Neurochemical model
Functional Ensemble of Temperament (FET) is a neurochemical model suggesting specific functional roles of main neurotransmitter systems in the regulation of behaviour.
Earlier theories.
Medications can adjust the release of brain neurotransmitters in cases of depression, anxiety disorder, schizophrenia and other mental disorders because an imbalance within neurotransmitter systems can emerge as consistent characteristics in behaviour compromising people's lives. All people have a weaker form of such imbalance in at least one of such neurotransmitter systems that make each of us distinct from one another. The impact of this weak imbalance in neurochemistry can be seen in the consistent features of behaviour in healthy people (temperament). In this sense temperament (as neuro-chemically-based individual differences) and mental illness represents varying degrees along the same continuum of neurotransmitter imbalance in neurophysiological systems of behavioural regulation.
In fact, multiple temperament traits (such as Impulsivity, sensation seeking, neuroticism, endurance, plasticity, sociability or extraversion) have been linked to brain neurotransmitters and hormone systems.
By the end of the 20th century, it became clear that the human brain operates with more than a dozen neurotransmitters and a large number of neuropeptides and hormones. The relationships between these different chemical systems are complex as some of them suppress and some of them induce each other's release during neuronal exchanges. This complexity of relationships devalues the old approach of assigning "inhibitory vs. excitatory" roles to neurotransmitters: the same neurotransmitters can be either inhibitory or excitatory depending on what system they interact with. It became clear that an impressive diversity of neurotransmitters and their receptors is necessary to meet a wide range of behavioural situations, but the links between temperament traits and specific neurotransmitters are still a matter of research. Several attempts were made to assign specific (single) neurotransmitters to specific (single) traits. For example, dopamine was proposed to be a neurotransmitter of the trait of Extraversion, noradrenaline was linked to anxiety, and serotonin was thought to be a neurotransmitter of an inhibition system. These assignments of neurotransmitter functions appeared to be an oversimplification when confronted by the evidence of much more diverse functionality. 
Research groups led by Petra Netter in Germany, Lars Farde in Karolinska Institute in Sweden and Trevor Robbins in Cambridge, UK had most extensive studies of the links between temperament/personality traits or dynamical properties of behavior and groups of neurotransmitters.
Development of the FET model.
The architecture of the Functional Ensemble of Temperament (FET) was developed by Trofimova as the Compact version of the Structure of Temperament Questionnaire (STQ-77) in 1997–2007. The differentiation between the rows of the FET inherits the activity-specific approach to the structure of temperament proposed by Rusalov in mid-1980s. According to this approach, the traits of temperament (and behavioural regulation) related to motor-physical, social-verbal and mental aspects of activities are based on different neurophysiological systems and should be assessed separately (so you can see a separation of traits into 3 rows related to these 3 types of activities). The 3-column structure of the FET framework follows Alexander Luria theory of three functional neuroanatomic systems (sensory-informational, programming and energetic) and is in line with functional constructivism approach. This approach views all behaviour as being constructed and generated anew based on individual's capacities and demands of the situation 
The final STQ-77/FET model considers 12 systems (and temperament traits): 9 systems (and traits) regulating the formal functional aspects of behaviour (energetic, dynamic and orientational, each assessed in three domains (intellectual, physical and social-verbal) together with 3 systems related to emotionality (Neuroticism, Impulsivity and a disposition of Satisfaction (formerly called Self-Confidence) (see Figure).
The differences between Trofimova's and Rusalov's models of temperament (and the structures of their versions of the STQ) are: 
In 2007–2013 this STQ-77 model of temperament was reviewed and compared to the main findings in neurophysiology, neurochemistry, clinical psychology and kinesiology resulting in the neurochemical FET model offered by Irina Trofimova, McMaster University. Trevor Robbins, Cambridge University who collaborated with Trofimova on this project in 2014–2016 suggested a revision of the part of the FET neurochemical hypothesis related to the trait of Intellectual (mental) Endurance (known in cognitive psychology also as "sustained attention"). This neurochemical component of the FET hypothesis was upgraded in 2015 by underlying a key role of acetylcholine and noradrenalin in sustained attention. In February 2018, by the suggestion of Dr Marina Kolbeneva (Institute of Psychology, Russian Academy of Sciences) the scale Self-Confidence was renamed as the scale of dispositional Satisfaction.
Overview of the model.
The final STQ-77/FET framework classifies temperament traits and their neurochemical biomarkers into 12 components: nine components regulating the formal functional aspects of behaviour (energetic, dynamic and orientational) each assessed in three domains (intellectual, physical and social-verbal); also three components related to emotionality (Neuroticism, Impulsivity and Satisfaction (Self-Confidence)) (see Figure) 
The FET framework summarized existing literature showing the nine non-emotionality traits are regulated by the monoamines (MA) (noradrenalin, dopamine, serotonin), acetylcholine and neuropeptide systems, whereas the three emotionality-related traits emerge as a dysregulation of opioid receptors systems that have direct control over MA systems. Importantly, the FET model suggests that there is no one-to-one correspondence between the neurotransmitter systems underlying temperament traits (or mental disorders) but instead specific ensemble relationships between these systems emerge as temperament traits. The FET framework is based only on the strongest consensus points in the research studying the role of neurotransmitter in behavioural regulation and the components of temperament; it doesn't list more controversial links between these multiple systems.
† Neurotransmitter systems: 5-HT: serotonin; DA: dopamine; NE: noradrenalin; ACh: acetylcholine; Glu: glutamate; OXY: oxytocin; VSP: vasopressin; NP: Neuropeptides; KOR, MOR, DOR: kappa-, mu- and delta-opioid receptors correspondingly; sANS - sympathetic autonomic nervous system; HPA - hypothalamic–pituitary–adrenal axis.
The FET points out that opioid receptor systems are involved not only in regulation of emotional dispositions but also amplify three non-emotionality aspects of behaviour (KOR for orientation, DOR for integration of actions and MOR of approval-maintenance of behaviour). This involvement was confirmed for MOR systems that bind endorphins: experiments show that MOR overstimulation influences hypothalamic serotonin and Brain-derived neurotrophic factor release and affecting endurance aspects of behaviour. The interplay within hormonal systems and its interaction with serotonin also appeared to be a factor is social emotions, such as shame and guilt 
FET model and classifications of psychiatric disorders.
FET framework was proposed to simplify classifications of psychiatric disorders (DSM, ICD) using 12 functional aspects of behaviour that this model highlights. Clinical studies showed good differential power of the FET framework for various diagnoses of psychopathology. For example, depressed people had low endurance and psychomotor slowdown in their temperament profiles. In contrast to them, patients with Generalized Anxiety Disorder had higher impulsivity and neuroticism. FET-developers suggested that every symptom in DSM/ICD diagnoses can be mapped into a specific FET code reflecting a disregulation within well-documented neurochemical systems.
References.
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