Revisiting clonidine: an innovative add-on option for attention-deficit/hyperactivity disorder.

Attention-deficit/hyperactivity disorder (ADHD) is a common neurobehavioral disorder characterized by developmentally inappropriate levels of inattention, hyperactivity and impulsivity. Although much evidence supports the use of psychostimulants as a first-line treatment in children and adolescents, up to 30% of patients may have an inadequate response to these medications. For these patients, addition of an α2-adrenoceptor agonist can further improve ADHD symptoms. The α2-adrenoceptor agonists may work in a synergistic fashion with stimulants through regulation of prefrontal cortex function. Early studies were completed with immediate-release clonidine (CLON-IR), which requires multiple daily doses and achieves a higher maximum concentration more rapidly than the more recently developed extended-release clonidine (CLON-XR). Pharmacokinetic properties of CLON-XR may be responsible for differences in efficacy and tolerability between the CLON-IR and CLON-XR formulations. Recent double-blind, placebo-controlled trials have shown that extended-release α2-adrenoceptor agonists are safe and effective, both as monotherapy and as adjunctive treatment with stimulants. This review will focus on clonidine used in conjunction with stimulants to optimize treatment of ADHD.

Non-specific binding and general cross-reactivity of Y receptor agonists are correlated and should importantly depend on their acidic sectors.

Non-specific binding of Y receptor agonists to intact CHO cells, and to CHO cell or rat brain particulates, is much greater for human neuropeptide Y (hNPY) compared to porcine peptide Y (pPYY), and especially relative to human pancreatic polypeptide (hPP). This binding of hNPY is reduced by alkali cations in preference to non-ionic chaotrope urea, while the much lower non-specific binding of pPYY is more sensitive to urea. The difference could mainly be due to the 10-16 stretch in 36-residue Y agonists (residues 8-14 in N-terminally clipped 34-peptides), located in the sector that contains all acidic residues of physiological Y agonists. Anionic pairs containing aspartate in the 10-16 zone could be principally responsible for non-specific attachments, but may also aid the receptor site binding. Two such pairs are found in hNPY, one in pPYY, and none in hPP. The hydroxyl amino acid residue at position 13 in mammalian PYY and PP molecules could lower conformational plasticity and the non-selective binding via intrachain hydrogen bonding. The acidity of this tract could also be important in agonist selectivity of the Y receptor subtypes. The differences point to an evolutionary reduction of promiscuous protein binding from NPY to PP, and should also be important for Y agonist selectivity within NPY receptor group, and correlate with partial agonism and out-of group cross-reactivity with other receptors.

Oligomerization of the heptahelical G protein coupling receptors: a case for association using transmembrane helices.

The heptahelical G protein coupling receptors oligomerize extensively via transmembrane domains, in association with heterotrimeric G proteins. This provides higher affinity for agonists, conformational stability necessary for signal transduction, and protection from intracellular proteinases. The oligomerization is relevant to organismic pathophysiology and could be targeted by natural or modified agonists.

Dimers of the neuropeptide Y (NPY) Y2 receptor show asymmetry in agonist affinity and association with G proteins.

In conditions precluding activation of G proteins, the binding of agonists to dimers of the neuropeptide Y (NPY) Y2 receptor shows two components of similar size, but differing in affinity. The dimers of all NPY receptors are solubilized as approximately 180-kDa complexes containing one G protein alpha beta gamma trimer. These heteropentamers are stable to excess agonists, chelators, and alkylators. However, dispersion in the weak surfactant cholate releases approximately 300-kDa complexes. These findings indicate that both protomers in the Y2 dimer are associated with G protein heterotrimers, but the extent of interaction depends on affinity for the agonist peptide. The G protein in contact with the first-liganded, higher-affinity protomer should have a stronger interaction with the receptor and a larger probability of activation.

Neuropeptide Y (NPY) Y2 receptors of rabbit kidney cortex are largely dimeric.

