Recent developments in research of melatonin and its potential therapeutic applications.

LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Confusing preclinical (predictive) drug screens with animal 'models' of psychiatric disorders, or 'disorder-like' behaviour, is undermining confidence in behavioural neuroscience.

Preclinical (predictive) screens for psychotropic drugs are often used, incorrectly, as animal 'models' of psychiatric disorders, or to study 'disorder-like' behaviours. This misunderstanding is contributing to poor translation and is undermining confidence in behavioural neuroscience. In this editorial, I discuss some of the reasons why the interpretation of results from many of these procedures is dubious because the criteria for validity of the test, as a model of the disorder, have been ignored. Arising from this, I propose that the description of any abnormal behaviour of rodents as a 'model' of a psychiatric disorder, or even 'disorder-like', without evidence-based justification, should be regarded as unacceptable in this journal.

A lack of functional NK1 receptors explains most, but not all, abnormal behaviours of NK1R-/- mice(1).

Mice lacking functional neurokinin-1 receptors (NK1R-/-) display abnormal behaviours seen in Attention Deficit Hyperactivity Disorder (hyperactivity, impulsivity and inattentiveness). These abnormalities were evident when comparing the behaviour of separate (inbred: 'Hom') wildtype and NK1R-/- mouse strains. Here, we investigated whether the inbreeding protocol could influence their phenotype by comparing the behaviour of these mice with that of wildtype (NK1R+/+) and NK1R-/- progeny of heterozygous parents ('Het', derived from the same inbred strains). First, we recorded the spontaneous motor activity of the two colonies/genotypes, over 7 days. This continuous monitoring also enabled us to investigate whether the diurnal rhythm in motor activity differs in the two colonies/genotypes. NK1R-/- mice from both colonies were hyperactive compared with their wildtypes and their diurnal rhythm was also disrupted. Next, we evaluated the performance of the four groups of mice in the 5-Choice Serial Reaction-Time Task (5-CSRTT). During training, NK1R-/- mice from both colonies expressed more impulsive and perseverative behaviour than their wildtypes. During testing, only NK1R-/- mice from the Hom colony were more impulsive than their wildtypes, but NK1R-/- mice from both colonies were more perseverative. There were no colony differences in inattentiveness. Moreover, a genotype difference in this measure depended on time of day. We conclude that the hyperactivity, perseveration and, possibly, inattentiveness of NK1R-/- mice is a direct consequence of a lack of functional NK1R. However, the greater impulsivity of NK1R-/- mice depended on an interaction between a functional deficit of NK1R and other (possibly environmental and/or epigenetic) factors.

The influence of test experience and NK1 receptor antagonists on the performance of NK1R-/- and wild type mice in the 5-Choice Serial Reaction-Time Task.

Genetically-altered mice, lacking functional NK1 receptors (NK1R-/-), express abnormal behaviours that are prominent in Attention Deficit Hyperactivity Disorder: namely, inattentiveness and impulsivity (indicated by their greater % omissions and premature responses in the 5-Choice Serial Reaction-Time Task (5-CSRTT) and locomotor hyperactivity. We investigated how behaviour in the 5-CSRTT is affected by repeated testing and whether the abnormalities expressed by NK1R-/- mice are mimicked by treating wild type mice with a NK1R antagonist (L 733060 or RP 67580; 5 or 10 mg/kg). Repeated testing with a variable (VITI) or fixed, prolonged (LITI) intertrial interval reduced % omissions. Premature responses also declined, but only in NK1R-/- mice, in the VITI test. By contrast, perseveration increased in both genotypes. RP 67580 (10 mg/kg) increased the % omissions in both genotypes in the VITI, an action which cannot be attributed to NK1R antagonism. Neither drug affected perseveration. However, for premature responses, the response profile suggested that the low and high doses of RP 67580 (VITI) and L 733060 (LITI) had opposing effects on this behaviour. We infer that the effect of NK1R antagonists in the 5-CSRTT is confounded by animals' test experience and non-specific drug effects at sites other than NK1R, possibly L-type Ca²âº(v) channels.

Risk of severe serotonin toxicity following co-administration of methylene blue and serotonin reuptake inhibitors: an update on a case report of post-operative delirium.

