The parabrachial nucleus is a critical link in the transmission of short latency nociceptive information to midbrain dopaminergic neurons

Chronic Neuropathic Pain and Comorbid Depression Syndrome: From Neural Circuit Mechanisms to Treatment

Chronic neuropathic pain and comorbid depression syndrome (CDS) is a major worldwide health problem that affects the quality of life of patients and imposes a tremendous socioeconomic burden. More than half of patients with chronic neuropathic pain also suffer from moderate or severe depression. Due to the complex pathogenesis of CDS, there are no effective therapeutic drugs available. The lack of research on the neural circuit mechanisms of CDS limits the development of treatments. The purpose of this article is to provide an overview of the various circuits involved in CDS. Notably, activating some neural circuits can alleviate pain and/or depression, while activating other circuits can exacerbate these conditions. Moreover, we discuss current and emerging pharmacotherapies for CDS, such as ketamine. Understanding the circuit mechanisms of CDS may provide clues for the development of novel drug treatments for improved CDS management.

In search of the locus coeruleus: guidelines for identifying anatomical boundaries and electrophysiological properties of the blue spot in mice, fish, finches, and beyond

Our understanding of human brain function can be greatly aided by studying analogous brain structures in other organisms. One brain structure with neurochemical and anatomical homology throughout vertebrate species is the locus coeruleus (LC), a small collection of norepinephrine (NE)-containing neurons in the brainstem that project throughout the central nervous system. The LC is involved in nearly every aspect of brain function, including arousal and learning, which has been extensively examined in rats and nonhuman primates using single-unit recordings. Recent work has expanded into putative LC single-unit electrophysiological recordings in a nonmodel species, the zebra finch. Given the importance of correctly identifying analogous structures as research efforts expand to other vertebrates, we suggest adoption of consensus anatomical and electrophysiological guidelines for identifying LC neurons across species when evaluating brainstem single-unit spiking or calcium imaging. Such consensus criteria will allow for confident cross-species understanding of the roles of the LC in brain function and behavior.

Substantia Nigra Dopamine Neuronal Responses to Habenular Stimulation and Foot Shock Are Altered by Lesions of the Rostromedial Tegmental Nucleus

Dopamine (DA) neurons of the substantia nigra (SN) and ventral tegmental area generally respond to aversive stimuli or the absence of expected rewards with transient inhibition of firing rates, which can be recapitulated with activation of the lateral habenula (LHb) and eliminated by lesioning the intermediating rostromedial tegmental nucleus (RMTg). However, a minority of DA neurons respond to aversive stimuli, such as foot shock, with a transient increase in firing rate, an outcome that rarely occurs with LHb stimulation. The degree to which individual neurons respond to these two stimulation modalities with the same response phenotype and the role of the RMTg is not known. Here, we record responses from single SN DA neurons to alternating activation of the LHb and foot shock in male rats. Lesions of the RMTg resulted in a shift away from inhibition to no response during both foot shock and LHb stimulation. Furthermore, lesions unmasked an excitatory response during LHb stimulation. The response correspondence within the same neuron between the two activation sources was no different from chance in sham controls, suggesting that external inputs rather than intrinsic DA neuronal properties are more important to response outcome. These findings contribute to a literature that shows a complex neurocircuitry underlies the regulation of DA activity and, by extension, behaviors related to learning, anhedonia, and cognition.

Separation of Channels Subserving Approach and Avoidance/Escape at the Level of the Basal Ganglia and Related Brainstem Structures

The basal ganglia have the key function of directing our behavior in the context of events from our environment and/or our internal state. This function relies on afferents targeting the main input structures of the basal ganglia, entering bids for action selection at the level of the striatum or signals for behavioral interruption at the level of the subthalamic nucleus, with behavioral reselection facilitated by dopamine signaling. Numerous experiments have studied action selection in relation to inputs from the cerebral cortex. However, less is known about the anatomical and functional link between the basal ganglia and the brainstem. In this review, we describe how brainstem structures also project to the main input structures of the basal ganglia, namely the striatum, the subthalamic nucleus and midbrain dopaminergic neurons, in the context of approach and avoidance (including escape from threat), two fundamental, mutually exclusive behavioral choices in an animal's repertoire in which the brainstem is strongly involved. We focus on three particularly well-described loci involved in approach and avoidance, namely the superior colliculus, the parabrachial nucleus and the periaqueductal grey nucleus. We consider what is known about how these structures are related to the basal ganglia, focusing on their projections toward the striatum, dopaminergic neurons and subthalamic nucleus, and explore the functional consequences of those interactions.

Negative Emotions Recruit the Parabrachial Nucleus Efferent to the VTA to Disengage Instrumental Food Seeking

The parabrachial nucleus (PBN) interfaces between taste and feeding systems and is also an important hub for relaying distress information and threats. Despite that the PBN sends projections to the ventral tegmental area (VTA), a heterogeneous brain region that regulates motivational behaviors, the function of the PBN-to-VTA connection remains elusive. Here, by using male mice in several behavioral paradigms, we discover that VTA-projecting PBN neurons are significantly engaged in contextual fear, restraint or mild stress but not palatable feeding, visceral malaise, or thermal pain. These results suggest that the PBN-to-VTA input may relay negative emotions under threat. Consistent with this notion, optogenetic activation of PBN-to-VTA glutamatergic input results in aversion, which is sufficient to override palatable feeding. Moreover, in a palatable food-reinforced operant task, we demonstrate that transient optogenetic activation of PBN-to-VTA input during food reward retrieval disengages instrumental food-seeking behaviors but spares learned action-outcome association. By using an activity-dependent targeting approach, we show that VTA DA neurons are disengaged by the PBN afferent activation, implicating that VTA non-DA neurons may mediate PBN afferent regulation. We further show that optogenetic activation of VTA neurons functionally recruited by the PBN input results in aversion, dampens palatable feeding, and disengages palatable food self-administration behavior. Finally, we demonstrate that transient activation of VTA glutamatergic, but not GABAergic, neurons recapitulates the negative regulation of the PBN input on food self-administration behavior. Together, we reveal that the PBN-to-VTA input conveys negative affect, likely through VTA glutamatergic neurons, to disengage instrumental food-seeking behaviors.SIGNIFICANCE STATEMENT The PBN receives multiple inputs and thus is well positioned to route information of various modalities to engage different downstream circuits to attend or respond accordingly. We demonstrate that the PBN-to-VTA input conveys negative affect and then triggers adaptive prioritized responses to address pertinent needs by withholding ongoing behaviors, such as palatable food seeking or intake shown in the present study. It has evolutionary significance because preparing to cope with stressful situations or threats takes priority over food seeking to promote survival. Knowing how appropriate adaptive responses are generated will provide new insights into circuitry mechanisms of various coping behaviors to changing environmental stimuli.

