Dynamic monitoring of NET activity in mature murine sympathetic terminals using a fluorescent substrate

Resting cardiac sympathetic firing frequencies suppress terminal norepinephrine transporter uptake

The prejunctional norepinephrine transporter (NET) is responsible for the clearance of released norepinephrine (NE) back into the sympathetic nerve terminal. NET regulation must be tightly controlled as variations could have important implications for neurotransmission. Thus far, the effects of sympathetic neuronal activity on NET function have been unclear. Here, we optically monitor single-terminal cardiac NET activity ex vivo in response to a broad range of sympathetic postganglionic action potential (AP) firing frequencies. Isolated murine left atrial appendages were loaded with a fluorescent NET substrate [Neurotransmitter Transporter Uptake Assay (NTUA)] and imaged with confocal microscopy. Sympathetic APs were induced with electrical field stimulation at 0.2-10 Hz (0.1-0.2 ms pulse width). Exogenous NE was applied during the NTUA uptake- and washout phases to investigate substrate competition and displacement, respectively, on transport. Single-terminal NET reuptake rate was rapidly suppressed in a frequency-dependent manner with an inhibitory EF₅₀ of 0.9 Hz. At 2 Hz, the effect was reversed by the α₂-adrenoceptor antagonist yohimbine (1 μM) (p < 0.01) with no further effect imposed by the muscarinic receptor antagonist atropine (1 μM). Additionally, high exogenous NE concentrations abolished NET reuptake (1 μM NE; p < 0.0001) and displaced terminal specific NTUA during washout (1-100 μM NE; p < 0.0001). We have also identified α₂-adrenoceptor-induced suppression of NET reuptake rate during resting stimulation frequencies, which could oppose the effect of autoinhibition-mediated suppression of exocytosis and thus amplify the effects of sympathetic drive on cardiac function.

Dynamic monitoring of single-terminal norepinephrine transporter rate in the rodent cardiovascular system: A novel fluorescence imaging method

Here, we validate the use of a novel fluorescent norepinephrine transporter (NET) substrate for dynamic measurements of transporter function in rodent cardiovascular tissue; this technique avoids the use of radiotracers and provides single-terminal resolution. Rodent (Wistar rats and C57BL/6 mice) hearts and mesenteric arteries (MA) were isolated, loaded with NET substrate Neurotransmitter Transporter Uptake Assay (NTUA) ex vivo and imaged with confocal microscopy. NTUA labelled noradrenergic nerve terminals in all four chambers of the heart and on the surface of MA. In all tissues, a temperature-dependent, stable linear increase in intra-terminal fluorescence upon NTUA exposure was observed; this was abolished by NET inhibitor desipramine (1 μM) and reversed by indirectly-acting sympathomimetic amine tyramine (10 μM). NET reuptake rates were similar across the mouse cardiac chambers. In both species, cardiac NET activity was significantly greater than in MA (by 62 ± 29% (mouse) and 21 ± 16% (rat)). We also show that mouse NET reuptake rate was twice as fast as that in the rat (for example, in the heart, by 94 ± 30%). Finally, NET reuptake rate in the mouse heart was attenuated with muscarinic agonist carbachol (10 μM) thus demonstrating the potential for parasympathetic regulation of norepinephrine clearance. Our data provide the first demonstration of monitoring intra-terminal NET function in rodent cardiovascular tissue. This straightforward method allows dynamic measurements of transporter rate in response to varying physiological conditions and drug treatments; this offers the potential to study new mechanisms of sympathetic dysfunction associated with cardiovascular disease.

Extracellular α-synuclein enters dopaminergic cells by modulating flotillin-1-assisted dopamine transporter endocytosis

