Response Of Dopaminergic Neurons Research Articles

A dual-omics approach on the effects of fibroblast growth factor-2 (FGF-2) on ventral tegmental area dopaminergic neurons in response to alcohol consumption in mice.

Harmful alcohol consumption is a major socioeconomic burden to the health system, as it can be the cause of mortality of heavy alcohol drinkers. The dopaminergic (DAergic) system is thought to play an important role in the pathogenesis of alcohol drinking behaviour; however, its exact role remains elusive. Fibroblast growth factor 2 (FGF-2), a neurotrophic factor, associated with both the DAergic system and alcohol consumption, may play an important role in DAergic neuroadaptations during alcohol abuse. Within this study, we aimed to clarify the role of endogenous FGF-2 on the DAergic system and whether there is a possible link to alcohol consumption. We found that lack of FGF-2 reduces the alcohol intake of mice. Transcriptome analysis of DAergic neurons revealed that FGF-2 knockout (FGF-2 KO) shifts the molecular fingerprint of midbrain dopaminergic (mDA) neurons to DA subtypes of the ventral tegmental area (VTA). In line with this, proteomic changes predominantly appear also in the VTA. Interestingly, these changes led to an altered regulation of the FGF-2 signalling cascades and DAergic pathways in a region-specific manner, which was only marginally affected by voluntary alcohol consumption. Thus, lack of FGF-2 not only affects the gene expression but also the proteome of specific brain regions of mDA neurons. Our study provides new insights into the neuroadaptations of the DAergic system during alcohol abuse and, therefore, comprises novel targets for future pharmacological interventions.

Near-Infrared Fluorescent Nanoprobes for Revealing the Role of Dopamine in Drug Addiction.

Brain imaging techniques enable visualizing the activity of central nervous system without invasive neurosurgery. Dopamine is an important neurotransmitter. Its fluctuation in brain leads to a wide range of diseases and disorders, like drug addiction, depression, and Parkinson's disease. We designed near-infrared fluorescence dopamine-responsive nanoprobes (DRNs) for brain activity imaging during drug abuse and addiction process. On the basis of light-induced electron transfer between DRNs and dopamine and molecular wire effect of the DRNs, we can track the dynamical change of the neurotransmitter level in the physiological environment and the releasing of the neurotransmitter in living dopaminergic neurons in response to nicotine stimulation. The functional near-infrared fluorescence imaging can dynamically track the dopamine level in the mice midbrain under normal or drug-activated condition and evaluate the long-term effect of addictive substances to the brain. This strategy has the potential for studying neural activity under physiological condition.

Targeting the noradrenergic system for anti-inflammatory and neuroprotective effects: implications for Parkinson's disease.

Degeneration of the locus coeruleus noradrenergic system is thought to play a key role in the pathogenesis of Parkinson's disease (PD), whereas pharmacological approaches to increase noradrenaline bioavailability may provide neuroprotection. Noradrenaline inhibits microglial activation and suppresses pro-inflammatory mediator production (e.g., tumor necrosis factor-α, interleukin-1β & inducible nitric oxide synthase activity), thus limiting the cytotoxicity of midbrain dopaminergic neurons in response to an inflammatory stimulus. Neighbouring astrocyte populations promote a neurotrophic environment in response to β2-adrenoceptor (β2-AR) stimulation via the production of growth factors (e.g., brain derived neurotrophic factor, cerebral dopamine neurotrophic factor & glial cell derived neurotrophic factor which have shown promising neuroprotective and neuro-restorative effects in the nigrostriatal dopaminergic system. More recent findings have demonstrated a role for the β2-AR in down-regulating expression levels of the human α-synuclein gene SNCA and relative α-synuclein protein abundance. Given that α-synuclein is a major protein constituent of Lewy body pathology, a hallmark neuropathological feature in Parkinson's disease, these findings could open up new avenues for pharmacological intervention strategies aimed at alleviating the burden of α-synucleinopathies in the Parkinsonian brain. In essence, the literature reviewed herein supports our hypothesis of a tripartite neuroprotective role for noradrenaline in combating PD-related neuropathology and motor dysfunction via (1) inhibiting nigral microglial activation & pro-inflammatory mediator production, (2) promoting the synthesis of neurotrophic factors from midbrain astrocytes and (3) downregulating α-synuclein gene expression and protein abundance in a β2-AR-dependent manner. Thus, taken together, either pharmacologically enhancing extra-synaptic noradrenaline bioavailability or targeting glial β2-ARs directly makes itself as a promising treatment option aimed at slowing/halting PD progression.

