Thalamic distribution of zinc-rich terminal fields and neurons of origin in the rat

The thalamic midline nucleus reuniens: potential relevance for schizophrenia and epilepsy

Anatomical, electrophysiological and behavioral studies in rodents have shown that the thalamic midline nucleus reuniens (RE) is a crucial link in the communication between hippocampal formation (HIP, i.e., CA1, subiculum) and medial prefrontal cortex (mPFC), important structures for cognitive and executive functions. A common feature in neurodevelopmental and neurodegenerative brain diseases is a dysfunctional connectivity/communication between HIP and mPFC, and disturbances in the cognitive domain. Therefore, it is assumed that aberrant functioning of RE may contribute to behavioral/cognitive impairments in brain diseases characterized by cortico-thalamo-hippocampal circuit dysfunctions. In the human brain the connections of RE are largely unknown. Yet, recent studies have found important similarities in the functional connectivity of HIP-mPFC-RE in humans and rodents, making cautious extrapolating experimental findings from animal models to humans justifiable. The focus of this review is on a potential involvement of RE in schizophrenia and epilepsy.

Topographical analysis of reactive zinc in the central nervous system of adult zebrafish (Danio rerio)

Reactive zinc (Zn) is crucial for neuronal signaling and is largely distributed within presynaptic vesicles of some axon terminals of distinct vertebrates. However, the distribution of reactive Zn throughout the central nervous system (CNS) is not fully explored. We performed a topographical study of CNS structures containing reactive Zn in the adult zebrafish (Danio rerio). Slices of CNS from zebrafish were stained by Neo-Timm and/or cresyl violet. The Zn specificity of Neo-Timm was evaluated with Zn chelants, N,N,N',N'-Tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), sodium diethyldithiocarbamate (DEDTC), Zn sulfide washing solution, and hydrochloric acid (HCl). Unfixed slices were also immersed in the fluorescent Zn probe (zinpyr-1). Yellow-to-brown-to-black granules were revealed by Neo-Timm in the zebrafish CNS. Telencephalon exhibited slightly stained regions, while rhombencephalic structures showed high levels of staining. Although stained granules were found on the cell bodies, rhombencephalic structures showed a neuropil staining profile. The TPEN produced a mild reduction in Neo-Timm staining, while HCl and mainly DEDTC abolished the staining, indicating a large Zn content. This result was also confirmed by the application of a Zn probe. The present topographical study revealed reactive Zn throughout the CNS in adult zebrafish that should be considered in future investigation of Zn in the brain on a larger scale.

Axonal branching patterns of ventral pallidal neurons in the rat

The ventral pallidum (VP) is a key component of the cortico-basal ganglia circuits that process motivational and emotional information, and also a crucial site for reward. Although the main targets of the two VP compartments, medial (VPm) and lateral (VPl) have already been established, the collateralization patterns of individual axons have not previously been investigated. Here we have fully traced eighty-four axons from VPm, VPl and the rostral extension of VP into the olfactory tubercle (VPr), using the anterograde tracer biotinylated dextran amine in the rat. Thirty to fifty percent of axons originating from VPm and VPr collateralized in the mediodorsal thalamic nucleus and lateral habenula, indicating a close association between the ventral basal ganglia-thalamo-cortical loop and the reward network at the single axon level. Additional collateralization of these axons in diverse components of the extended amygdala and corticopetal system supports a multisystem integration that may take place at the basal forebrain. Remarkably, we did not find evidence for a sharp segregation in the targets of axons arising from the two VP compartments, as VPl axons frequently collateralized in the caudal lateral hypothalamus and ventral tegmental area, the well-known targets of VPm, while VPm axons, in turn, also collateralized in typical VPl targets such as the subthalamic nucleus, substantia nigra pars compacta and reticulata, and retrorubral field. Nevertheless, VPl and VPm displayed collateralization patterns that paralleled those of dorsal pallidal components, confirming at the single axon level the parallel organization of functionally different basal ganglia loops.

