Retrograde, trans-synaptic and transneuronal transport of fragment C of tetanus toxin by sympathetic preganglionic neurons

Avoiding off-target effects in electrical stimulation of the cervical vagus nerve: Neuroanatomical tracing techniques to study fascicular anatomy of the vagus nerve

Vagus nerve stimulation (VNS) is a promising therapy for treatment of various conditions that are resistant to standard medication, such as heart failure, epilepsy, and depression. The vagus nerve is a complex nerve providing afferent and efferent innervation of the pharynx, larynx, heart, tracheobronchial tree and lungs, oesophagus, stomach, liver, pancreas, small intestine and proximal colon. It is therefore a prime target for intervention for VNS. Surprisingly, the fascicular organisation of the vagus nerve at the cervical level is still not well understood. This, along with the current stimulation techniques, results in the entire nerve being stimulated, which leads to unwanted off-target effects. Neuronal tracing is a promising method to delineate the organ-specific innervation by the vagus nerve, thereby providing valuable insight into the fascicular anatomy. In this review we discuss the current knowledge of vagus nerve anatomy and neuronal tracers used for mapping of its organ-specific projections in various species. Efferent vagal projections are a chain of two neurones (pre- and postganglionic), while afferent projections consist of only one pseudounipolar neurone with one branch terminating in the target organ/tissue directly and another in the brainstem. It would be feasible to retrogradely trace the afferent fibres from their respective visceral targets and identify them at the cervical level using non-transsynaptic neuronal tracers. Using this to create a map of the functional anatomical organisation of the vagus nerve will enable selective VNS ultimately allowing for the avoidance of the off-target effects and improving overall efficacy.

IGF-1:tetanus toxin fragment C fusion protein improves delivery of IGF-1 to spinal cord but fails to prolong survival of ALS mice

To improve delivery of human insulin-like growth factor-1 (hIGF-1) to brain and spinal cord, we generated a soluble IGF-1:tetanus toxin fragment C fusion protein (IGF-1:TTC) as a secreted product from insect cells. IGF-1:TTC exhibited IGF-1 and TTC activity in vitro; it increased levels of immunoreactive phosphoAkt in treated MCF-7 cells and bound to immobilized ganglioside GT1b. In mice, the fusion protein underwent retrograde transport by spinal cord motor neurons following intramuscular injection, and exhibited both TTC- and IGF-1 activity in the CNS following intrathecal infusion. Analogous to the case with TTC, intrathecal infusion of the fusion protein resulted in substantial levels of IGF-1:TTC in spinal cord tissue extracts. Tissue concentrations of hIGF-1 in lumbar spinal cords of mice infused with IGF-1:TTC were estimated to be approximately 500-fold higher than those in mice treated with unmodified recombinant hIGF-1 (rhIGF-1). Like rhIGF-1, infusion of IGF-1:TTC reduced levels of IGF-1 receptor immunoreactivity in the same extracts. Despite raising levels of exogenous hIGF-1 in spinal cord, intramuscular- or intrathecal administration of IGF-1:TTC had no significant effect on disease progression or survival of high-expressing SOD1(G93A) transgenic mice. IGF-1:TTC may prove to be neuroprotective in other animal models of CNS disease or injury known to be responsive to unmodified IGF-1.

GFP Reconstitution Across Synaptic Partners (GRASP) defines cell contacts and synapses in living nervous systems

The identification of synaptic partners is challenging in dense nerve bundles, where many processes occupy regions beneath the resolution of conventional light microscopy. To address this difficulty, we have developed GRASP, a system to label membrane contacts and synapses between two cells in living animals. Two complementary fragments of GFP are expressed on different cells, tethered to extracellular domains of transmembrane carrier proteins. When the complementary GFP fragments are fused to ubiquitous transmembrane proteins, GFP fluorescence appears uniformly along membrane contacts between the two cells. When one or both GFP fragments are fused to synaptic transmembrane proteins, GFP fluorescence is tightly localized to synapses. GRASP marks known synaptic contacts in C. elegans, correctly identifies changes in mutants with altered synaptic specificity, and can uncover new information about synaptic locations as confirmed by electron microscopy. GRASP may prove particularly useful for defining connectivity in complex nervous systems.

Autonomic impairment in tetanus: delayed baroreflex involvement

Autonomic nervous system impairment plays an important role in the clinical course of tetanus and is thought to be responsible for life-threatening complications. It is believed to be associated with predominance of sympathetic activity. Direct baroreflex involvement has not yet been reported. We hypothesized that impaired baroreflex may contribute to the autonomic cardiovascular dysregulation in tetanus. In a patient with tetanus baroreflex sensitivity was measured on the first 5 consecutive days non-invasively using a Finometer device. Baroreflex gain was calculated as sequential cross-correlation between heart rate and blood pressure. Short-time pulse interval standard deviations (SDNN) were derived. Additionally, heart rate and arterial blood pressure were monitored and recorded continuously. Baroreflex gain values and SDNN were compared to a sex- and age-matched control subject. Compared to the control subject the patient with tetanus initially did not show a significant difference in baroreflex gain values (mean 3.68 vs 3.15, p=0.1). However, in the course of the disease an almost complete baroreflex failure occurred (mean 1.0 vs 3.15 and 0.97 vs 3.15, both p<0.0001). No correlation was found between the dynamics of baroreflex gain values and blood pressure or heart rate variability expressed by standard deviation and variance. All 5 measurements in the tetanus patient showed decreased short-time SDNN when compared to the control subject and healthy standards. In our patient we found baroreflex impairment as a part of complex autonomic dysfunction in tetanus. Furthermore, baroreflex impairment occurred only delayed. Blood pressure instability could not be explained by baroreflex dynamics. We suggest that a shift towards sympathetic activity possibly overruled the effects of decreased baroreflex sensitivity on blood pressure regulation.

