Role of the mesoamygdaloid dopamine projection in emotional learning

Divergent anomaly in mesocorticolimbic dopaminergic circuits might be associated with different depressive behaviors, an animal study

BACKGROUND

The mesocorticolimbic dopamine system, which originates from the ventral tegmental area (VTA) and projects primarily to the prefrontal cortex (PFC), olfactory tubercle (OT), nucleus accumbens (NAc), dorsal striatum (ST), and the amygdala (AMy), plays a pivotal role in determining individual motivation and sensitivity to rewards, namely, anhedonia. Not all depressive individuals exhibited anhedonia, thus, it is natural to speculate that the heterogenous manifestations of depression might be related to the mesocorticolimbic dopamine system. Maternal deprivation (MD) and chronic unpredictable stress (CUPS) are two well-established depressogenic stressors, and they were proven to induce different depressive phenotypes.

METHODS

The depressive and anxiety-like behaviors of MD and CUPS-treated rats were measured by classical behavioral tests including open field, forced swimming, and sucrose preference test. The expression of D1-5 dopamine receptors and DAT mRNA and protein in the mesocorticolimbic dopamine system of rats exposed to MD and CUPS were measured by real-time PCR and Western blot, respectively.

RESULTS

Severe anhedonia was observed in MD but not CUPS rats. Divergent expression of D1 and D2 receptors and DAT mRNA and protein in the mesocorticolimbic dopamine system were found between MD and CUPS rats. Significant correlations between different depressive behaviors and D1-/D2-like receptors and DAT protein levels in the mesocorticolimbic dopamine system were observed.

CONCLUSION

Different depressive behaviors of rats such as anhedonia, passive coping behavior, and declined exploratory interest might be related to divergent dopaminergic pathways. Anhedonia is associated with the dysfunction of VTA-NAc and VTA-OT dopaminergic pathways, the passive coping behavior is related to the dysregulation of VTA-PFC and VTA-AMy pathways, and individual exploratory interest is associated with abnormal activity of VTA-PFC and VTA-ST pathways.

Sex and intrauterine growth restriction modify brain neurotransmitters profile of newborn piglets

The current study aimed to determine, using a swine model of intrauterine growth restriction (IUGR), whether short- and long-term neurological deficiencies and interactive dysfunctions of Low Birth-Weight (LBW) offspring might be related to altered pattern of neurotransmitters. Hence, we compared the quantities of different neurotransmitters (catecholamines and indoleamines), which were determined by HPLC, at brain structures related to the limbic system (hippocampus and amygdala) in 14 LBW and 10 Normal Body-Weight (NBW) newborn piglets. The results showed, firstly, significant effects of sex on the NBW newborns, with females having higher dopamine (DA) concentrations than males. The IUGR processes affected DA metabolism, with LBW piglets having lower concentrations of noradrenaline at the hippocampus and higher concentrations of the DA metabolites, homovanillic acid (HVA), at both the hippocampus and the amygdala than NBW neonates. The effects of IUGR were modulated by sex; there were no significant differences between LBW and NBW females, but LBW males had higher HVA concentration at the amygdala and higher concentration of 5-hydroxyindoleacetic acid, the serotonin metabolite, at the hippocampus than NBW males. In conclusion, the present study shows that IUGR is mainly related to changes, modulated by sex, in the concentrations of catecholamine neurotransmitters, which are related to adaptation to physical activity and to essential cognitive functions such as learning, memory, reward-motivated behavior and stress.