Serum or target deprivation-induced neuronal death causes oxidative neuronal accumulation of Zn2+ and loss of NAD+

Trophic deprivation-mediated neuronal death is important during development, after acute brain or nerve trauma, and in neurodegeneration. Serum deprivation (SD) approximates trophic deprivation in vitro, and an in vivo model is provided by neuronal death in the mouse dorsal lateral geniculate nucleus (LGNd) after ablation of the visual cortex (VCA). Oxidant-induced intracellular Zn(2+) release ([Zn(2+) ](i) ) from metallothionein-3 (MT-III), mitochondria or 'protein Zn(2+) ', was implicated in trophic deprivation neurotoxicity. We have previously shown that neurotoxicity of extracellular Zn(2+) required entry, increased [Zn(2+) ](i) , and reduction of NAD(+) and ATP levels causing inhibition of glycolysis and cellular metabolism. Exogenous NAD(+) and sirtuin inhibition attenuated Zn(2+) neurotoxicity. Here we show that: (1) Zn(2+) is released intracellularly after oxidant and SD injuries, and that sensitivity to these injuries is proportional to neuronal Zn(2+) content; (2) NAD(+) loss is involved - restoration of NAD(+) using exogenous NAD(+) , pyruvate or nicotinamide attenuated these injuries, and potentiation of NAD(+) loss potentiated injury; (3) neurons from genetically modified mouse strains which reduce intracellular Zn(2+) content (MT-III knockout), reduce NAD(+) catabolism (PARP-1 knockout) or increase expression of an NAD(+) synthetic enzyme (Wld(s) ) each had attenuated SD and oxidant neurotoxicities; (4) sirtuin inhibitors attenuated and sirtuin activators potentiated these neurotoxicities; (5) visual cortex ablation (VCA) induces Zn(2+) staining and death only in ipsilateral LGNd neurons, and a 1 mg/kg Zn(2+) diet attenuated injury; and finally (6) NAD(+) synthesis and levels are involved given that LGNd neuronal death after VCA was dramatically reduced in Wld(s) animals, and by intraperitoneal pyruvate or nicotinamide. Zn(2+) toxicity is involved in serum and trophic deprivation-induced neuronal death.

Zn2+ enhances the intrinsic bursting activity of a rat thalamic relay neuron

Although zinc ion (Zn2+) reduced the low-threshold T-type Ca2+ current of a rat thalamic relay neuron (TRN), we observed that Zn2+ increased a bursting activity of TRN by altering the generation and maintenance of low-threshold spike (LTS). Interestingly and importantly, Zn2+ shifted dramatically the voltage-dependence of both steady-state inactivation and activation of the transient A-type K+ current (I(A)) to a depolarizing direction. As I(A) is one of the main factors in shaping thalamic LTS, such alterations of gating properties of I(A) would contribute to the enhancement of TRN excitability under Zn2+.

Zn2+ Slows Down CaV3.3 Gating Kinetics: Implications for Thalamocortical Activity

We employed whole cell patch-clamp recordings to establish the effect of Zn2+ on the gating the brain specific, T-type channel isoform CaV3.3 expressed in HEK-293 cells. Zn2+ (300 μM) modified the gating kinetics of this channel without influencing its steady-state properties. When inward Ca2+ currents were elicited by step depolarizations at voltages above the threshold for channel opening, current inactivation was significantly slowed down while current activation was moderately affected. In addition, Zn2+ slowed down channel deactivation but channel recovery from inactivation was only modestly changed. Zn2+ also decreased whole cell Ca2+ permeability to 45% of control values. In the presence of Zn2+, Ca2+ currents evoked by mock action potentials were more persistent than in its absence. Furthermore, computer simulation of action potential generation in thalamic reticular cells performed to model the gating effect of Zn2+ on T-type channels (while leaving the kinetic parameters of voltage-gated Na+ and K+ unchanged) revealed that Zn2+ increased the frequency and the duration of burst firing, which is known to depend on T-type channel activity. In line with this finding, we discovered that chelation of endogenous Zn2+ decreased the frequency of occurrence of ictal-like epileptiform discharges in rat thalamocortical slices perfused with medium containing the convulsant 4-aminopyridine (50 μM). These data demonstrate that Zn2+ modulates CaV3.3 channel gating thus leading to increased neuronal excitability. We also propose that endogenous Zn2+ may have a role in controlling thalamocortical oscillations.

