Analysis of collateral projections with a double retrograde labeling technique

Intersectional, anterograde transsynaptic targeting of neurons receiving monosynaptic inputs from two upstream regions

A brain region typically receives inputs from multiple upstream areas. However, currently, no method is available to selectively dissect neurons that receive monosynaptic inputs from two upstream regions. Here, we developed a method to genetically label such neurons with a single gene of interest in mice by combining the anterograde transsynaptic spread of adeno-associated virus serotype 1 (AAV1) with intersectional gene expression. Injections of AAV1 expressing either Cre or Flpo recombinases and the Cre/Flpo double-dependent AAV into two upstream regions and the downstream region, respectively, were used to label postsynaptic neurons receiving inputs from the two upstream regions. We demonstrated this labelling in two distinct circuits: the retina/primary visual cortex to the superior colliculus and the bilateral motor cortex to the dorsal striatum. Systemic delivery of the intersectional AAV allowed the unbiased detection of the labelled neurons throughout the brain. This strategy may help analyse the interregional integration of information in the brain.

In this paper, a method is developed to genetically label neurons that receive monosynaptic inputs from two upstream regions of the brain. This could improve the analysis of interregional integration of information in neurons.

The overlap of spinothalamic and dorsal column nuclei projections in the ventrobasal complex of the rat thalamus: a double anterograde labeling study using light microscopy analysis

Projections from the spinal cord and the dorsal column nuclei (DCN) to the ventrobasal complex of the thalamus (VB) were studied in the rat by using double anterograde labeling strategy. This strategy was based on the injection of 3H-leucine into the DCN and of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the spinal cord and their subsequent transport. Adjacent 30-micron-thick sections were then processed differentially for autoradiography or for HRP by using tetramethyl benzidine (TMB) as a chromogen. Similar areas of the ventrobasal complex were labeled, in adjacent sections, after a large injection of 3H-leucine into the DCN and when wheat germ agglutinin-HRP had been injected in any part of the spinal cord. If, however, a small injection of the radioactive tracer was centered in the gracile nucleus and compared with an injection of WGA-HRP placed in the lumbar enlargement of the cord, the rostral and dorsal portions of the lateral VB were labeled from both sources. On the other hand, if tritiated leucine was injected into the cuneate nucleus, and WGA-HRP placed in the cervical enlargement, then the caudal and ventral portions of the lateral VB demonstrated overlap of both labels. The present results show that, in the rat, areas of termination of both the spinothalamic tract and the lemniscal pathway originating from the DCN overlap in the lateral VB. This overlap is somatotopically organized, thus indicating that the same area of the VB receives somatic inputs from one particular part of the body through both pathways. These results are discussed in comparison to those of comparable studies performed in the cat and in the monkey and with reference to the electrophysiological data that have demonstrated that, in the rat VB, neurons responding to noxious stimulation are intermingled with neurons exclusively responding to non-noxious stimulation.

Simultaneous labelling of two different central nervous system pathways with horseradish peroxidase and cobalt in Gnathonemus petersii and Rana esculenta

A new and simple simultaneous labelling procedure is described that gives the possibility of studying interneuronal connections and territorial overlap of different pathways. This method uses horseradish peroxidase (HRP) labelling in combination with the cobalt-filling technique. The blue colour of the HRP reaction end-product formed from 0-tolidine (Neuroscience, 4 (1979) 1805-1852) and the dark brown of cobalt-filled neurons and their processes allow easy distinction of the two different labels. Since both markers can be transported anterogradely and retrogradely by axons, this combination of the two techniques seems to be a very useful tool for neuroanatomical research. We have tested this method in a weakly electric fish, Gnathonemus petersii, and in frog, Rana esculenta.

Collateralization in the mammalian nervous system

After reviewing the loci of origin for neurons with collateralized axons, some hypotheses on their distribution in the mammalian nervous system, on their functional contributions and on their significance in the course of encephalization are discussed. In principle, the distribution of collateralized neurons seems to be restricted to anatomical circuits subserving unspecific activation of forebrain regions and controlling body balance and movements. Concerning the limbic system, a minor degree of collateralization seems to exist only in less encephalized species. Based on a number of anatomical and functional arguments, it is assumed that the significance of collateralization fades in the course of encephalization.

