Dopamine release in the nucleus accumbens is altered following traumatic brain injury

Serotonergic Regulation of Synaptic Dopamine Levels Mitigates L-DOPA-Induced Dyskinesia in a Mouse Model of Parkinson's Disease

Background

The serotonin (5-HT) system can manipulate the processing of exogenous L-DOPA in the DA-denervated striatum, resulting in the modulation of L-DOPA-induced dyskinesia (LID).

Objective

To characterize the effects of the serotonin precursor 5-hydroxy-tryptophan (5-HTP) or the serotonin transporter (SERT) inhibitor, Citalopram on L-DOPA-induced behavior, neurochemical signals, and underlying protein expressions in an animal model of Parkinson's disease.

Methods

MitoPark (MP) mice at 20 weeks of age, subjected to a 14-day administration of L-DOPA/Carbidopa, displayed dyskinesia, referred to as LID. Subsequent investigations explored the effects of 5-HT-modifying agents, such as 5-HTP and Citalopram, on abnormal involuntary movements (AIMs), locomotor activity, neurochemical signals, serotonin transporter activity, and protein expression in the DA-denervated striatum of LID MP mice.

Results

5-HTP exhibited duration-dependent suppressive effects on developing and established LID, especially related to abnormal limb movements observed in L-DOPA-primed MP mice. However, Citalopram, predominantly suppressed abnormal axial movement induced by L-DOPA in LID MP mice. We demonstrated that 5-HTP could decrease L-DOPA-upregulation of DA turnover rates while concurrently upregulating 5-HT metabolism. Additionally, 5-HTP was shown to reduce the expressions of p-ERK and p-DARPP-32 in the striatum of LID MP mice. The effect of Citalopram in alleviating LID development may be attributed to downregulation of SERT activity in the dorsal striatum of LID MP mice.

Conclusions

While both single injection of 5-HTP and Citalopram effectively mitigated the development of LID, the difference in mitigation of AIM subtypes may be linked to the unique effects of these two serotonergic agents on L-DOPA-derived DA and 5-HT metabolism.

PT320, a Sustained-Release GLP-1 Receptor Agonist, Ameliorates L-DOPA-Induced Dyskinesia in a Mouse Model of Parkinson's Disease

To determine the efficacy of PT320 on L-DOPA-induced dyskinetic behaviors, and neurochemistry in a progressive Parkinson's disease (PD) MitoPark mouse model. To investigate the effects of PT320 on the manifestation of dyskinesia in L-DOPA-primed mice, a clinically translatable biweekly PT320 dose was administered starting at either 5 or 17-weeks-old mice. The early treatment group was given L-DOPA starting at 20 weeks of age and longitudinally evaluated up to 22 weeks. The late treatment group was given L-DOPA starting at 28 weeks of age and longitudinally observed up to 29 weeks. To explore dopaminergic transmission, fast scan cyclic voltammetry (FSCV) was utilized to measure presynaptic dopamine (DA) dynamics in striatal slices following drug treatments. Early administration of PT320 significantly mitigated the severity L-DOPA-induced abnormal involuntary movements; PT320 particularly improved excessive numbers of standing as well as abnormal paw movements, while it did not affect L-DOPA-induced locomotor hyperactivity. In contrast, late administration of PT320 did not attenuate any L-DOPA-induced dyskinesia measurements. Moreover, early treatment with PT320 was shown to not only increase tonic and phasic release of DA in striatal slices in L-DOPA-naïve MitoPark mice, but also in L-DOPA-primed animals. Early treatment with PT320 ameliorated L-DOPA-induced dyskinesia in MitoPark mice, which may be related to the progressive level of DA denervation in PD.