Chronic Salsolinol Administration Prevents the Behavioral and Neurochemical Effects of l-DOPA in Rats

1-Methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol) is a well-known endogenous compound that has been proposed as a factor involved in the pathogenesis of Parkinson’s disease. In the present study, we investigated the impact of acute and chronic salsolinol (100 mg.kg i.p.) administration on l-DOPA-induced locomotor hyperactivity and neurochemical changes (the dopamine level and its metabolism in rat brain structures). Moreover, using the in vivo microdialysis technique, we measured the effect of acute and chronic salsolinol injection on l-DOPA-induced dopamine release in the rat striatum. The behavioral data demonstrated that both acute and chronic salsolinol administration antagonized l-DOPA-mediated hyperactivity. An ex vivo neurochemical experiment indicated that chronic but not acute salsolinol administration partially inhibited the l-DOPA-induced increases in the concentration of dopamine and all of its metabolites in dopaminergic structures. Additionally, the in vivo dopamine release data obtained from the microdialysis experiments clearly indicated that the differences in the effect of salsolinol on the activities of l-DOPA depended on the mode of salsolinol treatment. Acute injection of salsolinol enhanced the l-DOPA-induced elevation of dopamine release (by ~1200 %; P < 0.01), whereas chronic administration of salsolinol completely blocked the l-DOPA-induced elevation of dopamine release in the rat striatum. These data demonstrated that chronic administration of salsolinol significantly impaired the response of dopaminergic neurons to l-DOPA administration. In conclusion, we propose that an elevated salsolinol level in parkinsonian patients may represent a serious risk factor of the clinical efficacy of l-DOPA therapy.

Nuclear translocation of alpha-synuclein increases susceptibility of MES23.5 cells to oxidative stress

α-Synuclein (α-syn) and oxidative stress play pivotal roles in the pathogenesis of Parkinson's disease (PD). However, the mechanisms underlying the interaction between α-syn and oxidative stress remain poorly understood. The present study provides evidence to suggest that the nuclear translocation of α-syn increases death of dopaminergic neurons in response to oxidative stress. We found that administration of H2O2 induced a rapid cleavage and nuclear translocation of α-syn in cultured MES23.5 cells. Inhibition of calpain proteolysis, using a calpain inhibitor (MDL-28170), significantly blocked cleavage and nuclear translocation of α-syn and attenuated H2O2-induced cell death in MES23.5 cells. Expression of a truncated fragment of α-syn (58–140) significantly increased the cell death induced by H2O2 treatment. These results suggest that calpain proteolysis is involved in the process of nuclear translocation of α-syn in MES23.5 dopaminergic cells induced by oxidative stress, and that nuclear translocation of α-syn increases susceptibility of these cells to oxidative stress. Taken together, our findings provide new insight into the interaction between α-syn and oxidative stress through activation of calpain proteolytic activity.