VGLUT1 and VGLUT2 innervation in autonomic regions of intact and transected rat spinal cord

Fast excitatory neurotransmission to sympathetic and parasympathetic preganglionic neurons (SPN and PPN) is glutamatergic. To characterize this innervation in spinal autonomic regions, we localized immunoreactivity for vesicular glutamate transporters (VGLUTs) 1 and 2 in intact cords and after upper thoracic complete transections. Preganglionic neurons were retrogradely labeled by intraperitoneal Fluoro-Gold or with cholera toxin B (CTB) from superior cervical, celiac, or major pelvic ganglia or adrenal medulla. Glutamatergic somata were localized with in situ hybridization for VGLUT mRNA. In intact cords, all autonomic areas contained abundant VGLUT2-immunoreactive axons and synapses. CTB-immunoreactive SPN and PPN received many close appositions from VGLUT2-immunoreactive axons. VGLUT2-immunoreactive synapses occurred on Fluoro-Gold-labeled SPN. Somata with VGLUT2 mRNA occurred throughout the spinal gray matter. VGLUT2 immunoreactivity was not noticeably affected caudal to a transection. In contrast, in intact cords, VGLUT1-immunoreactive axons were sparse in the intermediolateral cell column (IML) and lumbosacral parasympathetic nucleus but moderately dense above the central canal. VGLUT1-immunoreactive close appositions were rare on SPN in the IML and the central autonomic area and on PPN. Transection reduced the density of VGLUT1-immunoreactive axons in sympathetic subnuclei but increased their density in the parasympathetic nucleus. Neuronal cell bodies with VGLUT1 mRNA occurred only in Clarke's column. These data indicate that SPN and PPN are densely innervated by VGLUT2-immunoreactive axons, some of which arise from spinal neurons. In contrast, the VGLUT1-immunoreactive innervation of spinal preganglionic neurons is sparse, and some may arise from supraspinal sources. Increased VGLUT1 immunoreactivity after transection may correlate with increased glutamatergic transmission to PPN.

A Means for Targeting Therapeutics to Peripheral Nervous System Neurons with Axonal Damage

OBJECTIVE

Delivery of biological therapeutics to motor and dorsal root ganglion neurons remains a major hurdle in the development of treatments for a variety of neurological processes, including peripheral nerve injury, pain, and motor neuron diseases. Because nerve cell bodies are important in initiating and controlling axonal regeneration, targeted delivery is an appealing strategy to deliver therapeutic proteins after peripheral nerve injury.

METHODS

Tet1 is a 12-aa peptide, isolated through phage display that is selected for tetanus toxin C fragment-like binding properties. In this study, we surveyed its uptake and retrograde transport using compartmented cultures and sciatic nerve injections. We then characterized the time course of this delivery. Finally, to confirm the retrograde transport involvement, a colchicine pretreatment was performed. We also performed competitive binding studies between Tet1 and a recombinant tetanus toxin C fragment using recombinant tetanus toxin C fragment enzyme-linked immunosorbent assay.

RESULTS

We were able to demonstrate efficient uptake and retrograde axonal transport of the Tet1 peptide in vitro and in vivo. Intraneural colchicine pretreatment partially blocked fluorescence detection in the spinal cord, revealing a retrograde axonal transport mechanism. Finally, a competitive enzyme-linked immunosorbent assay experiment revealed Tet1-specific binding to the recombinant tetanus toxin C fragment axon terminal trisialogangliosides receptor.

CONCLUSION

These properties of Tet1 can be applied to the development of therapeutic viral vectors and fusion proteins for neuronal targeting and enhanced spinal cord delivery in the treatment of nerve regeneration, neuroprotection, analgesia, and spasticity. Small peptides can be easily fused to larger proteins without significantly modifying their function and can be used to alter the binding and uptake properties of these proteins.

A glial cell line-derived neurotrophic factor (GDNF):tetanus toxin fragment C protein conjugate improves delivery of GDNF to spinal cord motor neurons in mice

Glial cell line-derived neurotrophic factor (GDNF) has shown robust neuroprotective and neuroreparative activities in various animal models of Parkinson's Disease or amyotrophic lateral sclerosis (ALS). The successful use of GDNF as a therapeutic in humans, however, appears to have been hindered by its poor bioavailability to target neurons in the central nervous system (CNS). To improve delivery of exogenous GDNF protein to CNS motor neurons, we employed chemical conjugation techniques to link recombinant human GDNF to the neuronal binding fragment of tetanus toxin (tetanus toxin fragment C, or TTC). The predominant species present in the purified conjugate sample, GDNF:TTC, had a molecular weight of approximately 80 kDa as determined by non-reducing SDS-PAGE. Like GDNF, addition of GDNF:TTC to culture media of neuroblastoma cells expressing GFRalpha-1/c-RET produced a dose-dependent increase in cellular phospho-c-RET levels. Treatment of cultured midbrain dopaminergic neurons with either GDNF or the conjugate similarly promoted both DA neuron survival and neurite outgrowth. However, in contrast to mice treated with GDNF by intramuscular injection, mice receiving GDNF:TTC revealed intense GDNF immunostaining associated with spinal cord motor neurons in fixed tissue sections. That GDNF:TTC provided neuroprotection of axotomized motor neurons in neonatal rats further revealed that the conjugate retained its GDNF activity in vivo. These results indicate that TTC can serve as a non-viral vehicle to substantially improve the delivery of functionally active growth factors to motor neurons in the mammalian CNS.

