Responses to alpha 2-adrenoceptor blockade by idazoxan in healthy male and female volunteers

Adrenoceptors: A Focus on Psychiatric Disorders and Their Treatments

Research into the involvement of adrenoceptor subtypes in the cause(s) of psychiatric disorders is particularly challenging. This is partly because of difficulties in developing animal models that recapitulate the human condition but also because no evidence for any causal links has emerged from studies of patients. These, and other obstacles, are outlined in this chapter. Nevertheless, many drugs that are used to treat psychiatric disorders bind to adrenoceptors to some extent. Direct or indirect modulation of the function of specific adrenoceptor subtypes mediates all or part of the therapeutic actions of drugs in various psychiatric disorders. On the other hand, interactions with central or peripheral adrenoceptors can also explain their side effects. This chapter discusses both aspects of the field, focusing on disorders that are prevalent: depression, schizophrenia, anxiety, attention-deficit hyperactivity disorder, binge-eating disorder, and substance use disorder. In so doing, we highlight some unanswered questions that need to be resolved before it will be feasible to explain how changes in the function of any adrenoceptor subtype affect mood and behavior in humans and other animals.

α2-Adrenoceptors are targets for antipsychotic drugs

RATIONALE

Almost all antipsychotic drugs (APDs), irrespective of whether they belong to the first-generation (e.g. haloperidol) or second-generation (e.g. clozapine), are dopamine D2 receptor antagonists. Second-generation APDs, which differ from first-generation APDs in possessing a lower propensity to induce extrapyramidal side effects, target a variety of monoamine receptors such as serotonin (5-hydroxytryptamine) receptors (e.g. 5-HT1A, 5-HT2A, 5-HT2C, 5-HT6, 5-HT7) and α1- and α2-adrenoceptors in addition to their antagonist effects at D2 receptors.

OBJECTIVE

This short review is focussed on the potential role of α2-adrenoceptors in the antipsychotic therapy.

RESULTS

Schizophrenia is characterised by three categories of symptoms: positive symptoms, negative symptoms and cognitive deficits. α2-Adrenoceptors are classified into three distinct subtypes in mammals, α2A, α2B and α2C. Whereas the α2B-adrenoceptor seems to play only a minor role in the brain, activation of postsynaptic α2A-adrenoceptors in the prefrontal cortex improves cognitive functions. Preclinical models such as D-amphetamine-induced locomotion, the conditioned avoidance response and the pharmacological N-methyl-D-aspartate receptor hypofunction model have shown that α2C-adrenoceptor blockade or the combination of D2 receptor antagonists with idazoxan (α2A/2C-adrenoceptor antagonist) could be useful in schizophrenia. A potential benefit of a treatment combination of first-generation APDs with the α2A/2C-adrenoceptor antagonists idazoxan or mirtazapine was also demonstrated in patients with schizophrenia.

CONCLUSIONS

It is concluded that α2-adrenoceptors may be promising targets in the antipsychotic therapy.

Long-term motor improvement after stroke is enhanced by short-term treatment with the alpha-2 antagonist, atipamezole

Drugs that increase central noradrenergic activity have been shown to enhance the rate of recovery of motor function in pre-clinical models of brain damage. Less is known about whether noradrenergic agents can improve the extent of motor recovery and whether such improvement can be sustained over time. This study was designed to determine if increasing central noradrenergic tone using atipamezole, an alpha-2 adrenoceptor antagonist, could induce a long-term improvement in motor performance in rats subjected to ischemic brain damage caused by permanent middle cerebral artery occlusion. The importance of pairing physical "rehabilitation" with enhanced noradrenergic activity was also investigated. Atipamezole (1 mg/kg, s.c.) or vehicle (sterile saline) was administered once daily on Days 2-8 post-operatively. Half of each drug group was housed under enriched environment conditions supplemented with daily focused activity sessions while the other half received standard housing with no focused activity. Skilled motor performance in forelimb reaching and ladder rung walking was assessed for 8 weeks post-operatively. Animals receiving atipamezole plus rehabilitation exhibited significantly greater motor improvement in both behavioral tests as compared to vehicle-treated animals receiving rehabilitation. Interestingly, animals receiving atipamezole without rehabilitation exhibited a significant motor improvement in the ladder rung walk test but not the forelimb reaching test. These results suggest that a short-term increase in noradrenergic activity can lead to sustained motor improvement following stroke, especially when paired with rehabilitation.

