Midbrain Dopamine System Research Articles

The lateral hypothalamus to lateral habenula projection, but not the ventral pallidum to lateral habenula projection, regulates voluntary ethanol consumption

The lateral habenula (LHb) is an epithalamic brain region implicated in aversive processing via negative modulation of midbrain dopamine (DA) and serotonin (5-HT) systems. Given the role of the LHb in inhibiting DA and 5-HT systems, it is thought to be involved in various psychiatric pathologies, including drug addiction. In support, it has been shown that LHb plays a critical role in cocaine- and ethanol-related behaviors, most likely by mediating drug-induced aversive conditioning. In our previous work, we showed that LHb lesions increased voluntary ethanol consumption and operant ethanol self-administration and blocked yohimbine-induced reinstatement of ethanol self-administration. LHb lesions also attenuated ethanol-induced conditioned taste aversion suggesting that a mechanism for the increased intake of ethanol may be reduced aversion learning. However, whether afferents to the LHb are required for mediating effects of the LHb on these behaviors remained to be investigated. Our present results show that lesioning the fiber bundle carrying afferent inputs to the LHb, the stria medullaris (SM), increases voluntary ethanol consumption, suggesting that afferent structures projecting to the LHb are important for mediating ethanol-directed behaviors. We then chose two afferent structures as the focus of our investigation. We specifically studied the role of the inputs from the lateral hypothalamus (LH) and ventral pallidum (VP) to the LHb in ethanol-directed behaviors. Our results show that the LH-LHb projection is necessary for regulating voluntary ethanol consumption. These results are an important first step towards understanding the functional role of afferents to LHb with regard to ethanol consumption.

Effects of city living on the mesolimbic reward system-An fmri study.

Based on higher prevalence rates of several mental disorders for city dwellers, psychosocial stress effects of urban living have been proposed as an environmental risk factor contributing to the development of mental disorders. Recently, it was shown that amygdala activation differs between city dwellers and rural residents in response to a cognitive-social stressor. Besides its influence on the amygdala, chronic stress also affects mesocorticolimbic brain regions involved in reward processing, and stress-related dysregulation of the mesocorticolimbic dopamine system is thought to contribute to onset and manifestation of psychiatric disorders. Here, we investigated differences in reward systems functioning in 147 healthy subjects living either in cities or in less urban areas by means of functional magnetic resonance imaging during performance of the desire-reason-dilemma paradigm, which permits a targeted investigation of bottom-up activation and top-down regulation of the reward circuit. Compared with subjects from less urban areas, city dwellers showed an altered activation and modulation capability of the midbrain (VTA) dopamine system. City dwellers also revealed increased responses in other brain regions involved in reward processing and in the regulation of stress and emotions, such as amygdala, orbitofrontal, and pregenual anterior cingulate cortex. These results provide further evidence for effects of an urban environment on the mesolimbic dopamine system and the limbic system which may increase the risk to develop mental disorders. Hum Brain Mapp 38:3444-3453, 2017. © 2017 Wiley Periodicals, Inc.

It's all about timing: An electrophysiological examination of feedback-based learning with immediate and delayed feedback

Feedback regarding an individual's action can occur immediately or with a temporal delay. Processing of feedback that varies in its delivery time is proposed to engage different brain mechanisms. fMRI data implicate the striatum in the processing of immediate feedback, and the medial temporal lobe (MTL) in the processing of delayed feedback. The present study offers an electrophysiological examination of feedback processing in the context of timing, by studying the effects of feedback timing on the feedback-related negativity (FRN), a product of the midbrain dopamine system, and elucidating whether the N170 ERP component could capture MTL activation associated with the processing of delayed feedback. Participants completed a word-object paired association learning task; they received feedback 500ms (immediate feedback condition) following a button press during the learning of two sets of 14 items, and at a delay of 6500ms (delayed feedback condition) during the learning of the other two sets. The results indicated that while learning outcomes did not differ under the two timing conditions, Event Related Potential (ERPs) pointed to differential activation of the examined ERP components. FRN amplitude was found to be larger following the immediate feedback condition when compared with the delayed feedback condition, and sensitive to valence and learning only under the immediate feedback condition. Additionally, the amplitude of the N170 was found larger following the delayed feedback condition when compared with the immediate feedback condition. Taken together, the findings of the present study support the contention that the processing of delayed feedback involves a shift away from midbrain dopamine activation to the recruitment of the MTL.