Immediate expression of Cdh2 is essential for efficient neural differentiation of mouse induced pluripotent stem cells

Induced pluripotent stem cells (iPSCs) exhibit reduced efficiency and higher variability in neural differentiation compared to embryonic stem cells (ESCs). In this study, we showed that mouse iPSCs failed to efficiently give rise to neuronal cells using conventional methods previously established for driving mouse ESC differentiation. We reported a novel approach which remarkably increases neural differentiation of mouse iPSCs. This novel approach initiated embryoid body (EB) formation directly from the whole cell clones isolated from the top of feeder cells. Compared to conventional neural induction methods such as single cell suspension or monolayer culture, the cell clone-derived EB method led to a pronounced increase in directed generation of various types of neural cells including neural stem cells, motoneurons and dopaminergic neurons in response to different inducers. Through gene expression microarray analysis, we identified 14 genes that were highly expressed in the cell clone-derived EBs. Among them, we found that Cdh2, also known as N-cadherin, played important roles in controlling the neural differentiation efficiency of mouse iPSCs. Forced expression of Cdh2 in iPSCs substantially enhanced the differentiation efficiency while knocking-down of Cdh2 by shRNA blocked the neural differentiation. Our results revealed a critical role of Cdh2 in the process of efficient neural differentiation of mouse iPS cells.

Neural signals of extinction in the inhibitory microcircuit of the ventral midbrain.

Midbrain dopaminergic (DA) neurons are thought to guide learning via phasic elevations of firing in response to reward predicting stimuli. The circuit mechanism for these signals remains unclear. Using extracellular recording during associative learning we show that inhibitory neurons in the ventral midbrain of mice respond to salient auditory stimuli with a burst of activity that occurs prior to the onset of the phasic response of DA neurons. This population of inhibitory neurons exhibited enhanced responses during extinction and was anti correlated with the phasic response of simultaneously recorded DA neurons. Optogenetic stimulation suggested that this population was in part derived from inhibitory projection neurons of the substantia nigra that provide a robust monosynaptic inhibition of DA neurons. Our results thus elaborate upon the dynamic upstream circuits that shape the phasic activity of DA neurons and suggest that the inhibitory microcircuit of the midbrain is critical for new learning in extinction.

Dopaminergic neurons inhibit striatal output through non-canonical release of GABA.

The substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) contain the two largest populations of dopamine (DA)-releasing neurons in the mammalian brain. These neurons extend elaborate projections in striatum, a large subcortical structure implicated in motor planning and reward-based learning. Phasic activation of dopaminergic neurons in response to salient or reward-predicting stimuli is thought to modulate striatal output via the release of DA to promote and reinforce motor action1–4. Here we show that activation of DA neurons in striatal slices rapidly inhibits action potential firing in both direct-and indirect-pathway striatal projection neurons (SPNs) through vesicular release of the inhibitory transmitter γ-aminobutyric acid (GABA). GABA is released directly from dopaminergic axons but in a manner that is independent of the vesicular GABA transporter VGAT. Instead GABA release requires activity of the vesicular monoamine transporter VMAT2, which is the vesicular transporter for DA. Furthermore, VMAT2 expression in GABAergic neurons lacking VGAT is sufficient to sustain GABA release. Thus, these findings expand the repertoire of synaptic mechanisms employed by DA neurons to influence basal ganglia circuits, reveal a novel substrate whose transport is dependent on VMAT2, and demonstrate that GABA can function as a bona fide co-transmitter in monoaminergic neurons.

Diazepam-induced neuronal plasticity attenuates locomotor responses to morphine and amphetamine challenges in mice

A single administration of benzodiazepine-site ligands of the inhibitory GABA(A) receptors has been shown to lead to persistently potentiated AMPA receptor-mediated responses in dopaminergic neurons of the ventral tegmental area (VTA). This plasticity has been suggested to be a common property of different kinds of addictive drugs. We now wanted to test if the plasticity induced by diazepam would also affect behaviors elicited by other drugs of abuse. Activity and plasticity of the VTA dopaminergic neurons are known to be essential for the initiation and/or sensitization of the psychomotor responses to morphine and amphetamine. The effect of diazepam pre-treatment (a single dose of 5 mg/kg) was studied 24-72 h later in behaving C57BL/6J mice on locomotor activity induced by acute and repeated administration of morphine (5 mg/kg) and amphetamine (2.5 mg/kg). The pre-treatment attenuated the locomotor-activating effect of morphine. On the other hand, it reduced the amphetamine-induced locomotor sensitization in male mice in N-methyl-d-aspartate (NMDA) receptor-dependent manner. The acute amphetamine effect was not affected. The results indicate that benzodiazepine-induced neural plasticity transiently reduces the sensitivity to psychomotor stimulation by opioids and stimulants.

