Effects of haloperidol on the activity and membrane physiology of substantia nigra dopamine neurons recorded in vitro

Different electrophysiological profiles of genetically labelled dopaminergic neurons in the mouse midbrain and olfactory bulb

Dopaminergic (DA) neurons play pivotal roles in diverse brain functions, spanning movement, reward processing and sensory perception. DA neurons are most abundant in the midbrain (Substantia Nigra pars compacta [SNC] and Ventral Tegmental Area [VTA]) and the olfactory bulb (OB) in the forebrain. Interestingly, a subtype of OB DA neurons is capable of regenerating throughout life, while a second class is exclusively born during embryonic development. Compelling evidence in SNC and VTA also indicates substantial heterogeneity in terms of morphology, connectivity and function. To further investigate this heterogeneity and directly compare form and function of midbrain and forebrain bulbar DA neurons, we performed immunohistochemistry and whole-cell patch-clamp recordings in ex vivo brain slices from juvenile DAT-tdTomato mice. After confirming the penetrance and specificity of the dopamine transporter (DAT) Cre line, we compared soma shape, passive membrane properties, voltage sags and action potential (AP) firing across midbrain and forebrain bulbar DA subtypes. We found that each DA subgroup within midbrain and OB was highly heterogeneous, and that DA neurons across the two brain areas are also substantially different. These findings complement previous work in rats as well as gene expression and in vivo datasets, further questioning the existence of a single "dopaminergic" neuronal phenotype.

Modulation of Functional Connectivity Between Dopamine Neurons of the Rat Ventral Tegmental Area in vitro

Micro Electrode Arrays were used to simultaneously record spontaneous extracellular action potentials from 10 to 30 dopamine neurons in acute brain slices from the lateral Ventral Tegmental Area (VTA) of the rat. The spike train of an individual neuron was used to characterize the firing pattern: firing rate, firing irregularity and oscillation frequency. Functional connectivity between a pair of neurons was quantified by the Paired Phase Consistency (PPC), taking the oscillation frequency as reference. Under baseline conditions the PPC was significantly different from zero and 42 of the 386 pairs of VTA neurons showed significant coupling. Fifty percent of the recorded dopamine neurons were part of the coupled VTA network. Raising extracellular potassium from 3.5 to 5 mM increased the mean firing rate of the dopamine neurons by 45%. The same increase could be induced by bath application of 300 μm glutamate. High potassium reduced the PPC, but it did not change during the glutamate application. Our findings imply that manipulating excitability has distinct and specific consequences for functional connectivity in the VTA network that cannot be directly predicted from the changes in neuronal firing rates. Functional connectivity reflects the spatial organization and synchronization of the VTA output and thus represents a unique element of the message that is sent to the mesolimbic projection area. It adds a dimension to pharmacological manipulation of the VTA micro circuit that might help to understand the pharmacological (side) effects of e.g., anti-psychotic drugs.

State-dependent effects of the D2 partial agonist aripiprazole on dopamine neuron activity in the MAM neurodevelopmental model of schizophrenia

Aripiprazole is an antipsychotic drug characterized by partial agonist activity at D₂ receptors to normalize both hyperdopaminergic and hypodopaminergic states. Traditional D₂ antagonist antipsychotic drugs have been shown previously to reduce dopamine neuron activity through action on D₂ autoreceptors to produce an overexcitation-induced cessation of cell firing, referred to as depolarization block. It is unclear whether aripiprazole reduces dopamine neuron activity via inhibition or, as seen following D₂ antagonist administration, depolarization block. The impact of acute and repeated aripiprazole treatment was examined in the methylazoxymethanol acetate (MAM) rodent model to observe its effects on a hyperdopaminergic system, compared to normal rats. We found that administration of aripiprazole acutely or after 1 or 7 days of withdrawal from 21-day repeated treatment led to a decrease in the number of spontaneously active dopamine neurons in MAM rats but not in controls. This reduction was not reversed by apomorphine (100-200 µg/kg i.p. or 20 µg/kg i.v.) administration, suggesting that it was not due to depolarization block. In contrast, 1 h after induction of depolarization block of dopamine neurons by acute haloperidol treatment (0.6 mg/kg i.p.), aripiprazole (1 mg/kg, i.p.) reversed the depolarization block state. Therefore, aripiprazole rapidly reduced the hyperdopaminergic activity selectively in MAM rats. The reduction is unlikely due to depolarization block and persists following 7-day withdrawal from repeated treatment. Aripiprazole also removes haloperidol-induced depolarization block in MAM rats, which may underlie the acute psychotic state often observed with switching to this treatment.