In a previous case report, published in this journal, we described a postoperative delirium in a patient during recovery from parathyroidectomy. We noted that the delirium resembled serotonin toxicity and that the patient had been taking paroxetine until 2 days before surgery. We offered several tentative explanations for this event, including an adverse interaction between paroxetine and other agent(s) used in the course of the anaesthesia. Recent developments in characterisation of serotonin toxicity have prompted us to re-examine the clinical details surrounding this life-threatening event. It is now known to be important that the patient was given methylene blue, pre-operatively, to enable visualisation of the parathyroid glands. Methylene blue has been found to be a potent inhibitor of monoamine oxidase (MAO), and several cases of serotonin toxicity have been reported recently following its administration. All these cases are consistent with the well-known risk of serotonin toxicity when drugs that augment serotonergic transmission are given in combination with an MAO inhibitor. Methylene blue is used in a variety of surgical settings as well as for treatment of various types of hypotensive shock and methemoglobinaemia. It is also being studied for treatment of Alzheimer's disease and malaria. In this paper, we outline the pharmacology of methylene blue and the aetiology of serotonin toxicity to help prevent further unintentional co-administration of drugs that risk precipitating this life-threatening drug interaction.

NK1 (TACR1) receptor gene 'knockout' mouse phenotype predicts genetic association with ADHD.

Mice with functional genetic ablation of the Tacr1 (substance P-preferring receptor) gene (NK1R-/-) are hyperactive. Here, we investigated whether this is mimicked by NK1R antagonism and whether dopaminergic transmission is disrupted in brain regions that govern motor performance. The locomotor activity of NK1R-/- and wild-type mice was compared after treatment with an NK1R antagonist and/or psychostimulant (d-amphetamine or methylphenidate). The inactivation of NK1R (by gene mutation or receptor antagonism) induced hyperactivity in mice, which was prevented by both psychostimulants. Using in vivo microdialysis, we then compared the regulation of extracellular dopamine in the prefrontal cortex (PFC) and striatum in the two genotypes. A lack of functional NK1R reduced (>50%) spontaneous dopamine efflux in the prefrontal cortex and abolished the striatal dopamine response to d-amphetamine. These behavioural and neurochemical abnormalities in NK1R-/- mice, together with their atypical response to psychostimulants, echo attention deficit hyperactivity disorder (ADHD) in humans. These findings prompted genetic studies on the TACR1 gene (the human equivalent of NK1R) in ADHD patients in a case-control study of 450 ADHD patients and 600 screened supernormal controls. Four single-nucleotide polymorphisms (rs3771829, rs3771833, rs3771856, and rs1701137) at the TACR1 gene, previously known to be associated with bipolar disorder or alcoholism, were strongly associated with ADHD. In conclusion, our proposal that NK1R-/- mice offer a mouse model of ADHD was borne out by our human studies, which suggest that DNA sequence changes in and around the TACR1 gene increase susceptibility to this disorder.

Areca nut, energy metabolism and hunger in Asian men.

BACKGROUND: The nut of the Areca catechu palm has long been attributed effects on hunger and the digestive process. OBJECTIVES: The objectives were to assess experimentally effects of areca nut on fasting and postprandial energy metabolism, substrate utilization and hunger. SUBJECTS AND METHODS: Two randomized, placebo-controlled, double-blind studies were undertaken. In study 1, eight Indian men received bioadhesive gels delivering 0, 5, 10 or 20 mg arecoline to the buccal sulcus after an overnight fast. Resting energy expenditure and substrate utilization were determined by ventilated hood calorimetry over 6 h during which hunger was rated on five occasions. In study 2, 15 Indian men received gels delivering 0 or 10 mg arecoline after consuming a 2.5 MJ meal, and the same protocol was then applied as in study 1. RESULTS: Fasting resting energy expenditures exceeded basal metabolic rate (BMR) by 5.4+/-0.8% (Mean+/-SE) after placebo, and 5.1+/-0.7% after 20 mg arecoline, but by 0.9+/-0.8% and 0.7+/-0.5% following 5 mg and 10 mg arecoline, respectively. Carbohydrate (CHO) utilization rates rose after areca nut compared to placebo (F(3,252)= 7.3, p< 0.001). Hunger varied across doses (chi(2) = 10.5, p < 0.02), being lowest after 10 mg and highest after 20 mg, and was influenced by interaction of dose with delta resting energy expenditure. In study 2, areca dose interacted with fat-free mass (FFM) to lower by 5.4+/-11.2% the thermic effect of a meal (F(1,28) = 4.9, p = 0.05), and retarded peak 'digestive-phase' thermogenesis by 60 min (F(1,58) = 5.7, p = 0.02). Postprandial delta CHO utilization was greater (F(1,28) = 4.5, p = 0.05), and hunger was lower (chi:(2) = 3.8, p = 0.05), after areca nut. The areca nut altered relationships of hunger to thermic effects of the meal, and to delta substrate utilization, in ways consistent with appetite suppression. CONCLUSION: Areca nut constituents modulate metabolic signals regulating appetite in man. This concurs with customary belief.