Therapeutic use of release-modifying drugs

Presynaptic inhibitory or facilitatory autoreceptors are targets for the endogenous neurotransmitter of the respective neuron, and also for exogenous agonists, partial agonists and antagonists which can produce pharmacological actions through changes in transmitter release. In addition, presynaptic inhibitory or facilitatory heteroreceptors can also be acted upon by exogenous agonists, partial agonists or antagonists to induce changes in transmitter release with useful therapeutic effects. This article summarizes drugs that are known or likely to produce their therapeutic effects through presynaptic modulation of neurotransmitter release. Included are drugs acting on alpha and beta adrenoceptors, dopamine receptors, angiotensin, opioid, cannabinoid, and nicotinic acetylcholine receptors. Also discussed are changes in presynaptic receptor mechanisms produced by drugs that inhibit transmitter re-uptake.

Chronic lithium administration enhances noradrenergic responses to intravenous administration of the alpha2 antagonist idazoxan in healthy volunteers

The acute and chronic effects of lithium carbonate administration at therapeutic blood levels on peripheral noradrenergic activity and sympathetic responses to alpha2 adrenoceptor blockade were examined in 10 medically and psychiatrically healthy volunteers. Supine resting levels of plasma norepinephrine and the increases in norepinephrine following intravenous infusion of 200 microg/kg of idazoxan, a selective alpha2 adrenoceptor antagonist, were determined before lithium (Li+) administration and after 5 days and after 4 weeks of daily Li+ treatment. Chronic Li+ treatment significantly increased mean resting plasma norepinephrine levels by 53.6%. The noradrenergic responses to infusions of idazoxan were slightly enhanced after 5 days of Li+ administration and significantly increased following 4 weeks of Li+ treatment. The possibility that Li+ produces functional alpha2 subsensitivity causing enhanced peripheral noradrenergic activity in humans is supported by the findings of increased mean resting plasma norepinephrine and increased response to idazoxan following chronic Li+ administration. Alteration of regulatory mechanisms in the noradrenergic system may be relevant to understanding the clinical effects of Li+ in manic-depressive illness.

Mirtazapine decreases stimulatory effects of reboxetine on cortisol, adrenocorticotropin and prolactin secretion in healthy male subjects

Reboxetine is a selective noradrenaline reuptake inhibitor, whereas mirtazapine acts as an antagonist at noradrenergic alpha(2), serotonin (5-HT(2)), 5-HT(3) and histamine H(1) receptors. In a former study we could demonstrate an inhibitory impact of mirtazapine on cortisol secretion. In the present investigation, the influence of combined administration of 15 mg mirtazapine and 4 mg reboxetine on the cortisol (COR), adrenocorticotropin (ACTH), growth hormone (GH), and prolactin (PRL) secretion was examined in 12 healthy male subjects, compared to reboxetine alone (4 mg). In a randomized order, the subjects received reboxetine (4 mg) alone or the combination of reboxetine (4 mg) and mirtazapine (15 mg) at 8:00 a.m. on two different days. After insertion of an intravenous catheter, blood samples were drawn 1 h prior to the administration of single reboxetine or the combination (reboxetine and mirtazapine), at time of administration, and during the time of 5 h thereafter in periods of 30 min. Serum concentrations of COR, GH, and PRL as well as plasma levels of ACTH were determined in each blood sample by means of double antibody RIA, fluoroimmunoassay and chemiluminescence immunometric assay methods. The area under the curve (AUC) was used as parameter for the COR, ACTH, GH, and PRL response. For statistical evaluation, the Wilcoxon signed-ranks test was performed. There was a pronounced stimulation of COR, ACTH, GH, and PRL concentrations after single administration of reboxetine. When reboxetine was given in combination with mirtazapine, a significant reduction of the COR, ACTH, and PRL stimulation was observed whereas GH secretion patterns remained unchanged, compared to single administration of reboxetine. Apparently, the stimulatory effects of reboxetine on pituitary hormone secretion via noradrenergic mechanisms are counteracted in part by the alpha(2)-blocking properties of mirtazapine and its inhibitory influence on cortisol secretion.