The neuropathological hallmarks of Parkinson's disease (PD) include the appearance of α-synuclein (α-SYN)-positive Lewy bodies (LBs) and the loss of catecholaminergic neurons. Thus, a potential mechanism promoting the uptake of extracellular α-SYN may exist in susceptible neurons. Of the various differentially expressed proteins, we are interested in flotillin (FLOT)-1 because this protein is highly expressed in the brainstem catecholaminergic neurons and is strikingly up-regulated in PD brains. In this study, we found that extracellular monomeric and fibrillar α-SYN can potentiate FLOT1-dopamine transporter (DAT) binding and pre-endocytic clustering of DAT on the cell surface, thereby facilitating DAT endocytosis and down-regulating its transporter activity. Moreover, we demonstrated that α-SYN itself exploited the DAT endocytic process to enter dopaminergic neuron-like cells, and both FLOT1 and DAT were found to be the components of LBs. Altogether, these findings revealed a novel role of extracellular α-SYN on cellular trafficking of DAT and may provide a rationale for the cell type-specific, functional, and pathologic alterations in PD.-Kobayashi, J., Hasegawa, T., Sugeno, N., Yoshida, S., Akiyama, T., Fujimori, K., Hatakeyama, H., Miki, Y., Tomiyama, A., Kawata, Y., Fukuda, M., Kawahata, I., Yamakuni, T., Ezura, M., Kikuchi, A., Baba, T., Takeda, A., Kanzaki, M., Wakabayashi, K., Okano, H., Aoki, M. Extracellular α-synuclein enters dopaminergic cells by modulating flotillin-1-assisted dopamine transporter endocytosis.

Peripheral cardiac sympathetic hyperactivity in cardiovascular disease: role of neuropeptides

High levels of sympathetic drive in several cardiovascular diseases including postmyocardial infarction, chronic congestive heart failure and hypertension are reinforced through dysregulation of afferent input and central integration of autonomic balance. However, recent evidence suggests that a significant component of sympathetic hyperactivity may also reside peripherally at the level of the postganglionic neuron. This has been studied in depth using the spontaneously hypertensive rat, an animal model of genetic essential hypertension, where larger neuronal calcium transients, increased release and impaired reuptake of norepinephrine in neurons of the stellate ganglia lead to a significant tachycardia even before hypertension has developed. The release of additional sympathetic cotransmitters during high levels of sympathetic drive can also have deleterious consequences for peripheral cardiac parasympathetic neurotransmission even in the presence of β-adrenergic blockade. Stimulation of the cardiac vagus reduces heart rate, lowers myocardial oxygen demand, improves coronary blood flow, and independently raises ventricular fibrillation threshold. Recent data demonstrates a direct action of the sympathetic cotransmitters neuropeptide Y (NPY) and galanin on the ability of the vagus to release acetylcholine and control heart rate. Moreover, there is as a strong correlation between plasma NPY levels and coronary microvascular function in patients with ST-elevation myocardial infarction being treated with primary percutaneous coronary intervention. Antagonists of the NPY receptors Y1 and Y2 may be therapeutically beneficial both acutely during myocardial infarction and also during chronic heart failure and hypertension. Such medications would be expected to act synergistically with β-blockers and implantable vagus nerve stimulators to improve patient outcome.

Angiotensin AT1 - α2C-Adrenoceptor Interaction Disturbs α2A-auto-Inhibition of Catecholamine Release in Hypertensive Rats

α₂-Adrenoceptors lower central sympathetic output and peripheral catecholamine release, and thus may prevent sympathetic hyperactivity and hypertension. α₂AR also influence vascular tension. These α₂AR are malfunctioning in spontaneously hypertensive rats (SHR). Here I tested if an interaction between α₂AR subtypes and the angiotensin AT1 receptor (AT₁R) precipitated these disorders. Blood pressure was monitored through a femoral artery catheter and cardiac output by ascending aorta flow in anesthetized rats. Catecholamine concentrations were determined in plasma collected at the end of a 15-min tyramine-infusion. Tyramine stimulates norepinephrine release through the re-uptake transporter, thus preventing re-uptake. Presynaptic control of vesicular release is therefore reflected as differences in overflow to plasma. Previous experiments showed surgical stress to activate some secretion of epinephrine, also subjected to α₂AR-auto-inhibition. Normotensive rats (WKY) and SHR were pre-treated with (1) vehicle or α₂AR-antagonist (L-659,066), followed by fadolmidine (α_2C>B>A + α₁AR-agonist), ST-91 (α_2non-A-selective agonist), or m-nitrobiphenyline (α_2CAR-agonist + α_2A+B-antagonist), or (2) AT1R-antagonist losartan, losartan + L-659,066, or losartan + clonidine. In WKY, L-659,066 alone, L-659,066 + agonist or losartan + L-659,066 increased catecholamine overflow to plasma after tyramine and eliminated the norepinephrine-induced rise in total peripheral vascular resistance (TPR). In SHR, L-659,066 + fadolmidine/ST-91/m-nitrobiphenyline and losartan + L-659,066 greatly increased, and losartan + clonidine reduced, catecholamine concentrations, and L-659,066 + ST-91, losartan + L-659,066 and losartan + clonidine eliminated the tyramine-induced rise in TPR. Separately, these drugs had no effect in SHR. In conclusion, peripheral α_2CAR-stimulation or AT₁R-inhibition restored failing α_2AAR-mediated auto-inhibition of norepinephrine and epinephrine release and control of TPR in SHR.