Neo-Timm staining in the thalamus of chronically epileptic rats

The thalamus is an important modulator of seizures and is severely affected in cholinergic models of epilepsy. In the present study, chronically epileptic rats had their brains processed for neo-Timm and acetylcholinesterase two months after the induction of status epilepticus with pilocarpine. Both controls and pilocarpine-treated animals presented neo-Timm staining in the anterodorsal nucleus, laterodorsal nucleus, reticular nucleus, most intralaminar nuclei, nucleus reuniens, and rhomboid nucleus of the thalamus, as well as in the zona incerta. The intensity of neo-Timm staining was similar in control and pilocarpine-treated rats, except for the nucleus reuniens and the rhomboid nucleus, which had a lower intensity of staining in the epileptic group. In animal models of temporal lobe epilepsy, zinc seems to modulate glutamate release and to decrease seizure activity. In this context, a reduction of neo-Timm-stained terminals in the midline thalamus could ultimately result in an increased excitatory activity, not only within its related nuclei, but also in anatomical structures that receive their efferent connections. This might contribute to the pathological substrate observed in chronic pilocarpine-treated epileptic animals.

Abnormal development of zinc-containing cortical circuits in the absence of the transcription factor Tailless

Absence of the transcription factor tailless (tlx) leads to premature laminar development and thinning of neocortex. We used zinc autometallography to determine if tailless deletion alters the organization of cortical circuits. In tlx-/- mice, layer 4 barrels, which normally lack synaptic zinc, are densely innervated by zinc-containing terminals. Furthermore, barrels with zinc inputs are constructed, in part, from zinc-sequestering neurons, a phenotype not normally found in layer 4.

Zinc accumulation after target loss: an early event in retrograde degeneration of thalamic neurons

Accumulation of cytoplasmic zinc is linked with a cascade of events leading to neuronal death. In many in vivo models of zinc-induced cell death, toxic concentrations of synaptically released zinc enter vulnerable neurons via neurotransmitter- or voltage-gated ion channels. In vitro studies demonstrate, in addition, that zinc can be liberated from intracellular stores following oxidative stress and contribute to cell death processes, including apoptosis. Here we describe accumulation of intracellular zinc in an in vivo model of cell death in the absence of presynaptic zinc release. We focused on the lateral geniculate nucleus (LGN) because LGN neurons undergo apoptosis when separated from their target, the primary visual cortex (V1), and the LGN is mostly devoid of zinc-containing presynaptic terminals. Infant and adult rats and adult mice received unilateral ablation of V1, either by aspiration or kainate injection. One to 14 days later, brain sections were stained with selenium autometallography or fluorescently labeled to localize zinc, or stained immunochemically for activated caspase-3. V1 lesions led to zinc accumulation in LGN neurons in infant and adult subjects. Zinc-containing neurons were evident 1-3 days after aspiration lesions, depending on age, but not until 14 days after kainate injection. Zinc accumulation was followed rapidly by immunostaining for activated caspase-3. Our data indicate that like neurotrauma and excitotoxicity, target deprivation leads to accumulation of zinc in apoptotic neurons. Moreover, zinc accumulation in vivo can occur in the absence of presynaptic zinc release. Together these findings suggest that accumulation of intracellular zinc is a ubiquitous component of the cell death cascade in neurons.

Zn2+ ions: modulators of excitatory and inhibitory synaptic activity

The role of Zn(2+) in the CNS has remained enigmatic for several decades. This divalent cation is accumulated by specific neurons into synaptic vesicles and can be released by stimulation in a Ca(2+)-dependent manner. Using Zn(2+) fluorophores, radiolabeled Zn(2+), and selective chelators, the location of this ion and its release pattern have been established across the brain. Given the distribution and possible release under physiological conditions, Zn(2+) has the potential to act as a modulator of both excitatory and inhibitory neurotransmission. Excitatory N-methyl-D-aspartate (NMDA) receptors are directly inhibited by Zn(2+), whereas non-NMDA receptors appear relatively unaffected. In contrast, inhibitory transmission mediated via GABA(A)receptors can be potentiated via a presynaptic mechanism, influencing transmitter release; however, although some tonic GABAergic inhibition may be suppressed by Zn(2+), most synaptic GABA receptors are unlikely to be modulated directly by this cation. In the spinal cord, glycinergic transmission may also be affected by Zn(2+) causing potentiation. Recently, the penetration of synaptically released Zn(2+) into neurons suggests that this ion has the potential to act as a direct transmitter, by affecting postsynaptic signaling pathways. Taken overall, present studies are broadly supportive of a neuromodulatory role for Zn(2+) at specific excitatory and inhibitory synapses.