Endocrinological effects of mirtazapine in healthy volunteers

OBJECTIVE

Unlike other antidepressants, mirtazapine does not inhibit the reuptake of norepinephrine or serotonin (5-HT) but acts as an antagonist at presynaptic alpha2-receptors and at postsynaptic 5-HT2, 5-HT3 and histamine H1-receptors. In the present investigation, the influence of acute oral administration of 15-mg mirtazapine on the cortisol (COR), adrenocorticotropin (ACTH), growth hormone (GH) and prolactin (PRL) secretion was examined in 12 healthy male subjects, compared to placebo.

METHODS

After insertion of an intravenous catheter, both the mean arterial blood pressure (MAP) and the heart rate were recorded and blood samples were drawn 1 h prior to the administration of mirtazapine or placebo (7:00 a.m.), at time of administration (8:00 a.m.) and during 5 h thereafter in periods of 30 min. Concentrations of COR, ACTH, GH and PRL were measured in each blood sample by double antibody radioimmunoassay and chemiluminescence immunoassay methods. The area under the curve (AUC; 0-300 min after mirtazapine or placebo administration) was used as parameter for the COR, ACTH, GH and PRL response. Furthermore, the urinary free cortisol excretion (UFC) was determined beginning at 8:00 a.m. (time of administration of placebo or mirtazapine) up to 8:00 a.m. the day after.

RESULTS

Two-sided t-tests for paired samples revealed significantly lower COR AUC, ACTH AUC, UFC and PRL AUC values after 15-mg mirtazapine compared to placebo, whereas no significant differences were found with respect to GH AUC, MAP and heart rate.

CONCLUSIONS

Since the acute inhibition of COR secretion in the healthy volunteers was paralleled by a simultaneous decrease of ACTH release, central mechanisms (e.g., inhibition of hypothalamic corticotropin releasing hormone (CRH) output) are suggested to be responsible for the inhibitory effects of mirtazapine on COR secretion. Our results are of particular interest in the light of the hypercortisolism observed in depressed patients and new pharmacological approaches such as CRH1 receptor antagonists.

The Future of Imaging in Drug Discovery

The number of new chemical entities being registered by drug companies each year is declining, while at the same time, the number of new compounds, and thereby potential therapeutics, is increasing at an exponential rate. The need to demonstrate the safety, efficacy, and the “value” of these new compounds to a sophisticated pharmaceutical market, driven in turn by the forces of healthcare economics, make drug development difficult, resulting in a very lengthy and complex series of steps in the development of a drug. Many aspects of clinical pharmacology are more art than science, and detecting pharmacological effects at the level of living integrated systems is difficult. These challenges are most evident when developing new therapeutics for neuropsychiatric illnesses. We may at last be entering a postmonoamine era, exemplified by compounds such as NK-1 antagonists and metatropic glutamate receptor agonists. Such developments hold significant promise for the treatment of severe mental illness, while at the same time being confronted with completely unknown clinical pharmacologies. Functional imaging may not only be useful for the development of new CNS compounds, but it may in fact be essential for helping to define their clinical pharmacology. Several examples will be addressed that highlight the utility of functional imaging in the development of potentially new CNS drugs.