Hemodynamic and emotional responses to a psychological stressor after cardiac transplantation.

OBJECTIVE: Because cardiac transplantation entails neuronal decentralization, cardiac responses to a psychological stressor in transplant patients would be expected to rely on circulating hormonal factors and therefore to be delayed and prolonged. We tested this prediction by comparing stress responses after transplantation with those in patients with coronary artery bypass grafts (to control for experience of surgery) or heart failure (to control for heart disease). METHODS: Fifty-six transplantation patients, 66 bypass patients, and 40 patients with heart failure underwent a 10-minute, computer-generated, Stroop color-word conflict test. Heart rate and systolic and diastolic blood pressures were recorded continuously for 1 minute before, during, and 12 minutes after the stressor. Emotional state was measured periodically by questionnaires. RESULTS: All hemodynamic variables were increased by the Stroop test. There was a pattern of blunted response to the Stroop test after cardiac transplantation, particularly in comparison with bypass patients, and slower recovery in comparison with both control groups. Emotional stress responses were similar in each group. CONCLUSIONS: This pattern cannot be attributed to the experience of major heart surgery or to cardiac disease. Nor can it be explained by differences in central processing of stress. Correspondingly the changed hemodynamic response to the Stroop test after cardiac transplantation evidently does not affect patients' emotional responses. The hemodynamic findings are consistent with an increased reliance on hormonal rather than neuronal hemodynamic regulation after cardiac transplantation.

Differences in central noradrenergic and behavioural responses of Maudsley non-reactive and Maudsley reactive inbred rats on exposure to an aversive novel environment.

The present experiments compared the noradrenaline and behavioural responses of inbred Maudsley reactive (MR) and non-reactive (MNRA) rats when they are exposed to the light or dark arena of a light/dark shuttle-box. Behavioural scores confirmed that both strains of rats perceived the light arena to be more aversive than the dark one. Using in vivo microdialysis, exposure to the light, but not the dark, arena was found to increase noradrenaline efflux in both the frontal cortex and the hypothalamus of MNRA and MR rats. However, whereas the increase in the frontal cortex of both strains and the hypothalamus of MR rats was transient, the hypothalamic response in MNRA rats was maintained throughout exposure to the test zone. Strain differences in activity/visit and time/visit were evident but it was not possible to discern whether this could be attributed to the strain difference in the hypothalamic noradrenaline response. Nevertheless, it remains possible that, by comparison with MR rats, the prolonged noradrenaline response in the hypothalamus of MNRA rats could contribute to their well-documented, greater resistance to aversive environmental stimuli.

A microdialysis study of the noradrenergic response in rat frontal cortex and hypothalamus to a conditioned cue for aversive, naturalistic environmental stimuli.

RATIONALE: Extracellular noradrenaline concentration in the rat forebrain is increased by aversive environmental stimuli. This study investigated whether conditioned cues for such stimuli have the same effect. METHODS: After training rats to associate a tone (conditioned cue) with transfer from a neutral zone to a brightly lit zone of a light/dark shuttle-box (unconditioned stimulus), microdialysis probes were implanted into the frontal cortex and hypothalamus under halothane anaesthesia. Changes in extracellular noradrenaline concentration were then monitored on exposure to the tone alone. Parallel experiments monitored rats' behaviour in the light arena. RESULTS: A single exposure to the light arena increased extracellular noradrenaline in the frontal cortex and the hypothalamus but neither a single, nor repeated, exposure to the tone alone had any effect. After conditioning trials, the tone alone increased extracellular noradrenaline in the frontal cortex but not the hypothalamus, whilst the tone+transfer to the light arena resulted in a prolonged increase in extracellular noradrenaline in both brain regions. The time that rats spent within the light arena was also prolonged. CONCLUSIONS: Noradrenergic neurones in the frontal cortex, but not the hypothalamus, respond to conditioned cues for aversive environmental stimuli. However, prolongation of the noradrenergic response in both brain regions could contribute to the behavioural adaptation to such unconditioned stimuli.

CNS drugs III: psychotherapeutics.

