Tyrosine enhances behavioral and mesocorticolimbic dopaminergic responses to aversive conditioning

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.

Diosgenin normalization of disrupted behavioral and central neurochemical activity after single prolonged stress

Introduction: Post-traumatic stress disorder (PTSD) is a chronic mental illness triggered by traumatic experiences such as wars, natural disasters, or catastrophes, and it is characterized by anxiety, depression and cognitive impairment. Diosgenin is a steroidal sapogenin with known neuroprotective and antioxidant properties. This study aimed to assess the pharmacological potential of diosgenin in a single prolonged stress (SPS) model of PTSD, plus other behavioral models along with any consequent alterations in brain neurochemistry in male mice. Methodology: SPS was induced by restraining animals for 2 h, followed by 20 min of forced swim, recuperation for 15 min, and finally, exposure to ether to induce anesthesia. The SPS-exposed animals were treated with diosgenin (20, 40, and 60 mg/kg) and compared with the positive controls, fluoxetine or donepezil, then they were observed for any changes in anxiety/depression-like behaviors, and cognitive impairment. After behavioral screening, postmortem serotonin, noradrenaline, dopamine, vitamin C, adenosine and its metabolites inosine and hypoxanthine were quantified in the frontal cortex, hippocampus, and striatum by high-performance liquid chromatography. Additionally, animal serum was screened for changes in corticosterone levels. Results: The results showed that diosgenin reversed anxiety- and depression-like behaviors, and ameliorated cognitive impairment in a dose-dependent manner. Additionally, diosgenin restored monoamine and vitamin C levels dose-dependently and modulated adenosine and its metabolites in the brain regions. Diosgenin also reinstated otherwise increased serum corticosterone levels in SPS mice. Conclusion: The findings suggest that diosgenin may be a potential candidate for improving symptoms of PTSD.

Neuronal adaptations in the lateral habenula during drug withdrawal: Preclinical evidence for addiction therapy

The epithalamic lateral habenula (LHb) regulates monoaminergic systems and contributes to the expression of both appetitive and aversive behaviours. Over the past years, the LHb has emerged as a vulnerable brain structure in mental illnesses including addiction. Behavioural and functional evidence in humans and rodents provide substantial support for a role of LHb in the negative affective symptoms emerging during withdrawal from addictive substances. Multiple forms of cellular and synaptic adaptations that take hold during drug withdrawal within the LHb are causally linked with the emergence of negative affective symptoms. These results indicate that targeting drug withdrawal-driven adaptations in the LHb may represent a potential strategy to normalize drug-related behavioural adaptations. In the current review we describe the mechanisms leading to functional alterations in the LHb, as well as the existing interventions used to counteract addictive behaviours. Finally, closing this loop we discuss and propose new avenues to potentially target the LHb in humans in light of the mechanistic understanding stemming from pre-clinical studies. Altogether, we provide an overview on how to leverage cellular-level understanding to envision clinically-relevant approaches for the treatment of specific aspects in drug addiction.

L-Tyrosine availability affects basal and stimulated catecholamine indices in prefrontal cortex and striatum of the rat

We previously found that L-tyrosine (L-TYR) but not D-TYR administered by reverse dialysis elevated catecholamine synthesis in vivo in medial prefrontal cortex (MPFC) and striatum of the rat (Brodnik et al., 2012). We now report L-TYR effects on extracellular levels of catecholamines and their metabolites. In MPFC, reverse dialysis of L-TYR elevated in vivo levels of dihydroxyphenylacetic acid (DOPAC) (L-TYR 250-1000 μM), homovanillic acid (HVA) (L-TYR 1000 μM) and 3-methoxy-4-hydroxyphenylglycol (MHPG) (L-TYR 500-1000 μM). In striatum L-TYR 250 μM elevated DOPAC. We also examined L-TYR effects on extracellular dopamine (DA) and norepinephrine (NE) levels during two 30 min pulses (P2 and P1) of K+ (37.5 mM) separated by t = 2.0 h. L-TYR significantly elevated the ratio P2/P1 for DA (L-TYR 125 μM) and NE (L-TYR 125-250 μM) in MPFC but lowered P2/P1 for DA (L-TYR 250 μM) in striatum. Finally, we measured DA levels in brain slices using ex-vivo voltammetry. Perfusion with L-TYR (12.5-50 μM) dose-dependently elevated stimulated DA levels in striatum. In all the above studies, D-TYR had no effect. We conclude that acute increases within the physiological range of L-TYR levels can increase catecholamine metabolism and efflux in MPFC and striatum. Chronically, such repeated increases in L-TYR availability could induce adaptive changes in catecholamine transmission while amplifying the metabolic cost of catecholamine synthesis and degradation. This has implications for neuropsychiatric conditions in which neurotoxicity and/or disordered L-TYR transport have been implicated.