Generation of reactive oxygen species in 1-methyl-4-phenylpyridinium (MPP+) treated dopaminergic neurons occurs as an NADPH oxidase-dependent two-wave cascade

BackgroundReactive oxygen species (ROS), superoxide and hydrogen peroxide (H2O2), are necessary for appropriate responses to immune challenges. In the brain, excess superoxide production predicts neuronal cell loss, suggesting that Parkinson's disease (PD) with its wholesale death of dopaminergic neurons in substantia nigra pars compacta (nigra) may be a case in point. Although microglial NADPH oxidase-produced superoxide contributes to dopaminergic neuron death in an MPTP mouse model of PD, this is secondary to an initial die off of such neurons, suggesting that the initial MPTP-induced death of neurons may be via activation of NADPH oxidase in neurons themselves, thus providing an early therapeutic target.MethodsNADPH oxidase subunits were visualized in adult mouse nigra neurons and in N27 rat dopaminergic cells by immunofluorescence. NADPH oxidase subunits in N27 cell cultures were detected by immunoblots and RT-PCR. Superoxide was measured by flow cytometric detection of H2O2-induced carboxy-H2-DCFDA fluorescence. Cells were treated with MPP+ (MPTP metabolite) following siRNA silencing of the Nox2-stabilizing subunit p22phox, or simultaneously with NADPH oxidase pharmacological inhibitors or with losartan to antagonize angiotensin II type 1 receptor-induced NADPH oxidase activation.ResultsNigral dopaminergic neurons in situ expressed three subunits necessary for NADPH oxidase activation, and these as well as several other NADPH oxidase subunits and their encoding mRNAs were detected in unstimulated N27 cells. Overnight MPP+ treatment of N27 cells induced Nox2 protein and superoxide generation, which was counteracted by NADPH oxidase inhibitors, by siRNA silencing of p22phox, or losartan. A two-wave ROS cascade was identified: 1) as a first wave, mitochondrial H2O2 production was first noted at three hours of MPP+ treatment; and 2) as a second wave, H2O2 levels were further increased by 24 hours. This second wave was eliminated by pharmacological inhibitors and a blocker of protein synthesis.ConclusionsA two-wave cascade of ROS production is active in nigral dopaminergic neurons in response to neurotoxicity-induced superoxide. Our findings allow us to conclude that superoxide generated by NADPH oxidase present in nigral neurons contributes to the loss of such neurons in PD. Losartan suppression of nigral-cell superoxide production suggests that angiotensin receptor blockers have potential as PD preventatives.

Dopaminergic modulation of rapid reality adaptation in thinking

Dopamine has long held a prominent role in the interpretation of schizophrenia and other psychoses. Clinical studies on confabulation and disorientation, disorders marked by a confusion of reality in thinking, indicated that the ability to keep thinking in phase with reality depends on a process suppressing the interference of upcoming memories that do not refer to ongoing reality. A host of animal studies and a recent clinical study suggested that this suppression might correspond to the phasic inhibition of dopaminergic neurons in response to the absence of expected outcomes. In this study, we tested healthy subjects with a difficult version of a memory paradigm on which confabulating patients had failed. Subjects participated in three test sessions, in which they received in double-blind, randomized fashion l-dopa, risperidone, or placebo. We found that l-dopa, in comparison with risperidone, impaired performance in a highly specific way, which corresponded to the pattern of patients with reality confusion. Specifically, they had an increase of false positive responses, while overall memory performance and reaction times were unaffected. We conclude that dopaminergic transmission influences the ability to rapidly adapt thinking to ongoing reality.

