Analysis of neuronal networks: a review of techniques for labeling axonal projections

Effect of estrogen on vagal afferent projections to the brainstem in the female

The effects of 17β-estradiol (E) on the distribution and density of brainstem projections of small or large diameter primary vagal afferents were investigated in Wistar rats using transganglionic transport of wheat germ agglutinin- (WGA; preferentially transported by non-myelinated afferent C-fibers; 2%), or cholera toxin B-subunit- (CTB, 5%; preferentially transported by large myelinated afferent A-fibers) conjugated horseradish peroxidase (HRP) in combination with the tetramethylbenzidine method in age matched ovariectomized (OVX) only or OVX and treated with E (OVX+E; 30 pg/ml plasma) females for 12 weeks. Additionally, these projections were compared to aged matched males. Unilateral microinjection of WGA-HRP into the nodose ganglion resulted in dense anterograde labeling bilaterally, with an ipsilateral predominance in several subnuclei of the nucleus of the solitary tract (NTS) and in area postrema that was greatest in OVX+E animals compared to OVX only and males. Moderately dense anterograde labeling was also observed in paratrigeminal nucleus (PAT) of the OVX+E animals. CTB-HRP produced less dense anterograde labeling in the NTS complex, but had a wider distribution within the brainstem including the area postrema, dorsal motor nucleus of the vagus, PAT, the nucleus ambiguus complex and ventrolateral medulla in all groups. The distribution of CTB-HRP anterograde labeling was densest in OVX+E, less dense in OVX only females and least dense in male rats. Little, if any, labeling was found within PAT in males using either WGA-or CTB-HRP. Taken together, these data suggest that small, non-myelinated (WGA-labeled) and large myelinated (CTB-labeled) diameter vagal afferents projecting to brainstem autonomic areas are differentially affected by circulating levels of estrogen. These effects of estrogen on connectivity may contribute to the sex differences observed in central autonomic mechanisms between gender, and in females with and without estrogen.

Locomotor training maintains normal inhibitory influence on both alpha- and gamma-motoneurons after neonatal spinal cord transection

Spinal cord injuries lead to impairments, which are accompanied by extensive reorganization of neuronal circuits caudal to the injury. Locomotor training can aid in the functional recovery after injury, but the neuronal mechanisms associated with such plasticity are only sparsely known. We investigated ultrastructurally the synaptic inputs to tibialis anterior motoneurons (MNs) retrogradely labeled in adult rats that had received a complete midthoracic spinal cord transection at postnatal day 5. A subset of the injured rats received locomotor training. Both γ- and α-MNs were studied. The total number of boutons apposing γ-MNs, but not α-MNs, was reduced after neonatal spinal cord transection. The proportion of inhibitory to excitatory boutons, however, was increased significantly in both α-MNs and γ-MNs in spinally transected rats, but with locomotor training returned to levels observed in intact rats. The specific densities and compositions of synaptic boutons were, however, different between all three groups. Surprisingly, we observed the atypical presence of both C- and M-type boutons apposing the somata of γ-MNs in the spinal rats, regardless of training status. We conclude that a neonatal spinal cord transection induces significant reorganization of synaptic inputs to spinal motoneurons caudal to the site of injury with a net increase in inhibitory influence, which is associated with poor stepping. Spinal cord injury followed by successful locomotor training, however, results in improved bipedal stepping and further synaptic changes with the proportion of inhibitory and excitatory inputs to the motoneurons being similar to that observed in intact rats.

The proportion of synapses formed by the axons of the lateral geniculate nucleus in layer 4 of area 17 of the cat