Shape analysis of the subcortical structures in North Korean refugees with post-traumatic stress disorder and major depressive disorder

BACKGROUND

Despite the growing number of refugees and their mental health issues, neurobiological mechanisms to explain clinical symptoms resulting from traumatic events, such as post-traumatic stress disorder (PTSD) or major depressive disorder (MDD), have not been extensively investigated. Research on the mental health of North Korean refugees (NKRs) who defected to South Korea for resettlement is still at an early stage but commonly reports structural and functional abnormalities in brain regions related to reward and motivational processing. The nucleus accumbens (NAc) and ventral pallidum (VP) are the major sites in subcortical structures that play key roles in reward and motivation.

METHODS

The present study examined subcortical structural abnormalities of 28 NKRs and age-, sex- matched South Korean Controls (SKCs) using shape analysis at the vertex level.

RESULTS

Among the 28 NKRs, 18 had psychiatric disorders, including PTSD and MDD. The NKRs showed significantly reduced volumes in the right NAc and bilateral VP compared to the SKRs. The volume of the right VP showed a significant negative correlation with current PTSD severity in the NKR group.

CONCLUSIONS

Our findings demonstrated that structural alterations of the NAc and VP may explain PTSD and MDD observed in the refugees and further suggest that the aftereffect of trauma, manifested as anhedonia and anxiety, may show chronically.

Structural and Functional Alterations of Substantia Nigra and Associations With Anxiety and Depressive Symptoms Following Traumatic Brain Injury

Backgrounds

Although there are a certain number of studies dedicated to the disturbances of the dopaminergic system induced by traumatic brain injury (TBI), the associations of abnormal dopaminergic systems with post-traumatic anxiety and depressive disorders and their underlying mechanisms have not been clarified yet. In the midbrain, dopaminergic neurons are mainly situated in the substantia nigra (SN) and the ventral tegmental area (VTA). Thus, we selected SN and VTA as regions of interest and performed a seed-based global correlation to evaluate the altered functional connectivity throughout the dopaminergic system post-TBI.

Methods

Thirty-three individuals with TBI and 21 healthy controls were recruited in the study. Anxiety and depressive symptoms were examined by the Hospital Anxiety and Depression Scale. All MRI data were collected using a Siemens Prisma 3.0 Tesla MRI system. The volume of SN and the global functional connectivity of the SN and VTA were analyzed.

Results

In the present study, patients with TBI reported more anxiety and depressive symptoms. More importantly, some structural and functional alterations, such as smaller SN and reduced functional connectivity in the left SN, were seen in individuals with TBI. Patients with TBI had smaller substantia nigra on both right and left sides, and the left substantia nigra was relatively small in contrast with the right one. Among these findings, functional connectivity between left SN and left angular gyrus was positively associated with post-traumatic anxiety symptoms and negatively associated with depressive symptoms.

Conclusions

The TBI causes leftward lateralization of structural and functional alterations in the substantia nigra. An impaired mesocortical functional connectivity might be implicated in post-traumatic anxiety and depression.

Phasic stimulation in the nucleus accumbens enhances learning after traumatic brain injury

Traumatic brain injury (TBI) is a significant cause of morbidity and mortality worldwide. Despite improvements in survival, treatments that improve functional outcome remain lacking. There is, therefore, a pressing need to develop novel treatments to improve functional recovery. Here, we investigated task-matched deep-brain stimulation of the nucleus accumbens (NAc) to augment reinforcement learning in a rodent model of TBI. We demonstrate that task-matched deep brain stimulation (DBS) of the NAc can enhance learning following TBI. We further demonstrate that animals receiving DBS exhibited greater behavioral improvement and enhanced neural proliferation. Treated animals recovered to an uninjured behavioral baseline and showed retention of improved performance even after stimulation was stopped. These results provide encouraging early evidence for the potential of NAc DBS to improve functional outcomes following TBI and that its effects may be broad, with alterations in neurogenesis and synaptogenesis.

Tetrabenazine Mitigates Aberrant Release and Clearance of Dopamine in the Nigrostriatal System, and Alleviates L-DOPA-Induced Dyskinesia in a Mouse Model of Parkinson's Disease

BACKGROUND

L-DOPA-induced dyskinesia (LID), occurring with aberrant processing of exogenous L-DOPA in the dopamine-denervated striatum, is a main complication of levodopa treatment in Parkinson's disease.