Immunoreactivity for cocaine- and amphetamine-regulated transcript in rat sympathetic preganglionic neurons projecting to sympathetic ganglia and the adrenal medulla

Many sympathetic preganglionic neurons (SPN) in the intermediolateral cell column (IML) contain cocaine- and amphetamine-regulated transcript (CART), but the function of these CART-immunoreactive (IR) neurons is unknown. To test the possibility that CART might mark SPN involved in cardiovascular regulation, we first established whether all CART neurons in the spinal cord were SPN by double-immunofluorescent labelling for CART and choline acetyltransferase (ChAT). All autonomic subnuclei contained SPN immunoreactive for ChAT plus CART. Occasional ChAT-negative, CART-positive neurons occurred adjacent to the IML, indicating the existence of CART-IR interneurons. We then retrogradely labelled SPN with cholera toxin subunit B (CTB) from a variety of targets and used double immunofluorescence to detect CTB and CART. Among SPN in the IML, 43% projecting to the coeliac ganglion, 34% projecting to the major pelvic ganglion, and about 15% projecting to the superior cervical ganglion or adrenal medulla contained CART. CART also occurred in most SPN projecting to the major pelvic ganglion from either the central autonomic area (63%) or the intercalated nucleus (58%). Finally, we used drug-induced hypotension in conscious rats to evoke Fos immunoreactivity in barosensitive SPN and immunostained to reveal Fos and CART. CART immunoreactivity was present in 41% of the Fos-IR barosensitive neurons, which were concentrated in the IML of segments T5-T13. CART-positive, Fos-negative neurons also occurred in the same segments. These results indicate that CART occurs in barosensitive SPN, nonbarosensitive SPN, and interneurons. Thus, CART is not an exclusive marker for cardiovascular SPN but is likely to influence many autonomic activities.

Tracing functionally identified neurones in a multisynaptic pathway in the hamster and rat using herpes simplex virus expressing green fluorescent protein

Using a genetically modified herpes simplex virus encoding green fluorescent protein we sought to establish if this viral modification could be used in transneuronal tracing studies of the sympathetic nervous system. The herpes simplex virus encoding green fluorescent protein was injected into the adrenal medulla of three hamsters and six rats. After a suitable survival period, neurones in the sympathetic intermediolateral cell column of the thoracolumbar spinal cord, rostral ventral medulla and paraventricular nucleus of the hypothalamus were clearly identified by the presence of a green fluorescence in the cytoplasm of the neurones of both species. Thus, herpes simplex virus encoding green fluorescent protein labelled chains of sympathetic neurones in the hamster and rat and therefore has the potential to be used in transneuronal tracing studies of autonomic pathways in these species.

Ultrastructural localization of the binding fragment of tetanus toxin in putative gamma-aminobutyric acidergic terminals in the intermediolateral cell column: a potential basis for sympathetic dysfunction in generalized tetanus

Tetanus toxin (TeTx) causes sympathetic hyperactivity, a major cause of mortality in generalized tetanus, apparently by obstructing the inhibition of sympathetic preganglionic neurons (SPNs). Neuroanatomic tracing and immunohistochemistry were used to investigate whether axon terminals in the intermediolateral cell column (IML) that synapse on SPNs and use the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) may be infected transsynaptically with TeTx. The binding fragment of TeTx (TTC; an atoxic surrogate of TeTx) and the cholera toxin B subunit (CTB; a retrograde tracer) were injected into the rat superior cervical ganglion and, over 16-48 hours, were transported to the ipsilateral IML in the caudal half of the last cervical and first three thoracic spinal cord segments. With light microscopy, diffuse CTB immunolabeling extended throughout SPN perikarya and dendrites. Punctate TTC and GABA immunolabeling were accumulated densely in the neuropil between and surrounding SPN processes. With electron microscopy, 54% of the axon terminals in the IML (n = 1,337 terminals) were TTC immunolabeled (TTC(+)), and 25% contained putative neurotransmitter levels of GABA immunolabeling (GABA(+)). On average, GABA(+) terminals had a 76% chance of also being TTC(+) and a 62% greater chance of being TTC(+) than GABA(-) terminals (P < 0.000001). Axon terminals were just as likely to be TTC(+) and/or GABA(+) regardless of whether the dendrites they synapsed on were large (>1 microM) or small in cross-sectional area or were labeled retrogradely. Sympathetic hyperactivity in tetanus may involve 1) retrograde and transsynaptic transport of TeTx by SPNs and 2) at least in part, an infection of GABAergic terminals in the IML.

Herpes simplex virus as a transneuronal tracer

Determining the connections of neural systems is critical for determining how they function. In this review, we focus on the use of HSV-1 and HSV-2 as transneuronal tracers. Using HSV to examine neural circuits is technically simple. HSV is injected into the area of interest, and after several days, the animals are perfused and processed for immunohistochemistry with antibodies to HSV proteins. Variables which influence HSV infection include species of host, age of host, titre of virus, strain of virus and phenotype of infected cell. The choice of strain of HSV is critically important. Several strains of HSV-1 and HSV-2 have been utilized for purposes of transneuronal tract-tracing. HSV has been used successfully to study neuronal circuitry in a variety of different neuroanatomical systems including the somatosensory, olfactory, visual, motor, autonomic and limbic systems.

Identification of lamina V and VII interneurons presynaptic to adrenal sympathetic preganglionic neurons in rats using a recombinant herpes simplex virus type 1

Although indirect evidence suggests that the control of sympathetic preganglionic neurons is mediated to a great extent through interneurons, little is known about the location, morphology or neurotransmitter phenotype of such interneurons. This limitation seriously impedes our understanding of spinal synaptic circuits crucial to control of arterial pressure and other visceral functions. We used a highly neurotropic, minimally cytopathic recombinant herpes simplex virus type-1 to study spinal "sympathetic" interneurons labelled by trans-synaptic transport of the virus from the adrenal gland in rats. Approximately 120-320 infected neurons/rat were identified by immunocytochemical detection of the viral antigen. We distinguished between virus-infected preganglionic neurons and infected interneurons by (i) their location within the spinal laminae, (ii) their size and shape and (iii) the presence or absence of immunoreactivity for the acetylcholine-synthesizing enzyme, choline acetyltransferase, a marker of sympathetic preganglionic neurons. Virus-labelled sympathetic preganglionic neurons were found within the known spinal preganglionic nuclei. Non-cholinergic, virus-labelled neurons were located throughout lamina VII and in the ventral portion of lamina V. These putative interneurons were found in the major spinal preganglionic nuclei, usually intermingled with the preganglionic neurons. Sometimes, they were located in clusters separate from the preganglionic neurons. The interneurons were approximately 15 microm in diameter, smaller than the average preganglionic neuron (diameter=25 microm), and had a few fine processes emanating from them. These non-cholinergic interneurons constituted approximately one-half of the population of virus-infected neurons. In summary, with the use of a recombinant herpes simplex virus, we identified a large number of non-cholinergic interneurons close to, or intermingled with, adrenal sympathetic preganglionic neurons. The neurotransmitter phenotype of these neurons remains to be determined but they likely integrate much of the supraspinal and primary afferent inputs to spinal preganglionic neurons that control arterial pressure and other visceral functions.