Converging circuits between pain and depression: the ventral tegmental area as a therapeutic hub

Chronic pain and depression are highly prevalent pathologies and cause a major socioeconomic burden to society. Chronic pain affects the emotional state of the individuals suffering from it, while depression worsens the prognosis of chronic pain patients and may diminish the effectiveness of pain treatments. There is a high comorbidity rate between both pathologies, which might share overlapping mechanisms. This review explores the evidence pinpointing a role for the ventral tegmental area (VTA) as a hub where both pain and emotional processing might converge. In addition, the feasibility of using the VTA as a possible therapeutic target is discussed. The role of the VTA, and the dopaminergic system in general, is highly studied in mood disorders, especially in deficits in reward-processing and motivation. Conversely, the VTA is less regarded where it concerns the study of central mechanisms of pain and its mood-associated consequences. Here, we first outline the brain circuits involving central processing of pain and mood disorders, focusing on the often-understudied role of the dopaminergic system and the VTA. Next, we highlight the state-of-the-art findings supporting the emergence of the VTA as a link where both pathways converge. Thus, we envision a promising part for the VTA as a putative target for innovative therapeutic approaches to treat chronic pain and its effects on mood. Finally, we emphasize the urge to develop and use animal models where both pain and depression-like symptoms are considered in conjunction.

Altered parabrachial nucleus nociceptive processing may underlie central pain in Parkinson's disease

The presence of central neuropathic pain in Parkinson's disease suggests that the brain circuits that allow us to process pain could be dysfunctional in the disorder. However, there is to date no clear pathophysiological mechanism to explain these symptoms. In this work, we present evidence that the dysfunction of the subthalamic nucleus and/or substantia nigra pars reticulata may impact nociceptive processing in the parabrachial nucleus (PBN), a low level primary nociceptive structure in the brainstem, and induce a cellular and molecular neuro-adaptation in this structure. In rat models of Parkinson's disease with a partial dopaminergic lesion in the substantia nigra compacta, we found that the substantia nigra reticulata showed enhanced nociceptive responses. Such responses were less impacted in the subthalamic nucleus. A total dopaminergic lesion produced an increase in the nociceptive responses as well as an increase of the firing rate in both structures. In the PBN, inhibited nociceptive responses and increased expression of GABA_A receptors were found following a total dopaminergic lesion. However, neuro-adaptations at the level of dendritic spine density and post-synaptic density were found in both dopaminergic lesion groups. These results suggest that the molecular changes within the PBN following a larger dopaminergic lesion, such as increased GABA_A expression, is a key mechanism to produce nociceptive processing impairment, whilst other changes may protect function after smaller dopaminergic lesions. We also propose that these neuro-adaptations follow increased inhibitory tone from the substantia nigra pars reticulata and may represent the mechanism generating central neuropathic pain in Parkinson's disease.

The Endogenous Cannabinoid and the Nitricoxidergic Systems in the Modulation of Stress Responses

The effects on stress-induced analgesia (SIA) from endogenous cannabinoid system (ECS) and nitric oxide (NO) interaction after 1 h of restraint stress were evaluated in male Wistar rats. The animals were subjected to 1 h of restraint and then injected with different combinations of cannabinoid receptor type 1 agonist anandamide (AEA) or antagonist AM251 along with an NO donor, NO precursor, or inhibitor of NO synthase. Nociception was evaluated using paw pressure (PP) or hot plate (HP) tests. AEA was administered immediately after the end of restraint-SIA (r-SIA). Administration of NO precursor reversed the pronociceptive effect of the CB1 agonist on r-SIA. Both the CB1 antagonist and the NOS inhibitor neutralized the pro-analgesic effect of L-arginine (L-arg). Administration of an NO donor, instead, increased r-SIA. Our experiments confirmed that the endogenous cannabinoid and the NO-ergic systems interact in the modulation of r-SIA. This interaction probably implies NO as a second messenger of the ECS.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

The Spino-Parabrachial Pathway for Itch

Itch-induced scratching is an evolutionarily conserved behavioral response that protects organisms from potential parasites/irritants in their immediate vicinity. How the exposure to a pruritogen is translated to the perception of itch and how that perception drives scratching directed towards the site of exposure remains poorly understood. In this review, we focus on the recent findings that shed light on the neural pathways in the brain that underlie itch-induced scratching. We compare the molecularly defined itch pathways with the known pain circuits as they have anatomical and functional overlap. We review the roles played by the neurons in the spinoparabrachial pathway-comprising of the neurons in the spinal cord and the parabrachial nucleus (PBN), which acts as a hub for transmitting itch information across the brain. Lastly, we deliberate on scratching as a behavioral measure of the intensity of itch and its implication in unraveling the underlying supraspinal mechanisms. In summary, we provide a resource on the recent advances and discuss a path forward on our understanding of the neural circuits for itch.

Molecular ontology of the parabrachial nucleus

This article has been removed because of a technical problem in the rendering of the PDF. 11 February 2022.

The possibility of alleviating chronic neuropathic pain and related behaviors by the direct suppression of the parabrachial nucleus

OBJECTIVE

This study aims to observe the effects of direct suppression of the parabrachial nucleus (PBN) on chronic neuropathic pain (CNP) and CNP-related behaviors in mice.

METHODS

A CNP model was established using partial sciatic nerve ligation (PSNL) in mice. Two groups were established: the experimental (PSNL) group and the control (sham) group. An assessment of PBN-region c-Fos expression was conducted following von Frey hair stimulation in the PSNL group and the sham group, and the effects of pain induction were detected using behavioral experiments. The PBN activity of the mice with CNP was manipulated using the designer receptors exclusively activated by designer drugs method. Effective and empty virus groups were used to study the effects of PBN activity inhibition on the pain threshold and pain-related behavior in mice with CNP.

RESULTS

The mechanical pain threshold (MPT) of the mice in the PSNL group was significantly lower than in the sham group. After von Frey stimulation, the c-Fos-positive, PBN-region neurons in the PSNL group were increased compared with the sham group. The central distance in the open field test and the time spent in the central area were lower in the PSNL group than in the sham group. The mice in the PSNL group had a lower duration and fewer entries in the open arm of the elevated plus-maze than the mice in the sham group. There was no difference in immobility time between the PSNL group and the sham group. PBN activity inhibition in mice with CNP did not affect their MPT or anxiety-like behavior.

