Biocytin: a neuronal tracer compatible with rapid decalcification procedures

Gene transfer in the nervous system and implications for transsynaptic neuronal tracing

IMPORTANCE OF THE FIELD

Neuronal circuitries are determined by specific synaptic connections and they provide the cellular basis of cognitive processes and behavioral functions. To investigate neuronal circuitries, tracers are typically used to identify the original neurons and their projection targets.

AREAS COVERED IN THIS REVIEW

Traditional tracing methods using chemical tracers have major limitations such as non-specificity. In this review, we highlight novel genetic tracing approaches that enable visualization of specific neuronal pathways by introducing cDNA encoding a transsynaptic tracer. In contrast to conventional tracing methods, these genetic approaches use cell-type-specific promoters to express transsynaptic tracers such as wheat germ agglutinin and C-terminal fragment of tetanus toxin, which allows labeling of either the input or output populations and connections of specific neuronal type.

WHAT THE READER WILL GAIN

Specific neuronal circuit information by these genetic approaches will allow more precise, comprehensive and novel information about individual neural circuits and their function in normal and diseased brains.

TAKE HOME MESSAGE

Using tracer gene transfer, neuronal circuit plasticity after traumatic injury or neurodegenerative diseases can be visualized. Also, this can provide a good marker for evaluation of therapeutic effects of neuroprotective or neurotrophic agents.

Improved neuronal tract tracing with stable biocytin-derived neuroimaging agents

One of the main characteristics of brains is their profuse connectivity at different spatial scales. Understanding brain function evidently first requires a comprehensive description of neuronal anatomical connections. Not surprisingly a large number of histological markers were developed over the years that can be used for tracing mono- or polysynaptic connections. Biocytin is a classical neuroanatomical tracer commonly used to map brain connectivity. However, the endogenous degradation of the molecule by the action of biotinidase enzymes precludes its applicability in long-term experiments and limits the quality and completeness of the rendered connections. With the aim to improve the stability of this classical tracer, two novel biocytin-derived compounds were designed and synthesized. Here we present their greatly improved stability in biological tissue along with retained capacity to function as neuronal tracers. The experiments, 24 and 96 h postinjection, demonstrated that the newly synthesized molecules yielded more detailed and complete information about brain networks than that obtained with conventional biocytin. Preliminary results suggest that the reported molecular designs can be further diversified for use as multimodal tracers in combined MRI and optical or electron microscopy experiments.

Use of biocytin as neuroanatomic tracer in harvested human pancreas: a confocal laser scanning microscopy analysis

INTRODUCTION

To identify central neuroanatomic structure, biocytin labeling has recently been used. To date, there are no bibliographic references about the use of this molecule in investigations of the peripheral nervous system. In the present study, fresh, harvested human pancreas was used to evidence pancreatic innervations by biocytin.

AIM

To investigate for the first time pancreatic innervation in harvested pancreas from human multiorgan cadaveric donors.

METHODOLOGY

Biocytin labeling was used as a neuroanatomic tracing method, and confocal laser scanning microscopy was used for analysis for description by means of high-resolution images.

RESULTS

The application of biocytin-avidin staining in harvested human pancreas revealed numerous bundles of nervous fibers, intrapancreatic ganglia, few small solitary neurons, and a large number of positive supporting cells (glial-like cells). Biocytin appeared to pass through gap junctions between glial elements and neurons and among the neurons. In human pancreas, biocytin is rapidly transported in both anterograde and retrograde directions, with consequent visualization of fine details of pancreatic innervation morphology. Indeed, evidence of anterograde and retrograde transportation of biocytin has been demonstrated in the extensive labeling of pancreatic preganglionic and postganglionic fibers as well as a great number of chemical buds that wind through exocrine tissue or undetermined target cells.

CONCLUSION

To our knowledge, this is the first report of the successful use of biocytin in neuronal retrograde and anterograde labeling in the human peripheral nervous system.

Rapid decalcification of temporal bones with preservation of ultrastructure

Decalcification of temporal bones, especially from primates, has routinely required long periods of time and has been a major deterrent to many types of morphological studies. In this investigation, temporal bones from the monkey, Macaca fuscata, were decalcified with ethylene diamine tetraacetic acid (EDTA) in a microwave oven. To isolate effects of microwaves on decalcification, tissue was fixed and embedded using routine methods; only decalcification was carried out in the microwave oven. The procedure is described in detail. Instead of months, decalcification was complete in two working days. Control procedures included decalcification at room temperature and use of a regular oven at a temperature equal to that reached in the microwave. The ultrastructure of cochlear tissue was equal to or better than that obtained with routine decalcification.

Vital staining from dye-coated microprobes identifies new olfactory interneurons for optical and electrical recording

A versatile technique for dye application in living tissue is described, which results in labeling of viable cells from which electrophysiological or optical recordings can be obtained. The dye-coated surface of a glass microelectrode tip is used to apply anatomical tracers or calcium sensitive probes with spatial precision. A total of three types of dyes have been applied in this way to find and record from olfactory interneurons in the terrestrial mollusc Limax maximus. Crystals of 1,1'-didodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) formed on the tips of glass microelectrodes were placed in the procerebral lobe, the major olfactory processing center of Limax. Somata in buccal and pedal ganglia with processes extending several 100 microm to the procerebral lobe were stained within 4-6 h. Intracellular recordings from DiI stained buccal (B(PC)) and pedal (P(PC)) cells were obtained. Cross correlograms of the oscillatory field potential in the procerebral lobe and spontaneous action potentials in P(PC) or B(PC) show that P(PC) activity is weakly coupled to the oscillation in the procerebral lobe, while B(PC) activity is clearly coupled to the oscillation. Stimulation of the procerebral lobe with nitric oxide activated P(PC) cells but suppressed activity in B(PC) cells. Calcium green-10Kdextran coated electrodes were used to place calcium green in the cell body layer of the procerebral lobe. Bursting and nonbursting procerebral neurons incorporated and transported the calcium green-dextran. Optical recordings of changes in fluorescence signals from several bursting cells recorded simultaneously were used to test alternative mechanisms of bursting cell coupling. Application of biotin 3Kdextran to the midline of the cerebral ganglion revealed a group of cells in each procerebral lobe with processes crossing the midline of the cerebral ganglion. These cells may couple right and left procerebral lobe activity during odor processing.