Patterns of axonal branching of neurons of the substantia nigra pars reticulata and pars lateralis in the rat

Axons from neurons of the rat substantia nigra pars reticulata (SNr) and pars lateralis (SNl) were traced after injecting their cell body with biotinylated dextran amine. Thirty-two single axons were reconstructed from serial sagittal sections with a camera lucida, whereas four other SNr axons were reconstructed in the coronal plane to determine whether they innervate the contralateral hemisphere. Four distinct types of SNr projection neurons were identified based on their main axonal targets: type I neurons that project to the thalamus; type II neurons that target the thalamus, the superior colliculus (SC), and the pedunculopontine tegmental nucleus (PPTg); type III neurons that project to the periaqueductal gray matter and the thalamus; and type IV neurons that target the deep mesencephalic nucleus (DpMe) and the SC. The axons of the SNl showed the same branching patterns as SNr axons of types I, II, and IV. The coronal reconstructions demonstrated that SNr neurons innervate the thalamus, the SC, and the DpMe bilaterally. At the thalamic level, SNr and SNl axons targeted preferentially the ventral medial, ventral lateral, paracentral, parafascicular, and mediodorsal nuclei. Axons reaching the SC arborized selectively within the deep layers of this structure. Our results reveal that the SNr and SNl harbor several subtypes of projection neurons endowed with a highly patterned set of axon collaterals. This organization allows single neurons of these output structures of the basal ganglia to exert a multifaceted influence on a wide variety of diencephalic and midbrain structures.

Distribution of zincergic neurons in the mouse forebrain

Synaptically released zinc is thought to play an important role in neuronal signaling by modulating excitatory and inhibitory receptors and intracellular signaling proteins. Consequently, neurons that release zinc have been implicated in synaptic plasticity underlying learning and memory as well as neuropathological processes such as epilepsy, stroke, and Alzheimer's disease. To characterize the distribution of these neurons, investigators have relied on a technique that involves the retrograde transport of zinc-selenium crystals from axonal boutons to the cell bodies of origin. However, one major problem with this method is that labeling of cell bodies is obscured by high levels of staining in synaptic boutons, particularly within forebrain structures where this staining is most intense. Here, we used a modification of the retrograde labeling method that eliminates terminal staining for zinc, thereby enabling a clear and comprehensive description of these neurons. Zincergic neurons were found in all cerebral cortical regions and were arranged in a distinct laminar pattern, restricted to layers 2/3, 5, and 6 with no labeling in layer 4. In the hippocampus, labeling was present in CA1, CA3, and the dentate gyrus but not in CA2. Labeled cell bodies were also observed in most amygdaloid nuclei, anterior olfactory nuclei, claustrum, tenia tecta, endopiriform region, lateral ventricle, lateral septum, zona incerta, superior colliculus, and periaqueductal gray. Moreover, retrograde labeling was also noted in the dorsomedial and lateral hypothalamus, regions that previously were thought to be devoid of neurons with a zincergic phenotype. Collectively these data show that zincergic neurons comprise a large population of neurons in the murine forebrain and will provide an anatomical framework for understanding the functional importance of these neurons in the mammalian brain.

Nutritional iron deprivation attenuates kainate-induced neurotoxicity in rats: implications for involvement of iron in neurodegeneration

There is evidence suggesting that oxidative stress contributes to kainate neurotoxicity. Since iron promotes oxidative stress, the present study explores how change in nutritional iron content modulates kainate-induced neurotoxicity. Rats received an iron-deficient diet (ID) from 22 days of age for 4 weeks. One control group received the same diet supplemented with iron and another control group received standard rodent diet. Cellular damage after subcutaneous kainate (10 mg/kg) was assessed by silver impregnation and gliosis by staining microglia. ID reduced cellular damage in piriform and entorhinal cortex, in thalamus, and in hippocampal layers CA1-3. ID also attenuated gliosis, except in the hippocampal CA1 layer. Given involvement of zinc in hippocampal neurotransmission and in oxidative stress, we tested for a possible interaction of nutritional iron with nutritional zinc. Rats were made iron-deficient and then assigned to supplementation with iron, zinc, or iron + zinc. Controls were continued on ID diet. After 2 weeks, rats were treated with kainate. Iron supplementation abolished the protective effect of ID in piriform and entorhinal cortex. In hippocampal CA1 and dorsal thalamus, neither iron nor zinc supplementation alone abolished the protective effect of ID against cellular damage. Iron + zinc supplementation abolished ID protection in dorsal thalamus, but not in reuniens nucleus. Kainate-induced gliosis in CA1 remained unaffected by nutritional treatments. Thus, in piriform and entorhinal cortex, nutritional iron has a major impact on cellular damage and gliosis. In hippocampal CA1, gliosis may associate with synaptic plasticity not modulated by nutritional iron, while cellular damage is sensitive to nutritional iron and zinc.