CONCLUSION

CNP can induce anxiety-like behavior and increase PBN-induced pain in mice. However, direct inhibition of the PBN neuron activity alone cannot improve CNP or CNP-related behavior.

Pain modulates dopamine neurons via a spinal-parabrachial-mesencephalic circuit

Pain decreases the activity of many ventral tegmental area (VTA) dopamine (DA) neurons, yet the underlying neural circuitry connecting nociception and the DA system is not understood. Here we show that a subpopulation of lateral parabrachial (LPB) neurons is critical for relaying nociceptive signals from the spinal cord to the substantia nigra pars reticulata (SNR). SNR-projecting LPB neurons are activated by noxious stimuli and silencing them blocks pain responses in two different models of pain. LPB-targeted and nociception-recipient SNR neurons regulate VTA DA activity directly through feed-forward inhibition and indirectly by inhibiting a distinct subpopulation of VTA-projecting LPB neurons thereby reducing excitatory drive onto VTA DA neurons. Correspondingly, ablation of SNR-projecting LPB neurons is sufficient to reduce pain-mediated inhibition of DA release in vivo. The identification of a neural circuit conveying nociceptive input to DA neurons is critical to our understanding of how pain influences learning and behavior.

Chronic Alcoholism and HHV-6 Infection Synergistically Promote Neuroinflammatory Microglial Phenotypes in the Substantia Nigra of the Adult Human Brain

Both chronic alcoholism and human herpesvirus-6 (HHV-6) infection have been identified as promoters of neuroinflammation and known to cause movement-related disorders. Substantia Nigra (SN), the dopaminergic neuron-rich region of the basal ganglia, is involved in regulating motor function and the reward system. Hence, we hypothesize the presence of possible synergism between alcoholism and HHV-6 infection in the SN region and report a comprehensive quantification and characterization of microglial functions and morphology in postmortem brain tissue from 44 healthy, age-matched alcoholics and chronic alcoholics. A decrease in the perivascular CD68+ microglia in alcoholics was noted in both the gray and white matter. Additionally, the CD68+/Iba1− microglial subpopulation was found to be the dominant type in the controls. Conversely, in alcoholics, dystrophic changes in microglia were seen with a significant increase in Iba1 expression and perivascular to diffuse migration. An increase in CD11b expression was noted in alcoholics, with the Iba1+/CD11b− subtype promoting inflammation. All the controls were found to be negative for HHV-6 whilst the alcoholics demonstrated HHV-6 positivity in both gray and white matter. Amongst HHV-6 positive alcoholics, all the above-mentioned changes were found to be heightened when compared with HHV-6 negative alcoholics, thereby highlighting the compounding relationship between alcoholism and HHV-6 infection that promotes microglia-mediated neuroinflammation.

A systems-neuroscience model of phasic dopamine

We describe a neurobiologically informed computational model of phasic dopamine signaling to account for a wide range of findings, including many considered inconsistent with the simple reward prediction error (RPE) formalism. The central feature of this PVLV framework is a distinction between a primary value (PV) system for anticipating primary rewards (Unconditioned Stimuli [USs]), and a learned value (LV) system for learning about stimuli associated with such rewards (CSs). The LV system represents the amygdala, which drives phasic bursting in midbrain dopamine areas, while the PV system represents the ventral striatum, which drives shunting inhibition of dopamine for expected USs (via direct inhibitory projections) and phasic pausing for expected USs (via the lateral habenula). Our model accounts for data supporting the separability of these systems, including individual differences in CS-based (sign-tracking) versus US-based learning (goal-tracking). Both systems use competing opponent-processing pathways representing evidence for and against specific USs, which can explain data dissociating the processes involved in acquisition versus extinction conditioning. Further, opponent processing proved critical in accounting for the full range of conditioned inhibition phenomena, and the closely related paradigm of second-order conditioning. Finally, we show how additional separable pathways representing aversive USs, largely mirroring those for appetitive USs, also have important differences from the positive valence case, allowing the model to account for several important phenomena in aversive conditioning. Overall, accounting for all of these phenomena strongly constrains the model, thus providing a well-validated framework for understanding phasic dopamine signaling. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Efferent projections of Vglut2, Foxp2, and Pdyn parabrachial neurons in mice

The parabrachial nucleus (PB) is a complex structure located at the junction of the midbrain and hindbrain. Its neurons have diverse genetic profiles and influence a variety of homeostatic functions. While its cytoarchitecture and overall efferent projections are known, we lack comprehensive information on the projection patterns of specific neuronal subtypes in the PB. In this study, we compared the projection patterns of glutamatergic neurons here with a subpopulation expressing the transcription factor Foxp2 and a further subpopulation expressing the neuropeptide Pdyn. To do this, we injected an AAV into the PB region to deliver a Cre-dependent anterograde tracer (synaptophysin-mCherry) in three different strains of Cre-driver mice. We then analyzed 147 neuroanatomical regions for labeled boutons in every brain (n = 11). Overall, glutamatergic neurons in the PB region project to a wide variety of sites in the cerebral cortex, basal forebrain, bed nucleus of the stria terminalis, amygdala, diencephalon, and brainstem. Foxp2 and Pdyn subpopulations project heavily to the hypothalamus, but not to the cortex, basal forebrain, or amygdala. Among the few differences between Foxp2 and Pdyn cases was a notable lack of Pdyn projections to the ventromedial hypothalamic nucleus. Our results indicate that genetic identity determines connectivity (and therefore, function), providing a framework for mapping all PB output projections based on the genetic identity of its neurons. Using genetic markers to systematically classify PB neurons and their efferent projections will enhance the translation of research findings from experimental animals to humans.

Electrogastrography for psychophysiological research: Practical considerations, analysis pipeline, and normative data in a large sample

Electrogastrography (EGG) is the noninvasive electrophysiological technique used to record gastric electrical activity by means of cutaneous electrodes placed on the abdomen. EGG has been so far mostly used in clinical studies in gastroenterology, but it represents an attractive method to study brain-viscera interactions in psychophysiology. Compared to the literature on electrocardiography for instance, where practical recommendations and normative data are abundant, the literature on EGG in humans remains scarce. The aim of this article is threefold. First, we review the existing literature on the physiological basis of the EGG, pathways of brain-stomach interactions, and experimental findings in the cognitive neuroscience and psychophysiology literature. We then describe practical issues faced when recording the EGG in young healthy participants, from data acquisition to data analysis, and propose a semi-automated analysis pipeline together with associated MATLAB code. The analysis pipeline aims at identifying a regular rhythm that can be safely attributed to the stomach, through multiple steps. Finally, we apply these recording and analysis procedures in a large sample (N = 117) of healthy young adult male and female participants in a moderate (<5 hr) to prolonged (>10 hr) fasting state to establish the normative distribution of several EGG parameters. Our results are overall congruent with the clinical gastroenterology literature, but suggest using an electrode coverage extending to lower abdominal locations than current clinical guidelines. Our results indicate a marginal difference in EGG peak frequency between male and female participants, and that the gastric rhythm becomes more irregular after prolonged fasting.

