Molecular dynamics-based simulation of trace amine membrane permeability

Trace amines are endogenous compounds, typified by 2-phenylethylamine (PE) and p-tyramine (TA), found in the vertebrate central nervous system. Although synthesized in pre-synaptic terminals, trace amines do not appear to act as neurotransmitters, but rather modulate responsivity to co-existing neurotransmitters. Trace amines are neither actively accumulated in synaptic vesicles, nor released in an activity-dependent manner. Further, Trace Amine-Associated Receptor 1 (TAAR1), which is selectively activated by PE and TA, is intracellular. As such, PE and TA need to cross cell membranes in order to exert their effects. This has been assumed to occur by simple diffusion, but has not previously been systematically examined. Experimental data were obtained using Fluorosome® technology. A permeability coefficient of 25.3 ± 3.8 Å/s (n = 6) was obtained for TA which was not significantly different from that obtained for the monoamine neurotransmitter noradrenaline (20.3 ± 3.8 Å/s, n = 8). PE was unsuitable for use with this system. We have also used molecular dynamics computer simulation techniques to determine the potential of mean force (PMF) associated with trace amine passage across lipid bilayers. A PMF peak barrier of 25 ± 6 kcal/mol (protonated) and 13 ± 1 kcal/mol (deprotonated) was obtained for PE. Protonated TA peak energy barriers were even greater at 31 ± 1 kcal/mol. Application of a homogeneous solubility-diffusion model combining the measured permeability coefficients and simulated PMF allows fitting of the diffusion coefficient for trace amines in the hydrophobic region of the lipid bilayer. The diffusion coefficients in other regions of the membrane were found to make negligible contributions to the permeability coefficient for the calculated PMF. The fit obtained a value for the diffusion coefficient of (163 ± 25) × 10−10 m2/s for TA+ in the hydrophobic core region. The diffusion coefficient for TA+ in the aqueous compartment was also calculated directly by simulation yielding a value of (0.62 ± 0.26) × 10−10 m2/s. The adopted simulation methods failed to yield diffusion coefficients in the core region indicating that further work will be required to accurately predict permeability coefficients for trace amines passing through membranes.