Chronic clonidine induces functional down-regulation of presynaptic alpha 2-adrenoceptors regulating [3H]noradrenaline and [3H]5-hydroxytryptamine release in the rat brain

A case of intracranial hemorrhage after a single dose of Yohimbine in a chronic user of clonidine

INTRODUCTION

Yohimbine, an alpha-2 adrenoreceptor antagonist found in a variety of supplements, has been historically used to treat libido, erectile dysfunction, xerostomia, and as a weight loss enhancement. Yohimbine toxicity causes a sympathomimetic syndrome as demonstrated by the case below of a female who developed an intracranial hemorrhage (ICH) following an ingestion of yohimbine.

CASE

This case follows a 39-year-old female who presented to the emergency department (ED) with complaints of nausea, emesis, and flushing following the ingestion of a female sexual enhancement supplement labeled to contain yohimbine (BioXgenic Nature's Desire) one hour prior. The patient took her prescribed 0.1 mg clonidine when the symptoms commenced. Upon arrival, the patient's blood pressure was 198/93. She developed neurological sequelae including a left-sided facial droop and weakness of her right extremities. A computed tomography scan demonstrated an acute basal ganglia hemorrhage with mild mass effect and mild subarachnoid hemorrhage. She was transferred to a regional referral hospital and discharged 16 days later to a rehabilitation center with persistent neurological sequelae.

DISCUSSION

This patient exhibited sympathetic toxicity temporally associated with yohimbine ingestion. Our patient also had a variety of risk factors that increased the likelihood of a poor outcome with yohimbine. Chronic use of clonidine is known to down-regulate alpha-2 receptors. This leads to dependence of clonidine to maintain adrenergic homeostasis and could potentiate the effects of yohimbine. To compound effects, our patient was also taking bupropion and desvenlafaxine, which inhibit norepinephrine reuptake, likely worsening our patient's sympathomimetic response. Despite the temporal relationship of our patient's ICH and ingestion of yohimbine, a definitive relationship cannot be inferred due to our lack of confirmatory testing of yohimbine content and possibility of adulterants. The U.S. Food and Drug Administration (FDA) does not regulate the supplement market strictly, with multiple studies illustrating variation among ingredients of supplements despite stated quantities on the labels.

CONCLUSION

Dietary supplements are not required by the FDA to undergo efficacy or safety testing, necessitating clear post-marketing communication regarding potential adverse events from various supplements. Users should be aware of yohimbine-containing products and the possible side effects of toxicity. It is crucial that physicians and patients be aware of possible drug-supplement interactions of yohimbine and the presentation of sympathomimetic syndromes.

Hypertensive Crisis in a Pediatric Patient Experiencing Clonidine Withdrawal

Background. Clonidine, a central alpha-adrenoreceptor agonist, was initially developed as an antihypertensive. Though no longer commonly used for its original indication due to rebound hypertension after discontinuation, it is currently widely prescribed as a treatment for many pediatric indications including sleep disorders, behavioral concerns, and attention deficit hyperactivity disorder. Case Report. We describe a girl who developed prolonged symptoms of clonidine withdrawal, including hypertension and elevated serum metanephrines. Discussion. Clonidine withdrawal in pediatric patient can present with hypertensive urgency and other signs of sympathetic stimulation. Withdrawal can also lead to dramatic elevation in serum metanephrines. Treatment with a clonidine taper will reduce development of withdrawal symptoms. Conclusion. Given the rise in clonidine use in pediatric patients, clinicians should be aware of the risk of clonidine withdrawal and how to recognize and avoid its development.