The neuropeptide Y (NPY) Y2 receptors and the pancreatic polypeptide Y4 receptors from rabbit kidney cortex are isolated largely as approximately 180 kDa complexes constituted of one receptor dimer and one G-protein heterotrimer, similar to NPY receptors expressed in the Chinese hamster ovary (CHO) cells. As expected, kidney and CHO cell Y2 dimers are converted into monomers by increasing concentrations of a selective agonist. Prevalence of dimeric Y2 receptors in the kidney could be related to low plasma levels of Y2 agonists, and possibly also to a relatively low concentration of Gi alpha subunits.

The neuropeptide Y (NPY) Y2 receptors are largely dimeric in the kidney, but monomeric in the forebrain.

The neuropeptide Y(NPY) Y2 receptors are detected largely as dimers in the clonal expressions in CHO cells and in particulates from rabbit kidney cortex. However, in two areas of the forebrain (rat or rabbit piriform cortex and hypothalamus), these receptors are found mainly as monomers. Evidence is presented that this difference relates to large levels of G proteins containing the Gi alpha -subunit in the forebrain areas. The predominant monomeric status of these Y2 receptors should also be physiologically linked to large synaptic inputs of the agonist NPY. The rabbit kidney and the human CHO cell-expressed Y2 dimers are converted by agonists to monomers in vitro at a similar rate in the presence of divalent cations.

Oligomerization of neuropeptide Y (NPY) Y2 receptors in CHO cells depends on functional pertussis toxin-sensitive G-proteins.

Human neuropeptide Y Y2 receptors expressed in CHO cells are largely oligomeric, and upon solubilization are recovered by density gradient centrifugation as approximately 180 kDa complexes of receptor dimers and G-protein heterotrimers. A large fraction of the receptors is inactivated in the presence of pertussis toxin, in parallel with inactivation of Gi alpha subunits (with half-periods of about 4 h for both). This is accompanied by a very long-lasting loss of receptor dimers and of masked surface Y2 sites (an apparent receptor reserve pre-coupled mainly to Gi alpha subunit-containing G-proteins). However, surface Y2 receptors accessible to large peptide agonists are much less sensitive to the toxin. All surface Y2 receptors are rapidly blocked by Y2 antagonist BIIE0246, with a significant loss of the dimers, but with little change of basal Gi activity. However, both dimers and Y2 receptor compartmentalization are restored within 24 h after removal of the antagonist. In CHO cells, the maintenance and organization of Y2 receptors appear to critically depend on functional pertussis toxin-sensitive G-proteins.

Self-regulation of agonist activity at the Y receptors.

Neuropeptide Y (NPY) is one of the most abundant neuropeptides, and is likely to be present at nanomolar levels over extended periods in the synaptic space of many forebrain areas. This might be linked to an evolved generalized toning activity through a number of other peptide receptors that use C-terminally amidated agonists (with LHRH and orexin receptors and GIR as examples). However, the Y1 and Y2 receptors (which constitute the bulk of Y receptors active in the neural matrix) possess subnanomolar affinities that, at saturating NPY levels, could produce excessive signaling, as well as receptor losses via repeated endocytosis. The related Y4 receptor shows an even higher agonist affinity, and faces the same problem in visceral and neural locations accessible to pancreatic polypeptide (PP). An examination of agonist peptide interaction with Y receptors shows that Y1 and Y4 receptors in particular (as located on either the intact cells, or on particulates derived from various cell types) develop a blockade dependent on ligand concentration, with the blocking ranks of [NPY]>>[peptide YY] (PYY) for the Y1, and [human PP]>>>[PYY-related Y4 agonist] for the Y4 receptor. This blockade is also echoed in a concentration-related reduction in biological activity of primary agonists (NPY and PP), resembling a partial agonism, and is influenced especially by the allosteric interactivity of agonists. With the Y2 receptor, the blocking by agonists is less pronounced, but the signaling by NPY-related peptides is apparently less than with PYY-related agonists. The extended occupancy and self-attenuation of primary agonist activity at Y receptors could represent an evolutionary solution contributing to a balancing of metabolic signaling, agonist clearance and receptor conservation.

Parallel inactivation of Y2 receptor and G-proteins in CHO cells by pertussis toxin.