New players in basal ganglia dysfunction in Parkinson's disease

The classical model of the basal ganglia (BG) circuit has been recently revised with the identification of other structures that play an increasing relevant role especially in the pathophysiology of Parkinson's disease (PD). Numerous studies have supported the spreading of the alpha-synuclein pathology to several areas beyond the BG and likely even before their involvement. With the aim of better understanding PD pathophysiology and finding new targets for treatment, the spinal cord, the pedunculopontine nucleus, the substantia nigra pars reticulata, the retina, the superior colliculus, the cerebellum, the nucleus parabrachialis and the Meynert's nucleus have been investigated both in animal and human studies. In this chapter, we describe the main anatomical and functional connections between the above structures and the BG, the relationship between their pathology and PD features, and the rational of applying neuromodulation treatment to improve motor and non-motor symptoms in PD. Some of these new players in the BG circuits might also have a potential intriguing role as early biomarkers of PD.

Three Rostromedial Tegmental Afferents Drive Triply Dissociable Aspects of Punishment Learning and Aversive Valence Encoding

Persistence of reward seeking despite punishment or other negative consequences is a defining feature of mania and addiction, and numerous brain regions have been implicated in such punishment learning, but in disparate ways that are difficult to reconcile. We now show that the ability of an aversive punisher to inhibit reward seeking depends on coordinated activity of three distinct afferents to the rostromedial tegmental nucleus (RMTg) arising from cortex, brainstem, and habenula that drive triply dissociable RMTg responses to aversive cues, outcomes, and prediction errors, respectively. These three pathways drive correspondingly dissociable aspects of punishment learning. The RMTg in turn drives negative, but not positive, valence encoding patterns in the ventral tegmental area (VTA). Hence, punishment learning involves pathways and functions that are highly distinct, yet tightly coordinated.

Sweet and bitter taste stimuli activate VTA projection neurons in the parabrachial nucleus

This study investigated neural projections from the parabrachial nucleus (PBN), a gustatory and visceral processing area in the brainstem, to the ventral tegmental area (VTA) in the midbrain. The VTA contains a large population of dopaminergic neurons that have been shown to play a role in reward processing. Anterograde neural tracing methods were first used to confirm that a robust projection from the caudal PBN terminates in the dorsal VTA; this projection was larger on the contralateral side. In the next experiment, we combined dual retrograde tracing from the VTA and the gustatory ventral posteromedial thalamus (VPMpc) with taste-evoked Fos protein expression, which labels activated neurons. Mice were stimulated through an intraoral cannula with sucrose, quinine, or water, and PBN sections were processed for immunofluorescent detection of Fos and retrograde tracers. The distribution of tracer-labeled PBN neurons demonstrated that the populations of cells projecting to the VTA or VPMpc are largely independent. Quantification of cells double labeled for Fos and either tracer demonstrated that sucrose and quinine were effective in activating both pathways. These results indicate that information about both appetitive and aversive tastes is delivered to a key midbrain reward interface via direct projections from the PBN.

Vagal Interoceptive Modulation of Motivated Behavior

In addition to regulating the ingestion and digestion of food, sensory feedback from gut to brain modifies emotional state and motivated behavior by subconsciously shaping cognitive and affective responses to events that bias behavioral choice. This focused review highlights evidence that gut-derived signals impact motivated behavior by engaging vagal afferents and central neural circuits that generally serve to limit or terminate goal-directed approach behaviors, and to initiate or maintain behavioral avoidance.

Collateral Projections from the Lateral Parabrachial Nucleus to the Central Amygdaloid Nucleus and the Ventral Tegmental Area in the Rat

Lateral parabrachial nucleus (LPB) is a critical region in the integration and transmission of peripheral nociceptive information. The parabrachio‐amygdaloid (P‐Amy) pathway and parabrachio‐ventral tegmental area (P‐VTA) pathway is thought to be significant in regulation of pain‐related negative emotions. In present study, retrograde tract tracers Fluoro‐gold (FG) and tetramethylrhodramine‐dextran (TMR) were stereotaxically injected into the right central amygdaloid nucleus (CeA) and right VTA, respectively. Then, part of these rats were performed with the spare nerve injury (SNI) in the controlateral side of FG and TMR injection. Afterwards, double‐ or triple‐immunofluorescent histochemistry was used to examine FG/TMR double‐ and FG/TMR/FOS or FG/TMR/CGRP triple‐labeled neurons in the LPB. The results showed that all of FG, TMR single‐ and FG/TMR double‐labeled neurons were distributed in the LPB bilaterally with an ipsilateral predominance. The proportion of FG/TMR double‐labeled neurons to the total number of FG‐ and TMR‐labeled neurons was 10.78% and 13.07%, respectively. Nearly all of the FG/TMR double‐labeled neurons (92.67%) showed calcitonin gene‐related peptide (CGRP) immunopositive. On the other hand, in the SNI rats, about 89.49% and 77.87% of FG‐ and TMR‐labeled neurons were FG/FOS‐ and TMR/FOS‐positive neurons; about 93.33% of the FG/TMR double‐labeled neurons were FOS‐LI. Our results suggest that the part of CGRP immunopositive neurons in the LPB send projection fibers to both the CeA and VTA by the way of axon collaterals, which are activated by the nociceptive stimulation in the SNI condition, and may play an important role in the transmission of peripheral nociceptive information. Anat Rec, 302:1178–1186, 2019. © 2018 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

A Neural Circuit for Gut-Induced Reward

The gut is now recognized as a major regulator of motivational and emotional states. However, the relevant gut-brain neuronal circuitry remains unknown. We show that optical activation of gut-innervating vagal sensory neurons recapitulates the hallmark effects of stimulating brain reward neurons. Specifically, right, but not left, vagal sensory ganglion activation sustained self-stimulation behavior, conditioned both flavor and place preferences, and induced dopamine release from Substantia nigra. Cell-specific transneuronal tracing revealed asymmetric ascending pathways of vagal origin throughout the CNS. In particular, transneuronal labeling identified the glutamatergic neurons of the dorsolateral parabrachial region as the obligatory relay linking the right vagal sensory ganglion to dopamine cells in Substantia nigra. Consistently, optical activation of parabrachio-nigral projections replicated the rewarding effects of right vagus excitation. Our findings establish the vagal gut-to-brain axis as an integral component of the neuronal reward pathway. They also suggest novel vagal stimulation approaches to affective disorders.