Axonal collateral-collateral transport of tract tracers in brain neurons: false anterograde labelling and useful tool

It is well established that some neuroanatomical tracers may be taken up by local axonal terminals and transported to distant axonal collaterals (e.g., transganglionic transport in dorsal root ganglion cells). However, such collateral-collateral transport of tracers has not been systematically examined in the central nervous system. We addressed this issue with four neuronal tracers--biocytin, biotinylated dextran amine, cholera toxin B subunit, and Phaseolus vulgaris-leucoagglutinin--in the cerebellar cortex. Labelling of distant axonal collaterals in the cerebellar cortex (indication of collateral-collateral transport) was seen after focal iontophoretic microinjections of each of the four tracers. However, collateral-collateral transport properties differed among these tracers. Injection of biocytin or Phaseolus vulgaris-leucoagglutinin in the cerebellar cortex yielded distant collateral labelling only in parallel fibres. In contrast, injection of biotinylated dextran amine or cholera toxin B subunit produced distant collateral labelling of climbing fibres and mossy fibres, as well as parallel fibres. The present study is the first systematic examination of collateral-collateral transport following injection of anterograde tracers in brain. Such collateral-collateral transport may produce false-positive conclusions regarding neural connections when using these tracers for anterograde transport. However, this property may also be used as a tool to determine areas that are innervated by common distant afferents. In addition, these results may indicate a novel mode of chemical communication in the nervous system.

Delivery of recombinant tetanus-superoxide dismutase proteins to central nervous system neurons by retrograde axonal transport

The nontoxic C fragment of tetanus toxin (TC) can transport other proteins from the circulation to central nervous system (CNS) motor neurons. Increased levels of CuZn superoxide dismutase (SOD) are protective in experimental models of stroke and Parkinson's disease, whereas mutations in SOD can cause motor neuron disease. We have linked TC to SOD and purified the active recombinant proteins in both the TC-SOD and SOD-TC orientations. Light microscopic immunohistochemistry and quantitative enzyme-linked immunosorbant assays (ELISA) of mouse brainstem, after intramuscular injection, demonstrate that the fusion proteins undergo retrograde axonal transport and transsynaptic transfer as efficiently as TC alone.

Transneuronal transport of intracellularly injected biotinamide in primary afferent axons

Transneuronal transport of biotinamide was observed following intracellular injection of biotinamide into rat jaw-muscle spindle afferent axons. Microelectrodes were advanced into the mesencephalic nucleus of the trigeminal nerve where jaw-muscle spindle afferent axons were identified by their increased firing during stretching of the jaw-elevator muscles. Biotinamide (Neurobiotin) was then injected into individual axons and the animals were maintained under anesthesia for 2-6 h. The animals were then killed via an overdose of anesthetic and the brainstem was processed histochemically. Biotinamide-filled axon collaterals and terminals were readily visible in the trigeminal motor nucleus, the trigeminal sensory nuclei, and adjacent reticular formation. In addition to these intracellularly stained axons, two to five neurons per animal (total of 36 in eight rats) were observed with a homogeneous gray reaction product distributed throughout their somata, proximal, and secondary dendrites. These neurons ranged in size from small (8-20 mu m, n - 26) to medium-sized (<30 mu m, n = 10) and were closely apposed by numerous (up to 20) biotinamide-stained spindle afferent boutons. Most of these neurons (n = 22) were located in the dorsomedial portion of the spinal trigeminal subnucleus interpolaris (Vi) 2.5-4.5 mm caudal to the intra-axonal injection site. Electron microscopic analysis in two rats suggests that the transneuronal biotinamide labeling occurred predominantly through asymmetric, axodendritic synapses between biotinamide-filled axon terminals and Vi neuronal dendrites. Although recent in vitro studies have reported that biotinamide permeates through gap junctions, in this study we found no evidence of biotinamide traversing the gap junctions which exist between trigeminal mesencephalic nucleus (Vme) neuronal somata. These results demonstrate that biotinamide can occasionally be transneuronally transported presumably via synapses; further information is needed to explain the seemingly sporadic nature of this transport.

CuZn superoxide dismutase (SOD-1):tetanus toxin fragment C hybrid protein for targeted delivery of SOD-1 to neuronal cells

Increased levels of CuZn superoxide dismutase (SOD-1) are cytoprotective in experimental models of neurological disorders associated with free radical toxicity (e.g. stroke, trauma). Targeted delivery of SOD-1 to central nervous system neurons may therefore be therapeutic in such diseases. The nontoxic C-fragment of tetanus toxin (TTC) possesses the nerve cell binding/transport properties of tetanus holotoxin and has been used as a vector to enhance the neuronal uptake of proteins including enzymes. We have now produced a recombinant, hybrid protein in Escherichia coli tandemly joining human SOD-1 to TTC. The expressed hybrid protein (SOD:Tet450) has a subunit molecular mass of 68 kDa and is recognized by both anti-SOD-1 and anti-TTC antibodies. Calculated per mol, SOD:Tet450 has approximately 60% of the expected SOD-1 enzymatic activity. Analysis of the hybrid protein's interaction with the neuron-like cell line, N18-RE-105, and cultured hippocampal neurons by enzyme immunoassay for human SOD-1 revealed that SOD:Tet451 association with cells was neuron-specific and dose-dependent. The hybrid protein was also internalized, but there was substantial loss of internalized hybrid protein over the first 24 h. Hybrid protein associated with cells remained enzymatically active. These results suggest that human SOD-1 and TTC retain their respective functional properties when expressed together as a single peptide. SOD:Tet451 may prove to be a useful agent for the targeted delivery of SOD-1 to neurons.