OBJECTIVE

To characterize the effects of the vesicular antagonist tetrabenazine (TBZ) on L-DOPA-induced behavior, neurochemical signals, and underlying protein expressions in an animal model of Parkinson's disease.

METHODS

20-week-old MitoPark mice were co-treated or separately administered TBZ and L-DOPA for 14 days. Abnormal involuntary movements (AIMs) and locomotor activity were analyzed. To explore dopamine (DA) transmission, fast scan cyclic voltammetry was used to assess presynaptic DA dynamics in striatal slices following treatments. PET imaging with 4-[18F]-PE2I, ADAM and immunoblotting assays were used to detect receptor protein changes in the DA-denervated striatum. Finally, nigrostriatal tissues were collected for HPLC measures of DA, serotonin and their metabolites.

RESULTS

A single injection of TBZ given in the interval between the two L-DOPA/Carbidopa treatments significantly attenuated L-DOPA-induced AIMs expression and locomotor hyperactivity. TBZ was shown to reduce tonic and phasic release of DA following L-DOPA treatment in DA-denervated striatal tissue. In the DA-depleted striatum, TBZ decreased the expression of L-DOPA-enhanced D1 receptors and the serotonin reuptake transporter. Neurochemical analysis indicated that TBZ attenuated L-DOPA-induced surges of DA levels by promoting DA turnover in the nigrostriatal system.

CONCLUSIONS

Our findings demonstrate that TBZ diminishes abnormal striatal DA transmission, which involves the ability of TBZ to modulate the presymptomatic dynamics of DA, and then mitigate aberrant release of exogenous L-DOPA from nerve terminals. The results support the potential of repositioning TBZ to counteract LID development.

Role of the Dopaminergic System in the Striatum and Its Association With Functional Recovery or Rehabilitation After Brain Injury

Disabilities are estimated to occur in approximately 2% of survivors of traumatic brain injury (TBI) worldwide, and disability may persist even decades after brain injury. Facilitation or modulation of functional recovery is an important goal of rehabilitation in all patients who survive severe TBI. However, this recovery tends to vary among patients because it is affected by the biological and physical characteristics of the patients; the types, doses, and application regimens of the drugs used; and clinical indications. In clinical practice, diverse dopaminergic drugs with various dosing and application procedures are used for TBI. Previous studies have shown that dopamine (DA) neurotransmission is disrupted following moderate to severe TBI and have reported beneficial effects of drugs that affect the dopaminergic system. However, the mechanisms of action of dopaminergic drugs have not been completely clarified, partly because dopaminergic receptor activation can lead to restoration of the pathway of the corticobasal ganglia after injury in brain structures with high densities of these receptors. This review aims to provide an overview of the functionality of the dopaminergic system in the striatum and its roles in functional recovery or rehabilitation after TBI.

Sustained Release GLP-1 Agonist PT320 Delays Disease Progression in a Mouse Model of Parkinson's Disease

GLP-1 agonists have become increasingly interesting as a new Parkinson's disease (PD) clinical treatment strategy. Additional preclinical studies are important to validate this approach and define the disease stage when they are most effective. We hence characterized the efficacy of PT320, a sustained release formulation of the long acting GLP-1 agonist, exenatide, in a progressive PD (MitoPark) mouse model. A clinically translatable biweekly PT320 dose was administered starting at 5 weeks of age and longitudinally evaluated to 24 weeks, and multiple behavioral/cellular parameters were measured. PT320 significantly improved spontaneous locomotor activity and rearing in MitoPark PD mice. "Motivated" behavior also improved, evaluated by accelerating rotarod performance. Behavioral improvement was correlated with enhanced cellular and molecular indices of dopamine (DA) midbrain function. Fast scan cyclic voltammetry demonstrated protection of striatal and nucleus accumbens DA release and reuptake in PT320 treated MitoPark mice. Positron emission tomography showed protection of striatal DA fibers and tyrosine hydroxylase protein expression was augmented by PT320 administration. Early PT320 treatment may hence provide an important neuroprotective therapeutic strategy in PD.

