attention and response inhibition, whereas α1 and β1/2 adrenergic receptor antagonists disrupted go performance and sustained attention, respectively. No relevant effects were obtained after targeting DA D1, D2 or D4 receptors, while both a D3 receptor agonist and antagonist improved post-error slowing and compulsive nose-poke behaviour, though generally impairing other task measures.

CONCLUSIONS

Our results suggest that the use of specific pharmacological agents targeting α2 and β noradrenergic receptors may improve existing treatments for attentional deficits and impulsivity, whereas DA D3 receptors may modulate error monitoring and perseverative behaviour.

Catecholamine synthesis and metabolism in the central nervous system of mice lacking alpha-adrenoceptor subtypes

BACKGROUND AND PURPOSE

This study investigates the role of alpha(2)-adrenoceptor subtypes, alpha(2A), alpha(2B) and alpha(2C), on catecholamine synthesis and catabolism in the central nervous system of mice.

EXPERIMENTAL APPROACH

Activities of the main catecholamine synthetic and catabolic enzymes were determined in whole brains obtained from alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptor knockout (KO) and C56Bl\7 wild-type (WT) mice.

KEY RESULTS

Although no significant differences were found in tyrosine hydroxylase activity and expression, brain tissue levels of 3,4-dihydroxyphenylalanine were threefold higher in alpha(2A)- and alpha(2C)-adrenoceptor KO mice. Brain tissue levels of dopamine and noradrenaline were significantly higher in alpha(2A) and alpha(2C)KOs compared with WT [WT: 2.8 +/- 0.5, 1.1 +/- 0.1; alpha(2A)KO: 6.9 +/- 0.7, 1.9 +/- 0.1; alpha(2B)KO: 2.3 +/- 0.2, 1.0 +/- 0.1; alpha(2C)KO: 4.6 +/- 0.8, 1.5 +/- 0.2 nmol.(g tissue)(-1), for dopamine and noradrenaline respectively]. Aromatic L-amino acid decarboxylase activity was significantly higher in alpha(2A) and alpha(2C)KO [WT: 40 +/- 1; alpha(2A): 77 +/- 2; alpha(2B): 40 +/- 1; alpha(2C): 50 +/- 1, maximum velocity (V(max)) in nmol.(mg protein)(-1).h(-1)], but no significant differences were found in dopamine beta-hydroxylase. Of the catabolic enzymes, catechol-O-methyltransferase enzyme activity was significantly higher in all three alpha(2)KO mice [WT: 2.0 +/- 0.0; alpha(2A): 2.4 +/- 0.1; alpha(2B): 2.2 +/- 0.0; alpha(2C): 2.2 +/- 0.0 nmol.(mg protein)(-1).h(-1)], but no significant differences were found in monoamine oxidase activity between all alpha(2)KOs and WT mice.

CONCLUSIONS AND IMPLICATIONS

In mouse brain, deletion of alpha(2A)- or alpha(2C)-adrenoceptors increased cerebral aromatic L-amino acid decarboxylase activity and catecholamine tissue levels. Deletion of any alpha(2)-adrenoceptor subtypes resulted in increased activity of catechol-O-methyltransferase. Higher 3,4-dihydroxyphenylalanine tissue levels in alpha(2A) and alpha(2C)KO mice could be explained by increased 3,4-dihydroxyphenylalanine transport.

The influence of modifications in imide fragment structure on 5-HT1A and 5-HT7 receptor affinity and in vivo pharmacological properties of some new 1-(m-trifluoromethylphenyl)piperazines

New, flexible (7, 9, 11 and 13) and rigid (8, 10, 12 and 14) imides with a 1-(m-trifluorophenyl)piperazine fragment and a tetramethylene or a 1e,4e-cyclohexylene spacer, respectively, showed very high affinity (K(i)=0.3-34 nM) and agonistic in vivo activity for 5-HT(1A) receptors. Flexible new compounds and the previously described 5 also bound to 5-HT(7) receptors (K(i)=21-134 nM). Selected glutarimide derivatives, that is, the most potent postsynaptic 5-HT(1A) receptor agonist rigid compound 8 and its flexible analogue 7, as well as the previously described full agonist-rigid compound 6 and the partial agonist-its flexible counterpart 5 exhibited moderate affinity for alpha(1)-adrenoceptors (K(i)=85 - 268 nM), but were practically devoid of any affinity for dopamine D(2) sites. Those glutarimides demonstrated anxiolytic- (5 and 7) and antidepressant-like (5, 6 and 8) activity in the four-plate and the swim tests in mice, respectively; at the same time, however, they inhibited the locomotor activity of mice. The antidepressant-like effect of 8 was significantly stronger than that induced by imipramine used as a reference antidepressant.

Alpha(1)-adrenergic and alpha(2)-adrenergic balance in the dorsal pons and gross behavioral activity of mice in a novel environment

RATIONALE

Central alpha(1)- and alpha(2)-adrenoceptors in a number of different brain regions are known to have opposing actions on gross behavioral activity, with the former stimulating and the latter inhibiting activity. Therefore, blockade of alpha(1)-receptors may induce inactivity by leading to unopposed alpha(2) activity.

OBJECTIVE

The aim of this study was to test if central blockade of alpha(2)-receptor function restores behavioral activity in alpha(1)-receptor-blocked mice.

METHODS

Dose-response studies were undertaken on the effects of alpha(1)- and alpha(2)-agonists and antagonists microinjected into the dorsal pons on gross behavioral activity in a novel cage test.

RESULTS

The behavioral inactivity resulting from blockade of alpha(1)-receptors in the pons with the antagonist, terazosin, was reversed by either a low dose of an alpha(2)-antagonist, atipamezole, or a low dose of an alpha(2)-agonist, dexmedetomidine, but was exacerbated by a high dose of the alpha(2)-agonist.

CONCLUSION

The results support the hypothesis that blockade of alpha(1)-receptors in the dorsal pons of mice produces inactivity by causing unopposed activity of alpha(2)-receptors. This condition may be relevant to inactive states seen after stress or during depressive illness.

Rate-dependent behavioral effects of stimulation of central motoric alpha(1)-adrenoceptors: hypothesized relation to depolarization blockade

AIM

The purpose of this review is to clarify how central alpha(1)-adrenoceptors control behavioral activity under varying conditions of activity and stress.

METHOD

The literature is reviewed regarding the behavioral actions of alpha(1)-agonists and antagonists, and alpha(2)-agonists and antagonists under conditions of high and low baseline activity and stress.