Identification of spinal interneurons antecedent to adrenal sympathetic preganglionic neurons using trans-synaptic transport of herpes simplex virus type 1

Control of sympathetic preganglionic neurons appears to be mediated, in part, through polysynaptic pathways using spinal interneurons. To identify spinal interneurons antecedent to adrenal sympathetic preganglionic neurons, we injected herpes simplex virus type 1 into the adrenal gland of hamsters as this virus is an effective trans-synaptic tracer of neural pathways. After a three day survival period, immunocytochemistry was used to visualize virus-infected spinal cord cells. Infected sympathetic preganglionic neurons with somata that were either kite-shaped, elliptical or fusiform and that had extensive dendrite arbors were identified as well as a group of smaller round cells with finer processes. For comparison, in additional hamsters, labelling with the retrograde tracer Fluoro-Gold and histochemical reactions for the enzyme nicotinamide adenine dinucleotide phosphate-diaphorase were used to identify sympathetic preganglionic neurons. Sympathetic preganglionic neurons identified with Fluoro-Gold or herpes virus were present mostly in the nucleus intermediolateralis, pars intermediolateralis and nucleus intermediolateralis, pars funicularis of the spinal cord. The smaller herpes virus-infected cells were found mostly medial to the preganglionic neurons in lamina VII and also dorsally in lamina V of the spinal cord. Assessing immunoreactivity for glial fibrillary acidic protein demonstrated that the smaller herpes virus-infected cells were not reactive astrocytes. Furthermore, these cells were immunoreactive for two neuronal markers, neuron-specific enolase and for microtubule-associated protein 2. These findings suggest that these smaller round cells with finer processes are distinct from sympathetic preganglionic neurons and astrocytes and may be interneurons antecedent to the sympathetic preganglionic neurons.

Transganglionic labelling of primary sensory afferents in the rat lumbar spinal cord: comparison between wheatgerm agglutinin and the I-B4 isolectin from Bandeiraea simplicifolia

We recently reported that the I-B4 isolectin from Bandeiraea simplicifolia could be used as a transganglionic neuronal tracer which appears to be selective for unmyelinated cutaneous afferents (C fibres) and their terminals in the superficial dorsal horn. As terminals in the superficial dorsal horn are also labelled by wheatgerm agglutinin, we sought to compare these two neuronal tracers. Three days after the injection of 1% wheatgerm agglutinin-HRP or 1% BSI-B4-HRP into the sciatic nerve of adult rats the lumbar spinal cord was processed for HRP reactivity. The majority of labelled structures was found in the superficial dorsal horn, with fewer labelled structures seen in the overlying white matter (including Lissauer's tract). In wheatgerm agglutinin-HRP experiments most labelled structures were synaptic terminals (63%) and unmyelinated axons (32%). About 3% of wheatgerm agglutinin-HRP-labelled structures were fine myelinated fibres (which were found only in lamina I and outer lamina II) and about 2% of label was located in neuronal somata. In contrast, label from BSI-B4-HRP experiments was found only in synaptic terminals (37%) and unmyelinated axons (63%). Previous studies have shown that small diameter dorsal root ganglion neurons and their terminals in the superficial dorsal horn express a range of structurally related carbohydrates that contain binding sites for BSI-B4 or wheatgerm agglutinin or both. Comparison of the labelling patterns produced by the two transganglionic tracers in the present study suggests that unmyelinated sciatic afferents express wheatgerm agglutinin and BSI-B4 binding sites, but some thin myelinated afferents, and a distinct form of synaptic terminal in lamina I/II outer, express the wheatgerm agglutinin binding site and not the BSI-B4 binding site.

Spinal cord lamina V and lamina VII interneuronal projections to sympathetic preganglionic neurons

This light and electron microscopic study sought to localize spinal cord interneurons that contribute to the normal and abnormal physiological regulation of spinal sympathetic preganglionic function. Sympathetic preganglionic neurons in caudal C8 through T4 of rat spinal cord were retrogradely labeled with wheat germ agglutinin (WGA) and/or cholera beta subunit (CT beta) following injections into the superior cervical ganglion (SCG). With two exceptions, the observed locations of retrogradely WGA- and CT beta-labeled sympathetic preganglionic neurons were as expected from previous studies. The exceptions were restricted populations of cells in caudal C8 and rostral T1 spinal segments. These neurons were classified as ventrolateral (vlSPN) and ventromedial (vmSPN) sympathetic preganglionic neurons; their somata and dendrites encircled dorsolateral lamina IX motoneurons. Only WGA was transported transneuronally following the retrograde labeling of sympathetic preganglionic neurons. Transneuronally WGA-labeled spinal interneurons were located principally in the reticulated division of lamina V and dorsolateral lamina VII. A strict segmental organization was observed. All transneuronally labeled interneurons were ipsilateral to, and coextensive with, retrogradely WGA-labeled sympathetic preganglionic neurons. Electron microscopic observations suggested that retrograde transsynaptic passage of WGA occurred within the sympathetic preganglionic neuropil and showed further that similar classes of organelles were WGA immunoreactive in retrogradely labeled sympathetic preganglionic neurons and in transneuronally labeled lamina V and lamina VII neurons: 1) cisternae and vesicles at the trans face of the Golgi apparatus, 2) large endosomes/dense bodies, and 3) multivesicular bodies. The data are consistent with two hypotheses: 1) Somatic and visceral primary afferent inputs to thoracic spinal cord modify segmental sympathetic preganglionic function through activation of a disynaptic pathway involving lamina V and/or lamina VII interneurons, and 2) long-loop propriospinal pathways access sympathetic preganglionic neurons through symmetrical, segmental interneuronal circuitry.

