The dopaminergic projection system, basal forebrain macrosystems, and conditioned stimuli

65 years of research on dopamine's role in classical fear conditioning and extinction: A systematic review

Dopamine, a catecholamine neurotransmitter, has historically been associated with the encoding of reward, whereas its role in aversion has received less attention. Here, we systematically gathered the vast evidence of the role of dopamine in the simplest forms of aversive learning: classical fear conditioning and extinction. In the past, crude methods were used to augment or inhibit dopamine to study its relationship with fear conditioning and extinction. More advanced techniques such as conditional genetic, chemogenic and optogenetic approaches now provide causal evidence for dopamine's role in these learning processes. Dopamine neurons encode conditioned stimuli during fear conditioning and extinction and convey the signal via activation of D1-4 receptor sites particularly in the amygdala, prefrontal cortex and striatum. The coordinated activation of dopamine receptors allows for the continuous formation, consolidation, retrieval and updating of fear and extinction memory in a dynamic and reciprocal manner. Based on the reviewed literature, we conclude that dopamine is crucial for the encoding of classical fear conditioning and extinction and contributes in a way that is comparable to its role in encoding reward.

Cocaine Augments Dopamine-Mediated Inhibition of Neuronal Activity in the Dorsal Bed Nucleus of the Stria Terminalis

The dorsal region of the bed nucleus of the stria terminalis (dBNST) receives substantial dopaminergic input which overlaps with norepinephrine input implicated in stress responses. Using ex vivo fast scan cyclic voltammetry in male C57BL6 mouse brain slices, we demonstrate that electrically stimulated dBNST catecholamine signals are of substantially lower magnitude and have slower uptake rates compared with caudate signals. Dopamine terminal autoreceptor activation inhibited roughly half of the catecholamine transient, and noradrenergic autoreceptor activation produced an ∼30% inhibition. Dopamine transporter blockade with either cocaine or GBR12909 significantly augmented catecholamine signal duration. We optogenetically targeted dopamine terminals in the dBNST of transgenic (TH:Cre) mice of either sex and, using ex vivo whole-cell electrophysiology, we demonstrate that optically stimulated dopamine release induces slow outward membrane currents and an associated hyperpolarization response in a subset of dBNST neurons. These cellular responses had a similar temporal profile to dopamine release, were significantly reduced by the D2/D3 receptor antagonist raclopride, and were potentiated by cocaine. Using in vivo fiber photometry in male C57BL/6 mice during training sessions for cocaine conditioned place preference, we show that acute cocaine administration results in a significant inhibition of calcium transient activity in dBNST neurons compared with saline administration. These data provide evidence for a mechanism of dopamine-mediated cellular inhibition in the dBNST and demonstrate that cocaine augments this inhibition while also decreasing net activity in the dBNST in a drug reinforcement paradigm.SIGNIFICANCE STATEMENT The dorsal bed nucleus of the stria terminalis (dBNST) is a region highly implicated in mediating stress responses; however, the dBNST also receives dopaminergic inputs from classically defined drug reward pathways. Here we used various techniques to demonstrate that dopamine signaling within the dBNST region has inhibitory effects on population activity. We show that cocaine, an abused psychostimulant, augments both catecholamine release and dopamine-mediated cellular inhibition in this region. We also demonstrate that cocaine administration reduces population activity in the dBNST, in vivo Together, these data support a mechanism of dopamine-mediated inhibition within the dBNST, providing a means by which drug-induced elevations in dopamine signaling may inhibit dBNST activity to promote drug reward.

Retrorubral field is a hub for diverse threat and aversive outcome signals

Adaptive fear scales to the degree of threat and requires diverse neural signals for threat and aversive outcome. We propose that the retrorubral field (RRF), a midbrain region containing A8 dopamine, is a neural origin of such signals. To reveal these signals, we recorded RRF single-unit activity while male rats discriminated danger, uncertainty, and safety. Many RRF neurons showed firing extremes to danger and safety that framed intermediate firing to uncertainty. The remaining neurons showed unique, threat-selective cue firing patterns. Diversity in firing direction, magnitude, and temporal characteristics led to the detection of at least eight functional neuron types. Neuron types defined with respect to threat showed unique firing patterns following aversive outcome. The result was RRF signals for foot shock receipt, positive prediction error, anti-positive prediction error, persistent safety, and persistent threat. The diversity of threat and aversive outcome signals points to a key role for the RRF in adaptive fear.