The role of dopamine-dependent negative feedback in the hippocampus-basal ganglia-thalamus-hippocampus loop in the extinction of responses

A mechanism for the extinction of the responses of hippocampal and dopaminergic neurons to repeated sensory stimuli is proposed, based on dopamine-dependent negative feedback in the hippocampus-basal ganglia-thalamus-hippocampus loop. Activation of hippocampal neurons evoked by a new stimulus facilitates the appearance of responses in dopaminergic neurons as a result of disinhibition via striopallidal cells of the nucleus accumbens and ventral pallidum. However, increases in dopamine levels and activation of D2 receptors on striopallidal cells, facilitating depression of hippocampal inputs, prevent disinhibition of dopaminergic neurons, such that their responses start to decline. Subsequent reductions in actions on D1 receptors lead to decreases in the efficiency of excitation both of neurons in hippocampal field CA1 and strionigral cells in the nucleus accumbens. The direct pathway via the basal ganglia mediates disinhibition of the thalamic nucleus reuniens, exciting neurons in field CA1, which leads to extinction of the responses of hippocampal neurons, decreases in disinhibition of dopaminergic cells, and further extinction of their responses.

Inhibition of the leucine-rich repeat protein LINGO-1 enhances survival, structure, and function of dopaminergic neurons in Parkinson's disease models

The nervous system-specific leucine-rich repeat Ig-containing protein LINGO-1 is associated with the Nogo-66 receptor complex and is endowed with a canonical EGF receptor (EGFR)-like tyrosine phosphorylation site. Our studies indicate that LINGO-1 expression is elevated in the substantia nigra of Parkinson's disease (PD) patients compared with age-matched controls and in animal models of PD after neurotoxic lesions. LINGO-1 expression is present in midbrain dopaminergic (DA) neurons in the human and rodent brain. Therefore, the role of LINGO-1 in cell damage responses of DA neurons was examined in vitro and in experimental models of PD induced by either oxidative (6-hydroxydopamine) or mitochondrial (N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) toxicity. In LINGO-1 knockout mice, DA neuron survival was increased and behavioral abnormalities were reduced compared with WT. This neuroprotection was accompanied by increased Akt phosphorylation (p-Akt). Similar neuroprotective in vivo effects on midbrain DA neurons were obtained in WT mice by blocking LINGO-1 activity using LINGO-1-Fc protein. Neuroprotection and enhanced neurite growth were also demonstrated for midbrain DA neurons in vitro. LINGO-1 antagonists (LINGO-1-Fc, dominant negative LINGO-1, and anti-LINGO-1 antibody) improved DA neuron survival in response to MPP+ in part by mechanisms that involve activation of the EGFR/Akt signaling pathway through a direct inhibition of LINGO-1's binding to EGFR. These results show that inhibitory agents of LINGO-1 activity can protect DA neurons against degeneration and indicate a role for the leucine-rich repeat protein LINGO-1 and related classes of proteins in the pathophysiological responses of midbrain DA neurons in PD.

Dopamine, Reward, and Attention

What is the functional significance of the fast-burst firing of midbrain dopaminergic neurons, and what are the normal afferents projecting to these cells that carry the information to which the neurons respond? Dommett et al. found that the superior colliculus is the major input source of short-latency visual responses of dopaminergic neurons. The induction of visual responses in dopaminergic neurons leads to an increase in dopamine release in the striatum. However, dopaminergic cells only responded to the novel visual stimuli when the colliculus was pharmacologically disinhibited. E. Dommett, V. Coizet, C. D. Blaha, J. Martindale, V. Lefebvre, N. Walton, J. E. W. Mayhew, P. G. Overton, P. Redgrave, How visual stimuli activate dopaminergic neurons at short latency. Science 307 , 1476-1479 (2005). [Abstract] [Full Text]

Potent Regulation of Midbrain Dopamine Neurons by the Bed Nucleus of the Stria Terminalis