A Role for the Amygdala in Impairments of Affective Behaviors Following Mild Traumatic Brain Injury

Mild traumatic brain injury (TBI) results in chronic affective disorders such as depression, anxiety, and fear that persist up to years following injury and significantly impair the quality of life for patients. Although a great deal of research has contributed to defining symptoms of mild TBI, there are no adequate drug therapies for brain-injured individuals. Preclinical studies have modeled these deficits in affective behaviors post-injury to understand the underlying mechanisms with a view to developing appropriate treatment strategies. These studies have also unveiled sex differences that contribute to the varying phenotypes associated with each behavior. Although clinical and preclinical studies have viewed these behavioral deficits as separate entities with unique neurobiological mechanisms, mechanistic similarities suggest that a novel approach is needed to advance research on drug therapy. This review will discuss the circuitry involved in the expression of deficits in affective behaviors following mild TBI in humans and animals and provide evidence that the manifestation of impairment in these behaviors stems from an amygdala-dependent emotional processing deficit. It will highlight mechanistic similarities between these different types of affective behaviors that can potentially advance mild TBI drug therapy by investigating treatments for the deficits in affective behaviors as one entity, requiring the same treatment.

Injury during adolescence leads to sex-specific executive function deficits in adulthood in a pre-clinical model of mild traumatic brain injury

Adolescents are more likely than adults to develop chronic symptoms, such as impulsivity and difficulty concentrating, following a mild traumatic brain injury (mTBI) which may relate to disruption of pre-frontal cortex (PFC development). During adolescence the PFC is undergoing extensive remodelling, driving maturation of executive functions incorporating attention, motivation and impulse control. In part maturation of the PFC is driven by outgrowth of dopaminergic neurons to the PFC under the guidance of specific axonal targeting cues, including netrin-1. How a mTBI in adolescence may alter the expression of these axonal targeting cues, and the influence on PFC development is not yet known. As such the effects of mTBI in mid-adolescence on executive functioning in adulthood (12 weeks) were examined via the 5-choice serial reaction task in both male and female Sprague Dawley rats. Animals at p35 (n = 12-16 per group) were injured via weight drop (100 g from 0.75 m) and injury confirmed by a significant increase in righting reflex. Interestingly, while a mid-adolescence mTBI in females led to significantly higher omissions and decreased accuracy when task difficulty was high (stimulus duration 1 s), males had significantly increased premature response rate when the intertrial interval was varied. Examination of levels of TH, as a reflection of dopaminergic innervation, found no difference in either gender post-TBI in the PFC, but a significant increase in the limbic system (nucleus accumbens) in males, but not females, chronically post-TBI, suggesting an imbalance between the regions. The increase in TH was accompanied by a chronic reduction in netrin-1 within the nucleus accumbens in males only. Taken together, these results indicate that mTBI in adolescence leads to sex specific effects in different domains of PFC function in adulthood, which may relate to subtle alterations in the developmental trajectory of the mesocortical limbic pathway in males only.

Mild traumatic brain injury impacts associations between limbic system microstructure and post-traumatic stress disorder symptomatology

BACKGROUND

Post-traumatic stress disorder (PTSD) is a psychiatric disorder that afflicts many individuals, yet the neuropathological mechanisms that contribute to this disorder remain to be fully determined. Moreover, it is unclear how exposure to mild traumatic brain injury (mTBI), a condition that is often comorbid with PTSD, particularly among military personnel, affects the clinical and neurological presentation of PTSD. To address these issues, the present study explores relationships between PTSD symptom severity and the microstructure of limbic and paralimbic gray matter brain regions, as well as the impact of mTBI comorbidity on these relationships.