Nonnoradrenergic mechanism of reflex cutaneous vasoconstriction in men

We tested for a nonnoradrenergic mechanism of reflex cutaneous vasoconstriction with whole body progressive cooling in seven men. Forearm sites (<1 cm(2)) were pretreated with: 1) yohimbine (Yoh; 5 mM id) to antagonize alpha-adrenergic receptors, 2) Yoh plus propranolol (5 mM Yoh-1 mM PR id) to block alpha- and beta-adrenergic receptors, 3) iontophoretic application of bretylium tosylate (BT) to block all sympathetic vasoconstrictor nerve effects, or 4) intradermal saline. Skin blood flow was measured by laser Doppler flowmetry and arterial pressure by finger photoplethysmography; cutaneous vascular conductance (CVC) was indexed as the ratio of the two. Whole body skin temperature (T(SK)) was controlled at 34 degrees C (water-perfused suit) for 10 min and then lowered to 31 degrees C over 15 min. During cooling, vasoconstriction was blocked at BT sites (P > 0.05). CVC at saline sites fell significantly beginning at T(SK) of 33.4 +/- 0.01 degrees C (P <0.05). CVC at Yoh-PR sites was significantly reduced beginning at TSK of 33.0 +/- 0.01 degrees C (P < 0.05). After cooling, iontophoretic application of norepinephrine (NE) confirmed blockade of adrenergic receptors by Yoh-PR. Because the effects of NE were blocked at sites showing significant reflex vasoconstriction, a nonnoradrenergic mechanism in human skin is indicated, probably via a sympathetic cotransmitter.

Neuroendocrine response to film-induced sexual arousal in men and women

The psychoneuroendocrine responses to sexual arousal have not been clearly established in humans. However, we have demonstrated previously that masturbation-induced orgasm stimulates cardiovascular activity and induces increases in catecholamines and prolactin in blood of both males and females. We presently investigated the role of orgasm in producing these effects. Therefore, in this study parallel analysis of prolactin, adrenaline, noradrenaline, and cortisol concentrations, together with cardiovascular variables of systolic/diastolic blood pressure and heart rate were undertaken during film-induced sexual arousal in nine healthy adult men and nine healthy adult women. Blood was drawn continuously via an indwelling cannula and connected tubing system passed through a mini-pump. In parallel, the cardiovascular parameters were recorded continuously via a computerised finger-cuff sensor. Subjective sexual arousal increased significantly in both men and women during the erotic film, with sexual arousal eliciting an increase in blood pressure in both males and females, and plasma noradrenaline in females only. In contrast, adrenaline, cortisol and prolactin levels were unaffected by sexual arousal. These data further consolidate the role of sympathetic activation in sexual arousal processes. Furthermore, they demonstrate that increases in plasma prolactin during sexual stimulation are orgasm-dependent, suggesting that prolactin may regulate a negative-feedback sexual-satiation mechanism.

Effects of mirtazapine on growth hormone, prolactin, and cortisol secretion in healthy male subjects

In the present study the effects of acute PO-administration of 15 mg mirtazapine on the growth hormone (GH), prolactin (PRL), and cortisol (COR) secretion were examined in eight physically and mentally healthy male subjects, compared to placebo. Mirtazapine is a new antidepressant agent which does not inhibit the reuptake of norepinephrine or serotonin but is an antagonist of presynaptic and, presumably, postsynaptic alpha 2-receptors as well as an antagonist of postsynaptic 5-HT2 and 5-HT3-receptors. After insertion of an i.v. catheter, blood samples were drawn 1 h prior to the administration of mirtazapine or placebo, at time of application, and during the time of 4 h after application in periods of 30 min. Plasma concentrations of GH, PRL, and COR were determined in each blood sample by double antibody RIA methods. The area under the curve (AUC) value was used as parameter for the GH, PRL, and COR response. With respect to GH and PRL secretion, mirtazapine did not show any effects in comparison with placebo. However, in all subjects, the COR concentrations were remarkably lower after mirtazapine compared to placebo, the difference being obvious in the mean value graphs 60 min after the application up to the end of the measurement period. The t-test for paired samples revealed a highly significant difference (P < 0.01) in COR-AUC-values between the mirtazapine group (mean COR-AUC: 1558.07 micrograms/100 ml x 240 min) and the placebo group (mean COR-AUC: 2698.86 micrograms/100 ml x 240 min). Further studies have to elucidate the question whether the demonstrated inhibition of COR secretion after application of 15 mg mirtazapine is caused by central or peripheral effects of this substance.