Revealing a novel nociceptive network that links the subthalamic nucleus to pain processing

Pain is a prevalent symptom of Parkinson's disease, and is effectively treated by deep brain stimulation of the subthalamic nucleus (STN). However, the link between pain and the STN remains unclear. In the present work, using in vivo electrophysiology in rats, we report that STN neurons exhibit complex tonic and phasic responses to noxious stimuli. We also show that nociception is altered following lesions of the STN, and characterize the role of the superior colliculus and the parabrachial nucleus in the transmission of nociceptive information to the STN, physiologically from both structures and anatomically in the case of the parabrachial nucleus. We show that STN nociceptive responses are abnormal in a rat model of PD, suggesting their dependence on the integrity of the nigrostriatal dopaminergic system. The STN-linked nociceptive network that we reveal is likely to be of considerable clinical importance in neurological diseases involving a dysfunction of the basal ganglia.

Anatomical Evidence for a Direct Projection from Purkinje Cells in the Mouse Cerebellar Vermis to Medial Parabrachial Nucleus

Cerebellar malformations cause changes to the sleep-wake cycle, resulting in sleep disturbance. However, it is unclear how the cerebellum contributes to the sleep-wake cycle. To examine the neural connections between the cerebellum and the nuclei involved in the sleep-wake cycle, we investigated the axonal projections of Purkinje cells in the mouse posterior vermis by using an adeno-associated virus (AAV) vector (serotype rh10) as an anterograde tracer. When an AAV vector expressing humanized renilla green fluorescent protein was injected into the cerebellar lobule IX, hrGFP and synaptophysin double-positive axonal terminals were observed in the region of medial parabrachial nucleus (MPB). The MPB is involved in the phase transition from rapid eye movement (REM) sleep to Non-REM sleep and vice versa, and the cardiovascular and respiratory responses. The hrGFP-positive axons from lobule IX went through the ventral spinocerebellar tract and finally reached the MPB. By contrast, when the AAV vector was injected into cerebellar lobule VI, no hrGFP-positive axons were observed in the MPB. To examine neurons projecting to the MPB, we unilaterally injected Fast Blue and AAV vector (retrograde serotype, rAAV2-retro) as retrograde tracers into the MPB. The cerebellar Purkinje cells in lobules VIII–X on the ipsilateral side of the Fast Blue-injected MPB were retrogradely labeled by Fast Blue and AAV vector (retrograde serotype), but no retrograde-labeled Purkinje cells were observed in lobules VI–VII and the cerebellar hemispheres. These results indicated that Purkinje cells in lobules VIII–X directly project their axons to the ipsilateral MPB but not lobules VI–VII. The direct connection between lobules VIII–X and the MPB suggests that the cerebellum participates in the neural network controlling the sleep-wake cycle, and cardiovascular and respiratory responses, by modulating the physiological function of the MPB.

Overlapping Brain Circuits for Homeostatic and Hedonic Feeding

Central regulation of food intake is a key mechanism contributing to energy homeostasis. Many neural circuits that are thought to orchestrate feeding behavior overlap with the brain's reward circuitry both anatomically and functionally. Manipulation of numerous neural pathways can simultaneously influence food intake and reward. Two key systems underlying these processes-those controlling homeostatic and hedonic feeding-are often treated as independent. Homeostatic feeding is necessary for basic metabolic processes and survival, while hedonic feeding is driven by sensory perception or pleasure. Despite this distinction, their functional and anatomical overlap implies considerable interaction that is often overlooked. Here, we argue that the neurocircuits controlling homeostatic feeding and hedonic feeding are not completely dissociable given the current data and urge researchers to assess behaviors extending beyond food intake in investigations of the neural control of feeding.

Threat Response System: Parallel Brain Processes in Pain vis-à-vis Fear and Anxiety

Pain is essential for avoidance of tissue damage and for promotion of healing. Notwithstanding the survival value, pain brings about emotional suffering reflected in fear and anxiety, which in turn augment pain thus giving rise to a self-sustaining feedforward loop. Given such reciprocal relationships, the present article uses neuroscientific conceptualizations of fear and anxiety as a theoretical framework for hitherto insufficiently understood pathophysiological mechanisms underlying chronic pain. To that end, searches of PubMed-indexed journals were performed using the following Medical Subject Headings' terms: pain and nociception plus amygdala, anxiety, cognitive, fear, sensory, and unconscious. Recursive sets of scientific and clinical evidence extracted from this literature review were summarized within the following key areas: (1) parallelism between acute pain and fear and between chronic pain and anxiety; (2) all are related to the evasion of sensory-perceived threats and are subserved by subcortical circuits mediating automatic threat-induced physiologic responses and defensive actions in conjunction with higher order corticolimbic networks (e.g., thalamocortical, thalamo-striato-cortical and amygdalo-cortical) generating conscious representations and valuation-based adaptive behaviors; (3) some instances of chronic pain and anxiety conditions are driven by the failure to diminish or block respective nociceptive information or unconscious treats from reaching conscious awareness; and (4) the neural correlates of pain-related conscious states and cognitions may become autonomous (i.e., dissociated) from the subcortical activity/function leading to the eventual chronicity. Identifying relative contributions of the diverse neuroanatomical sources, thus, offers prospects for the development of novel preventive, diagnostic, and therapeutic strategies in chronic pain patients.

Parabrachial complex links pain transmission to descending pain modulation

The rostral ventromedial medulla (RVM) has a well-documented role in pain modulation and exerts antinociceptive and pronociceptive influences mediated by 2 distinct classes of neurons, OFF-cells and ON-cells. OFF-cells are defined by a sudden pause in firing in response to nociceptive inputs, whereas ON-cells are characterized by a "burst" of activity. Although these reflex-related changes in ON- and OFF-cell firing are critical to their pain-modulating function, the pathways mediating these responses have not been identified. The present experiments were designed to test the hypothesis that nociceptive input to the RVM is relayed through the parabrachial complex (PB). In electrophysiological studies, ON- and OFF-cells were recorded in the RVM of lightly anesthetized male rats before and after an infusion of lidocaine or muscimol into PB. The ON-cell burst and OFF-cell pause evoked by noxious heat or mechanical probing were substantially attenuated by inactivation of the lateral, but not medial, parabrachial area. Retrograde tracing studies showed that neurons projecting to the RVM were scattered throughout PB. Few of these neurons expressed calcitonin gene-related peptide, suggesting that the RVM projection from PB is distinct from that to the amygdala. These data show that a substantial component of "bottom-up" nociceptive drive to RVM pain-modulating neurons is relayed through the PB. While the PB is well known as an important relay for ascending nociceptive information, its functional connection with the RVM allows the spinoparabrachial pathway to access descending control systems as part of a recurrent circuit.