Desensitization of alpha(2)-adrenoceptors which regulate noradrenaline synthesis and release after chronic treatment with clorgyline in the rat brain

The sensitivity of alpha(2)-adrenoceptors which regulate synthesis and release of noradrenaline was investigated in hippocampus, parietal cortex, and hypothalamus of rats treated with clorgyline. After administering a DOPA decarboxylase inhibitor, the in vivo tyrosine hydroxylase activity and the noradrenaline content were evaluated. Acute and chronic treatment with clorgyline led to both increases of noradrenaline levels and decreases of tyrosine hydroxylase activity, determined as the accumulation of DOPA. Whereas the alpha(2)-adrenoceptor agonist clonidine induced a similar reduction in tyrosine hydroxylase activity in the group subjected to the acute treatment and in the control group, it failed to do so after chronic clorgyline treatment. In hippocampal and cortical synaptosomes, a reduction in the sensitivity of alpha(2)-adrenoceptors which regulate [(3)H]noradrenaline release, reflected by the shift to the right of the concentration-effect curves for oxymetazoline, was also found after the repeated treatment. These results indicate a desensitization of alpha(2)-adrenoceptors after chronic treatment with clorgyline.

Distribution of alpha 2-adrenoceptors in the human locus coeruleus

Recent reports of specific topographic patterns of cell loss in the locus coeruleus (LC) in psychiatric and neurologic disorders underscores the need for detailed neurochemical analyses of this cell group. In this study, the anatomical distribution of alpha 2-adrenoceptors and its relationship to the distribution of noradrenergic neurons in the human LC was studied. Quantitative autoradiography was used to assess the binding of [125I]p-iodoclonidine ([125I]PIC) to alpha 2-adrenoceptors coordinately with counts of neuromelanin-containing cells in tissue sections cryocut at 10-13 levels along the rostrocaudal axis of the LC. Pontine brain tissue was obtained postmortem from 7 subjects dying of natural or accidental causes, ranging in age from 26 to 78 years. Both the binding of [125I]PIC and number of neuromelanin-containing cells were differentially distributed along the LC axis (P < 0.01) with almost identical topographical patterns. The highest concentration of binding and the greatest number of neuromelanin-containing cells per section occurred near the middle portion of the nucleus. There was a significant correlation between the number of neuromelanin-containing cells per section and the specific binding of [125I]PIC at any particular level of the LC (r2 = 0.56; P < 0.0001). The highest and lowest amounts of [125I]PIC binding in the LC were observed in the youngest and oldest subjects, respectively, and this trend was parallelled by a significant negative correlation between the number of neuromelanin-containing cells at a given level and age (r2 = 0.85; P < 0.003). The uneven distribution of alpha 2-adrenoceptors in the LC demonstrates the importance of anatomical specificity when performing quantitative studies of LC protein chemistry in psychiatric and neurologic disorders.

Increased alpha 2-adrenoceptor mediated regulation of adult rat brain noradrenaline overflow after chronic neonatal exposure to propranolol; a microdialysis study

Direct and persistent effects of chronic neonatal administration of the beta-adrenoceptor antagonist propranolol on brain noradrenergic activity were investigated by measuring tissue concentrations of noradrenaline and its metabolites and in vivo overflow of noradrenaline during adulthood. Rat pups were chronically treated with propranolol from postnatal day 1 to day 10. Determination of monoamine metabolism after the last injection showed an increase in noradrenaline metabolism in frontal cortex, limbic system and hippocampus of propranolol-exposed rats, but 47 days after this last injection it was apparent that these effects were not long-lasting. Moreover, basal noradrenaline overflow in vivo in the hippocampus of 40-55 day-old propranolol-exposed rats did not differ from that in controls. However, the regulation of noradrenaline release seemed to have been altered, since a pharmacological challenge with the alpha 2-adrenoceptor antagonist idazoxan induced an enhanced increase in the in vivo noradrenaline overflow in propranolol-exposed rats compared to controls. It is suggested that the neonatal beta-blockade induced a supersensitivity of the presynaptic alpha 2-adrenoceptor. The precise mechanism underlying this effect has to be elucidated.