Axonal transport and MR imaging: prospects for contrast agent development

Axonal transport plays a critical role in the physiology and pathology of neurons, yet there have been virtually no clinical tools for its evaluation in human subjects. A wide variety of molecules that can act as axonal transport facilitators have been discovered and, in many cases, used to deliver labels detectable with histologic methods. Recently a number of investigators have reported preliminary success in developing intraneural contrast agents based on various versions of dextran-coated magnetite that may render magnetic resonance imaging capable of depicting axonal transport. It is not yet clear whether any clinically useful agents will eventually be developed, but there has been considerable progress in identifying design factors for such a pharmaceutical agent.

Sciatic axotomy compromises axonal transport of transganglionic tracer BSI-B4 from the soma to the central terminals of C fibre afferents

The C afferent specific lectin BSI-B4 was used to examine the effects of sciatic axotomy on axonal transport by the C afferent subpopulation. From about 4 days after sciatic nerve lesion, BSI-B4 injected into the peripheral nerve is transported only as far as the neuronal cell bodies in the dorsal root ganglion. The previous demonstrations of A beta afferent terminal sprouting into lamina II, and the atrophy of lamina II terminals, in response to sciatic lesions may be related to the inability of C afferents to maintain transganglionic transport.

Identification of renal sympathetic preganglionic neurons in hamsters using transsynaptic transport of herpes simplex type 1 virus

Herpes viruses have been used as retrograde transsynaptic tracers to identify pathways from the CNS to specific target tissues. We used herpes simplex virus to identify central nervous system neurons responsible for control of the kidney. Herpes simplex type 1 or herpes simplex type 2 was injected into rat kidneys and herpes simplex type 1 was microinjected into hamster and guinea pig kidneys. After three to seven days, ganglia, spinal cords and brains were examined using immunohistochemistry to visualize the virus-infected neurons. Our first experiments demonstrated that rats were not susceptible to infection with neurotropic strains of herpes simplex type 1. Injections of a wildtype strain of herpes simplex type 2 into rat kidneys led to nonspecific infection of many central nervous system neurons and glia. In contrast, herpes simplex type 1 injections in hamsters and guinea pigs caused specific infection of limited numbers of neurons in approximately one-third of the animals and the study was continued using hamsters. Sympathetic preganglionic neuron labelling was found in the ipsilateral intermediolateral cell column of the spinal cord as well as the lateral funiculus. Most infected preganglionic neurons were located in the seventh to the ninth thoracic spinal segments. Infected neurons were not found in the dorsal or ventral horn of the spinal gray matter and only one or two cells were found in the brainstem. Sympathetic preganglionic neuron morphology was usually normal, showing detailed dendritic arborizations, and lysis was infrequent. Small infected cells were sometimes observed close to sympathetic preganglionic neurons. Because herpes simplex type 1 virus was not detected immunocytochemically in ganglionic neurons in these same hamsters, the polymerase chain reaction was used in some additional hamsters to detect viral DNA in the T12 and T13 chain ganglia and splanchnic ganglia ipsilateral to the kidney injected with herpes simplex type 1. Finally, the overall distribution of renal postganglionic and splanchnic preganglionic neurons in hamsters was examined for comparison to the number and locations of virus-labelled neurons. Retrograde transport of the fluorescent dye FluoroGold demonstrated that (i) renal postganglionic neurons are distributed in the T10-L1 chain ganglia and in the prevertebral splanchnic ganglion and (ii) splanchnic preganglionic neurons are located in the T3-T12 spinal segments, predominantly in the intermediolateral and funicular spinal autonomic nuclei. In conclusion, herpes simplex type 1 virus infected an exclusive population of "renal" neurons in hamsters without lysis and with little cellular reaction to the infection after a survival period of three days, permitting these neurons to be studied in detail.

Specificity of pseudorabies virus as a retrograde marker of sympathetic preganglionic neurons: implications for transneuronal labeling studies

The purpose of the present study was to examine the specificity of the Bartha strain of pseudorabies virus (PRV) as a CNS retrograde marker. This information is critical in assessing whether this virus has potential value as a specific transneuronal marker. The model system chosen for analysis was the intermediolateral cell column (IML)--the principal site of origin of sympathetic preganglionic neurons (SPNs). Two experiments were performed. The first experiment established the usefulness of this model system and the second examined the properties of PRV as a retrograde cell body marker. In the first experiment, injections of two different conventional retrograde cell body markers (cholera toxin-beta subunit (CTb) and Fluoro-Gold) were made in two ipsilateral sympathetic structures (viz., stellate ganglion and adrenal gland) in the same rat. This experiment established that (1) heterogenous SPNs originate in the same cell clusters that form the IML at the T4-T8 levels and 2) SPNs innervate specific sympathetic targets with almost none providing a dual innervation of the stellate ganglion and adrenal gland. This mosaic arrangement of target-specific SPNs makes the IML an excellent CNS site for this type of study. The second experiment followed the same paradigm: PRV was injected into the stellate ganglion and CTb into the adrenal gland (and vice versa). These experiments established that PRV infections of one functional class of SPNs did not produce infections in nearby, functionally unrelated SPNs and did not cause a reduction in the SPN cell population, except under conditions of severe gliosis. These two properties increase the probability that Bartha PRV may be used as a specific retrograde transneuronal marker of central autonomic pathways.