This two-day symposium reviewed specific aspects of current drug development for psychiatric disorders. The aim was to identify how existing drugs could he improved and what approaches could he adopted to develop new drugs acting by different mechanisms. The meeting was attended by approximately 50 delegates, mainly from multinational pharmaceutical companies or smaller private biotechnology based companies with a few academic clinicians and preclinical scientists. The topics covered on the first day included the influence of genomics and proteomics on drug discovery with reference to antipsychotics, GABAA receptor subunit specific compounds as a route to improved anxiolytics. AMPA receptor modulators as add-on therapy in schizophrenia and corticotrophin releasing factor receptors as targets in depression. The second day placed a greater emphasis on drugs influencing amine neurotransmission; an aspect already raised on the first day by discussion of 5-HT-moduline. The main areas covered were how serotonergic ligands could he used with greater effect in depression and schizophrenia, the therapeutic value of serotonin noradrenaline uptake inhibitors (SNRIs) and the potential clinical value of new amine re-uptake inhibitors. Finally, the meeting discussed the impact of various technological advances in genetics. neuroimaging and psychometric testing in psychotherapeutic drug discovery.

Plasma catecholamines, pharmacotherapy and mood of subjects with cardiovascular disorder.

This study investigated whether drug therapy explains why the concentration of arterial plasma catecholamines in patients who have received an orthotopic heart transplant (OHT) or coronary bypass and graft (CABG) is greater than in those with heart failure (HF). The results suggest that the differences in plasma catecholamine concentrations in these groups of patients could not be attributed to administration of any of the drugs studied here. An additional finding is that the use of aspirin is associated with a higher concentration of plasma noradrenaline, but not adrenaline. Patients who were taking aspirin also had a more positive mood, as rated by the Profile of Mood States; this was mainly because they had a lower fatigue score than did patients who were not taking this drug. In contrast, several agents (warfarin, Ca2+-channel blockers and 'mixed cardiac' drugs), which had no effects on catecholamine overspill, were linked with negative mood; this was expressed consistently as a higher tension score. These findings suggest that drugs which are administered for their effects in the periphery could also influence patients' psychological status. With the possible exception of aspirin, this does not involve changes in spillover of catecholamines in the periphery.

Postoperative delirium indicating an adverse drug interaction involving the selective serotonin reuptake inhibitor, paroxetine?

We report a postoperative delirium expressed by a 49-year-old female patient during recovery from anaesthesia. Prominent features of the delirium, which lasted for nearly 2 days, included agitation, confusion, uncontrolled limb movements, abnormal ocular function, hypertension, pyrexia, brisk reflexes, ankle clonus and raised creatine kinase. The delirium did not respond to naloxone, diazepam or flumazenil. The patient had not been prescribed neuroleptics but, before surgery, she had been taking the selective serotonin reuptake inhibitor, paroxetine, to relieve her depression. During surgery, she was given morphine, which increases release of the neurotransmitter, serotonin, and ondansetron, which blunts neuronal release of dopamine. Although there is no clear explanation for the delirium, it had many features in common with problems associated with paroxetine withdrawal, the serotonin syndrome and the malignant neuroleptic syndrome. We offer several alternative explanations for this event, all of which rest on disruption of serotonergic and/or dopaminergic transmission and which could also involve inhibition by paroxetine of the P450 enzyme, CYP2D6, which metabolizes ondansetron.

Modulation of sibutramine-induced increases in extracellular noradrenaline concentration in rat frontal cortex and hypothalamus by alpha2-adrenoceptors.

1. The effects of sibutramine (0.25 - 10 mg kg-1 i.p.) on extracellular noradrenaline concentration in the frontal cortex and hypothalamus of freely-moving rats were investigated using microdialysis. The role of presynaptic alpha2-adrenoceptors in modulating the effects of sibutramine in these brain areas was also determined. 2. Sibutramine induced an increase in extracellular noradrenaline concentration, the magnitude of which paralleled dose, in both brain areas. In the cortex, this increase was gradual and sustained, whereas in the hypothalamus it was more rapid and of shorter duration. 3. In both the cortex and hypothalamus, pretreatment of rats with the alpha2-adrenoceptor antagonist RX821002 (3 mg kg-1 i.p.) potentiated increases in the accumulation of extracellular noradrenaline induced by sibutramine (10 mg kg-1 i. p.), by 7 and 10 fold respectively. RX821002 also reduced the latency of sibutramine to reach its maximum effect in the cortex, but not in the hypothalamus. 4. Infusion of RX821002 (1 microM) via the probe increased the accumulation of extracellular noradrenaline induced by sibutramine (10 mg kg-1 i.p.) in both brain areas. In the hypothalamus, the effects of RX821002 on the accumulation of noradrenaline induced by sibutramine were 2 fold greater than those in the cortex. 5. These findings support evidence that sibutramine inhibits the reuptake of noradrenaline in vivo, but that the accumulation of extracellular noradrenaline is limited by noradrenergic activation of presynaptic alpha2-adrenoceptors. Furthermore, the data suggest that terminal alpha2-adrenoceptors in the hypothalamus exert a greater inhibitory effect over the control of extracellular noradrenaline accumulation than do those in the cortex.

