Exocytosis from large and small dense cored vesicles in noradrenergic nerve terminals

Catecholamine signaling is thought to modulate cognition in an inverted-U relationship, but the mechanisms are unclear. We measured norepinephrine and dopamine release, postsynaptic calcium responses, and interactions between tonic and phasic firing modes under various stimuli and conditions. High tonic activity in vivo depleted catecholamine stores, desensitized postsynaptic responses, and decreased phasic transmission. Together, these findings provide a more complete understanding of the inverted-U relationship, offering insights into psychiatric disorders and neurodegenerative diseases with impaired catecholamine signaling.

Melanin-concentrating hormone (MCH) is synthesized by neurons located in the hypothalamus and projecting to widespread regions of the brain, including the locus coeruleus (LC), through which MCH could modulate sleep-wake states. Yet MCH does not appear to exert direct postsynaptic effects on target neurons, including the noradrenergic LC neurons. Previous studies using in situ hybridization showed that MCH neurons synthesize glutamic acid decarboxylase (GAD) and could thus utilize GABA as a neurotransmitter. To determine whether MCH varicosities can release GABA, we examined by fluorescent microscopy in the LC, whether their terminals also contain the vesicular transporter for GABA (VGAT). In dual-immunostained sections, we found that approximately 6% of MCH varicosities was immunopositive for VGAT and a similar proportion for synaptophysin, the presynaptic marker for small synaptic vesicles, whereas <1% was positive for the vesicular glutamate transporter (VGluT2). Moreover, of the MCH varicosities, ∼5% abutted puncta that were immunostained for gephyrin, the postsynaptic marker for GABAergic synapses. In triple-immunostained sections viewed with confocal laser scanning microscopy, we established that MCH varicosities that also contained VGAT or abutted upon gephyrin puncta contacted the tyrosine hydroxylase-immunostained neurons of the LC. Our results suggest that although MCH neurons can influence noradrenergic LC neurons through paracrine release and indirect effects of their peptide, they can also do so through synaptic release and direct postsynaptic effects of GABA and thus serve to inhibit the LC neurons during sleep, when they are silent, and the MCH neurons discharge.

The review describes the initial experiments suggesting a fast intra-axonal transport of transmitter related substances, in addition to the “classic” slow flow. Early experiments were mainly conducted in the peripheral adrenergic system, focusing on transport of amine storage granules, the extent of the vast sympathetic adrenergic system and the importance of axonal transport of amine granules for the adrenergic system. Further, it describes important advances obtained from studies of other neuron systems regarding local axonal protein synthesis, motor proteins and new insights regarding relation between faults in the transport machinery and some neuropathological conditions.

The release of monoamine neurotransmitters from cell bodies and dendrites has an important role in behavior, but the mechanism (vesicular or nonvesicular) has remained unclear. Because the location of vesicular monoamine transporter 2 (VMAT2) defines the secretory vesicles capable of monoamine release, we have studied its trafficking to assess the potential for monoamine release by exocytosis. In neuroendocrine PC12 cells, VMAT2 localizes exclusively to large dense-core vesicles (LDCVs), and we now show that cytoplasmic signals target VMAT2 directly to LDCVs within the biosynthetic pathway. In neurons, VMAT2 localizes to a population of vesicles that we now find undergo regulated exocytosis in dendrites. Although hippocampal neurons do not express typical LDCV proteins, transfected chromogranins A, B, and brain-derived neurotrophic factor (BDNF) colocalize with VMAT2. VMAT2 thus defines a population of secretory vesicles that mediate the activity-dependent somatodendritic release of multiple retrograde signals involved in synaptic function, growth, and plasticity.

We identified CAPS1 (calcium-dependent activator protein for secretion) as a D2 dopamine receptor interacting protein (DRIP) in a yeast two-hybrid screen of a human brain library using the second intracellular domain of the human D2 receptor (D2IC2). CAPS1 is an evolutionarily conserved calcium binding protein essential for late-stage exocytosis of neurotransmitters from synaptic terminals. CAPS1 interaction was confirmed for both the long and short isoforms of the D2 receptor, but not with any other dopamine receptor subtype. Interaction between CAPS1 and the D2 receptor was validated using both pulldown and coimmunoprecipitation assays. Deletion mapping localized the D2 receptor binding site to a segment located within the C-terminal region of CAPS1 as well CAPS2. In PC12 cells, CAPS1 and D2 receptors were found to colocalize within both cytosolic and plasma membrane compartments. Overexpression of a truncated D2 receptor fragment caused a significant decrease in K⁺-evoked dopamine release from PC12 cells, whereas no effect on norepinephrine or BDNF release was observed. These results suggest that D2 dopamine receptors may modulate vesicle release from neuroendocrine cells via direct interaction with components of the exocytotic machinery

Exocytic fusion reactions triggered by Ca²⁺ are widespread in neural, endocrine, exocrine, hemapoetic and perhaps all cell types. These processes exhibit tremendous variation in latencies to fusion following a Ca²⁺ rise and in rates of fusion. We review reported differences for synaptic vesicle (SV) and dense-core vesicle (DCV) exocytosis and attempt to identify key features in the molecular mechanisms of docking, priming and fusion of SVs and DCVs that may account for differences in speed.