Recent studies have revealed an important role of the ventrolateral (subcommissural) aspect of the bed nucleus of the stria terminalis (vBNST) in motivational aspects of drug abuse (Delfs et al., 2000). Dopaminergic (DA) neurons in the ventral tegmental area (VTA) have also long been linked to motivation and drug abuse (Koob and Le Moal, 2001). The present study tested whether activity in the vBNST influences discharge of midbrain DA neurons. Responses of DA neurons in the VTA to activation of the vBNST were characterized in anesthetized rats using extracellular recording techniques. Electrical or chemical [10-50 mm glutamate (Glu)] stimulation of the vBNST consistently activated DA cells (122% increase in activity with 50 mm Glu). However, stronger chemical stimulation of the vBNST (100 mm Glu) completely inactivated DA neurons. In addition, apomorphine restored the activity of DA neurons that were inactivated by 100 mm Glu stimulation of the vBNST, indicating possible depolarization blockade of DA cells by vBNST activity. These findings reveal that the vBNST exerts a strong excitatory influence on DA neurons. Also striking was the finding that chemical stimulation (50 mm Glu) of the vBNST yielded long-lasting oscillatory activity (>15 min) in VTA DA neurons. These results indicate that the vBNST can generate long-lasting alterations in the activity of DA neurons in vivo.

Subthalamic stimulation-induced synaptic responses in substantia nigra pars compacta dopaminergic neurons in vitro.

The subthalamic nucleus (STN) is one of the principal sources of excitatory glutamatergic input to dopaminergic neurons of the substantia nigra, yet stimulation of the STN produces both excitatory and inhibitory effects on nigral dopaminergic neurons recorded extracellularly in vivo. The present experiments were designed to determine the sources of the excitatory and inhibitory effects. Synaptic potentials were recorded intracellularly from substantia nigra pars compacta dopaminergic neurons in parasagittal slices in response to stimulation of the STN. Synaptic potentials were analyzed for onset latency, amplitude, duration, and reversal potential in the presence and absence of GABA and glutamate receptor antagonists. STN-evoked depolarizing synaptic responses in dopaminergic neurons reversed at approximately -31 mV, intermediate between the expected reversal potential for an excitatory and an inhibitory postsynaptic potential (EPSP and IPSP). Blockade of GABA(A) receptors with bicuculline caused a positive shift in the reversal potential to near 0 mV, suggesting that STN stimulation evoked a near simultaneous EPSP and IPSP. Both synaptic responses were blocked by application of the glutamate receptor antagonist, 6-cyano-7-nitroquinoxalene-2,3-dione. The confounding influence of inhibitory fibers of passage from globus pallidus and/or striatum by STN stimulation was eliminated by unilaterally transecting striatonigral and pallidonigral fibers 3 days before recording. The reversal potential of STN-evoked synaptic responses in dopaminergic neurons in slices from transected animals was approximately -30 mV. Bath application of bicuculline shifted the reversal potential to approximately 5 mV as it did in intact animals, suggesting that the source of the IPSP was within substantia nigra. These data indicate that electrical stimulation of the STN elicits a mixed EPSP-IPSP in nigral dopaminergic neurons due to the coactivation of an excitatory monosynaptic and an inhibitory polysynaptic connection between the STN and the dopaminergic neurons of substantia nigra pars compacta. The EPSP arises from a direct monosynaptic excitatory glutamatergic input from the STN. The IPSP arises polysynaptically, most likely through STN-evoked excitation of GABAergic neurons in substantia nigra pars reticulata, which produces feed-forward GABA(A)-mediated inhibition of dopaminergic neurons through inhibitory intranigral axon collaterals.

Alternative sulfonylurea receptor expression defines metabolic sensitivity of K-ATP channels in dopaminergic midbrain neurons.