Importance of kynurenine 3-monooxygenase for spontaneous firing and pharmacological responses of midbrain dopamine neurons: Relevance for schizophrenia

Kynurenine 3-monooxygenase (KMO) is an essential enzyme of the kynurenine pathway, converting kynurenine into 3-hydroxykynurenine. Inhibition of KMO increases kynurenine, resulting in elevated levels of kynurenic acid (KYNA), an endogenous N-methyl-d-aspartate and α*7-nicotinic receptor antagonist. The concentration of KYNA is elevated in the brain of patients with schizophrenia, possibly as a result of a reduced KMO activity. In the present study, using in vivo single cell recording techniques, we investigated the electrophysiological characteristics of ventral tegmental area dopamine (VTA DA) neurons and their response to antipsychotic drugs in a KMO knock-out (K/O) mouse model. KMO K/O mice exhibited a marked increase in spontaneous VTA DA neuron activity as compared to wild-type (WT) mice. Furthermore, VTA DA neurons showed clear-cut, yet qualitatively opposite, responses to the antipsychotic drugs haloperidol and clozapine in the two genotypes. The anti-inflammatory drug parecoxib successfully lowered the firing activity of VTA DA neurons in KMO K/O, but not in WT mice. Minocycline, an antibiotic and anti-inflammatory drug, produced no effect in this regard. Taken together, the present data further support the usefulness of KMO K/O mice for studying distinct aspects of the pathophysiology and pharmacological treatment of psychiatric disorders such as schizophrenia.

Determining Whether a Definitive Causal Relationship Exists Between Aripiprazole and Tardive Dyskinesia and/or Dystonia in Patients With Major Depressive Disorder, Part 2: Preclinical and Early Phase Human Proof of Concept Studies

This series of columns has 3 main goals: (1) to explain class warnings as used by the United States Food and Drug Administration, (2) to increase awareness of movement disorders that may occur in patients treated with antipsychotic medications, and (3) to understand why clinicians should refrain from immediately assuming a diagnosis of tardive dyskinesia/dystonia (TD) in patients treated with antipsychotics. The first column in this series began with the case of a 76-year-old man with major depressive disorder who developed orofacial dyskinesias while being treated with aripiprazole as an antidepressant augmentation strategy. It was alleged that a higher than intended dose of aripiprazole (ie, 20 mg/d for 2 wk followed by 10 mg/d for 4 wk instead of the intended dose of 2 mg/d) was the cause of the dyskinetic movements in this man, and the authors were asked to review the case and give their opinion. The principal basis for this theory of causation was the class warning about TD in the package insert for aripiprazole. The rationale for concluding aripiprazole caused TD in the 76-year-old man led to this series of columns about aripiprazole, its potential--if any--to cause TD, and the presence of a class warning about TD in its package insert. The central point is to illustrate why class warnings exist and their implications for practice. The first column in this series focused on the historical background, incidence, prevalence, risk factors, and clinical presentations of tardive and spontaneous dyskinesias and concluded with a discussion of diagnostic considerations explaining why clinicians should avoid making a diagnosis of TD until a thorough differential diagnosis has been considered. This second column in the series reviews the pharmacology of aripiprazole and the preclinical and phase I translational human studies that suggest aripiprazole should have a low to nonexistent risk of causing TD compared with other antipsychotics. The third column in the series will review the systematic clinical trial data and "real-world" data on TD and the use of aripiprazole as adjunctive treatment with antidepressants for major depressive disorder to see whether these data support the conclusion of a low to nonexistent relationship between aripiprazole treatment and the development of TD. The fourth and final column in the series will consider the type of study that would need to be performed to avoid a specific class warning, focusing on the TD class warning as an example and discussing why such studies are rarely done.

Implications of cellular models of dopamine neurons for schizophrenia

Midbrain dopamine neurons are pacemakers in vitro, but in vivo they fire less regularly and occasionally in bursts that can lead to a temporary cessation in firing produced by depolarization block. The therapeutic efficacy of antipsychotic drugs used to treat the positive symptoms of schizophrenia has been attributed to their ability to induce depolarization block within a subpopulation of dopamine neurons. We summarize the results of experiments characterizing the physiological mechanisms underlying the ability of these neurons to enter depolarization block in vitro, and our computational simulations of those experiments. We suggest that the inactivation of voltage-dependent Na(+) channels, and, in particular, the slower component of this inactivation, is critical in controlling entry into depolarization block. In addition, an ether-a-go-related gene (ERG) K(+) current also appears to be involved by delaying entry into and speeding recovery from depolarization block. Since many antipsychotic drugs share the ability to block this current, ERG channels may contribute to the therapeutic effects of these drugs.