RESULTS

It was found that alpha(1)-receptor stimulation of active behavior has a number of similarities to rate dependency including: (1) a dependence on low-active, low-stress conditions or on the prior depletion of endogenous brain catecholamines; (2) a nonmonotonic dose-response relationship with high doses producing a fall-off or actual depression of activity; (3) a failure to be blocked at high agonist doses by alpha(1)-antagonists; and (4) a facilitation by alpha(2)-adrenoceptor agonists which produce an opposing hyperpolarization.

DISCUSSION

To explain these findings, it is proposed that high levels of stimulation of central alpha(1)-receptors produce, in host neurons, a depolarization block that impedes nerve impulse generation and inhibits active behavior. This effect is assumed to be precluded or mitigated by low-active, low-stress conditions, depletion of brain catecholamines, and by hyperpolarizing alpha(2)-agonists, and to be reversed at high agonist doses by alpha(1)-antagonists.

CONCLUSION

Because brain alpha(1)-receptors are not only involved in motor activity but also in the mechanism of action of antidepressant and stimulant drugs, arousal, anxiety, stress and psychosis, a depolarization block from intense stimulation of these receptors could have broad psychopharmacological consequences and underlie rate dependency to a variety of stimulant drugs.

Effects of antisense oligodeoxynucleotide to the alpha2A-adrenoceptors on the plasma corticosterone level and on elevated plus-maze behavior in rats

Antisense strategy was used to investigate the role of alpha2A-adrenoceptor (alpha2A-AR) subtype in anxiety-related behavior. A 18-mer phosphorothioate oligodeoxynucleotide (AS-ODN) complementary to the alpha2A-AR mRNA was administered to the adult male rats for 3 days (1 nmol/5 microl/day) into the region of locus coeruleus (LC). Control groups received infusions of either oligodeoxynucleotide of a random sequence (RS-ODN) or saline. Treatment with AS-ODN significantly reduced the levels of alpha2A-AR mRNA in the brain stem. At the same time, AS-ODN treatment caused only a small reduction in [(3)H]clonidine binding (by 26-32%) in the brain stem which was not significant. Compared to both RS-ODN and saline controls, treatment with AS-ODN significantly increased the percentage of open arm entries in the elevated plus-maze while the total number of arm entries was unaltered. Also, AS-ODN treatment elevated basal levels of plasma corticosterone by 217% and 96% compared to both RS-ODN and saline controls. These changes in the hormone concentrations were at a level of marginal significance (p<0.1 versus random group). Taken together, the data indicate that administration of AS-ODN against alpha2A-ARs in the LC significantly reduced expression of alpha2A-AR mRNA in brain stem, moderately increased plasma corticosterone and had anxiolytic-like effect in the elevated plus-maze.

Genetic differences in the synthesis and reception of noradrenaline in the mouse brain and behavior in a novel environment

The activity of tyrosine hydroxylase, the key enzyme in catecholamine biosynthesis, was studied along with adrenoceptor density in the brains of male CBA/Lac, BALB/cLac, and C57BL/6J mice, which show different responses to novel environments. C57BL mice showed the highest level of movement activity and the lowest level of emotionality in a novel environment. Mice of this line also showed the highest brainstem tyrosine hydroxylase activity. At the same time, the density of beta-adrenoceptors in the cortex and hypothalamus of C57BL mice was lower than in the other two lines of mice, while the density of alpha2-adrenoceptors in these parts of the brain was lower than in CBA mice. In BALB mice, movement activity was twice as high as in CBA mice, while levels of emotionality were similar in these two lines. Tyrosine hydroxylase activity was higher in the cerebral cortex of BALB mice, while the density of alpha2-adrenoceptors was lower than in CBA mice. These results show that increased investigative activity and decreased emotionality were seen in animals with higher levels of noradrenaline synthesis and decreased density of adrenergic receptors in the brain.

Brain alpha 1-adrenergic neurotransmission is necessary for behavioral activation to environmental change in mice

Terazosin, a water-soluble alpha 1 antagonist that can be administered in high doses intraventricularly was used to study the relationship between brain alpha 1 adrenoceptor neurotransmission and behavioral activation in the mouse. The antagonist was found to produce a dose-dependent, complete inhibition of motor activity and catalepsy which were reversed preferentially by coinfusion of an alpha 1 agonist (phenylephrine) compared to a D1 (SKF38393) or a D2 agonist, (quinpirole). Blockade of central beta-1 (betaxolol), alpha-2 (RX821002) or beta-2 (ICI 118551) adrenoceptors had smaller or non-significant effects. Terazosin's selectivity for alpha 1 receptors versus dopaminergic receptors was verified under the present conditions by showing that the intraventricularly administered antagonist protected striatal and cerebral cortical alpha 1 receptors but not striatal or cortical D1 receptors from in vivo alkylation by N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroxyquinoline. That its effect was due to blockade of brain rather than peripheral receptors was shown by the finding that intraperitoneal doses of terazosin three to 66 times greater than the maximal intraventricular dose produced less behavioral inhibition. Intraventricular terazosin also produced hypothermia and a reduced respiratory rate suggestive of a reduced sympathetic outflow. However, external heat did not affect the inactivity, and captopril, a hypotensive agent, did not mimic it. Terazosin did not impair performance on a horizontal wire test or the ability to make co-ordinated movements in a swim test suggesting that its activity-reducing effect was not due to sedation and may have a motivational or sensory gating component. It is concluded that central alpha 1-noradrenergic neurotransmission is required for behavioral activation to environmental change in the mouse and may operate on sensorimotor and motivational processes.

Antinociception induced by amitriptyline and imipramine is mediated by alpha2A-adrenoceptors

The involvement of alpha2-adrenoceptors in the antinociception induced by the tricyclic antidepressants amitriptyline and imipramine was investigated in mice by using the hot-plate and abdominal constriction tests. The antinociception produced by amitriptyline (15 mg/kg, i.p.) and imipramine (15 mg/kg, i.p.) was prevented by reserpine (2 mg/kg, i.p.) and yohimbine (3-10 mg/kg, i.p.) but not by naloxone (1 mg/kg, i.p.), atropine (5 mg/kg, i.p.), CGP 35348 (100 mg/kg, i.p.) and prazosin (1 mg/kg, i.p.). On the basis of the above data, it can be postulated that amitriptyline and imipramine exerted their antinociceptive effect by activation of alpha2-adrenoceptors. Administration of the alpha2A-adrenoceptor antagonist BRL 44408 (1 mg/kg, i.p.) prevented amitriptyline and imipramine antinociception, whereas the alpha2B/C-adrenoceptor antagonist ARC 239 (10 mg/kg, i.p.) was ineffective. These data indicate that the enhancement of the pain threshold produced by amitriptyline and imipramine is mediated by activation of alpha2A-adrenoceptors. Neither tricyclic antidepressants nor the antagonists used impaired mouse performance evaluated by the rota-rod and hole-board tests.