Emotional memory impairments induced by AAV-mediated overexpression of human α-synuclein in dopaminergic neurons of the ventral tegmental area

Parkinson's disease (PD) is associated with extensive degeneration of dopaminergic neurons originating in the substantia nigra pars compacta, but neuronal loss is also found in the ventral tegmental area (VTA). The VTA projects to areas involved in cognitive and emotional processes, including hippocampus, amygdala, nucleus accumbens and prefrontal cortex, and has thus been proposed to play a role in emotional memory impairments in PD. Since the formation of α-synuclein inclusions throughout the central nervous system is a pathological hallmark of PD, we studied the progressive effects of α-synuclein overexpression in the VTA on motor functions, emotional behaviour and emotional memory. Adeno-associated viral (AAV) vectors encoding either human α-synuclein or green fluorescent protein (GFP) were injected stereotactically into the VTA, and behaviour was monitored 3 and 8 weeks following AAV injection. At week 8, there was a 22% reduction of TH+ neurons in the VTA. We demonstrate that α-synuclein overexpression in dopaminergic neurons of the VTA induced mild motor deficits that appeared 3 weeks following AAV-α-synuclein injection and were aggravated at week 8. No depressive- or anxiety-like behaviours were found. To address emotional memory, we used the passive avoidance test, a one-trial associative learning paradigm based on contextual conditioning which requires minimal training. Interestingly, emotional memory impairments were found in α-synuclein overexpressing animals at week 8. These findings indicate that α-synuclein overexpression induces progressive memory impairments likely caused by a loss of function of mesolimbic dopaminergic projections.

Dopamine signals and physiological origin of cognitive dysfunction in Parkinson's disease

The pathological hallmark of Parkinson's disease (PD) is the degeneration of midbrain dopamine neurons. Cognitive dysfunction is a feature of PD patients even at the early stages of the disease. Electrophysiological studies on dopamine neurons in awake animals provide contradictory accounts of the role of dopamine. These studies have established that dopamine neurons convey a unique signal associated with rewards rather than cognitive functions. Emphasizing their role in reward processing leads to difficulty in developing hypothesis as to how cognitive impairments in PD are associated with the degeneration of dopamine circuitry. A hint to resolve this contradiction came from recent electrophysiological studies reporting that dopamine neurons transmit more diverse signals than previously thought. These studies suggest that dopamine neurons are divided into at least two functional subgroups, one signaling "motivational value" and the other signaling "salience." The former subgroup fits well with the conventional reward theory, whereas the latter subgroup has been shown to transmit signals related to salient but non-rewarding experiences such as aversive stimulations and cognitively demanding situations. This article reviews recent advances in understanding the non-reward functions of dopamine, and then discusses the possibility that cognitive dysfunction in PD is at least partially caused by the degeneration of the dopamine neuron subgroup signaling the salience of events in the environment.

A motivation model for interaction between parent and child based on the need for relatedness

In parent-child communication, emotions are evoked by various types of intrinsic and extrinsic motivation. Those emotions encourage actions that promote more interactions. We present a motivation model of infant-caregiver interactions, in which relatedness, one of the most important basic psychological needs, is a variable that increases with experiences of emotion sharing. Besides being an important factor of pleasure, relatedness is a meta-factor that affects other factors such as stress and emotional mirroring. The proposed model is implemented in an artificial agent equipped with a system to recognize gestures and facial expressions. The baby-like agent successfully interacts with an actual human and adversely reacts when the caregiver suddenly ceases facial expressions, similar to the “still-face paradigm” demonstrated by infants in psychological experiments. In the simulation experiment, two agents, each controlled by the proposed motivation model, show relatedness-dependent emotional communication that mimics actual human communication.

Protein kinase G linked to dopamine D3 receptors in the dorsal striatum controls dopamine release, ΔFosB expression and locomotor activity after repeated cocaine administration

Protein kinase G (PKG) has been implicated in a variety of physiological functions including synaptic plasticity in the brain. This study investigated the involvement of dopamine D3 (D3) receptors in PKG-regulated dopamine release, long-term changes in gene expression and behavioral sensitization after repeated cocaine administration. Repeated systemic injections of cocaine (20mg/kg), once a day for seven consecutive days, increased extracellular dopamine concentrations in the dorsal striatum. Inhibition of neuronal nitric oxide synthase, cGMP or PKG, stimulation of D3 receptors, and simultaneous inhibition of each of them with D3 receptor stimulation decreased the repeated cocaine-induced increase in dopamine concentrations and locomotor activity. Similarly, inhibition of PKG and simultaneous inhibition of PKG with D3 receptor stimulation decreased ΔFosB immunoreactivity elevated by repeated cocaine administration, however stimulation of D3 receptors alone did not. These findings suggest that activation of PKG after repeated cocaine administration is more sensitive to interact with D3 receptors in the dopamine terminals than those in medium spiny neurons. This interaction may result in the development of behavioral sensitization by the upregulation of dopamine releases in the dorsal striatum.