The Y(2) receptor for neuropeptide Y (NPY) interacts with pertussis toxin (PTX)-sensitive G-proteins, but little is known about interdependence of their levels and functions. We found that PTX reduces Y(2) receptors expressed in CHO cells in parallel to inactivation of Gi G-proteins, to loss of inhibition by Y(2) agonists of forskolin-stimulated adenylyl cyclase, and to decrease in the binding of GTP-gamma-S. These losses were attenuated by the endosome alkalinizer ammonium chloride. Affinity of the Y(2) receptor was not changed by PTX treatment. Prolonged treatment induced a large decrease of Y(2) receptor immunoreactivity (more than 70% in 48 h). The Gi(3) alpha-subunit immunoreactivity decreased slowly (about 46% in 48 h). There was a significant increase in Gq alpha immunoreactivity and in fraction of Y(2) binding sensitive to a Gq-selective antagonist. Possibly linked to that, the surface Y(2) sites and the internalization of the Y(2) receptor were less than 40% reduced. However, the abundant masked Y(2) sites were eliminated by the toxin, and could be mainly coupled to PTX-sensitive G-proteins.

Altered mesolimbocortical and thalamic dopamine in Tourette syndrome.

We used [F-18]fallypride PET in six adults with Tourette syndrome and age-matched controls to assess extrastriatal dopamine 2 (D2) receptors. D2 receptor availability was significantly lower in the orbitofrontal cortex, primary motor cortex, anterior cingulate gyrus, mediodorsal nucleus of thalamus, and hippocampus, areas important for motivation and reward, sensory gating, movement, and attention. Altered dopaminergic function in mesolimbocortical systems and thalamus may contribute to increased motivational salience of tics.

Atomoxetine treatment in children and adolescents with ADHD and comorbid tic disorders.

OBJECTIVE: To test the hypothesis that atomoxetine does not significantly worsen tic severity relative to placebo in children and adolescents with attention deficit/hyperactivity disorder (ADHD) and comorbid tic disorders. METHODS: Study subjects were 7 to 17 years old, met Diagnostic and Statistical Manual of Mental Disorders-IV criteria for ADHD, and had concurrent Tourette syndrome or chronic motor tic disorder. Patients were randomly assigned to double-blind treatment with placebo (n = 72) or atomoxetine (0.5 to 1.5 mg/kg/day, n = 76) for up to 18 weeks. RESULTS: Atomoxetine treatment was associated with greater reduction of tic severity at endpoint relative to placebo, approaching significance on the Yale Global Tic Severity Scale total score (-5.5 +/- 6.9 vs -3.0 +/- 8.7, p = 0.063) and Tic Symptom Self-Report total score (-4.7 +/- 6.5 vs -2.9 +/- 5.2, p = 0.095) and achieving significance on the Clinical Global Impressions (CGI) tic/neurologic severity scale score (-0.7 +/- 1.2 vs -0.1 +/- 1.0, p = 0.002). Atomoxetine patients also showed greater improvement on the ADHD Rating Scale total score (-10.9 +/- 10.9 vs -4.9 +/- 10.3, p < 0.001) and CGI severity of ADHD/psychiatric symptoms scale score (-0.8 +/- 1.1 vs -0.3 +/- 1.0, p = 0.015). Discontinuation rates were not significantly different between treatment groups. Atomoxetine patients had greater increases in heart rate and decreases of body weight, and rates of treatment-emergent decreased appetite and nausea were higher. No other clinically relevant treatment differences were seen in any other vital sign, adverse event, or electrocardiographic or laboratory measures. CONCLUSIONS: Atomoxetine did not exacerbate tic symptoms. Rather, there was some evidence of reduction in tic severity with a significant reduction of attention deficit/hyperactivity disorder symptoms. Atomoxetine treatment appeared safe and well tolerated.

Expression of the glucocorticoid-induced receptor mRNA in rat brain.