METHODS

Structural and diffusion MRI data were acquired from 102 male veterans who were diagnosed with current PTSD. Diffusion data were analyzed with free-water imaging to quantify average CSF-corrected fractional anisotropy (FA) and mean diffusivity (MD) in 18 limbic and paralimbic gray matter regions. Associations between PTSD symptom severity and regional average dMRI measures were examined with repeated measures linear mixed models. Associations were studied separately in veterans with PTSD only, and in veterans with PTSD and a history of military mTBI.

RESULTS

Analyses revealed that in the PTSD only cohort, more severe symptoms were associated with higher FA in the right amygdala-hippocampus complex, lower FA in the right cingulate cortex, and lower MD in the left medial orbitofrontal cortex. In the PTSD and mTBI cohort, more severe PTSD symptoms were associated with higher FA bilaterally in the amygdala-hippocampus complex, with higher FA bilaterally in the nucleus accumbens, with lower FA bilaterally in the cingulate cortex, and with higher MD in the right amygdala-hippocampus complex.

CONCLUSIONS

These findings suggest that the microstructure of limbic and paralimbic brain regions may influence PTSD symptomatology. Further, given the additional associations observed between microstructure and symptom severity in veterans with head trauma, we speculate that mTBI may exacerbate the impact of brain microstructure on PTSD symptoms, especially within regions of the brain known to be vulnerable to chronic stress. A heightened sensitivity to the microstructural environment of the brain could partially explain why individuals with PTSD and mTBI comorbidity experience more severe symptoms and poorer illness prognoses than those without a history of brain injury. The relevance of these microstructural findings to the conceptualization of PTSD as being a disorder of stress-induced neuronal connectivity loss is discussed.

Experimental Traumatic Brain Injury during Adolescence Enhances Cocaine Rewarding Efficacy and Dysregulates Dopamine and Neuroimmune Systems in Brain Reward Substrates

Although clinical studies identify traumatic brain injury (TBI) as a risk factor for the development of substance use disorder, much remains unknown about the possible underlying pathogenesis and age-specific effects. Thus, the aim of this study is to test the hypothesis that at an age of ongoing maturation, adolescent TBI alters elements of the reward pathway, resulting in increased sensitivity to the rewarding effects of a subthreshold dose of cocaine that does not induce significant behavioral changes in naïve, non-injured mice. Specifically, these results were derived from the combination of the controlled cortical impact model of TBI, performed on either adolescent (6 weeks) or young adult (8 weeks) mice, followed by the cocaine-induced conditioned place preference assay 2 weeks later. Using three-dimensional isosurface rendering and volumetric image analysis, TBI was found to induce neuromorphological changes such as decreased dendritic complexity and reduced spine density in brain regions essential for reward perception and processing of drug-induced euphoria. Further, we demonstrated that these neuronal changes may affect the differential expression of dopamine-associated genes. Our analysis also provided evidence for age-related differences in immune response and the distinct involvement of augmented microglial phagocytic activity in the remodeling of neuronal structures in the adolescent TBI brain. Our studies suggest that TBI during adolescence, a period associated with ongoing maturation of dopaminergic systems, may subsequently enhance the abuse liability of cocaine in adulthood.

Delayed Dopamine Dysfunction and Motor Deficits in Female Parkinson Model Mice

This study analyzed gender differences in the progressive dopamine (DA) deficiency phenotype in the MitoPark (MP) mouse model of Parkinson’s disease (PD) with progressive loss of DA release and reuptake in midbrain DA pathways. We found that the progressive loss of these DA presynaptic parameters begins significantly earlier in male than female MP mice. This was correlated with behavioral gender differences of both forced and spontaneous motor behavior. The degeneration of the nigrostriatal DA system in MP mice is earlier and more marked than that of the mesolimbic DA system, with male MP mice again being more strongly affected than female MP mice. After ovariectomy, DA presynaptic and behavioral changes in female mice become very similar to those of male animals. Our results suggest that estrogen, either directly or indirectly, is neuroprotective in the midbrain DA system. Our results are compatible with epidemiological data on incidence and symptom progression in PD, showing that men are more strongly affected than women at early ages.