Yohimbine interacts with the dopaminergic system to reverse sexual satiation: further evidence for a role of sexual motivation in sexual exhaustion

The possible interaction of yohimbine with the dopaminergic system in the mediation of sexual behaviour expression in sexually exhausted male rats was investigated. The behavioural effects of the simultaneous injection of yohimbine (500 microg/kg) plus apomorphine (50 microg/kg) and those of the combined treatment of haloperidol (125 microg), a nonspecific dopamine receptor antagonist, with an effective dose of yohimbine (2000 microg/kg) on sexually satiated rats were evaluated. Data show that yohimbine and apomorphine, per se, dose-dependently reverse sexual exhaustion by increasing the percentage of sexually satiated rats copulating and resuming copulation after ejaculation. Injection of haloperidol simultaneous to an effective dose of yohimbine, blocked the ability of the latter to reverse sexual satiation. The combined treatment with subthreshold doses of apomorphine and yohimbine synergised to reverse the sexual inhibition characteristic of sexual exhaustion. Data suggest that the dopaminergic system might be the final pathway for the yohimbine-induced sexual behaviour expression in satiated rats. The possible role of sexual motivation in the sexual exhaustion phenomenon is discussed.

The role of adrenoreceptors in control of stereotyped oral behavior in restricted-fed fowls

Effects on environmentally induced oral stereotypies (object pecking and drinker-directed activity) of preferential antagonists and agonists of adrenoreceptor subtypes were examined in individually caged broiler breeder fowls subjected to chronic food restriction. Three drugs in each category were injected intravenously at three doses, and their effects compared with a saline control treatment. With the antagonists, object pecking was suppressed more by prazosin (alpha 1) and propranolol (beta) than by yohimbine (alpha 2), while drinker-directed activity showed delayed stimulation with yohimbine and propranolol. With the agonists, drinker-directed activity was suppressed more by clonidine (alpha 2) than by isoproterenol (beta) and phenylephrine (alpha 1), while object pecking was inhibited by the high doses of clonidine and isoproterenol but showed delayed stimulation with the low dose of clonidine and (nonsignificantly) the high dose of phenylephrine. Initial suppression of both oral stereotypies by the high doses of yohimbine and isoproterenol, and high and medium doses of clonidine, may have been due to sedation, because in those instances it coincided with increased sitting, an activity not normally seen. Increased standing with clonidine and the medium dose of yohimbine may also reflect sedation. When there were no significant increases in sitting or standing to indicate sedation, responses of both stereotypies were essentially the same with all three adrenoreceptor subtypes; i.e., object pecking was inhibited by the antagonist but not the agonist, while drinker-directed activity was inhibited by the agonist but not the antagonist. It is concluded that alpha 1, alpha 2, and beta adrenoreceptors are all implicated in expression of these stereotypes, and that the two activities may be differentially controlled.

Noradrenergic alpha-1 and alpha-2 antagonists block L-dopa-induced air-stepping in neonatal rats

Five-day-old rat pups suspended in air and administered L-3,4-dihydroxyphenylalanine (L-DOPA) engage in a highly stereotyped and coordinated locomotor behavior termed air-stepping. L-DOPA is a precursor for dopamine and noradrenaline and one or both of these neurotransmitters could play a role in L-DOPA-induced air-stepping. The role of noradrenaline was investigated by assessing the abilities of the alpha-1 noradrenergic receptor antagonist prazosin and the alpha-2 noradrenergic receptor antagonist idazoxan to block L-DOPA-induced air-stepping in 5-day-old rats. Both antagonists decreased the duration of air-stepping. In addition, prazosin altered the topography of air-stepping by interfering with coordination of the hindlimbs. The results suggest that alpha-1 and alpha-2 noradrenergic receptor subtypes are involved in L-DOPA-induced air-stepping.

Influence of idazoxan on the dopamine D2 receptor agonist-induced behavioural effects in rats

The behavioural effects in rats of the dopamine D2 receptor agonists, lisuride, B-HT 920 and SND 919, were variously influenced by pre-treatment with the selective alpha 2-adrenoceptor antagonist, idazoxan (2 mg/kg), depending on the nature of the effect in question and the doses of agonist employed. The influence of idazoxan on drug-induced stretching-yawning, penile erection, sedation, stereotyped behaviour, aggressiveness and mounting is described and tentatively interpreted in neurochemical terms, account being taken of the activity of respective alpha 2-adrenoceptor antagonist and dopamine receptor agonists used, at alpha 2-adrenoceptors and at different dopamine D2 receptor subtypes, pre- and postsynaptically located.

Effect of locus coeruleus lesion on c-fos expression in the cerebral cortex caused by yohimbine injection or stress

The injection of the alpha-2 adrenoceptor antagonist, yohimbine, has been shown to increase c-fos immunoreactivity in the rat cerebral cortex. To determine the extent to which this response is mediated by the central noradrenergic system, the present studies examined it in rats previously given unilateral 6-OHDA lesions of the locus coeruleus. The lesions were found to produce a significant attenuation of the response. A similar effect on the c-fos immunoreactive response to restraint stress was found. It is concluded that the noradrenergic system plays a necessary role in the above c-fos responses in the cortex to yohimbine and to stress. The c-fos protein therefore appears to be involved in the effects of noradrenergic neurotransmission in the CNS.

Suppression of central noradrenergic neuronal activity inhibits hyperglycemia

Hypothalamic noradrenergic neuronal activity (NNA) and hepatic glucose output are stimulated by stress. The aim of the present investigation was to examine whether the blockade of noradrenergic responses to stress might suppress the associated hyperglycemia. Mass spectrometry was used for analysis of norepinephrine (NE) and its neuronal metabolite 3,4-dihydroxyphenylglycol (DHPG) in rat hypothalamus, and the ratio DHPG/NE was used as an index of NNA. Treatment of rats with 2-deoxy-D-glucose (500 mg/kg ip, -30 min), yohimbine (10 mg/kg ip, -20 min), or neostigmine (2 micrograms icv, -60 min) increased both NNA and serum glucose (P < 0.05). When rats were additionally pretreated with pentobarbital (60 mg/kg ip; -60 min), the NNA responses were blocked (P < 0.01). At the same time the hyperglycemic responses were also inhibited (P < 0.01). In rats that had reduced NNA due to 7 days "gentling," serum glucose levels were also significantly reduced (P < 0.001) compared with naive controls. The data demonstrate that inhibition of central noradrenergic activity is also associated with an inhibition of hyperglycemia, raising the concept that therapies aimed at reducing central NNA may have a role in the management of diseases with excessive hepatic glucose output such as non-insulin-dependent diabetes mellitus.

