Intermolecular Association Provides Specific Optical and NMR Signatures for Serotonin at Intravesicular Concentrations

Abstract

Neurotransmitter vesicles contain biomolecules at extraordinarily high concentrations (hundreds of millimoles/liter). Such concentrations can drive intermolecular associations, which may affect vesicular osmolarity and neuronal signaling. Here we investigate whether aqueous serotonin (a monoamine neurotransmitter) forms oligomers at intravesicular concentrations and whether these oligomers have specific spectroscopic signatures that can potentially be used for monitoring neuronal storage and release. We report that, as serotonin concentration is increased from 60 microM to 600 mM, the normalized fluorescence spectrum of serotonin displays a growing long-wavelength tail, with an isoemissive point at 376 nm. The fluorescence decay is monoexponential with a lifetime of 4 ns at low concentrations but is multiexponential with an average lifetime of 0.41 ns at 600 mM. A 600 mM serotonin solution has 30% less osmolarity than expected for monomeric serotonin, indicating oligomer formation. The proton NMR chemical shifts move upfield by as much as 0.3 ppm at 600 mM compared to those at 10 mM, indicating a stacking of the serotonin indole moieties. However, no intermolecular crosspeak is evident in the two-dimensional NMR rotating frame Overhauser effect spectroscopy spectrum even at 600 mM, suggesting that oligomeric structures are possibly weakly coupled. The appearance of a single peak for each proton suggests that the rate of interconversion between the monomeric and the oligomeric structures is faster than 240 Hz. A stopped-flow kinetic experiment also confirms that the rate of dissociation is faster than 100 ms. We conclude that serotonin forms oligomers at intravesicular concentrations but becomes monomeric quickly on dilution. NMR signatures of the oligomers provide potential contrast agents for monitoring the activity of serotonergic neurons in vivo.

Three‐photon microscopy shows that somatic release can be a quantitatively significant component of serotonergic neurotransmission in the mammalian brain

“…Rh‐123 accumulates specifically in the mitochondria of living cells (Johnson et al, 1980; Darzynkiewicz et al, 1981). This dye emits at a wavelength at about 550 nm and can be separated from serotonin, which emits at a shorter wavelength (Nag et al, 2008). We labeled the cells with rh‐123 and recorded images with a multiphoton microscope setup with two detection channels (Fig.…”

Spectroscopic signatures and structural motifs in isolated and hydrated serotonin: a computational study

“…1,2 It is involved in the regulation of mood, stress, sleep, pain, aggression, anger, learning and regulation of body temperature and blood pressure. 1,[5][6][7][8][9][10] Moreover, serotonin is the target, at least partially, for most drugs that are currently used for the treatment of psychiatric disorders (e.g., depression and schizophrenia). 1,[5][6][7][8][9][10] Moreover, serotonin is the target, at least partially, for most drugs that are currently used for the treatment of psychiatric disorders (e.g., depression and schizophrenia).…”

“…1,[5][6][7][8][9][10] Moreover, serotonin is the target, at least partially, for most drugs that are currently used for the treatment of psychiatric disorders (e.g., depression and schizophrenia). 2,6,11,12 It is generally accepted that some particular conformation of a biologically active molecule at the receptor site is decisive in order to trigger a specic biological response. The biological signicance of this molecule has drawn considerable theoretical and experimental interest for exploring its structure in the gas phase.…”

A Molecular View of the Synapse

“…Serotonin is an aromatic compound with a nitrogen-containing cycle (indole) bearing an OH group and an ethylamine chain ( Fig. 23 Like glutamate, serotonin can interact with either ionotropic or metabotropic receptors. At physiological pH, the amine group is positively charged.…”

“…The same sialic acid has the methyl of its N-acetyl group oriented toward the aromatic ring of serotonin, consistent with the establishment of a CH-π interaction. In the specific case of serotonin, which displays self-aggregating properties in water, 23 this process would also be a simple way to prevent aggregation, ensuring the efficient delivery of serotonin monomers to its postsynaptic receptors. 33 It has been hypothesized that the transient binding of serotonin to the gangliosides of the postsynaptic membrane take place immediately after the release of the neurotransmitter in the synaptic cleft.…”