Differential mitochondrial morphology in ventral striatal projection neuron subtypes

The two striatal projection neuron subtypes (medium spiny neurons- MSNs), those enriched in dopamine receptor 1 versus 2 (D1-MSNs and D2-MSNs), display dichotomous properties at the level of the transcriptome, projections, morphology, and electrophysiology. Recent work illustrates dichotomous mitochondrial length in NAc MSN subtype dendrites after cocaine self-administration, with a shift toward smaller mitochondria, due to enhanced fission, occurring in D1-MSN dendrites and a shift toward larger mitochondria in D2-MSN dendrites. However, to date there has been no comparison of mitochondrial morphological properties between MSN subtypes. In this study, we examine mitochondrial morphology in NAc D1-MSNs versus D2-MSNs. We observe an increase in the frequency of smaller length mitochondria in D2-MSN dendrites relative to D1-MSN dendrites, while D1-MSN dendrites display an increase in larger length mitochondria. The differences in mitochondrial length occur in both NAc core and shell, although to a greater extent in NAc core. Finally, we demonstrate that the mitochondrial fusion molecule, Opa1, is differentially expressed in NAc MSN subtypes, with D1-MSNs displaying higher expression of Opa1 ribosome-associated mRNA. The difference in Opa1 levels may account for the bias toward enhanced smaller mitochondria in D2-MSNs and enhanced larger mitochondria in D1-MSNs. Collectively, our study demonstrates differential mitochondrial size and a potential molecular mediator of these mitochondrial differences in NAc MSN subtypes.

Executive (dys)function after traumatic brain injury: special considerations for behavioral pharmacology

Executive function is an umbrella term that includes cognitive processes such as decision-making, impulse control, attention, behavioral flexibility, and working memory. Each of these processes depends largely upon monoaminergic (dopaminergic, serotonergic, and noradrenergic) neurotransmission in the frontal cortex, striatum, and hippocampus, among other brain areas. Traumatic brain injury (TBI) induces disruptions in monoaminergic signaling along several steps in the neurotransmission process - synthesis, distribution, and breakdown - and in turn, produces long-lasting deficits in several executive function domains. Understanding how TBI alters monoamingeric neurotransmission and executive function will advance basic knowledge of the underlying principles that govern executive function and potentially further treatment of cognitive deficits following such injury. In this review, we examine the influence of TBI on the following measures of executive function - impulsivity, behavioral flexibility, and working memory. We also describe monoaminergic-systems changes following TBI. Given that TBI patients experience alterations in monoaminergic signaling following injury, they may represent a unique population with regard to pharmacotherapy. We conclude this review by discussing some considerations for pharmacotherapy in the field of TBI.

Brain interrupted: Early life traumatic brain injury and addiction vulnerability

Recent reports provide evidence for increased risk of substance use disorders (SUD) among patients with a history of early-life traumatic brain injury (TBI). Preclinical research utilizing animal models of TBI have identified injury-induced inflammation, blood-brain barrier permeability, and changes to synapses and neuronal networks within regions of the brain associated with the perception of reward. Importantly, these reward pathway networks are underdeveloped during childhood and adolescence, and early-life TBI pathology may interrupt ongoing maturation. As such, maladaptive changes induced by juvenile brain injury may underlie increased susceptibility to SUD. In this review, we describe the available clinical and preclinical evidence that identifies SUD as a persistent psychiatric consequence of pediatric neurotrauma by discussing (1) the incidence of early-life TBI, (2) how preclinical studies model TBI and SUD, (3) TBI-induced neuropathology and neuroinflammation in the corticostriatal regions of the brain, and (4) the link between childhood or adolescent TBI and addiction in adulthood. In summary, preclinical research utilizes an innovative combination of models of early-life TBI and SUD to recapitulate clinical features and to determine how TBI promotes a risk for the development of SUD. However, causal processes that link TBI and SUD remain unclear. Additional research to identify and therapeutically target underlying mechanisms of aberrant reward pathway development will provide a launching point for TBI and SUD treatment strategies.