CyPPA, a Positive SK3/SK2 Modulator, Reduces Activity of Dopaminergic Neurons, Inhibits Dopamine Release, and Counteracts Hyperdopaminergic Behaviors Induced by Methylphenidate

Dopamine (DA) containing midbrain neurons play critical roles in several psychiatric and neurological diseases, including schizophrenia and attention deficit hyperactivity disorder, and the substantia nigra pars compacta neurons selectively degenerate in Parkinson’s disease. Pharmacological modulation of DA receptors and transporters are well established approaches for treatment of DA-related disorders. Direct modulation of the DA system by influencing the discharge pattern of these autonomously firing neurons has yet to be exploited as a potential therapeutic strategy. Small conductance Ca²⁺-activated K⁺ channels (SK channels), in particular the SK3 subtype, are important in the physiology of DA neurons, and agents modifying SK channel activity could potentially affect DA signaling and DA-related behaviors. Here we show that cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine (CyPPA), a subtype-selective positive modulator of SK channels (SK3 > SK2 > > > SK1, IK), decreased spontaneous firing rate, increased the duration of the apamin-sensitive afterhyperpolarization, and caused an activity-dependent inhibition of current-evoked action potentials in DA neurons from both mouse and rat midbrain slices. Using an immunocytochemically and pharmacologically validated DA release assay employing cultured DA neurons from rats, we show that CyPPA repressed DA release in a concentration-dependent manner with a maximal effect equal to the D2 receptor agonist quinpirole. In vivo studies revealed that systemic administration of CyPPA attenuated methylphenidate-induced hyperactivity and stereotypic behaviors in mice. Taken together, the data accentuate the important role played by SK3 channels in the physiology of DA neurons, and indicate that their facilitation by CyPPA profoundly influences physiological as well as pharmacologically induced hyperdopaminergic behavior.

Antipsychotic drug-induced increases in ventral tegmental area dopamine neuron population activity via activation of the nucleus accumbens-ventral pallidum pathway

Acute administration of antipsychotic drugs increases dopamine (DA) neuron activity and DA release via D2 receptor blockade. However, it is unclear whether the DA neuron activation produced by antipsychotic drugs is due to feedback from post-synaptic blockade or is due to an action on DA neuron autoreceptors. This was evaluated using two drugs: the first-generation antipsychotic drug haloperidol that has potent D2 blocking properties, and the second-generation drug sertindole, which is unique in that it is reported to fail to reverse the apomorphine-induced decrease in firing rate typically associated with DA neuron autoreceptor stimulation. Using single-unit extracellular recordings from ventral tegmental area (VTA) DA neurons in anaesthetized rats, both drugs were found to significantly increase the number of spontaneously active DA neurons (population activity). Apomorphine administered within 10 min either before or after sertindole reversed the sertindole-induced increase in population activity, but had no effect when administered 1 h after sertindole. Moreover, both sertindole- and haloperidol-induced increase in population activity was prevented when nucleus accumbens feedback was interrupted by local infusion of the GABAA antagonist bicuculline into the ventral pallidum. Taken together, these data suggest that antipsychotics increase DA neuron population activity via a common action on the nucleus accumbens-ventral pallidum-VTA feedback pathway and thus provide further elucidation on the mechanism by which antipsychotic drugs affect DA neuron activity. This provides an important insight into the relationship between altered DA neuron activity and potential antipsychotic efficacy.

Long term exposure to the chemokine CCL2 activates the nigrostriatal dopamine system: a novel mechanism for the control of dopamine release

Accumulating evidence show that chemokines can modulate the activity of neurons through various mechanisms. Recently, we demonstrated that CCR2, the main receptor for the chemokine CCL2, is constitutively expressed in dopamine neurons in the rat substantia nigra. Here we show that unilateral intranigral injections of CCL2 (50 ng) in freely moving rats increase extracellular concentrations of dopamine and its metabolites and decrease dopamine content in the ipsilateral dorsal striatum. Furthermore, these CCL2 injections are responsible for an increase in locomotor activity resulting in contralateral circling behavior. Using patch-clamp recordings of dopaminergic neurons in slices of the rat substantia nigra, we observed that a prolonged exposure (>8 min) to 10 nM CCL2 significantly increases the membrane resistance of dopaminergic neurons by closure of background channels mainly selective to potassium ions. This leads to an enhancement of dopaminergic neuron discharge in pacemaker or burst mode necessary for dopamine release. We provide here the first evidence that application of CCL2 on dopaminergic neurons increases their excitability, dopamine release and related locomotor activity.

