[Citation misuse and its effects on public health]

In order to effectively contribute to scientific knowledge, biomedical observations have to be validated and debated by scientists in the relevant field. Along this debate that mainly takes place in the scientific literature, citation of previous studies plays a major role. However, only a few academic studies have quantitatively evaluated the suitability and accuracy of scientific citations. Here we review these academic studies. Two types of misuse have been pointed out: Citation bias and citation distortion. First, scientific citations favor positive results and those supporting authors' conclusion. Second, many statements linked to a reference actually misrepresent the referenced findings. About 10% of all citations in biomedicine are strongly inaccurate and misleading for the reader. Finally, we give two examples illustrating how some citation misuses do affect public health: The opioid crisis in the USA and the unjustified fostering of hydroxychloroquine for Covid-19 treatment in France.

[Efficacy of psychodynamic therapies: A systematic review of the recent literature]

AIM

A French governmental institute published, in February 2004, a report assessing the efficacy of psychotherapies in the light of the biomedical literature. It concluded that cognitive psychotherapies effectively cure common mental disorders, while the efficacy of psychodynamic therapies is not proven by scientific studies. Because many French mental health professionals are practicing with reference to psychoanalysis, this conclusion stirred up heated controversy. Since February 2004, numerous studies assessing psychodynamic therapies have been published in peer-reviewed biomedical journals. Moreover, these primary studies have been meta-analyzed in dozens of review articles. Here, we systematically review these meta-analysis articles.

METHODS

A systematic search for meta-analyses assessing psychodynamic therapies was performed using PubMed and identified 71 articles published from January 2004 to December 2019. Among them, 25 articles were judged to be relevant because they reported meta-analyses assessing the symptoms of common mental disorders in at least three distinct cohorts of adult patients. Although the primary studies included in these 25 meta-analysis articles often overlap, the selection criteria, calculation methods and results always differ between them. Therefore, we reviewed all of them without further selection. From all the meta-analyses reported in these 25 articles, we systematically present here the most compelling ones, i.e. those calculated from the largest number of primary studies. Results were quantified in terms of effect size (i.e. standardized mean difference). Effect sizes below 0.25 were considered as without clinical significance, whereas those superior to 0.8 were regarded as robust. Because short-term psychodynamic therapies had been assessed in 20 meta-analysis articles published until 2017, we did not search for more recent primary studies. However, because the most recent meta-analysis article about long-term psychodynamic therapies was published in 2013, we also searched, using PubMed, for primary studies assessing psychodynamic therapies lasting for at least one year and published from January 2013 to December 2019. Among the 57 publications retrieved by PubMed, three were identified as randomized controlled trials not included in meta-analyses and were extensively described here.

RESULTS

Eight meta-analysis articles have assessed symptom improvement at treatment termination by comparing with baseline symptoms. According to all of them, psychodynamic therapies alleviate symptoms and their effect sizes are always robust. Three meta-analysis articles compared psychodynamic therapies with inactive treatments (e.g. placebo medication, waiting list) and reported clinically significant differences in favor of psychodynamic therapies. Ten meta-analysis articles compared, at treatment termination, psychodynamic therapies to active treatments, including medication and cognitive psychotherapies. Nine of them reported no difference. Only one article concluded that psychodynamic therapies are clinically inferior to cognitive psychotherapies (d=-0.28). Seven meta-analysis articles compared psychodynamic therapies to active treatment at follow-up (i.e. months or years after treatment termination). Five of them reported no significant difference, one reported a medium effect size in favor of psychodynamic therapies over various active treatments (d=0.38), while the other reported a clinically significant difference in favor of cognitive psychotherapies (d=-0.55). Because short-term treatments are often insufficient to prevent relapse, investigations about long-term treatments (i.e. more than one year) are needed, but such published studies are still scarce. Five meta-analysis articles and three primary studies published since 2013 compared long-term psychodynamic therapies to various active treatments of similar duration. According to them, psychodynamic therapies were at least as effective as other active treatments.

CONCLUSION

A systematic review about psychodynamic therapies, published in 2015 in Lancet Psychiatry, included 64 randomized controlled trials of which 37 were published after 2003. Therefore, most quality studies assessing psychodynamic therapies have been published since 2003 and have been reviewed in recent meta-analysis articles. All together, this recent literature leads to the conclusion that psychodynamic therapies are as effective as active treatments, including cognitive psychotherapies, to help patients suffering from common mental disorders (unipolar depression, anxiety disorders, eating disorders and personality disorders). Beside this overall conclusion, it appears that randomized controlled trials are not well suited for answering why psychotherapies work in some patients but not in others, and how they work in general. Other approaches are needed, including case studies.

Where to find accurate information on attention-deficit hyperactivity disorder? A study of scientific distortions among French websites, newspapers, and television programs

Attention-deficit/hyperactivity disorder is the most frequent mental disorder among school-age children. This condition has given rise to a large mediatic coverage, which contributed to the shaping of the lay public's perceptions. We therefore conducted two studies on the way attention-deficit/hyperactivity disorder was portrayed in the TV programs and the lay-public press in France between 1995 and 2015, but the growing part played by the Internet required an additional study to analyze and compare the scientific material which is available to the French lay public depending on the source of information used. We studied the 50 first French websites dedicated to attention-deficit/hyperactivity as indexed by Google^® search engine using a structured quantitative content analysis for the web. We illustrate our results with excerpts derived from the websites. The conceptions of attention-deficit/hyperactivity disorder available on the Internet are essentially biomedical and comprise an important level of scientific distortion. Findings concerning other mass media such as television programs and the press also demonstrate massive and systematic distortions caused by the role of experts and the pharmaceutical industry. Furthermore, the most consulted media present the highest level of scientific distortions.

Do newspapers preferentially cover biomedical studies involving national scientists?

News value theory rates geographical proximity as an important factor in the process of issue selection by journalists. But does this apply to science journalism? Previous observational studies investigating whether newspapers preferentially cover scientific studies involving national scientists have generated conflicting answers. Here we used a database of 123 biomedical studies, 113 of them involving at least one research team working in eight countries (Australia, Canada, France, Ireland, Japan, New Zealand, the United Kingdom, and the United States). We compiled all the newspaper articles covering these 123 studies and published in English, French, and Japanese languages. In all eight countries, we found that newspapers preferentially covered studies involving a national team. Moreover, these "national" studies on average gave rise to a larger number of newspaper articles than "foreign" studies. Finally, our study resolves the conflict with previous conclusions by providing an alternative interpretation of published observations.

How French media have portrayed ADHD to the lay public and to social workers

Two models of attention deficit hyperactivity disorder (ADHD) coexist: the biomedical and the psychosocial. We identified in nine French newspapers 159 articles giving facts and opinions about ADHD from 1995 to 2015. We classified them according to the model they mainly supported and on the basis of what argument. Two thirds (104/159) mainly supported the biomedical model. The others either defended the psychodynamic understanding of ADHD or voiced both models. Neurological dysfunctions and genetic risk factors were mentioned in support of the biomedical model in only 26 and eight articles, respectively. These biological arguments were less frequent in the most recent years. There were fewer articles mentioning medication other than asserting that medication must be combined with psychosocial interventions (14 versus 57 articles). Only 11/159 articles claimed that medication protects from school failure. These results were compared to those of our two previous studies. Thus, both French newspapers and the specialized press read by social workers mainly defended either the psychodynamic understanding of ADHD or a nuanced version of the biomedical model. In contrast, most French TV programmes described ADHD as an inherited neurological disease whose consequences on school failure can be counteracted by a very effective medication.

Poor replication validity of biomedical association studies reported by newspapers

Objective

To investigate the replication validity of biomedical association studies covered by newspapers.

Methods

We used a database of 4723 primary studies included in 306 meta-analysis articles. These studies associated a risk factor with a disease in three biomedical domains, psychiatry, neurology and four somatic diseases. They were classified into a lifestyle category (e.g. smoking) and a non-lifestyle category (e.g. genetic risk). Using the database Dow Jones Factiva, we investigated the newspaper coverage of each study. Their replication validity was assessed using a comparison with their corresponding meta-analyses.