The glucocorticoid-induced receptor (GIR) is an orphan G-protein-coupled receptor awaiting pharmacological characterization. GIR was originally identified in murine thymoma cells, and shows a widespread, yet not completely complementary distribution in mouse and human brain. Expression of the mouse GIR gene is modulated by dexamethasone in the brain and periphery, suggesting that GIR function is directly responsive to glucocorticoid signals. The rat GIR was cloned from rat prefrontal cortex by our group and was shown to be up-regulated following chronic amphetamine. The physiological role of GIR in the rat is not known at present. In order to gain a clearer understanding of the potential functions of GIR in the rat, we performed a detailed mapping of GIR mRNA expression in the rat brain. GIR mRNA showed widespread distribution in forebrain limbic and thalamic structures, and a more restricted distribution in hindbrain areas such as the spinal trigeminal nucleus and the median raphe nucleus. Areas with moderate to high levels of GIR include olfactory regions such as the nucleus of olfactory tract, hippocampus, various thalamic nuclei, cortical layers, and some hypothalamic nuclei. In comparison with previous studies, significant regional differences exist in GIR distribution in mouse and rat brain, particularly in the thalamus, striatum and in hippocampus at a cellular level. Overall, the expression of GIR in rat brain more closely approaches that seen previously in human than mouse, suggesting that rat models may be more informative for understanding the role of GIR in glucocorticoid physiology and glucocorticoid-related disease states. GIR mRNA distribution in the rat indicates a potential role of this receptor in the control of feeding and ingestive behavior, regulation of stress and emotional behavior, learning and memory, and, drug reinforcement and reward.

Tic reduction with pergolide in a randomized controlled trial in children.

OBJECTIVE: To determine whether pergolide, a mixed D1/D2/D3 dopamine agonist, is efficacious and safe in the treatment of children with chronic tic disorders and Tourette syndrome. BACKGROUND: Neuroleptics, which block dopamine transmission, are currently used to treat children with severe tics, but major side effects and limited efficacy reduce clinical utility. Prior open-label and crossover studies of pergolide suggest potential benefit. METHODS: The authors enrolled 57 children and adolescents, ages 7 to 17 years, randomizing them in a 2:1 ratio to either pergolide (0.15 to 0.45 mg per day) or placebo. Tic symptoms had to be >30 on the Yale Global Tic Severity Scale (YGTSS). The primary outcome measure was change in tic severity assessed by YGTSS. RESULTS: Compared to placebo treatment, pergolide treatment was associated with lower tic severity scores (treatment effect 8.8, pergolide vs placebo; 95% CI 0.1 to 17.6; p = 0.05) and attention-deficit hyperactivity disorder symptoms scores (treatment effect 3.8; 95% CI 0.7 to 6.8; p = 0.02). No patient had a serious adverse event and pergolide was well tolerated. CONCLUSIONS: In this randomized, placebo-controlled trial, pergolide appeared to be an efficacious and safe medication for tic reduction in children, and may also improve attention-deficit hyperactivity disorder symptoms.

Atomoxetine in the treatment of children and adolescents with attention-deficit/hyperactivity disorder: a randomized, placebo-controlled, dose-response study.

OBJECTIVE: Atomoxetine is an investigational, nonstimulant pharmacotherapy being studied as potential treatment for attention-deficit/hyperactivity disorder (ADHD). It is thought to act via blockade of the presynaptic norepinephrine transporter in the brain. We assessed the efficacy of 3 doses of atomoxetine compared with placebo in children and adolescents with ADHD. METHODS: A total of 297 children and adolescents who were 8 to 18 years of age and had ADHD as defined by the Diagnostic and Statistical Manual of Mental Disorders, 4th edition, were randomized to placebo or atomoxetine dosed on a weight-adjusted basis at 0.5 mg/kg/day, 1.2 mg/kg/day, or 1.8 mg/kg/day for an 8-week period. ADHD symptoms, affective symptoms, and social and family functioning were assessed using parent and investigator rating scales. RESULTS: Approximately 71% of children enrolled were male, approximately 67% met criteria for mixed subtype (both inattentive and hyperactive/impulsive symptoms), and the only common psychiatric comorbidity was oppositional defiant disorder (approximately 38% of the sample). At baseline, symptom severity was rated as moderate to severe for most children. At endpoint, atomoxetine 1.2 mg/kg/day and 1.8 mg/kg/day were consistently associated with superior outcomes in ADHD symptoms compared with placebo and were not different from each other. The dose of 0.5 mg/kg/day was associated with intermediate efficacy between placebo and the 2 higher doses, suggesting a graded dose-response. Social and family functioning also were improved in the atomoxetine groups compared with placebo with statistically significant improvements in measures of children's ability to meet psychosocial role expectations and parental impact. Discontinuations as a result of adverse events were <5% for all groups. CONCLUSION: Among children and adolescents aged 8 to 18, atomoxetine was superior to placebo in reducing ADHD symptoms and in improving social and family functioning symptoms. Atomoxetine was associated with a graded dose-response, and 1.2 mg/kg/day seems to be as effective as 1.8 mg/kg/day and is likely to be the appropriate initial target dose for most patients. Treatment with atomoxetine was safe and well tolerated.