Chronic clonidine treatment and its termination: effects on penile erection and ejaculation in the dog

The effects of chronic administration (4 weeks) of the alpha-2 adrenoceptor agonist clonidine (CL) and its termination on penile erection and ejaculation were investigated in male dogs. Penile erection and ejaculation were elicited by manual penile stimulation (for 5 min). CL (10 micrograms/kg/hr, s.c.) was delivered via osmotic minipump (Alza, 2ML-4). 3 or 7 days after the minipump implantation, CL caused a significant decrease in the amount of ejaculate produced by the genital stimulation without affecting the erectile potency. Ejaculatory ability returned to pretreatment levels despite continued CL administration, becoming evident in tests 14 days after initiation of treatment. Further, chronic CL (23 days) antagonized the inhibitory effects of acute administration of CL (0.05 mg/kg, i.p.). These data indicate tolerance to continued delivery of low doses as well as to acute administration of a higher dose. In the acute drug experiments, the ejaculatory inhibition elicited by CL (0.05 mg/kg, i.p.) was completely antagonized by pretreatment with yohimbine (0.05 and 0.10 mg/kg, i.p.), an alpha-2 adrenoceptor antagonist, but not with naloxone (1.0 mg/kg, i.p.), an opioid receptor antagonist. Furthermore, DG-5128 (1.0 and 2.0 mg/kg, i.p.), a selective alpha-2 adrenoceptor antagonist that poorly penetrates the blood-brain barrier, failed to antagonize the CL-induced ejaculatory inhibition. This study suggests that functional alterations in the central alpha-2 adrenoceptor mechanism may be related to the changes in the ejaculatory capacity during chronic treatment with CL.

Early postnatal clonidine treatment results in altered regional catecholamine utilisation in adult rat brain

Clonidine is a clinically used antihypertensive which has been suggested to produce physiological changes in children after exposure in utero. The aim of our study was to test the hypothesis that chronic exposure of the developing brain to an alpha 2-adrenergic agonist like clonidine would influence the adult neurochemical setting of central monoamine neurotransmitter systems. Male rat pups were treated from postnatal day 8 to 21 twice daily with saline or with 0.1 mg/kg clonidine. After the last injection on day 21, brain regional catecholamine utilisation was determined using synthesis inhibition with alpha-methyl-p-tyrosine in a subgroup of the pups. The expected decrease in noradrenaline utilisation after clonidine was observed, although statistical significance was not reached in a number of brain regions. Dopamine utilisation was not affected. The other pups were left to reach young adulthood and catecholamine utilisation was measured on day 90. Noradrenaline utilisation on day 90 was significantly decreased in two regions: the medulla-pons and the mesolimbic (dopamine projection) areas. Dopamine utilisation was decreased in the hypothalamus and increased in the amygdala and the cerebellum. These adult neurochemical alterations corroborate previous findings of adult behavioural, physiological and central biochemical alterations in rats exposed to clonidine in early postnatal life.

Clonidine inhibition of potassium-evoked release of glutamate and aspartate from rat cortical synaptosomes

Release of endogenous glutamic acid (Glu), aspartic acid (Asp) and gamma-aminobutyric acid (GABA) has been investigated using synaptosomes prepared from rat cerebral cortex. Exposure in superfusion to a depolarizing concentration of KCl (30 mM) evoked 3-, 2- and 2-fold increases in Glu, Asp and GABA release, respectively. More than 70% of Glu and Asp overflow were calcium-dependent, although 67% of the GABA overflow was calcium-independent. Clonidine inhibited the K(+)-evoked overflow of Glu and Asp in a concentration-dependent manner, but the GABA overflow was not inhibited. Clonidine inhibited K(+)-evoked Glu and Asp overflow to 40 and 30% of the control with a potency (IC50) of 11 and 36 nM, respectively. Similarly, norepinephrine inhibited the K(+)-evoked overflow of Glu and Asp, although phenylephrine and isoproterenol showed no effect. Rauwolscine, yohimbine and idazoxan counteracted the effects of clonidine on Glu and Asp overflow. The data suggest that the depolarization-evoked overflow of excitatory amino acids is regulated in an inhibitory fashion by alpha 2 adrenoceptors, which are located on the nerve terminals of Glu and Asp neurons in rat cortex.