Results

Among the 5029 articles of our database, 156 primary studies (of which 63 were lifestyle studies) and 5 meta-analysis articles were reported in 1561 newspaper articles. The percentage of covered studies and the number of newspaper articles per study strongly increased with the impact factor of the journal that published each scientific study. Newspapers almost equally covered initial (5/39 12.8%) and subsequent (58/600 9.7%) lifestyle studies. In contrast, initial non-lifestyle studies were covered more often (48/366 13.1%) than subsequent ones (45/3718 1.2%). Newspapers never covered initial studies reporting null findings and rarely reported subsequent null observations. Only 48.7% of the 156 studies reported by newspapers were confirmed by the corresponding meta-analyses. Initial non-lifestyle studies were less often confirmed (16/48) than subsequent ones (29/45) and than lifestyle studies (31/63). Psychiatric studies covered by newspapers were less often confirmed (10/38) than the neurological (26/41) or somatic (40/77) ones. This is correlated to an even larger coverage of initial studies in psychiatry. Whereas 234 newspaper articles covered the 35 initial studies that were later disconfirmed, only four press articles covered a subsequent null finding and mentioned the refutation of an initial claim.

Conclusion

Journalists preferentially cover initial findings although they are often contradicted by meta-analyses and rarely inform the public when they are disconfirmed.

Why most biomedical findings echoed by newspapers turn out to be false: the case of attention deficit hyperactivity disorder

Context

Because positive biomedical observations are more often published than those reporting no effect, initial observations are often refuted or attenuated by subsequent studies.

Objective

To determine whether newspapers preferentially report on initial findings and whether they also report on subsequent studies.

Methods

We focused on attention deficit hyperactivity disorder (ADHD). Using Factiva and PubMed databases, we identified 47 scientific publications on ADHD published in the 1990s and soon echoed by 347 newspapers articles. We selected the ten most echoed publications and collected all their relevant subsequent studies until 2011. We checked whether findings reported in each “top 10” publication were consistent with previous and subsequent observations. We also compared the newspaper coverage of the “top 10” publications to that of their related scientific studies.

Results

Seven of the “top 10” publications were initial studies and the conclusions in six of them were either refuted or strongly attenuated subsequently. The seventh was not confirmed or refuted, but its main conclusion appears unlikely. Among the three “top 10” that were not initial studies, two were confirmed subsequently and the third was attenuated. The newspaper coverage of the “top 10” publications (223 articles) was much larger than that of the 67 related studies (57 articles). Moreover, only one of the latter newspaper articles reported that the corresponding “top 10” finding had been attenuated. The average impact factor of the scientific journals publishing studies echoed by newspapers (17.1 n = 56) was higher (p<0.0001) than that corresponding to related publications that were not echoed (6.4 n = 56).

Conclusion

Because newspapers preferentially echo initial ADHD findings appearing in prominent journals, they report on uncertain findings that are often refuted or attenuated by subsequent studies. If this media reporting bias generalizes to health sciences, it represents a major cause of distortion in health science communication.

Stimulation of serotonin2C receptors elicits abnormal oral movements by acting on pathways other than the sensorimotor one in the rat basal ganglia

Serotonin2C (5-HT(2C)) receptors act in the basal ganglia, a group of sub-cortical structures involved in motor behavior, where they are thought to modulate oral activity and participate in iatrogenic motor side-effects in Parkinson's disease and Schizophrenia. Whether abnormal movements initiated by 5-HT(2C) receptors are directly consequent to dysfunctions of the motor circuit is uncertain. In the present study, we combined behavioral, immunohistochemical and extracellular single-cell recordings approaches in rats to investigate the effect of the 5-HT(2C) agonist Ro-60-0175 respectively on orofacial dyskinesia, the expression of the marker of neuronal activity c-Fos in basal ganglia and the electrophysiological activity of substantia nigra pars reticulata (SNr) neuron connected to the orofacial motor cortex (OfMC) or the medial prefrontal cortex (mPFC). The results show that Ro-60-0175 (1 mg/kg) caused bouts of orofacial movements that were suppressed by the 5-HT(2C) antagonist SB-243213 (1 mg/kg). Ro-60-0175 (0.3, 1, 3 mg/kg) dose-dependently enhanced Fos expression in the striatum and the nucleus accumbens. At the highest dose, it enhanced Fos expression in the subthalamic nucleus, the SNr and the entopeduncular nucleus but not in the external globus pallidus. However, the effect of Ro-60-0175 was mainly associated with associative/limbic regions of basal ganglia whereas subregions of basal ganglia corresponding to sensorimotor territories were devoid of Fos labeling. Ro-60-0175 (1-3 mg/kg) did not affect the electrophysiological activity of SNr neurons connected to the OfMC nor their excitatory-inhibitory-excitatory responses to the OfMC electrical stimulation. Conversely, Ro-60-0175 (1 mg/kg) enhanced the late excitatory response of SNr neurons evoked by the mPFC electrical stimulation. These results suggest that oral dyskinesia induced by 5-HT(2C) agonists are not restricted to aberrant signalling in the orofacial motor circuit and demonstrate discrete modifications in associative territories.

Nigrostriatal lesion induces D2-modulated phase-locked activity in the basal ganglia of rats

There is a debate as to what modifications of neuronal activity underlie the clinical manifestations of Parkinson's disease and the efficacy of antiparkinsonian pharmacotherapy. Previous studies suggest that release of GABAergic striatopallidal neurons from D2 receptor-mediated inhibition allows spreading of cortical rhythms to the globus pallidus (GP) in rats with 6-hydroxydopamine-induced nigrostriatal lesions. Here this abnormal spreading was thoroughly investigated. In control urethane-anaesthetized rats most GP neurons were excited during the active part of cortical slow waves ('direct-phase' neurons). Two neuronal populations having opposite phase relationships with cortical and striatal activity coexisted in the GP of 6-hydroxydopamine-lesioned rats. 'Inverse-phase' GP units exhibited reduced firing coupled to striatal activation during slow waves, suggesting that this GP oscillation was driven by striatopallidal hyperactivity. Half of the pallidonigral neurons identified by antidromic stimulation exhibited inverse-phase activity. Therefore, spreading of inverse-phase oscillations through pallidonigral axons might contribute to the abnormal direct-phase cortical entrainment of basal ganglia output described previously. Systemic administration of the D2 agonist quinpirole to 6-hydroxydopamine-lesioned rats reduced GP inverse-phase coupling with slow waves, and this effect was reversed by the D2 antagonist eticlopride. Because striatopallidal hyperactivity was only slightly reduced by quinpirole, other mechanisms might have contributed to the effect of quinpirole on GP oscillations. These results suggest that antiparkinsonian efficacy may rely on other actions of D2 agonists on basal ganglia activity. However, abnormal slow rhythms may promote enduring changes in functional connectivity along the striatopallidal axis, contributing to D2 agonist-resistant clinical signs of parkinsonism.

Differences in ultrastructural localization of dopaminergic D1 receptors between dorsal striatum and nucleus accumbens in the rat

Dopaminergic receptors of the D1 type are highly expressed in the dorsal striatum and nucleus accumbens. In the dorsal striatum, they are rarely observed on presynaptic terminals. However, their subcellular localization in the nucleus accumbens core and shell had not been compared to that of dorsal striatum. Here we investigated the subcellular localization of D1 receptors in these three brain regions using immunogold labeling and electron microscopy. We showed that, among all presynaptic terminals forming asymmetric contact with dendritic processes, the percentage of D1R immunoreactive terminals was low in the dorsal striatum (8.2%), but reached in the nucleus accumbens core and shell 25.5 and 29%, respectively. These observations are consistent with electrophysiological studies, which showed that D1 stimulation inhibits the response of target neurons to glutamatergic input via presynaptic mechanisms in the nucleus accumbens but not in the dorsal striatum.