Ventrolateral periaqueductal gray neurons prioritize threat probability over fear output

Faced with potential harm, individuals must estimate the probability of threat and initiate an appropriate fear response. In the prevailing view, threat probability estimates are relayed to the ventrolateral periaqueductal gray (vlPAG) to organize fear output. A straightforward prediction is that vlPAG single-unit activity reflects fear output, invariant of threat probability. We recorded vlPAG single-unit activity in male, Long Evans rats undergoing fear discrimination. Three 10 s auditory cues predicted unique foot shock probabilities: danger (p=1.00), uncertainty (p=0.375) and safety (p=0.00). Fear output was measured by suppression of reward seeking over the entire cue and in one-second cue intervals. Cued fear non-linearly scaled to threat probability and cue-responsive vlPAG single-units scaled their firing on one of two timescales: at onset or ramping toward shock delivery. VlPAG onset activity reflected threat probability, invariant of fear output, while ramping activity reflected both signals with threat probability prioritized.

Pleasure: The missing link in the regulation of sleep

Although largely unrecognized by sleep scholars, sleeping is a pleasure. This report aims first, to fill the gap: sleep, like food, water and sex, is a primary reinforcer. The levels of extracellular mesolimbic dopamine show circadian oscillations and mark the "wanting" for pro-homeostatic stimuli. Further, the dopamine levels decrease during waking and are replenished during sleep, in opposition to sleep propensity. The wanting of sleep, therefore, may explain the homeostatic and circadian regulation of sleep. Accordingly, sleep onset occurs when the displeasure of excessive waking is maximal, coinciding with the minimal levels of mesolimbic dopamine. Reciprocally, sleep ends after having replenished the limbic dopamine levels. Given the direct relation between waking and mesolimbic dopamine, sleep must serve primarily to gain an efficient waking. Pleasant sleep (i.e. emotional sleep), can only exist in animals capable of feeling emotions. Therefore, although sleep-like states have been described in invertebrates and primitive vertebrates, the association sleep-pleasure clearly marks a difference between the sleep of homeothermic vertebrates and cool blooded animals.

Reward and loss anticipation in panic disorder: An fMRI study

Anticipatory anxiety and harm avoidance are essential features of panic disorder (PD) and may involve deficits in the reward system of the brain, in particular in the ventral striatum. While neuroimaging studies on PD have focused on fearful and negative affective stimulus processing, no investigations have directly addressed deficits in reward and loss anticipation. To determine whether the ventral striatum shows abnormal neural activity in PD patients during anticipation of loss or gain, an event-related functional magnetic resonance imaging experiment using a monetary incentive delay task was employed in 10 patients with PD and 10 healthy controls. A repeated-measures ANOVA to identify effects of group (PD vs. Control) and condition (anticipation of loss vs. gain vs. neutral outcome) revealed that patients with PD showed significantly reduced bilateral ventral striatal activation during reward anticipation but increased activity during loss anticipation. Within the patient group, the degree of activation in the ventral striatum during loss-anticipation was positively correlated with harm avoidance and negatively correlated with novelty seeking. These findings suggest that behavioural impairments in panic disorder may be related to abnormal neural processing of motivational cues.

The Effects of Age, from Young to Middle Adulthood, and Gender on Resting State Functional Connectivity of the Dopaminergic Midbrain

Dysfunction of the dopaminergic ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) is implicated in psychiatric disorders including attention-deficit/ hyperactivity disorder (ADHD), addiction, schizophrenia and movement disorders such as Parkinson's disease (PD). Although the prevalence of these disorders varies by age and sex, the underlying neural mechanism is not well understood. The objective of this study was to delineate the distinct resting state functional connectivity (rsFC) of the VTA and SNc and examine the effects of age, from young to middle-adulthood, and sex on the rsFC of these two dopaminergic structures in a data set of 250 healthy adults (18-49 years of age, 104 men). Using blood oxygenation level dependent (BOLD) signals, we correlated the time course of the VTA and SNc to the time courses of all other brain voxels. At a corrected threshold, paired t-test showed stronger VTA connectivity to bilateral angular gyrus and superior/middle and orbital frontal regions and stronger SNc connectivity to the insula, thalamus, parahippocampal gyrus (PHG) and amygdala. Compared to women, men showed a stronger VTA/SNc connectivity to the left posterior orbital gyrus. In linear regressions, men but not women showed age-related changes in VTA/SNc connectivity to a number of cortical and cerebellar regions. Supporting shared but also distinct cerebral rsFC of the VTA and SNc and gender differences in age-related changes from young and middle adulthood in VTA/SNc connectivity, these new findings help advance our understanding of the neural bases of many neuropsychiatric illnesses that implicate the dopaminergic systems.