APP+, a fluorescent analogue of the neurotoxin MPP+, is a marker of catecholamine neurons in brain tissue, but not a fluorescent false neurotransmitter

We have previously introduced fluorescent false neurotransmitters (FFNs) as optical reporters that enable visualization of individual dopaminergic presynaptic terminals and their activity in the brain. In this context, we examined the fluorescent pyridinium dye 4-(4-dimethylamino)phenyl-1-methylpyridinium (APP+), a fluorescent analogue of the dopaminergic neurotoxin MPP+, in acute mouse brain tissue. APP+ is a substrate for the dopamine transporter (DAT), norepinephrine transporter (NET), and serotonin transporter (SERT), and as such represented a candidate for the development of new FFN probes. Here we report that APP+ labels cell bodies of catecholaminergic neurons in the midbrain in a DAT- and NET-dependent manner, as well as fine dopaminergic axonal processes in the dorsal striatum. APP+ destaining from presynaptic terminals in the dorsal striatum was also examined under the conditions inducing depolarization and exocytotic neurotransmitter release. Application of KCl led to a small but significant degree of destaining (approximately 15% compared to control), which stands in contrast to a nearly complete destaining of the new generation FFN agent, FFN102. Electrical stimulation of brain slices at 10 Hz afforded no significant change in the APP+ signal. These results indicate that the majority of the APP+ signal in axonal processes originates from labeled organelles including mitochondria, whereas only a minor component of the APP+ signal represents the releasable synaptic vesicular pool. These results also show that APP+ may serve as a useful probe for identifying catecholaminergic innervations in the brain, although it is a poor candidate for the development of FFNs.

Distinct modes of dopamine and GABA release in a dual transmitter neuron

We now know of a surprising number of cases where single neurons contain multiple neurotransmitters. Neurons that contain a fast-acting neurotransmitter, such as glutamate or GABA, and a modulatory transmitter, such as dopamine, are a particularly interesting case because they presumably serve dual signaling functions. The olfactory bulb contains a large population of GABA- and dopamine-containing neurons that have been implicated in normal olfaction as well as in Parkinson's disease. Yet, they have been classified as nonexocytotic catecholamine neurons because of the apparent lack of vesicular monoamine transporters. Thus, we examined how dopamine is stored and released from tyrosine hydroxylase-positive GFP (TH(+)-GFP) mouse periglomerular neurons in vitro. TH(+) cells expressed both VMAT2 (vesicular monoamine transporter 2) and VGAT (vesicular GABA transporter), consistent with vesicular storage of both dopamine and GABA. Carbon fiber amperometry revealed that release of dopamine was quantal and calcium-dependent, but quantal size was much less than expected for large dense core vesicles, suggesting that release originated from small clear vesicles identified by electron microscopy. A single action potential in a TH(+) neuron evoked a brief GABA-mediated synaptic current, whereas evoked dopamine release was asynchronous, lasting for tens of seconds. Our data suggest that dopamine and GABA serve temporally distinct roles in these dual transmitter neurons.