Establishing causality for dopamine in neural function and behavior with optogenetics

Dopamine (DA) is known to play essential roles in neural function and behavior. Accordingly, DA neurons have been the focus of intense experimental investigation that has led to many important advances in our understanding of how DA influences these processes. However, it is becoming increasingly appreciated that delineating the precise contributions of DA neurons to cellular, circuit, and systems-level phenomena will require more sophisticated control over their patterns of activity than conventional techniques can provide. Specifically, the roles played by DA neurons are likely to depend on their afferent and efferent connectivity, the timing and length of their neural activation, and the nature of the behavior under investigation. Recently developed optogenetic tools hold great promise for disentangling these complex issues. Here we discuss the use of light-sensitive microbial opsins in the context of outstanding questions in DA research. A major technical advance offered by these proteins is the ability to bidirectionally modulate DA neuron activity in in vitro and in vivo preparations on a time scale that more closely approximates those of neural, perceptual and behavioral events. In addition, continued advances in rodent genetics and viral-mediated gene delivery have contributed to the ability to selectively target DA neurons or their individual afferent and efferent connections. Further, these tools are suitable for use in experimental subjects engaged in complex behaviors. After reviewing the strengths and limitations of optogenetic methodologies, we conclude by describing early efforts in the application of this valuable new approach that demonstrate its potential to improve our understanding of the neural and behavioral functions of DA. This article is part of a Special Issue entitled Optogenetics (7th BRES).

Effects of sensitization on the detection of an instrumental contingency

While prior exposure to drugs of abuse permanently changes many behaviors, the underlying psychological mechanisms are relatively obscure. Here, the effects of sensitization on the detection of an action-outcome relationship were assessed, using a particularly stringent contingency degradation procedure. Rats were trained to leverpress until the probability of reinforcement for a response on one lever, or alternative reinforcement for a response on a second lever was reduced to 0.05 per second. Sensitization was then carried out (1mg/kg d-amphetamine/day for 7 days). Then, one reinforcer was also made available for a lack of response on either lever (p=0.05/s), maintaining its contiguity with the original response but eliminating its contingent relationship. Sensitized animals were more active, particularly early in the contingency degradation phase, but reduced responding directed at the degraded action-outcome contingency at a similar rate as controls. However, controls also reduced responding directed at the nondegraded contingency until very late in training, while sensitized animals maintained nondegraded responding at baseline levels. It was suggested that the relatively specific response shown by sensitized animals may reflect either improved action-outcome utilization or discrimination of relevant task features.

Dopamine in motivational control: rewarding, aversive, and alerting

Midbrain dopamine neurons are well known for their strong responses to rewards and their critical role in positive motivation. It has become increasingly clear, however, that dopamine neurons also transmit signals related to salient but nonrewarding experiences such as aversive and alerting events. Here we review recent advances in understanding the reward and nonreward functions of dopamine. Based on this data, we propose that dopamine neurons come in multiple types that are connected with distinct brain networks and have distinct roles in motivational control. Some dopamine neurons encode motivational value, supporting brain networks for seeking, evaluation, and value learning. Others encode motivational salience, supporting brain networks for orienting, cognition, and general motivation. Both types of dopamine neurons are augmented by an alerting signal involved in rapid detection of potentially important sensory cues. We hypothesize that these dopaminergic pathways for value, salience, and alerting cooperate to support adaptive behavior.