A role for central postsynaptic alpha 2-adrenoceptors in glucoregulation

Central or peripheral administration of the alpha 2-adrenoceptor agonist clonidine causes marked hyperglycemia in the rat. It is not clear whether this effect is mediated within the brain at either pre- or postsynaptic alpha 2-adrenoceptors or whether it is due to peripheral alpha 2-agonist actions. We employed computerized mass spectrometry to measure noradrenaline (NA) and its primary neuronal metabolite 3,4-dihydroxyphenylglycol (DHPG) in the medial basal hypothalamus of rats treated acutely with clonidine, the alpha 2-antagonist yohimbine, the postganglionic noradrenergic blocker guanethidine and the neuroglycopenic agent 2-deoxy-D-glucose (2-DG). That clonidine's hyperglycemic effect was due, in part, to an action at central alpha 2-adrenoceptors was indicated by the ability of guanethidine to significantly inhibit the glucose response. Because of clonidine's inhibition of hypothalamic NA release (assessed by the DHPG/NA ratio), presumably by presynaptic agonism, these data indicated that postsynaptic receptor stimulation by clonidine was involved in activating glucose release. Yohimbine markedly increased the hypothalamic DHPG/NA ratio, reflecting presynaptic stimulation of NA release, but at the same time inhibited the hyperglycemic response due to 2-DG administration. This latter effect to block hyperglycemia is consistent with antagonism of postsynaptic alpha 2-adrenoceptors involved in mediating hepatic glucose output. These data indicate a major role for postsynaptic alpha 2-adrenoceptors in glucoregulation.

The pharmacology of fluparoxan: a selective alpha 2-adrenoceptor antagonist

1. This paper describes the pharmacology of the novel alpha 2-adrenoceptor antagonist fluparoxan (GR 50360) which is currently being studied clinically as a potential anti-depressant. Idazoxan and yohimbine were included in many studies for comparison. 2. In the rat isolated, field-stimulated vas deferens and the guinea-pig isolated, field-stimulated ileum preparations, fluparoxan was a reversible competitive antagonist of the inhibitory responses to the alpha 2-adrenoceptor agonist UK-14304 with pKB values of 7.87 and 7.89 respectively. In the rat isolated anococcygeus muscle, fluparoxan was a much weaker competitive antagonist of the contractile response to the alpha 1-adrenoceptor agonist phenylephrine with a pKB of 4.45 giving an alpha 2: alpha 1-adrenoceptor selectivity ratio of greater than 2500. 3. In the conscious mouse, fluparoxan (0.2-3.0 mg kg-1) was effective by the oral route and of similar potency to idazoxan in preventing clonidine-induced hypothermia and antinociception. In the rat, UK-14304-induced hypothermia (ED50 = 1.4 mg kg-1, p.o. or 0.5 mg kg-1, i.v.) and rotarod impairment (ED50 = 1.1 mg kg-1 p.o. or 1.3 mg kg-1, i.v.) were antagonized by fluparoxan. Fluparoxan, 0.67-6 mg kg-1, p.o., also prevented UK-14304-induced sedation and bradycardia in the dog. 4. In specificity studies fluparoxan had low or no affinity for a wide range of neurotransmitter receptor sites at concentrations up to at least 1 x 10(-5) M. It displayed weak affinity for 5-HT1A (pIC50 = 5.9) and 5-HT1B (pKi = 5.5) binding sites in rat brain. 5. We conclude that fluparoxan is a highly selective and potent alpha 2-adrenoceptor antagonist. The density of rat brain [3H]-dihydroalprenolol binding sites was reduced by 26% when fluparoxan was administered chronically for 6 days at a dose of 12 mg kg- 1 orally twice daily. The down-regulation of beta-adrenoceptors by fluparoxan is consistent with its antidepressant potential.

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.

Alpha 1 and alpha 2-adrenergic receptor blockade influences angiotensin II facilitation of avoidance behavior and stereotypy in rats

Pretreatment of rats with prazosin (PRA), an alpha 1-adrenergic receptor blocker, abolished the increased rate of learning of conditioned avoidance responses stimulated by intracerebroventricular angiotensin II (AII) administration. Yohimbine (YOH), an alpha 2-receptor blocker, reversed the effect of AII. PRA did not affect, and YOH abolished, the improvement of recall of a passive avoidance behavior caused by AII. The stereotypies produced by apomorphine (APO) and amphetamine (AMP) were enhanced by AII. PRA changed neither stereotypy, but it abolished the AII effect in both cases. YOH did not alter APO stereotypy and abolished the enhancement of that behavior caused by AII. YOH increased AMP stereotypy and had an additive effect with AII. No significant changes of exploratory motor activity were caused by PRA, YOH, or their combination, with AII. These findings indicate that functioning alpha 1- and alpha 2-adrenergic receptors are necessary for the facilitation of learning by AII, while only alpha 2-receptors appear to be involved in AII improvement of recall. The central dopaminergic system may in part be responsible for the modulation by PRA and YOH of the effects of AII on learning and recall.

Antagonism of the hypothermic effect of clozapine in mice by centrally-active alpha 2-adrenergic antagonists and alpha 1-adrenergic agonists

Hypothermia induced by either clozapine or clonidine in mice was blocked by the alpha 2-adrenergic antagonists yohimbine, idazoxan, CH-38083, SKF 86466, and L-657,743. These effects were dose related, and the ID50 values for inhibition of clozapine- or clonidine-induced hypothermia were fairly comparable. The order of potency for blocking clonidine-induced hypothermia was: L-657,743 greater than CH-38083 greater than yohimbine greater than idazoxan greater than SKF 86466. A very similar blockade hierarchy for clozapine-induced hypothermia was observed, with the order of the two most effective compounds being reversed. Hypothermia induced by either compound was not blocked by the peripherally-acting, selective alpha 2-adrenergic antagonist, L-659,066, indicating that blockade by the other compounds occurred centrally. The centrally-acting, alpha 1-adrenergic agonists St 587, cirazoline, and SKF 89748 were very effective in blocking the response to clozapine, but ineffective in antagonizing clonidine-induced hypothermia. The ED50 values for the blockade of this response to clozapine, however, did not correlate with their reported potencies in stimulating either peripheral or central alpha 1-adrenergic receptors. This indicates that clozapine-induced hypothermia in mice is not a suitable model for evaluating the properties of central alpha 1-adrenergic compounds. Moreover, since the clonidine-induced hypothermia is not influenced by alpha 1-adrenergic agonists, this paradigm is preferable to clozapine-induced hypothermia in the assessment of alpha 2-adrenergic antagonism The ability of alpha 2-adrenergic antagonists to block clozapine-induced hypothermia may result from the central overflow of norepinephrine, which is known to be brought about by this group of compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormone and norepinephrine: effects on alpha-2, beta, and reuptake sites in cerebral cortex and heart