Selective labelling of primary sensory afferent terminals in lamina II of the dorsal horn by injection of Bandeiraea simplicifolia isolectin B4 into peripheral nerves

The I-B4 isolectin from Bandeiraea simplicifolia exhibits specific binding to a subpopulation of rat dorsal root ganglion neurons of small diameter which terminate in the substantia gelatinosa of the dorsal horn. Recent double-labelling experiments in the rat have demonstrated that only primary afferents which innervate the skin are recognized by the I-B4 lectin [Plenderleith and Snow (1993) Neurosci. Lett. (in press)]. As the I-B4 lectin appears to bind selectively to a subset of small-diameter primary afferents with cutaneous peripheral projections, we sought to determine whether it could be used as a transganglionic tracer which selectively labels the spinal terminations of cutaneous afferents in superficial dorsal horn. We now report that the I-B4-horseradish peroxidase conjugate labels synaptic terminals in lamina II of the dorsal horn following the injection of the conjugate into the sciatic and saphenous nerves in the rat. Electron-microscopic examination of the dorsal horn revealed many examples of labelled synaptic terminals and unmyelinated axons, but in no cases was label observed in myelinated axons. No label was observed outside of the substantia gelatinosa; thus the I-B4 isolectin is unique among lectins used for transganglionic tracing in that it does not retrogradely label motoneurons. These results, together with previous studies of lectin binding properties of primary sensory afferents, suggest that injection of I-B4 conjugates into peripheral nerves enables the visualization of the central terminations of cutaneous C-fibres. Transganglionic labelling with the I-B4 isolectin from Bandeiraea simplicifolia should facilitate further examination of synaptic relationships of nociceptive cutaneous afferents in the superficial dorsal horn.

Essential organization of the sympathetic nervous system

The sympathetic nervous system consists of efferent neurones supplying the viscera. The cell bodies of preganglionic neurones are located in four areas in the thoracolumbar cord; however, the majority are found in the IML. Various tracing techniques have provided information concerning the location of the cell bodies of sympathetic preganglionic neurones projecting into various nerves and ganglia and regulating the adrenal gland, the kidney and the sympathetic supply to skeletal muscle. Numerous supraspinal neurones project to the neuropil surrounding sympathetic preganglionic neurones and may form synaptic contacts with these neurones. The areas of the brain that project to the IML appear to be part of a network of reciprocally connected supraspinal cell groups. Although much emphasis has been placed on the importance of the RVLM in the mediation of tonic and phasic inputs to sympathetic preganglionic neurones, it appears that other areas are of significant import; the RVLM should not be considered to be 'the vasomotor centre'. Spinal and cranial afferents influence the sympathetic nervous system. Baroreceptor afferents terminate in the NTS and may utilize an excitatory amino acid as their neurotransmitter. However, a number of neuropeptides are also associated with these afferents. Neurones within the NTS project to a number of brain stem areas thought to be involved in the regulation of sympathetic activity; consequently the baroreceptor reflex may be mediated over a number of parallel pathways involving both supraspinal and spinal sites of inhibition. Many neurotransmitters are thought to regulate the activity of sympathetic preganglionic neurons: monoamines, peptides and amino acids. Matching the chemical content of the cell bodies of neurones within a particular cell group with physiological characteristics is a challenging task; some barosensitive neurones of the RVLM do not appear to be adrenergic although they are in the midst of the C1 adrenergic cell group. Besides acetylcholine and noradrenaline, neurotransmission in the periphery appears to involve numerous peptides and ATP.

Transneuronal transfer of herpes simplex virus type 1 (HSV 1) from mixed limb nerves to the CNS. I. Sequence of transfer from sensory, motor, and sympathetic nerve fibres to the spinal cord

The time course of transneuronal transfer of Herpes simplex virus type 1 (HSV 1) from sensory, motor, and sympathetic nerve fibres to connected spinal neurones was examined. After injection of a constant number of infectious units into distal forelimb or hindlimb nerves of inbred rats of the same age, the extent of viral transfer was strictly dependent on the survival time postinoculation (p.i.). Retrograde transport to somatic motoneurones occurred at 28-29 hours p.i. (stage 1), in synchrony with anterograde transneuronal transfer via small cutaneous afferents (to laminae I-II). At 36-43 hours p.i. (stage 2), retrograde transneuronal transfer from sympathetic nerve fibres first labelled sympathetic preganglionic neurones. At 48-51 hours p.i. (stage 3), transfer via sensory and sympathetic axons became more extensive, labelling laminae III-IV and other preganglionic neurones. Transneuronal transfer from large muscle afferents and motoneurones (to Clarke's columns and the spinal intermediate zone) occurred only at 66-78 hours p.i. (stage 4). Further increases in distribution (stages 5-6) obtained between 78 and 97 hours p.i. may reflect both specific labelling of second and third order neurones and a gradual local loss of specificity. These results indicate that transfer of HSV 1 occurs through all main classes of peripheral axons, but that both anterograde and retrograde transneuronal transfer from small (unmyelinated and fine myelinated) cutaneous and sympathetic axons precedes transfer from large (myelinated) cutaneous and muscle afferents and motor axons. Analysis of viral transfer at sequential intervals is required to distinguish serially connected neurones, determine the route of labelling, and ensure its specificity.

Sympathetic and sensory neurons projecting into the cervical sympathetic trunk in the chicken

The cell bodies of the sensory and sympathetic pre- and postganglionic neurons projecting into the cervical sympathetic trunk were retrogradely labeled with horseradish peroxidase in the chicken. Preganglionic neurons were located in the spinal segments T1-T6 (maximum T2), postganglionic neurons in the paravertebral ganglia T1-T3 (maximum T1) and sensory neurons in the dorsal root ganglia T1-T4 (maximum T1). Labeled preganglionic neurons were widely distributed across the intermediate gray matter and lateral funiculus, but the majority of them were located in the intermediomedial area dorsolateral to the central canal. The short and long axis diameters of labeled preganglionic neurons in this area decreased caudally. From the data of the present study, it is estimated that about 4190 preganglionic, about 450 postganglionic and about 390 sensory neurons project into the cervical sympathetic trunk cranial to the paravertebral ganglion T1 in the chicken.