Reward-based contextual learning supported by anterior cingulate cortex.

The anterior cingulate cortex (ACC) is commonly associated with cognitive control and decision making, but its specific function is highly debated. To explore a recent theory that the ACC learns the reward values of task contexts (Holroyd & McClure in Psychological Review, 122, 54-83, 2015; Holroyd & Yeung in Trends in Cognitive Sciences, 16, 122-128, 2012), we recorded the event-related brain potentials (ERPs) from participants as they played a novel gambling task. The participants were first required to select from among three games in one "virtual casino," and subsequently they were required to select from among three different games in a different virtual casino; unbeknownst to them, the payoffs for the games were higher in one casino than in the other. Analysis of the reward positivity, an ERP component believed to reflect reward-related signals carried to the ACC by the midbrain dopamine system, revealed that the ACC is sensitive to differences in the reward values associated with both the casinos and the games inside the casinos, indicating that participants learned the values of the contexts in which rewards were delivered. These results highlight the importance of the ACC in learning the reward values of task contexts in order to guide action selection.

Beyond the Classic VTA: Extended Amygdala Projections to DA-Striatal Paths in the Primate

The central extended amygdala (CEA) has been conceptualized as a ‘macrosystem’ that regulates various stress-induced behaviors. Consistent with this, the CEA highly expresses corticotropin-releasing factor (CRF), an important modulator of stress responses. Stress alters goal-directed responses associated with striatal paths, including maladaptive responses such as drug seeking, social withdrawal, and compulsive behavior. CEA inputs to the midbrain dopamine (DA) system are positioned to influence striatal functions through mesolimbic DA-striatal pathways. However, the structure of this amygdala-CEA-DA neuron path to the striatum has been poorly characterized in primates. In primates, we combined neuronal tracer injections into various arms of the circuit through specific DA subpopulations to assess: (1) whether the circuit connecting amygdala, CEA, and DA cells follows CEA intrinsic organization, or a more direct topography involving bed nucleus vs central nucleus divisions; (2) CRF content of the CEA-DA path; and (3) striatal subregions specifically involved in CEA-DA-striatal loops. We found that the amygdala-CEA-DA path follows macrostructural subdivisions, with the majority of input/outputs converging in the medial central nucleus, the sublenticular extended amygdala, and the posterior lateral bed nucleus of the stria terminalis. The proportion of CRF+ outputs is >50%, and mainly targets the A10 parabrachial pigmented nucleus (PBP) and A8 (retrorubal field, RRF) neuronal subpopulations, with additional inputs to the dorsal A9 neurons. CRF-enriched CEA-DA projections are positioned to influence outputs to the ‘limbic-associative’ striatum, which is distinct from striatal regions targeted by DA cells lacking CEA input. We conclude that the concept of the CEA is supported on connectional grounds, and that CEA termination over the PBP and RRF neuronal populations can influence striatal circuits involved in associative learning.

Genetic inhibition of neurotransmission reveals role of glutamatergic input to dopamine neurons in high-effort behavior

Midbrain dopamine neurons are crucial for many behavioral and cognitive functions. As the major excitatory input, glutamatergic afferents are important for control of the activity and plasticity of dopamine neurons. However, the role of glutamatergic input as a whole onto dopamine neurons remains unclear. Here we developed a mouse line in which glutamatergic inputs onto dopamine neurons are specifically impaired, and utilized this genetic model to directly test the role of glutamatergic inputs in dopamine-related functions. We found that while motor coordination and reward learning were largely unchanged, these animals showed prominent deficits in effort-related behavioral tasks. These results provide genetic evidence that glutamatergic transmission onto dopaminergic neurons underlies incentive motivation, a willingness to exert high levels of effort to obtain reinforcers, and have important implications for understanding the normal function of the midbrain dopamine system.