Thyroid hormone produces metabolic effects similar to those of stimulation of noradrenergic receptors. It has been reported, however, that norepinephrine turnover is reduced during thyrotoxicosis and that beta-noradrenergic receptor number is increased. Metabolic effects of thyroid hormone may therefore reduce noradrenergic activity. We examined effects of thyroid hormone administration or production of hypothyroidism with methimazole on receptors associated with regulation of noradrenergic function. Treatment with thyroid hormone increased beta-receptor binding, increased alpha-2 receptor binding, and decreased desipramine binding, opposite to effects of hypothyroidism produced by methimazole. Heart was more sensitive than brain to these effects. These data are consistent with reduced noradrenergic activity during hyperthyroidism, possibly mediated by an increase in autoreceptor function.

Behavioural effects of the alpha 2-adrenoceptor antagonists idazoxan and yohimbine in rats: comparisons with amphetamine

Although yohimbine has long been known to increase arousal, reactivity and anxiety in animals and humans, little is known about the behavioural effects of more selective alpha 2-adrenoceptor antagonists such as idazoxan. In a recent experiment, however, it was found that in rats both yohimbine and idazoxan increased low rates of lever pressing, an effect also produced by amphetamine. The purpose of the present study was to investigate further the effects of yohimbine and idazoxan in comparison with those of d-amphetamine on the operant behaviour of rats. In rats trained to press a lever on a FI 60s schedule to obtain food both yohimbine and idazoxan increased response rates, although the effect of yohimbine was considerably greater than that of idazoxan. Lower doses of d-amphetamine had no consistent effect on overall rates of responding whereas a higher dose suppressed responding. Characteristically, d-amphetamine increased responding during early portions of the intervals and decreased responding during the final portions. Idazoxan and yohimbine tended to increase responding throughout the intervals except immediately after reinforcement. When idazoxan was administered in combination with prazosin FI response rates were markedly decreased. Administration of DSP4 did not alter the response rate-increasing effects of either yohimbine or idazoxan. In rats trained to discriminate d-amphetamine from saline both idazoxan and yohimbine gave rise to responding on the saline associated lever. Combination of idazoxan with d-amphetamine did not antagonise the amphetamine cue but produced substantial reductions in response rates, probably due to toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 1- and alpha 2-adrenoreceptor antagonists differentially influence locomotor and stereotyped behaviour induced by d-amphetamine and apomorphine in the rat

The importance of dopamine (DA) in mediating locomotor, exploratory and stereotyped behaviour in rodents is well established. Evidence also indicates a modulatory role for noradrenaline (NA) although, due to nonspecificity. of previously available agents, a precise role remains undefined. The effects of the specific and selective alpha-adrenoreceptor antagonists idazoxan (alpha 2) and prazosin (alpha 1) on behaviour induced by amphetamine and apomorphine have been investigated in the rat. d-Amphetamine (2 mg/kg) induced hyperactive locomotion and exploration. Pretreatment with prazosin (1 mg/kg) markedly reduced these responses. In contrast, pretreatment with idazoxan (20 mg/kg) only marginally altered d-amphetamine hyperactivity. Apomorphine (0.5 mg/kg) induced biphasic locomotor and exploratory activity. Neither alpha-antagonist affected the initial burst of activity (60 min), although prazosin inhibited whereas idazoxan potentiated the secondary phase (90-180 min). At higher dosage, amphetamine (6 mg/kg) and apomorphine (2 mg/kg) induced stereotyped behaviours. Prazosin pretreatment enhanced stereotyped gnawing and decreased sniffing and locomotion, whereas idazoxan increased locomotion and decreased amphetamine-induced mouth movements. These data indicate that DA-induced locomotor and stereotyped behaviours are differentially influenced (in opposite directions) by both alpha1- and alpha 2-adrenoreceptor antagonists. NA may thus modulate the expression and character of behaviour by influencing DA function in certain brain areas.

Yohimbine-induced alterations of monoamine metabolism in the spontaneously hypertensive rat of the Okamoto strain (SHR). II. The central nervous system (CNS)

Steady state levels of monoamine neurotransmitters were examined in SHR, a genetic model of hypertension and compared to its normotensive control (WKY). SHR and WKY were also challenged with alpha 2-adrenergic antagonists, (yohimbine, YOH, idazoxan) or an alpha 1-antagonist (prazosin) and alterations in CNS monoamine metabolism evaluated. SHR were found to have elevated levels of NE and 5-HT in a number of brain regions involved in cardiovascular control when compared to WKY. DA levels and metabolism were also altered in the SHR. Blockade of alpha 2-adrenoceptors and other direct and indirect actions of YOH exacerbated the abnormalities in central monoaminergic neurotransmission in SHR. Significant decreases in NE content were produced by YOH or idazoxan treatment in both SHR and WKY, presumably the result of the inhibition of alpha 2-adrenoceptor medicated presynaptic control of NE release. YOH treatment abolished the differences in steady state levels of NE between SHR and WKY, however, idazoxan did not. YOH administration resulted in significant increases in DA and 5-HT in a number of brain regions of both SHR and WKY. Idazoxan or prazosin produced few changes in DA and 5-HT metabolism except for increases in DA content in the spinal cord and brainstem of SHR given idazoxan. The YOH-induced increases in DA and 5-HT content of SHR were of a greater magnitude than the WKY in several brain regions. DOPAC levels were significantly elevated by YOH in both WKY and SHR, reflecting the antidopaminergic properties of YOH. 5-HIAA content was significantly reduced by YOH in a number of brain regions in both SHR and WKY, however, this effect was attenuated in several brain regions in SHR. The results of the present study demonstrate the multifarious nature of the alterations in CNS monoamine metabolism in SHR.