Computational model predicts a role for ERG current in repolarizing plateau potentials in dopamine neurons: implications for modulation of neuronal activity

Blocking the small-conductance (SK) calcium-activated potassium channel promotes burst firing in dopamine neurons both in vivo and in vitro. In vitro, the bursting is unusual in that spiking persists during the hyperpolarized trough and frequently terminates by depolarization block during the plateau. We focus on the underlying plateau potential oscillation generated in the presence of both apamin and TTX, so that action potentials are not considered. We find that although the plateau potentials are mediated by a voltage-gated Ca(2+) current, they do not depend on the accumulation of cytosolic Ca(2+), then use a computational model to test the hypothesis that the slowly voltage-activated ether-a-go-go-related gene (ERG) potassium current repolarizes the plateaus. The model, which includes a material balance on calcium, is able to reproduce the time course of both membrane potential and somatic calcium concentration, and can also mimic the induction of plateau potentials by the calcium chelator BAPTA. The principle of separation of timescales was used to gain insight into the mechanisms of oscillation and its modulation using nullclines in the phase space. The model predicts that the plateau will be elongated and ultimately result in a persistent depolarization as the ERG current is reduced. This study suggests that the ERG current may play a role in burst termination and the relief of depolarization block in vivo.

D2 autoreceptors chronically enhance dopamine neuron pacemaker activity

Activation of D2 autoreceptors on midbrain dopamine neurons has been shown previously to acutely open K+ channels to inhibit intrinsically generated pacemaker activity. Here we report that D2 autoreceptors act chronically to produce an opposite action: to increase the speed and regularity of repetitive action potential firing. Voltage-, current-, and dynamic-clamp experiments, using conventional whole-cell and perforated patch-clamp recording, with cultured rat midbrain dopamine neurons show that a change in the number of functional A-type K+ channels alters firing rate and susceptibility to irregularity produced by other channels. cAMP and protein kinase A mediate the long-term action of D2 receptors in a manner that counters the short-term effect of this signaling pathway on K+ channel gating. We conclude that D2 autoreceptors, in addition to mediating acute negative feedback, are responsible for long-term enhancement of the rate and fidelity of dopamine neuron pacemaker activity.

Quetiapine increases the firing rate of rat substantia nigra and ventral tegmental area dopamine neurons in vitro

The antipsychotic drug quetiapine increases the firing rate of dopamine neurons in the substantia nigra and the ventral tegmental area of the rat. In the present study we used an in vitro midbrain slice preparation and found that 3 microM quetiapine increases the firing rate of dopamine neuron in both structures by approximately 30%. The magnitude of the increase was not correlated with the basal firing rate of the dopamine neurons. In addition, quetiapine was not able to antagonize the inhibition of the firing evoked by the dopamine D2 receptor agonist quinpirole. Only with a very high concentration (30 microM), quetiapine was able to counteract the amphetamine-induced inhibition of the firing of the ventral tegmental area neurons; this effect was less pronounced in substantia nigra neurons. These findings indicate that the increase in firing rate induced by quetiapine cannot solely be mediated through an interaction with the dopamine D2-like autoreceptor present on the dopamine neurons.

A Ca2+-independent slow afterhyperpolarization in substantia nigra compacta neurons

The discharge properties of dopaminergic neurons in substantia nigra are influenced by slow adaptive responses, which have not been fully identified. The present study describes, in a slice preparation from the rat, a complex afterhyperpolarization (AHP), elicited by action potential trains. The AHP could be subdivided into a fast component (AHP(f)), which was generated near action potential threshold, relaxed within approximately 1 s, and had highest amplitude when evoked by short-lasting (0.1 s) depolarizations, and a slow component (AHP(s)), which lasted several seconds, was evoked from subthreshold potentials, and required prolonged depolarizing stimuli (>0.1 s). A large proportion of the AHP(f) was sensitive to (i) 0.1 microM apamin, (ii) the Ca(2+) antagonists, Cd(2+) (0.2 mM) and Ni(2+) (0.3 mM), (iii) low (0.2 mM) extracellular Ca(2+) concentration, and (iv), Ca(2+) chelation with intracellular EGTA. The AHP(s) was resistant to the above treatments, and it was insensitive to 25 microM dantrolene or prolonged exposure to 1 microM thapsigargin. The reversal potential of the AHP(s) (-97 mV) was close to the K(+) equilibrium potential. It was significantly inhibited by 5 mM 4-aminopyridine, 5 microM haloperidol, 10 microM terfenadine, or high extracellular Mg(2+) (10 mM), but not by 30 mM tetraethylammonium chloride, 50 microM carbachol, 0.5 microM glipizide, 2 microM (-)sulpiride, 100 microM N-allyl-normetazocine, or 100 microM pentazocine. Haloperidol reduced the post-stimulus inhibitory period seen during spontaneous discharge, but had no detectable effect on spike frequency adaptation. It is concluded that the SK-type Ca(2+)-activated K(+) channels underlies a major component of the AHP(f), whereas the AHP(s) is Ca(2+)-independent and relies, in part, on a voltage-dependent K(+) current with properties resembling the ether-a-go-go-related gene K(+) channel. The latter component exerts a slow, spike-independent, inhibitory influence on repetitive discharge and contributes to the prolonged decrease in excitability following sustained depolarizing stimuli.