Toward a systems-oriented approach to the role of the extended amygdala in adaptive responding

Research into the structure and function of the basal forebrain macrostructure called the extended amygdala (EA) has recently seen considerable growth. This paper reviews that work, with the objectives of identifying underlying themes and developing a common goal towards which investigators of EA function might work. The paper begins with a brief review of the structure and the ontological and phylogenetic origins of the EA. It continues with a review of research into the role of the EA in both aversive and appetitive states, noting that these two seemingly disparate avenues of research converge on the concept of reinforcement - either negative or positive - of adaptive responding. These reviews lead to a proposal as to where the EA may fit in the organization of the basal forebrain, and an invitation to investigators to place their findings in a unifying conceptual framework of the EA as a collection of neural ensembles that mediate adaptive responding.

Transcranial Sonography Findings in Depression in Association With Psychiatric and Neurologic Diseases: A Review

The transcranial sonography (TCS) finding of reduced echogenicity of brainstem raphe (hypoechogenic BR) has been associated with depressive states. Here, we review the TCS studies in subjects with depressive disorders and with depression related to degenerative brain diseases, and compare the frequency and clinical correlates of hypoechogenic BR in these reports. Summarizing the data published so far, hypoechogenic BR is present in 67% (range, 37-95%) of depressed but only in 15% (5-36%) of nondepressed subjects without history of neurodegenerative disease. The finding of hypoechogenic BR in these subjects is associated with a relative risk of 3.03 (95% CI, 2.44-3.75; P < .001) of being diagnosed with depression. In patients with Parkinson's disease, hypoechogenic BR is present in 63% (35-92%) of depressed but only in 27% (10-62%) of nondepressed patients, resulting in a relative risk of 2.18 (95% CI, 1.80-2.66; P < .001) of being diagnosed with depression. Hypoechogenic BR is associated with depression in a number of neurological disorders such Huntington's disease, idiopathic Rapid Eye Movement (REM) sleep behavior disorder, myotonic dystrophies, and cerebral small vessel disease. Although some studies did not show any relationship between BR echogenicity and severity of depression, others suggest an association with higher severity of depression, or even with suicidal ideation. In one study BR hypoechogenicity was found to be associated with better responsivity to serotonin reuptake inhibitors. Further studies are warranted to compare the TCS findings of BR alteration with post-mortem histopathological findings, and with genetic variants related to cerebral serotonin metabolism.

Axonal branching patterns of ventral pallidal neurons in the rat

The ventral pallidum (VP) is a key component of the cortico-basal ganglia circuits that process motivational and emotional information, and also a crucial site for reward. Although the main targets of the two VP compartments, medial (VPm) and lateral (VPl) have already been established, the collateralization patterns of individual axons have not previously been investigated. Here we have fully traced eighty-four axons from VPm, VPl and the rostral extension of VP into the olfactory tubercle (VPr), using the anterograde tracer biotinylated dextran amine in the rat. Thirty to fifty percent of axons originating from VPm and VPr collateralized in the mediodorsal thalamic nucleus and lateral habenula, indicating a close association between the ventral basal ganglia-thalamo-cortical loop and the reward network at the single axon level. Additional collateralization of these axons in diverse components of the extended amygdala and corticopetal system supports a multisystem integration that may take place at the basal forebrain. Remarkably, we did not find evidence for a sharp segregation in the targets of axons arising from the two VP compartments, as VPl axons frequently collateralized in the caudal lateral hypothalamus and ventral tegmental area, the well-known targets of VPm, while VPm axons, in turn, also collateralized in typical VPl targets such as the subthalamic nucleus, substantia nigra pars compacta and reticulata, and retrorubral field. Nevertheless, VPl and VPm displayed collateralization patterns that paralleled those of dorsal pallidal components, confirming at the single axon level the parallel organization of functionally different basal ganglia loops.