CD24 expression does not affect dopamine neuronal survival in a mouse model of Parkinson's disease.

Parkinson’s disease (PD) is a progressive neurodegenerative condition that is characterised by the loss of specific populations of neurons in the brain. The mechanisms underlying this selective cell death are unknown but by using laser capture microdissection, the glycoprotein, CD24 has been identified as a potential marker of the populations of cells that are affected in PD. Using in situ hybridization and immunohistochemistry on sections of mouse brain, we confirmed that CD24 is robustly expressed by many of these subsets of cells. To determine if CD24 may have a functional role in PD, we modelled the dopamine cell loss of PD in Cd24 mutant mice using striatal delivery of the neurotoxin 6-OHDA. We found that Cd24 mutant mice have an anatomically normal dopamine system and that this glycoprotein does not modulate the lesion effects of 6-OHDA delivered into the striatum. We then undertook in situ hybridization studies on sections of human brain and found—as in the mouse brain—that CD24 is expressed by many of the subsets of the cells that are vulnerable in PD, but not those of the midbrain dopamine system. Finally, we sought to determine if CD24 is required for the neuroprotective effect of Glial cell-derived neurotrophic factor (GDNF) on the dopaminergic nigrostriatal pathway. Our results indicate that in the absence of CD24, there is a reduction in the protective effects of GDNF on the dopaminergic fibres in the striatum, but no difference in the survival of the cell bodies in the midbrain. While we found no obvious role for CD24 in the normal development and maintenance of the dopaminergic nigrostriatal system in mice, it may have a role in mediating the neuroprotective aspects of GDNF in this system.

PDE Inhibitors for the Treatment of Schizophrenia.

Schizophrenia is a pervasive neuropsychiatric disorder affecting over 1% of the world's population. Dopamine system dysfunction is strongly implicated in the etiology of schizophrenia. Data support the long-standing concept of schizophrenia as a disease characterized by hyperactivity within midbrain (striatal D2) dopamine systems. In addition, there is now considerable evidence that glutamate neurotransmission, mediated through NMDA-type receptors, is deficient in patients with schizophrenia and that hypoactivity in cortical dopamine and glutamate pathways is a key feature of this serious mental disorder. While current antipsychotic medications-with a common mechanism involving dopamine D2 receptor antagonism or pre-synaptic partial agonism-adequately address positive symptoms of the disease, such as the acute hallucinations and delusions, they fail to substantially improve negative features, such as social isolation, and can further compromise poor cognitive function associated with schizophrenia. In fact, cognitive impairment is a core feature of schizophrenia. The treatment of cognitive impairment and other residual symptoms associated with schizophrenia, therefore, remains a significant unmet medical need. With current cell-surface receptor-based pharmacology falling short of addressing these core cognitive symptoms, more recent approaches to treatment development have focused on processes within the cell. In this review, we discuss the importance of cyclic nucleotide (cNT) phosphodiestereases (PDEs)-intracellular enzymes that control the activity of key second messenger signaling pathways in the brain-which have been proposed as targets for new schizophrenia therapies. We also discuss the challenge facing those developing drugs to target specific PDE enzymes involved in psychopathology without involving other systems that produce concomitant side effects.

Cross-hemispheric dopamine projections have functional significance

Dopamine signaling occurs on a subsecond timescale, and its dysregulation is implicated in pathologies ranging from drug addiction to Parkinson's disease. Anatomic evidence suggests that some dopamine neurons have cross-hemispheric projections, but the significance of these projections is unknown. Here we report unprecedented interhemispheric communication in the midbrain dopamine system of awake and anesthetized rats. In the anesthetized rats, optogenetic and electrical stimulation of dopamine cells elicited physiologically relevant dopamine release in the contralateral striatum. Contralateral release differed between the dorsal and ventral striatum owing to differential regulation by D2-like receptors. In the freely moving animals, simultaneous bilateral measurements revealed that dopamine release synchronizes between hemispheres and intact, contralateral projections can release dopamine in the midbrain of 6-hydroxydopamine-lesioned rats. These experiments are the first, to our knowledge, to show cross-hemispheric synchronicity in dopamine signaling and support a functional role for contralateral projections. In addition, our data reveal that psychostimulants, such as amphetamine, promote the coupling of dopamine transients between hemispheres.