Cortical inputs and GABA interneurons imbalance projection neurons in the striatum of parkinsonian rats

The striatum receives massive cortical excitatory inputs and is densely innervated by dopamine. Striatal projection neurons form either the direct or indirect pathways. Models of Parkinson's disease propose that dopaminergic degeneration imbalances both pathways, although direct electrophysiological evidence is lacking. Here, striatal neurons were identified by electrophysiological criteria and Neurobiotin labeling combined with either immunohistochemistry or in situ hybridization. Their spontaneous discharge activity and spike response to cortical stimulation were recorded in vivo in anesthetized rats rendered hemi-parkinsonian by 6-hydroxydopamine. We showed that striatonigral neurons (direct pathway) were inhibited whereas striatopallidal neurons (indirect pathway) were activated by dopaminergic lesion. We also identified, with antidromic stimulations, corticostriatal neurons that preferentially innervate striatonigral or striatopallidal neurons and showed that dopaminergic depletion selectively decreased the spontaneous activity of the former. Therefore, dopamine degeneration induces a cascade of imbalances that spread out of the basal ganglia and affect the whole basal ganglia-thalamo-cortical circuits. Fast-spiking GABA interneurons provide potent feedforward inhibition of striatal projection neurons. We showed here that these interneurons narrowed the time window of the responses of projection neurons to cortical stimulation. In the dopamine-depleted striatum, because the intrinsic activity of these interneurons was not altered, their feedforward inhibition worsened the striatal imbalance. Indeed, the time window of the evoked responses was narrower for striatonigral neurons and wider for striatopallidal neurons. Therefore, after dopaminergic depletion, cortical inputs and GABA interneurons might imbalance striatal projection neurons and represent two novel nondopaminergic mechanisms that might secondarily contribute to the pathophysiology of Parkinson's disease.

Feedforward inhibition of projection neurons by fast-spiking GABA interneurons in the rat striatum in vivo

Discharge activities and local field potentials were recorded in the orofacial motor cortex and in the corresponding rostrolateral striatum of urethane-anesthetized rats. Striatal projection neurons were identified by antidromic activation and fast-spiking GABAergic interneurons (FSIs) by their unique characteristics: briefer spike and burst responses. Juxtacellular injection of neurobiotin combined with parvalbumin immunohistochemistry validated this identification. Spontaneous activities and spike responses to cortical stimulation were recorded during both states of cortical activity: slow waves and desynchronization. Both FSI and projection neurons spontaneously discharged synchronously with slow waves at the maximum of cortical activity, but, on average, FSIs were much more active. Cortical desynchronization enhanced FSI activity and facilitated their spike responses to cortical stimulation, whereas opposite effects were observed regarding projection neurons. Experimental conditions favoring FSI discharge were always associated with a decrease in the firing activity of projection neurons. Spike responses to cortical stimulation occurred earlier (latency difference, 4.6 ms) and with a lower stimulation current for FSIs than for projection neurons. Moreover, blocking GABA(A) receptors by local picrotoxin injection enhanced the spike response of projection neurons, and this increase was larger in experimental conditions favoring FSI responses. Therefore, on average, FSIs exert in vivo a powerful feedforward inhibition on projection neurons. However, a few projection neurons were actually more sensitive to cortical stimulation than FSIs. Moreover, picrotoxin, which revealed FSI inhibition, preferentially affected projection neurons exhibiting the weakest sensitivity to cortical stimulation. Thus, feedforward inhibition by FSIs filters cortical information effectively transmitted by striatal projection neurons.

The rate of cocaine administration alters gene regulation and behavioral plasticity: implications for addiction

The rapid delivery of drugs of abuse to the brain is thought to promote addiction, but why this occurs is unknown. In the present study, we characterized the influence of rate of intravenous cocaine infusion (5-100 sec) on three effects thought to contribute to its addiction liability: its ability to block dopamine (DA) uptake, to activate immediate early gene expression, and to produce psychomotor sensitization. Rapid infusions potentiated the ability of cocaine to block DA reuptake, to induce c-fos and arc mRNA expression, especially in mesocorticolimbic regions, and to produce psychomotor sensitization. Thus, the rate at which cocaine is delivered influences both its neurobiological impact and its ability to induce a form of drug experience-dependent plasticity implicated in addiction. We propose that rapidly delivered cocaine may be more addictive, in part, because this more readily induces forms of neurobehavioral plasticity that lead to the compulsive pursuit of drugs.

Presynaptic regulation of dopaminergic neurotransmission

The development of electrochemical recordings with small carbon-fiber electrodes has significantly advanced the understanding of the regulation of catecholamine transmission in various brain areas. Recordings in vivo or in slice preparations monitor diffusion of catecholamine following stimulated synaptic release into the surrounding tissue. This synaptic 'overflow' is defined by the amount of release, by the activity of reuptake, and by the diffusion parameters in brain tissue. Such studies have elucidated the complex regulation of catecholamine release and uptake, and how psychostimulants and anti-psychotic drugs interfere with it. Moreover, recordings with carbon-fiber electrodes from cultured neurons have provided analysis of catecholamine release and its plasticity at the quantal level.

D5 (not D1) dopamine receptors potentiate burst-firing in neurons of the subthalamic nucleus by modulating an L-type calcium conductance

Dopamine is a crucial factor in basal ganglia functioning. In current models of basal ganglia, dopamine is postulated to act on striatal neurons. However, it may also act on the subthalamic nucleus (STN), a key nucleus in the basal ganglia circuit. The data presented here were obtained in brain slices using whole-cell patch clamp. They reveal that D5 dopamine receptors strengthen electrical activity in the subset of subthalamic neurons endowed with burst-firing capacity, resulting in longer discharges of spontaneous or evoked bursts. To distinguish between D1 and D5 subtypes, the action of agonists in the D1/D5 receptor family was first investigated on rat subthalamic neurons. Single-cell reverse transcription-PCR profiling showed that burst-competent neurons only expressed D5 receptors. Accordingly, receptors localized in postsynaptic membranes within the STN were labeled by a D5-specific antibody. Second, agonists in the D1/D5 family were tested in mouse brain slices. It was found that these agonists were active in D1 receptor knock-out mice in a similar way to wild-type mice or rats. This proved that D5 rather than D1 receptors were involved. Pharmacological tools (dihydropyridines, omega-conotoxins, and calciseptine) were used to identify the target of D5 receptors as an L-type channel. This was reached via G-protein and protein kinase A. The action of dopamine on D5 receptors therefore shapes neuronal activity. It contributes to normal information processing in basal ganglia outside striatum. This finding may be useful in drug therapy for various disorders involving changes in STN activity, such as Parkinson's disease and related disorders.

Catecholamine Metabolism in Locus Coeruleus Neurons: A Study of its Activation by Sciatic Nerve Stimulation in the Rat

Sciatic nerve stimulation, which strongly activates noradrenergic locus coeruleus (NA-LC) neurons, was used in anaesthetized rats as a model to study the transneuronal control of catechol metabolism in this nucleus. We show, using in vivo electrochemistry and biochemical post-mortem assays, that a prolonged (20 min) unilateral sciatic nerve electrical stimulation led to a reversible enhancement (80 - 130%) of both endogenous and in vivo extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) within the contralateral LC region. An elevation in DOPAC levels was also observed in the ipsilateral nucleus but was always significantly lower. The response was abolished by a pretreatment with kynurenic acid, a non-selective excitatory amino acid (EAA) antagonist known to block footshock-induced excitations of NA-LC neurons: in antagonist-treated rats, the stimulation induced a non-significant effect (+ 30%) on endogenous DOPAC levels, which contrasted with the highly significant effect (+ 113%) observed in vehicle-treated animals. As the major source of EAA afferents to the LC originates in the nucleus paragigantocellularis, we made an attempt to suppress activation by a section of these fibres. An incision performed obliquely (45 degrees ) between LC and PGi greatly and significantly attenuated, but did not totally suppress, the increase in DOPAC endogenous content due to the stimulation. These experiments indicate that a peripheral stimulus provokes an activation of catecholamine metabolism within the soma - dendritic region of the NA-LC cells. They suggest that this effect may be mediated, at least in part, by afferent pathways originating from the medulla which utilize an EAA as transmitter.

