15. Measurement of dopamine

For various reasons, the measurement of dopamine in relation to psychological contexts is not possible for diagnostic or treatment purposes.
Dopamine cannot cross the blood-brain barrier.

• 15.1. Dopamine measurements in peripheral body fluids or cerebral spinal fluid
• 15.2. Blink rate as an indicator of dopamine level changes
• 15.3. Retina and increased extracellular dopamine
• 15.4. Dopamine measurement in the laboratory
  • 15.4.1. Electrochemical measurement methods
  • 15.4.2. Measurement of whole cells
  • 15.4.3. Imaging procedures
  • 15.4.4. Dopamine release at surface level
• 15.5. DAT promoter methylation in blood could predict DAT expression in the striatum

15.1. Dopamine measurements in peripheral body fluids or cerebral spinal fluid¶

Measuring the concentration of dopamine and its metabolites (e.g. homovanillic acid, HVA) in peripheral body fluids or in cerebral spinal fluid (CSF) is not sensitive enough to say anything about dopamine activity in the brain.
A comparison of HVA concentrations in four brain regions (dorsal frontal cortex, orbital frontal cortex, caudate nucleus and putamen), CSF and blood plasma found a single significant correlation (between CSF and dorsal frontal cortex), so that measurements of HVA concentration in the raw plasma (even when influences by diet or anesthesia are excluded ) show barely any benefit for the assessment of central dopamine metabolism and turnover.¹

Peripheral dopamine levels say little about brain dopamine levels because dopamine is not only synthesized and released in the brain, but also by various peripheral tissues, e.g. ²

• Pancreas
• Adrenal medulla
• Kidney
• peripheral leukocytes.

This also applies to the measurement³

• the amount of the degradation enzyme MAO in the blood
• the prolactin stress response

Finally, dopaminergic-mediated influences on behavior do not necessarily require a change in dopamine levels or its metabolites. They can also result from mere changes in receptors or dopamine fluxes.

15.2. Blink rate as an indicator of dopamine level changes¶

The blink rate (Eyeblink)⁴

• correlates with the activity of DRD1 and DRD2⁵⁶
  • basal blink rate may correlate more strongly with DRD2
• can indicate reduced or increased dopamine activity⁷ and the normalization of this activity after treatment
  • especially striatal dopamine⁸
• can reliably predict individual differences in performance on many cognitive tasks, particularly in relation to reward-driven behavior and cognitive flexibility

Two studies found no correlation between blink rate and dopamine activity.⁹¹⁰

15.3. Retina and increased extracellular dopamine¶

A non-invasive analysis of retinal responses to light revealed an increased extracellular dopamine level resulting from a genetically determined increased DAT dopamine efflux.¹¹

15.4. Dopamine measurement in the laboratory¶

Dopamine can be measured in vitro (on tissue samples) in various ways:³

• Measurement of the electrical activity of downstream neurons in response to stimulation of dopaminergic neurons
• Measurement of the firing rate of individual dopaminergic neurons in response to dopamine-relevant stimuli
• Measuring the effect of dopamine agonists and dopamine antagonists
• SPECT examinations measure the binding behavior of radioactively labeled dopamine ligands; this allows an indirect conclusion to be drawn about the dopamine concentration
  • dopamine release can be detected by means of the radioligand [11C]-raclopride by reducing the D2R binding potential
• Imaging studies can show activity of dopaminergic neurons in response to dopamine-related stimuli

15.4.1. Electrochemical measurement methods¶

Cyclic fast scan voltammetry (FSCV)¹²

• Measurement of the change in dopamine release
  • FSCV requires the subtraction of a baseline current, therefore only suitable for recording dopamine changes
• In vivo, in vitro
• Carbon fiber electrode is inserted into tissue and triangular wave is applied (-0.6 to 1 V)
• Fast sampling rate (> 400 V/s)
  • Sampling rate is relatively slow compared to the speed of exocytosis
• Detection rate 10 to 100 Hz
• Different compounds generate oxidation and reduction currents at different voltages during the scan; peak current for dopamine at approx. 0.6 V

Amperometry¹²

• Similar to FSCV, but uses a constant potential (~0.6 V for dopamine) at the electrode.
• High temporal resolution, limited only by sampling rate.
• Limited specificity for dopamine. Can therefore only be used if dopamine is the most important electroactive substance.
• Oxidation consumes dopamine and can therefore contribute to the signal decay.

Microdialysis¹²

• Measurement of the absolute dopamine level
• Usually with subsequent high-performance liquid chromatography
• In vivo
• Sensitive enough to measure basal dopamine levels in the brains of living animals
• Low temporal resolution, therefore unsuitable for measuring fast dopamine transients
• Tissue damage due to probe; may affect measurements

15.4.2. Measurement of whole cells¶

D2 IPSC recording¹²

• Indicates the activation of DRD2.
  • other dopamine release is not measured
• Method uses GPCR signal transduction, which causes a delay of approx. 50 to 100 ms between dopamine release and detection.
• GIRK (G-protein-activated inwardly rectifying potassium channels) generate an inhibitory postsynaptic current (IPSC) when activated by dopamine, which can be measured.
• DRD2 is coupled to GIRK in midbrain dopamine neurons
• In the striatum, DRD2s are not coupled to GIRKs. However, GIRKs can be virally expressed to signal D2 activation.

LGC-53¹²

• dopamine-sensitive chloride channel in C. elegans
• Measurement of the chloride current with good sensitivity and specificity for dopamine

15.4.3. Imaging procedures¶

VMAT-pHluorin¹²

• Measurement of the fusion of individual vesicles from individual varicosities
• VMAT-pHluorin is a pH-sensitive fluorophore that is intraluminally bound to VMAT2
• Fusion of VMAT-pHluorin-labeled vesicles with plasma membrane increases fluorescence signal
• Specificity limited; result shows vesicular release of all neurotransmitters released from dopamine neurons

Fluorescent false neurotransmitters (FFN)¹²

• FFN are VMAT2 substrates that are selectively loaded into monoamine-containing vesicles
• Upon release of vesicles, FFN diffusion reduces fluorescence in the varicosities
• Analysis of individual varicosities
• Sensitivity is limited, as even with strong stimulation only a small proportion of the vesicles of a terminal are released

Genetically encoded dopamine sensors (dLight; GrabDA)¹²

• Analysis of dopamine receptors
• Fluorescent indicators obtained by engineering dopamine receptors
• Labeling with circularly permuted GFP
• In vivo; in vitro
• High sensitivity
• High spatio-temporal resolution

Nanotube sensors¹²

• Investigation of the spatial and temporal properties of dopamine release
• Single-walled carbon nanotubes
  • Conjugated with single-stranded oligonucleotides
  • Fluorescent in the near infrared range
• In vivo; in vitro
• Direct insertion into tissue
• Strong increase in fluorescence after binding to dopamine
• Fast response
• Very sensitive
• Specificity limited, as other catecholamines and ascorbic acid are also detected

15.4.4. Dopamine release at surface level¶

• Previously, dopamine could only be measured selectively. This only made it possible to record a quantity value at a single point. In the meantime, methods have been developed with which dopamine release can be measured and recorded on an area level. One method describes the observation of dopamine release from whole cells,¹³ another the area-based observation of dopamine release down to dendrite level.¹⁴
  These new techniques enable considerable gains in knowledge.

15.5. DAT promoter methylation in blood could predict DAT expression in the striatum¶

A measurement of DAT promoter methylation in the blood could possibly serve as an indicator of DAT expression in the striatum.¹⁵