Blockade of nociceptin/orphanin FQ receptor signaling in rat substantia nigra pars reticulata stimulates nigrostriatal dopaminergic transmission and motor behavior

A multidisciplinary approach was followed to investigate whether the opioid-like peptide nociceptin/orphanin FQ (N/OFQ) regulates the nigrostriatal dopaminergic pathway and motor behavior. Nigrostriatal dopaminergic cells, which express N/OFQ peptide (NOP) receptors, are located in the substantia nigra pars compacta and extend their dendrites in the substantia nigra pars reticulata, thereby modulating the basal ganglia output neurons. In vitro electrophysiological recordings demonstrated that N/OFQ hyperpolarized the dopaminergic cells of the substantia nigra pars compacta and inhibited their firing activity. In vivo dual-probe microdialysis showed that N/OFQ perfused in the substantia nigra pars reticulata reduced dopamine release in the ipsilateral striatum, whereas UFP-101 ([Nphe1,Arg14,Lys15]N/OFQ(1-13)-NH2) (a selective NOP receptor peptide antagonist) stimulated it. N/OFQ microinjected in the substantia nigra pars reticulata impaired rat performance on a rotarod apparatus, whereas UFP-101 enhanced it. Electromyography revealed that N/OFQ and UFP-101 oppositely affected muscle tone, inducing relaxation and contraction of triceps, respectively. The selective NOP receptor nonpeptide antagonist J-113397 (1-[3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one), either injected intranigrally or given systemically, also elevated striatal dopamine release and facilitated motor activity, confirming that these effects were caused by blockade of endogenous N/OFQ signaling. The inhibitory role played by endogenous N/OFQ on motor activity was additionally strengthened by the finding that mice lacking the NOP receptor gene outperformed wild-type mice on the rotarod. We conclude that NOP receptors in the substantia nigra pars reticulata, activated by endogenous N/OFQ, drive a physiologically inhibitory control on motor behavior, possibly via modulation of the nigrostriatal dopaminergic pathway.

Clozapine modulates midbrain dopamine neuron firing via interaction with the NMDA receptor complex

The mode of action by which the atypical antipsychotic drug clozapine exerts its superior efficacy to ameliorate both positive and negative symptoms is still relatively unknown. A recent study shows that a pharmacologically increased concentration of brain kynurenic acid, an endogenous antagonist at the glycine-site of the NMDA receptor as well as at the alpha7* nicotinic receptor, reverses the excitatory effects of clozapine on ventral tegmental area (VTA) dopamine (DA) neurons into an inhibitory action. In the present in vivo electrophysiological study, we further investigated the mechanisms of action of clozapine on VTA DA neurons. In control rats intravenously administered clozapine (1.25-10 mg/kg) was associated with increased firing rate and burst firing activity of VTA DA neurons. However, administration of the N-methyl-D-aspartate (NMDA)-receptor antagonist MK 801 blocked the excitatory action of clozapine. Moreover, in rats pretreated with the antagonist of the glycine-site of the NMDA receptor, L-701,324, the effects of clozapine on VTA DA neurons were converted to purely inhibitory responses, including a decrease in firing rate and burst firing activity. Pretreatment with the alpha7* nicotinic receptor antagonist MLA did not affect the excitatory action of clozapine on VTA DA neurons. The results of the present study suggest that clozapine interacts with the NMDA receptor complex. In this regard, clozapine could affect the glycine site of the NMDA receptor or tentatively inhibit the glycine transporter. The inhibitory action of clozapine on VTA DA neurons may account for its beneficial effects in ameliorating symptoms of schizophrenia and may suggest further studies to investigate a role of the glycine site of the NMDA receptor as a target for novel antipsychotics.

Inhibitory action of clozapine on rat ventral tegmental area dopamine neurons following increased levels of endogenous kynurenic acid

The mode of action by which the atypical antipsychotic drug clozapine exerts its superior efficacy to ameliorate both positive and negative symptoms is still unknown. In the present in vivo electrophysiological study, we investigate the effects of haloperidol (a typical antipsychotic drug) and clozapine on ventral tegmental area (VTA) dopamine (DA) neurons in a situation of hyperdopaminergic activity in order to mimic tentatively a condition similar to that seen in schizophrenia. Increased DA transmission was induced by elevating endogenous levels of the N-methyl-D-aspartate receptor and alpha7(*) nicotinic receptor antagonist kynurenic acid (KYNA; by means of PNU 156561A, 40 mg /kg, i.v.). In control rats, i.v. administered haloperidol (0.05-0.8 mg/kg) or clozapine (1.25-10 mg/kg) was associated with increased firing rate and burst firing activity of VTA DA neurons. However, in rats displaying hyperdopaminergia (induced by elevated levels of KYNA), the effects of clozapine on VTA DA neurons were converted into pure inhibitory responses, including decrease in burst firing activity. In contrast, haloperidol still produced an excitatory action on VTA DA neurons in rats with elevated levels of endogenous brain KYNA. The results of the present study suggest that clozapine facilitates or inhibits VTA DA neurotransmission, depending on brain concentration of KYNA. Such an effect of clozapine may be related to its unique effect in also ameliorating negative symptoms of schizophrenia.