Combined immunohistochemical and retrograde tracing reveals little evidence of innervation of the rat dentate gyrus by midbrain dopamine neurons

Although the functional neuroanatomy of the midbrain dopamine (mDA) system has been well characterized, the literature regarding its capacity to innervate the hippocampal formation has been inconsistent. The lack of expression of definitive markers for dopaminergic fibers, such as the dopamine transporter, in the hippocampus has complicated studies in this area. Here we have used immunohistochemical techniques to characterize the tyrosine hydroxylase expressing fiber network in the rat hippocampus, combined with retrograde tracing from the dentate gyrus to assess the capacity for afferent innervation by mDA neurons. The results indicate that virtually all tyrosine hydroxylase fibers throughout the hippocampus are of a noradrenergic phenotype, while the overlying cortex contains both dopaminergic and noradrenergic fiber networks. Furthermore, retrograde tracing from the dentate gyrus robustly labels tyrosine hydroxylase-immunoreactive noradrenergic neurons in the locus coeruleus but not mDA neurons.

Reward Sensitivity of ACC as an Intermediate Phenotype between DRD4-521T and Substance Misuse.

The development and expression of the midbrain dopamine system is determined in part by genetic factors that vary across individuals such that dopamine-related genes are partly responsible for addiction vulnerability. However, a complete account of how dopamine-related genes predispose individuals to drug addiction remains to be developed. Adopting an intermediate phenotype approach, we investigated whether reward-related electrophysiological activity of ACC-a cortical region said to utilize dopamine reward signals to learn the value of extended, context-specific sequences of goal-directed behaviors-mediates the influence of multiple dopamine-related functional polymorphisms over substance use. We used structural equation modeling to examine whether two related electrophysiological phenomena associated with the control and reinforcement learning functions of ACC-theta power and the reward positivity-mediated the relationship between the degree of substance misuse and genetic polymorphisms that regulate dopamine processing in frontal cortex. Substance use data were collected from 812 undergraduate students. One hundred ninety-six returned on a subsequent day to participate in an electrophysiological experiment and to provide saliva samples for DNA analysis. We found that these electrophysiological signals mediated a relationship between the DRD4-521T dopamine receptor genotype and substance misuse. Our results provide a theoretical framework that bridges the gap between genes and behavior in drug addiction and illustrate how future interventions might be individually tailored for specific genetic and neurocognitive profiles.

ID 373 – The Feedback related negativity in ADHD: A meta-analytic review

To review current findings of the feedback related negativity component (FRN) in the ADHD population. A systematic search in PubMed, PsycINFO, and Science Direct databases (up to December 15, 2014) identified 33 published articles of EEG studies with ADHD patients that were tested within a reward paradigm, that reported FRN component on a FCZ location, 250 ms following feedback onset. Six studies measured the FRN within a reward context met all inclusion criteria, while five studies measured the FRN within loss context. ADHD population showed altered FRN in a reward context relative to typical control groups while FRN in a loss context did not differ between groups. This neuronal alteration showed enlarged amplitudes in the FRN waveform for the ADHD groups in a reward context. Results are consistent with previous work that reported a greater sensitivity to gain than loss context (e.g. Sambrook et al. 2012) Clearly, this meta-analysis brings evidence that this effect is also observable in clinical population. Moreover, our results are compatible with previous ADHD models that suggest abnormalities in the midbrain dopamine system (e.g. Dopamine Transfer Deficit Model). Feedback Related Negativity (FRN), Reward system, Attention Deficit Hyperactivity Disorder (ADHD).