Yohimbine-induced alterations of monoamine metabolism in the spontaneously hypertensive rat. I. The peripheral nervous system

The effects of alpha 2 adrenoreceptor blockade with YOH on blood pressure, plasma catecholamines and norepinephrine (NE) stores in kidney, adrenal and spleen of spontaneously hypertensive rats of the Okamoto strain (SHR) and Wistar-Kyoto (WKY) control animals were examined. YOH administration resulted in a significant (p less than 0.001) reduction in arterial pressure in both SHR and WKY. Plasma NE and EPI were significantly (p less than 0.05) elevated by YOH treatment in both SHR and WKY, but SHR exhibited a significantly (p less than 0.05) greater percent increase in plasma NE than WKY. YOH produced significant decreases in splenic NE content in both SHR and WKY but reduced renal NE content in the SHR only. SHR had significantly higher basal renal NE and DA content and fewer NE uptake (3H-desmethylimipramine binding) sites (p less than 0.05) than WKY. Treatment of SHR or WKY with either the alpha 2-adrenergic antagonist, idazoxan, or the alpha 1-antagonist, prazosin, failed to significantly alter renal NE levels from those found after saline injection. The enhanced YOH-induced renal NE depletion in SHR suggests an alteration in the presynaptic control of NE release in the genetically hypertensive rat, however, the effects of YOH in the SHR may be mediated by mechanisms unrelated to alpha 2-adrenergic receptors.

Behavioural and biochemical evidence for the release of noradrenaline in mouse brain by TRH and some of its biologically stable analogues

Small doses of clonidine probably induce hypoactivity (a distinct form of sedation) by stimulating presynaptic alpha 2-adrenoceptors. This was attenuated by injection of 0.1-10 mg/kg of thyrotropin releasing hormone (TRH) or its biologically stable analogues, CG3509, CG3703 and RX77368, when these were given 10 min before clonidine. This effect was dose-dependent in all cases, but the analogues were more potent than TRH. The TRH metabolites, TRH acid and histidyl-proline diketopiperazine (10 mg/kg) were without effect. This response was still attenuated by the analogues, but not TRH, when these were given 1 hr before clonidine. The results, therefore, suggested that it was the basic tripeptide structure which was active and TRH was less potent than its analogues because of rapid metabolism. Attenuation of hypoactivity by TRH and analogues was not due to increased dopaminergic function because apomorphine (5 mg/kg) was ineffective. Thyrotropin releasing hormone (20 mg/kg), CG3509 (10 mg/kg) and CG3703 (1 mg/kg) also induced locomotor activity and produced various other behavioural changes. This was inhibited by prazosin (3 mg/kg) and haloperidol (0.5 mg/kg) but not by yohimbine (1 mg/kg). Apomorphine (5 mg/kg)-induced activity was inhibited by haloperidol and yohimbine but not by prazosin. This indicated that the activity produced by the TRH compounds, but not apomorphine, was partly mediated by alpha 1-adrenoceptors. Both CG3509 (10(-5) and 10(-4) M) and RX77368 (10(-4) M) evoked the release of endogenous noradrenaline from slices of hypothalamus in vitro. The TRH analogues, however, had no affinity for alpha 1- or alpha 2-adrenoceptors in ligand-receptor binding experiments. Viewed overall, the data showed that TRH and its analogues induced the release of noradrenaline in the brain. In addition, a comparison of the behavioural effects of TRH compounds with dopamine and alpha 1-adrenoceptor agonists suggested that in mice these behavioural responses resulted from stimulation of both noradrenergic and dopaminergic function.

Brain Na+,K+-ATPase: alteration of ligand affinities and conformation by chronic ethanol and noradrenergic stimulation in vivo

These experiments examined effects of chronic ethanol, repeated noradrenergic stimulation or inhibition, and ethanol combined with the noradrenergic treatments on regulation of Na+,K+-ATPase. Chronic treatment with ethanol reduced the sensitivity of K+-p-nitrophenyl-phosphatase to ethanol, increased affinity for K+, reduced the sensitivity of K+ affinity to ATP or ethanol, and reduced delta H and delta S for K+ activation and for the E1-E2 transition. These effects were all opposite to those of ethanol added in vitro. Treatment with yohimbine had the opposite effects on ethanol sensitivity, K+ affinity, K+ interactions with ethanol and ATP, and thermodynamic parameters for cation activation or conformational change. These effects were similar to those of norepinephrine in vitro. The effects of yohimbine treatment were eliminated or reduced in rats also treated with ethanol. Depletion of norepinephrine had effects opposite to those of yohimbine. These data are consistent with a reduction in membrane fluidity, at least in the vicinity of Na+,K+-ATPase, during ethanol tolerance. Exposure to norepinephrine, in vitro or in vivo, had effects on Na+,K+-ATPase that were similar to those of increased membrane fluidity.

Effect of clonidine and yohimbine on sleep in man: polygraphic study and EEG analysis by normalized slope descriptors

An all-night polygraphic study of sleep was carried out in 6 healthy volunteers acutely medicated with clonidine (225 micrograms), yohimbine (15 mg), a combination of both drugs or with placebo at random order under simple blind conditions. The conventional scoring of sleep stages was extended by an on-line analysis of the EEG according to Hjorth. Clonidine, an agonist at alpha2 receptors, induced increases of the stages 2 and 3 + 4 but a decrease of REM sleep; the reduction of the parameters mobility and complexity during non-REM sleep signified synchronization of the EEG. Yohimbine, an antagonist at the same receptor sites, enhancing noradrenaline release, increased stage 1 and REM sleep, but decreased the stages 2 and 3 + 4; increases of mobility during REM and non-REM sleep signified desynchronization. The combined treatment with clonidine + yohimbine reduced REM sleep, but not as extensively as clonidine alone; the Hjorth parameters were without significant alterations compared to placebo. The physiological hypotonia was enhanced by clonidine and the combined treatment with clonidine + yohimbine. These results favour the hypothesis of a noradrenergic facilitation of REM sleep.

Comparison of the effects of recently developed alpha 2-adrenergic antagonists with yohimbine and rauwolscine on monoamine synthesis in rat brain

The effects of two recently developed alpha 2-adrenergic antagonists, RX 781094 and WY 26703, on the synthesis of norepinephrine (NE), dopamine (DA) and serotonin (5-HT) in rat brain were compared to those of yohimbine, its diastereoisomer rauwolscine, and mianserin. Intraperitoneal administration of these compounds increased cortical NE synthesis with the potency order: yohimbine, RX 781094, WY 26703 greater than rauwolscine greater than mianserin. Within a similar dose range, yohimbine, rauwolscine and WY 26703 also stimulated striatal DA synthesis and decreased hypothalamic 5-HT synthesis, while RX 781094 and mianserin were very weak or inactive. Yohimbine and the structurally-related WY 26703 were also active as DA antagonists in the gamma-butyrolactone model for DA autoreceptor function. Based on the drug-induced changes in monoamine synthesis as indication of receptor-mediated events, RX 781094 has greater selectivity as an alpha 2-antagonist than compounds structurally related to yohimbine.