Time-dependent changes in gene expression profiles of midbrain dopamine neurons following haloperidol administration

Antipsychotic drugs require a treatment regimen of several weeks before clinical efficacy is achieved in patient populations. While the biochemical mechanisms underlying the delayed temporal profile remain unclear, molecular adaptations in specific neuroanatomical loci are likely involved. Haloperidol-induced changes in gene expression in various brain regions have been observed; however, alterations in distinct neuronal populations have remained elusive. The present study examined changes in gene expression profiles of ventral tegmental area (VTA) and substantia nigra (SN) tyrosine hydroxylase immunopositive neurons following 1, 10 or 21 days of haloperidol administration (0.5 mg/kg/day). Macroarrays were used to study the expression of receptors, signaling proteins, transcription factors and pre- and post-synaptic proteins. Data were analyzed using conventional statistical procedures as well as self-organizing maps (SOM) to elucidate conserved patterns of expression changes. Results show statistically significant haloperidol-induced and time-dependent alterations in 17 genes in the VTA and 25 genes in the SN, including glutamate and GABA receptor subunits, signaling proteins and transcription factors. SOMs revealed distinct patterns of gene expression changes in response to haloperidol. Understanding how gene expression is altered over a clinically relevant time course of haloperidol administration may provide insight into the development of antipsychotic efficacy as well as the underlying pathology of schizophrenia.

Effects of estrogen on brain development and neuroprotection--implications for negative symptoms in schizophrenia

Increasing evidence during the last few years suggests that there are gender-specific differences in schizophrenia, influencing the age of onset, treatment outcome and the prevalence of negative symptoms. With respect to the latter in postmortem brain and cerebrospinal fluid of schizophrenic patients with negative symptoms a reduction of dopaminergic activity became evident. Measures of noradrenergic activity, dopamine beta-hydroxylase and the metabolite MHPG, appear to decrease with brain atrophy seen in patients with negative symptoms. Serotonergic activity tends to be low in patients with impaired cognitive function as is seen in negative schizophrenia. In these patients ventricular enlargement is associated with the severity of negative symptoms, low monoamine activity and low cerebral glucose metabolism. On the other hand atypical antipsychotic drugs that modulate also glutamate receptor activity, suggest an additional alternative mechanism of antipsychotic action beyond aminergic neurotransmitters. These drugs improve glutamatergic transmission and decrease negative symptoms; this suggests a glutamatergic deficiency as an extension of the dopamine model. The glutamate-dopamine interaction illustrates the importance of cross-talk between projections to the cortex, striatum, and lower brainstem for the expression of negative symptomatology. On the other hand, estradiol-17beta the most potent female sex hormone influences not only primary and secondary sexual characteristics but also embryonal and fetal growth as well as development of the brain aminergic networks, which are involved in schizophrenia. Estradiol-l7beta possesses neuroprotective properties, which are relevant for the course of schizophrenia and this may explain the pronounced gender differences with respect to progression and therapeutic response of schizophrenia. The present review attempts an update and synthesis of the information about the hormonal influence on neuronal pathways in negative symptoms of schizophrenia. It shows that estradiol-l7beta influences transporters and receptors as well as the morphological appearance of neuronal systems and that it may be an integral part of the neuroprotective system ameliorating schizophrenia.

In vitro modulation of the firing rate of dopamine neurons in the rat substantia nigra pars compacta and the ventral tegmental area by antipsychotic drugs

An in vitro experimental midbrain slice preparation is described which allows simultaneous extracellular recordings of the (spontaneous) electrical activity of dopamine neurons in the rat substantia nigra (SN) and the ventral tegmental area (VTA). Under identical in vitro circumstances the mean firing frequency of the SN dopamine neurons was higher than that of the VTA dopamine neurons (2.1 vs. 1.4Hz). With this slice preparation, modulation of the electrical activity of SN and VTA dopamine neurons by (new) drugs can be quickly determined. Experiments with the selective D2 receptor agonist quinpirole and the selective D2 receptor antagonist (-)-sulpiride indicated that dopamine neurons in the SN and VTA hardly differ in their pharmacological properties for the D2-like (auto)receptor. (-)-Sulpiride and to a lesser extent risperidone induced a small increase in firing rate in SN and VTA neurons, which was reversible upon wash-out. Olanzapine-induced increase in firing rate was persistent in SN and VTA neurons, whereas the clozapine-induced increase in firing rate was only completely recovered upon wash-out in SN neurons. The difference in firing rates of SN and VTA dopamine neurons could have consequences for the effectiveness of dopaminergic drugs acting at the D2-like dopamine (auto)receptor on these neurons.

