Delineation of the rat visual system by the axonal iontophoresis-cobalt sulfide precipitation technique

Horseradish peroxidase and cobalt chloride as neuromarkers in the hypoglossal nucleus

The hypoglossal nucleus in 129 REJ normal mouse strains was investigated using two neuroanatomical markers, namely the cobalt chloride (CoCl2) and the horseradish peroxidase (HRP) techniques. CoCl2 was introduced through the cut end of the hypoglossal nerve. In one set of experiments HRP was injected into the hypoglossal nerve, while in the other it was injected into the tongue musculature. Results show that with these techniques the hypoglossal neurons are conspicuously stained and can be easily located among series of brainstem sections. The mean number +/- SD of neurons in the hypoglossal nucleus was 1,417 +/- 37, 846 +/- 28 and 1,272 +/- 42 using CoCl2 and HRP injected into the tongue musculature or the hypoglossal nerve, respectively. The estimated length of the nucleus was 0.92 mm with the CoCl2 technique.

A whole-mount horseradish peroxidase study of the retinal central projection in normal and monocular rats

Anatomical mapping of the retinal central projection in albino rats was performed by whole-mount HRP histochemistry, after intraocular injection of HRP. The overall features of the retinal central pathways from the optic chiasma to the superior colliculus as well as the accessory optic systems in normal and monocular rats were clearly visualized, following application of a modified tetramethyl benzidine method on the whole-mounted brain stems including the diencephalon and the mid-brain. When combined with a partial retinal lesion, the whole-mount HRP method made feasible detection of the spatial retinotopy of the collicular projection.

Retinal projections in the goldfish: a study using cobaltous-lysine

Cobaltous lysine applied to the distal stump of a severed optic nerve was used to study the retinal projections of normal adult goldfish. Both the termination areas of optic axons and the pathways they traveled were established. Contrary to previous descriptions of the goldfish visual system, the optic nerves do not decussate completely at the optic chiasm. Fascicles that entered the ipsilateral optic tract innervated targets in the ipsilateral thalamus and optic tectum. Other optic fibers crossed the posterior commissure from the contralateral side of the brain and also innervated the ipsilateral tectum and thalamus. In addition, optic fibers bilaterally innervated a hypothalamic target in close proximity to the infundibulum that may correspond to the nucleus tuberis lateralis. The contralateral preoptic region contained two discrete areas of innervation, each served by separate fascicles. The ipsilateral preoptic region was similarly innervated, but more sparsely. Fibers that entered the controlateral ventral thalamus originated from three fascicles and terminated in three distinct targets. In contrast, three targets in the contralateral dorsal thalamus were served by one fascicle, and fibers passed from one nucleus to the other two. Innervation of the ipsilateral thalamus was similar to that seen contralaterally. Each main optic tract divided into three tracts, two of which entered the optic tectum, while the other innervated several pretectal areas. Other fibers innervated an accessory optic nucleus located near nucleus glomerulosus. The contralateral tectum contained numerous radially oriented optic fascicles. These fascicles represented optic fibers that left thalamic and pretectal targets to enter the optic tectum from beneath the stratum periventriculare. Optic fibers were also observed in the transverse commissure, tractus rotundus, horizontal commissure, tectobulbar tract, and fasciculus retroflexus. Therefore, it appears that many of the anomalous projections seen after tectal ablation or after optic nerve crush are not in fact aberrant. Such projections probably reflect the presence of unusually large numbers of optic fibers in tracts that normally contain optic axons, as well as increased innervation of areas that normally receive sparse retinal projections. Filled tectal cells that could represent cells projecting to the retina were not observed in either tectal lobe. The ipsilateral retinal projections could not be attributed to cobaltous-lysine being transneuronally transported in readily detectable amounts.

The cholinergic pathway to cerebral blood vessels. I. Morphological studies

The application of cobalt chloride to the peripheral cut end of the greater superficial petrosal nerve (g.s.p.n.) in rats revealed that only a few fibers in the plexus of nerves on the adventitial surface of the internal carotid artery were in axonal continuity with the g.s.p.n. A similarly small contribution of cholinergic fibers to cerebral blood vessels from this nerve was suggested by the observation that section of the g.s.p.n. resulted in an insignificant reduction in the density of the AChE-staining plexus in the internal carotid and cerebral arteries and in the incidence of at most 2% degenerate terminals of those observed on the middle cerebral artery. Alternative explanations of the results are discussed: that the AChE-staining fibers are postganglionic, that the time course for degeneration is unusually slow and that non-cholinergic fibers stain non-specifically for AChE. It is concluded that a cholinergic dilator pathway is most probably carried by the g.s.p.n. but that it is not unique.