Retrograde double labeling of neurons: the combined use of horseradish peroxidase and diamidino yellow dihydrochloride (DY·2HCl) compared with true blue and DY·2HCl in rat descending brainstem pathways

Three series of double-labeling experiments were carried out in a study of the collateralization of brainstem nuclei which project to the spinal cord in the rat. The fluorescent tracer Diamidino Yellow Dihydrochloride (DY X 2HCl) was injected in one half of the spinal gray and white matter at T7-T8 or T13-L1. Subsequently, either True Blue (TB), wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) or free HRP were injected ipsilaterally in the gray matter at C5-C8. The distributions of single and double retrogradely labeled neurons were studied in the following cell groups: red nucleus, interstitial nucleus of Cajal, ventrolateral pontine tegmentum, nuclei coeruleus and subcoeruleus and nuclei raphe magnus and raphe pallidus including the adjoining ventral reticular formation. The numbers of TB- or HRP-labeled neurons present in those nuclei were counted and the percentages of double-labeled neurons were calculated when TB or HRP had been used in combination with DY X 2HCl. The results indicate: The HRP-TMB reaction product and the DY X 2HCl fluorescence can be visualized simultaneously in retrogradely single- or double-labeled neurons. The distributions of single- and double-labeled neurons in the various brainstem nuclei were entirely comparable when using TB with DY X 2HCl or HRP with DY X 2HCl. The percentages of double-labeled neurons obtained with HRP and DY X 2HCl were consistent over a series of cases, and were comparable to those obtained with TB and DY X 2HCl in several structures. However, in the red nucleus slightly lower percentages of double-labeled neurons were obtained using HRP and DY X 2HCl as compared with the percentages obtained using TB and DY X 2HCl.

Simultaneous visualization of Nuclear yellow and iron-dextran complex for demonstration of branched neurons by retrograde axonal transport

Retrogradely transported iron-dextran complex and Nuclear yellow could be demonstrated simultaneously on the same sections of rat CNS. Using combined bright-field and ultraviolet illuminations, double labeled neurons were observed. They exhibited a fluorescent nucleus and cytoplasmic Prussian blue granules. Thus, the combination of these two tracers appeared to be a convenient method for the anatomical demonstration of collaterals.

Bilateral serotonergic projections to the dorsal hippocampus of the rat: simultaneous localization of 3H-5HT and HRP after retrograde transport

Horseradish peroxidase (HRP, Sigma VI, 30-70 nl of a 10-15% solution in saline) or 3H-5HT (30 Ci/mmole, 2.5 X 10 -3 M containing 3.3 X 10(-3) M norepinephrine in saline, 50-100 nl) was injected unilaterally into the dorsal hippocampus in separate groups of rats. HRP-labeled cells were seen in the hippocampus, medial septal nucleus, nucleus of the diagonal band, supramammilary nucleus, median raphe nucleus, interfascicular portion of the dorsal raphe nucleus, and the locus coeruleus. In contrast, 3H-5HT-labeled cells were largely restricted to the raphe nuclei. In this nucleus an equal number of ipsilateral and bilateral cells were found. Occasionally, these labeled cells stretched across the midline (bridge pattern). In another series, the 3H-5HT and HRP were injected into the same hippocampus either as a mixture or sequentially. This resulted in double labeling of the median and dorsal raphe neurons. A final group of rats received injections of 3H-5HT and HRP into opposing hippocampi. Double-labeled cells accounted for 10% of the neurons labeled. In addition, closely paired neurons composed of an HRP- and 3H-5HT-containing cell were found. In summary, the serotonergic fibers may play a key role in harmonizing the electrical activity of the hippocampi by use of bilateral projections, paired neurons with differential projections, and bridging neurons stretching across the midline but with unilateral projections.

A retrograde double label tracing technique using horseradish peroxidase and the fluorescent dye 4'-,6-diamidino-2-phenylindole 2HC1 (DAPI)

Retrograde transport of HRP and the fluorescent dye DAPI were used to double label spinothalamic tract cells with dual projections to thalamus and cerebellum. Cells containing both retrograde tracers were found in the intermediate gray zone and in the lateral spinal nucleus of the rat lumbosacral spinal cord. Double labeled cells were easily identified by the presence of granular HRP reaction, product and the blue fluorescence of DAPI. The use of HRP and DAPI provides an effective double labeling technique for the localization of neurons with nonoverlapping projections in the central and/or peripheral nervous system.

