Introduction
Personality, acting as a significant differentiating factor among individuals, is intimately associated with the underlying neurochemical substrates (Clauss et al.,
2015). Cloninger's theory of personality suggests that temperament, a heritable aspect of personality, is measurable using psychometric scales such as the Temperament and Character Inventory (TCI) (Cloninger,
1986). It highlights four fundamental temperamental domains: harm avoidance, reward dependence, novelty seeking, and persistence. Research has linked these dimensions to the modulation of specific neurotransmitters, particularly dopamine, whose variations in synthesis capacity are associated with behavioural disinhibition (Cloninger et al.,
1993; Wingo et al.,
2016).
Gambling disorder (GD) is a prevalent behavioural addiction globally, underpinned by increasing evidence pointing to neurobiological dysfunction in regions such as the prefrontal cortex and basal ganglia (Raimo et al.,
2021; Verdejo-García et al.,
2008). This disorder, similar to complex conditions like alcohol dependence, exhibits a broad range of manifestations across various patient demographics, encompassing extremes of both high and low impulsivity and novelty seeking (Nordin & Nylander,
2007; Pettorruso et al.,
2023). Based on this concept, it is feasible to associate early and late-onset GD with Cloninger's Type II (characterized by high novelty seeking) and Type I (defined by high harm avoidance) temperamental profiles, respectively (Janiri et al.,
2007; Shin et al.,
2009). The acknowledgment of such heterogeneity is crucial for further understanding of the neurobiological bases underlying addiction.
Being vital to motivation and pleasure, dopamine influences distinct personality and behavioural facets that forge individual temperament. The interaction between dopamine and GD is yet to be fully elucidated, with existing research revealing conflicting results about presynaptic, postsynaptic structures, and dopamine synthesis (Clark et al.,
2018; Pettorruso et al.,
2020c). These discrepancies may stem from heterogeneous imaging methodologies (i.e., functional MRI, PET, and SPECT, the latter two employing various radioligands) and studied populations (Potenza,
2018). The dopamine transporter (DAT), an integral component of the dopaminergic system, has sparked considerable interest due to its implications in GD. Recent findings have demonstrated a relationship between reduced DAT availability and GD (Pettorruso et al.,
2019b), thus suggesting that changes in DAT distribution and lateralization might affect reward preference and risk propensity (Guerra et al.,
2023). While significant DAT asymmetries have been related to Parkinson's Disease (PD) (Cilia et al.,
2010; Di Giuda et al.,
2012), the role of physiological lateralization of DAT in non-Parkinsonian individuals remains a widely unexplored field (Garrido et al.,
2020).
The existing literature on the subject is limited (Guerra et al.,
2023; Kaasinen et al.,
2023; Pettorruso et al.,
2019b), and does not allow for a clear identification of the dopaminergic pathways underlying the different temperamental aspects of GD. Therefore, this study aimed at investigating potential correlations between striatal DAT asymmetry index and lateralization, as evaluated by
123I-FP-CIT SPECT imaging, and temperamental dimensions in GD patients.
Discussion
The present study investigated the potential relationships between striatal DAT lateralization, asymmetry index, assessed by 123I-FP-CIT SPECT, and temperamental dimensions measured by TCI in subjects diagnosed with GD. The results revealed interesting correlations between DAT binding asymmetry in the caudate and putamen and the temperament dimensions of harm avoidance and novelty seeking. More specifically, higher novelty seeking scores coincided with increased DAT binding in the left caudate compared to the right. Conversely, higher harm avoidance scores corresponded with increased DAT binding in the right putamen relative to the left.
Over the years, numerous attempts have been made to decipher the intricate relationship between neuronal pathways and personality traits. Cloninger initially proposed three personality dimensions — harm avoidance, novelty seeking, and reward dependence — which he posited were influenced by imbalances in the serotonin, dopamine, and noradrenaline systems, respectively (Cloninger,
1986). As far as the authors are aware, this study represents the first exploration of the potential interaction between temperamental dimensions and DAT availability in the basal ganglia of GD patients.
Dopamine transporters, acting as the gatekeepers of the dopaminergic synaptic function, regulate the reuptake of the extracellular neurotransmitter by presynaptic neurons (Vaughan & Foster,
2013). The caudate nucleus, an integral part of numerous neural pathways, interacts extensively with the dorsolateral prefrontal cortex (DLPFC), which modulates cognitive and emotional control (Koechlin et al.,
2003; Rahm et al.,
2013). Increased DAT expression could suggest heightened dopamine availability and greater activation of the DLPFC-basal ganglia pathway.
Harm avoidance, an inheritable predisposition towards behaviour inhibition in response to signals of punishment or non-reward, is distinguished by fear of uncertainty, social inhibition, shame, and pessimistic worry in anticipation of problems (Petrosini et al.,
2017). Our findings suggest that enhanced function of the right caudate might correspond to hyperactivation of the DLPFC-putamen-basal ganglia-thalamus pathway, potentially resulting in over-activation of the right hemisphere. This over-activation may serve to mitigate emotional “noise”, fear of action, and avoidance of problem-solving in the right hemisphere.