Inhibition of dopamine release via presynaptic D2 receptors: time course and functional characteristics in vivo

Most neurotransmitters inhibit their own release through autoreceptors. However, the physiological functions of these presynaptic inhibitions are still poorly understood, in part because their time course and functional characteristics have not been described in vivo. Dopamine inhibits its own release through D2 autoreceptors. Here, the part played by autoinhibition in the relationship between impulse flow and dopamine release was studied in vivo in real time. Dopamine release was evoked in the striatum of anesthetized mice by electrical stimulation of the medial forebrain bundle and was continuously monitored by amperometry using carbon fiber electrodes. Control experiments performed in mice lacking D2 receptors showed no autoinhibition of dopamine release. In wild-type mice, stimulation at 100 Hz with two to six pulses linearly inhibited further release, whereas single pulses were inefficient. Dopaminergic neurons exhibit two discharge patterns: single spikes forming a tonic activity below 4 Hz and bursts of two to six action potentials at 15 Hz. Stimulation mimicking one burst (four pulses at 15 Hz) promoted extracellular dopamine accumulation and thus inhibited further dopamine release. This autoinhibition was maximal between 150 and 300 msec after stimulation and disappeared within 600 msec. This delayed and prolonged time course is not reflected in extracellular DA availability and thus probably attributable to mechanisms downstream from autoreceptor stimulation. Thus, in physiological conditions, autoinhibition has two important roles. First, it contributes to the attenuation of extracellular dopamine during bursts. Second, autoinhibition elicited by one burst transiently attenuates further dopamine release elicited by tonic activity.

Amphetamine distorts stimulation-dependent dopamine overflow: effects on D2 autoreceptors, transporters, and synaptic vesicle stores

Amphetamine (AMPH) is known to raise extracellular dopamine (DA) levels by inducing stimulation-independent DA efflux via reverse transport through the DA transporter and by inhibiting DA re-uptake. In contrast, recent studies indicate that AMPH decreases stimulation-dependent vesicular DA release. One candidate mechanism for this effect is the AMPH-mediated redistribution of DA from vesicles to the cytosol. In addition, the inhibition of stimulation-dependent release may occur because of D2 autoreceptor activation by DA that is released via reverse transport. We used the D2 receptor antagonist sulpiride and mice lacking the D2 receptor to address this issue. To evaluate carefully AMPH effects on release and uptake, we recorded stimulated DA overflow in striatal slices by using continuous amperometry and cyclic voltammetry. Recordings were fit by a random walk simulation of DA diffusion, including uptake with Michaelis-Menten kinetics, that provided estimates of DA concentration and uptake parameters. AMPH (10 microm) promoted the overflow of synaptically released DA by decreasing the apparent affinity for DA uptake (K(m) increase from 0.8 to 32 microm). The amount of DA released per pulse, however, was decreased by 82%. This release inhibition was prevented partly by superfusion with sulpiride (47% inhibition) and was reduced in D2 mutant mice (23% inhibition). When D2 autoreceptor activation was minimal, the combined effects of AMPH on DA release and uptake resulted in an enhanced overflow of exocytically released DA. Such enhancement of stimulation-dependent DA overflow may occur under conditions of low D2 receptor activity or expression, for example as a result of AMPH sensitization.

Adaptive changes in the nigrostriatal pathway in response to increased 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurodegeneration in the mouse

Although several adaptive mechanisms have been identified that mask the existence of Parkinson's disease and delay the onset and aggravation of motor symptoms, the timescale and implications of this compensatory process remain an enigma. In order to examine: (i) the nature of the dopaminergic adaptive mechanisms that come into action; (ii) their sequential activation in relation to the severity of degeneration; and (iii) their efficacy with regard to the maintenance of a normal level of basal ganglia activity, we analysed the brains of mice treated daily with 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP, 4 mg/kg, i.p.) and killed at 5-day intervals from day 0 (D0) to D20. Our results demonstrate the sequential activation of two compensatory mechanisms: (i) an increase in striatal tyrosine hydroxylase (TH) protein content attested by the persistence of TH immunolabelling up to D15, contrasting with the decrease observed in both the number of nigral TH-immunoreactive neurons (-70.2%) and striatal dopamine content (-38.4%); (ii) a downregulation of DA uptake in surviving terminals at D20 (73.4% of nigral degeneration). At this point, the failure of adaptive mechanisms to maintain striatal dopaminergic homeostasis is also illustrated by an increase in the cytochrome oxidase activity of substantia nigra pars reticulata, a marker of neuronal function. It has been postulated that an increase in dopamine release per pulse could constitute an adaptive mechanism. The data we present from our MPTP mice model infirm this hypothesis. This study explores the link between the degree of nigral degeneration and the sequential activation of dopaminergic compensatory mechanisms in the nigrostriatal pathway and, in so doing, proposes a rethink of the paradigm applied to these mechanisms.

Release and elimination of dopamine in vivo in mice lacking the dopamine transporter: functional consequences

In mice lacking the dopamine transporter (DAT), the amplitude of dopamine (DA) release and the kinetics of dopamine elimination were measured in vivo using carbon fibre electrodes combined with amperometry. DA release was evoked by electrical stimulation of the medial forebrain bundle. The amplitude of DA release per pulse was lower (7% in striatum and 21% in nucleus accumbens) than in wild-type mice. Inhibition of monoamine oxidases (MAOs) by pargyline, but not of catechol-O-methyltransferase (COMT) by tolcapone, slowed down DA elimination in knockout mice. As DA half-life was two orders of magnitude higher in these mice, the DA diffusion distance was 10-times higher than in wild-types (100 and 10 microm, respectively). In knockout mice, alpha-methyl-p-tyrosine induced a much faster decline of DA release and haloperidol was less effective in potentiating DA release. Therefore, DA release was more dependent on DA synthesis than in normal animals but was less influenced by D2 autoregulation. Dopaminergic neurons exhibit two kinds of discharge activity, i.e. single spikes and bursts of 2-6 action potentials. In wild-type mice, stimuli mimicking bursts evoked significant increases in extracellular DA over its basal level sustained by tonic activity. However, in mice lacking the DAT, low frequency firing resulted in consistently high extracellular DA levels that could not be distinguished from DA levels achieved by high frequency firing. Therefore, the burst firing activity cannot be specifically translated into phasic changes in extracellular DA. This deficit might contribute to the difficulties of these mice in spatial cognitive function.

Increased rewarding properties of morphine in dopamine-transporter knockout mice

The activation of dopamine (DA) neurotransmission plays a crucial role in the behavioural responses to drugs of abuse. In particular, increased extracellular levels of DA within the mesolimbic pathway have been implicated in the rewarding and locomotor stimulatory properties of morphine. We investigated the behavioural responses to morphine in mice with a genetic disruption of the DA transporter (DAT), resulting in a constitutively high level of extrasynaptic DA. In the conditioned place preference test, DAT-/- mice exhibited a stronger rewarding response to morphine (5 mg/kg, s.c.) compared with control littermates. However, the same dose of morphine failed to increase locomotor activity in DAT-/- mice, whilst enhancing locomotion in DAT+/- and DAT+/+ animals. Morphine-induced analgesia was unaffected in mutant mice, but the behavioural expression of naloxone-induced withdrawal signs was blunted. In vivo voltammetry in the shell of the nucleus accumbens revealed that morphine was able to stimulate DA neurons in DAT-/- mice, resulting in the accumulation of higher extracellular DA levels compared with control animals. Morphine also induced a higher rate of c-fos transcription in the shell of the nucleus accumbens in mutant mice. We conclude that morphine-induced rewarding responses are firmly established in DAT mutant mice despite a DA transmission that is already tonically activated, and independently of any effect on locomotion. These particular behavioural responses to morphine may be associated with the action of the drug on DA release and c-fos expression in the shell of the nucleus accumbens of DAT-/- mice.

Dopamine tone regulates D1 receptor trafficking and delivery in striatal neurons in dopamine transporter-deficient mice

In vivo, G protein-coupled receptors (GPCR) for neurotransmitters undergo complex intracellular trafficking that contribute to regulate their abundance at the cell surface. Here, we report a previously undescribed alteration in the subcellular localization of D1 dopamine receptor (D1R) that occurs in vivo in striatal dopaminoceptive neurons in response to chronic and constitutive hyperdopaminergia. Indeed, in mice lacking the dopamine transporter, D1R is in abnormally low abundance at the plasma membrane of cell bodies and dendrites and is largely accumulated in rough endoplasmic reticulum and Golgi apparatus. Decrease of striatal extracellular dopamine concentration with 6-hydroxydopamine (6- OHDA) in heterozygous mice restores delivery of the receptor from the cytoplasm to the plasma membrane in cell bodies. These results demonstrate that, in vivo, in the central nervous system, the storage in cytoplasmic compartments involved in synthesis and the membrane delivery contribute to regulate GPCR availability and abundance at the surface of the neurons under control of the neurotransmitter tone. Such regulation may contribute to modulate receptivity of neurons to their endogenous ligands and related exogenous drugs.