Effect of clonidine on blood pressure and GABAergic mechanisms in spontaneously hypertensive rats

The action of clonidine on blood pressure and on the functional state of the gamma-aminobutyric acid-ergic (GABAergic) system was studied. A single injection of clonidine (1, 5, 10, 20 micrograms.kg-1) induced a dose-dependent decrease of blood pressure. Chronic administration of clonidine, 10 micrograms.kg-1, produced the maximum effect after the third injection. The effect was maintained for the duration of the study. Single or chronic clonidine injections, at the dose of 10 micrograms.kg-1 enhanced the GABA content in the brain and hypothalamus. This effect was less pronounced in the hippocampus. The drug administered according to the same regimen stimulated glutamic acid decarboxylase activity only in the hypothalamus. Clonidine caused a marked enhancement of specific [3H]GABA binding in the hypothalamus. These data suggest that the hypotensive action of clonidine is related to stimulation of the GABAergic system.

A microassay for measuring synaptosomal 3H-dopamine and 3H-metabolite release

A modified synaptosomal superfusion apparatus is described which uses less than 10 micrograms of tissue per replicate sample and facilitates the routine separation of 3H-DA, 3H-DOPAC, and 3H-HVA. A flow rate of 1.5 ml/min allows superfusion without the use of reuptake or monoamine oxidase inhibitors. Superfusate samples are collected directly onto alumina columns for the separation of 3H-DA and its acid metabolites. Total recovery of authentic 3H-DA applied via superfusion was 87.63(1.10) percent [Mean(SEM)]. Contamination of the acetic acid eluate fraction, containing 80.98(1.15)% of the total DA, by DOPAC and HVA was less than 0.1%. To illustrate the utility of this technique, the relative proportions of 3H-DA and 3H-metabolites released from synaptosomes by 6 mM potassium and 1 microM reserpine were compared.

Down-regulation of alpha 2-adrenoceptors involved in growth hormone control in the hypothalamus of infant rats receiving short-term clonidine administration

In infant rats short-term administration of the alpha 2-adrenoceptor agonist, clonidine (CLO), induces refractoriness to the growth hormone (GH)-releasing effect of an acute CLO challenge. CLO reportedly stimulates GH release via increased release of GH-releasing hormone (GHRH) from the hypothalamus. Based on these premises, in this study we investigated the possibility that repeated CLO administration may induce down-regulation of hypothalamic alpha 2-adrenoceptors, involved in GH control, thus prohibiting the GH-releasing effect of the drug. alpha 2-Adrenoceptor binding was determined in different brain regions of 10-day-old rats pretreated for 5 days with CLO (150 micrograms/kg, b.i.d.) and killed 14 h after last CLO administration. [3H]p-Aminoclonidine [( 3H]PAC) was used as the specific ligand of alpha 2-adrenoceptors. Treatment with CLO decreased by about 30% the maximum number of binding sites (Bmax) in areas of the mediobasal hypothalamus (MBH) involved in the stimulatory control of GH secretion, i.e. nucleus periventricularis arcuatus, nucleus ventromedialis hypothalami and nucleus lateralis hypothalami. Reduction of Bmax for [3H]PAC binding was observed also in the nucleus periventricularis hypothalami, an area involved in the inhibitory control of GH secretion and, among extrahypothalamic areas, only in the cortex piriformis. In no brain areas was the affinity constant (Kd) for [3H]PAC binding significantly changed after CLO pretreatment. Binding studies performed with a specific ligand of alpha 1-adrenoceptors, [3H]prazosin, showed that the effect of CLO was specific since no changes in the Bmax or Kd were present in either hypothalamic or extrahypothalamic regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic dexmedetomidine, a selective alpha 2-agonist, decreases serotonin but not noradrenaline turnover in rat brainstem nuclei