Fluorescent dopamine tracer resolves individual dopaminergic synapses and their activity in the brain

We recently introduced fluorescent false neurotransmitters (FFNs) as optical tracers that enable the visualization of neurotransmitter release at individual presynaptic terminals. Here, we describe a pH-responsive FFN probe, FFN102, which as a polar dopamine transporter substrate selectively labels dopamine cell bodies and dendrites in ventral midbrain and dopaminergic synaptic terminals in dorsal striatum. FFN102 exhibits greater fluorescence emission in neutral than acidic environments, and thus affords a means to optically measure evoked release of synaptic vesicle content into the extracellular space. Simultaneously, FFN102 allows the measurement of individual synaptic terminal activity by following fluorescence loss upon stimulation. Thus, FFN102 enables not only the identification of dopamine cells and their processes in brain tissue, but also the optical measurement of functional parameters including dopamine transporter activity and dopamine release at the level of individual synapses. As such, the development of FFN102 demonstrates that, by bringing together organic chemistry and neuroscience, molecular entities can be generated that match the endogenous transmitters in selectivity and distribution, allowing for the study of both the microanatomy and functional plasticity of the normal and diseased nervous system.

Ganglion-specific impairment of the norepinephrine transporter in the hypertensive rat

Hypertension is associated with enhanced cardiac sympathetic transmission, although the exact mechanisms underlying this are still unknown. We hypothesized that defective function of the norepinephrine uptake transporter (NET) may contribute to the sympathetic phenotype of the spontaneously hypertensive rat, and that this may occur before the development of hypertension itself. The dynamic kinetics of NET were monitored temporally using a novel fluorescent assay of the transporter in cultured postganglionic sympathetic neurons from the cardiac stellate ganglion, the superior cervical ganglion, the celiac ganglia/superior mesenteric ganglia, and the renal sympathetic chain. All NET activity was blocked by desipramine. NET rate was significantly impaired in cardiac stellate sympathetic neurons from the prehypertensive spontaneously hypertensive rat compared with age-matched normotensive Wistar-Kyoto rats. A similar response was seen in hypertensive spontaneously hypertensive rats stellate sympathetic neurons. However, no reduction in transporter rate was observed at either age in the other major noncardiac sympathetic ganglia. Depolarization of cardiac stellate neurons by electrical field stimulation further potentiated the difference in transporter rate observed between the hypertensive and normotensive rats at both developmental ages. In conclusion, dysregulation of the norepinephrine transporter in the hypertensive rat is ganglion-specific, where NET impairment in the stellate neurons may contribute to the increased cardiac norepinephrine spillover seen in hypertension.

Mechanisms involved in nicotinic acetylcholine receptor-induced neurotransmitter release from sympathetic nerve terminals in the mouse vas deferens

Prejunctional nicotinic acetylcholine receptors (nAChRs) amplify postganglionic sympathetic neurotransmission, and there are indications that intraterminal Ca²⁺ stores might be involved. However, the mechanisms by which nAChR activation stimulates neurotransmitter release at such junctions is unknown. Rapid local delivery (picospritzing) of the nAChR agonist epibatidine was combined with intracellular sharp microelectrode recording to monitor spontaneous and field-stimulation-evoked neurotransmitter release from sympathetic nerve terminals in the mouse isolated vas deferens. Locally applied epibatidine (1 µM) produced ‘epibatidine-induced depolarisations’ (EIDs) that were similar in shape to spontaneous excitatory junction potentials (SEJPs) and were abolished by nonselective nAChR antagonists and the purinergic desensitizing agonist α,β-methylene ATP. The amplitude distribution of EIDs was only slightly shifted towards lower amplitudes by the selective α7 nAChR antagonists α-bungarotoxin and methyllcaconitine, the voltage-gated Na⁺ channel blocker tetrodotoxin or by blocking voltage-gated Ca²⁺ channels with Cd²⁺. Lowering the extracellular Ca²⁺ concentration reduced the frequency of EIDs by 69%, but more surprisingly, the Ca²⁺-induced Ca²⁺ release blocker ryanodine greatly decreased the amplitude (by 41%) and the frequency of EIDs by 36%. Ryanodine had no effect on electrically-evoked neurotransmitter release, paired-pulse facilitation, SEJP frequency, SEJP amplitude or SEJP amplitude distribution. These results show that activation of non-α7 nAChRs on sympathetic postganglionic nerve terminals induces high-amplitude junctional potentials that are argued to represent multipacketed neurotransmitter release synchronized by intraterminal Ca²⁺-induced Ca²⁺ release, triggered by Ca²⁺ influx directly through the nAChR. This nAChR-induced neurotransmitter release can be targeted pharmacologically without affecting spontaneous or electrically-evoked neurotransmitter release.