D1 and D2 dopaminergic systems in the rat basolateral amygdala are involved in anxiogenic-like effects induced by histamine

Involvement of the dopamine receptors in the basolateral amygdala (BLA) in the effects of histamine on anxiety-like behaviors of the elevated plus maze in male Wistar rats was investigated. The results showed that bilateral intra-BLA injections of histamine (2.5, 5 and 7.5 µg/rat) induced an anxiogenic-like effect, revealed by decreases in percentage of open arm time (%OAT) and open arm entries (%OAE). Intra-BLA administration of dopamine D1 receptor agonist, SKF38393 (0.25 µg/rat), and dopamine D2 receptor agonist, quinpirole (0.03 and 0.05 µg/rat), decreased %OAT but not %OAE. Conversely, intra-BLA administration of dopamine D1 receptor antagonist, SCH23390 (0.5 and 1 µg/rat), and dopamine D2 receptor antagonist, sulpiride (0.3 and 0.5 µg/rat), increased %OAT and %OAE, suggesting an anxiolytic-like effect for both drugs. Interestingly, co-administration of a silent dose of SCH23390 or sulpiride prevented anxiogenic-like effects of SKF38393 and quinpirole, respectively. Conjoint administration of a sub-effective dose of SKF38393 (0.125 µg/rat) or quinpirole (0.01 µg/rat) along with lower doses of histamine (1 and 2.5 µg/rat) induced anxiolytic-like effects. On the other hand, intra-BLA pretreatment with a silent dose of SCH23390 (0.25 µg/rat) or sulpiride (0.1 µg/rat) prevented the anxiogenic-like effect of higher doses of histamine (5 and 7.5 µg/rat). No significant change was observed in total closed arm entries, as an index for motor activity of the animals. It can be concluded that the dopamine D1 and D2 receptors in the BLA may be involved in the anxiogenic-like effects induced by histamine.

Inputs to the midbrain dopaminergic complex in the rat, with emphasis on extended amygdala-recipient sectors

The midbrain dopaminergic neuronal groups A8, A9, A10, and A10dc occupy, respectively, the retrorubral field (RRF), substantia nigra compacta (SNc), ventral tegmental area (VTA), and ventrolateral periaqueductal gray (PAGvl). Collectively, these structures give rise to a mixed dopaminergic and nondopaminergic projection system that essentially permits adaptive behavior. However, knowledge is incomplete regarding how the afferents of these structures are organized. Although the VTA is known to receive numerous afferents from cortex, basal forebrain, and brainstem and the SNc is widely perceived as receiving inputs mainly from the striatum, the afferents of the RRF and PAGvl have yet to be assessed comprehensively. This study was performed to provide an account of those connections and to seek a better understanding of how afferents might contribute to the functional interrelatedness of the VTA, SNc, RRF, and PAGvl. Ventral midbrain structures received injections of retrograde tracer, and the resulting retrogradely labeled structures were targeted with injections of anterogradely transported Phaseolus vulgaris leucoagglutinin. Whereas all injections of retrograde tracer into the VTA, SNc, RRF, or PAGvl produced labeling in many structures extending from the cortex to caudal brainstem, pronounced labeling of structures making up the central division of the extended amygdala occurred following injections that involved the RRF and PAGvl. The anterograde tracing supported this finding, and the combination of retrograde and anterograde labeling data also confirmed reports from other groups indicating that the SNc receives robust input from many of the same structures that innervate the VTA, RRF, and PAGvl.

Supranuclear control of swallowing

Swallowing is an act requiring complex sensorimotor integration. Using a variety of methods first used to study limb physiology, initial efforts to study swallowing have yielded information that multiple cortical and subcortical regions are active participants. Not surprisingly, the regions activated appear to overlap those involved in both oral and nonoral motor behaviors. This review offers a perspective that considers the supranuclear control of swallowing in light of these physiological similarities.

Anxiolytic-like effect induced by the cannabinoid CB1 receptor agonist, arachydonilcyclopropylamide (ACPA), in the rat amygdala is mediated through the D1 and D2 dopaminergic systems

In the present study the influence of the dopaminergic system(s) of the amygdala on the anxiolytic-like effect of the cannabinoid CB1 receptor agonist, arachydonilcyclopropylamide (ACPA), in male Wistar rats was investigated. An elevated plus-maze test of anxiety was used to assess anxiety-like behaviors. The results showed that bilateral intra-amygdala injections of ACPA (0.125, 1.25 and 5 ng/rat) and the mixed dopamine D1/D2 receptor agonist, apomorphine, at different doses (0.001, 0.01 and 0.1 µg/rat) increased percentage open arm time (%OAT) and percentage open arm entries (%OAE), indicating an anxiolytic-like effect for both of the drugs. In contrast, intra-amygdala administration of the dopamine D1 receptor antagonist SCH23390 (0.5 and 1 µg/rat) and the dopamine D2 receptor antagonist, sulpiride (2 and 3 µg/rat) decreased %OAT and %OAE, suggesting an anxiogenic-like effect for both of the drugs. Interestingly, pretreatment with a sub-effective dose of apomorphine (0.0005 µg/rat) increased, while SCH23390 (0.25 µg/rat) and sulpiride (1.5 µg/rat) decreased the anxiolytic-like effect of ACPA. It can be concluded that the dopaminergic system of the amygdala may be involved, at least partly, in the anxiolytic-like effects induced by ACPA in the rat amygdala.