Facilitation and depression of ATP and noradrenaline release from sympathetic nerves of rat tail artery

1. Excitatory junction currents (EJCs) were used to measure ATP release; noradrenaline (NA) oxidation currents and fractional overflow of labelled NA, [3H]NA, were used to monitor the release of endogenous and exogenous NA, respectively, from post-ganglionic sympathetic nerves of rat tail artery. 2. During nerve stimulation with 100 pulses at 5-20 Hz the EJCs initially grew in size (maximally by 23 %, at 2-10 Hz), and then depressed, maximally by 68 % at 20 Hz. 3. The peak amplitude of NA oxidation currents in response to nerve stimulation with 100 pulses at 2-20 Hz grew in size with frequency, while the area was independent of frequency and roughly constant. 4. The size of the NA oxidation currents evoked by nerve stimulation with 4-100 pulses at 20 Hz grew linearly with train length between pulses 4-16. Between pulses 20-100 there was a train length-dependent depression of the signal. 5. Fractional overflow of [3H]NA in response to nerve stimulation with 5-100 pulses at 20 Hz behaved similarly to the EJCs. It initially grew roughly linearly between pulses 5-25, and then showed a dramatic depression similar to that of the EJCs. 6. The alpha2-adrenoceptor antagonists rauwolscine and yohimbine increased the overflow of [3H]NA and the amplitude of NA oxidation currents, but not that of the EJCs. 7. It is concluded that during high-frequency stimulation (i) the release of ATP and NA is first briefly facilitated then markedly depressed, (ii) facilitation and depression of the two transmitters are similar in magnitude and time course, and (iii) alpha2-adrenoceptor antagonists differentially modify EJCs and the NA signals. The results obtained in the absence of drugs are compatible with the hypothesis that ATP and NA are released in parallel, while the effects of alpha2-adrenoceptor antagonists seem to suggest dissociated release.

Opposite tonic modulation of dopamine and adenosine on c-fos gene expression in striatopallidal neurons

The impulse flow-dependent dopamine release in the striatum was acutely blocked by unilateral lesion of the medial forebrain bundle with 6-hydroxydopamine. Within 45 min this disruption reduced the striatal extracellular dopamine levels by 80% as determined by in vivo voltammetry. A strong induction of c-fos messenger RNA was detected in the ipsilateral dorsolateral striatum 75 min after 6-hydroxydopamine injection by in situ hybridization. Double labelling demonstrates that this induction was confined to neurons expressing the dopamine D2 receptor messenger RNA. At this time-point, there were no changes in the striatal levels of either tyrosine hydroxylase immunoreactivity or dopamine D2 receptor messenger RNA. The c-fos messenger RNA expression induced by acute 6-hydroxydopamine injection was abolished by intraperitoneal pretreatment with the dopamine D2 receptor agonist, quinelorane (2 mg/kg) and strongly reduced by administration of the selective adenosine A2A receptor antagonist SCH-58261 (5 mg/kg). The results reported here show, by using a novel methodological approach, that an acute decrease of dopamine release causes an induction of c-fos messenger RNA in dopamine D2 receptor-containing striatopallidal neurons. This, together with previous findings, demonstrates that the c-fos gene expression is tonically inhibited by the impulse flow-dependent dopamine release via D2 receptors. In addition, this study provides evidence that endogenous adenosine, acting via adenosine A2A receptors, induces striatal c-fos messenger RNA when extracellular dopamine levels are strongly reduced. Thus endogenous dopamine and adenosine exert opposite effects on the activity of the D2-containing striatopallidal neurons.

Paired pulse analysis of ATP and noradrenaline release from sympathetic nerves of rat tail artery and mouse vas deferens: effects of K+ channel blockers

1. The paired pulse stimulus paradigm - two pulses of equal strength delivered at variable interpulse intervals was used to study the release of ATP and noradrenaline (NA) from post ganglionic sympathetic nerves of rat tail artery and mouse vas deferens. 2. Excitatory junction currents (EJCs) were used to measure the release of ATP, and differential pulse amperometry to measure that of NA. 3. At interpulse intervals of 0.1 - 1 s paired pulse stimulation caused an increase in the size of the second EJC, both in rat tail artery and mouse vas deferens. As the interpulse interval was increased to 10 s or more, the two EJCs became of equal size. 4. In both preparations the K+ channel blockers tetraethylammonium (TEA, 20 mM) and 4-aminopyridine (4-AP, 1 mM) prolonged the duration of the nerve terminal spike and greatly amplified the first EJC of the pair. 5. In the presence of TEA and 4-AP in rat tail artery paired pulse stimulation caused a dramatic depression of the second EJC without markedly affecting the nerve terminal spike. The depression of the second EJC decreased with increasing interpulse intervals, and also when external Ca2+ was reduced to 0.2 mM. In mouse vas deferens, TEA and 4-AP caused only a modest depression of the second EJC. 6. In rat tail artery in the presence of TEA and 4-AP paired pulse stimulation caused a depression of the NA oxidation current evoked by the second pulse, which was similar in magnitude and time course to that of the EJC. Similar TEA and 4-AP induced depression of the second pulse response was also observed when the purinergic and noradrenergic components of the contractile response were investigated. 7. The results show that in rat tail artery K+ channel blockers cause a dramatic paired pulse depression of the release of ATP and NA. The similarity in the depression of the EJC, the NA oxidation current, and the purinergic and noradrenergic components of the contractile response is compatible with the hypothesis that ATP and NA are released in parallel from the same neuronal sources.

Internalization of D1 dopamine receptor in striatal neurons in vivo as evidence of activation by dopamine agonists

To investigate how dopamine influences the subcellular localization of the dopamine receptors in the striatal dopaminoceptive neurons, we have used immunohistochemistry to detect D1 dopamine receptors (D1R) after modifications of the dopamine environment. In normal rats, D1R are located mostly extrasynaptically at the plasma membrane of the cell bodies, dendrites, and spines. The intrastriatal injection of the full D1R agonist SKF-82958 and the intraperitoneal injection of the same molecule or of amphetamine (which induces a massive release of dopamine in the striatum) induce modifications of the pattern of D1R immunoreactivity in the dorsal and ventral striatum. Whereas normal rats display homogenous staining of the neuropile with staining of the plasma membrane of the cell bodies, either treatment provokes the appearance of an intense immunoreactivity in the cytoplasm and the proximal dendrites. The labeling pattern is heterogeneous and more intense in the striosomes than in the matrix. Analysis of semithin sections and electron microscopy studies demonstrates a translocation of the labeling from the plasma membrane to endocytic vesicles and endosomes bearing D1R immunoreactivity in the cytoplasm of cell bodies and dendrites. Injection of D1R antagonist (SCH-23390) alone or injection of D1R antagonist, together with amphetamine or SKF-82958, do not provoke modification of the immunoreactivity, as compared with normal rat. Our results demonstrate that, in vivo, the acute activation of dopamine receptors by direct agonists or endogenously released dopamine provokes dramatic modifications of their subcellular distribution in neurons, including internalization in the endosomal compartment in the cytoplasm. This suggests that modifications of the localization of neurotransmitter receptors, including extrasynaptic ones, may be a critical event that contributes to the postsynaptic response in vivo.