Higher striatal dopamine transporter density in PTSD: an in vivo SPECT study with [(99m)Tc]TRODAT-1

RATIONALE

Some evidence suggests a hyperdopaminergic state in posttraumatic stress disorder (PTSD). The 9-repetition allele (9R) located in the 3' untranslated region of the dopamine transporter (DAT) gene (SLC6A3) is more frequent among PTSD patients. In vivo molecular imaging studies have shown that healthy 9R carriers have increased striatal DAT binding. However, no prior study evaluated in vivo striatal DAT density in PTSD.

OBJECTIVES

The objective of this study was to evaluate in vivo striatal DAT density in PTSD.

METHODS

Twenty-one PTSD subjects and 21 control subjects, who were traumatized but asymptomatic, closely matched comparison subjects evaluated with the Clinician-Administered PTSD Scale underwent a single-photon emission computed tomography scan with [(99m)TC]-TRODAT-1. DAT binding potential (DAT-BP) was calculated using the striatum as the region of the interest and the occipital cortex as a reference region.

RESULTS

PTSD patients had greater bilateral striatal DAT-BP (mean ± SD; left, 1.80 ± 0.42; right, 1.78 ± 0.40) than traumatized control subjects (left, 1.62 ± 0.32; right, 1.61 ± 0.31; p = 0.039 for the left striatum and p = 0.032 for the right striatum).

CONCLUSIONS

These results provide the first in vivo evidence for increased DAT density in PTSD. Increases in DAT density may reflect higher dopamine turnover in PTSD, which could contribute to the perpetuation and potentiation of exaggerated fear responses to a given event associated with the traumatic experience. Situations that resemble the traumatic event turn to be interpreted as highly salient (driving attention, arousal, and motivation) in detriment of other daily situations.

Prefrontocortical dopamine loss in rats delays long-term extinction of contextual conditioned fear, and reduces social interaction without affecting short-term social interaction memory

Prefrontal dopamine loss delays extinction of cued fear conditioning responses, but its role in contextual fear conditioning has not been explored. Medial prefrontal lesions also enhance social interaction in rats, but the role of prefrontal dopamine loss on social interaction memory is not known. Besides, a role for subcortical accumbal dopamine on mnesic changes after prefrontal dopamine manipulation has been proposed but not explored. The objective was to study the involvement of dopaminergic neurotransmission in the medial prefrontal cortex (mPFC) and nucleus accumbens in two mnesic tasks: contextual fear conditioning and social interaction memory. For contextual fear conditioning, short- and long-term freezing responses after an electric shock were studied, as well as extinction retention. Regarding social interaction memory, the recognition of a juvenile, a very sensitive short-term memory test, was used. Dopamine loss was carried out by injection of 6-hydroxydopamine, and postmortem catecholamine levels were analyzed by high-performance liquid chromatography. Prefrontocortical dopamine loss (>76%) led to a reactive enhancement of accumbal dopamine content (p<0.01), supporting the hypothesis that a hyperdopaminergic tone emerges in the nucleus accumbens after prefrontocortical dopamine loss. In lesioned rats, long-term extinction of contextual fear conditioning was significantly delayed and extinction retention was impaired without changes in acquisition and short-term contextual fear conditioning and, on the other hand, acquisition and short-term social interaction memory were not affected, although time spent on social interaction was significantly reduced. Added dopamine loss in the nucleus accumbens (>76%) did not alter these behavioral changes. In summary, the results of the present study indicate that the dopaminergic network in the mPFC (but not in the nucleus accumbens) coordinates the normal long-term extinction of contextual fear conditioning responses without affecting their acquisition, and it is involved in time spent on social interaction, but not acquisition and short-term social interaction memory.

