Membrane permeability of trace amines: Evidence for a regulated, activity-dependent, nonexocytotic, synaptic release

Abstract

Both pre- and post-synaptic effects of trace amines have been demonstrated. The putative intracellular location of Trace Amine-Associated Receptors necessitate that membrane transport processes be present in order for post-synaptic effects to occur. Here we examine the ability of trace amines to cross synthetic (Fluorosomes) and native (synaptosomes) lipid bilayer membranes. Trace amines readily crossed Fluorosome membranes by simple diffusion, p-tyramine (P = 0.01) and tryptamine (P = 0.0004) showing significantly faster diffusion than dopamine and 5-HT, respectively, with diffusion half-lives of 13.5 ± 4.1 (p-tyramine) and 6.8 ± 0.7 seconds (tryptamine). Similarly, release of [(3) H]p-tyramine and [(3) H]2-phenylethylamine from pre-loaded synaptosomes occurred significantly quicker than did [(3) H]dopamine (P = 0.0001), with half lives of 38.9 (p-tyramine), 7.8 (2-phenylethylamine) and 133.6 seconds (dopamine). This was, however, significantly slower than the diffusion mediated passage across Fluorosome membranes (P = 0.0001), suggesting a role for transporters in mediating trace amine release. Further, a pronounced shoulder region was observed in the synaptosome [(3) H]p-tyramine release curve, suggesting that multiple processes regulate release. No such shoulder region was present for [(3) H]dopamine release. Surprisingly, both [(3) H]p-tyramine (P = 0.001) and [(3) H]2-phenylethylamine (P = 0.0001) release from synaptosomes was significantly decreased under depolarizing conditions. As expected, depolarization significantly increased [(3) H]dopamine release. The data presented indicate that the release of p-tyramine and 2-phenylethylamine from neuronal terminals occurs by a different mechanism than dopamine, and does not involve classical exocytosis. The data are consistent with an initial release of trace amines by simple diffusion, followed by an activity-dependent regulation of synaptic levels via one or more transporter proteins.

Trace Amines and Their Receptors

“…Unlike their neurotransmitter structural analogs, PEA, TYR, and TRP do not appear to be stored. All three readily diffuse across synthetic lipid bilayers in the absence of transporters, with diffusion half-lives of 15 seconds or less (Berry et al, 2013). Consistent with this, PEA passage across the blood-brain barrier occurs by a mechanism consistent with passive diffusion (Mosnaim et al, 2013), whereas TYR passage across intestinal epithelial cells also appears to be diffusion mediated (Tchercansky et al, 1994).…”

“…Consistent with this, PEA passage across the blood-brain barrier occurs by a mechanism consistent with passive diffusion (Mosnaim et al, 2013), whereas TYR passage across intestinal epithelial cells also appears to be diffusion mediated (Tchercansky et al, 1994). Release of PEA from the whole brain (Henry et al, 1988), striatal slices (Dyck, 1989), or synaptosomes (Berry et al, 2013) is not increased by K + -induced depolarization. Similar results were also obtained with TYR (Henry et al, 1988;Berry et al, 2013), with only a small increase in release in response to K + -induced depolarization observed in brain slices (Dyck, 1989).…”

“…Release of PEA from the whole brain (Henry et al, 1988), striatal slices (Dyck, 1989), or synaptosomes (Berry et al, 2013) is not increased by K + -induced depolarization. Similar results were also obtained with TYR (Henry et al, 1988;Berry et al, 2013), with only a small increase in release in response to K + -induced depolarization observed in brain slices (Dyck, 1989). Likewise, no effect of depolarization on TRP release from striatal slices was Trace Amines and Their Receptors present (Dyck, 1989).…”

“…Although there are still many open questions, through these research efforts a picture has emerged of TAARs being intricately linked to the detection of olfactory cues associated with species-specific survival mechanisms. That TAARs are also distributed throughout the body (Table 4) and at least some of their endogenous ligands readily cross cell membranes (Berry et al, 2013) provides an interesting perspective, which potentially allows the same molecule that provides the environmental cue to directly affect various physiologic processes that could conceivably further aid in adaptation responses. Rather than existing as distinct fields, it is expected that in the upcoming years much closer links will be forged between the olfactory and nonolfactory TAAR communities, which will further speed the filling of current knowledge gaps with respect to the basic underlying physiology of TAAR systems.…”

“…Second, studies have so far assumed that all responses are downstream of olfactory epithelium TAAR activation. At least some of the ligands (e.g., PEA) have been shown to readily cross lipid bilayers (Berry et al, 2013), and individual TAARs are present in peripheral cell populations (see previous sections). As such, it is possible that the physiologic responses to some olfactory delivered trace amines may occur due to a combination of downstream central nervous system signaling after olfactory epithelium TAAR activation, and systemic responses after ligand entry in to the bloodstream after passage across pulmonary membranes.…”