Zinc transporter 3 and zinc ions in the rodent superior cervical ganglion neurons

Previous studies have revealed that zinc-enriched (ZEN) terminals are present in all parts of the CNS though with great differences in intensity. The densest populations of both ZEN terminals and ZEN somata are found in telencephalic structures, but also structures like the spinal cord demonstrate impressive ZEN systems spreading terminals several segments around the respective ZEN somata. The present study evaluates whether sympathetic neurons in the superior cervical ganglia (SCG) are ZEN neurons, i.e. contain vesicles that have zinc transporter 3 (ZnT3) proteins in their membranes and contain zinc ions. ZnT3 immunoreactivity (IR) was found in the somata and processes in the postganglionic neurons of mouse SCG. Only a small fraction of neurons (less than 5%), expressed varying degrees of ZnT3. Colchicine treatment, however, increased the number of ZnT3-positive neurons three-fold, suggesting an accumulation of ZnT3 protein in the somata. A small proportion of the postganglionic axons revealed dotted accumulations of ZnT3 IR along their courses. Double labeling showed that all ZnT3-positive neurons and axons were also tyrosine hydroxylase-positive with strong immunofluorescence, while no colocalization was found between ZnT3 and the vesicular acetylcholine transporter (VAChT) or neuropeptide Y IR. VAChT-positive preganglionic neurons were found to terminate on ZnT3 neuronal somata. 6-Methoxy 8-para toluene sulfonamide quinoline fluorescence and zinc selenium autometallography (ZnSe(AMG)) revealed that a subgroup of SCG cells contained free or loosely bound zinc ions. It is therefore concluded that ZnT3 and zinc ions are present in a subpopulation of TH-positive, NPY-negative neurons in the rodent SCG, supporting the notion that vesicular zinc ions may play a special role in the peripheral sympathetic adrenergic system.

Retrograde transport of sodium selenite and intracellular injection of micro-ruby: a combined method to describe the morphology of zinc-rich neurones

Zinc is found in synaptic vesicles in a large number of glutamatergic systems. Its involvement in neurotransmission and neurological disorders has been suggested. There are methods for tracing these circuits, but they do not fill the dendritic tree. In this study, extracellular selenite injections in vivo were combined with intracellular injection of fluorochromes in fixed tissue to reveal the morphology of these zinc-rich neurones. Intraperitoneal and intracerebral injections of sodium selenite alone or intracerebral injections of selenite combined with bisbenzimide were made in the visual cortex of the rat in order to locate the somata of zinc-rich neurones. After 24 h of retrograde transport, animals were killed and fluorescent markers were injected intracellularly into fixed slices to show neuronal morphology: (a) Lucifer Yellow (LY) followed by biocytin, (b) LY coupled to biocytin or (c) micro-ruby (MR) (dextranamines bound to rhodamine and biotin). Double-labelled somata (selenite+fluorochrome) were plotted. Details of the dendritic morphology were then revealed by incubation in avidin-biotin complex and development in 3,3'-diaminobenzidine and H(2)O(2). Camera lucida drawings showed that zinc-rich neurones in layers II-III involved in cortico-cortical visual projections were typical pyramidal neurones. This technique is noteworthy for its analysis of the morphology (and connections) of zinc-rich neurones.

Distinct pattern of c-fos mRNA expression after systemic and intra-accumbens amphetamine and MK-801

Pharmacological manipulation of both dopamine and glutamate systems affects motor responses in laboratory animals. The two systems, however, seem to act in opposite ways, since direct or indirect activation of dopamine receptors induces similar stimulatory effects to those seen following blockade of N-methyl-D-aspartate receptors. In the present study we compared the pattern of c-fos activation induced by systemic and intra-accumbens administration of the non-competitive N-methyl-D-aspartate antagonist MK-801 and the indirect dopamine agonist amphetamine. Systemic MK-801 induced c-fos mRNA expression in the motor cortex and preferentially in the motor thalamus, i.e. ventrolateral nucleus. Systemic amphetamine, on the other hand, enhanced c-fos mRNA expression in the shell of the accumbens and in limbic thalamic nuclei such as the anteroventral and anterodorsal nuclei. The main effect observed after intra-accumbens administrations of either drug was enhanced c-fos expression in the thalamus, somewhat similar to what seen following systemic administration. In fact also in this case there was a preferential activation of the limbic thalamus by amphetamine and the motor thalamus by MK-801. The present results confirm that different neural substrates underlie behavioral effects induced by systemic administrations of N-methyl-D-aspartate receptor antagonists and dopamine agonists. Further they suggest that intra-accumbens manipulation of the two neural systems could affect different efferent pathways from this structure activating different thalamic targets.