Locus coeruleus phasic discharge is essential for stimulus-induced gamma oscillations in the prefrontal cortex

The locus coeruleus (LC) noradrenergic (NE) neuromodulatory system is critically involved in regulation of neural excitability via its diffuse ascending projections. Tonic NE release in the forebrain is essential for maintenance of vigilant states and increases the signal-to-noise ratio of cortical sensory responses. The impact of phasic NE release on cortical activity and sensory processing is less explored. We previously reported that LC microstimulation caused a transient desynchronization of population activity in the medial prefrontal cortex (mPFC), similar to noxious somatosensory stimuli. The LC receives nociceptive information from the medulla and therefore may mediate sensory signaling to its forebrain targets. Here we performed extracellular recordings in LC and mPFC while presenting noxious stimuli in urethane-anesthetized rats. A brief train of foot shocks produced a robust phasic response in the LC and a transient change in the mPFC power spectrum, with the strongest modulation in the gamma (30-90 Hz) range. The LC phasic response preceded prefrontal gamma power increase, and cortical modulation was proportional to the LC excitation. We also quantitatively characterized distinct cortical states and showed that sensory responses in both LC and mPFC depend on the ongoing cortical state. Finally, cessation of the LC firing by bilateral local iontophoretic injection of clonidine, an α₂-adrenoreceptor agonist, completely eliminated sensory responses in the mPFC without shifting cortex to a less excitable state. Together, our results suggest that the LC phasic response induces gamma power increase in the PFC and is essential for mediating sensory information along an ascending noxious pathway. NEW & NOTEWORTHY Our study shows linear relationships between locus coeruleus phasic excitation and the amplitude of gamma oscillations in the prefrontal cortex. Results suggest that the locus coeruleus phasic response is essential for mediating sensory information along an ascending noxious pathway.

Chronic Pain and Chronic Stress: Two Sides of the Same Coin?

Pain and stress share significant conceptual and physiological overlaps. Both phenomena challenge the body's homeostasis and necessitate decision-making to help animals adapt to their environment. In addition, chronic stress and chronic pain share a common behavioral model of failure to extinguish negative memories. Yet, they also have discrepancies such that the final brain endophenotype of posttraumatic stress disorder, depression, and chronic pain appears to be different among the three conditions, and the role of the hypothalamic-pituitary-adrenal axis remains unclear in the physiology of pain. Persistence of either stress or pain is maladaptive and could lead to compromised well-being. In this brief review, we highlight the commonalities and differences between chronic stress and chronic pain, while focusing particularly on the central role of the limbic brain. We assess the current attempts in the field to conceptualize and understand chronic pain, within the context of knowledge gained from the stress literature. The limbic brain-including hippocampus, amygdala, and ventromedial pre-frontal cortex-plays a critical role in learning. These brain areas integrate incoming nociceptive or stress signals with internal state, and generate learning signals necessary for decision-making. Therefore, the physiological and structural remodeling of this learning circuitry is observed in conditions such as chronic pain, depression, and posttraumatic stress disorder, and is also linked to the risk of onset of these conditions.

From Belly to Brain: Targeting the Ghrelin Receptor in Appetite and Food Intake Regulation

Ghrelin is the only known peripherally-derived orexigenic hormone, increasing appetite and subsequent food intake. The ghrelinergic system has therefore received considerable attention as a therapeutic target to reduce appetite in obesity as well as to stimulate food intake in conditions of anorexia, malnutrition and cachexia. As the therapeutic potential of targeting this hormone becomes clearer, it is apparent that its pleiotropic actions span both the central nervous system and peripheral organs. Despite a wealth of research, a therapeutic compound specifically targeting the ghrelin system for appetite modulation remains elusive although some promising effects on metabolic function are emerging. This is due to many factors, ranging from the complexity of the ghrelin receptor (Growth Hormone Secretagogue Receptor, GHSR-1a) internalisation and heterodimerization, to biased ligand interactions and compensatory neuroendocrine outputs. Not least is the ubiquitous expression of the GHSR-1a, which makes it impossible to modulate centrally-mediated appetite regulation without encroaching on the various peripheral functions attributable to ghrelin. It is becoming clear that ghrelin’s central signalling is critical for its effects on appetite, body weight regulation and incentive salience of food. Improving the ability of ghrelin ligands to penetrate the blood brain barrier would enhance central delivery to GHSR-1a expressing brain regions, particularly within the mesolimbic reward circuitry.

Visual Cue-Discriminative Dopaminergic Control of Visuomotor Transformation and Behavior Selection

Animals behave differently in response to visual cues with distinct ethological meaning, a process usually thought to be achieved through differential visual processing. Using a defined zebrafish escape circuit as a model, we found that behavior selection can be implemented at the visuomotor transformation stage through a visually responsive dopaminergic-inhibitory circuit module. In response to non-threatening visual stimuli, hypothalamic dopaminergic neurons and their positively regulated hindbrain inhibitory interneurons increase activity, suppressing synaptic transmission from the visual center to the escape circuit. By contrast, threatening visual stimuli inactivate some of these neurons, resulting in dis-inhibition of the visuomotor transformation and escape generation. The distinct patterns of dopaminergic-inhibitory neural module's visual responses account for this stimulus-specific visuomotor transformation and behavioral control. Thus, our study identifies a behavioral relevance-dependent mechanism that controls visuomotor transformation and behavior selection and reveals that neuromodulation can be tuned by visual cues to help animals generate appropriate responses.