Transneuronal labeling of cochlear nucleus neurons by HRP-labeled auditory nerve fibers and olivocochlear branches in mice

Auditory nerve fibers were labeled by extracellular injections of horseradish peroxidase into the spiral ganglion in mice. The labeled fibers were traced in an anterograde direction through the auditory nerve into the cochlear nucleus. In almost half of the injections, the labeled endings of auditory nerve fibers contacted cochlear nucleus neurons that were also labeled with horseradish peroxidase and were presumably transneuronally labeled. Only darkly labeled endings were associated with transneuronally labeled neurons, but not all darkly labeled endings had targets that were transneuronally labeled. Transneuronal labeling occurred almost exclusively in the ventral cochlear nucleus, often between endbulbs and bushy cells. Both "modified" endbulbs and the larger endbulbs of Held transneuronally labeled the bushy cells that they contacted. At the ultrastructural level, transneuronal labeling was evident as a darkening of ribosomes and the membrane surfaces of mitochondria, endoplasmic reticulum, and the nucleus. Transneuronal labeling occurred rarely in octopus, small, and stellate cells, and in neurons of the dorsal cochlear nucleus. Spiral ganglion injections also label olivocochlear fibers, efferent fibers that pass through the ganglion en route to the hair cells. These fibers give off branches to the cochlear nucleus that were rarely associated with transneuronal labeling. In eight instances, the targets of olivocochlear branches were stellate cells or small cells. We suggest that in our mouse preparation, horseradish peroxidase is effective as a transneuronal marker because the short distance from injection site to the cochlear nucleus results in a high concentration of horseradish peroxidase in the endings of the auditory nerve fibers.

Neuronal lysosomal enzyme replacement using fragment C of tetanus toxin

Development of a strategy for efficient delivery of exogenous enzyme to neuronal lysosomes is essential to achieve enzyme replacement in neurodegenerative lysosomal storage diseases. We tested whether effective lysosomal targeting of the human enzyme beta-N-acetylhexosaminidase A (Hex A; beta-N-acetyl-D-hexosaminide N-acetylhexosaminohydrolase, EC 3.2.1.52) can be obtained by coupling it via disulfide linkage to the atoxic fragment C of tetanus toxin (TTC) that is bound avidly by neuronal membrane. TTC-Hex A conjugation resulted in neuronal surface binding and enhanced endocytosis of enzyme as observed in immunofluorescence studies with rat brain cultures. In immunoelectrophoretic quantitative uptake studies, rat neuronal cell cultures contained 16- and 40-fold greater amounts of enzyme after incubation with TTC-Hex A than with nonderivatized Hex A. In cerebral cortex cell cultures from a feline model of human GM2 gangliosidosis (Tay-Sachs and Sandhoff diseases), binding and uptake patterns of the enzymes were similar to those in the rat brain cell cultures. After exposure to extracellular concentrations of enzyme attainable in vivo, lysosomal storage of immunodetectable GM2 ganglioside was virtually eliminated in neurons exposed to TTC-Hex A, whereas a minimal effect was observed with Hex A. These findings demonstrate the usefulness of TTC adducts for effective neuronal lysosomal enzyme replacement.

Glycine-like immunoreactive input to sympathetic preganglionic neurons

The organization of glycine-like immunoreactive (GLY-LIR) processes was investigated within the sympathetic preganglionic neuropils of male Sprague-Dawley rats and pigeons (Columba livia). Sympathetic preganglionic neurons were retrogradely labeled with horseradish peroxidase following injections into the superior cervical ganglion in rats or into the avian homologue of the mammalian stellate ganglion (paravertebral ganglion 14) in pigeons. Glycine-like immunoreactivity was visualized using postembedding immunoperoxidase and immunogold labeling methods. The neuropils surrounding pigeon sympathetic preganglionic neurons in the principal preganglionic cell column (nucleus of Terni) and in the nucleus intercalatus contained numerous GLY-LIR puncta. Many of these processes appeared to be 'terminal-like' swellings which closely apposed retrogradely labeled preganglionic perikarya and proximal dendrites. GLY-LIR somal and dendritic processes were intermingled among retrogradely labeled preganglionic neurons in the nucleus of Terni. None of these GLY-LIR cells were retrogradely labeled. The neuropils surrounding sympathetic preganglionic neurons in the rat also contained numerous GLY-LIR puncta; there were, however, qualitative differences in the density of such profiles across the preganglionic subnuclei. Within the central autonomic and intercalated regions there were numerous GLY-LIR processes, many of which closely apposed retrogradely labeled sympathetic preganglionic somas and proximal dendrites. Within the principal preganglionic cell column, the nucleus intermediolateralis pars principalis (Ilp), there were very few GLY-LIR 'terminal-like' swellings closely apposed to cell bodies in regions of high somal packing density. In regions were this density diminished, GLY-LIR puncta closely apposed retrogradely labeled perikarya and proximal dendritic processes. GLY-LIR spinal neurons were never observed to be within Ilp proper but were present in areas immediately dorsal (lateral lamina V), medial and ventral (lateral lamina VII). GLY-LIR neurons were never retrogradely labeled. The ultrastructural features of GLY-LIR terminals within the sympathetic preganglionic neuropils of both vertebrates were nearly identical. GLY-LIR terminal boutons formed synaptic contacts with retrogradely labeled preganglionic somas as well as with large and medium-sized proximal dendrites. The majority of identified GLY-LIR terminals, however, contacted non-retrogradely labeled medium and small caliber dendrites within the preganglionic neuropils. Ninety-eight percent of GLY-LIR synapses formed symmetric specializations with the postsynaptic element. Ninety-six percent of the GLY-LIR terminal boutons contained some combination of pleomorphic vesicles. These light and electron microscopic observations support the hypothesis that glycine is localized in terminals presynaptic to sympathetic preganglionic perikarya and dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)