Experience-dependent plasticity of zinc-containing cortical circuits during a critical period of postnatal development

Distinctive subsets of glutamatergic neurons in cerebral cortex sequester the transition metal zinc within the synaptic vesicles of their axon terminals. In the present study we used histochemical localization of synaptic zinc to investigate normal postnatal development and experience-dependent plasticity of zinc-containing circuits in somatosensory barrel cortex of rats. First, we found that zinc-containing cortical circuits are dynamically reorganized between postnatal day (P) 0 and P28. Whereas most cortical laminae exhibited idiosyncratic increases in zinc histochemical staining with advancing age, lamina IV barrels were darkly reactive early in life and then lost much of their complement of synaptic zinc during postnatal weeks 2-4. Second, we established that sensory experience plays a major role in sculpting the zinc-containing innervation of cortical barrels. Trimming a particular facial whisker arrested the normal postnatal decline in synaptic zinc in the corresponding, deprived barrel. This resulted in more intense zinc staining in deprived barrels compared with adjacent, nondeprived barrels. Notably, the influence of experience on development of zinc circuits was most robust during a critical period extending from about P14, when an effect of whisker trimming first could be observed, through P28, after which time chronic deprivation no longer resulted in heightened levels of synaptic zinc in lamina IV. These findings indicate that sensory input can have a marked influence on development of cortical circuits, including those within lamina IV, throughout the first postnatal month.

Localization of zinc-enriched neurons in the mouse peripheral sympathetic system

Growing evidence supports the notion that zinc ions located in the synaptic vesicles of zinc-enriched neurons (ZEN) play important physiological roles and are involved in certain pathological changes in the central nervous system. Here we present data revealing the distribution of zinc ions and the co-localization of zinc transporter 3 (ZnT3) and tyrosine hydroxylase (TH) in crush-operated sciatic nerves and lumbar sympathetic ganglia of mice, using zinc selenide autometallography (ZnSe(AMG)) and ZnT3 immunofluorescence combined with confocal scanning microscopy, respectively. Six hours after the crush operation, ZnSe(AMG) grains and ZnT3 immunoreactivity were predominantly present in a subpopulation of thin unmyelinated sciatic nerve axons. In order to identify the type(s) of ZEN axons involved, double labeling with ZnT3 and (1) TH, (2) vesicular acetylcholine transporter (VAChT), (3) calcitonin gene-related peptide (CGRP), and (4) neuropeptide Y (NPY) was performed. Confocal microscopic observations showed that ZnT3 was located in a subpopulation of sciatic axons in distended parts proximal and distal to the crush site. Most, if not all, ZnT3-positive axons contained TH immunofluorescence, a few showed co-localization of ZnT3 and VAChT with very weak immunostaining, while no congruence was observed between ZnT3 and CGRP or NPY. Studies of the lumbar sympathetic ganglia showed that not more than 5% of the neurons were ZnT3-positive and that almost all of these were TH-positive. Furthermore, approximately 5% of total lumbar sympathetic ganglionic cells were ZnSe(AMG) positive, 48 h after a local injection of sodium selenide into the sciatic nerve. The present data support the notion that a subgroup of mouse sympathetic postganglionic neurons are ZEN neurons.

Is the postganglionic sympathetic neuron zinc-enriched? A stop-flow nerve crush study on rat sciatic nerve

Axonal transport of endogenous zinc ions in the rat sciatic nerve was studied by a stop-flow/nerve crush technique combined with zinc selenide autometallography (ZnSeAMG) at light and electron microscopic levels. Distinct accumulations of ZnSeAMG grains were detected, in particular proximal but also distal to the crushes, 1.5 h after the operation, and the amounts of zinc ions increased further in the following 3-8 h. Ultrastructurally, ZnSeAMG grains were located predominantly in unmyelinated axons. The data suggest that a subpopulation of sciatic nerve axons contains and transports zinc ions both antero- and retrogradely, indicating that the second neuron in the sympathetic nervous system is zinc enriched (ZEN).