A subpopulation of neurochemically-identified ventral tegmental area dopamine neurons is excited by intravenous cocaine

Systemic administration of cocaine is thought to decrease the firing rates of ventral tegmental area (VTA) dopamine (DA) neurons. However, this view is based on categorizations of recorded neurons as DA neurons using preselected electrophysiological characteristics lacking neurochemical confirmation. Without applying cellular preselection, we recorded the impulse activity of VTA neurons in response to cocaine administration in anesthetized adult rats. The phenotype of recorded neurons was determined by their juxtacellular labeling and immunohistochemical detection of tyrosine hydroxylase (TH), a DA marker. We found that intravenous cocaine altered firing rates in the majority of recorded VTA neurons. Within the cocaine-responsive neurons, half of the population was excited and the other half was inhibited. Both populations had similar discharge rates and firing regularities, and most neurons did not exhibit changes in burst firing. Inhibited neurons were more abundant in the posterior VTA, whereas excited neurons were distributed evenly throughout the VTA. Cocaine-excited neurons were more likely to be excited by footshock. Within the subpopulation of TH-positive neurons, 36% were excited by cocaine and 64% were inhibited. Within the subpopulation of TH-negative neurons, 44% were excited and 28% were inhibited. Contrary to the prevailing view that all DA neurons are inhibited by cocaine, we found a subset of confirmed VTA DA neurons that is excited by systemic administration of cocaine. We provide evidence indicating that DA neurons are heterogeneous in their response to cocaine and that VTA non-DA neurons play an active role in processing systemic cocaine.

Parkinson's disease: disturbed vestibular function and levodopa

Evidence indicates Levodopa effects central postural control. As electrophysiological postural control biomarkers, sensory oto-acoustic features were extracted from Electrovestibulography (EVestG) data to identify 20 healthy age and gender matched individuals as Controls from 20 PD subjects before (PDlowmed) and 18 after (PDmed) morning doses of Levodopa. EVestG data was collected using a single tilt stimulus applied in the pitch plane. The extracted features were based on the measured firing pattern, interval histogram and the shape of the average field potential response. An unbiased cross validated classification accuracy of 88%, 88% and 79% was achieved using combinations of 2 features for separating PDlowmed from control, control from PD (combined PDlowmed and PDmed), and PDlowmed from PDmed groups respectively. One feature showed significant correlations (p<0.05) with the Modified Hoehn and Yahr PD staging scale. The results indicate disturbed vestibular function is observed in both the PDmed and PDlowmed conditions, and these are separable. The implication is that Levodopa may also affect peripheral as well as central postural control.

Heterogeneity of dopamine neuron activity across traits and states

Midbrain dopamine neurons fire irregularly, with interspersed clusters of high-frequency spikes, commonly called 'bursts'. In this review we examine such heterogeneity in activity, and provide insight into how it can participate in psychiatric conditions such as drug addiction. We first describe several techniques used to evaluate dopamine neuron activity, and comment on the different measures that each provides. We next describe the activity of dopamine neurons in 'basal' conditions. Specifically, we discuss how the use of anesthesia and reduced preparations may alter aspects of dopamine cell activity, and how there is heterogeneity across species and regions. We also describe how dopamine cell firing changes throughout the peri-adolescent period and how dopamine neuron activity differs across the population. In the final section, we discuss how dopamine neuron activity changes in response to life events. First, we focus attention on drugs of abuse. Drugs themselves change firing activity through a variety of mechanisms, with effects on firing while drug is present differing from those seen after drug discontinuation. We then review how stimuli that are rewarding, aversive, or salient can evoke changes in firing rate and discharge pattern of dopamine neurons, and provide behavioral relevance of dopamine signaling. Finally, we discuss how stress can modulate dopamine neuron firing and how this may contribute to the role that stressful experiences play in psychiatric disorders such as addiction and depression.

Pedunculopontine tegmental nucleus neurons provide reward, sensorimotor, and alerting signals to midbrain dopamine neurons

Dopamine (DA) neurons in the midbrain are crucial for motivational control of behavior. However, recent studies suggest that signals transmitted by DA neurons are heterogeneous. This may reflect a wide range of inputs to DA neurons, but which signals are provided by which brain areas is still unclear. Here we focused on the pedunculopontine tegmental nucleus (PPTg) in macaque monkeys and characterized its inputs to DA neurons. Since the PPTg projects to many brain areas, it is crucial to identify PPTg neurons that project to DA neuron areas. For this purpose we used antidromic activation technique by electrically stimulating three locations (medial, central, lateral) in the substantia nigra pars compacta (SNc). We found SNc-projecting neurons mainly in the PPTg, and some in the cuneiform nucleus. Electrical stimulation in the SNc-projecting PPTg regions induced a burst of spikes in presumed DA neurons, suggesting that the PPTg-DA (SNc) connection is excitatory. Behavioral tasks and clinical tests showed that the SNc-projecting PPTg neurons encoded reward, sensorimotor and arousal/alerting signals. Importantly, reward-related PPTg neurons tended to project to the medial and central SNc, whereas sensorimotor/arousal/alerting-related PPTg neurons tended to project to the lateral SNc. Most reward-related signals were positively biased: excitation and inhibition when a better and worse reward was expected, respectively. These PPTg neurons tended to retain the reward value signal until after a reward outcome, representing 'value state'; this was different from DA neurons which show phasic signals representing 'value change'. Our data, together with previous studies, suggest that PPTg neurons send positive reward-related signals mainly to the medial-central SNc where DA neurons encode motivational values, and sensorimotor/arousal signals to the lateral SNc where DA neurons encode motivational salience.

Differential suppression of intracranial self-stimulation, food-maintained operant responding, and open field activity by paw incision and spinal nerve ligation in rats

BACKGROUND

Detection of ongoing spontaneous pain behaviors in laboratory animals remains a research challenge. Most preclinical pain studies measure elicited behavioral responses to an external noxious stimulus; however, ongoing spontaneous pain in humans and animals may be unrelated to hypersensitivity, and likely diminishes many behaviors, particularly motivated behaviors, that we hypothesize will decrease after induction of acute and chronic pain.

METHODS

In this study, 201 male rats were subjected to paw incision (INC), L5/L6 spinal nerve ligation (SNL), or INC in SNL rats, and the effects on paw withdrawal threshold (PWT) were assessed. For comparison, the behavioral-decreasing effects on nonevoked measures, including lever pressing for rewarding electrical stimulation of the ventral tegmental area intracranial self-stimulation (VTA ICSS) or food reinforcement (FR), and open field activity (OFA), were also assessed in these same rats.

RESULTS

INC decreased PWT for 4 days, decreased VTA ICSS for 2 days, and FR for 1 day but did not alter OFA. SNL decreased PWT similarly to INC but did not decrease VTA ICSS or FR; SNL did however decrease OFA. INC in SNL rats reduced PWT, VTA ICSS, and FR similarly to INC alone and did not decrease OFA compared with SNL alone.