Comparison of changes in the extracellular concentration of noradrenaline in rat frontal cortex induced by sibutramine or d-amphetamine: modulation by alpha2-adrenoceptors.

1. The effects of sibutramine (0.25 - 10 mg kg-1, i.p.) on extracellular noradrenaline concentration in the frontal cortex of halothane-anaesthetized rats were compared with those of d-amphetamine (1 - 3 mg kg-1, i.p.) using in vivo microdialysis. The role of presynaptic alpha2-adrenoceptors in modulating the effects of these drugs on extracellular noradrenaline concentration were also investigated by pretreating rats with the selective alpha2-adrenoceptor antagonist, RX821002. 2. Sibutramine induced a gradual and sustained increase in extracellular noradrenaline concentration. The dose-response relationship was described by a bell-shaped curve with a maximum effect at 0.5 mg kg-1. In contrast, d-amphetamine induced a rapid increase in extracellular noradrenaline concentration, the magnitude of which paralleled drug dose. 3. Pretreatment with the alpha2-adrenoceptor antagonist, RX821002 (dose 3 mg kg-1, i.p.) increased by 5 fold the accumulation of extracellular noradrenaline caused by sibutramine (10 mg kg-1) and reduced the latency of sibutramine to reach its maximum effect from 144 - 56 min. 4. RX821002-pretreatment increased by only 2.5 fold the increase in extracellular noradrenaline concentration caused by d-amphetamine alone (10 mg kg-1) and had no effect on the latency to reach maximum. 5. These findings support evidence that sibutramine acts as a noradrenaline uptake inhibitor in vivo and that the effects of this drug are blunted by indirect activation of presynaptic alpha2-adreno-ceptors. In contrast, the rapid increase in extracellular noradrenaline concentration induced by d-amphetamine is consistent with this being mainly due to an increase in Ca2+-independent release of noradrenaline.

Effect of novel environmental stimuli on rat behaviour and central noradrenaline function measured by in vivo microdialysis.

RATIONALE: Although physically aversive stimuli induce functional changes in central noradrenergic neurones, little is known about the noradrenergic response to environmentally aversive stimuli. OBJECTIVES: The first aim was to characterise environmental features that are perceived as stressful by rats. The second was to investigate whether changes in the concentration of extracellular noradrenaline are induced by these environmental features. METHODS: A light/dark shuttle-box was used to test rats' behavioural response to a range of stimuli (novelty, bright light, and the presence of an unfamiliar rat), either before or after microdialysis probe implantation. Changes in the concentration of extracellular noradrenaline in the frontal cortex and hypothalamus in vivo were then evaluated on exposure to these same test conditions. RESULTS: Naive rats spent less time in a brightly-lit test arena than a dark one. However, the behavioural response to the light arena was attenuated by the presence of an unfamiliar rat. Probe implantation intensified the response to the light arena but did not affect behaviour in the dark arena. In the microdialysis studies, there was no change in the concentration of extracellular noradrenaline on transfer of rats to the dark arena but there was an increase in both the frontal cortex (+45%) and hypothalamus (+75%) on exposure to the light arena. A similar increase was induced in both brain regions when the light arena contained an unfamiliar rat. CONCLUSIONS: Implantation of a microdialysis probe modifies the behavioural responses to certain environmental stimuli. Regardless of this, the extent to which rats perceive a novel environment as aversive is not the only determinant of the noradrenergic response to such stimuli. However, differences in stimulus controllability in the microdialysis and the behavioural experiments could influence the apparent intensity of the stress.

Psychological adjustment after cardiac transplantation.