Increased expression of NGFI-A mRNA in the rat striatum following burst stimulation of the medial forebrain bundle

Using in situ hybridization, we examined the mRNA expression for several immediate early genes in dopamine-innervated brain areas following electrical burst vs. regular stimulation of the medial forebrain bundle in anaesthetized rats. Two hours after 5 Hz burst stimulation, the expression of the nerve growth factor-inducible clone A (NGFI-A) mRNA was increased in the medial part of the striatum. This increase was prevented by pretreatment with the dopamine-D1 receptor antagonist, SCH23390 (0.1 mg/kg i.p.). After 8 Hz burst stimulation, NGFI-A mRNA expression was increased in the medial, central and lateral parts of the striatum. Induction occurred predominantly in cells expressing mRNAs for the dopamine-D1 receptor, substance P and dopamine and cAMP-regulated phosphoprotein (DARP-32). Regular stimulation had no effect on NGFI-A mRNA expression. The induction of NGFI-A was related to the levels of dopamine released by burst or regular stimulation as demonstrated with in vivo amperometry. Two hours after stimulation, the expression of none of the other genes studied was altered. One hour after 8 Hz burst stimulation, the expression of NGFI-A, NGFI-B and jun-B mRNAs was increased in the striatum and that of NGFI-A, NGFI-B, c-fos, fos-B and jun-B mRNAs was variably increased in the nucleus accumbens and lateral septum. These results provide additional support for the physiological importance of burst firing activity in midbrain dopamine neurons for the activation of their target cells. They demonstrate a spatial and temporal specificity as regards the brain region, the gene activated, the receptor involved and the phenotype of the cells affected.

Prolonged and extrasynaptic excitatory action of dopamine mediated by D1 receptors in the rat striatum in vivo

The spatiotemporal characteristics of the dopaminergic transmission mediated by D1 receptors were investigated in vivo. For this purpose dopamine (DA) release was evoked in the striatum of anesthetized rats by train electrical stimulations of the medial forebrain bundle (one to four pulses at 15 Hz), which mimicked the spontaneous activity of dopaminergic neurons. The resulting dopamine overflow was electrochemically monitored in real time in the extracellular space. This evoked DA release induced a delayed increase in discharge activity in a subpopulation of single striatal neurons. This excitation was attributable to stimulation of D1 receptors by released DA because it was abolished by acute 6-hydroxydopamine lesion and strongly reduced by the D1 antagonist SCH 23390. Striatal neurons exhibiting this delayed response were also strongly excited by intravenous administration of the D1 agonist SKF 82958. Whereas the DA overflow was closely time-correlated with stimulation, the excitatory response mediated by DA started 200 msec after release and lasted for up to 1 sec. Moreover, functional evidence presented here combined with previous morphological data show that D1 receptors are stimulated by DA diffusing up to 12 micron away from release sites in the extrasynaptic extracellular space. In conclusion, DA released by bursts of action potentials exerts, via D1 receptors, a delayed and prolonged excitatory influence on target neurons. This phasic transmission occurs outside synaptic clefts but still exhibits a high degree of spatial specificity.

Exocytosis in single chromaffin cells: regulation by a secretory granule-associated Go protein

1. Besides having a role in signal transduction, trimeric G proteins may also be involved in membrane trafficking events. In chromaffin cells, G alpha o has been found associated with the membrane of secretory granules. Here we examined the role of Go in regulated exocytosis using pressure microinjection combined with amperometric measurement of catecholamine secretion from individual chromaffin cells. 2. Microinjection of GTP gamma S and mastoparan strongly inhibits the amperometric response to either nicotine or high K+. 3. The presence of mastoparan in the cell incubation medium had no effect on K(+)-evoked secretion, suggesting that mastoparan blocks the exocytotic machinery through an intracellular target protein not located just beneath the plasma membrane. 4. Microinjection of anti-G alpha o antibodies potentiates by more than 50% the K(+)-evoked secretion, whereas anti-G alpha i1/2 antibodies have no effect. 5. Thus an inhibitory Go protein, probably associated with secretory granules, controls exocytosis in chromaffin cells. The intracellular proteins controlling organelle-associated G proteins are currently unknown. The neuronal cytosolic protein GAP-43 stimulates G alpha o in purified chromaffin granule membranes and inhibits exocytosis in permeabilized cells. We show here that microinjection of a synthetic peptide corresponding to the domain of GAP-43 that interacts with Go inhibits secretion. We suggest that GAP-43 or a related cytosolic protein controls the exocytotic priming step in chromaffin, cells by stimulating a granule-associated Go protein.

Neurochemical characteristics of amisulpride, an atypical dopamine D2/D3 receptor antagonist with both presynaptic and limbic selectivity

The benzamide derivative amisulpride shows a unique therapeutic profile being antipsychotic, at high doses, and disinhibitory, at low doses, while giving rise to only a low incidence of extrapyramidal side effects. In vitro, amisulpride has high affinity and selectivity for the human dopamine D2 (Ki = 2.8 nM) and D3 (Ki = 3.2 nM) receptors. Amisulpride shows antagonist properties toward D3 and both pre- and postsynaptic D2-like dopamine receptors of the rat striatum or nucleus accumbens in vitro. At low doses (< or = 10 mg/kg) amisulpride preferentially blocks presynaptic dopamine autoreceptors that control dopamine synthesis and release in the rat, whereas at higher doses (40-80 mg/kg) postsynaptic dopamine D2 receptor occupancy and antagonism is apparent. In contrast, haloperidol is active in all of these paradigms within the same dose range. Amisulpride preferentially inhibits in vivo binding of the D2/D3 antagonist [3H]raclopride to the limbic system (ID50 = 17 mg/kg) in comparison to the striatum (ID50 = 44 mg/kg) of the rat, increases striatal and limbic tissue 3,4-dihydroxyphenylacetic acid levels with similar potency and efficacy, and preferentially increases extracellular 3,4-dihydroxyphenylacetic acid levels in the nucleus accumbens when compared to the striatum. Haloperidol shows similar potency for the displacement of in vivo [3H]raclopride binding in striatal and limbic regions and preferentially increases striatal tissue 3,4-dihydroxyphenylacetic acid levels. The present data characterize amisulpride as a specific dopamine receptor antagonist with high and similar affinity for the dopamine D2 and D3 receptor. In vivo, it displays a degree of limbic selectivity and a preferential effect, at low doses, on dopamine D2/D3 autoreceptors. This atypical profile may explain the therapeutic efficacy of amisulpride in the treatment of both positive and negative symptoms of schizophrenia.

Excitatory effects of dopamine released by impulse flow in the rat nucleus accumbens in vivo

Dopamine is generally considered to be an inhibitory neurotransmitter in the central nervous system. Dopamine release in the nucleus accumbens can be evoked by chemical stimulation of the afferent cell bodies using N-methyl-D-aspartate microinjection in the ventral tegmental area. We report here that following such injections most neurons of the nucleus accumbens were excited. This excitation was abolished if dopaminergic neurons were lesioned and was blocked by antagonists of the D1 dopamine receptors. Finally, excitatory responses to electrical stimulation of the hippocampus were strongly facilitated by endogenously released dopamine. We suggest, therefore, that under physiological conditions, dopamine acting on D1 receptors is actually an excitatory neurotransmitter.

Burst stimulation of the medial forebrain bundle selectively increase Fos-like immunoreactivity in the limbic forebrain of the rat

The present study was designed to evaluate the postsynaptic functional consequences of different presynaptic activity patterns in midbrain dopamine systems using electrical stimulation of the rat medial forebrain bundle and subsequent determination of c-fos expression, used as a marker for neuronal activation, in dopamine target areas, by means of Fos immunohistochemistry. Nerve terminal dopamine release evoked by electrical stimulation of the medial forebrain bundle was monitored in the same animals using in vivo voltammetry. A 5 Hz stimulation consisting of 60 trains of five pulses and lasting 1 min was applied to the medial forebrain bundle. This stimulation was repeated 15 times every 3 min. Its pattern was defined by the interpulse interval which was either 70 ms or 200 ms for burst or regularly spaced stimulation, respectively. Our results show that burst stimulation of the medial forebrain bundle, which increase release of dopamine in target areas, increases the basal Fos-like immunoreactivity in the stimulated hemisphere, while regular stimulation does not affect expression of this protein. Moreover, the increase in Fos-like immunoreactivity induced by burst stimulation is restricted to limbic related structures, i.e. nucleus accumbens shell and intermediate aspect of the lateral septum, and the major island of Calleja, but is not observed in motor related structures (nucleus accumbens core and striatum). Pretreatment with the D1 dopamine receptor antagonist, SCH23390 (0.1 mg/kg, i.p.), blocked the increase in Fos-like immunoreactivity induced by burst stimulation of the medial forebrain bundle, suggesting a role for these receptors in the observed effects. Pretreatment with the 5-hydroxytryptamine2A/2C receptor antagonist ritanserin (0.4 mg/kg, i.p.) did not affect the increase in Fos-like immunoreactivity induced by burst stimulation in the nucleus accumbens shell or in the lateral septum, although it blocked the stimulated enhancement of Fos-like immunoreactivity in the major island of Calleja. The present data indicate that, rather than the absolute mean discharge rate of midbrain dopamine neurons, the temporal organization of the action potentials they generate conveys information to their target areas.