Central noradrenergic involvement in yohimbine excitation of acoustic startle: effects of DSP4 and 6-OHDA

It was previously shown that i.p. administration of the alpha 2-adrenergic antagonist yohimbine increased the magnitude of the acoustic startle response in rats. The purpose of the present study was to determine possible central noradrenergic involvement in yohimbine's effect on startle. Pretreatment with N-(2-chloroethyl)-N-ethyl-2-bromo-benzylamine (DSP4; 50 mg/kg, i.p.; 1-2 days before testing) completely blocked the excitatory effect of yohimbine on startle. DSP4 reduced forebrain and spinal cord NE levels by 47% and 56%, respectively, without affecting forebrain or spinal serotonin (5-HT), or forebrain dopamine (DA). Pretreatment with the NE reuptake blocker desmethylimipramine (DMI; 20 mg/kg, i.p.; 30 min before DSP4) prevented the ability of DSP4 to block the yohimbine effect. DMI partially reversed the NE-depleting effects of DSP4. Neither bilateral adrenalectomy nor intravenously administered 6-hydroxydopamine (6-OHDA; 20 mg/kg; 1-2 days before testing) altered the excitatory effect of yohimbine, indicating that peripheral NE is not involved. 6-OHDA (2 X 200 micrograms) injected into the lateral ventricles blocked yohimbine's effect, and depleted NE by 95% (spinal cord) and 86% (forebrain), without affecting 5-HT in either region. 6-OHDA also depleted forebrain DA levels by 49%. Finally, intrathecal administration of 6-OHDA (20 micrograms; 14 days before testing) into the subarachnoid space of the lumbar spinal cord blocked the excitatory effect of yohimbine, and produced an extensive (94%) depletion of spinal cord NE. Intrathecal 6-OHDA did not alter spinal levels of 5-HT or forebrain levels of NE, 5-HT or DA. In summary, these data indicate that central descending NE neurons are necessary for yohimbine's excitatory effect on startle.

Possible function of alpha 1-adrenoceptors in the CNS in anaesthetized and conscious animals

The influence of St 587 (2-(2-chloro-5-trifluoromethylphenylimino)imidazolidine), a selective alpha 1-adrenoceptor agonist which easily penetrates the blood-brain barrier, was tested on behavior and cardiovascular functions, respectively. The substance (up to 10 mg/kg subcutaneously (s.c.)) did not increase the exploratory activity of naive mice. The hexobarbitone 'sleeping' time in mice was reduced in a dose-dependent manner (St 587 ED50 = 14.4 mg/kg s.c.). Haloperidol 10 mg/kg s.c. induced catalepsy which was antagonized by St 587 in a dose-dependent manner (ED50 = 2.7 mg/kg i.p.). Conversely, the alpha 1-adrenoceptor-blocking agents prazosin and corynanthine elicited catalepsy in mice which had been treated with a subthreshold dose (2 mg/kg s.c.) of haloperidol; the ED50 values of the antagonists were 0.26 and 4.7 mg/kg i.p., respectively. In anaesthetized cats blood pressure and heart rate were not affected by 100 micrograms/kg St 587 injected into the left vertebral artery. In conscious dogs with beta-adrenoceptors blocked, the drug was without effect (100 micrograms/kg intracisternally) on vagally mediated reflex bradycardia, as evoked by intravenous noradrenaline injection. As a positive control the alpha 2-adrenoceptor agonist B-HT 920 which is equipotent to St 587 with respect to peripheral vasopressor effects in rats was injected with 10 micrograms/kg intracisternally and facilitated the reflex bradycardia. It is concluded that alpha 1-adrenoceptors within the brain mediate behavioral activation in states of CNS depression but remain without effect on cardiovascular centers.

B-HT 958 stimulates dopamine autoreceptors but blocks noradrenaline autoreceptors in the brain

B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo 5,4-d azepine) blocked the gamma-butyrolactone-induced increase in the synthesis of dopamine and slowed down the alpha-methyltyrosine-induced disappearance of dopamine in the mouse brain by haloperidol-sensitive mechanisms. In reserpine-treated mice, B-HT 958 produced a most a weak locomotion and no change in the apomorphine-induced increase in motor activity. The motor activity of normal mice was reduced by B-HT 958. At high doses, B-HT 958 accelerated the alpha-methyltyrosine-induced disappearance of noradrenaline and it inhibited the effects of clonidine on the turnover and on the synthesis of noradrenaline in the mouse brain. The findings indicate that the dopamine autoreceptors can be selectively stimulated by B-HT 958 but that the alpha 2-adrenoceptors can be blocked following high doses.

The effectiveness of yohimbine in blocking rat central dopamine autoreceptors in vivo

The influence of various alpha-adrenoceptor antagonists (10 mg/kg i.p.) upon the rate of turnover of dopamine (DA) in the rat brain was investigated. Taking the levels of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) as a measure of the rate of DA turnover, it was found that prazosin and phenoxybenzamine decreased, whereas piperoxane and yohimbine increased the turnover rate both in the corpus striatum and in the tuberculum olfactorium. Azapetine, phentolamine and tolazoline as well as the beta-adrenoceptor antagonist propranolol were without a significant effect, whereas the DA antagonist haloperidol increased DOPAC and HVA levels and decreased the levels of DA itself. The possibility that the yohimbine-induced increase in the DA turnover rate was produced by a direct blockade of DA autoreceptors, was investigated under conditions where influences other than those elicited via DA autoreceptors are thought to be eliminated, i.e. in rats treated with reserpine or gamma-butyrolactone (GBL). In rats that were pretreated with reserpine, yohimbine (10 mg/kg i.p.) was found to be ineffective in antagonizing the reduction of the accumulation of 3,4-dihydroxyphenylalanine (DOPA) following decarboxylase inhibition, that was produced by the DA agonist apomorphine (2.0 mg/kg i.p.). In rats pretreated with reserpine, yohimbine (10 mg/kg i.p.) was also ineffective in antagonizing the reduction of the DOPAC and HVA levels produced by apomorphine (2.0 mg/kg i.p.), but it was effective in antagonizing the reduction of the HVA level that was produced by the selective DA autoreceptor agonist N,N-di-n-propyl-7-hydroxy-2-aminotetralin (DP-7-AT, 1.0 mg/kg i.p.).(ABSTRACT TRUNCATED AT 250 WORDS)

