Label-free imaging of neurotransmitters in live brain tissue by multi-photon ultraviolet microscopy

Visualizing small biomolecules in living cells remains a difficult challenge. Neurotransmitters provide one of the most frustrating examples of this difficulty, as our understanding of signaling in the brain critically depends on our ability to follow the neurotransmitter traffic. Last two decades have seen considerable progress in probing some of the neurotransmitters, e.g. by using false neurotransmitter mimics, chemical labeling techniques, or direct fluorescence imaging. Direct imaging harnesses the weak UV fluorescence of monoamines, which are some of the most important neurotransmitters controlling mood, memory, appetite, and learning. Here we describe the progress in imaging of these molecules using the least toxic direct excitation route found so far, namely multi-photon (MP) imaging. MP imaging of serotonin, and more recently that of dopamine, has allowed researchers to determine the location of the vesicles, follow their intracellular dynamics, probe their content, and monitor their release. Recent developments have even allowed ratiometric quantitation of the vesicular content. This review shows that MP ultraviolet (MP-UV) microscopy is an effective but underutilized method for imaging monoamine neurotransmitters in neurones and brain tissue.

Fluorogenic Detection of Monoamine Neurotransmitters in Live Cells

Monoamine neurotransmission is key to neuromodulation, but imaging monoamines in live neurons has remained a challenge. Here we show that externally added ortho-phthalaldehyde (OPA) can permeate live cells and form bright fluorogenic adducts with intracellular monoamines (e.g., serotonin, dopamine, and norepinephrine) and with L-DOPA, which can be imaged sensitively using conventional single-photon excitation in a fluorescence microscope. The peak excitation and emission wavelengths (λ_ex = 401 nm and λ_em = 490 nm for serotonin; λ_ex = 446 nm and λ_em = 557 nm for dopamine; and λ_ex = 446 nm and λ_em = 544 nm for norepinephrine, respectively) are accessible to most modern confocal imaging instruments. The identity of monoamine containing structures (possibly neurotransmitter vesicles) in serotonergic RN46A cells is established by quasi-simultaneous imaging of serotonin using three-photon excitation microscopy. Mass spectrometry of cell extracts and of in vitro solutions helps us identify the chemical nature of the adducts and establishes the reaction mechanisms. Our method has low toxicity, high selectivity, and the ability to directly report the location and concentration of monoamines in live cells.

Late Emergence of Parkinsonian Phenotype and Abnormal Dopamine Transporter Scan in Chorea-Acanthocytosis

Chorea-acanthocytosis (ChAc) is a neurodegenerative condition predominantly manifesting with chorea and often acanthocytes on peripheral blood film. Abnormal appearances with ¹²³I-FP-CIT single-photon emission computed tomography (SPECT) have not previously been reported in ChAc. We describe 2 cases with typical presentations of ChAc and late development of parkinsonism with asymmetric reduction in presynaptic striatal uptake on ¹²³I-FP-CIT SPECT. Case 1, a 50-year-old male, developed micrographia and limb bradykinesia 14 years after initial presentation at the age of 30. Case 2, a 42-year-old female presenting with vocal tics and generalized dystonia at the age of 25, developed tremor, bradykinesia, and rigidity 11 years into the disease course. These cases represent the best description to date of the natural history of ChAc, in which the early hyperkinetic clinical syndromes give way to a parkinsonian phenotype. This is consistent with a gradual deterioration of presynaptic nigrostriatal projections, reflected in the clinical parkinsonism and abnormal ¹²³I FP-CIT SPECT.