Release parameters during progressive degeneration of dopamine neurons in a mouse model reveal earlier impairment of spontaneous than forced behaviors

To determine the role of reduced dopaminergic transmission for declines of forced versus spontaneous behavior, we used a model of Parkinson's disease with progressive degeneration of dopamine (DA) neurons, the MitoPark mouse. Mice were subjected to rotarod tests of motor coordination, and open field and cylinder tests for spontaneous locomotor activity and postural axial support. To measure DA release in dorsal striatum and the shell of Nucleus Accumbens (NAc), we used ex vivo fast-scan cyclic voltammetry in 6- to 24-week-old mice. To determine decline of DA transporter function, we used 18FE-PE2I positron emission tomography. We show here that fast-scan cyclic voltammetry is a sensitive tool to detect evoked DA release dysfunction in MitoPark mice and that electrically evoked DA release is affected earlier in nigrostriatal than mesolimbic DA systems. DA reuptake was also affected more slowly in NAc shell. Positron emission tomography data showed DA uptake to be barely above detection levels in 16- and 20-week-old MitoPark mice. Rotarod performance was not impaired until mice were 16 weeks old, when evoked DA release in striatum had decreased to ≈ 40% of wild-type levels. In contrast, impairment of open field locomotion and rearing began at 10 weeks, in parallel with the initial modest decline of evoked DA release. We conclude that forced behaviors, such as motivation not to fall, can be partially maintained even when DA release is severely compromised, whereas spontaneous behaviors are much more sensitive to impaired DA release, and that presumed secondary non-dopaminergic system alterations do not markedly counteract or aggravate effects of severe impairment of DA release. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Nicotine-Induced Conditional Place Preference Is Affected by Head Injury: Correlation with Dopamine Release in the Nucleus Accumbens Shell

BACKGROUND

Traumatic brain injury is known to impact dopamine-mediated reward pathways, but the underlying mechanisms have not been fully established.

METHODS

Nicotine-induced conditional place preference was used to study rats exposed to a 6-psi fluid percussion injury with and without prior exposure to nicotine. Preference was quantified as a score defined as (C1 - C2) / (C1 + C2), where C1 is time in the nicotine-paired compartment and C2 is time in the saline-paired compartment. Subsequent fast-scan cyclic voltammetry was used to analyze the impact of nicotine infusion on dopamine release in the shell portion of the nucleus accumbens. To further determine the influence of brain injury on nicotine withdrawal, nicotine infusion was administered to the rats after fluid percussion injury. The effects of fluid percussion injury on conditional place preference after prior exposure to nicotine and abstinence or withdrawal from nicotine were also assessed.

RESULTS

After traumatic brain injury, dopamine release was reduced in the nucleus accumbens shell, and nicotine-induced conditional place preference preference was significantly impaired. Preference scores of control, sham-injured, and fluid percussion injury groups were 0.1627±0.04204, 0.1515±0.03806, and -0.001300±0.04286, respectively. Nicotine-induced conditional place preference was also seen in animals after nicotine pretreatment, with a conditional place preference score of 0.07805±0.02838. Nicotine preexposure substantially increased tonic dopamine release in sham-injured animals, but it did not change phasic release; nicotine exposure after fluid percussion injury enhanced phasic release, though not to the same levels seen in sham-injured rats. Conditioned preference was related not only to phasic dopamine release (r=0.8110) but also to the difference between tonic and phasic dopamine levels (r=0.9521).

CONCLUSIONS

Traumatic brain injury suppresses dopamine release from the shell portion of the nucleus accumbens, which in turn significantly alters reward-seeking behavior. These results have important implications for tobacco and drug use after traumatic brain injury.