Amphetamine blocks long-term synaptic depression in the ventral tegmental area

The mesolimbic dopamine system is essential for reward-seeking behavior, and drugs of abuse are thought to usurp the normal functioning of this pathway. A growing body of evidence suggests that glutamatergic synapses on dopamine neurons in the ventral tegmental area (VTA) are modified during exposure to addictive drugs, producing sensitization, a progressive augmentation in the rewarding properties of psychostimulant drugs with repeated exposure. We have tested the hypothesis that psychostimulant exposure interferes with the synaptic plasticity of glutamatergic inputs to the VTA. We find that excitatory synapses onto VTA dopamine neurons exhibit long-term depression (LTD) in response to low-frequency stimulation and modest depolarization. LTD in the VTA is NMDA receptor-independent but is dependent on intracellular Ca(2+) and can be induced by driving Ca(2+) into the dopamine neuron. Brief exposure to amphetamine entirely blocks LTD at glutamatergic synapses in the VTA, by releasing endogenous dopamine that acts at D2 dopamine receptors. The block of LTD is selective, because amphetamine has no effect on hippocampal LTD. The LTD we have discovered in the VTA is likely to be an important component of excitatory control of the reward pathway; amphetamine will inhibit LTD, removing this normal brake on the glutamatergic drive to dopamine neurons. This effect of amphetamine represents an important mechanism by which normal function of the brain reward system may be impaired during substance abuse.

Neurons exhibiting dopamine D2 receptor immunoreactivity in the substantia nigra of the mutant weaver mouse

Neurons exhibiting D2 receptor-like immunoreactivity were investigated in the substantia nigra pars compacta of weaver mice at the light and electron microscope levels using immunocytochemical techniques. At the light microscope level, there was significant loss of D2-like immunoreactive cells in weaver mice and the remaining labeled cells exhibited less intense immunoreactivity. At the ultrastructural level, there was a decrease in the number of immunoreactive profiles and fewer synapses were observed abutting labeled dendritic profiles. In addition, degenerative changes were noted in some of the D2 receptor-like immunoreactive profiles. Double labeling with D2 and tyrosine hydroxylase indicated that the majority of the labeled profiles were double labeled. Eight-week-old homozygous weavers were paired with wild-type littermates as controls and perfused with a buffered solution of acrolein/paraformadehyde. Midbrain sections were reacted immunocytochemically either with an antiserum to D2 or with antisera to D2 and tyrosine hydroxylase, using a double-labeling technique. Sections were processed for light and electron microscopy by standard methods. The results of this study confirm the autoreceptor-like activity of D2 receptors on nigral dopamine neurons. The cell degeneration, down-regulation of D2 receptors, and decreased dendritic and synaptic components in the neuropil suggest that the synaptic integrity of the substantia nigra has been compromised, which in turn would affect the functional efficacy of the basal ganglia circuitry. This altered circuity is expressed in the Parkinson-like symptoms displayed by this mutant mouse.

Clozapine does activate nigrostriatal dopamine neurons in unanesthetized rats

Antipsychotic drugs are traditionally classified as typical or atypical on the basis of their property to cause or not to cause extrapyramidal side-effects. A widely accepted selectivity for the mesolimbic, vs. the nigrostriatal, dopaminergic system is postulated to underlie the existence of fewer or no extrapyramidal side-effects during treatment with atypical neuroleptics. In order to verify this hypothesis we examined the effect of acute clozapine on nigrostriatal dopaminergic neurons recorded from non-anaesthetised and from chloral hydrate-anaesthetised rats. Extracellular single-unit recording coupled with antidromic activation from the neostriatum was used. Intravenous administration of cumulative doses of clozapine (1.25-10 mg/kg) increased the firing rate of nigrostriatal dopaminergic neurons in non-anaesthetised rats, but failed to significantly modify the activity of the same units under chloral hydrate anesthesia. These results indicate that acute clozapine activates nigrostriatal dopamine cells in non-anaesthetised rats and cast doubts about a direct link between the lack of significant extrapyramidal side-effects and the selectivity of atypical neuroleptics, such as clozapine, for the mesolimbic dopamine system.