Loss of VGLUT3 Produces Circadian-Dependent Hyperdopaminergia and Ameliorates Motor Dysfunction and l-Dopa-Mediated Dyskinesias in a Model of Parkinson's Disease

The striatum is essential for many aspects of mammalian behavior, including motivation and movement, and is dysfunctional in motor disorders such as Parkinson's disease. The vesicular glutamate transporter 3 (VGLUT3) is expressed by striatal cholinergic interneurons (CINs) and is thus well positioned to regulate dopamine (DA) signaling and locomotor activity, a canonical measure of basal ganglia output. We now report that VGLUT3 knock-out (KO) mice show circadian-dependent hyperlocomotor activity that is restricted to the waking cycle and is due to an increase in striatal DA synthesis, packaging, and release. Using a conditional VGLUT3 KO mouse, we show that deletion of the transporter from CINs, surprisingly, does not alter evoked DA release in the dorsal striatum or baseline locomotor activity. The mice do, however, display changes in rearing behavior and sensorimotor gating. Elevation of DA release in the global KO raised the possibility that motor deficits in a Parkinson's disease model would be reduced. Remarkably, after a partial 6-hydroxydopamine (6-OHDA)-mediated DA depletion (∼70% in dorsal striatum), KO mice, in contrast to WT mice, showed normal motor behavior across the entire circadian cycle. l-3,4-dihydroxyphenylalanine-mediated dyskinesias were also significantly attenuated. These findings thus point to new mechanisms to regulate basal ganglia function and potentially treat Parkinson's disease and related disorders. Dopaminergic signaling is critical for both motor and cognitive functions in the mammalian nervous system. Impairments, such as those found in Parkinson's disease patients, can lead to severe motor deficits. Vesicular glutamate transporter 3 (VGLUT3) loads glutamate into secretory vesicles for neurotransmission and is expressed by discrete neuron populations throughout the nervous system. Here, we report that the absence of VGLUT3 in mice leads to an upregulation of the midbrain dopamine system. Remarkably, in a Parkinson's disease model, the mice show normal motor behavior. They also show fewer abnormal motor behaviors (dyskinesias) in response to l-3,4-dihydroxyphenylalanine, the principal treatment for Parkinson's disease. The work thus suggests new avenues for the development of novel treatment strategies for Parkinson's disease and potentially other basal-ganglia-related disorders.

Development and function of the midbrain dopamine system: what we know and what we need to.

The past two decades have seen an explosion in our understanding of the origin and development of the midbrain dopamine system. Much of this work has been focused on the aspects of dopamine neuron development related to the onset of movement disorders such as Parkinson's disease, with the intent of hopefully delaying, preventing or fixing symptoms. While midbrain dopamine degeneration is a major focus for treatment and research, many other human disorders are impacted by abnormal dopamine, including drug addiction, autism and schizophrenia. Understanding dopamine neuron ontogeny and how dopamine connections and circuitry develops may provide us with key insights into potentially important avenues of research for other dopamine-related disorders. This review will provide a brief overview of the major molecular and genetic players throughout the development of midbrain dopamine neurons and what we know about the behavioral- and disease-related implications associated with perturbations to midbrain dopamine neuron development. We intend to combine the knowledge of two broad fields of neuroscience, both developmental and behavioral, with the intent on fostering greater discussion between branches of neuroscience in the service of addressing complex cognitive questions from a developmental perspective and identifying important gaps in our knowledge for future study.

Heterogeneity of the midbrain dopamine system: Implications for Parkinson disease.

The midbrain dopaminergic (DA) system is composed primarily of neurons in the substantia nigra pars compacta (SNc, area A10) and ventral tegmental area (VTA, area A9). These include at least 5 neuron subpopulations with distinct patterns of gene expression, electrophysiologic properties, projections to the striatum and cerebral cortex, and response to environmental stimuli. They are involved in several key brain functions, including action selection, motor performance, motivation and reward-based learning, working memory, and cognition. Midbrain DA neurons are activated by alerting signals involved in rapid detection of potentially relevant sensory cues. Most increase their activity in response to reward stimuli and are inhibited by aversive stimuli, thereby coding motivational value for learning reward-oriented behavior. Others are activated by aversive stimuli and code motivational salience of the stimulus. These different types of midbrain DA neurons are anatomically segregated and have specific afferent and efferent connections supporting specific aspects of behavior. Neurons encoding motivational value are located in the VTA and medial SNc and project primarily to the nucleus accumbens (NAc) and ventromedial prefrontal cortex and support Pavlovian learning of actions leading to successful outcomes. Neurons encoding motivational salience are located in the dorsolateral SNc and project to the dorsomedial striatum (primarily the caudate) and dorsolateral prefrontal cortex and modulate goal-oriented behavior. Neurons in the ventral tier of the SNc project to the dorsolateral striatum (putamen) and regulate habit formation and automatic motor behavior. Several features of ventral tier SNc neurons render them susceptible to oxidative stress and the subsequent neurodegeneration underlying Parkinson disease (PD). Involvement of other SNc and VTA neurons may contribute to nonmotor aspects of PD, such as depression, cognitive dysfunction, and impulse dyscontrol, in response to DA agonists. All these topics have been extensively discussed in several reviews.1–18