Uptake of dopamine released by impulse flow in the rat mesolimbic and striatal systems in vivo

The release of dopamine in the striatum, nucleus accumbens, and olfactory tubercle of anesthetized rats was evoked by electrical stimulation of the mesolimbic dopaminergic pathway (four pulses at 15 Hz or four pulses at 200 Hz). Carbon fiber electrodes were implanted in these regions to monitor evoked dopamine overflow by continuous amperometry. The kinetics of dopamine elimination were estimated by measuring the time to 50% decay of the dopamine oxidation current after stimulation ceased. This time ranged from 64 ms in the striatum to 113 ms in the nucleus accumbens. Inhibition of dopamine uptake by nomifensine (2-20 mg/kg), GBR 12909 (20 mg/kg), cocaine (20 mg/kg), mazindol (10 mg/kg), or bupropion (25 mg/kg) enhanced this decay time by up to +602%. Uptake inhibition also produced an increase in the maximal amplitude of dopamine overflow evoked by four pulses at 15 Hz. This latter effect was larger in the striatum (+420%) than in mesolimbic areas (+140%). These results show in vivo that these uptake inhibitors actually slow the clearance of dopamine released by action potentials and suggest that dopaminergic transmission is both prolonged and potentiated strongly by these drugs, in particular in the striatum.

A novel electrophysiological approach to monitor pulse by pulse the concentration of released noradrenaline at the presynaptic alpha 2-adrenoceptors of sympathetic nerves in rat tail artery

The excitatory junction current (EJC) evoked by electrical stimulation of postganglionic sympathetic nerves of rat tail artery with 100 pulses at 2 Hz, at 1.3 mmol/l external Ca2+, was used as a measure of the per pulse release of ATP. In controls the EJCs were initially facilitated, then gradually depressed during the stimulus train. The first EJC was slightly depressed by the alpha 2-adrenoceptor antagonist yohimbine, but starting from the 4th pulse the EJCs were enhanced. Yohimbine increased the early facilitation without markedly modifying the subsequent depression. The yohimbine-induced enhancement of EJCs caused by pulses 11-100 was, thus, constant. The noradrenaline reuptake blocker cocaine depressed the EJCs, abolished the early facilitation and slightly enhanced the depression. These effects of cocaine were reversed by further addition of yohimbine. The alpha 2-adrenoceptor agonist xylazine (1 and 10 mumol/l) dose dependently depressed the EJCs starting from the first pulse. The inhibitory effect of 1 mumol/l xylazine, but not that of 10 mumol/l xylazine, declined with train length. The inhibition of individual EJCs caused by activation of presynaptic alpha 2-adrenoceptors was used to monitor the concentration of released noradrenaline at these receptors. The ratio of individual EJCs in the presence and absence of yohimbine was assumed to reflect, pulse by pulse, the relative concentration of released noradrenaline at the presynaptic alpha 2-adrenoceptors, and hence termed [NA]alpha 2. For comparison, the concentration of endogenous noradrenaline was monitored electrochemically by differential pulse amperometry with a carbon fibre microelectrode; this signal is termed [NA]CF. [NA]alpha 2 and [NA]CF grew during the first 7-10 or 14-16 pulses, respectively, and then remained relatively constant throughout the stimulus train. Cocaine caused [NA]alpha 2 and [NA]CF to continue to grow during the first 35 and 50 pulses, and enhanced their peak levels by 180% and 320%, respectively. For comparison with the effects on the EJCs mediated via presynaptic alpha 2-adrenoceptors, those caused by varying external Ca2+ level were examined. At 0.65 mmol/l Ca2+ the amplitude of the first EJC was smaller than that at 1.3 mmol/l Ca2+, but the facilitation of later EJCs was enhanced and the subsequent depression reduced. An increase in external Ca2+ to 2.6 mmol/l had the opposite effects. All effects on EJCs caused by changes in external Ca2+ were maximal for the first EJC and then declined with the train length.(ABSTRACT TRUNCATED AT 400 WORDS)

Nonlinear relationship between impulse flow, dopamine release and dopamine elimination in the rat brain in vivo

Central dopaminergic neurons exhibit two kinds of discharge activity: single spikes and bursts of two to six action potentials. Since these neurons can switch from one discharge pattern to the other whereas the mean discharge rate remains little affected, bursts may be more potent in triggering the release of their neurotransmitter, dopamine. Electrical stimulations mimicking the bursting pattern were actually twice as potent as regularly spaced stimulations to enhance the dopamine extracellular concentration. This suggested that dopamine release might be facilitated by increasing the impulse flow frequency. The high extracellular overflow evoked by a burst might also be due to accumulation of the released dopamine whereas, at lower frequencies, dopamine might be readily eliminated between every action potential. In the present study the dopamine overflow evoked by electrical stimulation of the dopaminergic pathway was measured in vivo by carbon fibre electrodes combined with continuous amperometry. We observed a small facilitation of the release per pulse during stimulations mimicking a burst but only in mesolimbic areas. The high extra-cellular dopamine level evoked by a burst was mainly due to accumulation of the released dopamine.

Continuous in vivo monitoring of evoked dopamine release in the rat nucleus accumbens by amperometry

The release of dopamine in the nucleus accumbens of anaesthetized rats was evoked either by electrical stimulation of the mesolimbic dopaminergic pathway or by local ejection of N-methyl-D-aspartate in the ventral tegmental area. Untreated carbon-fibre electrodes implanted in the nucleus accumbens were held at +400 mV versus a reference electrode, and the oxidation current was continuously monitored. Despite a poor selectivity to dopamine versus other oxidizable compounds such as ascorbic acid, the evoked responses were solely due to dopamine overflow in the extracellular fluid since they were closely correlated with the stimulations and exhibited all the expected characteristics related to a dopamine release. First, these effects were closely consistent with the anatomy of the mesolimbic dopaminergic system. Second, the responses to electrical stimulations were abolished by a tetrodotoxin ejection in the vicinity of the carbon-fibre electrode and they were strongly, but reversibly, diminished (60% decrease) when cadmium was substituted for calcium in an artificial cerebrospinal fluid ejected close to the electrode. Third, their maximal amplitudes were enhanced by amphetamine, pargyline, nomifensine and haloperidol. Fourth, inhibition of dopamine reuptake by nomifensine induced a five-fold decrease in the rate of decline of the evoked oxidation current. Fifth, contribution of noradrenaline and serotonin to the observed effects seems unlikely since specific reuptake blockers (desipramine and sertraline, respectively) did not alter them. Dopaminergic neurons discharge either in a single spike mode with a mean firing rate below 5 Hz or in a bursting pattern (intraburst frequency: 10 to 20 Hz).(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve activity-dependent variations in clearance of released noradrenaline: regulatory roles for sympathetic neuromuscular transmission in rat tail artery