Compensatory mechanisms in experimental and human parkinsonism: towards a dynamic approach

This paper provides an overview of the compensatory mechanisms which come into action during experimental and human parkinsonism. The intrinsic properties of the dopaminergic neurones of the substantia nigra pars compacta (SNc) which degenerate during Parkinson's disease are described in detail. It is generally considered that the nigrostriatal pathway is principally responsible for the compensatory preservation of dopaminergic function. It is also becoming clear that the morphological characteristics of dopaminergic neurones and the dual character, synaptic and asynaptic, of striatal dopaminergic innervation engender two modes of transmission, wiring and volume, and that both these modes play a role in the preservation of dopaminergic function. The plasticity of the dopamine neurones, extrinsic or intrinsic to the striatum, can thus be regarded as another compensatory mechanism. Recent anatomical and electrophysiological studies have shown that the SNc receives both glutamatergic and cholinergic inputs. The dynamic role this innervation plays in compensatory mechanisms in the course of the disease is explained and discussed. Recent developments in the field of compensatory mechanisms speak for the urgence to develop a valid chronic model of Parkinson's disease, integrating all the clinical features, even resting tremor, and illustrating the gradual evolution of nigral degeneration observed in human Parkinson's disease. Only a dynamic approach to the physiopathological study of compensatory mechanisms in the basal ganglia will be capable of elucidating these complex questions.

Depletion of dopamine in the prefrontal cortex decreases the basal electrophysiological activity of mesolimbic dopamine neurons

One hypothesis regarding the etiology of schizophrenia proposes that disruption of the dopaminergic innervation of the prefrontal cortex leads to an increase in dopamine (DA) transmission in subcortical regions. In the present study, we examined the effect of 6-hydroxydopamine lesions of the medial prefrontal cortex (mPFC) dopamine innervation on the spontaneous electrophysiological activity of ventral tegmental DA neurons recorded in vivo. DA cell activity was assessed along three dimensions: (1) the relative proportion of DA neurons exhibiting spontaneous activity, (2) their basal firing rate, and (3) the mean percentage of spikes fired in bursts. In lesioned rats, DA neurons in the ventral tegmental area (VTA) exhibited a significantly slower mean firing rate, as well as a significant reduction in the percentage of spikes fired in bursts relative to controls. In contrast, depletion of DA in the mPFC did not have a significant effect on the relative proportion of VTA DA neurons exhibiting spontaneous activity. We suggest that by reducing the basal electrophysiological activity of VTA DA neurons, mPFC DA depletion may lead to an increase in the level of responsivity of the system to excitatory stimuli. Thus, the magnitude of increase in action potential-dependent DA release that occurs in response to a challenge may be augmented in lesioned rats.

Burst firing in midbrain dopaminergic neurons

Midbrain dopaminergic (DA) neurons fire bursts of activity in response to sensory stimuli, including those associated with primary reward. They are therefore conditional bursters - the bursts conveying, amongst other things, motivationally relevant information to the forebrain. In the forebrain, bursts give rise to a supra-additive release of dopamine, and possibly favour the release of co-localised neuropeptides. Evidence is presented that in rat DA neurons, bursts are engendered by the activity of cortically-regulated afferents. Certain factors are identified which, in combination, lead to burst production: (1) A burst of activity in EAAergic afferents to DA neurons arising from non-cortical sources, but controlled by the medial prefrontal cortex; (2) N-methyl-D-aspartate receptor activation, producing a slow depolarising wave in the recipient neuron; (3) activation of a high threshold, dendritically located calcium conductance which produces a 'plateau potential'; (4) activation of a calcium-activated potassium conductance, which terminates the burst. These factors are argued to operate in the context of an 'optimal' level of intracellular calcium buffering for bursting. Other factors which appear to be involved in bursting in other systems, in particular a low threshold calcium conductance, are rejected as being necessary for bursting in DA neurons. The factors which do play a crucial role in burst production in DA neurons are integrated into a theory from which arises a series of hypotheses amenable to empirical investigation. Additional factors are discussed which may modulate bursting. These may either act indirectly through changes in membrane potential (or intracellular calcium concentration), or they may act directly through an interaction with certain conductances, which appear to promote or inhibit burst firing in DA neurons.

Dopamine-cell depolarization block as a model for the therapeutic actions of antipsychotic drugs

Antipsychotic drugs used in the treatment of schizophrenia have in common the property of being dopamine-receptor antagonists. However, the rapid timecourse of receptor blockade produced upon drug administration does not correlate with the emergence of clinical actions, which typically require weeks of treatment to become manifest. Studies in rats have shown that repeated antipsychotic drug treatment results in a delayed inactivation of dopamine-neuron firing in the midbrain due to depolarization block. Furthermore, the therapeutic efficacy of antipsychotic drugs in humans correlates with their ability to induce depolarization block of mesolimbic dopamine neurons, whereas their potential to produce extrapyramidal side effects correlates with their propensity for inducing depolarization block in the nigrostriatal dopamine system. Therefore, dopamine-cell depolarization block is an effective model for evaluating antipsychotic drug efficacy, and provides a potential mechanism to account for their therapeutic impact on a dysregulated dopamine system.