The effects of a chronic treatment with dexmedetomidine, the active d-isomer of a selective alpha 2-agonist, medetomidine, on the metabolism and turnover of catecholamines and indoleamines in discrete rat brain stem and mesencephalic nuclei were investigated. Chronic dexmedetomidine (3 or 10 micrograms/kg per h for 14 days, delivered by osmotic minipumps) did not change the basal concentrations of noradrenaline, dopamine or 3,4-dihydroxyphenylacetic acid, or the rates of accumulation of 3,4-dihydroxyphenylalanine after inhibition of decarboxylase by NSD 1015. In contrast, the concentrations of 5-hydroxyindoleacetic acid (5-HIAA) and/or 5-HIAA/5-HT were significantly reduced in the nucleus tractus solitarii, raphe dorsalis, locus coeruleus, A1-C1 area, A9 and A10 areas after 10 micrograms/kg per h of dexmedetomidine. The accumulation of 5-hydroxytryptophan was significantly reduced in the locus coeruleus and nucleus tractus solitarii. It is concluded that chronic dexmedetomidine treatment significantly decreases the turnover of 5-hydroxytryptamine in discrete brainstem and mesencephalic nuclei, while the rate of synthesis of noradrenaline in the same areas is not changed.

Modulation of histamine release and synthesis in the brain mediated by alpha 2-adrenoceptors

The adrenergic regulation of histamine release was studied in rat brain slices labeled with L-[3H]histidine. Noradrenaline in increasing concentrations progressively inhibited K+-evoked [3H]histamine release from cortical slices, whereas phenylephrine and isoprenaline were ineffective. Yohimbine, a preferential alpha 2-adrenoceptor antagonist, reversed the noradrenaline effect in an apparently competitive manner and with a mean Ki value of 30 nM. Phentolamine reversed the noradrenaline effect with a similar potency, whereas propranolol was ineffective. The imidazolines clonidine and oxymetazoline acted as partial agonists, oxymetazoline even behaving as an apparent antagonist. In vivo clonidine also inhibited [3H]histamine formation in cerebral cortex, an effect reversed by the administration of yohimbine. However, yohimbine failed to increase significantly [3H]histamine release in vitro and [3H]histamine formation in vivo, suggesting that adrenergic receptors are not activated by endogenous noradrenaline released under basal conditions. It is concluded that adrenergic alpha 2-adrenoceptors presumably located on histaminergic axons control release and synthesis of histamine in the brain.

Acute and long-term regulation of brain alpha 2-adrenoceptors after manipulation of noradrenergic transmission in the rat

The specific binding of [3H]clonidine (KD and Bmax) to rat brain membranes was used as a biochemical index to directly evaluate alpha 2-adrenoceptor changes after manipulation of synaptic noradrenaline (NA) pools or stimulation or blockade of the receptor. Acute (2 h) and prolonged (7 days) inhibition of NA synthesis with alpha-methyl-p-tyrosine (150 mg/kg) or acute (2 h) and chronic (14 days) treatment with reserpine (0.1-0.5 mg/kg) reduced the NA content by 15-90%, which also resulted in marked reductions (35-55%) of the KD values for [3H]clonidine in all brain regions studied. In contrast to alpha-methyl-p-tyrosine, chronic reserpine treatment did not alter the Bmax values for [3H]clonidine or [3H]UK 14304 in any brain region. In the hypothalamus and cerebral cortex, acute (2 h) and chronic (7-14 days) treatment with the monoamine oxidase (MAO) inhibitors clorgyline (1 mg/kg) or tranylcypromine (5 mg/kg) increased the content of NA by 6-100%, which led to marked reductions (20-50%) of Bmax without altering the KD values for [3H]clonidine. Similarly, prolonged (21 days) inhibition of NA neuronal uptake with cocaine or protriptyline (10 mg/kg) also resulted in decreases in Bmax (20-25%) with no alterations in KD in the hypothalamus. In various brain regions, chronic (14 days) but not short-term (1 day) treatment with clonidine (0.1 mg/kg) or yohimbine (10 mg/kg) resulted in decreases (30-40%) and increases (15-20%), respectively, in Bmax without altering the KD values for [3H]clonidine. The results indicate that drugs which deplete endogenous NA up-regulate alpha 2-adrenoceptors (increased affinity of [3H]clonidine binding sites) while drugs which increase the intraneuronal and/or synaptic NA pools down-regulate the receptors (decreased number of [3H]clonidine binding sites). These adaptive receptor changes appear to be dependent on NA availability.