Expanding the role of striatal cholinergic interneurons and the midbrain dopamine system in appetitive instrumental conditioning.

The basal ganglia are a collection of subcortical nuclei thought to underlie a wide variety of vertebrate behavior. Although a great deal is known about the functional and physiological properties of the basal ganglia, relatively few models have been formally developed that have been tested against both behavioral and physiological data. Our previous work (Ashby FG, Crossley MJ. J Cogn Neurosci 23: 1549-1566, 2011) showed that a model grounded in the neurobiology of the basal ganglia could account for basic single-neuron recording data, as well as behavioral phenomena such as fast reacquisition that constrain models of conditioning. In this article we show that this same model accounts for a variety of appetitive instrumental conditioning phenomena, including the partial reinforcement extinction (PRE) effect, rapid and slowed reacquisition following extinction, and renewal of previously extinguished instrumental responses by environmental context cues.

Prefrontal Regulation of Neuronal Activity in the Ventral Tegmental Area.

Growing evidence indicates that midbrain dopamine (DA) cells integrate reward expectancy-related information from the prefrontal cortex to properly compute errors in reward prediction. Here we investigated how 2 major prefrontal subregions, the orbitofrontal cortex (OFC) and the medial prefrontal cortex (mPFC), contributed to DAergic prediction errors while rats performed a delay discounting task on a T-maze. Most putative DA cells in the task showed phasic responses to salient cues that predicted delayed rewards, but not to the actual rewards. After temporary inactivation of the OFC, putative DA cells exhibited strikingly reduced phasic responses to reward-predicting cues but increased responses to rewards. In contrast, mPFC inactivation significantly elevated DA responses to both predictive cues and rewards. In addition, OFC, but not mPFC, inactivation disrupted the activity of putative non-DA cells that encoded expected reward values during waiting periods. These results suggest that the 2 prefrontal subregions differentially regulate DAergic prediction errors and the OFC conveys value signals to midbrain dopamine systems.

Serotonergic interaction between medial prefrontal cortex and mesotelencephalic DA system underlies cognitive and affective deficits in hemiparkinsonian rats

Parkinson’s disease (PD) patients not only exhibit motor impairments, but also characteristic deficits in cognitive and affective functions. Such functions have consistently been associated with the medial prefrontal cortex (mPFC). To determine whether there is an interaction between the midbrain dopamine system (MDS) and the mPFC underlying the cognitive and emotional deficits seen in rats, we administered a disconnection procedure of these structures by applying lesions to the mPFC (N-methyl-d-aspartic acid (NMDA)) and the medial forebrain bundle (6-hydroxydopamine (6-OHDA)) either in the same or opposite hemispheres. The results indicate a functional interaction of the MDS and the mPFC: Disconnection effects on behavior were observed with respect to memory-, anxiety- and depression-related behaviors. A disconnection of the mPFC and MDS had promnestic, antidepressant- and anxiolytic-like effects. In order to determine whether this circuit between the mPFC and MDS involves serotonergic mechanisms, we also utilized serotonin-specific disconnections of the mPFC by applying the 5-HT-specific agent 5,7-dihydroxytryptamine (5,7-DHT) into the mPFC and 6-OHDA into the medial forebrain bundle, again either in the same or opposite hemispheres. The behavioral effects observed here resembled those incurred by the unspecific disconnection of the mPFC, demonstrating a significant contribution of serotonergic mechanisms to the interplay between the MDS and the mPFC. Taken together, these experiments provide evidence for an interaction of the MDS and the mPFC in the control of cognitive and affective processes known to be impaired in PD and point toward a prominent involvement of the serotonergic system. A disconnection of the mPFC and the MDS had promnestic, antidepressant- and anxiolytic-like behavioral effects. These findings may impact therapeutic approaches in the treatment of cognitive and neuropsychiatric symptoms seen in PD.