Psychosocial function improves after cardiac transplantation but the extent of improvement is not established. Neither are the factors established that account for variability in function between patients following successful transplantation. We therefore compared illness-related dysfunction in patients following orthotopic cardiac transplantation (OCT) with that in angina-free patients following coronary artery bypass graft (CABG) and stable patients in heart failure awaiting transplantation (HF). We also measured two factors that might contribute to variation in function: emotional distress and concern with physical symptoms. Psychosocial function was as good in OCT as in CABG patients and, in both, was better than in HF patients. Differences in emotional distress and in physical symptoms showed a similar pattern and contributed to, but did not completely account for, differences in function. Concern with physical symptoms contributed to variability in functional impairment in HF but not CABG patients; transplantation strengthened this relationship. The results show that successful cardiac transplantation enhances psychosocial function to a level comparable with that after CABG, and suggest targets for psychological or educational intervention to improve quality of life after transplantation in patients whose recovery is inhibited by concern about physical symptoms.

Evidence from microdialysis and synaptosomal studies of rat cortex for noradrenaline uptake sites with different sensitivities to SSRIs.

1. Microdialysis of the frontal cortex of freely-moving rats and uptake of [3H]noradrenaline into cortical synaptosomes were used to evaluate changes in efflux of noradrenaline in vivo and uptake of [3H]noradrenaline in vitro, respectively, induced by the selective serotonin reuptake inhibitors (SSRIs), fluoxetine and citalopram, and the tricyclic antidepressant, desipramine. 2. Noradrenaline efflux was increased during local infusion into the cortex of each of these drugs. All three agents also inhibited synaptosomal uptake of [3H]noradrenaline; this inhibition was unaffected by a substantial (50%) lesion of central 5-hydroxytrytaminergic neurones induced by intracerebroventricular infusion of 5,7-DHT (150 microg). 3. A noradrenergic lesion (70%), induced by pretreatment with the selective neurotoxin, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, 40 mg kg(-1) i.p.), 5 days earlier, abolished the increase in noradrenaline efflux caused by local infusion of fluoxetine. In contrast, the desipramine-induced increase in efflux was greater than in non-lesioned rats whereas the effect of citalopram on noradrenaline efflux was unaffected by DSP-4 pretreatment. 4. The combined results of all these experiments suggest that there could be more than one, functionally distinct, noradrenaline uptake site in rat frontal cortex which can be distinguished by their different sensitivities to desipramine and the SSRIs, fluoxetine and citalopram.

The anxiogenic agents, yohimbine and FG 7142, disrupt the noradrenergic response to novelty.

Whether or not abnormal noradrenergic transmission can be a causal factor in anxiety is controversial. The present experiments examined this question by comparing the effects of two anxiogenic agents on noradrenaline efflux in the frontal cortex of freely moving rats. A single anxiogenic dose of either yohimbine (2.5 or 5 mg/kg) or FG 7142 (10 or 20 mg/kg) was administered to rats by i.p. injection. Yohimbine increased spontaneous efflux of noradrenaline, but FG 7142 had no effect. However, subsequent exposure of rats to a novel environment increased noradrenaline efflux in vehicle-, but not drug-treated rats. Calculation of the net change in noradrenaline efflux caused by transfer to the novel environment showed that this was reduced by yohimbine, whereas FG 7142 increased it. These two compounds also had different effects on locomotor activity in the novel environment. The results suggest that anxiety is unlikely to be invariably associated with increased noradrenergic transmission, in the frontal cortex at least. However, it remains possible that any disruption of the noradrenergic response to stress could be an underlying feature of anxiety.

A partial noradrenergic lesion induced by DSP-4 increases extracellular noradrenaline concentration in rat frontal cortex: a microdialysis study in vivo.

The effect of systemic administration of the selective neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) on noradrenaline efflux in the frontal cortex was studied in freely-moving rats using microdialysis in vivo. Five days after treatment with DSP-4 (40 mg/kg i.p.), the noradrenaline content of the frontal cortex was reduced by 75%. Yet, noradrenaline efflux in the frontal cortex was nearly two-fold greater in DSP-4 treated rats than in saline-injected controls. Local infusion of the noradrenaline-selective uptake blocker, desipramine (5 microM), via the microdialysis probe, increased noradrenaline efflux in rats from both groups. Perfusion of Ringer's solution, containing 80 mM K+, also increased noradrenaline efflux in both groups, but the increase after DSP-4 pretreatment was greater than in the controls. In contrast, removal of Ca2+ from the infusion medium reduced noradrenaline efflux in both treatment groups. These results indicate that, at this dose, DSP-4 increases the extracellular concentration of noradrenaline in rat frontal cortex despite causing a partial lesion of noradrenergic neurones. This is due to an increase in the release of noradrenaline, although reduced clearance is also likely. These data challenge the assumption that depletion of noradrenaline content after treatment with DSP-4 invariably translates into diminished noradrenergic transmission.