Double retrograde neuronal labeling through divergent axon collaterals, using two fluorescent tracers with the same excitation wavelength which label different features of the cell

Recent studies show that several fluorescent substances are transported retrogradely through axons to their parent cell bodies and label in different colors different features of the cell at the same 360 nm excitation wavelength. Thus, Bisbenzimide (Bb) and "Nuclear Yellow" (NY; Hoechst S 769121) produce green and golden-yellow retrograde labeling of the neuronal nucleus. "True Blue" (TB) and "Fast Blue" (FB) produce blue retrograde labeling of the neuronal cytoplasm. In the present study the possibility of retrograde double labeling of neurons by way of divergent axon collaterals using combinations of Bb or NY with TB or FB has been explored in rat and cat. The findings show that in these animals these tracer combinations are transported retrogradely through two axon collaterals to one and the same cell. Neurons which are retrogradely double-labeled with these tracer combinations display a blue fluorescent cytoplasm and a white or golden-yellow fluorescent nucleus at the same 360 nm excitation wavelength. Therefore, these tracer combinations can be successfully used to demonstrate the existence of divergent axon collaterals in the brain.

Generation of theta rhythm in medial entorhinal cortex of freely moving rats

A regular slow wave theta rhythm can be recorded in the medial entorhinal cortex (MEC) of freely moving rats during voluntary behaviors and paradoxical sleep. Electrode penetrations normal to the cortical layers proceeding from the deeper to the more superficial layers reveal a continuous theta rhythm in layers IV-III (deep MEC theta rhythm) with an amplitude maximum in layer III, a null between the outer one-third of layer III and the inner one-half of layer I, and a continuous phase-reversed theta rhythm in layers II-I (superficial MEC theta rhythm) with an amplitude maximum there. Deep MEC theta rhythm is similar in phase and wave shape to CA1 theta rhythm; superficial MEC theta rhythm is similar in phase to DG theta rhythm. Laminar profiles throughout MEC show that the theta rhythm is generated there; it is not volume conducted from hippocampus.

Double labelling of blanched neurons in the central nervous system of the rat by retrograde axonal transport of horseradish peroxidase and iron dextran complex

The retrograde axonal transport of an iron-dextran complex leads to a labelling of neural cell bodies in the central nervous system (CNS) of the rat. This tracer and horseradish peroxidase (HRP) can both be demonstrated histochemically in same cell bodies of intralaminar thalamic neurons in the central lateral nucleus, after injection of iron-dextran in the striatum and injection of HRP in the motor cortex. This is made possible by processing the sections first for HRP and then for ferric ions by Perl's reaction. This method allows an accurate demonstration of divergent axonal projections and is compatible with cytoarchitectonic studies on the same sections.

Fluorescent retrograde double labeling: axonal branching in the ascending raphe and nigral projections

Red fluorescent Evans blue and blue fluorescent DAPI-primuline were injected into the anterior-medial and lateral-caudal forebrains, respectively, of the same rats. Separate clusters of cells labeled by retrograde transport were observed in the substantia nigra, while in the dorsal raphe many cells were double-labeled. Thus, single raphe cells send divergent axon collaterals to widespread forebrain areas.

Features of foreign proteins affecting their retrograde transport in axons of the visual system

The retrograde axoplasmic transport of foreign proteins in the rat visual system shows certain specificities. The molecular features of these proteins which may underlie their entry into the retrograde phase have been examined using biochemical and morphologic techniques. Of the isoenzymes of horseradish peroxidase (HRP), the basic isoenzyme C is strongly transported, while the acidic isoenzymes Abeta and Aalpha are transported weakly and not at all, respectively. Decreasing the isoelectric point (pI) of isoenzyme C from 8.2 to 4.4 decreases its transport, but a basic pI is not the sole requisite for transportability since two other basic peroxidases (turnip isoenzyme P7 and lactoperoxidase) are not transported in retrograde. The sugar component as a whole of isoenzyme C does not appear to be required for determining transport. Isoenzyme C and the other proteins which are transported enter multivesicular bodies in axons and axon terminals, as well as synaptic and coated vesicles and fine tubules in axon terminals. The non-transported proteins enter only the vesicular organelles thought to be involved in neurotransmitter recycling in axon terminals and do not enter multivesicular bodies. Thus the two systems of axonal membraneous compartments involved in local synaptic recycling versus the retrograde phase of transport do not show the same specificity of uptake of extracellular tracers and can be dissociated by the experimental use of these peroxidases.