Differences in Nicotine Encoding Dopamine Release between the Striatum and Shell Portion of the Nucleus Accumbens

The aim of this work was to determine the effect of nicotine desensitization on dopamine (DA) release in the dorsal striatum and shell of the nucleus accumbens (NAc) from brain slices. In vitro fast-scan cyclic voltammetry analysis was used to evaluate dopamine release in the dorsal striatum and the NAc shell of Sprague–Dawley rats after infusion of nicotine, a nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine (Mec), and an α4β2 cholinergic receptor antagonist (DHβe). DA release related to nicotine desensitization in the striatum and NAc shell was compared. In both structures, tonic release was suppressed by inhibition of the nicotine receptor (via Mec) and the α4β2 receptor (via DHβe). Paired-pulse ratio (PPR) was facilitated in both structures after nicotine and Mec infusion, and this facilitation was suppressed by increasing the stimulation interval. After variable frequency stimulation (simulating phasic burst), nicotine infusion induced significant augmentation of DA release in the striatum that was not seen in the absence of nicotine. In contrast, nicotine reduced phasic DA release in NAc, although frequency augmentation was seen both with and without nicotine. Evaluation of DA release evoked by various trains (high-frequency stimulation (HFS) 100 Hz) of high-frequency stimulation revealed significant enhancement after a train of three or more pulses in the striatum and NAc. The concentration differences between tonic and phasic release related to nicotine desensitization were more pronounced in the NAc shell. Nicotine desensitization is associated with suppression of tonic release of DA in both the striatum and NAc shell that may occur via the α4β2 subtype of nAChR, whereas phasic frequency-dependent augmentation and HFS-related gating release is more pronounced in the striatum than in the NAc shell. Differences between phasic and tonic release associated with nicotine desensitization may underlie processing of reward signals in the NAc shell, and this may have major implications for addictive behavior.

Effect of traumatic brain injury on nicotine-induced modulation of dopamine release in the striatum and nucleus accumbens shell

Background

Traumatic brain injury is associated with substantial alterations in reward processing, but underlying mechanisms are controversial.

Objective

A better understanding of alterations in dopamine (DA) release patterns from the dorsal striatum and nucleus accumbens shell (NAc) may provide insights into posttraumatic reward pathology.

Materials and Methods

The patterns of DA release with or without exposure to nicotine in brain slices with striatum and NAc, isolated from Sprague-Dawley rats with 6 psi fluid percussion (FPI) or sham injury were analysis by using fast-scan cyclic voltammetry. Tonic and phasic DA releases were assessed using single pulse and 10 pulses at 25 Hz, respectively. DA release relative to stimulation intensity, frequency, number of pulses, and paired-pulse facilitation was evaluated to determine release probability and response to bursting.

Results

There was a profound suppression in tonic DA release after nicotine desensitization after FPI, and the input/output curve for the DA release based on stimulation intensity was shifted to the right. FPI was associated with a significant decrease in frequency-dependent DA release augmentation, DA release induced by high frequency stimulation trains, and DA release in response to paired-pulse facilitation. The effect of nicotine desensitization was similar in FPI and sham-injured animals, although significantly smaller after FPI. Nicotine desensitization–induced differences between phasic and tonic release concentrations that contrasted with the reward-related signals then became less prominent in NAc after FPI.

Conclusions

TBI blunts DA release from mesolimbic reward centers, and more intense stimuli are required to produce context-dependent DA release sufficient to have a physiological effect.

Implications

The nicotine desensitization-related suppression in tonic DA release was profound with right-ward shift of the input/output curve for DA release after FPI. FPI was associated with a significant decrease in frequency-dependent DA release augmentation, DA release induced by high frequency stimulation trains, and DA release in response to paired-pulse facilitation. Nicotine desensitization–induced differences between phasic and tonic release concentrations that contrasted with the reward-related signals then became less prominent in NAc after FPI. TBI thus blunts DA release from mesolimbic reward centers, and more intense stimuli are required to produce context-dependent DA release sufficient to have a physiological effect.