Yohimbine and rauwolscine reduce food intake of genetically obese (obob) and lean mice

Multiple behavioral and neurochemical abnormalities are found in the genetically obese mouse, obob , including hyperphagia, elevated hypothalamic norepinephrine (NE) levels, and increases alpha-1 receptor density. The obese mutant also responds abnormally to neuropharmacological agents. In the current study the alpha-2 receptor blockers yohimbine and rauwolscine were administered to food-restricted (6-hour food access) obob and lean mice. Yohimbine and rauwolscine significantly reduced the 3- and 6-hour food intake of both obob and lean mice. The obob mice were, however, more sensitive to this anorectic effect than lean mice. Effective anorectic doses of yohimbine did not affect water intake in water-deprived lean mice, suggesting a specific effect of the drug upon food intake. Low doses (50 and 100 micrograms) of the alpha-2 agonist clonidine increased the 1-hour food intake of obob mice, but did not affect the food intake of lean mice. No differences were found between obob and lean mice in the number of alpha-receptors in the hypothalamus. The results suggest that modification of NE release by manipulation of alpha-2 receptor can alter food intake, and that the obob mutant is particularly sensitive to this effect.

Relationships between brain noradrenergic activity and blood glucose

Glucose is the principal energy substrate for the brain and studies have shown that the brain is able to increase glucose availability in the face of glucose starvation (neuroglycopaenia). The mechanisms, believed to be hypothalamic, that may be involved in a brain/blood glucose control system have not yet been identified. We have used novel techniques for assessing brain monoamine neuronal activity to investigate its relationship to blood glucose concentrations in the rat. We describe here two important relationships which emerge from these studies. One is that activation of hypothalamic noradrenaline (NA) activity following stress is associated with concurrent increases in plasma glucose concentrations. This relationship is linear and independent of the adrenal or pituitary glands. The second is an inverse relationship between plasma glucose concentration and hypothalamic NA neuronal activity--high blood glucose levels significantly inhibited the hypothalamic NA activity responses to stress, alpha 2-adrenergic blockade and adrenalectomy. Thus glucose (or a metabolite of it) seems to provide a negative feedback signal sensed by hypothalamic NA neuronal systems which, in turn, appear to stimulate liver glucose output by a neural mechanism.

The cortical and medullary blood flow at different levels of renal nerve activity

The effect of (1) renal denervation and (2) stimulation of the renal nerve on the regional renal blood flow were determined by the Rb uptake method. Under control conditions the total renal blood flow was 3.64 +/- 0.09 ml X min-1 X g-1 tissue increasing significantly (p less than 0.02) to 4.39 +/- 0.28 ml X min-1 X g-1 after denervation. Upon stimulation of the peripheral portions of the sectioned renal nerves the blood flow decreased almost linearly with the frequency of stimulation reaching 0.99 +/- 0.24 ml X min-1 X g-1 at 10 Hz. Utilizing the relation between blood flow and stimulation frequency the control blood flow correspond to a spontaneous activity of 1.5 Hz. As expected the cortical blood flow responded in the same way as for the total renal blood flow. In the renal medulla denervation gave a much more pronounced response where e.g. the inner medullary flow increased from 0.88 +/- 0.09 to 1.30 +/- 0.16 ml X min-1 X g-1, i.e. a 50% increase (p less than 0.05). Stimulation with 2 Hz produced a steep fall in the blood flow, whereafter it decreased linearly with the stimulation frequency reaching 0.11 ml X min-1 X g-1 at 10 Hz stimulation. This demonstrates again that the renal medulla is sensitive to renal nerve activity primarily in the low level range. It should be remarked, however, that the 86-Rb uptake method reflects the effective blood flow, which might differ from the blood flow in absolute terms.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha-adrenoceptors: recent development and some comparative aspects

On anatomical and functional bases, alpha-adrenoceptors have been divided into pre- and postsynaptic alpha-adrenoceptors. Recently, alpha-adrenoceptors have been classified as alpha 1 and alpha 2 according to their pharmacological responses, irrespective of their anatomical location. The presynaptic alpha-adrenoceptors, which have been recognized as alpha 2, determine the frequency of the nerve impulses travelling along the axon and also the amount of transmitter released per nerve impulse from the varicose terminal. Postsynaptic alpha-adrenoceptors have been recognized in various tissues including smooth muscle, pancreatic islets, fat cells, platelets and other tissues. Both alpha 1- and alpha 2-adrenoceptors have been located postsynaptically. alpha-Adrenoceptors have been found also in the central nervous system. Generally, they fall into the same categories (alpha 1 and alpha 2) as the peripheral alpha-adrenoceptors. A new class of drugs, the so called calcium blockers, inhibit the postsynaptic response to alpha 2 stimulation but not the alpha 1-mediated response, indicating that the alpha 2-adrenoceptors are dependent on Ca2+ ions for their function. In the most primitive group of vertebrates, the fishes, alpha-adrenoceptors seem to be different in as much as they do not respond to many of the classical drugs employed to distinguish between alpha-adrenoceptors in mammals. In reptiles and amphibians alpha 2-adrenoceptors have been shown to exist. These receptors are involved in the regulation of melanocytes. In the most advanced non-mammalian vertebrates (birds) both peripheral and central alpha-adrenoceptors seem to be qualitatively similar to the mammalian types.

Formation of deaminated metabolites of dopamine in noradrenaline neurons

The deaminated monoamine metabolites 3-methoxy-4-hydroxyphenylethyleneglycol (MOPEG), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were determined electrochemically following organic solvent extraction and reverse-phase, high performance, liquid chromatography in four regions of the mouse brain. In the noradrenaline (NA)-predominant regions (hemispheres, brain stem), the ratio of the concentrations of DOPAC plus HVA to NA plus dopamine (DA) was approximately the same as in the DA-predominant regions (corpus striatum, limbic system). Yohimbine and reserpine elevated the concentrations of DOPAC and HVA both in the NA- and the DA-predominant regions. The effect of yohimbine was somewhat enhanced by the alpha 1-receptor blocking agent prazosin in the NA-predominant regions. The concentration of MOPEG was increased by yohimbine and decreased by reserpine. The concentrations of DOPAC and HVA were lowered by clonidine, but not by apomorphine in the NA-predominant regions of reserpine-treated mice. In the DA-predominant regions, apomorphine, but not clonidine, reduced the concentrations of DOPAC and HVA. The effects of clonidine and apomorphine were reversed by yohimbine and haloperidol, respectively. The results indicate that the concentrations of the acid DA metabolites DOPAC and HVA in the NA-predominant regions reflect the rate of synthesis of DA in the NA neurons.