The connection between the dorsal lateral geniculate nucleus (dLGN) and area 17 of the cat is a classical model for studying thalamocortical relations. We investigated the proportion of asymmetric synapses in layer 4 of area 17 of cats formed by axons of the dLGN, because this is an important morphological parameter in understanding the impact of dLGN axons on their target neurons. Although the present consensus is that this proportion is small, the exact percentage remains in doubt. Most previous work estimated that the thalamus contributes less than 10% of excitatory synapses in layer 4, but one estimate was as high as 28%. Two issues contribute to these widely different estimates, one being the tracers used, the other being the use of biased stereological approaches. We have addressed both of these issues. Thalamic axons were labeled in vivo by injections of biotinylated dextran amine into the A lamina of the dLGN of anesthetized cats. After processing, the brain was cut serially and prepared for light and electron microscopy. The density of asymmetric synapses in the neuropil and the density of synapses formed by labeled dLGN boutons were measured by using an unbiased sampling method called the physical disector. Our counts indicate that, in the fixed cat brain, there are 5.9 x 10(8) +/- 0.9 x 10(8) asymmetric synapses per cubic millimeter of layer 4 in area 17, and the dLGN input provides only 6% of all asymmetric synapses in layer 4. The vast majority of synapses of layer 4 probably originate from other neurons in area 17.

Differential synaptic inputs to the cell body and proximal dendrites of preganglionic parasympathetic neurons in the rat conus medullaris

Preganglionic parasympathetic neurons (PPNs) reside in the intermediolateral (IML) nucleus of the rat lumbosacral spinal cord and contribute to the autonomic control of visceral pelvic organs. PPNs provide the final common pathway for efferent parasympathetic information originating in the spinal cord. We examined the detailed ultrastructure of the type and organization of synaptic inputs to the cell body and proximal dendrites of PPNs in the rat conus medullaris. The PPNs were retrogradely labeled by a systemic administration of the B subunit of cholera toxin conjugated to horseradish peroxidase. We demonstrate four distinct types of synaptic boutons in apposition with PPN somata and proximal dendrites: S-type boutons show clear, spheroid vesicles; F-type boutons show flattened vesicles; dense-cored vesicle-type (DCV-type) boutons show a mixture of clear and dense-cored vesicles; L-type boutons were rare, but large, exhibited clear spheroid vesicles, and were only encountered in apposition with the PPN dendrites in our sample. The membrane surface covered by apposed boutons was markedly higher for the proximal dendrites of PPNs, compared with their somata. The inhibitory synaptic influence was markedly higher over the PPN somata compared with their proximal dendrites, as suggested by the higher proportion of putative inhibitory F-type boutons in apposition with the soma and a higher frequency of S-type boutons per membrane length for the proximal dendrites. Our studies suggest that the synaptic input to PPNs originates from multiple distinct sources and is differentially distributed and integrated over the cell membrane surface.

Glial reactions and degeneration of myelinated processes in spinal cord gray matter in chronic experimental autoimmune encephalomyelitis

Multiple sclerosis and experimental autoimmune encephalomyelitis (EAE) result in inflammatory white matter lesions in the CNS. However, information is sparse with regard to the effects of autoimmune demyelinating disease on gray matter regions. Therefore, we studied the late effects of chronic EAE in C57BL/6 mice on the spinal cord gray matter using immunohistochemistry. Here, EAE induced marked astrocytic, microglial, and macrophage activation in the ventral horn gray matter, without any motoneuron loss. Activated caspase-3 was also increased in the ventral horn gray matter. Furthermore, activated poly (ADP-ribose) polymerase (PARP), another apoptotic marker, co-localized with myelin basic protein (MBP) of oligodendrocyte processes, but not with the oligodendroglial cell body marker, adenomatous polyposis coli gene clone CC1 (APC-CC1), or with neurofilament marker (RT-97) or synaptophysin of axonal arbors. However, there was no associated increase in the number of terminal deoxynucleotidyl transferase (TdT) mediated-dUTP nick end labeling positive nuclei in the spinal cord gray matter of EAE mice. In addition, co-localization of MBP and the low-affinity neurotrophin receptor, p75, was demonstrated, further supporting the notion of apoptotic oligodendrocyte process degeneration in the gray matter of EAE mice.