CONCLUSIONS

The acute effects of INC on decreasing lever pressing for VTA ICSS and FR (1-2 days after incision) correspond to the timeframe in which ongoing spontaneous pain is expected to occur after INC. Therefore, these decreases are likely mediated by ongoing spontaneous pain, which may be unrelated to mechanical hypersensitivity that persists for up to 4 days after INC. PWT is decreased similarly by SNL, yet operant behavior (lever pressing for VTA ICSS and FR) was not decreased by SNL. SNL, but not INC, decreased rearing behavior but not total distance traveled during OFA. This further indicates that the presence and the extent of hypersensitivity are not predictive of many behavioral changes in rats thought to be mediated by the presence of ongoing pain. Surprisingly, the behavioral effects of INC are not exacerbated in SNL rats. These data support the growing belief that acute pain models produce short-lived spontaneous pain behaviors that are often less pronounced or absent in neuropathic pain models, and highlight the need for assessment of both evoked and nonevoked pain behaviors in developing future therapies for acute and chronic pain.

An update on the connections of the ventral mesencephalic dopaminergic complex

This review covers the intrinsic organization and afferent and efferent connections of the midbrain dopaminergic complex, comprising the substantia nigra, ventral tegmental area and retrorubral field, which house, respectively, the A9, A10 and A8 groups of nigrostriatal, mesolimbic and mesocortical dopaminergic neurons. In addition, A10dc (dorsal, caudal) and A10rv (rostroventral) extensions into, respectively, the ventrolateral periaqueductal gray and supramammillary nucleus are discussed. Associated intrinsic and extrinsic connections of the midbrain dopaminergic complex that utilize gamma-aminobutyric acid (GABA), glutamate and neuropeptides and various co-expressed combinations of these compounds are considered in conjunction with the dopamine-containing systems. A framework is provided for understanding the organization of massive afferent systems descending and ascending to the midbrain dopaminergic complex from the telencephalon and brainstem, respectively. Within the context of this framework, the basal ganglia direct and indirect output pathways are treated in some detail. Findings from rodent brain are briefly compared with those from primates, including humans. Recent literature is emphasized, including traditional experimental neuroanatomical and modern gene transfer and optogenetic studies. An attempt was made to provide sufficient background and cite a representative sampling of earlier primary papers and reviews so that people new to the field may find this to be a relatively comprehensive treatment of the subject.

Sensory regulation of dopaminergic cell activity: Phenomenology, circuitry and function

Dopaminergic neurons in a range of species are responsive to sensory stimuli. In the anesthetized preparation, responses to non-noxious and noxious sensory stimuli are usually tonic in nature, although long-duration changes in activity have been reported in the awake preparation as well. However, in the awake preparation, short-latency, phasic changes in activity are most common. These phasic responses can occur to unconditioned aversive and non-aversive stimuli, as well as to the stimuli which predict them. In both the anesthetized and awake preparations, not all dopaminergic neurons are responsive to sensory stimuli, however responsive neurons tend to respond to more than a single stimulus modality. Evidence suggests that short-latency sensory information is provided to dopaminergic neurons by relatively primitive subcortical structures - including the midbrain superior colliculus for vision and the mesopontine parabrachial nucleus for pain and possibly gustation. Although short-latency visual information is provided to dopaminergic neurons by the relatively primitive colliculus, dopaminergic neurons can discriminate between complex visual stimuli, an apparent paradox which can be resolved by the recently discovered route of information flow through to dopaminergic neurons from the cerebral cortex, via a relay in the colliculus. Given that projections from the cortex to the colliculus are extensive, such a relay potentially allows the activity of dopaminergic neurons to report the results of complex stimulus processing from widespread areas of the cortex. Furthermore, dopaminergic neurons could acquire their ability to reflect stimulus value by virtue of reward-related modification of sensory processing in the cortex. At the forebrain level, sensory-related changes in the tonic activity of dopaminergic neurons may regulate the impact of the cortex on forebrain structures such as the nucleus accumbens. In contrast, the short latency of the phasic responses to sensory stimuli in dopaminergic neurons, coupled with the activation of these neurons by non-rewarding stimuli, suggests that phasic responses of dopaminergic neurons may provide a signal to the forebrain which indicates that a salient event has occurred (and possibly an estimate of how salient that event is). A stimulus-related salience signal could be used by downstream systems to reinforce behavioral choices.

Feelings about food: the ventral tegmental area in food reward and emotional eating

Overconsumption of high caloric food plays an important role in the etiology of obesity. Several factors drive such hedonic feeding. High caloric food is often palatable. In addition, when an individual is sated, stress and food-related cues can serve as potent feeding triggers. A better understanding of the neurobiological underpinnings of food palatability and environmentally triggered overconsumption would aid the development of new treatment strategies. In the current review we address the pivotal role of the mesolimbic dopamine reward system in the drive towards high caloric palatable food and its relation to stress- and cue-induced feeding. We also discuss how this system may be affected by both established and potential anti-obesity drug targets.

Bidirectional modulation of substantia nigra activity by motivational state

A major output nucleus of the basal ganglia is the substantia nigra pars reticulata, which sends GABAergic projections to brainstem and thalamic nuclei. The GABAergic (GABA) neurons are reciprocally connected with nearby dopaminergic neurons, which project mainly to the basal ganglia, a set of subcortical nuclei critical for goal-directed behaviors. Here we examined the impact of motivational states on the activity of GABA neurons in the substantia nigra pars reticulata and the neighboring dopaminergic (DA) neurons in the pars compacta. Both types of neurons show short-latency bursts to a cue predicting a food reward. As mice became sated by repeated consumption of food pellets, one class of neurons reduced cue-elicited firing, whereas another class of neurons progressively increased firing. Extinction or pre-feeding just before the test session dramatically reduced the phasic responses and their motivational modulation. These results suggest that signals related to the current motivational state bidirectionally modulate behavior and the magnitude of phasic response of both DA and GABA neurons in the substantia nigra.

Pain relief produces negative reinforcement through activation of mesolimbic reward-valuation circuitry

Relief of pain is rewarding. Using a model of experimental postsurgical pain we show that blockade of afferent input from the injury with local anesthetic elicits conditioned place preference, activates ventral tegmental dopaminergic cells, and increases dopamine release in the nucleus accumbens. Importantly, place preference is associated with increased activity in midbrain dopaminergic neurons and blocked by dopamine antagonists injected into the nucleus accumbens. The data directly support the hypothesis that relief of pain produces negative reinforcement through activation of the mesolimbic reward-valuation circuitry.