The aim of this study was to find out if clearance of noradrenaline released from sympathetic nerve terminals in rat isolated tail artery is a physiological variable and if so, to determine its role for the noradrenaline-mediated neurogenic contraction. The per pulse release of noradrenaline induced by electrical nerve stimulation and the fluctuations of the level of noradrenaline at the receptors driving the contractions were assessed from the electrochemically determined noradrenaline oxidation current at a carbon fibre electrode at the surface of the artery. Both were compared with the noradrenaline-mediated neurogenic contraction. The effects on these parameters of cocaine or desipramine, or of corticosterone, were used to assess the relative roles of neuronal and extraneuronal uptake, respectively. The effects of cocaine or desipramine, which enhance the noradrenaline level at the receptors by blocking neuronal reuptake, were compared with those of yohimbine, presumed to act exclusively by enhancing the per pulse release of noradrenaline. The results seem to support the following tentative conclusions. Clearance of released noradrenaline occurs by neuronal uptake and diffusion, while extraneuronal uptake is negligible. The noradrenaline-induced neurogenic contraction is mediated via adrenoceptors on cells near the plane of the nerve plexus; the excitation spreads from these cells throughout the syncytium. The contractile response to exogenous noradrenaline may also be mediated via receptors on the innervated key cells. Reuptake of noradrenaline into the releasing varicosities, i.e. in "active junctions", is highly efficient for single quanta but rapidly saturated by repeated release, while reuptake of noradrenaline in the "surround" of active junctions is probably rarely saturated and more independent of nerve activity. Saturation of the transporter by repeated release of quanta from the same varicosity and the consequent accumulation of "residual" noradrenaline and increased diffusion out of the junction and recruitment of noradrenaline receptors in the surround may be the cause of the rapid growth of the contraction during a high frequency train. Diffusion of released noradrenaline away from the postjunctional receptors is restricted by a local nerve activity-dependent buffering mechanism which, in spite of fading of the per pulse release, helps maintain the noradrenaline concentration at the receptors and the contraction during long high-frequency trains. Reactivation of the clearance mechanisms upon cessation of nerve activity accelerates the relaxation.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatiotemporal pattern of quantal release of ATP and noradrenaline from sympathetic nerves: consequences for neuromuscular transmission

The recent explosive development in research concerning the fundamental mechanisms of synaptic transmission helps put the present paper in context. It is now evident that not all transmitter vesicles in a nerve terminal, not even all those docked at its active zones, are immediately available for release (36). We watch, fascinated, the unraveling of the amazingly complex cellular mechanisms and molecular machinery that determine whether or not a vesicle is "exocytosis-competent" (77,78,39,79). Studies on quantal release in different systems show that neurons are fundamentally similar in one respect: that transmitter release from individual active zones is monoquantal (2). But they also show that active zones in different neurons differ drastically in the probability of monoquantal release and in the number of quanta immediately available for release (3). This implies that one should not extrapolate directly from transmitter release in one set of presynaptic terminals (e.g., in neuromuscular endplate or squid giant synapse) to that in other nerve terminals, especially if they have a very different morphology. As shown here, one should not even extrapolate from transmitter release in sympathetic nerves in one tissue (e.g., rat tail artery) to that in other tissues or species (e.g., mouse vas deferens). It is noteworthy that most studies of quantal release are based on electrophysiological analysis and therefore deal with release of fast, ionotropic transmitters from small synaptic vesicles at the active zones, especially in neurons in which these events may be examined with high resolution (49,48,46,33,32). Such data are useful as general models of the release of both fast and slow transmitters from small synaptic vesicles at active zones in other systems, provided that these transmitters are released in parallel, as are apparently ATP and NA in sympathetic nerves. They tell us little or nothing, however, about the release of transmitters (e.g., neuropeptides) from the large vesicles, nor about the spatiotemporal pattern of monoquantal release from small synaptic vesicles in the many neurons that have boutons-en-passent terminals. They show that the time course of effector responses to fast, rapidly inactivated transmitters such as ACh or ATP is necessarily release related. But they do not even address the possibility that the effector responses to slow transmitters such as NA, co-released from the same terminals, may obey completely different rules and perhaps rather be clearance related (7).(ABSTRACT TRUNCATED AT 400 WORDS)

Intermittent release of noradrenaline by single pulses and release during short trains at high frequencies from sympathetic nerves in rat tail artery

As shown by electrophysiological analysis, the release of the sympathetic co-transmitter adenosine 5'-triphosphate (ATP) from individual release sites is monoquantal and intermittent; the average release probability may be as low as 0.01. Indirect evidence from biochemical studies of noradrenaline overflow is compatible with a similar monoquantal, low probability release of noradrenaline as well. In the present study our first aim was to address this issue more directly in rat tail artery, using continuous amperometry to monitor in real time the release of noradrenaline from a relatively small number of sympathetic nerve varicosities. The results seem to provide the first direct evidence that noradrenaline, similarly to ATP, may be released intermittently during nerve stimulation at low frequency. Our second aim was to use the same technique to study the release of noradrenaline caused by nerve stimulation with single pulses or short trains (two to eight pulses) at high frequencies. The results show that during stimulation at 20 Hz the peak amplitude of the noradrenaline oxidation current response grew linearly with the train length, but at 50 Hz the curve describing this growth was sigmoid in shape.

Kinetics of ATP- and noradrenaline-mediated sympathetic neuromuscular transmission in rat tail artery

Electrophysiological, electrochemical and mechanical recordings were employed to study the kinetics of the release and clearance of adenosine 5'-triphosphate (ATP) and noradrenaline (NA) as sympathetic co-transmitters and of the neurogenic and non-neurogenic contractions in rat isolated tail artery. The life-time of ATP and NA released by a single pulse or 10 pulses at 50 Hz was brief (< 100 ms, or < 3 s, respectively); the neurogenic contractile responses occurred largely after the transmitters had been removed from the extracellular space. The ATP-induced neurogenic contractile responses to a single pulse or 10 pulses at 50 Hz were similar in time-course to the responses to direct muscle stimulation at low voltage; both seemed to be caused by activation of nifedipine-sensitive voltage-gated L-type Ca2+ channels. The alpha 1- and alpha 2-adrenoceptor-mediated components of the NA-induced neurogenic contractile response to 10 pulses at 50 Hz were more delayed and prolonged and determined by properties of the post-receptor mechanisms. The per pulse release of both ATP and NA faded rapidly during long high-frequency trains. So did the ATP level at the receptors and the ATP-induced neurogenic contraction. The NA levels and the contractile responses induced via alpha 1- and alpha 2-adrenoceptors were much better maintained during ongoing stimulation at 20 Hz but relaxed rapidly afterwards, suggesting that nerve activity suppressed, and cessation of nerve activity reactivated NA clearance.

Kinetics of noradrenaline released by sympathetic nerves

At the skeletal neuromuscular junction the released neurotransmitter, acetylcholine, is eliminated within some milliseconds. This time course is known with great precision through the electrical response of target cells. At the sympathetic neuroeffector junction the fast electrical response is not mediated by noradrenaline but by a cotransmitter: ATP. The slow electrical response and the slow component of smooth muscle contraction are principally mediated by noradrenaline. These responses are two orders of magnitude slower than the electrical response to ATP. Therefore, great uncertainty remains regarding the kinetics of noradrenaline appearance and elimination. Here, the local noradrenaline concentration at the surface of the isolated rat tail artery was electrochemically monitored in real time using a carbon fibre electrode. We have shown that the time course of the neurogenically released noradrenaline is at least one order of magnitude faster than the resulting contraction. The kinetics of noradrenaline inactivation by neuronal reuptake were also precisely measured.

High sensitivity measurement of brain catechols and indoles in vivo using electrochemically treated carbon-fiber electrodes

The combination of electrochemically treated carbon-fiber electrodes with DPV, DNPV or DPA represents a wide range of possibilities. As shown in this review, the choice of treatment and measurement technique depends on the purpose. As regards in vivo monitoring of 5-HIAA or DOPAC from very small brain nuclei, electrochemically treated carbon-fiber electrodes appear very potent and inexpensive. The main limitation of the established electrochemical techniques, including those discussed here, is that the unequivocal measurement of the basal extracellular neurotransmitter level cannot be achieved unless animals are treated with pargyline. On the other hand, this monitoring is feasible with in vivo dialysis. Therefore, electrochemical techniques, on the one hand, and in vivo dialysis, on the other hand, present different advantages. The former are much more potent than the latter in two respects. First, due to the much smaller size of the sensor, electrochemical techniques are more suitable for studying small brain nuclei. Second, since electrochemical techniques exhibit a better temporal resolution, they are recommended for investigating the relationship between impulse flow and neurotransmitter release. However, when high anatomical or temporal resolution is not required, in vivo dialysis is more suitable for recording the basal monoamine release.