Novelty seeking, in contrast, reflects a predisposition towards exploratory activity in response to novelty, impulsive decision-making, and quick loss of temper (Petrosini et al.,
2017). Analogously, the activation of the left caudate and subsequent left basal ganglia hyperactivation could be associated with the hyperactivation of the DLPFC-caudate-basal ganglia-thalamus circuits. This might result in the initiation of left cerebral pathways primarily involved in impulsive and fearless behaviours. Persistent activation in the left hemisphere could amplify the ipsilateral basal ganglia circuit, reducing the background noise of impulsivity.
Therefore, we suggest that the observed DAT asymmetry in the basal ganglia is a direct manifestation of the hemispheric asymmetry in emotion perception and behavioural guidance. Long-term stimulation of the basal ganglia on one side of the brain leads to a surge in dopamine levels in the caudate, which in turn, triggers an upregulation of dopamine transporters, key players in regulating neurotransmitter quantities in the synaptic cleft.
In line with our theoretical proposal, numerous studies have highlighted the substantial role of basal ganglia asymmetry in temperament dimensions, specifically in harm avoidance and novelty seeking (Laricchiuta et al.,
2014). Indeed, in studies focused on DAT levels in PD, researchers observed a decrease in novelty-seeking behaviours among PD patients compared to healthy controls. This was particularly pronounced in instances where there was a more significant reduction of dopamine in the left hemisphere. On the other hand, higher levels of harm avoidance were found only among PD patients exhibiting reduced dopamine levels in the right hemisphere when compared to healthy controls (Porat et al.,
2014; Tomer & Aharon-Peretz,
2004).
In a study by Liang et al. (
2017) that involved opioid-dependent patients, an inverted-U-shaped correlation was identified between the personality trait of novelty seeking and DAT availability. Concurrently, a clinical study conducted by Menza et al. (
1995) involving nine PD patients reported a positive association between novelty seeking scores and [18F]dopa uptake in the left caudate, an association that was not observed in the right caudate.
Collectively, these findings lend credence to the theoretical proposition that each hemisphere specializes in distinct aspects of emotion and behaviour regulation. The left hemisphere seems to primarily oversee motivation control and cognitive approaches pertaining to behaviour. Increased activity in the left frontal region has been linked to extroversion and sociability (Pujol et al.,
2000). In contrast, a larger right anterior cingulate area has been associated with the harm avoidance dimension of temperament (Pujol et al.,
2002).
Interestingly, these findings introduce the compelling prospect of dopamine-related personality traits significantly impacting the outcomes of neuromodulation treatments (Pettorruso et al.,
2020a). Such treatments are typically applied to unilateral target regions (Pettorruso et al.,
2021) and have been proven to effectively modulate both dopamine levels (Strafella et al.,
2001) and DAT (Pettorruso et al.,
2019a; Xu et al.,
2023) in striatal regions. The observed asymmetries in striatal dopaminergic transmission, which run parallel to the range of temperamental facets in GD, provide a promising basis for potential advancements in personalized therapies (Pettorruso et al.,
2024). Consequently, these therapeutic interventions could be diversified and tailored to individual patients, based on their distinct neurobiological profiles (Spano et al.,
2019). Future research are positioned to probe deeper into this hypothesis, thereby shedding light on the heterogeneous responses to neuromodulation exhibited by patients with GD, and addictions in general (Martinotti et al.,
2019,
2022).
Despite these encouraging findings, the current study does present with several limitations. A critical issue to consider is the limited sample size, which diminishes the robustness and statistical power of the findings (de Winter et al.,
2016; Schönbrodt & Perugini,
2013). Moreover, the absence of a control group also constrains our ability to predict the impact of addiction on the observed findings. Additionally, the assessment of the dopaminergic synapses in the basal ganglia was exclusively carried out through DAT, a presynaptic transporter involved in the reuptake of neurotransmitters in the synaptic cleft. In the future, expanding the scope of this initial study to encompass the analysis of postsynaptic receptors could enhance our understanding of the role dopamine plays in the basal ganglia circuit.
Declarations
G.M. has been a consultant and a speaker and has received research grants from Angelini, Doc Generici, Janssen-Cilag, Lundbeck, Otsuka, Pfizer, Servier, and Recordati. J. E. G. has received research grants from Janssen, Boehringer Ingelheim, and Biohaven Pharmaceuticals. He receives yearly compensation from Springer Publishing for acting as Editor-in Chief of the Journal of Gambling Studies and has received royalties from Oxford University Press, American Psychiatric Publishing, Inc., Norton Press, and McGraw Hill. The other Authors have nothing to declare.
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