Inhibition of ischemia-induced brain catecholamine alterations by clonidine

The effect of clonidine, an alpha 2-agonist, on ischemia-induced alterations in brain catecholamine and metabolite levels was studied in Mongolian gerbils subjected to 180 min of unilateral cerebral ischemia. The gerbils were randomly assigned to four treatment groups: sham-operated or unilateral carotid lesion; each pretreated with clonidine 0.4 mg/kg IP, or untreated. All animals were neurologically assessed and categorized as asymptomatic, neurological deficit or seizure activity at the time of sacrifice. Hemispheric levels of noradrenaline (NA), dopamine (DA), homovanillic acid (HVA), and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured using high pressure liquid chromatography with electrochemical detection. No changes from control were found in animals that remained asymptomatic regardless of treatment. In untreated gerbils that exhibited neurological deficits, marked reductions in both NA and DA and increases in HVA occurred in the ischemic hemisphere. These alterations were greater in gerbils that developed seizures during the observation period. Ischemic animals pretreated with clonidine did not show any significant alterations in catecholamine or metabolite levels from clonidine-treated, sham-operated controls in spite of the presence of neurological deficits. Although significant reductions in NA and DA still occurred in pretreated animals that developed seizures, the changes were markedly less than in untreated gerbils. These results indicate that alpha 2-adrenoceptor stimulation is an effective approach for inhibition of ischemia-induced brain catecholamine alterations, and thus may provide a useful method for assessing the role of catecholamine release in the production of acute ischemic neuronal damage.

Idazoxan and brain alpha 2-adrenoceptors in the rabbit

The effect of intravenous infusion of idazoxan on the depressor response to intracisternal clonidine 1 microgram/kg and on [3H]yohimbine binding in the fore- and hindbrain of the rabbit was examined. Idazoxan was infused either acutely (30 min) or chronically (5 days) at doses of 0.56 or 1.1 mg/h. Idazoxan 1.1 mg/h reduced the fall in blood pressure after clonidine. This attenuation of the depressor response was observed in the groups that were given the higher dose of idazoxan both acutely and chronically. The extent of attenuation was not modified by the duration of treatment. The low dose of idazoxan given acutely had no significant effect on the response to clonidine but the chronically infused group showed an enhanced response. A significant increase in the number of [3H]yohimbine binding sites (83%) was observed in the forebrain after 5 days infusion of 1.1 mg/h idazoxan with no change in the hindbrain. The lower dose of infusion did not cause any significant change in [3H]yohimbine binding in either brain region. Thus it appears that the susceptibility of the alpha 2-adrenoceptor binding sites to up-regulation by idazoxan may depend on the brain region observed.

Long-term effects of basal forebrain lesions on cholinergic, noradrenergic and serotonergic markers in mouse neocortex

Lesions of basal forebrain cholinergic neurons projecting to cerebral cortex and hippocampus have recently been exploited as animal models for some of the neurochemical and behavioral deficits of Alzheimer's disease. We have observed that electrolytic lesions of cholinergic basal forebrain nuclei can lead to morphological plasticity in adult mouse cortex. In the present study, the acute and chronic sequelae of basal forebrain electrolytic lesion on cortical synaptic chemistry have been examined. In addition to choline acetyltransferase (ChAT) activity, levels of norepinephrine and of serotonin were reduced within a week after the lesion. Recovery of ChAT activity and of serotonin levels began within a month after the lesion. Serotonin type 2 receptor binding exhibited an acute reduction after the lesion in ipsilateral cortex, followed later by a chronic bilateral decrease. No significant changes in beta-adrenergic receptors were apparent at any time after the lesion despite a permanent and bilateral reduction of norepinephrine levels after the lesion. The potential significance of these results for cortical plasticity regulation and Alzheimer's disease is discussed.