Impact of prenatal nicotine on the structure of midbrain dopamine regions in the rat.

In utero exposure of rats to nicotine (NIC) provides a useful animal model for studying the impact of smoking during pregnancy on human offspring. Certain sequelae of prenatal NIC exposure suggest an impact on the development of the midbrain dopamine (DA) system, which receives a robust cholinergic innervation from the mesopontine tegmentum. We therefore investigated whether prenatal NIC induced structural changes in cells and synapses within the midbrain that persisted into adulthood. Osmotic minipumps delivering either sodium bitartrate (vehicle; VEH) or NIC bitartrate at 2mg/kg/day were implanted into nine timed-pregnant dams at E4. At birth, rat pups were culled to litters of six males each, and the litters were cross-fostered. Plasma levels of NIC and cotinine from killed pups provided evidence of NIC exposure in utero. Pups separated from dams at weaning showed a trend toward reduced locomotor activity at this time point but not when tested again in adulthood. Adult rats were killed for anatomical studies. Estimates of brain size and volume did not vary with NIC treatment. Midbrain sections stained for Nissl or by immunoperoxidase for tyrosine hydroxylase and analyzed using unbiased stereology revealed no changes in volume or cell number in the substantia nigra compacta or ventral tegmental area as a result of NIC exposure. Within the ventral tegmental area, electron microscopic physical disector analysis showed no significant differences in the number of axon terminals or the number of asymmetric (putative excitatory) or symmetric (putative inhibitory) synapses. Although too infrequent to estimate by unbiased stereology, no obvious difference in the proportion of cholinergic axons was noted in NIC- versus VEH-treated animals. These data suggest that activation of nicotinic receptors during prenatal development induces no significant modifications in the structure of cells in the ventral midbrain when assessed in adulthood.

Selective lesions of the cholinergic neurons within the posterior pedunculopontine do not alter operant learning or nicotine sensitization.

Cholinergic neurons within the pedunculopontine tegmental nucleus have been implicated in a range of functions, including behavioral state control, attention, and modulation of midbrain and basal ganglia systems. Previous experiments with excitotoxic lesions have found persistent learning impairment and altered response to nicotine following lesion of the posterior component of the PPTg (pPPTg). These effects have been attributed to disrupted input to midbrain dopamine systems, particularly the ventral tegmental area. The pPPTg contains a dense collection of cholinergic neurons and also large numbers of glutamatergic and GABAergic neurons. Because these interdigitated populations of neurons are all susceptible to excitotoxins, the effects of such lesions cannot be attributed to one neuronal population. We wished to assess whether the learning impairments and altered responses to nicotine in excitotoxic PPTg-lesioned rats were due to loss of cholinergic neurons within the pPPTg. Selective depletion of cholinergic pPPTg neurons is achievable with the fusion toxin Dtx-UII, which targets UII receptors expressed only by cholinergic neurons in this region. Rats bearing bilateral lesions of cholinergic pPPTg neurons (>90% ChAT+ neuronal loss) displayed no deficits in the learning or performance of fixed and variable ratio schedules of reinforcement for pellet reward. Separate rats with the same lesions had a normal locomotor response to nicotine and furthermore sensitized to repeated administration of nicotine at the same rate as sham controls. Previously seen changes in these behaviors following excitotoxic pPPTg lesions cannot be attributed solely to loss of cholinergic neurons. These findings indicate that non-cholinergic neurons within the pPPTg are responsible for the learning deficits and altered responses to nicotine seen after excitotoxic lesions. The functions of cholinergic neurons may be related to behavioral state control and attention rather than learning.