Demonstration of disturbed activity of the lateral amygdaloid nucleus projection neurons in depressed patients by the AgNOR staining method

BACKGROUND

The aim to find a morphological biomarker of disturbed activity of the lateral amygdaloid nucleus in depression was approached by a karyometric analysis of projection neurons.

METHODS

The study was performed on paraffin-embedded brains from 19 depressed patients from both the major depressive disorder (MDD) and the bipolar disorder (BD) diagnostic groups, including 10 suicides, and 24 matched controls. The karyometric parameters of the lateral amygdaloid nucleus (La) projection neurons bilaterally were evaluated by the argyrophilic nucleolar organiser region (AgNOR) silver staining method.

RESULTS

An increased AgNOR number was found in the right La in suicides compared to controls. The intra-group comparisons between the hemispheres suggest a disturbed amygdaloid lateralisation in depressed patients. The effects were independent from psychotropic medication. There was a strong positive correlation between the nuclear area in La projection neurons and prefrontal limbic areas pyramidal neurons in the right hemisphere specific for suicide and MDD.

LIMITATIONS

A major limitation of this study is the relatively small number of cases. A further limitation is given by the lack of data on drug exposure across the entire lifespan.

CONCLUSION

The results suggest that depressed patients from both the MDD and BD diagnostic groups exhibit an increased activity of the La output neurons specific for suicidal patients. The distinctness of the diagnostic groups of mood disorders was accentuated in the correlation analysis. This putative hyperactivity was specific for the right hemisphere and psychotropic medication most likely did not counteract it.

Fos after single and repeated self-administration of cocaine and saline in the rat: emphasis on the Basal forebrain and recalibration of expression

The effects of addictive psychostimulant drugs on the brain change over repeated administrations. We evaluated a large sample of brain structures, particularly ones comprising basal forebrain macrosystems, and determined in which the immediate-early gene product, Fos, is expressed following a single and repeated self-administrations of cocaine. The caudate-putamen and accumbens, comprising the basal ganglia input structures, and the hypothalamic supraoptic and paraventricular nuclei, lateral and medial habenula, mesopontine rostromedial tegmental nucleus and anterior cingulate cortex exhibited Fos expression enhanced by acute self-administration of cocaine (SAC), but desensitized after repeated administrations. Fos expression was mainly enhanced by acutely self-administered cocaine in basal ganglia output and intrinsic structures and the intermediate nucleus of lateral septum, medial division of the central amygdaloid nucleus and zona incerta, but, in contrast, was sensitized in these structures after repeated administrations. Acute and repeated SAC left Fos expression unaffected or marginally enhanced in most extended amygdala structures, of which nearly all, however, exhibited robustly increased Fos expression after repeated saline self-administration, occasionally to levels exceeding those elicited by cocaine. Thus, self-administered cocaine mainly elicits Fos expression, which persists or increases with repeated administrations in some structures, but declines in others. In addition, Fos expression is sensitized in most extended amygdala structures merely by the act of repeated self-administering. Similar spatiotemporal patterns of cocaine- or saline-elicited Fos expression characterize functionally related clusters of structures, such as, eg, basal ganglia input structures, basal ganglia output structures, extended amygdala and structures in the brainstem to which forebrain macrosystems project.

The lateral habenula: no longer neglected

In this contribution to the CNS Spectrums neuroanatomy series, Stefanie Geisler, MD, discusses the lateral habenula (LHb). This nuclear complex is one of the areas of the brain that forms part of the cross-talk between limbic fore-brain and some important ascending modulatory pathways. Situated at the caudal end of the dorsal diencephalon and classically regarded as projecting largely to the brainstem, including the serotoninergic raphe nuclei, the LHb receives afferents from widespread forebrain areas. Therefore, the LHb is able to influence serotonin tone in the brain, and has long interested neuroanatomists as a potential limbic-motor interface. Nonetheless, the LHb was not much discussed outside neuroanatomical circles until recently, when it was discovered that its impact on the mesotelencephalic dopamine system is probably much greater than had been assumed. The LHb has become a hot topic. This article-addresses these developments and emphasizes the clinical relevance of this interesting brain structure.