Fear-like biochemical and behavioral responses in rats to the predator odor, TMT, are dependent on the exposure environment

Several laboratories have reported that exposure to predator odor can result in stress-like effects in rodents. While some laboratories have reported fear-like alterations in behavior, other laboratories, including our own, have failed to consistently observe fearful behaviors in rats exposed to the predator odor TMT. One potential contributing factor to this discrepancy is the handling of the rat and its test environment. In the current report, we examine biochemical, endocrinological, and behavioral effects of TMT in two distinct open fields: one small, familiar, and dimly lit, while the other was large, novel, and brightly lit. Only exposure to TMT in the large, novel open field resulted in fearful behavior; however, no increase in dopamine turnover was noted compared to no odor and control odor rats. As expected, the different open fields resulted in some biochemical and behavioral differences, including more horizontal locomotion and less grooming, higher serum corticosterone, and increased dopamine turnover in the ventral prefrontal cortex in the large open field. Finally, compared to the same open field controls, TMT exposure elevated rat serum corticosterone levels in both open fields and dopamine turnover in the dorsal and ventral medial prefrontal cortex and amygdala of rats only in the small, familiar open field. These results indicate that the TMT-induced biochemical activation of may occur without detectable fearful behaviors and may indicate a mechanism that prepares the animal for the expression of a fearful response if additional provocative stimuli are present.

TMT, a predator odor, elevates mesoprefrontal dopamine metabolic activity and disrupts short-term working memory in the rat

Working memory has been proposed to require the proper functioning of the medial prefrontal cortex and its dopaminergic innervation. The dopaminergic input to the medial prefrontal cortex has been demonstrated to be sensitive to physical and psychological stress. In this report, we demonstrate that a brief exposure to 2, 5-dihydro-2,4,5-trimethylthiazoline (TMT), an odor derived from a predator of the rat, the fox, resulted in elevated dopamine metabolism in the medial prefrontal cortex and elevated serum corticosterone. We tested the effects of this olfactory stress on working memory using a spontaneous, delayed, non-matching-to-sample task using object recognition methods. Rats were exposed to one set of objects and, after a delay of 1, 15 or 60 min, later demonstrated a robust working memory of the familiar object compared to a novel object. When rats were exposed to TMT during the 15-min delay, working memory was disrupted without altering exploratory behavior. We conclude from these studies that (1) TMT selectively activates mesoprefrontal dopamine neurons, (2) TMT exposure can disrupt working memory and (3) this disruption in working memory is not due to an overall suppression of exploratory behavior but may involve altered mesoprefrontal dopaminergic activity.

The predator odor, TMT, displays a unique, stress-like pattern of dopaminergic and endocrinological activation in the rat

Predator odors may provide a species relevant aversive stimuli to study the central effects of stress in rats and may have several benefits over currently applied models. Here, we examined one such odor, TMT, isolated from the fox, a predator of the rat, on fear-induced behaviors, serum corticosterone, and central dopamine metabolism. Habituated rats were exposed to TMT, or a control odor, butyric acid, in an open field. For comparison, other rats were subjected to a model of conditioned fear - a traditional fear model. Several similarities between the two stresses were observed including increased serum corticosterone and increased dopamine metabolism in the medial prefrontal cortex. Differences were also observed. TMT, but not conditioned fear, activated dopamine metabolism in the amygdala, but not the nucleus accumbens core and shell. Rats exposed to conditioned fear, but not TMT odor, demonstrated altered behaviors associated with fear, including locomotion, grooming and immobility. Finally, rats reexposed to TMT after a 24-h delay did not demonstrate any of the changes observed with acute exposure to TMT. These data indicate acute exposure to a predator odor, TMT, can result in a unique pattern of biochemical activation that is similar, but not identical, to conditioned fear. The differences may indicate unique features of a central 'fear arousal' pathway that responds to innate, unlearned stressful stimuli, such as predator odors.