Cloning, expression, and regulation of a glucocorticoid-induced receptor in rat brain: effect of repetitive amphetamine.

Behavioral sensitization to psychostimulants involves neuroadaptation of stress-responsive systems. We have identified and sequenced a glucocorticoid-induced receptor (GIR) cDNA from rat prefrontal cortex. The full-length GIR cDNA encodes a 422 amino acid protein belonging to G-protein-coupled receptor superfamily. Although the ligand for GIR is still unknown, the dendrogram construction indicates that GIR may belong to peptide receptor subfamily (e.g., substance P receptor), with more distant relationship to subfamilies of glycoprotein hormone receptors (e.g., thyrotropin receptor) and biogenic amine receptors (e.g., dopamine receptor). GIR shares 31-34% amino acid identity to the tachykinin receptors (substance P receptor, neurokinin A receptor, and neurokinin B receptor). GIR mRNA is expressed preferentially in brain, and its neuronal expression is relegated to limbic brain regions, particularly in forebrain. GIR transcript levels are increased significantly and persistently in prefrontal cortex for 7 d after discontinuation of chronic amphetamine exposure. The induction of GIR expression by amphetamine is associated with augmented behavioral activation. These findings suggest that modulation of GIR expression may be involved in behavioral sensitization, and GIR may play a role at the interface between stress and neuroadaptation to psychostimulants.

Transcranial magnetic stimulation (TMS) as a research tool in Tourette syndrome and related disorders.

TMS is a technology with much promise for understanding brain function in health and disease. This chapter has reviewed the physics and safety of this new tool. Much basic work remains to be done in order to understand exactly how TMS affects neurons, and the roles that intensity, frequency, and location have on brain activity. Recent work combining TMS with imaging is showing the method for future advances. In the area of TS, TMS has already had a significant impact; 3 studies have hinted that TS patients have a deficient inhibitory system. Other investigators are exploring whether TMS might be used in a therapeutic manner to alter the behavior of dysfunctional circuits. These areas of applying TMS to understanding and perhaps treating TS show much promise. Nonetheless, a great deal of basic work is likely needed before TMS can be fully used as a research tool or treatment modality for TS.

Yohimbine challenge in children with anxiety disorders.

OBJECTIVE: The authors evaluated the neurohormonal and subjective mood response of children with anxiety disorders who were challenged with yohimbine. METHOD: Seventeen children with DSM-IV diagnoses of anxiety disorders and 15 normal comparison children were given yohimbine orally (0.1 mg/kg). Neurohormonal measures and visual analog self-reports of tenseness were recorded over a 150-minute period. RESULTS: Yohimbine was uniformly well tolerated, and it behaviorally differentiated children with anxiety disorders from normal comparison children with higher maximum change (Deltamax) ratings of anxiety in the patients (mean=17.4 mm, SD=29.8) than in the comparison subjects (mean=0.3 mm, SD=4.4). Yohimbine-stimulated Deltamax growth hormone (GH) for children with anxiety disorders (mean=-1.5 ng/ml, SD=5.9) was significantly reduced compared to that of normal comparison children (mean=2.7 ng/ml, SD=4.5). CONCLUSIONS: Yohimbine selectively elevates self-rated anxiety in children with anxiety disorders and is associated with the blunting of GH in those children relative to that of comparison children. Presence of a blunted GH response to yohimbine in children with anxiety disorders is reminiscent of findings in adults with anxiety disorders, particularly panic disorder. These findings support enhanced central adrenergic sensitivity in children with anxiety disorders, as demonstrated by yohimbine-exacerbated anxiety. The findings should be reconciled with the absence of clonidine-related GH blunting in the same cohort.