Ultrastructural synaptic features differ between alpha- and gamma-motoneurons innervating the tibialis anterior muscle in the rat

We investigated the synaptology of retrogradely labeled spinal motoneurons after injection of horseradish peroxidase into the tibialis anterior (TA) muscle of adult rat. In total, 32 TA motoneurons were investigated in the electron microscope and demonstrated a bimodal size distribution with cell diameter peaks at 40 microm and 20 microm, likely representing alpha- and gamma-motoneurons, respectively. Both alpha- and gamma-motoneurons were apposed by S- and F-type synaptic boutons, whereas only alpha-motoneurons demonstrated inputs by the large M- and C-type boutons. The proportion of cell body membrane covered by synaptic inputs was surprisingly indistinguishable between alpha-motoneurons (72.2%) and gamma-motoneurons (63.5%). The ratio between the number of F- and S-type boutons in apposition with the motoneuron cell body (F/S ratio) and the ratio between the soma membrane coverage provided by F- and S-type boutons were both significantly higher in alpha- than in gamma-motoneurons. When comparing our data with previous findings in other species, we conclude that rat TA alpha-motoneurons are similar to cat and primate alpha-motoneurons with regard to synaptic terminal morphology, frequency, and distribution. However, rat gamma-motoneurons show a markedly higher total synaptic coverage and frequency than cat gamma-motoneurons, although both species exhibit appositions made by the same synaptic types and similar ratios between inhibitory and excitatory inputs.

Improved detection of fluorogold-labeled neurons in long-term studies

Fluorogold (FG) is a widely used neuroanatomical tracer. However, because FG-labeled neurons become undetectable over time, its use has been limited in long-term studies. We investigated whether the detection of FG in retrogradely labeled neurons in long-term studies can be improved by immunohistochemistry (IHC) using an antibody to FG. We performed intraperitoneal injections of a FG solution to retrogradely label all parasympathetic preganglionic neurons (PPNs) and motoneurons (MNs) in the S1 spinal cord segment in adult rats. At 1, 6, and 12 weeks after the tracer injection, sections were immunohistochemically processed for FG and choline acetyltransferase (ChAT), an endogenous marker for all PPNs and MNs. Stereological counts demonstrated no cell loss of FG-labeled PPNs and MNs at 6 and 12 weeks. Cell size measurements showed that FG-immunolabeled neurons were smaller at 12 weeks, but not at 6 weeks. However, it is likely that there was no neuronal atrophy, but loss/degradation of the dye at a timepoint between 6 and 12 weeks, as ChAT-immunolabeled neurons showed no cell size reduction at 12 weeks. Our results suggest that the use of an antibody against FG improves the detection of FG for reliable neuronal counts and that the dye is not toxic to the retrogradely labeled neurons. We conclude that FG-labeling is a useful tool to determine neuronal counts in long-term studies, but should be used cautiously for neuronal size measurements.

Systemic administration of cholera toxin B subunit conjugated to horseradish peroxidase in the adult rat labels preganglionic autonomic neurons, motoneurons, and select primary afferents for light and electron microscopic studies

Retrograde and transganglionic labeling techniques are commonly used to visualize subsets of neurons and sensory afferent projections in the nervous system. These methods commonly require anesthesia and surgical methods. However, some tracers can also be administered systemically in awake animals, thus reducing risks associated with anesthesia and surgery and allowing for labeling of neuronal populations that are difficult to label with local tracer injections. Here, we demonstrate in the adult rat that intraperitoneal administration of cholera toxin subunit B conjugated to horseradish peroxidase labels preganglionic autonomic neurons, motoneurons, and the terminal projections of select primary afferents for both light and electron microscopic studies. We demonstrate also that this method can be combined with post-embedding immunogold labeling to detect amino acid transmitters in synaptic boutons.

Quantitative analysis of synaptic distribution along thalamocortical axons in adult mouse barrels

Quantitative data on thalamocortical synapses in adult mouse barrels have been obtained largely by using lesion-nduced degeneration to label thalamic afferents. By the time degenerating axons can be identified with the electron microscope, they have broken up into many separate pieces, making it impossible to assess the distribution of synapses along unbroken lengths of afferent. Here, this deficiency is rectified by examining intact lengths of axon labeled by the injection of biotinylated dextran amine into ipsilateral thalamus. Serial thin section reconstructions were analyzed to determine the numbers of synapses per axon length made with dendritic spines vs. shafts and the locations of synapses with respect to axonal varicosities. Results for seven axonal segments from six mice showed an average of 0.2 synapses/microm; 80% were made with spines and 20% with dendritic shafts. Just over two-thirds of axonal varicosities formed one synapse; most of the remainder formed two and rarely three, whereas 8% formed none. Although most synapses occurred at varicosities (88%), more than 12% were made at cylindrically shaped regions of the reconstructed axonal segments. These results serve as a caveat for the use of light microscopy to quantify synapses, wherein the usual approach is to equate one varicosity with one synapse. For thalamocortical afferents to mouse barrels, equating one varicosity with one synapse would prove to be incorrect more than 30% of the time and would exclude the roughly 12% of synaptic connections made at cylindrical regions of thalamocortical afferents.