Synapses of optic nerve afferents in the rat suprachiasmatic nucleus. I. Identification, qualitative description, development and distribution

Synapses of optic nerve afferents (optic synapses) in the rat suprachiasmatic nucleus (SCN) have been identified ultrastructurally. They are easily distinguished from other types of synapses. The optic boutons are characterized by the presence of large mitochondria with a swollen electron lucent matrix and an interconnected tubular system formed by their inner membrane. Other, more variable features include: 1) a scattered pattern of synaptic vesicles which are found throughout the entire presynaptic element with relatively little accumulation near the active zones; 2) the occurrence of dense core vesicles and glycogen granules; 3) the active zones, the majority of which is Gray-type I, but a minority can obviously be classified as Gray's type II; 4) the innervation of smaller peripheral dendrites and dendritic spines. Boutons of this kind are exclusively filled with anterogradely transported horseradish peroxidase injected into both eyes. Very few neuronal elements containing the typical mitochondria have been observed in the SCN on the 6th day post partum, increasingly more on the 9th and 12th day, but considerably higher numbers after opening of the eyes on the 17th and the following days. The location of normal and degenerating optic boutons was examined light- and electron microscopically. In the rostral third of the SCN there are relatively few optic synapses which are found close to the optic chiasma. In the middle portion of the SCN optic synapses increase in number; they are found not only in the ventral part of the nucleus but also in lateral regions. This becomes particularly obvious in the caudal third of the SCN.

Bilateral ganglion cell branches in the normal rat: a demonstration with electrophysiological collision and cobalt tracing methods

The application of microelectrode recording and anatomical tracing methods to the subcortical optic projections of the normal rat shows that the uncrossed pathway from the retina of this mammal is substantially composed of branches of crossed axons. In the recording experiments, paired stimuli to the optic tracts produces an attenuated antidromic response in the optic nerve which is best explained by the collision of impulses which travel in branches of the same parent axon. Cobalt injection of one optic tract results in retrograde filling of axons in the entire contralateral optic nerve and filling of axons in restricted regions of the ipsilateral optic nerve. This procedure also results in the filling of axons in the opposite optic tract. The locations of the filled axons in the opposite tract correspond to the positions of crossed and uncrossed projections from the temporal retinae: ventrolateral and dorsomedial, respectively. The position of the temporal retinal projections in both optic tracts was determined by applying silver degeneration methods after small lesions of the retina.

The seventh cranial nerve of the rat. Visualization of efferent and afferent pathways by cobalt precipitation

The cobalt sulphide precipitation technique, in conjunction with Timm's intensification procedure, was used to delineate the afferent and efferent intramedullary pathways of the seventh cranial nerve complex in the rat. The branchial motor nucleus with the accompanying first part, genu, and second part of the root are described. The motor branches to the superficial facial musculature do not contain fibres of geniculate ganglion origin or fibres which terminate in the spinal trigeminal nucleus. The motor branches to the deep facial muscles arise from the dorsal part of the branchial motor nucleus and traverse to the midline medial to the genu, then project under the genu into the lateral reticular formation before exiting with the facial nerve. The salivatory and lacrimal nuclei and their intramedullary pathways are described. Sensory fibres from the cutaneous auricular branch enter the spinal trigeminal tract and most of the chorda tympani gustatory fibres enter the fasciculus solitarius. A smaller number of gustatory fibres extend medially to the region of the salivatory nucleus. Fibres of greater superficial petrosal origin also enter the fasciculus solitarius as well as the medial reticular formation. These findings are discussed in relation to previous anatomical, physiological and clinical reports.

Early eye removal produces excessive bilateral branching in the rat: application of cobalt filling method

When one eye of a rat is removed at birth, axons from the remaining eye form an excess of branches which are directed to both sides of the brain. This finding, which is based on a novel application of cobalt tracing methods, provides an explanation for previous reports of expanded uncrossed projections after early eye removal.

Trigeminal projections in snakes possessing infrared sensitivity

The first order of projections of the trigeminal nerve were studied in four species of snakes, three of which are infrared sensitive, utilizing silver degeneration and cobalt iontophoresis techniques. The trigeminal nerve projected ipsilaterally to a main sensory nucleus, descending spinal nucleus, ventral trigeminal field and solitary nucleus with a contralateral projection to the base of the descending spinal nucleus in all four species. A major additional projection to the lateral nucleus of the trigeminal was present in all three infra-red sensitive species but absent in the other. Iontophoresis of cobalt chloride into a trigeminal branch composed entirely of infrared sensitive fibers and into a trigeminal branch containing no infrared fibers indicated that the lateral nucleus of the trigeminal nerve was the sole first target of the infrared system.

Retinohypothalamic projection in the mouse: electron microscopic and iontophoretic investigations of hypothalamic and optic centers

The problem of the direct retinohypothalamic projection in mammals (Moore, 1973) was reinvestigated in the laboratory mouse by electron microscopy and cobalt chloride-iontophoresis. The time-course of the axonal degeneration in the suprachiasmatic nucleus was studied 3, 6 and 12 h, 1, 2, 4, 6, 9 and 12 days after unilateral retinectomy. Specificity of the degenerative changes was controlled by investigation of the superficial layers of the superior colliculus. The ratio of crossed to uncrossed optic fibers could could be determined by counting degenerating structures (axons and terminals) in the optic chiasma and the ipsilateral and contralateral areas of the optic tract, the suprachiasmatic nucleus, and the superior colliculus. The number of degenerating axons in the suprachiasmatic nucleus showed a maximum one day after unilateral retinectomy and was, at all stages studied, two to three times higher in the contralateral than in the ipsilateral nuclear area. In the optic tract and in the superior colliculus the number of degenerating profiles was three times higher in the contralateral than in the ipsilateral area. Retinohypothalamic connections and crossing pattern of retinal fibers were studied light microscopically using impregnation with cobalt sulfide in whole mounts of brains. Most of the optic fibers in the laboratory mouse are crossed crossed (70-80%). A bundle of predominantly crossed optic fibers runs to the suprachiasmatic nucleus.