ATP-sensitive potassium (K-ATP) channels couple the metabolic state to cellular excitability in various tissues. Several isoforms of the K-ATP channel subunits, the sulfonylurea receptor (SUR) and inwardly rectifying K channel (Kir6.X), have been cloned, but the molecular composition and functional diversity of native neuronal K-ATP channels remain unresolved. We combined functional analysis of K-ATP channels with expression profiling of K-ATP subunits at the level of single substantia nigra (SN) neurons in mouse brain slices using an RT-multiplex PCR protocol. In contrast to GABAergic neurons, single dopaminergic SN neurons displayed alternative co-expression of either SUR1, SUR2B or both SUR isoforms with Kir6.2. Dopaminergic SN neurons expressed alternative K-ATP channel species distinguished by significant differences in sulfonylurea affinity and metabolic sensitivity. In single dopaminergic SN neurons, co-expression of SUR1 + Kir6.2, but not of SUR2B + Kir6.2, correlated with functional K-ATP channels highly sensitive to metabolic inhibition. In contrast to wild-type, surviving dopaminergic SN neurons of homozygous weaver mouse exclusively expressed SUR1 + Kir6.2 during the active period of dopaminergic neurodegeneration. Therefore, alternative expression of K-ATP channel subunits defines the differential response to metabolic stress and constitutes a novel candidate mechanism for the differential vulnerability of dopaminergic neurons in response to respiratory chain dysfunction in Parkinson's disease.

A predictive reinforcement model of dopamine neurons for learning approach behavior.

A neural network model of how dopamine and prefrontal cortex activity guides short- and long-term information processing within the cortico-striatal circuits during reward-related learning of approach behavior is proposed. The model predicts two types of reward-related neuronal responses generated during learning: (1) cell activity signaling errors in the prediction of the expected time of reward delivery and (2) neural activations coding for errors in the prediction of the amount and type of reward or stimulus expectancies. The former type of signal is consistent with the responses of dopaminergic neurons, while the latter signal is consistent with reward expectancy responses reported in the prefrontal cortex. It is shown that a neural network architecture that satisfies the design principles of the adaptive resonance theory of Carpenter and Grossberg (1987) can account for the dopamine responses to novelty, generalization, and discrimination of appetitive and aversive stimuli. These hypotheses are scrutinized via simulations of the model in relation to the delivery of free food outside a task, the timed contingent delivery of appetitive and aversive stimuli, and an asymmetric, instructed delay response task.

Striatal dopaminergic changes depend on the attractive or aversive value of stimulus.

Using a conditioned aversion paradigm we reported previously that dopaminergic (DA) responses were opposite in the core part of the nucleus accumbens depending on the affective value of the stimulus. The opposite responses were more marked in the left hemisphere. Anatomo-functional similarities between the core part of the nucleus accumbens and the dorsal striatum have been described. In the present study we tested the hypothesis that DA responses in the left ventromedial part of dorsal striatum (VMS) depend on the affective value of the stimulus. Responses of DA neurons were studied using in vivo voltammetry coupled with computer-assisted numerical analysis. A rapid and marked DA increase and a delayed DA decrease were observed in rats presented to the appetitive and the aversive olfactory stimulus, respectively.

Cellular Quantification of Tyrosine Hydroxylase in the Rat Brain by Immunoautoradiography

We developed a rapid and sensitive radioimmunohistochemical method for the quantification of tyrosine hydroxylase (TH) at both the anatomical and cellular level. Coronal tissue sections from fresh-frozen rat brains were incubated in the presence of a TH monoclonal antibody. The reaction was revealed with a 35S-labeled secondary antibody. TH content was quantified in catecholaminergic brain areas by measuring optical density on autoradiographic films or silver grain density on autoradiographic emulsion-coated sections. Regional TH concentrations determined in the locus ceruleus (LC), substantia nigra pars compacta (SNC), and ventral tegmental area (VTA) were significantly increased by 45% after reserpine treatment in the LC but unchanged in the SNC and VTA. Microscopic examination of TH radioimmunolabeling showed a heavy accumulation of silver grains over catecholaminergic cell bodies. In the LC, grain density per cell was heterogeneous and higher in the ventral than in the dorsal part of the structure. After reserpine treatment, TH levels were significantly increased (57%) in the neurons of the LC but not in those of the SNC or VTA. The data support the validity of this radioimmunohistochemical method as a tool for quantifying TH protein at the cellular level and they confirm that TH protein content is differentially regulated in noradrenergic and dopaminergic neurons in response to reserpine.