Quantitative estimation of 3-D fiber course in gross histological sections of the human brain using polarized light

Series of polarized light images can be used to achieve quantitative estimates of the angles of inclination (z-direction) and direction (in xy-plane) of central nervous fibers in histological sections of the human brain. (1) The corpus callosum of a formalin-fixed human brain was sectioned at different angles of inclination of nerve fibers and at different thicknesses of the samples. The minimum, and maximum intensities, and their differences revealed a linear relationship to the angle of inclination of fibers. It was demonstrated that sections with a thickness of 80--120 microm are best suited for estimating the angle of inclination. (2) Afterwards the optic tracts of eight formalin-fixed human brains were sliced at different angles of fiber inclination at 100 microm. Measurements of intensity in 30 pixels in each section were used to calculate a linear function of calibration. The maximum intensities and the differences between maximum and minimum values measured with two polars only were best suited for estimation of fiber inclination. (3) Gross histological brain slices of formalin-fixed human brains were digitized under azimuths from 0 to 80 degrees using two polars only. These sequences were used to estimate the inclination of fibers (in z-direction). The same slices were digitized under azimuths from 0 to 160 degrees in steps of 20 degrees using a quarter wave plate additionally. These sequences were used to estimate the direction of the fibers in xy-direction. The method can be used to produce maps of fiber orientation in gross histological sections of the human brain similar to the fiber orientation maps derived by diffusion weighted magnetic resonance imaging.

Visualization of nerve fiber orientation in gross histological sections of the human brain

Diffusion weighted magnetic resonance imaging (DWMRI) allows visualization of the orientation of the nervous fibers in the living brain. For comparison, a method was developed to examine the orientation of fibers in histological sections of the human brain. Serial sections through the entire human brain were analyzed regarding fiber orientation using polarized light. Direction of fibers in the cutting plane was obtained by measuring the azimuth with the lowest intensity value at each point, and inclination of fibers in the section was evaluated using fuzzy logic approximations. Direction and inclination of fibers revealing their three-dimensional orientation were visualized by colored arrows mapped into the images. Using this procedure, various fiber tracts were identified (pyramidal tract, radiatio optica, radiatio acustica, arcuate fascicle, and 11 more). Intermingled fibers could be separated from each other. The orientation of the fiber tracts derived from polarized light microscopy was validated by confocal laser scanning microscopy in a defined volume of the internal capsule, where the fiber orientation was studied in four human brains. The polarization method visualizes the high degree of intermingled fiber bundles in the brain, so that distinct fiber pathways cannot be understood as solid, compact tracts: Neighbouring bundles of fibers can belong to different systems of fibers distinguishable by their orientation.

Electron microscopic imaging of multiple markers in glutaraldehyde fixed CNS tissue of Macaca fascicularis: maximizing information from a single experimental animal

Sensory and motor pathways in the central nervous system (CNS) of macaque monkeys were visualized by anterograde or retrograde axonal transport of wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) reacted with the chromagen tetramethylbenzidine (TMB), or by the use of anterograde degeneration after specific ablation lesions. To maximize information from each animal we combined the results of the anterograde and retrograde axonal transport with several pre- and post-embedding markers at both the light and electron microscopic levels while maintaining good preservation of tissue. Pre-embedding techniques included those for cytochrome oxidase activity and the calcium-binding proteins calbindin D-28k and parvalbumin. Post-embedding techniques included immunocytochemistry for gamma-aminobutyric acid (GABA) or other amino acid neurotransmitters. We believe that the methods described here provide superior tissue preservation, thus permitting a more detailed analysis of tissue prepared after experiments concerned with neural circuitry.