Attenuation of tolerance to opioid-induced antinociception and protection against morphine-induced decrease of neurofilament proteins by idazoxan and other I2-imidazoline ligands

1. Agmatine, the proposed endogenous ligand for imidazoline receptors, has been shown to attenuate tolerance to morphine-induced antinociception (Kolesnikov el al., 1996). The main aim of this study was to assess if idazoxan, an alpha2-adrenoceptor antagonist that also interacts with imidazoline receptors, could also modulate opioid tolerance in rats and to establish which type of imidazoline receptors (or other receptors) are involved. 2. Antinociceptive responses to opioid drugs were determined by the tail-flick test. The acute administration of morphine (10 mg kg(-1), i.p., 30 min) or pentazocine (10 mg kg(-1), i.p., 30 min) resulted in marked increases in tail-flick latencies (TFLs). As expected, the initial antinociceptive response to the opiates was lost after chronic (13 days) treatment (tolerance). When idazoxan (10 mg kg(-1), i.p.) was given chronically 30 min before the opiates it completely prevented morphine tolerance and markedly attenuated tolerance to pentazocine (TFLs increased by 71-143% at day 13). Idazoxan alone did not modify TFLs. 3. The concurrent chronic administration (10 mg kg(-1), i.p., 13 days) of 2-BFI, LSL 60101, and LSL 61122 (valldemossine), selective and potent I2-imidazoline receptor ligands, and morphine (10 mg kg(-1), i.p.), also prevented or attenuated morphine tolerance (TFLs increased by 64 172% at day 13). This attenuation of morphine tolerance was still apparent six days after discontinuation of the chronic treatment with LSL 60101-morphine. The acute treatment with these drugs did not potentiate morphine-induced antinociception. These drugs alone did not modify TFLs. Together, these results indicated the specific involvement of I2-imidazoline receptors in the modulation of opioid tolerance. 4. The concurrent chronic (13 days) administration of RX821002 (10 mg kg(-1), i.p.) and RS-15385-197 (1 mg kg(-1), i.p.), selective alpha2-adrenoceptor antagonists, and morphine (10 mg kg(-1), i.p.), did not attenuate morphine tolerance. Similarly, the concurrent chronic treatment of moxonidine (1 mg kg(-1), i.p.), a mixed I(1)-imidazoline receptor and alpha2-adrenoceptor agonist, and morphine (10 mg kg(-1), i.p.), did not alter the development of tolerance to the opiate. These results discounted the involvement of alpha2-adrenoceptors and I(1)-imidazoline receptors in the modulatory effect of idazoxan on opioid tolerance. 5. Idazoxan and other imidazol(ine) drugs fully inhibited [3H]-(+)-MK-801 binding to N-methyl-D-aspartate (NMDA) receptors in the rat cerebral cortex with low potencies (Ki: 37-190 microM). The potencies of the imidazolines idazoxan, RX821002 and moxonidine were similar, indicating a lack of relationship between potency on NMDA receptors and ability to attenuate opioid tolerance. These results suggested that modulation of opioid tolerance by idazoxan is not related to NMDA receptors blockade. 6. Chronic treatment (13 days) with morphine (10 mg kg(-1), i.p.) was associated with a marked decrease (49%) in immunolabelled neurofilament proteins (NF-L) in the frontal cortex of morphine-tolerant rats, suggesting the induction of neuronal damage. Chronic treatment (13 days) with idazoxan (10 mg kg(-1)) and LSL 60101 (10 mg kg(-1)) did not modify the levels of NF-L proteins in brain. Interestingly, the concurrent chronic treatment (13 days) of idazoxan or LSL 60101 and morphine, completely reversed the morphine-induced decrease in NF-L immunoreactivity, suggesting a neuroprotective role for these drugs. 7. Together, the results indicate that chronic treatment with I2-imidazoline ligands attenuates the development of tolerance to opiate drugs and may induce neuroprotective effects on chronic opiate treatment. Moreover, these findings offer the I2-imidazoline ligands as promising therapeutic coadjuvants in the management of chronic pain with opiate drugs.