The role of mesoprefrontal dopamine neurons in the acquisition and expression of conditioned fear in the rat

The mesoprefrontal dopamine neurons are sensitive to physical, pharmacological and psychological stressors. In this report, the role of these neurons in the response to classical fear conditioning was investigated. 6-Hydroxydopamine lesions to the medial prefrontal cortex reduced dopamine levels to about 13% of controls but did not alter behavior during the acquisition of fear conditioning. As expected, conditioned fear increased dopamine metabolism (3,4-dihydroxyphenylacetic acid/dopamine ratio) in the nucleus accumbens in sham-lesion rats. The medial prefrontal 6-hydroxydopamine lesions did not alter this effect. During the expression, however, lesioned rats demonstrated a delayed extinction of the conditioned response without an overall increase in the initial conditioned response. This effect was consistent in rats receiving 6-hydroxydopamine lesions before or after the acquisition period. The calculated rates of extinction showed that the 6-hydroxydopamine lesioned rats had a reduced rate of extinction, but not acquisition, of fear conditioning. The results presented in this manuscript indicate that the mesoprefrontal dopamine neurons are involved in co-ordinating the normal extinction of a fear response but do not alter the acquisition of fearful behaviors. These data are consistent with the conclusion that the mesoprefrontal dopamine neurons are involved in maintaining the animal's response adaptability with regards to stress-related changes in the external environment.

An antisense oligonucleotide reverses the footshock-induced expression of fos in the rat medial prefrontal cortex and the subsequent expression of conditioned fear-induced immobility

The immediate-early genes, including c-fos, have been proposed to be involved in learning and memory. In this report, we examine stress-induced Fos-like immunoreactivity (Fos-li) in subregions of the prefrontal cortex during a conditioned fear paradigm. During the acquisition phase, the rats were conditioned to fear a formerly neutral tone by pairing the tone with a mild footshock. The rats were then tested for fearful behavior by reexposure to the tone without additional footshock. During acquisition, Fos-li was increased in the medial prefrontal cortex (infralimbic and prelimbic) but not the anterior cingulate and M1 motor cortex. However, during the extinction phase, no significant increase in Fos-li was observed in any region. These findings indicate that acquisition, but not extinction, of conditioned fear is associated with an increase in Fos-li in subregions of the medial prefrontal cortex. In other animals, an antisense oligonucleotide directed against the c-fos mRNA was injected into the infralimbic/prelimbic cortex 12 or 72 hr before the acquisition session. Antisense treatment given 12, but not 72, hr earlier suppressed Fos production without altering behavior during the acquisition session. Three days after the acquisition session, rats were tested for fearful behavior as before. The antisense oligonucleotide blockade of Fos production during acquisition was associated with a significantly less fearful response during the extinction session. These results support a role for Fos in the medial prefrontal cortex during the acquisition of aversive learning.

Mesoprefrontal dopaminergic neurons: can tyrosine availability influence their functions?

The dopamine (DA) neurons projecting to the prefrontal cortex (PFC) are thought to be involved in working memory, stress response, and the pathogenesis of schizophrenia. In this commentary, we review the current evidence supporting a precursor tyrosine dependence of these mesoprefrondal DN neurons. Several studies in rats employing different experimental paradigms [i.e. experimental diabetes and early-treated phenylketonuria (PKU) model] have shown that reduced tyrosine levels in brain can affect markedly the physiology and functions of these DA neurons. However, supplemental tyrosine is effective in enhancing functional transmitter outflow from mesoprefrontal DA neurons only under conditions where their physiological activity is enhanced and DA synthesis and release are uncoupled from intrinsic regulatory controls. Recent studies in humans have also suggested that variations in brain tyrosine levels can affect significantly higher cortical functions subserved by the PFC. In early-treated PKU patients with mildly reduced tyrosine levels, marked impairments in cognitive functions dependent on the dorsolateral PFC could be detected. In drug-treated schizophrenic patients, supplemental tyrosine was shown to have a disruptive effects on the smooth-pursuit eye movement performance task. Furthermore, tyrosine administration was effective in restoring impaired working memory in humans following cold stress paradigm, as assessed by a computer-based delayed matching to-sample memory task. These human studies, together with the current evidence obtained from animal experiments, suggest that the functions of the mesoprefrontal DA neurons can, under certain circumstances, be readily influenced by the availability of the precursor tyrosine.