Involvement of alpha-adrenoceptors in para-hydroxyamphetamine-induced head-twitch response

The effect of some drugs with a higher selectivity for either the alpha 1- or alpha 2-adrenoceptors on the head-twitches induced by intracerebroventricular administration of p-hydroxyamphetamine (p-OHA) in mice, have been studied. Pretreatment with yohimbine increased the number of head-twitches induced by p-OHA, whereas pretreatment with clonidine or prazosin reduced the number of responses. The decrease in head-twitches produced by clonidine was completely antagonized by pretreatment with yohimbine. Pretreatment with 6-hydroxydopamine prior to the combined treatment with clonidine and p-OHA, resulted in recovery of the reduced level head-twitches to the level induced by p-OHA alone. Pretreatment with 6-hydroxydopamine alone resulted in a marked increase in the number of p-OHA-induced head-twitches. These results clearly indicate that a noradrenaline system in the brain may, at least in part, be involved in the p-OHA-induced head-twitches in mouse, most probably by modulating a serotonergic system which is responsible for the head-twitch response.

Chronic clonidine treatment produces desensitisation of post- but not presynaptic alpha 2-adrenoceptors

Rats were treated chronically with clonidine by osmotic minipump (500 micrograms/kg daily for 14 days). One day after the end of the infusion period, the animals were pithed, and alpha 2-presynaptic and -postsynaptic adrenoceptor responses evaluated using inhibition of cardioaccelerator response to sympathetic stimulation by guanabenz, and pressor response to guanabenz respectively. Chronic clonidine treatment had no effect on the presynaptic effect of guanabenz, but the postsynaptic response to this agonist was decreased. In a separate group of pithed rats treated chronically with clonidine, there was no alteration in the increment in plasma noradrenaline concentration produced by sympathetic stimulation, and no alteration in the yohimbine-induced enhancement of plasma noradrenaline response to sympathetic stimulation. These data argue against desensitisation of peripheral presynaptic alpha 2-adrenoceptors by chronic administration of alpha 2-adrenoceptor agonist. Sympathetic hyperactivity following withdrawal of chronic clonidine treatment may be mediated by down-regulation of postsynaptic alpha 2-adrenoceptors in the CNS.

Clonidine treatment for short stature

34 pubertal children with constitutional growth delay (CGD) were treated with clonidine orally twice a day. In 25 of the children the height velocity rose on clonidine treatment, and in 21 of them by more than 2 cm/yr during the first 6 months of treatment (mean [SD] growth increment 4.4 [0.5] cm/yr). Of the 22 who were treated for 12 months the increment in height velocity was maintained in 13 (3.4[0.4] cm/yr). Withdrawal of clonidine for 6 months did not stop the stimulatory effect of the drug on linear growth in 6 children, but in the other 8 children height velocities fell to pretreatment levels or below. In a few children reinstitution of clonidine for 2-4 months resulted in a new increment in height velocity. A high height standard deviation score and low growth velocity before treatment were predictive of a good growth response to clonidine. Clonidine did not induce noticeable side-effects. It may be a useful form of therapy for children with CGD.

Electrophysiological evidence for locus coeruleus norepinephrine autoreceptor subsensitivity following subchronic administration of D-amphetamine

The effect produced by subchronic administration of D-amphetamine (D-AMP) on the sensitivity of norepinephrine (NE) autoreceptors in the rat locus coeruleus (LC) was studied by means of single unit recording and microiontophoretic techniques. Twice daily i.p. administration of 5 mg/kg D-AMP for one week markedly reduced the ability of i.v. D-AMP and microiontophoretic application of clonidine to suppress the firing of LC NE neurons, suggesting strongly that NE autoreceptors became subsensitive. In addition, the firing pattern of NE neurons became 'disorganized' following subchronic AMP treatment.