Fluoxetine-related death in a child with cytochrome P-450 2D6 genetic deficiency.

The clinical course of a 9-year-old diagnosed with attention-deficit hyperactivity disorder, obsessive-compulsive disorder, and Tourette's disorder and treated with a combination of methylphenidate, clonidine, and fluoxetine is described. The patient experienced over a 10-month period, signs and symptoms suggestive of metabolic toxicity marked by bouts of gastrointestinal distress, low-grade fever, incoordination, and disorientation. Generalized seizures were observed, and the patient lapsed into status epilepticus followed by cardiac arrest and subsequently expired. At autopsy, blood, brain, and other tissue concentrations of fluoxetine and norfluoxetine were several-fold higher than expected based on literature reports for overdose situations. The medical examiner's report indicated death caused by fluoxetine toxicity. As the child's adoptive parents controlled medication access, they were investigated by social welfare agencies. Further genetic testing of autopsy tissue revealed the presence of a gene defect at the cytochrome P450 CYP2D locus, which results in poor metabolism of fluoxetine. As a result of this and other evidence, the investigation of the adoptive parents was terminated. This is the first report of a fluoxetine-related death in a child with a confirmed genetic polymorphism of the CYP2D6 gene that results in impaired drug metabolism. Issues relevant to child and adolescent psychopharmacology arising from this case are discussed.

Tourette's syndrome improvement with pergolide in a randomized, double-blind, crossover trial.

OBJECTIVE: To determine whether pergolide, a mixed D1-D2-D3 dopamine agonist, is efficacious and safe in the treatment of children with Tourette's syndrome. BACKGROUND: Neuroleptics, which block dopamine transmission, are currently used for treatment of children with severe tics, but major side effects and limited efficacy reduce clinical utility. Prior open-label reports of pergolide suggest potential benefit. METHODS: The authors enrolled 24 children age 7 to 17 years with Tourette's disorder, chronic motor tic disorder, or chronic vocal tic disorder by Diagnostic and Statistical Manual of Mental Disorders (4th ed.) criteria, plus severity criteria on the Yale Global Tic Severity Scale (YGTSS) of > or =20, in a double-blind, placebo-controlled, crossover study. Children were randomized to receive either placebo or up to 300 microg/day pergolide for the first 6-week treatment period, with a 2-week placebo washout, followed by crossover to the alternate treatment. The primary outcome measure was tic severity assessed by YGTSS. RESULTS: Compared with placebo treatment, pergolide treatment was associated with significantly lower YGTSS scores (42.0 +/- 20.4 versus 23.5 +/- 18.7; F = 12.0, df = 1, 17, p = 0.0011). No patient had a serious adverse event and pergolide was well tolerated. CONCLUSIONS: In this randomized, placebo-controlled, crossover trial, pergolide appeared to be a safe and efficacious treatment for Tourette's syndrome in children.

Ziprasidone treatment of children and adolescents with Tourette's syndrome: a pilot study.

OBJECTIVE: To evaluate the efficacy and tolerability of ziprasidone in children and adolescents with Tourette's syndrome and chronic tic disorders. METHOD: Twenty-eight patients aged 7 to 17 years were randomly assigned to ziprasidone or placebo for 56 days. Ziprasidone was initiated at a dose of 5 mg/day and flexibly titrated to a maximum of 40 mg/day. RESULTS: Ziprasidone was significantly more effective than placebo in reducing the Global Severity (p = .016) and Total Tic (p = .008) scores on the Yale Global Tic Severity Scale. Compared with placebo, ziprasidone significantly reduced tic frequencies as determined by blind videotape tic counts (p = .039). The mean (+/- SD) daily dose of ziprasidone during the last 4 weeks of the trial was 28.2 +/- 9.6 mg. Mild transient somnolence was the most common adverse event. No clinically significant effects were observed on specific ratings of extrapyramidal symptoms, akathisia, or tardive dyskinesia. CONCLUSIONS: In this limited sample, ziprasidone (5-40 mg/day) appears to be effective and well tolerated in the treatment of Tourette's syndrome. Ziprasidone may be associated with a lower risk of extrapyramidal side effects in children. However, additional studies are necessary to evaluate more fully its safety and efficacy in children with tic disorders.